Symmetric C66H36 multihelicenes

Guillaume Dauvergne, Mohammed Bin Jassar, Florian Rigoulet, Albert Artigas, Yannick Carissan, Denis Hagebaum-Reignier, Yoann Coquerel

► To cite this version:

Guillaume Dauvergne, Mohammed Bin Jassar, Florian Rigoulet, Albert Artigas, Yannick Carissan, et al. Symmetric C66H36 multihelicenes. Canadian Journal of Chemistry, inPress, 10.1139/cjc-2024-0234. hal-05207607

HAL Id: hal-05207607

https://hal.science/hal-05207607v1

Submitted on 12 Aug 2025

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Distributed under a Creative Commons Attribution 4.0 International License
Symmetric C<sub>66</sub>H<sub>36</sub> multihelicenes

Guillaume Dauvergne, Mohammed Bin Jassar<sup>[a]</sup>, Florian Rigoulet, Albert Artigas<sup>[b]</sup>, Yannick Carissan*, Denis Hagebaum-Reignier*, Yoann Coquerel*

Aix Marseille Univ, CNRS, Centrale Méditerranée, ISM2, 13397 Marseille, France.

E-mail: yannick.carissan@univ-amu.fr ; denis.hagebaum-reignier@univ-amu.fr ; yoann.coquerel@univ-amu.fr

**Abstract**

English version:

The relative stability and the conformational behavior of symmetric multihelicene isomers of formula C<sub>66</sub>H<sub>36</sub> are analyzed by computational means. Our efforts to synthesize some of these molecules are also reported, in particular the observation of unexpected debromination when attempting to synthesize 13,14-dibromo[5]phenacene, and of unexpected reactivity of 13,14-dibromo[5]aphene when attempting its cyclotrimerization promoted by a nickel(0) complex.

<footnote>[a]current address: PASTEUR, Département de Chimie, École Normale Supérieure, PSL University, Sorbonne University, CNRS, 75005 Paris, France</footnote>

<footnote>[b]current address: Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona (UdG), Facultat de Ciències, Girona, Catalunya 17003, Spain</footnote>
Version française:

La stabilité relative et le comportement conformationnel des isomères symétriques de multihélicènes de formule C₆₆H₃₆ sont analysés par le calcul. Nos efforts pour synthétiser certaines de ces molécules sont également rapportés, en particulier l'observation d'une débromation inattendue lors de la tentative de synthèse du 13,14-dibromo[5]phénacène, et d'une réactivité inattendue du 13,14-dibromo[5]aphène lors de tentatives de sa cyclotrimérisation promue par un complexe de nickel(0).

<figure><img src="image_2.png" /></figure>

Keywords

Multihelicene, regioisomers, diastereomers, London dispersion, cyclotrimerization
Introduction

The atomically precise controlled organic synthesis of nanographenes, i.e. fragments of
graphene with at least one dimension of 1–100 nm, and the examination of their properties,
have become extremely active fields of research.¹ Especially, scientists from several fields are
exploring the relationships between the size and the shape of nanographenes, and thus of
their π-conjugated electronic system, with their opto-electronic or magnetic properties, or
their aromaticity. The structural diversity of benzenoid nanographenes, i.e. nanographenes
composed only of 6-membered rings of carbon atoms, is extraordinary. The fusion of one or
several additional benzene rings to a given benzenoid molecule offers an exponentially
growing number of possible π-extended isomeric molecules, possibly existing under different
conformations.²

Because of the steric hindrance of peripheric hydrogen atoms, nanographenes with
fjord regions at their edges, i.e. with carbo[5]helicene fragments, are contorted and become
three-dimensional. The carbo[n]helicenes<sup>3,4,5,6,7,8</sup> represent a family of synthetic benzenoid
compounds, that for n ≥ 6 they can no longer be conceptualized as graphene fragments due
to the superposition of either two carbon atoms (for n = 6) or rings (for n > 6). Of paramount
importance, carbo[n]helicenes are chiral conformationally stable π-conjugated molecules that
display peculiar chiral properties. As such, they are at the heart of research on circular
dichroism (CD), circularly-polarized luminescence (CP-OLED) and chiral-induced spin
selectivity (CISS).<sup>9</sup> As for nanographenes, fusing rings onto carbohelicenes can provide a
diversity of π-extended and expanded helically twisted chiral benzenoid molecule with new
properties.<sup>10,11,12,13,14,15</sup> Especially, several carbohelicene fragments with various sizes, twists,
and degrees of π-extension can be introduced on the same molecule, the so-called
multihelicenes, resulting in extraordinary 3D benzenoid
architectures.¹⁶,¹⁷,¹⁸,¹⁹,²⁰,²¹,²²,²³,²⁴,²⁵,²⁶,²⁷,²⁸,²⁹,³⁰,³¹

