# [​4+2] versus [​2+2] Homodimerization in P(V) Derivatives of 2,4-Disubstituted Phospholes

Authors: Guillaume Bousrez; Emmanuel Nicolas; Agathe Martinez; Sylviane Chevreux; Florian Jaroschik
Journal: Heteroatom Chemistry
 (2019)

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## Abstract

Phosphole P(V) derivatives are interesting building blocks for various applications from ligand synthesis to material sciences. We herein describe the preparation and characterisation of new 2,4-disubstituted oxo-, thiooxo-, and selenooxophospholes. The nature of the substituents on the phosphole ring determines the reactivity of these compounds towards homodimerization reactions. Aryl and trimethylsilyl substituted oxophospholes undergo selective[4+2] dimerization, whereas, for thiooxo- and selenooxophospholes, light-induced, selective [2+2] head-to-head dimerization occurs in the case of aryl substituents. DFT calculations provide some insights on these differences in reactivity.


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## Body

## 1. Introduction

Phospholes have found widespread interest in many areas, including catalysis, material sciences, and biological applications, due to the ready modification of their electronic, steric, and physicochemical properties and their different coordination modes to metals [1–14]. Among the various possibilities, changing the substituents or the substitution pattern on the phosphole ring or on the phosphorus atom can modify the stability and the aromaticity of these heterocycles [15–17]. A major change is often induced by oxidation of the weakly aromatic P(III) compounds to non- or anti-aromatic P(V) derivatives employing oxygen/peroxides, sulphur, or selenium [18]. In material sciences and biological applications, the P(V) phospholes are mainly employed [8–14], whereas for catalysis and coordination chemistry purposes the P(III) heterocycles act as phosphine ligands or cyclopentadienyl analogues, or a combination of both [4–7].Some years ago, we reported the synthesis of a series of 2,4-disubstituted phospholes1 through a highly selective transformation of a mixture of 2,4- and 2,5-disubstituted zirconacyclopentadienes (Scheme 1) [19]. In this reaction, the addition of PPhCl2 led only to the formation of phospholes1; no other regioisomer was observed. Quenching the reaction mixture with HCl yielded the 1,4-disubstituted 1,3-butadienes, which could be readily separated from1. This methodology was successfully applied to aryl, alkyl, and trimethylsilyl groups on the phosphole ring.Scheme 1
Selective formation of 2,4-disubstituted phospholes1.We herein show that the oxidation of some of these heterocycles to the corresponding oxo-, thiooxo-, and selenooxophospholes can lead to various reactivity behaviours with respect to homodimerization processes, i.e.,[4+2] or [2+2] cycloaddition reactions.


## 2. Materials and Methods

### 2.1. General Considerations

Dichloromethane was collected under argon from a PURSOLV MD-3 (Innovative Technologie Inc.) solvent purification unit.m-chloroperbenzoic acid, sulphur, and selenium were purchased from Aldrich or Alfa Aesar. Phospholes1a-d were prepared according to the literature procedure [19]. 1H, 13C, 19F, 29Si, 31P, and 77Se NMR spectra were recorded in CDCl3, unless specified, on a 500 MHz Bruker Avance III spectrometer equipped with a BBFO+ probe. Chemical shifts are reported in delta (δ) units, expressed in parts per million (ppm). High resolution ESI-MS spectra were recorded on a hybrid tandem quadrupole/time-of-flight (Q-TOF) instrument, equipped with a pneumatically assisted electrospray (Z-spray) ion source (Micromass, Manchester, UK) operated in positive mode. High resolution EI-MS spectra were obtained on a GCT-TOFmass spectrometer (Micromass, Manchester, UK) with EI source. Single crystals of6d were coated in Paratone-N oil and mounted on a loop. Data were collected at 150.0(1) K on a Nonius Kappa CCD diffractometer using a Mo Kα (λ= 0.71070 Å) X-ray source and a graphite monochromator. All data were measured using phi and omega scans. The crystal structure was solved using SIR 97 and refined using Shelx 2016 [20, 21]. DFT calculations were performed using the Gaussian09 suite of software (full details are provided in the SI (available here)).


### 2.2. General Procedure for the Synthesis of Oxophospholes2 and the [4+2] Dimers3

In a flask equipped with a magnetic stir bar, phosphole1a-c (5 mmol), dichloromethane (5 mL), andm-chloroperbenzoic acid (mCPBA) (6 mmol) were introduced. After stirring for 5 minutes at room temperature, the solution was filtered and the solvent evaporated under reduced pressure. The 31P NMR spectrum (CDCl3) of the crude residue showed complete conversion of starting phosphole and formation of oxophospholes2a-c and several nonidentified by-products. After a given time (several hours to days) compounds2a,b transform to the dimers3a,b in solution or in the solid state.


### 2.3. General Procedure for the Synthesis of Thiooxophospholes4and the [2+2]dimers5

In a flask equipped with a magnetic stir bar, phosphole1a-d (5 mmol), elemental sulphur (S8) (1 mmol), and dichloromethane (5 mL) were introduced. After stirring overnight at room temperature, the solution was filtered, and then the solvent evaporated under reduced pressure. The 31P NMR spectrum (CDCl3) of the crude product showed complete conversion to4a-d. Under the influence of natural light, compounds4a,d transformed to the dimers5a,d after several days in solution in a classical NMR tube or in the solid state.


### 2.4. General Procedure for the Synthesis of Selenooxophospholes6and the [2+2] Dimer7

In a flask equipped with a magnetic stir bar, phosphole1a-c (5 mmol), elemental selenium (Se) (6 mmol), and dichloromethane (5 mL) were introduced. After stirring overnight at room temperature, the solution was filtered and the solvent evaporated under reduced pressure. The 31P NMR spectra (CDCl3) of the crude product showed complete conversion to6a-c. Under the influence of natural light, compound6a transformed to the dimer7a after several days in solution in a classical NMR tube or in the solid state.


#### 2.4.1. 1,10-Diphenyl-2,4,6,8-tetrakis(trimethysilyl)-1,10-diphosphatricyclo[5.6.5]deca-2,6-diene-1,10-dioxide (3b)

H   1 (500 MHz, C D C l 3 ). -0.13 (s, 9H), -0.08 (s, 9H), 0.13 (s, 9H), 0.37 (s, 9H), 3.31 (d, JP-H = 4.5 Hz, 1H), 3.45 (s, 1H), 6.70 (d, JP-H = 49.0 Hz, 1H), 6.96 (d, JP-H = 14.0 Hz, 1H), 7.31-7.34 (m, 2H), 7.41-7.50 (m, 4H), 7.64-7.68 (m, 2H), 7.78 (dd, JH-H = 8.0 Hz, JH-H = 8.0 Hz, 2H).C   13 (125 MHz, C D C l 3 ). -2.2 (CH3), -0.6 (CH3), 0.10 (d, JP-C = 2.6 Hz, CH3), 1.2 (CH3), 46.3 (d, JP-C = 45.3 Hz, C), 51.7 (d, JP-C = 8.4 Hz, CH), 52.5 (d, JP-C = 60.6 Hz, CH), 53.1 (dd, JP-C = 15.0 Hz, JP-C = 15.0 Hz, C), 127.8 (d, JP-C = 10.3 Hz, CH), 128.3 (d, JP-C = 11.4 Hz, CH), 129.3 (d, JP-C = 83.3 Hz, C), 131.5 (d, JP-C = 10.1 Hz, CH), 131.5 (d, JP-C = 3.0 Hz, CH), 131.7 (d, JP-C = 2.4 Hz, CH), 134.2 (d, JP-C = 7.0 Hz, CH), 135.5 (d, JP-C = 91.0 Hz, C), 136.2 (d, JP-C = 61.5 Hz, C), 142.5 (d, JP-C = 8.5 Hz, C), 143.9 (dd, JP-C = 10.3 Hz, JP-C = 5.5 Hz, CH), 164.4 (dd, JP-C = 17.9 Hz, JP-C = 10.3 Hz, CH).P   31 (200 MHz, C D C l 3 ). 64.4 (d, JP-P = 37.0 Hz), 83.2 (d, JP-P = 37.0 Hz).S i   29 (100 MHz, C D C l 3 ). -8.3 (d, JP-Si = 6.4 Hz), -6.7 (dd, JP-Si = 15.5 Hz, JP-Si = 2.1 Hz), 1.8 (s), 8.1 (d, JP-Si = 5.5 Hz).


#### 2.4.2. 1,2,4-Triphenyl-thiooxophosphole (4a)

H   1 (500 MHz, Acetone- d 6 ). 6.85 (dd, JP-H = 31.0 Hz, JH-H = 1.5 Hz, 1H), 7.31-7.33 (m, 2H), 7.51-7.54 (m, 5H), 7.95 (dd, JH-H = 8.5 Hz, JH-H = 1.5 Hz, 1H), 7.97-7.99 (m, 3H), 8.13 (dd, JP-H = 40.5 Hz, JH-H = 1.5 Hz, 1H).C   13 (125 MHz, Acetone- d 6 ).123.1 (d, JP-C = 86.1 Hz, CH), 128.0 (CH), 128.1 (d, JP-C = 6.4 Hz, CH), 129.6 (CH), 129.7 (C), 129.9 (CH), 130.0 (d, JP-C = 2.3 Hz, CH), 130.1 (d, JP-C = 7.5 Hz, CH), 131.3 (d, JP-C = 11.7 Hz, CH), 133.1 (d, JP-C = 3.0 Hz, CH), 133.3 (d, JP-C = 12.3 Hz, C), 134.9 (d, JP-C = 20.1 Hz, CH), 135.3 (d, JP-C = 16.5 Hz, C), 144.6 (d, JP-C = 74.0 Hz, C), 152.6 (d, JP-C = 17.1 Hz, C).P   31 (200 MHz, Acetone- d 6 ). 52.7 (s)


#### 2.4.3. 1-Phenyl-2,4-bis(trimethylsilyl)-thiooxophosphole (4b)

H   1 (500 MHz, C D C l 3 ). 0.05 (s, 9H), 0.25 (s, 9H), 6.73 (dd, JP-H = 39.5 Hz, JH-H = 1.0 Hz, 1H), 7.14 (dd, JP-H = 48.0 Hz, JH-H = 1.0 Hz, 1H), 7.40 (ddd, JH-H = 7.5 Hz, JH-H = 7.5 Hz, JH-H = 3.0 Hz, 2H), 7.47 (ddd, JH-H = 7.5 Hz, JH-H = 7.5 Hz, JH-H = 2.0 Hz, 1H), 7.75 (dd, JP-H = 13.5 Hz, JH-H = 7.0 Hz, 2H).C   13 (125 MHz, C D C l 3 ). -1.9 (CH3), -0.8 (d, JP-C = 1.3 Hz, CH3), 126.6 (d, JP-C = 70.1 Hz, C), 128.7 (d, JP-C = 12.0 Hz, CH), 130.4 (d, JP-C = 11.3 Hz, CH), 131.8 (d, JP-C = 3.0 Hz, CH), 143.0 (d, JP-C = 63.1 Hz, CH), 145.5 (d, JP-C = 41.3 Hz, C), 150.4 (d, JP-C = 18.5 Hz, CH), 158.3 (d, JP-C = 11.3 Hz, C).P   31 (200 MHz, C D C l 3 ). 66.1 (s)S i   29 (100 MHz, C D C l 3 ). -5.6 (d, JP-Si = 10.9 Hz), -5.4 (d, JP-Si = 7.8 Hz).HRMS (EI) for C16H25PSSi2: calcd. (m/z) 336.0953; found (m/z) 336.0963.


#### 2.4.4. 2,4-Bis(tert-butyl)-1-phenyl-thiooxophosphole (4c)

H   1 (500 MHz, C D C l 3 ).1.14 (s, 9H), 1.21 (s, 9H), 5.83 (dd, JP-H = 33.0 Hz, JH-H = 1.5 Hz, 1H), 6.74 (dd, JP-H = 43.5 Hz, JH-H = 1.5 Hz, 1H), 7.41 (ddd, JH-H = 7.5 Hz, JH-H = 7.5 Hz, JH-H = 3.0 Hz, 2H), 7.45-7.48 (m, 1H), 7.83 (dd, JP-H = 14.0 Hz, JH-H = 7.5 Hz, 2H).C   13 (125 MHz, C D C l 3 ).28.3 (CH3), 30.9 (d, JP-C = 3.8 Hz, CH3), 34.9 (d, JP-C = 13.3 Hz, C), 36.4 (d, JP-C = 10.5 Hz, C), 119.3 (d, JP-C = 84.6 Hz, CH), 128.1 (d, JP-C = 72.1 Hz, C), 128.7 (d, JP-C = 12.3 Hz, CH), 130.4 (d, JP-C = 11.4 Hz, CH), 131.6 (d, JP-C = 3.0 Hz, CH), 133.7 (d, JP-C = 26.1 Hz, CH), 155.5 (d, JP-C = 66.3 Hz, C),164.1 (d, JP-C = 14.8 Hz, C).P   31 (200 MHz, C D C l 3 ). 51.7 (s)HRMS (EI) for C18H25PS: calcd. (m/z) 304.1415; found (m/z) 304.1417.