Hexabenzotriphenylene (HBTP) is an archetypal symmetric benzenoid triple
carbo[5]helicene with three carbo[5]helicene fragments fused within a benzenic central ring
(Figure 1). Because of the symmetrical arrangement of three stereogenic helical fragments,
whose helicity is noted (P) for right-handed helicity and (M) for left-handed helicity, HBTP can
exist as two diastereomers with C₂ and D₃ symmetry. Synthetically, HBTP can be regarded as
a phenanthrene cyclotrimer through the formation of its central six-membered ring (with sky
blue highlight in Figure 1).³²,³³ Accordingly, the group of Pascal reported that D₃-HBTP could
be obtained through the thermolysis of phenantrene-9,10-dicarboxylic anhydride, probably
involving an uncatalyzed [2 + 2 + 2] cycloaddition of 9,10-phenanthryne, a short-lived aryne
intermediate generated in-situ.³⁴ Contemporaneously, Pérez, Guitián, and co-workers
reported that both diasteromers C₂-HBTP and D₃-HBTP could be obtained efficiently by a
palladium-catalyzed [2 + 2 + 2] cycloaddition of 9,10-phenanthryne, now generated under
mild conditions from its Kobayashi-type precursor.³⁵,³⁶ This study experimentally confirmed
Pascal's prediction that D₃-HBTP is the thermodynamic diastereomer, and demonstrated that
C₂-HBTP is the kinetic diastereomer of the [2 + 2 + 2] cyclotrimerization of 9,10-phenanthryne.

Herein, we compare the relative stability and the conformational behavior of the
symmetrically π-extended hexabenzo analogs of HBTP of formula C₆₆H₃₆, the multihelicenes
5H, 5P, and 5A, through computational investigations supported by experimental data when
available. Note that the six stereogenic carbo[5]helicene fragments in 5H induce up to ten
possible diastereomers, with only the two most stable ones discussed herein. Multihelicenes $D_3-5H,^{37,38,39} C_2-5H,^{37}$ and $C_2-5P^{40,41}$ could recently be synthesized, while $D_3-5P, C_2-$ and $D_3-5A$ have resisted synthesis efforts to date, which is also analyzed.

## Computational work

Geometries of all stationary points were optimized without symmetry constraints with the Gaussian 16 program$^{42}$ using the DFT B3LYP hybrid exchange-correlation functional.$^{43}$ The 6-31G(d) basis set was employed for all atoms. The D3 Grimme energy corrections for dispersion with the original damping function were added to the calculations.$^{44}$ Analytical Hessians were computed to determine the nature of stationary points (one and zero imaginary frequencies for transition states and minima, respectively) and to calculate unscaled zero-point energies (ZPEs) as well as thermal corrections and entropy effects using the standard statistical-mechanics relationships for an ideal gas.$^{45}$ These two latter terms were computed at 298.15 K and 1 atm. As a summary, the reported Gibbs energies ($\Delta G$) were computed in the gas phase at the B3LYP-D3/6-31G(d) level of theory unless otherwise mentioned.

## Experimental section

### General

Reactions were generally carried out under an argon atmosphere in oven-dried reaction vessels in anhydrous solvents. All reagents were weighed and handled in air at room
temperature unless otherwise mentioned, and all commercially available reagents were used
as received unless otherwise mentioned. Anhydrous methanol was obtained by refluxing over
magnesium turning and then distilled. Anhydrous tetrachloromethane was obtained by initial
drying with CaSO4, followed by fractional distillation and stored on 3Å molecular sieves under
argon. Anhydrous dichloromethane, diethyl ether, tetrahydrofuran and toluene were dried by
filtration over solid dehydrating agents using a commercial solvent purification system.
Reactions were monitored by thin layer chromatography carried out on Merck Kieselgel 60
F254 0.2 mm plates. Visualization was accomplished using ultraviolet light (254 and 365 nm)
and/or chemical staining with an ethanolic solution of para-anisaldehyde with sulphuric acid
as appropriate. Purifications were routinely performed using flash chromatography with 40-
63 µm silica gel generally eluted with a mixture of pentane/ethyl acetate or pentane/diethyl
ether. NMR data were generally recorded in deuterated chloroform with a 9.4 T spectrometer
at 298 ± 3 K using as internal standards the residual chloroform signal for 1H NMR (δ = 7.26
ppm) and the deuterated solvent signal for 13C NMR (δ = 77.16 ppm). Chemical shifts (δ) are
in ppm, coupling constants (J) are in Hertz (Hz) and the classical abbreviations are used to
describe the signal multiplicities. 13C DEPT135 experiments were used to support assignments.
High resolution mass spectra (HRMS) were recorded in triplicate at the Spectropole
(https://fr-chimie.univ-amu.fr/spectropole/) on a Waters Synapt G2 HDMS apparatus using a
positive electrospray (ESI) ionization source. Melting points were recorded using Büchi
Melting Point B-545 apparatus.

Compounds **2**, **3**, **10**, and **11** were prepared as previously described in comparable yields and purity as analyzed by NMR spectroscopy.⁴⁶
## Compound 4

To a stirred suspension of **3** (1.5 g, 4.84 mmol) in MeOH (50 mL) was slowly added NaBH₄ (367 mg, 10.16 mmol) at 0 °C under argon atmosphere. After addition, the solution was warmed to room temperature. After 1 h, the reaction was quenched with concentrated HCl solution until pH = 1. The resulting precipitate was filtered, triturated with MeOH cooled with an ice-bath, filtered, and dried under vacuum to afford **4** as a white solid (1.52 g, 99%) requiring no further purification. **Mp** 171–173 °C (amorphous); **TLC** (pentane/EtOAc = 5:1) R<sub>f</sub> = 0.41; **HRMS** (ESI+) *m/z* calcd for C<sub>22</sub>H<sub>12</sub>O<sub>2</sub>Na<sup>+</sup> [M+Na]<sup>+</sup> 337.1199, found 337.1194; <sup>1</sup>H NMR (400 MHz, CDCl<sub>3</sub>) δ 8.26 (d, J = 8.3 Hz, 2H), 7.93 (dd, J = 8.0, 1.0 Hz, 2H), 7.85 (d, J = 8.4 Hz, 2H), 7.62 (ddd, J = 8.8, 6.9, 1.3 Hz, 2H), 7.56 (ddd, J = 8.4, 7.0, 1.0 Hz, 2H), 7.30 (d, J = 8.3 Hz, 2H), 4.95 (d, J = 12.0 Hz, 2H), 4.35 (d, J = 12.0 Hz, 2H), 3.68 (broad s, 2H); <sup>13</sup>C<sup>{1H}</sup> NMR (101 MHz, CDCl<sub>3</sub>) δ 139.2 (2C), 134.1 (2C), 133.5 (2C), 132.3 (2C), 128.8 (2CH), 128.5 (2CH), 128.5 (2CH), 127.4 (2CH), 126.2 (2CH), 124.6 (2CH), 59.0 (2CH<sub>2</sub>).

## Compound 5

To a solution of **4** (1.52 g, 4.84 mmol) in anhydrous dichloromethane (20 mL) at 0 °C under an argon atmosphere was added dropwise PBr<sub>3</sub> (1.15 mL, 12.1 mmol). After addition, the resulting yellow solution was warmed to room temperature. After 2 h, the reaction was carefully quenched with a saturated NaHCO<sub>3</sub> aqueous solution at 0 °C. The aqueous phase was extracted with EtOAc and the combined organic layers were washed with water, brine, and dried over anhydrous Na<sub>2</sub>SO<sub>4</sub> before filtration and concentration under reduced pressure. The resulting material (2.13 g, quantitative) required no further purification. **Mp** 217–219 °C (amorphous); **TLC** (pentane/Et<sub>2</sub>O = 10:1) R<sub>f</sub> = 0.63; **HRMS** (ESI+) *m/z* calcd for C<sub>22</sub>H<sub>16</sub>Br<sub>2</sub>Ag<sup>+</sup> [M+Ag]<sup>+</sup> 546.8650, found 546.8649; <sup>1</sup>H NMR (400 MHz, CDCl<sub>3</sub>) δ 8.26 (dd, J = 8.4, 0.5 Hz, 2H),
7.97 (dd, J = 10.5, 8.0 Hz, 2H), 7.72 (ddd, J = 9.5, 7.0, 1.3 Hz, 2H), 7.61 (ddd, J = 8.6, 7.0, 1.1 Hz, 2H), 7.45 (d, J = 8.5 Hz, 2H), 4.85 (d, J = 10.5 Hz, 2H), 4.66 (d, J = 10.5 Hz, 2H); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 138.9 (2C), 133.8 (2C), 131.5 (2C), 130.7 (2C), 129.4 (2CH), 129.1 (2CH), 127.4 (2CH), 127.4 (2CH), 126.6 (2CH), 124.4 (2CH), 29.2 (2CH₂).