#### 2.4.5. 2,4-Bis(4-fluorophenyl)-1-phenyl-thiooxophosphole (4d)

H   1 (500 MHz, C D C l 3 ). 6.53 (d, JP-H = 30.5 Hz, 1H), 6.97 (dd, JH-H = 8.5 Hz, JH-H = 8.5 Hz, 2H), 7.16 (dd, JH-H = 8.5 Hz, JH-H = 8.5 Hz, 2H), 7.44 (ddd, JP-H = 7.5 Hz, JH-H = 7.5 Hz, JP-H = 3.0 Hz, 2H), 7.51 (ddd, JP-H = 7.5 Hz, JH-H = 7.0 Hz, JP-H = 1.0 Hz, 1H), 7.59 (dd, JP-H = 39.5 Hz, JH-H = 1.0 Hz, 1H), 7.66-7.70 (m, 4H), 7.91 (dd, JP-H = 14.5 Hz, JH-H = 8.0 Hz, 2H).C   13 (125 MHz, C D C l 3 ).116.1 (dd, JP-C = 22.1 Hz, JF-C = 22.7 Hz, CH), 121.3 (d, JP-C = 86.4 Hz, CH), 127.1 (C), 127.7 (C), 128.1 (dd, JP-C = 12.3 Hz, JF-C = 3.5 Hz, C), 129.0 (dd, JP-C = 6.8 Hz, JF-C = 7.4 Hz, CH), 129.2 (d, JP-C = 12.6 Hz, CH), 130.5 (d, JP-C = 11.8 Hz, CH), 131.0 (dd, JP-C = 6.1 Hz, JF-C = 5.8 Hz, CH), 132.4 (d, JP-C = 2.9 Hz, CH), 132.7 (d, JP-C = 22.9 Hz, CH), 142.9 (d, JP-C = 74.4 Hz, C), 150.8 (d, JP-C = 17.3 Hz, C), 162.5 (d, JF-C = 81.8 Hz, C), 164.5 (d, JF-C = 82.8 Hz, C).F   19 (470 MHz, C D C l 3 ). -109.5 (s), -110.8 (s).P   31 (200 MHz, C D C l 3 ). 53.5 (s)HRMS (EI) for C22H15F2PS: calcd. (m/z) 380.0600; found (m/z) 380.0611.


#### 2.4.6. [2+2] Dimer of 1,2,4-triphenyl-thiooxophosphole (5a)

H   1 (600 MHz, C D C l 3 ).4.40 (d, JP-H = 21.6 Hz, 1H), 6.64 (d, JP-H = 40.2 Hz, 1H), 6.97 (d, JH-H = 7.2 Hz, 1H), 7.05 (dd, JH-H = 7.2 Hz, JH-H = 7.8 Hz, 2H), 7.10 (d, JH-H = 7.2 Hz, 1H), 7.25 (dd, JH-H = 6.6 Hz, JH-H = 7.2 Hz, 2H), 7.32 (dd, JH-H = 6.6 Hz, JH-H = 7.2 Hz, 2H), 7.35-7.38 (m, 5H), 7.63-7.66 (m, 2H).C   13 (150 MHz, C D C l 3 ). 40.6 (dd, JP-C = 64.8 Hz, JP-C = 13.5 Hz, CH), 65.8 (dd, JP-C = 6.0 Hz, JP-C = 6.0 Hz, C), 126.9 (CH), 127.2 (CH), 127.5 (dd, JP-C = 2.1 Hz, JP-C = 1.9 Hz, CH), 128.1 (CH), 128.6 (CH), 128.9 (d, JP-C = 6.3 Hz, CH), 128.9 (d, JP-C = 2.9 Hz, CH), 129.3 (CH), 130.6 (d, JP-C = 9.9 Hz, CH), 131.3 (dd, JP-C = 4.9 Hz, JP-C = 4.9 Hz, CH), 132.2 (d, JP-C = 15.5 Hz, CH), 132.2 (CH), 132.3 (d, JP-C = 2.5 Hz, CH), 133.4 (d, JP-C = 19.4 Hz, CH), 134.5 (d, JP-C = 13.6 Hz, C), 138.0 (CH), 137.9 (d, JP-C = 7.5 Hz, C), 138.4 (d, JP-C = 7.4 Hz, C), 144.6 (dd, JP-C = 10.1 Hz, JP-C = 9.9 Hz, CH), 151.9 (d, JP-C = 14.3 Hz, C).P   31 (200 MHz, C D C l 3 ). 71.2 (s)HRMS (ESI) for C44H35P2S2 [M+H]: calcd. (m/z) 689.1655; found (m/z) 689.1661.


#### 2.4.7. [2+2] Dimer of 2,4-bis(4-fluorophenyl)-1-phenyl-thiooxophosphole (5d)

H   1 (500 MHz, C D C l 3 ). 4.40 (d, JP-H = 21.0 Hz, 1H), 6.58 (d, JP-H = 39.5 Hz, 1H), 6.86 (dd, JH-H = 9.0 Hz, JF-H = 8.5 Hz, 2H), 7.08 (dd, JH-H = 9.0 Hz, JF-H = 5.0 Hz, 2H), 7.17 (dd, JH-H = 8.5 Hz, JF-H = 8.5 Hz, 2H), 7.36 (dd, JH-H = 8.0 Hz, JP-H = 2.0 Hz, 2H), 7.38-7.40 (m, 3H), 7.65-7.69 (m, 2H).C   13 (125 MHz, C D C l 3 ).40.9 (dd, JP-C = 65.4 Hz, JP-C = 14.4 Hz, CH), 65.1 (dd, JP-C = 5.9 Hz, JP-C = 6.0 Hz, C), 115.9 (d, JP-C = 21.6 Hz, CH), 116.4 (d, JP-C = 21.3 Hz, CH), 128.1 (ddd, JP-C = 5.5 Hz, JF-C = 5.5 Hz, JP-C = 3.5 Hz, C), 128.9 (d, JP-C = 7.9 Hz, CH), 129.1 (dd, JP-C = 6.3 Hz, JP-C = 6.3 Hz, CH), 129.3 (dd, JP-C = 2.8 Hz, JP-C = 2.3 Hz, CH), 129.3 (dd, JP-C = 2.5 Hz, JP-C = 2.5 Hz, CH), 131.1 (dd, JP-C = 5.6 Hz, JF-C = 6.1 Hz, CH), 132.5 (CH), 133.8 (d, JP-C = 2.9 Hz, C), 137.8 (d, JP-C = 72.0 Hz, C), 143.5 (dd, JP-C = 12.3 Hz, JP-C = 11.6 Hz, CH), 150.9 (d, JP-C = 3.3 Hz, C), 161.8 (d, JF-C = 97.5 Hz, C), 163.8 (d, JF-C = 98.1 Hz, C).F   19 (470 MHz, C D C l 3 ). -111.5 (s), -113.0 (s).P   31 (200 MHz, C D C l 3 ). 71.2 (s)HRMS (ESI) for C44H30F4P2S2 [M+H]: calcd. (m/z) 761.1279; found (m/z) 761.1288.


#### 2.4.8. 1,2,4-Triphenyl-selenooxophosphole (6a)

H   1 (500 MHz, C D C l 3 ).6.72 (dd, JP-H = 32.0 Hz, JH-H = 1.0 Hz, 1H), 7.29-7.34 (m, 3H), 7.45-7.47 (m, 2H), 7.48-7.52 (m, 4H), 7.73-7.75 (m, 4H), 7.77 (d, JP-H = 24.0 Hz, 1H), 7.99 (dd, JP-H = 14.5 Hz, JH-H = 1.5 Hz, 1H), 8.01 (d, JP-H = 14.5 Hz, 1H).C   13 (125 MHz, C D C l 3 ).121.8 (d, JP-C = 78.3 Hz, CH), 126.9 (CH), 127.2 (d, JP-C = 6.5 Hz, CH), 128.8 (CH), 129.0 (CH), 129.1 (CH), 129.2 (CH), 130.2 (CH), 131.2 (d, JP-C = 12.1 Hz, CH), 131.7 (d, JP-C = 12.4 Hz, C), 132.4 (d, JP-C = 2.8 Hz, CH), 133.0 (d, JP-C = 22.1 Hz, CH), 134.2 (d, JP-C = 15.9 Hz, C), 143.7 (d, JP-C = 66.8 Hz, C), 151.7 (d, JP-C = 15.3 Hz, C).P   31 (200 MHz, C D C l 3 ). 39.4 (JP-Se = 733 Hz).S e   77 (96 MHz, C D C l 3 ). -397 (d, JSe-P = 733 Hz).HRMS (EI) for C22H17PSe: calcd. (m/z) 392.0233; found (m/z) 392.0230.


#### 2.4.9. 1-Phenyl-2,4-bis(trimethylsilyl)-selenooxophosphole (6b)

H   1 (500 MHz, C D C l 3 ). 0.08 (s, 9H), 0.26 (s, 9H), 6.82 (d, JP-H = 41.0 Hz, 1H), 7.13 (dd, JP-H = 47.5 Hz, JH-H = 1.0 Hz, 1H), 7.40 (dt, JP-H = 7.5 Hz, JH-H = 7.5 Hz, JH-H = 3.0 Hz, 2H), 7.46-7.49 (m, 1H), 7.76 (dd, JP-H = 14.0 Hz, JH-H = 1.0 Hz, 1H), 7.78 (d, JP-H = 13.5 Hz, 1H).C   13 (125 MHz, C D C l 3 ). -1.8 (CH3), -0.5 (CH3), 124.8 (d, JP-C = 62.6 Hz, CH), 128.8 (d, JP-C = 12.1 Hz, CH), 131.0 (d, JP-C = 11.8 Hz, C), 132.0 (d, JP-C = 3.0 Hz, CH), 143.2 (d, JP-C = 55.4 Hz, CH), 145.4 (d, JP-C = 32.9 Hz, C), 150.5 (d, JP-C = 16.9 Hz, CH), 158.1 (d, JP-C = 9.5 Hz, C).P   31 (200 MHz, C D C l 3 ). 52.0 (JP-Se = 716 Hz).S i   29 (100 MHz, C D C l 3 ). -5.1 (d, JP-Si = 20.2 Hz), -5.6 (d, JP-Si = 22.1 Hz).S e   77 (96 MHz, C D C l 3 ). -466 (d, JSe-P = 716 Hz).HRMS (EI) for C16H25PSeSi2: calcd. (m/z) 384.0398; found (m/z) 384.0408.


#### 2.4.10. 2,4-Bis(tert-butyl)-1-phenyl-selenooxophosphole (6c)

H   1 (500 MHz, C D C l 3 ).1.16 (s, 9H), 1.21 (s, 9H), 5.89 (dd, JP-H = 34.5 Hz, JH-H = 1.5 Hz, 1H), 6.75 (dd, JP-H = 43.0 Hz, JH-H = 2.0 Hz, 1H), 7.39 (dd, JH-H = 7.5 Hz, JP-H = 2.5 Hz, 2H), 7.44 (dd, JH-H = 7.0 Hz, JP-H = 1.0 Hz, 1H), 7.83 (dd, JP-H = 14.0 Hz, JH-H = 7.0 Hz, 2H).C   13 (125 MHz, C D C l 3 ).28.3 (CH3), 31.2 (d, JP-C = 3.8 Hz, CH3), 34.8 (d, JP-C = 12.9 Hz, C), 36.5 (d, JP-C = 10.6 Hz, C), 119.4 (d, JP-C = 77.1 Hz, CH), 126.0 (d, JP-C = 64.3 Hz, C), 128.7 (d, JP-C = 12.3 Hz, CH), 130.9 (d, JP-C = 11.8 Hz, CH), 131.7 (d, JP-C = 2.8 Hz, CH), 133.8 (d, JP-C = 25.0 Hz, CH), 155.0 (d, JP-C = 58.1 Hz, C), 164.0 (d, JP-C = 13.1 Hz, C).P   31 (200 MHz, C D C l 3 ). 36.9 (JP-Se = 724 Hz).S e   77 (96 MHz, C D C l 3 ). -430 (d, JSe-P = 724 Hz).


#### 2.4.11. [2+2] Dimer of 1,2,4-Triphenyl-selenooxophosphole (7a)

H   1 (500 MHz, C D C l 3 ).4.55 (d, JP-H = 23.0 Hz, 1H), 6.69 (d, JP-H = 40.0 Hz, 1H), 7.03-7.10 (m, 2H), 7.15-7.23 (m, 5H), 7.31-7.38 (m, 5H), 7.40-7.53 (m, 10H), 7.76-7.81 (m, 2H).C   13 (125 MHz, C D C l 3 ).42.5 (dd, JP-C = 60.0 Hz, JP-C = 15.0 Hz, CH), 67.2 (dd, JP-C = 6.5 Hz, JP-C = 5.0 Hz, C), 126.8 (CH), 127.2 (CH), 127.7 (CH), 128.1 (CH), 128.5 (2CH), 128.9 (d, JP-C = 10.0 Hz, CH), 129.0 (d, JP-C = 5.0 Hz, CH), 129.2 (d, JP-C = 6.5 Hz, CH), 129.3 (d, JP-C = 8.8 Hz, CH), 129.4 (2CH), 131.9 (dd, JP-C = 6.5 Hz, JP-C = 5.0 Hz, CH), 132.3 (CH), 133.3 (d, JP-C = 25.0 Hz, C), 137.0 (C), 137.6 (C), 138.3 (CH), 144.4 (dd, JP-C = 11.3 Hz, JP-C = 11.3 Hz, CH), 146.2 (d, JP-C = 23.8 Hz, C).P   31 (200 MHz, C D C l 3 ). 64.5 (JP-Se = 759 Hz).S e   77 (96 MHz, C D C l 3 ). -385 (d, JSe-P = 759 Hz).


## 2.1. General Considerations

Dichloromethane was collected under argon from a PURSOLV MD-3 (Innovative Technologie Inc.) solvent purification unit.m-chloroperbenzoic acid, sulphur, and selenium were purchased from Aldrich or Alfa Aesar. Phospholes1a-d were prepared according to the literature procedure [19]. 1H, 13C, 19F, 29Si, 31P, and 77Se NMR spectra were recorded in CDCl3, unless specified, on a 500 MHz Bruker Avance III spectrometer equipped with a BBFO+ probe. Chemical shifts are reported in delta (δ) units, expressed in parts per million (ppm). High resolution ESI-MS spectra were recorded on a hybrid tandem quadrupole/time-of-flight (Q-TOF) instrument, equipped with a pneumatically assisted electrospray (Z-spray) ion source (Micromass, Manchester, UK) operated in positive mode. High resolution EI-MS spectra were obtained on a GCT-TOFmass spectrometer (Micromass, Manchester, UK) with EI source. Single crystals of6d were coated in Paratone-N oil and mounted on a loop. Data were collected at 150.0(1) K on a Nonius Kappa CCD diffractometer using a Mo Kα (λ= 0.71070 Å) X-ray source and a graphite monochromator. All data were measured using phi and omega scans. The crystal structure was solved using SIR 97 and refined using Shelx 2016 [20, 21]. DFT calculations were performed using the Gaussian09 suite of software (full details are provided in the SI (available here)).


## 2.2. General Procedure for the Synthesis of Oxophospholes2 and the [4+2] Dimers3

In a flask equipped with a magnetic stir bar, phosphole1a-c (5 mmol), dichloromethane (5 mL), andm-chloroperbenzoic acid (mCPBA) (6 mmol) were introduced. After stirring for 5 minutes at room temperature, the solution was filtered and the solvent evaporated under reduced pressure. The 31P NMR spectrum (CDCl3) of the crude residue showed complete conversion of starting phosphole and formation of oxophospholes2a-c and several nonidentified by-products. After a given time (several hours to days) compounds2a,b transform to the dimers3a,b in solution or in the solid state.


## 2.3. General Procedure for the Synthesis of Thiooxophospholes4and the [2+2]dimers5

In a flask equipped with a magnetic stir bar, phosphole1a-d (5 mmol), elemental sulphur (S8) (1 mmol), and dichloromethane (5 mL) were introduced. After stirring overnight at room temperature, the solution was filtered, and then the solvent evaporated under reduced pressure. The 31P NMR spectrum (CDCl3) of the crude product showed complete conversion to4a-d. Under the influence of natural light, compounds4a,d transformed to the dimers5a,d after several days in solution in a classical NMR tube or in the solid state.


## 2.4. General Procedure for the Synthesis of Selenooxophospholes6and the [2+2] Dimer7

In a flask equipped with a magnetic stir bar, phosphole1a-c (5 mmol), elemental selenium (Se) (6 mmol), and dichloromethane (5 mL) were introduced. After stirring overnight at room temperature, the solution was filtered and the solvent evaporated under reduced pressure. The 31P NMR spectra (CDCl3) of the crude product showed complete conversion to6a-c. Under the influence of natural light, compound6a transformed to the dimer7a after several days in solution in a classical NMR tube or in the solid state.


### 2.4.1. 1,10-Diphenyl-2,4,6,8-tetrakis(trimethysilyl)-1,10-diphosphatricyclo[5.6.5]deca-2,6-diene-1,10-dioxide (3b)

H   1 (500 MHz, C D C l 3 ). -0.13 (s, 9H), -0.08 (s, 9H), 0.13 (s, 9H), 0.37 (s, 9H), 3.31 (d, JP-H = 4.5 Hz, 1H), 3.45 (s, 1H), 6.70 (d, JP-H = 49.0 Hz, 1H), 6.96 (d, JP-H = 14.0 Hz, 1H), 7.31-7.34 (m, 2H), 7.41-7.50 (m, 4H), 7.64-7.68 (m, 2H), 7.78 (dd, JH-H = 8.0 Hz, JH-H = 8.0 Hz, 2H).C   13 (125 MHz, C D C l 3 ). -2.2 (CH3), -0.6 (CH3), 0.10 (d, JP-C = 2.6 Hz, CH3), 1.2 (CH3), 46.3 (d, JP-C = 45.3 Hz, C), 51.7 (d, JP-C = 8.4 Hz, CH), 52.5 (d, JP-C = 60.6 Hz, CH), 53.1 (dd, JP-C = 15.0 Hz, JP-C = 15.0 Hz, C), 127.8 (d, JP-C = 10.3 Hz, CH), 128.3 (d, JP-C = 11.4 Hz, CH), 129.3 (d, JP-C = 83.3 Hz, C), 131.5 (d, JP-C = 10.1 Hz, CH), 131.5 (d, JP-C = 3.0 Hz, CH), 131.7 (d, JP-C = 2.4 Hz, CH), 134.2 (d, JP-C = 7.0 Hz, CH), 135.5 (d, JP-C = 91.0 Hz, C), 136.2 (d, JP-C = 61.5 Hz, C), 142.5 (d, JP-C = 8.5 Hz, C), 143.9 (dd, JP-C = 10.3 Hz, JP-C = 5.5 Hz, CH), 164.4 (dd, JP-C = 17.9 Hz, JP-C = 10.3 Hz, CH).P   31 (200 MHz, C D C l 3 ). 64.4 (d, JP-P = 37.0 Hz), 83.2 (d, JP-P = 37.0 Hz).S i   29 (100 MHz, C D C l 3 ). -8.3 (d, JP-Si = 6.4 Hz), -6.7 (dd, JP-Si = 15.5 Hz, JP-Si = 2.1 Hz), 1.8 (s), 8.1 (d, JP-Si = 5.5 Hz).


### 2.4.2. 1,2,4-Triphenyl-thiooxophosphole (4a)

H   1 (500 MHz, Acetone- d 6 ). 6.85 (dd, JP-H = 31.0 Hz, JH-H = 1.5 Hz, 1H), 7.31-7.33 (m, 2H), 7.51-7.54 (m, 5H), 7.95 (dd, JH-H = 8.5 Hz, JH-H = 1.5 Hz, 1H), 7.97-7.99 (m, 3H), 8.13 (dd, JP-H = 40.5 Hz, JH-H = 1.5 Hz, 1H).C   13 (125 MHz, Acetone- d 6 ).123.1 (d, JP-C = 86.1 Hz, CH), 128.0 (CH), 128.1 (d, JP-C = 6.4 Hz, CH), 129.6 (CH), 129.7 (C), 129.9 (CH), 130.0 (d, JP-C = 2.3 Hz, CH), 130.1 (d, JP-C = 7.5 Hz, CH), 131.3 (d, JP-C = 11.7 Hz, CH), 133.1 (d, JP-C = 3.0 Hz, CH), 133.3 (d, JP-C = 12.3 Hz, C), 134.9 (d, JP-C = 20.1 Hz, CH), 135.3 (d, JP-C = 16.5 Hz, C), 144.6 (d, JP-C = 74.0 Hz, C), 152.6 (d, JP-C = 17.1 Hz, C).P   31 (200 MHz, Acetone- d 6 ). 52.7 (s)


### 2.4.3. 1-Phenyl-2,4-bis(trimethylsilyl)-thiooxophosphole (4b)

H   1 (500 MHz, C D C l 3 ). 0.05 (s, 9H), 0.25 (s, 9H), 6.73 (dd, JP-H = 39.5 Hz, JH-H = 1.0 Hz, 1H), 7.14 (dd, JP-H = 48.0 Hz, JH-H = 1.0 Hz, 1H), 7.40 (ddd, JH-H = 7.5 Hz, JH-H = 7.5 Hz, JH-H = 3.0 Hz, 2H), 7.47 (ddd, JH-H = 7.5 Hz, JH-H = 7.5 Hz, JH-H = 2.0 Hz, 1H), 7.75 (dd, JP-H = 13.5 Hz, JH-H = 7.0 Hz, 2H).C   13 (125 MHz, C D C l 3 ). -1.9 (CH3), -0.8 (d, JP-C = 1.3 Hz, CH3), 126.6 (d, JP-C = 70.1 Hz, C), 128.7 (d, JP-C = 12.0 Hz, CH), 130.4 (d, JP-C = 11.3 Hz, CH), 131.8 (d, JP-C = 3.0 Hz, CH), 143.0 (d, JP-C = 63.1 Hz, CH), 145.5 (d, JP-C = 41.3 Hz, C), 150.4 (d, JP-C = 18.5 Hz, CH), 158.3 (d, JP-C = 11.3 Hz, C).P   31 (200 MHz, C D C l 3 ). 66.1 (s)S i   29 (100 MHz, C D C l 3 ). -5.6 (d, JP-Si = 10.9 Hz), -5.4 (d, JP-Si = 7.8 Hz).HRMS (EI) for C16H25PSSi2: calcd. (m/z) 336.0953; found (m/z) 336.0963.


### 2.4.4. 2,4-Bis(tert-butyl)-1-phenyl-thiooxophosphole (4c)

H   1 (500 MHz, C D C l 3 ).1.14 (s, 9H), 1.21 (s, 9H), 5.83 (dd, JP-H = 33.0 Hz, JH-H = 1.5 Hz, 1H), 6.74 (dd, JP-H = 43.5 Hz, JH-H = 1.5 Hz, 1H), 7.41 (ddd, JH-H = 7.5 Hz, JH-H = 7.5 Hz, JH-H = 3.0 Hz, 2H), 7.45-7.48 (m, 1H), 7.83 (dd, JP-H = 14.0 Hz, JH-H = 7.5 Hz, 2H).C   13 (125 MHz, C D C l 3 ).28.3 (CH3), 30.9 (d, JP-C = 3.8 Hz, CH3), 34.9 (d, JP-C = 13.3 Hz, C), 36.4 (d, JP-C = 10.5 Hz, C), 119.3 (d, JP-C = 84.6 Hz, CH), 128.1 (d, JP-C = 72.1 Hz, C), 128.7 (d, JP-C = 12.3 Hz, CH), 130.4 (d, JP-C = 11.4 Hz, CH), 131.6 (d, JP-C = 3.0 Hz, CH), 133.7 (d, JP-C = 26.1 Hz, CH), 155.5 (d, JP-C = 66.3 Hz, C),164.1 (d, JP-C = 14.8 Hz, C).P   31 (200 MHz, C D C l 3 ). 51.7 (s)HRMS (EI) for C18H25PS: calcd. (m/z) 304.1415; found (m/z) 304.1417.