## Compound 6

To a 50 mL Schlenk tube was added 5 (500 mg, 1.14 mmol), N-bromosuccinimide (1.22 g, 6.84 mmol), benzoyl peroxide (42 mg, 15 mol%) and freshly distilled CCl₄ (10 mL). The mixture was heated in an oil bath at 80 °C with a vigorous stirring. After 60 h, N-bromosuccinimide (406 mg, 2.28 mmol) and benzoyl peroxide (14 mg, 5 mol%) were added again to the red mixture which was stirred for additional 36 h at 80 °C. After cooling down to room temperature, filtration over silica gel washed with hot chloroform, and removal of the solvent under vacuum afforded a residue which was taken up in dichloromethane and water. The aqueous phase was separated and extracted twice with dichloromethane. The combined organic layers were washed with a saturated Na₂CO₃ aqueous solution, dried over anhydrous Na₂SO₄, filtered, and concentrated under reduced pressure. Flash column chromatography on silica gel (eluted with pentane/Et₂O 10:1) afforded compound 6 (544 mg, 80%) as a white solid. Mp 238-240 °C (CDCl₃); TLC (pentane/Et₂O = 10:1) R_f = 0.89; HRMS (ESI+) m/z calcd for C₂₂H₁₄Br₄Ag⁺ [M+Ag]⁺ 704.6841, found 704.6834; ¹H NMR (400 MHz, CDCl₃) δ 9.13 (dd, J = 8.7, 0.8 Hz, 2H), 8.00 (ddd, J = 10.4, 9.0, 0.7 Hz, 4H), 7.81 (ddd, J = 8.6, 7.0, 1.5 Hz, 2H), 7.67 (ddd, J = 8.9, 7.0, 1.0 Hz, 2H), 7.24 (d, J = 8.4 Hz, 2H), 6.95 (s, 2H); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 134.8 (2C), 133.3 (2C), 133.3 (2C), 131.3 (2CH), 130.7 (2C), 129.3 (2CH), 127.3 (2CH), 126.9 (2CH), 126.8 (2CH), 125.4 (2CH), 36.2 (2CH).
Recrystallization of **6** from *d*-chloroform (slow evaporation in air) afforded crystalline needles suitable for single crystal X-ray diffraction analysis, CCDC 2395137.

## 13-Bromo[5]phenacene (7)

To a suspension of **6** (20 mg, 0.033 mmol) in freshly distilled DMF (2 mL) at 0 °C under an argon atmosphere was added potassium *tert*-butoxide (7.5 mg, 0.067 mmol). The resulting brown solution was warmed to room temperature and vigorously stirred until the starting material is no longer detectable by TLC (1 h). Then, the solution was poured into an 6M HCl aqueous solution and extracted three times with ethyl acetate. The combined organic layers were washed with water, brine, and dried over Na₂SO₄, before filtration and concentration under reduced pressure. The resulting crude material was purified by flash column chromatography on silica gel (eluted with pentane/Et₂O 10:1) to provide 13-bromo[5]phenacene (7) a white solid (11 mg, 97%). Mp 262–264 °C (amorphous); **TLC** (pentane/Et₂O = 10:1) R_f = 0.75; **HRMS** (ESI+) *m/z* calcd for C₂₂H₁₃BrAg⁺ [M+Ag]⁺ 464.9235, found 704.6834; **¹H NMR** (400 MHz, CDCl₃) δ 9.96–9.94 (m, 1H), 9.26 (s, 1H), 8.75 (dd, *J* = 9.6 Hz, 2H), 8.69 (d, *J* = 9.0 Hz, 1H), 8.05–7.99 (m, 4H), 7.76 (ddd, *J* = 8.3, 7.0, 1.3 Hz, 1H), 7.72–7.66 (m, 3H); **¹³C{¹H} NMR** (101 MHz, CDCl₃) δ 133.2 (C), 132.3 (C), 131.5 (C), 130.2 (C), 129.6 (CH), 129.4 (C), 129.0 (C), 128.9 (CH), 128.6 (CH), 128.3 (CH), 128.2 (CH), 128.1 (C), 127.5 (CH), 127.3 (CH), 127.3 (CH), 127.3 (C), 127.1 (CH), 125.3 (CH), 123.3 (CH), 121.6 (CH), 121.5 (CH), 118.4 (C).

## Compound 12

To a solution of **11** (700 mg, 1.89 mmol) in anhydrous THF (10 mL) was added dropwise DIBAL-H (1 M in THF, 18.9 mmol) at 0 °C. The reaction mixture was then heated at reflux until the starting material is no longer detectable by TLC (2 h). After cooling at 0 °C, the resulting brown
mixture was slowly poured into 1M HCl and stirred for 30 min, resulting in precipitation. The resulting solid (the intermediate diol) was filtered, washed with water and Et₂O and dried under vacuum. This material was directly suspended in AcOH and HBr (50% w/v in AcOH, 9.45 mmol) was added at room temperature, resulting in an orange coloration of the solution. The mixture was heated at reflux for 2 h and cooled down at 0 °C with moderate stirring to provide a white precipitate. After filtration, this material was washed with water and dried under vacuum to afford **12** (482 mg, 58% after two steps) as a white solid, requiring no additional purification. **Mp** 171–173 °C (amorphous); **TLC** (pentane/EtOAc = 20:1) *R*<sub>f</sub> = 0.58; **HRMS** (ESI+) *m/z* calcd for C<sub>22</sub>H<sub>16</sub>Br<sub>2</sub>Ag<sup>+</sup> [M+Ag]<sup>+</sup> 546.8650, found 546.8653; <sup>1</sup>H NMR (400 MHz, CDCl<sub>3</sub>) δ 8.26 (dd, *J* = 8.4, 0.5 Hz, 2H), 7.97 (dd, *J* = 10.5, 8.0 Hz, 2H), 7.72 (ddd, *J* = 9.5, 7.0, 1.3 Hz, 2H), 7.61 (ddd, *J* = 8.6, 7.0, 1.1 Hz, 2H), 7.45 (d, *J* = 8.5 Hz, 1H), 4.85 (d, *J* = 10.5 Hz, 2H), 4.66 (d, *J* = 10.5 Hz, 2H); <sup>13</sup>C<sup>{1H}</sup> NMR (101 MHz, CDCl<sub>3</sub>) δ 136.9 (2C), 133.4 (2C), 133.3 (2C), 132.9 (2C), 130.3 (2CH), 130.0 (2CH), 128.0 (2CH), 128.0 (2CH), 127.2 (2CH), 126.8 (2CH), 32.9 (2CH2).

**Compound 13**

To a 50 mL Schlenk tube was added **12** (440 mg, 1.0 mmol), *N*-bromosuccinimide (890 mg, 5.0 mmol), benzoyl peroxide (48 mg, 20 mol%) in freshly distilled CCl₄ (10 mL). The mixture was heated with an oil bath at 80 °C with a vigorous stirring until the starting material is no longer detectable by TLC analysis (2 days) whereupon a saturated Na₂CO₃ aqueous solution was added to the reaction mixture. The phases were separated and the aqueous phase was extracted twice with dichloromethane. The combined organic phases were washed with water, brine, dried over Na₂SO₄, filtered, and concentrated under reduced pressure. Flash column chromatography on silica gel (eluted with pentane/Et₂O 10:1) afforded compound **13** (449 mg, 75%) as a white solid. **Mp** 222–224 °C (amorphous); **TLC** (pentane/Et₂O = 10:1) *R*<sub>f</sub> =
0.62; **HRMS** (ESI+) *m/z* calcd for C₂₂H₁₈NBr₄⁺ [M+NH₄]⁺ 615.8128, found 615.8130; **¹H NMR** (400 MHz, CDCl₃) δ 8.70 (s, 2H), 8.04 (dd, *J* = 8.2, 0.9 Hz, 2H), 7.87 (dd, *J* = 8.0, 1.5 Hz, 2H), 7.73 (s, 2H), 7.65–7.58 (m, 4H), 6.44 (s, 2H); **¹³C{¹H} NMR** (101 MHz, CDCl₃) δ 138.5 (2C), 133.5 (2C), 133.2 (2C), 131.8 (2C), 130.8 (2CH), 129.3 (2CH), 128.5 (2CH), 128.2 (2CH), 127.9 (2CH), 127.5 (2CH), 39.1 (2CH).

## 13,14-dibromo[5]aphene (14)

To a suspension of **13** (20 mg, 0.033 mmol) in freshly distilled DMF (2 mL) at 0 °C under an argon atmosphere was added potassium *tert*-butoxide (7.5 mg, 0.067 mmol). The resulting brown solution was warmed to room temperature and vigorously stirred until the starting material is no longer detectable by TLC (1 h). Then, the solution was poured into an 6M HCl aqueous solution and extracted with ethyl acetate. The organic layer was washed with water, brine, and dried over Na₂SO₄ before filtration and concentration under reduced pressure. The essentially pure 13,14-dibromo[5]aphene (14) was further purified by flash column chromatography on silica gel (eluted with pentane/EtOAc = 10:1 to pentane/THF 1:1) to provide a yellow solid (14 mg, quantitative). **Mp** 253–255 °C (amorphous); **TLC** (pentane/EtOAc = 10:1) *R*<sub>f</sub> = 0.43; **HRMS** (ESI+) *m/z* calcd for C₂₂H₁₃Br₂⁺ [M+H]⁺ 436.9359, found 436.9356; **¹H NMR** (400 MHz, CDCl₃/CS₂ = 1:1) δ 9.19 (s, 2H), 8.90 (s, 2H), 8.12 (ddd, *J* = 12.0, 7.0, 2.0 Hz, 4H), 7.66–7.59 (m, 4H); **¹³C{¹H} NMR** (101 MHz, CDCl₃/CS₂ = 1:1) δ 132.7 (2C), 132.4 (2C), 129.5 (2CH), 129.4 (2C), 128.5 (2CH), 128.4 (2C), 128.0 (2CH), 127.2 (2C), 127.1 (2CH), 126.8 (2CH), 122.3 (2CH).