### 2.4.5. 2,4-Bis(4-fluorophenyl)-1-phenyl-thiooxophosphole (4d)

H   1 (500 MHz, C D C l 3 ). 6.53 (d, JP-H = 30.5 Hz, 1H), 6.97 (dd, JH-H = 8.5 Hz, JH-H = 8.5 Hz, 2H), 7.16 (dd, JH-H = 8.5 Hz, JH-H = 8.5 Hz, 2H), 7.44 (ddd, JP-H = 7.5 Hz, JH-H = 7.5 Hz, JP-H = 3.0 Hz, 2H), 7.51 (ddd, JP-H = 7.5 Hz, JH-H = 7.0 Hz, JP-H = 1.0 Hz, 1H), 7.59 (dd, JP-H = 39.5 Hz, JH-H = 1.0 Hz, 1H), 7.66-7.70 (m, 4H), 7.91 (dd, JP-H = 14.5 Hz, JH-H = 8.0 Hz, 2H).C   13 (125 MHz, C D C l 3 ).116.1 (dd, JP-C = 22.1 Hz, JF-C = 22.7 Hz, CH), 121.3 (d, JP-C = 86.4 Hz, CH), 127.1 (C), 127.7 (C), 128.1 (dd, JP-C = 12.3 Hz, JF-C = 3.5 Hz, C), 129.0 (dd, JP-C = 6.8 Hz, JF-C = 7.4 Hz, CH), 129.2 (d, JP-C = 12.6 Hz, CH), 130.5 (d, JP-C = 11.8 Hz, CH), 131.0 (dd, JP-C = 6.1 Hz, JF-C = 5.8 Hz, CH), 132.4 (d, JP-C = 2.9 Hz, CH), 132.7 (d, JP-C = 22.9 Hz, CH), 142.9 (d, JP-C = 74.4 Hz, C), 150.8 (d, JP-C = 17.3 Hz, C), 162.5 (d, JF-C = 81.8 Hz, C), 164.5 (d, JF-C = 82.8 Hz, C).F   19 (470 MHz, C D C l 3 ). -109.5 (s), -110.8 (s).P   31 (200 MHz, C D C l 3 ). 53.5 (s)HRMS (EI) for C22H15F2PS: calcd. (m/z) 380.0600; found (m/z) 380.0611.


### 2.4.6. [2+2] Dimer of 1,2,4-triphenyl-thiooxophosphole (5a)

H   1 (600 MHz, C D C l 3 ).4.40 (d, JP-H = 21.6 Hz, 1H), 6.64 (d, JP-H = 40.2 Hz, 1H), 6.97 (d, JH-H = 7.2 Hz, 1H), 7.05 (dd, JH-H = 7.2 Hz, JH-H = 7.8 Hz, 2H), 7.10 (d, JH-H = 7.2 Hz, 1H), 7.25 (dd, JH-H = 6.6 Hz, JH-H = 7.2 Hz, 2H), 7.32 (dd, JH-H = 6.6 Hz, JH-H = 7.2 Hz, 2H), 7.35-7.38 (m, 5H), 7.63-7.66 (m, 2H).C   13 (150 MHz, C D C l 3 ). 40.6 (dd, JP-C = 64.8 Hz, JP-C = 13.5 Hz, CH), 65.8 (dd, JP-C = 6.0 Hz, JP-C = 6.0 Hz, C), 126.9 (CH), 127.2 (CH), 127.5 (dd, JP-C = 2.1 Hz, JP-C = 1.9 Hz, CH), 128.1 (CH), 128.6 (CH), 128.9 (d, JP-C = 6.3 Hz, CH), 128.9 (d, JP-C = 2.9 Hz, CH), 129.3 (CH), 130.6 (d, JP-C = 9.9 Hz, CH), 131.3 (dd, JP-C = 4.9 Hz, JP-C = 4.9 Hz, CH), 132.2 (d, JP-C = 15.5 Hz, CH), 132.2 (CH), 132.3 (d, JP-C = 2.5 Hz, CH), 133.4 (d, JP-C = 19.4 Hz, CH), 134.5 (d, JP-C = 13.6 Hz, C), 138.0 (CH), 137.9 (d, JP-C = 7.5 Hz, C), 138.4 (d, JP-C = 7.4 Hz, C), 144.6 (dd, JP-C = 10.1 Hz, JP-C = 9.9 Hz, CH), 151.9 (d, JP-C = 14.3 Hz, C).P   31 (200 MHz, C D C l 3 ). 71.2 (s)HRMS (ESI) for C44H35P2S2 [M+H]: calcd. (m/z) 689.1655; found (m/z) 689.1661.


### 2.4.7. [2+2] Dimer of 2,4-bis(4-fluorophenyl)-1-phenyl-thiooxophosphole (5d)

H   1 (500 MHz, C D C l 3 ). 4.40 (d, JP-H = 21.0 Hz, 1H), 6.58 (d, JP-H = 39.5 Hz, 1H), 6.86 (dd, JH-H = 9.0 Hz, JF-H = 8.5 Hz, 2H), 7.08 (dd, JH-H = 9.0 Hz, JF-H = 5.0 Hz, 2H), 7.17 (dd, JH-H = 8.5 Hz, JF-H = 8.5 Hz, 2H), 7.36 (dd, JH-H = 8.0 Hz, JP-H = 2.0 Hz, 2H), 7.38-7.40 (m, 3H), 7.65-7.69 (m, 2H).C   13 (125 MHz, C D C l 3 ).40.9 (dd, JP-C = 65.4 Hz, JP-C = 14.4 Hz, CH), 65.1 (dd, JP-C = 5.9 Hz, JP-C = 6.0 Hz, C), 115.9 (d, JP-C = 21.6 Hz, CH), 116.4 (d, JP-C = 21.3 Hz, CH), 128.1 (ddd, JP-C = 5.5 Hz, JF-C = 5.5 Hz, JP-C = 3.5 Hz, C), 128.9 (d, JP-C = 7.9 Hz, CH), 129.1 (dd, JP-C = 6.3 Hz, JP-C = 6.3 Hz, CH), 129.3 (dd, JP-C = 2.8 Hz, JP-C = 2.3 Hz, CH), 129.3 (dd, JP-C = 2.5 Hz, JP-C = 2.5 Hz, CH), 131.1 (dd, JP-C = 5.6 Hz, JF-C = 6.1 Hz, CH), 132.5 (CH), 133.8 (d, JP-C = 2.9 Hz, C), 137.8 (d, JP-C = 72.0 Hz, C), 143.5 (dd, JP-C = 12.3 Hz, JP-C = 11.6 Hz, CH), 150.9 (d, JP-C = 3.3 Hz, C), 161.8 (d, JF-C = 97.5 Hz, C), 163.8 (d, JF-C = 98.1 Hz, C).F   19 (470 MHz, C D C l 3 ). -111.5 (s), -113.0 (s).P   31 (200 MHz, C D C l 3 ). 71.2 (s)HRMS (ESI) for C44H30F4P2S2 [M+H]: calcd. (m/z) 761.1279; found (m/z) 761.1288.


### 2.4.8. 1,2,4-Triphenyl-selenooxophosphole (6a)

H   1 (500 MHz, C D C l 3 ).6.72 (dd, JP-H = 32.0 Hz, JH-H = 1.0 Hz, 1H), 7.29-7.34 (m, 3H), 7.45-7.47 (m, 2H), 7.48-7.52 (m, 4H), 7.73-7.75 (m, 4H), 7.77 (d, JP-H = 24.0 Hz, 1H), 7.99 (dd, JP-H = 14.5 Hz, JH-H = 1.5 Hz, 1H), 8.01 (d, JP-H = 14.5 Hz, 1H).C   13 (125 MHz, C D C l 3 ).121.8 (d, JP-C = 78.3 Hz, CH), 126.9 (CH), 127.2 (d, JP-C = 6.5 Hz, CH), 128.8 (CH), 129.0 (CH), 129.1 (CH), 129.2 (CH), 130.2 (CH), 131.2 (d, JP-C = 12.1 Hz, CH), 131.7 (d, JP-C = 12.4 Hz, C), 132.4 (d, JP-C = 2.8 Hz, CH), 133.0 (d, JP-C = 22.1 Hz, CH), 134.2 (d, JP-C = 15.9 Hz, C), 143.7 (d, JP-C = 66.8 Hz, C), 151.7 (d, JP-C = 15.3 Hz, C).P   31 (200 MHz, C D C l 3 ). 39.4 (JP-Se = 733 Hz).S e   77 (96 MHz, C D C l 3 ). -397 (d, JSe-P = 733 Hz).HRMS (EI) for C22H17PSe: calcd. (m/z) 392.0233; found (m/z) 392.0230.


### 2.4.9. 1-Phenyl-2,4-bis(trimethylsilyl)-selenooxophosphole (6b)

H   1 (500 MHz, C D C l 3 ). 0.08 (s, 9H), 0.26 (s, 9H), 6.82 (d, JP-H = 41.0 Hz, 1H), 7.13 (dd, JP-H = 47.5 Hz, JH-H = 1.0 Hz, 1H), 7.40 (dt, JP-H = 7.5 Hz, JH-H = 7.5 Hz, JH-H = 3.0 Hz, 2H), 7.46-7.49 (m, 1H), 7.76 (dd, JP-H = 14.0 Hz, JH-H = 1.0 Hz, 1H), 7.78 (d, JP-H = 13.5 Hz, 1H).C   13 (125 MHz, C D C l 3 ). -1.8 (CH3), -0.5 (CH3), 124.8 (d, JP-C = 62.6 Hz, CH), 128.8 (d, JP-C = 12.1 Hz, CH), 131.0 (d, JP-C = 11.8 Hz, C), 132.0 (d, JP-C = 3.0 Hz, CH), 143.2 (d, JP-C = 55.4 Hz, CH), 145.4 (d, JP-C = 32.9 Hz, C), 150.5 (d, JP-C = 16.9 Hz, CH), 158.1 (d, JP-C = 9.5 Hz, C).P   31 (200 MHz, C D C l 3 ). 52.0 (JP-Se = 716 Hz).S i   29 (100 MHz, C D C l 3 ). -5.1 (d, JP-Si = 20.2 Hz), -5.6 (d, JP-Si = 22.1 Hz).S e   77 (96 MHz, C D C l 3 ). -466 (d, JSe-P = 716 Hz).HRMS (EI) for C16H25PSeSi2: calcd. (m/z) 384.0398; found (m/z) 384.0408.


### 2.4.10. 2,4-Bis(tert-butyl)-1-phenyl-selenooxophosphole (6c)

H   1 (500 MHz, C D C l 3 ).1.16 (s, 9H), 1.21 (s, 9H), 5.89 (dd, JP-H = 34.5 Hz, JH-H = 1.5 Hz, 1H), 6.75 (dd, JP-H = 43.0 Hz, JH-H = 2.0 Hz, 1H), 7.39 (dd, JH-H = 7.5 Hz, JP-H = 2.5 Hz, 2H), 7.44 (dd, JH-H = 7.0 Hz, JP-H = 1.0 Hz, 1H), 7.83 (dd, JP-H = 14.0 Hz, JH-H = 7.0 Hz, 2H).C   13 (125 MHz, C D C l 3 ).28.3 (CH3), 31.2 (d, JP-C = 3.8 Hz, CH3), 34.8 (d, JP-C = 12.9 Hz, C), 36.5 (d, JP-C = 10.6 Hz, C), 119.4 (d, JP-C = 77.1 Hz, CH), 126.0 (d, JP-C = 64.3 Hz, C), 128.7 (d, JP-C = 12.3 Hz, CH), 130.9 (d, JP-C = 11.8 Hz, CH), 131.7 (d, JP-C = 2.8 Hz, CH), 133.8 (d, JP-C = 25.0 Hz, CH), 155.0 (d, JP-C = 58.1 Hz, C), 164.0 (d, JP-C = 13.1 Hz, C).P   31 (200 MHz, C D C l 3 ). 36.9 (JP-Se = 724 Hz).S e   77 (96 MHz, C D C l 3 ). -430 (d, JSe-P = 724 Hz).


### 2.4.11. [2+2] Dimer of 1,2,4-Triphenyl-selenooxophosphole (7a)

H   1 (500 MHz, C D C l 3 ).4.55 (d, JP-H = 23.0 Hz, 1H), 6.69 (d, JP-H = 40.0 Hz, 1H), 7.03-7.10 (m, 2H), 7.15-7.23 (m, 5H), 7.31-7.38 (m, 5H), 7.40-7.53 (m, 10H), 7.76-7.81 (m, 2H).C   13 (125 MHz, C D C l 3 ).42.5 (dd, JP-C = 60.0 Hz, JP-C = 15.0 Hz, CH), 67.2 (dd, JP-C = 6.5 Hz, JP-C = 5.0 Hz, C), 126.8 (CH), 127.2 (CH), 127.7 (CH), 128.1 (CH), 128.5 (2CH), 128.9 (d, JP-C = 10.0 Hz, CH), 129.0 (d, JP-C = 5.0 Hz, CH), 129.2 (d, JP-C = 6.5 Hz, CH), 129.3 (d, JP-C = 8.8 Hz, CH), 129.4 (2CH), 131.9 (dd, JP-C = 6.5 Hz, JP-C = 5.0 Hz, CH), 132.3 (CH), 133.3 (d, JP-C = 25.0 Hz, C), 137.0 (C), 137.6 (C), 138.3 (CH), 144.4 (dd, JP-C = 11.3 Hz, JP-C = 11.3 Hz, CH), 146.2 (d, JP-C = 23.8 Hz, C).P   31 (200 MHz, C D C l 3 ). 64.5 (JP-Se = 759 Hz).S e   77 (96 MHz, C D C l 3 ). -385 (d, JSe-P = 759 Hz).