## Compounds 15 and 16
In a glovebox under an argon atmosphere, a 10 mL tubular reaction vessel was charged with 13,14-dibromo[5]aphene (14, 100 mg, 0.23 mmol), 2,2'-bipyridine (93 mg, 0.60 mmol), degassed cis-cycloocta-1,5-diene (164 mg, 1.52 mmol), degassed dry THF (2 mL), and Ni(cod)₂ (170 mg, 0.39 mmol). The reaction vessel was sealed in the glovebox and then heated under microwave irradiation for 30 min at 130 °C, resulting in a dark purple solution. The reaction mixture was concentrated under reduced pressure and the resulting material (501 mg) was dissolved in 20 mL of boiling chloroform and filtrated on a short silica plug. The silica plug was washed with hot chloroform until the filtrate is colorless. Concentration of the filtrate under reduced pressure afforded 101 mg of a brown solid. TLC analysis (pentane/EtOAc = 10:1) revealed the presence of two compounds exhibiting blue fluorescence under 254 and 365 nm with respectively R<sub>f1</sub> = 0.93 and R<sub>f2</sub> = 0.62. Flash column chromatography on silica gel (eluted with pentane/EtOAc = 20:1) afforded [5]aphene (16, white solid, 18 mg, 28%) as the first eluted compound, and atropisomer 15 (beige solid, 37 mg, 57%) as the second eluted compound. For 15: Mp 198-200 °C (amorphous); TLC (pentane/EtOAc = 10:1) R<sub>f</sub> = 0.62; HRMS (ESI+) *m/z* calcd for C<sub>44</sub>H<sub>26</sub>Ag<sup>+</sup> [M+Ag]<sup>+</sup> 663.1085, found 663.1090; <sup>1</sup>H NMR (400 MHz, CDCl<sub>3</sub>) δ 9.44 (s, 2H), 9.43 (s, 2H), 8.35 (s, 2H), 8.26 (dd, *J* = 8.4, 1.5 Hz, 2H), 8.18 (d, *J* = 8.3 Hz, 2H), 8.10 (s, 2H), 8.08 (dd, *J* = 7.5, 1.5 Hz, 2H), 7.89 (s, 2H), 7.67-7.58 (m, 6H), 7.54 (ddd, *J* = 8.6, 6.8, 1.3 Hz, 2H), 7.38 (ddd, *J* = 9.0, 7.0, 1.0 Hz, 2H); <sup>13</sup>C<sub>1</sub>H<sub>7</sub> NMR (101 MHz, CDCl<sub>3</sub>) δ 137.7 (2C), 132.8 (2C), 132.6 (2C), 132.4 (2C), 132.2 (2C), 131.5 (2C), 131.2 (2C), 130.6 (2C), 129.4 (2CH), 129.1 (2CH), 128.7 (2CH), 128.4 (2CH), 128.2 (2CH), 127.9 (2CH), 127.3 (2CH), 126.7 (4CH), 126.3 (2CH), 126.0 (2CH), 122.4 (2C), 122.3 (2CH), 122.1 (2CH).