## 2.4.1. 1,10-Diphenyl-2,4,6,8-tetrakis(trimethysilyl)-1,10-diphosphatricyclo[5.6.5]deca-2,6-diene-1,10-dioxide (3b)

H   1 (500 MHz, C D C l 3 ). -0.13 (s, 9H), -0.08 (s, 9H), 0.13 (s, 9H), 0.37 (s, 9H), 3.31 (d, JP-H = 4.5 Hz, 1H), 3.45 (s, 1H), 6.70 (d, JP-H = 49.0 Hz, 1H), 6.96 (d, JP-H = 14.0 Hz, 1H), 7.31-7.34 (m, 2H), 7.41-7.50 (m, 4H), 7.64-7.68 (m, 2H), 7.78 (dd, JH-H = 8.0 Hz, JH-H = 8.0 Hz, 2H).C   13 (125 MHz, C D C l 3 ). -2.2 (CH3), -0.6 (CH3), 0.10 (d, JP-C = 2.6 Hz, CH3), 1.2 (CH3), 46.3 (d, JP-C = 45.3 Hz, C), 51.7 (d, JP-C = 8.4 Hz, CH), 52.5 (d, JP-C = 60.6 Hz, CH), 53.1 (dd, JP-C = 15.0 Hz, JP-C = 15.0 Hz, C), 127.8 (d, JP-C = 10.3 Hz, CH), 128.3 (d, JP-C = 11.4 Hz, CH), 129.3 (d, JP-C = 83.3 Hz, C), 131.5 (d, JP-C = 10.1 Hz, CH), 131.5 (d, JP-C = 3.0 Hz, CH), 131.7 (d, JP-C = 2.4 Hz, CH), 134.2 (d, JP-C = 7.0 Hz, CH), 135.5 (d, JP-C = 91.0 Hz, C), 136.2 (d, JP-C = 61.5 Hz, C), 142.5 (d, JP-C = 8.5 Hz, C), 143.9 (dd, JP-C = 10.3 Hz, JP-C = 5.5 Hz, CH), 164.4 (dd, JP-C = 17.9 Hz, JP-C = 10.3 Hz, CH).P   31 (200 MHz, C D C l 3 ). 64.4 (d, JP-P = 37.0 Hz), 83.2 (d, JP-P = 37.0 Hz).S i   29 (100 MHz, C D C l 3 ). -8.3 (d, JP-Si = 6.4 Hz), -6.7 (dd, JP-Si = 15.5 Hz, JP-Si = 2.1 Hz), 1.8 (s), 8.1 (d, JP-Si = 5.5 Hz).


## 2.4.2. 1,2,4-Triphenyl-thiooxophosphole (4a)

H   1 (500 MHz, Acetone- d 6 ). 6.85 (dd, JP-H = 31.0 Hz, JH-H = 1.5 Hz, 1H), 7.31-7.33 (m, 2H), 7.51-7.54 (m, 5H), 7.95 (dd, JH-H = 8.5 Hz, JH-H = 1.5 Hz, 1H), 7.97-7.99 (m, 3H), 8.13 (dd, JP-H = 40.5 Hz, JH-H = 1.5 Hz, 1H).C   13 (125 MHz, Acetone- d 6 ).123.1 (d, JP-C = 86.1 Hz, CH), 128.0 (CH), 128.1 (d, JP-C = 6.4 Hz, CH), 129.6 (CH), 129.7 (C), 129.9 (CH), 130.0 (d, JP-C = 2.3 Hz, CH), 130.1 (d, JP-C = 7.5 Hz, CH), 131.3 (d, JP-C = 11.7 Hz, CH), 133.1 (d, JP-C = 3.0 Hz, CH), 133.3 (d, JP-C = 12.3 Hz, C), 134.9 (d, JP-C = 20.1 Hz, CH), 135.3 (d, JP-C = 16.5 Hz, C), 144.6 (d, JP-C = 74.0 Hz, C), 152.6 (d, JP-C = 17.1 Hz, C).P   31 (200 MHz, Acetone- d 6 ). 52.7 (s)


## 2.4.3. 1-Phenyl-2,4-bis(trimethylsilyl)-thiooxophosphole (4b)

H   1 (500 MHz, C D C l 3 ). 0.05 (s, 9H), 0.25 (s, 9H), 6.73 (dd, JP-H = 39.5 Hz, JH-H = 1.0 Hz, 1H), 7.14 (dd, JP-H = 48.0 Hz, JH-H = 1.0 Hz, 1H), 7.40 (ddd, JH-H = 7.5 Hz, JH-H = 7.5 Hz, JH-H = 3.0 Hz, 2H), 7.47 (ddd, JH-H = 7.5 Hz, JH-H = 7.5 Hz, JH-H = 2.0 Hz, 1H), 7.75 (dd, JP-H = 13.5 Hz, JH-H = 7.0 Hz, 2H).C   13 (125 MHz, C D C l 3 ). -1.9 (CH3), -0.8 (d, JP-C = 1.3 Hz, CH3), 126.6 (d, JP-C = 70.1 Hz, C), 128.7 (d, JP-C = 12.0 Hz, CH), 130.4 (d, JP-C = 11.3 Hz, CH), 131.8 (d, JP-C = 3.0 Hz, CH), 143.0 (d, JP-C = 63.1 Hz, CH), 145.5 (d, JP-C = 41.3 Hz, C), 150.4 (d, JP-C = 18.5 Hz, CH), 158.3 (d, JP-C = 11.3 Hz, C).P   31 (200 MHz, C D C l 3 ). 66.1 (s)S i   29 (100 MHz, C D C l 3 ). -5.6 (d, JP-Si = 10.9 Hz), -5.4 (d, JP-Si = 7.8 Hz).HRMS (EI) for C16H25PSSi2: calcd. (m/z) 336.0953; found (m/z) 336.0963.


## 2.4.4. 2,4-Bis(tert-butyl)-1-phenyl-thiooxophosphole (4c)

H   1 (500 MHz, C D C l 3 ).1.14 (s, 9H), 1.21 (s, 9H), 5.83 (dd, JP-H = 33.0 Hz, JH-H = 1.5 Hz, 1H), 6.74 (dd, JP-H = 43.5 Hz, JH-H = 1.5 Hz, 1H), 7.41 (ddd, JH-H = 7.5 Hz, JH-H = 7.5 Hz, JH-H = 3.0 Hz, 2H), 7.45-7.48 (m, 1H), 7.83 (dd, JP-H = 14.0 Hz, JH-H = 7.5 Hz, 2H).C   13 (125 MHz, C D C l 3 ).28.3 (CH3), 30.9 (d, JP-C = 3.8 Hz, CH3), 34.9 (d, JP-C = 13.3 Hz, C), 36.4 (d, JP-C = 10.5 Hz, C), 119.3 (d, JP-C = 84.6 Hz, CH), 128.1 (d, JP-C = 72.1 Hz, C), 128.7 (d, JP-C = 12.3 Hz, CH), 130.4 (d, JP-C = 11.4 Hz, CH), 131.6 (d, JP-C = 3.0 Hz, CH), 133.7 (d, JP-C = 26.1 Hz, CH), 155.5 (d, JP-C = 66.3 Hz, C),164.1 (d, JP-C = 14.8 Hz, C).P   31 (200 MHz, C D C l 3 ). 51.7 (s)HRMS (EI) for C18H25PS: calcd. (m/z) 304.1415; found (m/z) 304.1417.


## 2.4.5. 2,4-Bis(4-fluorophenyl)-1-phenyl-thiooxophosphole (4d)

H   1 (500 MHz, C D C l 3 ). 6.53 (d, JP-H = 30.5 Hz, 1H), 6.97 (dd, JH-H = 8.5 Hz, JH-H = 8.5 Hz, 2H), 7.16 (dd, JH-H = 8.5 Hz, JH-H = 8.5 Hz, 2H), 7.44 (ddd, JP-H = 7.5 Hz, JH-H = 7.5 Hz, JP-H = 3.0 Hz, 2H), 7.51 (ddd, JP-H = 7.5 Hz, JH-H = 7.0 Hz, JP-H = 1.0 Hz, 1H), 7.59 (dd, JP-H = 39.5 Hz, JH-H = 1.0 Hz, 1H), 7.66-7.70 (m, 4H), 7.91 (dd, JP-H = 14.5 Hz, JH-H = 8.0 Hz, 2H).C   13 (125 MHz, C D C l 3 ).116.1 (dd, JP-C = 22.1 Hz, JF-C = 22.7 Hz, CH), 121.3 (d, JP-C = 86.4 Hz, CH), 127.1 (C), 127.7 (C), 128.1 (dd, JP-C = 12.3 Hz, JF-C = 3.5 Hz, C), 129.0 (dd, JP-C = 6.8 Hz, JF-C = 7.4 Hz, CH), 129.2 (d, JP-C = 12.6 Hz, CH), 130.5 (d, JP-C = 11.8 Hz, CH), 131.0 (dd, JP-C = 6.1 Hz, JF-C = 5.8 Hz, CH), 132.4 (d, JP-C = 2.9 Hz, CH), 132.7 (d, JP-C = 22.9 Hz, CH), 142.9 (d, JP-C = 74.4 Hz, C), 150.8 (d, JP-C = 17.3 Hz, C), 162.5 (d, JF-C = 81.8 Hz, C), 164.5 (d, JF-C = 82.8 Hz, C).F   19 (470 MHz, C D C l 3 ). -109.5 (s), -110.8 (s).P   31 (200 MHz, C D C l 3 ). 53.5 (s)HRMS (EI) for C22H15F2PS: calcd. (m/z) 380.0600; found (m/z) 380.0611.


## 2.4.6. [2+2] Dimer of 1,2,4-triphenyl-thiooxophosphole (5a)

H   1 (600 MHz, C D C l 3 ).4.40 (d, JP-H = 21.6 Hz, 1H), 6.64 (d, JP-H = 40.2 Hz, 1H), 6.97 (d, JH-H = 7.2 Hz, 1H), 7.05 (dd, JH-H = 7.2 Hz, JH-H = 7.8 Hz, 2H), 7.10 (d, JH-H = 7.2 Hz, 1H), 7.25 (dd, JH-H = 6.6 Hz, JH-H = 7.2 Hz, 2H), 7.32 (dd, JH-H = 6.6 Hz, JH-H = 7.2 Hz, 2H), 7.35-7.38 (m, 5H), 7.63-7.66 (m, 2H).C   13 (150 MHz, C D C l 3 ). 40.6 (dd, JP-C = 64.8 Hz, JP-C = 13.5 Hz, CH), 65.8 (dd, JP-C = 6.0 Hz, JP-C = 6.0 Hz, C), 126.9 (CH), 127.2 (CH), 127.5 (dd, JP-C = 2.1 Hz, JP-C = 1.9 Hz, CH), 128.1 (CH), 128.6 (CH), 128.9 (d, JP-C = 6.3 Hz, CH), 128.9 (d, JP-C = 2.9 Hz, CH), 129.3 (CH), 130.6 (d, JP-C = 9.9 Hz, CH), 131.3 (dd, JP-C = 4.9 Hz, JP-C = 4.9 Hz, CH), 132.2 (d, JP-C = 15.5 Hz, CH), 132.2 (CH), 132.3 (d, JP-C = 2.5 Hz, CH), 133.4 (d, JP-C = 19.4 Hz, CH), 134.5 (d, JP-C = 13.6 Hz, C), 138.0 (CH), 137.9 (d, JP-C = 7.5 Hz, C), 138.4 (d, JP-C = 7.4 Hz, C), 144.6 (dd, JP-C = 10.1 Hz, JP-C = 9.9 Hz, CH), 151.9 (d, JP-C = 14.3 Hz, C).P   31 (200 MHz, C D C l 3 ). 71.2 (s)HRMS (ESI) for C44H35P2S2 [M+H]: calcd. (m/z) 689.1655; found (m/z) 689.1661.


## 2.4.7. [2+2] Dimer of 2,4-bis(4-fluorophenyl)-1-phenyl-thiooxophosphole (5d)

H   1 (500 MHz, C D C l 3 ). 4.40 (d, JP-H = 21.0 Hz, 1H), 6.58 (d, JP-H = 39.5 Hz, 1H), 6.86 (dd, JH-H = 9.0 Hz, JF-H = 8.5 Hz, 2H), 7.08 (dd, JH-H = 9.0 Hz, JF-H = 5.0 Hz, 2H), 7.17 (dd, JH-H = 8.5 Hz, JF-H = 8.5 Hz, 2H), 7.36 (dd, JH-H = 8.0 Hz, JP-H = 2.0 Hz, 2H), 7.38-7.40 (m, 3H), 7.65-7.69 (m, 2H).C   13 (125 MHz, C D C l 3 ).40.9 (dd, JP-C = 65.4 Hz, JP-C = 14.4 Hz, CH), 65.1 (dd, JP-C = 5.9 Hz, JP-C = 6.0 Hz, C), 115.9 (d, JP-C = 21.6 Hz, CH), 116.4 (d, JP-C = 21.3 Hz, CH), 128.1 (ddd, JP-C = 5.5 Hz, JF-C = 5.5 Hz, JP-C = 3.5 Hz, C), 128.9 (d, JP-C = 7.9 Hz, CH), 129.1 (dd, JP-C = 6.3 Hz, JP-C = 6.3 Hz, CH), 129.3 (dd, JP-C = 2.8 Hz, JP-C = 2.3 Hz, CH), 129.3 (dd, JP-C = 2.5 Hz, JP-C = 2.5 Hz, CH), 131.1 (dd, JP-C = 5.6 Hz, JF-C = 6.1 Hz, CH), 132.5 (CH), 133.8 (d, JP-C = 2.9 Hz, C), 137.8 (d, JP-C = 72.0 Hz, C), 143.5 (dd, JP-C = 12.3 Hz, JP-C = 11.6 Hz, CH), 150.9 (d, JP-C = 3.3 Hz, C), 161.8 (d, JF-C = 97.5 Hz, C), 163.8 (d, JF-C = 98.1 Hz, C).F   19 (470 MHz, C D C l 3 ). -111.5 (s), -113.0 (s).P   31 (200 MHz, C D C l 3 ). 71.2 (s)HRMS (ESI) for C44H30F4P2S2 [M+H]: calcd. (m/z) 761.1279; found (m/z) 761.1288.


## 2.4.8. 1,2,4-Triphenyl-selenooxophosphole (6a)

H   1 (500 MHz, C D C l 3 ).6.72 (dd, JP-H = 32.0 Hz, JH-H = 1.0 Hz, 1H), 7.29-7.34 (m, 3H), 7.45-7.47 (m, 2H), 7.48-7.52 (m, 4H), 7.73-7.75 (m, 4H), 7.77 (d, JP-H = 24.0 Hz, 1H), 7.99 (dd, JP-H = 14.5 Hz, JH-H = 1.5 Hz, 1H), 8.01 (d, JP-H = 14.5 Hz, 1H).C   13 (125 MHz, C D C l 3 ).121.8 (d, JP-C = 78.3 Hz, CH), 126.9 (CH), 127.2 (d, JP-C = 6.5 Hz, CH), 128.8 (CH), 129.0 (CH), 129.1 (CH), 129.2 (CH), 130.2 (CH), 131.2 (d, JP-C = 12.1 Hz, CH), 131.7 (d, JP-C = 12.4 Hz, C), 132.4 (d, JP-C = 2.8 Hz, CH), 133.0 (d, JP-C = 22.1 Hz, CH), 134.2 (d, JP-C = 15.9 Hz, C), 143.7 (d, JP-C = 66.8 Hz, C), 151.7 (d, JP-C = 15.3 Hz, C).P   31 (200 MHz, C D C l 3 ). 39.4 (JP-Se = 733 Hz).S e   77 (96 MHz, C D C l 3 ). -397 (d, JSe-P = 733 Hz).HRMS (EI) for C22H17PSe: calcd. (m/z) 392.0233; found (m/z) 392.0230.


## 2.4.9. 1-Phenyl-2,4-bis(trimethylsilyl)-selenooxophosphole (6b)

H   1 (500 MHz, C D C l 3 ). 0.08 (s, 9H), 0.26 (s, 9H), 6.82 (d, JP-H = 41.0 Hz, 1H), 7.13 (dd, JP-H = 47.5 Hz, JH-H = 1.0 Hz, 1H), 7.40 (dt, JP-H = 7.5 Hz, JH-H = 7.5 Hz, JH-H = 3.0 Hz, 2H), 7.46-7.49 (m, 1H), 7.76 (dd, JP-H = 14.0 Hz, JH-H = 1.0 Hz, 1H), 7.78 (d, JP-H = 13.5 Hz, 1H).C   13 (125 MHz, C D C l 3 ). -1.8 (CH3), -0.5 (CH3), 124.8 (d, JP-C = 62.6 Hz, CH), 128.8 (d, JP-C = 12.1 Hz, CH), 131.0 (d, JP-C = 11.8 Hz, C), 132.0 (d, JP-C = 3.0 Hz, CH), 143.2 (d, JP-C = 55.4 Hz, CH), 145.4 (d, JP-C = 32.9 Hz, C), 150.5 (d, JP-C = 16.9 Hz, CH), 158.1 (d, JP-C = 9.5 Hz, C).P   31 (200 MHz, C D C l 3 ). 52.0 (JP-Se = 716 Hz).S i   29 (100 MHz, C D C l 3 ). -5.1 (d, JP-Si = 20.2 Hz), -5.6 (d, JP-Si = 22.1 Hz).S e   77 (96 MHz, C D C l 3 ). -466 (d, JSe-P = 716 Hz).HRMS (EI) for C16H25PSeSi2: calcd. (m/z) 384.0398; found (m/z) 384.0408.


## 2.4.10. 2,4-Bis(tert-butyl)-1-phenyl-selenooxophosphole (6c)

H   1 (500 MHz, C D C l 3 ).1.16 (s, 9H), 1.21 (s, 9H), 5.89 (dd, JP-H = 34.5 Hz, JH-H = 1.5 Hz, 1H), 6.75 (dd, JP-H = 43.0 Hz, JH-H = 2.0 Hz, 1H), 7.39 (dd, JH-H = 7.5 Hz, JP-H = 2.5 Hz, 2H), 7.44 (dd, JH-H = 7.0 Hz, JP-H = 1.0 Hz, 1H), 7.83 (dd, JP-H = 14.0 Hz, JH-H = 7.0 Hz, 2H).C   13 (125 MHz, C D C l 3 ).28.3 (CH3), 31.2 (d, JP-C = 3.8 Hz, CH3), 34.8 (d, JP-C = 12.9 Hz, C), 36.5 (d, JP-C = 10.6 Hz, C), 119.4 (d, JP-C = 77.1 Hz, CH), 126.0 (d, JP-C = 64.3 Hz, C), 128.7 (d, JP-C = 12.3 Hz, CH), 130.9 (d, JP-C = 11.8 Hz, CH), 131.7 (d, JP-C = 2.8 Hz, CH), 133.8 (d, JP-C = 25.0 Hz, CH), 155.0 (d, JP-C = 58.1 Hz, C), 164.0 (d, JP-C = 13.1 Hz, C).P   31 (200 MHz, C D C l 3 ). 36.9 (JP-Se = 724 Hz).S e   77 (96 MHz, C D C l 3 ). -430 (d, JSe-P = 724 Hz).


## 2.4.11. [2+2] Dimer of 1,2,4-Triphenyl-selenooxophosphole (7a)

H   1 (500 MHz, C D C l 3 ).4.55 (d, JP-H = 23.0 Hz, 1H), 6.69 (d, JP-H = 40.0 Hz, 1H), 7.03-7.10 (m, 2H), 7.15-7.23 (m, 5H), 7.31-7.38 (m, 5H), 7.40-7.53 (m, 10H), 7.76-7.81 (m, 2H).C   13 (125 MHz, C D C l 3 ).42.5 (dd, JP-C = 60.0 Hz, JP-C = 15.0 Hz, CH), 67.2 (dd, JP-C = 6.5 Hz, JP-C = 5.0 Hz, C), 126.8 (CH), 127.2 (CH), 127.7 (CH), 128.1 (CH), 128.5 (2CH), 128.9 (d, JP-C = 10.0 Hz, CH), 129.0 (d, JP-C = 5.0 Hz, CH), 129.2 (d, JP-C = 6.5 Hz, CH), 129.3 (d, JP-C = 8.8 Hz, CH), 129.4 (2CH), 131.9 (dd, JP-C = 6.5 Hz, JP-C = 5.0 Hz, CH), 132.3 (CH), 133.3 (d, JP-C = 25.0 Hz, C), 137.0 (C), 137.6 (C), 138.3 (CH), 144.4 (dd, JP-C = 11.3 Hz, JP-C = 11.3 Hz, CH), 146.2 (d, JP-C = 23.8 Hz, C).P   31 (200 MHz, C D C l 3 ). 64.5 (JP-Se = 759 Hz).S e   77 (96 MHz, C D C l 3 ). -385 (d, JSe-P = 759 Hz).


## 3. Results and Discussion

### 3.1. Oxophospholes

The reaction of phospholes1 withm-chloroperbenzoic acid (mcpba) in dichloromethane at room temperature led immediately to the formation of 2,4-disubstituted oxophospholes2 as shown by 31P NMR spectroscopy (Scheme 2, Table 1). Within several hours, the aryl-substituted compound2a transformed to the corresponding [4+2] cycloadduct3a in a highly selective endo-anti fashion. For2bcontaining two trimethylsilyl groups, the dimerization took several days, whereas with the bulky tert-butyl groups in2c no dimerization occurred.Table 1
31P NMR chemical shifts (ppm) (CDCl3) of 1, 2, and 3.
Entry R 1 2 3 1 Ph(a) 11.3 47.8 55.8 (d,J3P-P = 37.6 Hz)76.3 (d, J3P-P = 37.6 Hz) 2 Trimethylsilyl(b) 31.9 59.0 64.4 (d,J3P-P = 37.0 Hz)83.2 (d, J3P-P = 37.0 Hz) 3 t-Butyl(c) 1.4 47.5 No dimerScheme 2
Phosphole oxidation with mcpba then[4+2] dimerization.The values observed in the31P NMR spectrum for2 and3are close to other values in the literature [22, 23]. The oxidation of1a and1c produced nonidentified side-products, which did not allow a full characterisation of these compounds and the dimer3a. In contrast, compound3bcould be obtained as a pure product and was fully characterized by multinuclear NMR spectroscopy. In Table 2, the 13C NMR data of product3b is compared to the previously described [4+2] dimer of 3-methylphosphole oxideA, which had also been characterized by X-ray diffraction analysis [22]. The good correlation of the data between compounds3b andA led us to the assumption that in our case the same endo-anti product was formed as major product.Table 2
Comparison of13C NMR chemical shifts (ppm) (CDCl3) of 3b and A.
3b A  Cn δ (ppm) J P-C (Hz) δ (ppm) J P-C (Hz) C1 135.5 91.0 130.4 103.5 C2 164.4 17.9; 10.3 154.6 24.2; 9.0 C3 53.1 15.0; 15.0 51.4 12.8; 12.8 C4 52.2 62.5 53.8 67.9 C5 142.5 8.5 134.6 11.4 C6 143.9 10.3; 5.5 122.5 12.2 C7 46.3 45.3 49.8 64.6 C8 51.7 68.8; 8.6 43.7 79.5; 15.0