## Results and Discussion
The development of steric effects and resulting torsion in the isomeric multihelicenes **5H**, **5P**, and **5A** can be evaluated by comparing the relative energy of their diastereomers calculated without accounting for London dispersion interactions, e.g., using the B3LYP/6-31G(d) level of theory. Relative energies (ΔG) were thus found in the order: *D*₃-*5A* (0.0 kJ·mol⁻¹), *C*₂-*5A* (24.7 kJ·mol⁻¹), *D*₃-*5H* (96.9 kJ·mol⁻¹), *C*₂-*5H* (130.2 kJ·mol⁻¹), *C*₂-*5P* (132.4 kJ·mol⁻¹), and *D*₃-*5P* (149.0 kJ·mol⁻¹), indicating a net increase in destabilizing steric effects and resulting torsion from **5A** to **5H** to **5P**.<sup>37,40</sup> However, the compactness of these molecules also induces the development of intramolecular attractive London dispersion forces, which must be accounted for in the calculations to obtain realistic relative stability order, as well as reliable estimations of the barriers to diastereomers interconversion.<sup>47,48</sup> Using the D3 Grimme energy corrections for dispersion,<sup>44</sup> e.g., with the B3LYP-D3/6-31G(d) level of theory, it was found that *D*₃-*5A* is the most stable isomer (0.0 kJ·mol⁻¹), followed by *C*₂-*5A* (16.9 kJ·mol⁻¹), *C*₂-*5P* (44.5 kJ·mol⁻¹), *D*₃-*5H* (70.8 kJ·mol⁻¹), *D*₃-*5P* (73.3 kJ·mol⁻¹), and *C*₂-*5H* (102.2 kJ·mol⁻¹, Figure 2). This indicates a major contribution of London dispersion to the relative stability of these isomeric multihelicenes, which is especially important for *D*₃- and *C*₂-*5P*, with 75.7 and 87.9 kJ·mol⁻¹ stabilization energies, respectively. Indeed, *D*₃- and *C*₂-*5P* show the most compact geometries, with overlapping benzene rings and strong π-π interactions within the carbo[7]helicene fragments. Interestingly, the energy of planar [16]phenacene, possibly the most stable polycyclic aromatic hydrocarbon of formula *C*₆₆H₃₆,<sup>49</sup> is comparatively ΔG = -152.7 kJ·mol⁻¹ (or -36.3 kcal·mol⁻¹) illustrating the wide energy range that can be covered by stable isomeric PAHs. The relative energies of the *D*₃- vs the *C*₂-symmetric diastereomers of both **5H** and **5A** compare with those of the parent **HBTP**: the *D*₃-symmetric diastereomer is the thermodynamic diastereomer. The situation is otherwise for **5P**, now with *C*₂-*5P* being the
thermodynamic diastereomer,⁴⁰ a situation previously observed for related overcrowded triphenylene derivatives.⁵⁰,⁵¹,⁵² The barriers to diastereomers interconversion, from the less stable diastereomer to the more stable one, were also examined. It was found that C₂-5A only requires ΔGdia‡ = 103.9 kJ·mol⁻¹ to convert to its thermodynamic diastereomer D₃-5A, corresponding to a rate kdia = 3.9·10⁻⁶ s⁻¹ at 25 °C (t₁/₂ = 50 h), a situation comparable with HBTP for which ΔGdia‡ = 109 kJ·mol⁻¹ as experimentally determined at 328 K in d₂-tetrachloroethane.³⁵b The barriers to diastereomers interconversion for 5H³⁷,³⁸ and 5P⁴⁰ have previously been examined at different levels of theory, they have been recomputed at the B3LYP-D3/6-31G(d) level of theory herein for comparative purposes: ΔGdia‡ = 120.6 and 180.5 kJ·mol⁻¹ for 5H and 5P, respectively. Note for 5H, the diastereomer interconversion is a three-step process, corresponding to the inversion of configuration of three carbo[5]helicene fragments in C₂-5H, and only the rate limiting transition state is shown in Figure 2.

Synthetically, C₂-5H was obtained at room temperature by a palladium-catalyzed [2 + 2 + 2] of 7,8-carbo[5]helicyne, generated in-situ from its Kobayashi-type precursor, and D₃-5H was obtained from C₂-5H by a thermal diastereomer interconversion at 100 °C.³⁷ Alternatively, D₃-5H could be directly obtained by a nickel-promoted Yamamoto cyclotrimerization from 7,8-dibromocarbo[5]helicene at 120 °C.³⁸ Thus, for the synthesis of 5A and 5P, two routes can be considered, namely the aryne [2 + 2 + 2] and the Yamamoto cyclotrimerization reactions. The synthesis of dibromo[5]phenacene (8) was attempted as a potential synthetic precursor of 5P through a Yamamoto cyclotrimerization reaction. However, as described hereafter, our attempts for the synthesis of 8 have remained unsuccessful with the observation for an unexpected debromination in the final step. Happily, some of us could instead successfully implement the aryne [2 + 2 + 2] route for the synthesis of C₂-5P from 13,14-[5]phenacyne.⁴⁰
Our attempts to synthesize D₃-5A by the Yamamoto cyclotrimerization approach were also unproductive, which is detailed hereafter.

It was devised that 13,14-dibromo[5]phenacene (8) could be obtained by a base-promoted cyclization/elimination sequence from the tetrabromo precursor 6 (Scheme 1), following the same approach employed for the synthesis of 7,8-dibromo-carbo[5]helicene.⁵³ Thus, bis(aldehyde) 3 was prepared in two steps from 2-hydroxy-1-naphthaldehyde (1).⁴⁶ Two-fold reduction of 3 with sodium borohydride afforded diol 4, which was converted into dibromide 5. Bis-bromination of 5 under free-radical conditions afforded tetrabromide 6. Unexpectedly, treatment of tetrabromide 6 with two equivalents of potassium *tert*-butoxide afforded 13-bromo[5]phenacene (7) efficiently, with no detectable amount of the expected 13,14-dibromo[5]phenacene (8). Changing the reaction temperature (from -50 °C to 50 °C), the amount of potassium *tert*-butoxide (from 1 to 15 equiv), and the nature of the bases (for instance, KHMDS and NaNH₂) did not improve the reaction: in no case we could detect the formation of the expected product 8, but only variable amounts of 7. The origin of the unexpected debromination occurring from 6 to 7 is not clear. However, it can reasonably be attributed to the development of unfavorable steric factors induced by the formation of bay regions at both sides of the expected ortho-dibromoaryl fragment in 8.

Following the same approach, the synthesis of one or both diastereomers of 5A was then envisioned (Scheme 2). Thus, the tetrabromide 13 was prepared in five steps from methyl 3-hydroxy-2-naphthoate (9). Treatment of 13 with two equivalents of potassium *tert*-butoxide now afforded the desired product 13,14-dibromo[5]aphene (14) quantitatively. The cyclotrimerization of 14 was then attempted under standard Yamamoto conditions, i.e. the
reaction conditions that were efficient for the synthesis of D3-5H from 7,8-dibromo[5]helicene.³⁸ Surprisingly, only dimeric 13,13'-bi[5]aphenyl (15) and [5]aphene (16) itself could be obtained in good overall yield, with no detectable amount of the expected cyclotrimer multihelicene products C2-5A or D3-5A. Despite our efforts to change the outcome of this reaction by modification of the experimental conditions, i.e., using different temperatures, using DMF as the solvent, or changing the proportions of the nickel(0) complex and its ligands, no significant changes could be observed: 13,13'-bi[5]aphenyl (15) was systematically obtained as the major product (39–57% yield) together with [5]aphene (16) as a minor product (21–28%). This indicates that the stoichiometric organonickel intermediates formed during this reaction are not amenable to cyclotrimerization, probably due to unfavorable conformations.

## Conclusions

The synthesis of both C2-5H and D3-5H could be achieved by cyclotrimerization reactions using either the [2 + 2 + 2] aryne approach or the Yamamoto approach. This is remarkable considering that these multihelicenes are amongst the highest in energy of this series of molecules with formula C66H36. While the C66H36 multihelicene C2-5P could be synthesized by the [2 + 2 + 2] aryne approach, the synthesis of its diastereomer D3-5P is difficult to foresee through a cyclotrimerization approach because it is neither the kinetic nor the thermodynamic diastereomer. The synthesis of the C66H36 multihelicene C2-5A also appears challenging, especially because it is not the thermodynamic diastereomer with a low barrier to diastereomer interconversion, inducing rapid formation of the thermodynamically preferred
diastereomer *D*<sub>3</sub>-5*A*. We show here that formation of *D*<sub>3</sub>-5*A* under Yamamoto cyclotrimerization conditions is not occurring, but its synthesis through the [2 + 2 + 2] aryne approach or another approach seems feasible.

## Acknowledgments

This work was funded by the French Agence Nationale de la Recherche–ANR (ANR-19-CE07-0041). Institutional financial support from Aix-Marseille University and the CNRS is also acknowledged. A.A. also acknowledges the Spanish Ministerio de Universidades and the EU for a Margarita Salas grant (REQ2021_A_02). The authors thank Dr. Michel Giorgi (Aix Marseille Univ) for the structural analysis of compound 6.

## Supporting Information available

Geometries of the stationary points, copies of the NMR spectra, details of the single crystal X-ray diffraction analysis for 6 (CCDC 2395137).

## Author contribution

G. Dauvergne, M. Bin Jassar, F. Rigoulet, A. Artigas performed investigations – synthesis and calculations, and writing – review & editing. Y. Carissan, D. Hagebaum-Reignier, Y. Coquerel
performed conceptualization, methodology, supervision, project administration, funding
acquisition, and writing – original draft, review & editing.

**Competing Interests**

The authors declare no competing interest.

**Data Availability**

Data generated or analyzed during this study are available from the corresponding author
upon reasonable request. Crystallographic data generated during this study are available in
the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe
Access Structures Service, www.ccdc.cam.ac.uk/structures, under deposition number
2395137.

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<figure><img src="image_3.png" /><figcaption>**Figure 1.** The two diastereomers of hexabenzotriphenylene (HBTP) and their symmetrically π-extended hexabenzo analogs of formula C<sub>66</sub>H<sub>36</sub>.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Relative stability and barriers to diastereomers interconversion of multihelicenes 5A, 5P, and 5H. $ΔG$ in kJ·mol$^{-1}$ computed at the B3LYP-D3/6-31G(d) level of theory.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Attempted synthesis of 13,14-dibromo[5]phenacene (8)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Scheme 2.** Attempt for the synthesis of D₃-5A.</figcaption></figure>