### 3.2. Thiooxophospholes

The oxidation of phospholes1 with sulfur in dichloromethane was complete after one night stirring at room temperature, as shown by 31P NMR spectroscopy, yielding the corresponding thiooxophospholes4 (Scheme 3, Table 3). In the case of aryl-substituted compounds4a and4d, a new signal appeared in the 31P NMR spectrum after workup. This singlet increased steadily upon leaving the sample exposed to natural light with a concomitant decrease of the signal for4 until nearly full conversion after several days. No other products appeared in the spectrum. The new products could be identified as [2+2] head-to-head dimers5a and5d through multinuclear NMR spectroscopy and X-ray diffraction studies for5d. In the case of4b and4c, no further reaction was observed. When compounds4a and4d were stored in the dark the corresponding products5aand5d did not form, whereas exposure to direct sunlight accelerated the reaction.Table 3
31P NMR chemical shifts (ppm) (CDCl3) of 4 and 5 (​∗done in acetone-d6).
Entry R 1 4 5 1 Ph(a) 11.3 52.7 ∗ 71.2 2 Trimethylsilyl(b) 31.9 66.1 No dimer 3 t-Butyl(c) 1.4 51.7 No dimer 4 4-F-Ph(d) 11.9 53.5 71.2Scheme 3
Phosphole oxidation with S8 then [2+2] dimerization.The31P NMR values for4 are in agreement with literature data [24–27]. A comparison of the 1H and 13C NMR data of compounds1d,4d, and5d is shown in Table 4 confirming the head-to-head dimerization of compound4d. Interesting observations are the changes in the coupling constants 1JP-C for Cα and Cα′’ upon oxidation from1d(0 and 1.8 Hz) to4d(86.4 and 74.0 Hz) [24]. Upon dimerization to5d, the 1JP-C for Cα′’ is most impacted (down to 3.3 Hz), whereas for Cα a high value remains (65.4 Hz).Table 4
Chemical shifts (ppm) and coupling constants (Hz) of1d, 4d, and 5d.
1H 13C Entry R Hα JP-H2 H β ′ JP-H3 Cα JP-C1 C α ′ JP-C1 Cβ JP-C2 C β ′ JP-C2  1 1d 6.97(39.0) 7.48(12.5) 127.4(0) 153.1(1.8) 149.3(7.6) 131.5(10.1) 2 4d 6.53(30.5) 7.59(39.5) 121.3(86.4) 142.9(74.0) 150.8(17.3) 132.7(22.9) 3 5d 4.40(21.0) 6.58(39.5) 40.9(65.4) 150.9(3.3) 65.1(5.9) 143.5(12.3)Crystals of compound5d suitable for X-ray diffraction studies were obtained through slow evaporation of the chloroform solvent.5d crystallised in the monoclinic space group C2/c with one disordered solvent molecule in the unit cell (Figure 1). The tricyclic [5;4;5] pattern shows a syn-anti arrangement with respect to the phosphole units and the substituents on the cyclobutene ring. There are only two other structurally characterised compounds with this arrangement in the literature, i.e., the dimerised helical phosphoindole oxides which have no substituents on the C1 and C2 position, reported by Marinetti and Voituriez [28]. The cyclobutane ring in5d is quasi-rectangular (angles C1–C2–C2a 88.32(8)° and C2–C1–C1a 90.95(8)°) with a considerable deviation from planarity (dihedral angle C1–C2–C2a-C1a 9.1°). The biggest differences with respect to the reported structures are the bond lengths within the cyclobutane ring. Whereas C1-C2 and C1-C1a are in the expected range (1.562(2)Å and 1.547(3)Å), the C2-C2a bond (1.619(3) Å) is considerably longer, which is probably due to the adjacent phenyl groups. For the latter bond, the corresponding bond lengths in the reported nonsubstituted phosphoindole oxides are 1.563(5) Å and 1.564(18) Å [28].Figure 1
Molecular structure of5d: (a) top-view of whole molecule with 50% probability ellipsoids; hydrogens and solvent molecule omitted for clarity; (b) side-view of tricyclic [5;4;5] pattern showing the syn-anti arrangement.
(a) (b)


### 3.3. Selenooxophospholes

When selenium was employed for the oxidation of phospholes1 in dichloromethane the corresponding selenooxophospholes6 were obtained quantitatively after 18h at room temperature (Scheme 4). The substituents on the phosphole ring influence strongly the 31P NMR shifts and, in this case, also the 77Se NMR values (Table 5). Leaving compound6a in a standard NMR tube for several days exposed to natural light led to a good but not full conversion to the [2+2] head-to-head dimer7a as shown by 31P NMR.Table 5
31P and 77Se NMR chemical shifts (ppm) (CDCl3) and coupling constants of 6 and 7.
6 7 Entry R δ (31P) δ (77Se) δ (31P) δ (77Se) 1 Ph(a) 39.4 -397(d, JSe-P1 = 733 Hz) 64.5 -385(d, JSe-P1 = 759 Hz) 2 Trimethylsilyl(b) 52.0 -466(d, JSe-P1 = 716 Hz) No dimer 3 t-Butyl(c) 36.9 -430(d, JSe-P1 = 724 Hz) No dimerScheme 4
Phosphole oxidation with Se then[2+2] dimerization.It has previously been shown that the coupling constant1JSe-P can provide information on the σ-donor ability of phospholes [4–7, 29–32]. According to Table 5, phosphole1b having the trimethylsilyl groups in positions 2 and 4 is the strongest σ-donor as6bhas the smallest coupling constant with 716 Hz, close to the value for 1-phenyl-3,4-dimethylphosphole (713 Hz). The value for compound6a is smaller compared to the corresponding 1,2,5-triphenylselenooxophosphole (742 Hz) [4–7], indicating a certain influence of the position of the ring substituents on the σ-donor ability. Interestingly, the dimer7a has a considerably higher coupling constant with 759 Hz.


### 3.4. DFT Calculations

In an attempt to correlate the observed reactivity of the different phosphole P(V) derivatives with their electronic properties, we carried out DFT calculations at the B3LYP-D3/6-31G(d) level of theory (see SI) to determine the HOMO-LUMO gaps and the NICS(0) values (Table6). Phospholes1 are often considered as weakly aromatic compounds [15–17] and this is reflected in the small negative values for the NICS(0) (-2.83, -3.09, and -2.62 ppm for1a,1b, and1c, respectively). In contrast, phosphole P=O compounds2 tend towards anti-aromatic systems with positive NICS(0) values (1.41, 1.79, and 1.22 ppm for2a,2b, and2c, respectively), which makes them more reactive towards further dimerization reactions [18]. The steric bulk of the substituents can strongly influence these reactions. The P=S and P=Se analogues4 and6 are best described as non-aromatic or slightly anti-aromatic (NICS values ranging from 0.20 to 0.92 ppm for4a-c and5a-c) and they are more stable towards dimerization. However, the aryl-substituted yellow compounds4a and4d can absorb visible light and undergo [2+2] reactions, in agreement with the calculated HOMO-LUMO gaps (3.555 and 3.541 eV, respectively; see SI for4d). The colourless compounds4b and4c show no reactivity, which is also in good agreement with the calculations (HOMO-LUMO gaps of 3.848 and 4.142 eV, respectively). Neither phospholes1 nor phosphole oxides2 undergo [2+2] dimerisation under ambient conditions in agreement with the too large HOMO-LUMO gaps, all above 3.698 eV. It is worthwhile noting that for all computed P(V) compounds the HOMO corresponds to the π system, with a contribution of the O, S, or Se atom, while the LUMO corresponds to the π∗ system. Single-electron excitation is thus consistent with the visible-light-induced [2+2] dimerization that is observed.Table 6
Comparison of NICS(0) (ppm) and HOMO-LUMO gap (eV) for three disubstituted phospholes and their oxides.
X lone pair(1) NICS(0) -2.83 -3.09 -2.62 Δ(HOMO-LUMO) 3.968 4.770 4.898  O(2) NICS(0) 1.41 1.79 1.22 Δ(HOMO-LUMO) 3.698 4.458 4.646  S(4) NICS(0) 0.58 0.92 0.40 Δ(HOMO-LUMO) 3.555 3.848 4.142  Se(6) NICS(0) 0.34 0.62 0.20 Δ(HOMO-LUMO) 3.262 3.475 3.761


### 3.5. Discussion

#### 3.5.1. [4+2] Dimerization

The propensity of phosphole derivatives to undergo[4+2] homodimerization reactions has been a long-standing research issue and the selectivity of such transformations has been investigated by synthetic and theoretical means [1, 33–35]. It concerns mainly, but not exclusively, phosphole oxides and metal-coordinated phospholes [22, 23, 36]. The steric bulk and the position of the substituents play an important role. Whereas 3,4-disubstituted phospholes are prone to homodimerization, 2,5-disubstituted phospholes are stable towards this reaction. The latter can nevertheless react under more stringent conditions with other dienophiles [37]. In our case, the 2,4-disubstituted phosphole oxides are just borderline: with aryl groups and the bulky, but flexible trimethylsilyl groups homodimerization occurs, albeit slowly, whereas the bulky t-butyl group prevents this reaction. A very good regioselectivity with the formation of mainly one [4+2] dimer, the endo-anti isomer, is observed.


#### 3.5.2. [2+2] Dimerization

Until recently, thermal or light-induced[2+2] dimerization reactions were mainly restricted to phosphole derivatives coordinated to metals [38–41]. In 2012, Marinetti and Voituriez reported the first metal-free, head-to-head [2+2] photocyclizations with nonsubstituted helical phosphoindole oxides [28]. More recently, a helical phosphinamide substituted in the C2 position was examined, providing the head to tail [2+2] dimer in solution under sunlight. Furthermore, the reaction took also place in the solid state under sunlight or X-ray radiation [42]. In our case, the 2,4-disubstituted thiooxo- and selenooxophospholes4a, 4d and6a are the first examples for phosphole P=S and P=Se derivatives to undergo metal-free head-to-head [2+2] homodimerization reactions, despite the presence of substituents in the C2 position. These transformations are highly regio- and stereoselective, yielding a single isomer. The aryl groups have a crucial role in this case, as they allow the absorption of visible light by the phosphole moiety and, as can be seen from the DFT calculations, they lower the HOMO-LUMO gaps just under the threshold for visible light energy, so that [2+2] dimerization can occur. In contrast, trimethylsilyl or t-butyl substituted phospholes4b,c and5b,c do not absorb in the visible light region and, in most cases, have too wide HOMO-LUMO gaps for visible light mediated reactions. These findings open the way to further light-driven transformations of phosphole P(V) derivatives.


## 3.1. Oxophospholes

The reaction of phospholes1 withm-chloroperbenzoic acid (mcpba) in dichloromethane at room temperature led immediately to the formation of 2,4-disubstituted oxophospholes2 as shown by 31P NMR spectroscopy (Scheme 2, Table 1). Within several hours, the aryl-substituted compound2a transformed to the corresponding [4+2] cycloadduct3a in a highly selective endo-anti fashion. For2bcontaining two trimethylsilyl groups, the dimerization took several days, whereas with the bulky tert-butyl groups in2c no dimerization occurred.Table 1
31P NMR chemical shifts (ppm) (CDCl3) of 1, 2, and 3.
Entry R 1 2 3 1 Ph(a) 11.3 47.8 55.8 (d,J3P-P = 37.6 Hz)76.3 (d, J3P-P = 37.6 Hz) 2 Trimethylsilyl(b) 31.9 59.0 64.4 (d,J3P-P = 37.0 Hz)83.2 (d, J3P-P = 37.0 Hz) 3 t-Butyl(c) 1.4 47.5 No dimerScheme 2
Phosphole oxidation with mcpba then[4+2] dimerization.The values observed in the31P NMR spectrum for2 and3are close to other values in the literature [22, 23]. The oxidation of1a and1c produced nonidentified side-products, which did not allow a full characterisation of these compounds and the dimer3a. In contrast, compound3bcould be obtained as a pure product and was fully characterized by multinuclear NMR spectroscopy. In Table 2, the 13C NMR data of product3b is compared to the previously described [4+2] dimer of 3-methylphosphole oxideA, which had also been characterized by X-ray diffraction analysis [22]. The good correlation of the data between compounds3b andA led us to the assumption that in our case the same endo-anti product was formed as major product.Table 2
Comparison of13C NMR chemical shifts (ppm) (CDCl3) of 3b and A.
3b A  Cn δ (ppm) J P-C (Hz) δ (ppm) J P-C (Hz) C1 135.5 91.0 130.4 103.5 C2 164.4 17.9; 10.3 154.6 24.2; 9.0 C3 53.1 15.0; 15.0 51.4 12.8; 12.8 C4 52.2 62.5 53.8 67.9 C5 142.5 8.5 134.6 11.4 C6 143.9 10.3; 5.5 122.5 12.2 C7 46.3 45.3 49.8 64.6 C8 51.7 68.8; 8.6 43.7 79.5; 15.0


## 3.2. Thiooxophospholes

The oxidation of phospholes1 with sulfur in dichloromethane was complete after one night stirring at room temperature, as shown by 31P NMR spectroscopy, yielding the corresponding thiooxophospholes4 (Scheme 3, Table 3). In the case of aryl-substituted compounds4a and4d, a new signal appeared in the 31P NMR spectrum after workup. This singlet increased steadily upon leaving the sample exposed to natural light with a concomitant decrease of the signal for4 until nearly full conversion after several days. No other products appeared in the spectrum. The new products could be identified as [2+2] head-to-head dimers5a and5d through multinuclear NMR spectroscopy and X-ray diffraction studies for5d. In the case of4b and4c, no further reaction was observed. When compounds4a and4d were stored in the dark the corresponding products5aand5d did not form, whereas exposure to direct sunlight accelerated the reaction.Table 3
31P NMR chemical shifts (ppm) (CDCl3) of 4 and 5 (​∗done in acetone-d6).
Entry R 1 4 5 1 Ph(a) 11.3 52.7 ∗ 71.2 2 Trimethylsilyl(b) 31.9 66.1 No dimer 3 t-Butyl(c) 1.4 51.7 No dimer 4 4-F-Ph(d) 11.9 53.5 71.2Scheme 3
Phosphole oxidation with S8 then [2+2] dimerization.The31P NMR values for4 are in agreement with literature data [24–27]. A comparison of the 1H and 13C NMR data of compounds1d,4d, and5d is shown in Table 4 confirming the head-to-head dimerization of compound4d. Interesting observations are the changes in the coupling constants 1JP-C for Cα and Cα′’ upon oxidation from1d(0 and 1.8 Hz) to4d(86.4 and 74.0 Hz) [24]. Upon dimerization to5d, the 1JP-C for Cα′’ is most impacted (down to 3.3 Hz), whereas for Cα a high value remains (65.4 Hz).Table 4
Chemical shifts (ppm) and coupling constants (Hz) of1d, 4d, and 5d.
1H 13C Entry R Hα JP-H2 H β ′ JP-H3 Cα JP-C1 C α ′ JP-C1 Cβ JP-C2 C β ′ JP-C2  1 1d 6.97(39.0) 7.48(12.5) 127.4(0) 153.1(1.8) 149.3(7.6) 131.5(10.1) 2 4d 6.53(30.5) 7.59(39.5) 121.3(86.4) 142.9(74.0) 150.8(17.3) 132.7(22.9) 3 5d 4.40(21.0) 6.58(39.5) 40.9(65.4) 150.9(3.3) 65.1(5.9) 143.5(12.3)Crystals of compound5d suitable for X-ray diffraction studies were obtained through slow evaporation of the chloroform solvent.5d crystallised in the monoclinic space group C2/c with one disordered solvent molecule in the unit cell (Figure 1). The tricyclic [5;4;5] pattern shows a syn-anti arrangement with respect to the phosphole units and the substituents on the cyclobutene ring. There are only two other structurally characterised compounds with this arrangement in the literature, i.e., the dimerised helical phosphoindole oxides which have no substituents on the C1 and C2 position, reported by Marinetti and Voituriez [28]. The cyclobutane ring in5d is quasi-rectangular (angles C1–C2–C2a 88.32(8)° and C2–C1–C1a 90.95(8)°) with a considerable deviation from planarity (dihedral angle C1–C2–C2a-C1a 9.1°). The biggest differences with respect to the reported structures are the bond lengths within the cyclobutane ring. Whereas C1-C2 and C1-C1a are in the expected range (1.562(2)Å and 1.547(3)Å), the C2-C2a bond (1.619(3) Å) is considerably longer, which is probably due to the adjacent phenyl groups. For the latter bond, the corresponding bond lengths in the reported nonsubstituted phosphoindole oxides are 1.563(5) Å and 1.564(18) Å [28].Figure 1
Molecular structure of5d: (a) top-view of whole molecule with 50% probability ellipsoids; hydrogens and solvent molecule omitted for clarity; (b) side-view of tricyclic [5;4;5] pattern showing the syn-anti arrangement.
(a) (b)


## 3.3. Selenooxophospholes

When selenium was employed for the oxidation of phospholes1 in dichloromethane the corresponding selenooxophospholes6 were obtained quantitatively after 18h at room temperature (Scheme 4). The substituents on the phosphole ring influence strongly the 31P NMR shifts and, in this case, also the 77Se NMR values (Table 5). Leaving compound6a in a standard NMR tube for several days exposed to natural light led to a good but not full conversion to the [2+2] head-to-head dimer7a as shown by 31P NMR.Table 5
31P and 77Se NMR chemical shifts (ppm) (CDCl3) and coupling constants of 6 and 7.
6 7 Entry R δ (31P) δ (77Se) δ (31P) δ (77Se) 1 Ph(a) 39.4 -397(d, JSe-P1 = 733 Hz) 64.5 -385(d, JSe-P1 = 759 Hz) 2 Trimethylsilyl(b) 52.0 -466(d, JSe-P1 = 716 Hz) No dimer 3 t-Butyl(c) 36.9 -430(d, JSe-P1 = 724 Hz) No dimerScheme 4
Phosphole oxidation with Se then[2+2] dimerization.It has previously been shown that the coupling constant1JSe-P can provide information on the σ-donor ability of phospholes [4–7, 29–32]. According to Table 5, phosphole1b having the trimethylsilyl groups in positions 2 and 4 is the strongest σ-donor as6bhas the smallest coupling constant with 716 Hz, close to the value for 1-phenyl-3,4-dimethylphosphole (713 Hz). The value for compound6a is smaller compared to the corresponding 1,2,5-triphenylselenooxophosphole (742 Hz) [4–7], indicating a certain influence of the position of the ring substituents on the σ-donor ability. Interestingly, the dimer7a has a considerably higher coupling constant with 759 Hz.


## 3.4. DFT Calculations

In an attempt to correlate the observed reactivity of the different phosphole P(V) derivatives with their electronic properties, we carried out DFT calculations at the B3LYP-D3/6-31G(d) level of theory (see SI) to determine the HOMO-LUMO gaps and the NICS(0) values (Table6). Phospholes1 are often considered as weakly aromatic compounds [15–17] and this is reflected in the small negative values for the NICS(0) (-2.83, -3.09, and -2.62 ppm for1a,1b, and1c, respectively). In contrast, phosphole P=O compounds2 tend towards anti-aromatic systems with positive NICS(0) values (1.41, 1.79, and 1.22 ppm for2a,2b, and2c, respectively), which makes them more reactive towards further dimerization reactions [18]. The steric bulk of the substituents can strongly influence these reactions. The P=S and P=Se analogues4 and6 are best described as non-aromatic or slightly anti-aromatic (NICS values ranging from 0.20 to 0.92 ppm for4a-c and5a-c) and they are more stable towards dimerization. However, the aryl-substituted yellow compounds4a and4d can absorb visible light and undergo [2+2] reactions, in agreement with the calculated HOMO-LUMO gaps (3.555 and 3.541 eV, respectively; see SI for4d). The colourless compounds4b and4c show no reactivity, which is also in good agreement with the calculations (HOMO-LUMO gaps of 3.848 and 4.142 eV, respectively). Neither phospholes1 nor phosphole oxides2 undergo [2+2] dimerisation under ambient conditions in agreement with the too large HOMO-LUMO gaps, all above 3.698 eV. It is worthwhile noting that for all computed P(V) compounds the HOMO corresponds to the π system, with a contribution of the O, S, or Se atom, while the LUMO corresponds to the π∗ system. Single-electron excitation is thus consistent with the visible-light-induced [2+2] dimerization that is observed.Table 6
Comparison of NICS(0) (ppm) and HOMO-LUMO gap (eV) for three disubstituted phospholes and their oxides.
X lone pair(1) NICS(0) -2.83 -3.09 -2.62 Δ(HOMO-LUMO) 3.968 4.770 4.898  O(2) NICS(0) 1.41 1.79 1.22 Δ(HOMO-LUMO) 3.698 4.458 4.646  S(4) NICS(0) 0.58 0.92 0.40 Δ(HOMO-LUMO) 3.555 3.848 4.142  Se(6) NICS(0) 0.34 0.62 0.20 Δ(HOMO-LUMO) 3.262 3.475 3.761


## 3.5. Discussion

### 3.5.1. [4+2] Dimerization

The propensity of phosphole derivatives to undergo[4+2] homodimerization reactions has been a long-standing research issue and the selectivity of such transformations has been investigated by synthetic and theoretical means [1, 33–35]. It concerns mainly, but not exclusively, phosphole oxides and metal-coordinated phospholes [22, 23, 36]. The steric bulk and the position of the substituents play an important role. Whereas 3,4-disubstituted phospholes are prone to homodimerization, 2,5-disubstituted phospholes are stable towards this reaction. The latter can nevertheless react under more stringent conditions with other dienophiles [37]. In our case, the 2,4-disubstituted phosphole oxides are just borderline: with aryl groups and the bulky, but flexible trimethylsilyl groups homodimerization occurs, albeit slowly, whereas the bulky t-butyl group prevents this reaction. A very good regioselectivity with the formation of mainly one [4+2] dimer, the endo-anti isomer, is observed.


### 3.5.2. [2+2] Dimerization

Until recently, thermal or light-induced[2+2] dimerization reactions were mainly restricted to phosphole derivatives coordinated to metals [38–41]. In 2012, Marinetti and Voituriez reported the first metal-free, head-to-head [2+2] photocyclizations with nonsubstituted helical phosphoindole oxides [28]. More recently, a helical phosphinamide substituted in the C2 position was examined, providing the head to tail [2+2] dimer in solution under sunlight. Furthermore, the reaction took also place in the solid state under sunlight or X-ray radiation [42]. In our case, the 2,4-disubstituted thiooxo- and selenooxophospholes4a, 4d and6a are the first examples for phosphole P=S and P=Se derivatives to undergo metal-free head-to-head [2+2] homodimerization reactions, despite the presence of substituents in the C2 position. These transformations are highly regio- and stereoselective, yielding a single isomer. The aryl groups have a crucial role in this case, as they allow the absorption of visible light by the phosphole moiety and, as can be seen from the DFT calculations, they lower the HOMO-LUMO gaps just under the threshold for visible light energy, so that [2+2] dimerization can occur. In contrast, trimethylsilyl or t-butyl substituted phospholes4b,c and5b,c do not absorb in the visible light region and, in most cases, have too wide HOMO-LUMO gaps for visible light mediated reactions. These findings open the way to further light-driven transformations of phosphole P(V) derivatives.


## 3.5.1. [4+2] Dimerization

The propensity of phosphole derivatives to undergo[4+2] homodimerization reactions has been a long-standing research issue and the selectivity of such transformations has been investigated by synthetic and theoretical means [1, 33–35]. It concerns mainly, but not exclusively, phosphole oxides and metal-coordinated phospholes [22, 23, 36]. The steric bulk and the position of the substituents play an important role. Whereas 3,4-disubstituted phospholes are prone to homodimerization, 2,5-disubstituted phospholes are stable towards this reaction. The latter can nevertheless react under more stringent conditions with other dienophiles [37]. In our case, the 2,4-disubstituted phosphole oxides are just borderline: with aryl groups and the bulky, but flexible trimethylsilyl groups homodimerization occurs, albeit slowly, whereas the bulky t-butyl group prevents this reaction. A very good regioselectivity with the formation of mainly one [4+2] dimer, the endo-anti isomer, is observed.


## 3.5.2. [2+2] Dimerization

Until recently, thermal or light-induced[2+2] dimerization reactions were mainly restricted to phosphole derivatives coordinated to metals [38–41]. In 2012, Marinetti and Voituriez reported the first metal-free, head-to-head [2+2] photocyclizations with nonsubstituted helical phosphoindole oxides [28]. More recently, a helical phosphinamide substituted in the C2 position was examined, providing the head to tail [2+2] dimer in solution under sunlight. Furthermore, the reaction took also place in the solid state under sunlight or X-ray radiation [42]. In our case, the 2,4-disubstituted thiooxo- and selenooxophospholes4a, 4d and6a are the first examples for phosphole P=S and P=Se derivatives to undergo metal-free head-to-head [2+2] homodimerization reactions, despite the presence of substituents in the C2 position. These transformations are highly regio- and stereoselective, yielding a single isomer. The aryl groups have a crucial role in this case, as they allow the absorption of visible light by the phosphole moiety and, as can be seen from the DFT calculations, they lower the HOMO-LUMO gaps just under the threshold for visible light energy, so that [2+2] dimerization can occur. In contrast, trimethylsilyl or t-butyl substituted phospholes4b,c and5b,c do not absorb in the visible light region and, in most cases, have too wide HOMO-LUMO gaps for visible light mediated reactions. These findings open the way to further light-driven transformations of phosphole P(V) derivatives.


## 4. Conclusions

We have shown that P(V) derivatives of 2,4-disubstituted phospholes have intriguing properties with respect to[4+2] and [2+2] homodimerization reactions, which are highly dependent on (a) the heteroatom on phosphorus (O vs S, Se) and (b) the substituents on the phosphole ring (aryl vs. trimethylsilyl vs. t-butyl). Their reactivity and their properties lie in-between the corresponding 2,5- and 3,4-disubstituted phospholes, as, for example, shown with the 1JPSe coupling constants. Particularly, the light-driven [2+2] photocyclization requires further in-depth studies to explore its full potential towards other derivatization reactions. Further transformations of the new [2+2] dimers (reduction of the P=S bond and chiral resolution) could provide the platform for a new family of chiral ligands for asymmetric catalysis.



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