# Regiospecific Methylalumination of Silacyclopropenes: The Access to Z-1-Silyl-2-Aluminyl-Disubstituted Olefins

A Selvaraj, Alejandra Acuña, Floriane Leboucher, Marie Cordier, Claire

Roiland, Gilles Alcaraz, Marc Devillard

► To cite this version:

A Selvaraj, Alejandra Acuña, Floriane Leboucher, Marie Cordier, Claire Roiland, et al. Regiospecific Methylalumination of Silacyclopropenes: The Access to Z-1-Silyl-2-Aluminyl-Disubstituted Olefins. Inorganic Chemistry, 2024, 63 (48), pp.22962-22972. 10.1021/acs.inorgchem.4c04049. [https://doi.org/10.1021/acs.inorgchem.4c04049](https://doi.org/10.1021/acs.inorgchem.4c04049). [CC-BY-NC-ND 4.0](https://creativecommons.org/licenses/by-nc-nd/4.0/)

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Regiospecific Methylalumination of
Silacyclopropenes: the Access to Z-1-Silyl-2-Aluminyl-Disubstituted Olefins

Akshitha Selvaraj,Alejandra Acuña,Floriane Leboucher,Marie Cordier,Claire Roiland,
Gilles Alcaraz,Marc Devillard*

ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, Université de Rennes, CNRS
263 av. du général Leclerc, F-35042 Rennes (France)

**KEYWORDS.** silacyclopropene - methylalumination - intramolecular interaction - regioselectivity - polymer synthesis

**ABSTRACT:** A methodology to access Z-1-silyl-2-aluminyl-disubstituted olefins is developed. It relies on the uncatalyzed ring opening of silacyclopropenes in presence of a stoichiometric amount of trimethylaluminum. The resulting heterosubstituted alkenes exhibit a particular interaction between the electron-rich [Si-CH₃] moiety and the electron-deficient diorganoaluminyl group, resulting in similar geometrical features due to the proximity of these two centers. This interaction is rationalized by using experimental and theoretical descriptors. The regioselectivity and functional tolerance of the methylalumination was also studied in the case of unsymmetrical silacyclopropenes and the methodology ultimately transposed to the synthesis of a polymeric organoalane incorporating tricoordinated aluminum centers.
## Introduction

Organoalanes are extensively studied organometallic species due to the metal's low cost, high natural abundance, and their strong Lewis acidity widely exploited in organic and polymerization chemistry.¹,² The synthesis of highly substituted olefins, a particularly active area of research, is a good example where vinylaluminium³ compounds are widely used, usually complementing vinylsilanes.⁴ However, 1-silyl-2-alumینyl-substituted olefins, that combine the two reactive functions in adjacent positions, are relatively less common due to regioselectivity issues during their synthesis. While the conventional routes involving mainly the hydroalumination of alkynes⁵⁻⁸ can be applied to access 1-silyl-2-alumینyl E isomers from silylacetylenes, their formation often competes with that of the Si / Al gem-disubstituted olefins, due to the electronic influence of silicon⁹⁻¹² and only few methods have been reported to selectively access these species.¹³,¹⁴ The case of Z-1-silyl-2-alumینyl-substituted olefins is even more complex, as no access route is known to date, leaving their chemistry unexplored.¹⁵

In this work, and following our research program on silicon heterocyclic chemistry,¹⁶,¹⁷ we unveil the ring-opening of silacyclopropenes¹⁸ (silirenes) by methylalumination as an access to this type of compound (Figure 1). The corresponding heterodisubstituted Z-olefins prepared thanks to this strategy have been in-depth studied as they display a peculiar intramolecular interaction. The regioselectivity of the methylalumination reaction of the silirenes as well as synthetic possibilities offered by the methodology will also be discussed, including its application to polymer synthesis.

**Figure 1.** a) Traditional hydroalumination route for the synthesis of E or gem silyl-alumینyl-disubstituted olefins; b) The methylalumination of silacyclopropenes as a new route for the synthesis of Z-1-silyl-2-alumینyl-disubstituted olefins.
a) Previous reports

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

b) This work

<figure><img src="image_3.png" /></figure>

**Results and Discussion**

We started our study with the symmetrical model silirene 1,1-di-tert-butyl-2,3-diphenylsilacyclopropene A. Heating a solution of A in presence of 1.05 equivalent of trimethylaluminum (TMA) in toluene for 24 hours at 50 °C leads selectively to the targeted Z-1-silyl-2-aluminyl-disubstituted alkene **1-AlMe₂** that could be isolated as highly air sensitive colorless crystals in 53 % yield (Figure 2a).¹⁹,²⁰ The nature of **1-AlMe₂** was unambiguously determined by multinuclear NMR, HR-MS and elemental analysis. The X-ray structure of **1-AlMe₂** was determined at 150 K (Figure 2a). In the solid state, **1-AlMe₂** exists as a monomeric species which is surprising given the tendency of methylaluminums to associate via Al–Me–Al bridges.²¹⁻²⁶ **1-AlMe₂** adopts a Z configuration which is in agreement with the data from the analysis of the compound in solution. The -AlMe₂ fragment is close to orthogonality with the plane of the molecule (85.7 °) (planes Si1C10C17Al1 and C24Al1C25) and faces the Si–CH₃ bond with a C9Al distance of 2.657(4) Å, which is longer than the sum of the single-bond covalent radii of Al and C [Σcov(Al,C) = 2.01 Å]²⁷ but significantly shorter than the sum of their vdW radii [ΣvdW(Al,C) = 3.8 Å].²⁸,²⁹ In addition, the aluminum center is slightly pyramidalized toward the C9 carbon of the methyl group [Σα(C-Al-C) = 357.6 °].
**Figure 2.** a) Synthesis of 1-AlMe2 by methylalumination of silirene A with trimethylaluminum and X-ray structure of 1-AlMe2 with displacement ellipsoid plot at 50 % probability level, hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°]: Si1-C9 1.884(5), Si1-C1 1.917(5), Si1-C5 1.927(4), Si1-C10 1.890(4), Al-C24 1.960(5), Al-C25 1.948(5), Al-C17 1.976(4), C10-C17 1.362(6); C9-Si1-C10 105.68(19), C9-Si1-C1 105.2(2), C10-Si1-C1 110.52(19), C9-Si1-C5 105.0(2), C10-Si1-C5 113.37(19), C1-Si1-C5 115.96(19), C25-Al1-C24 120.0(2), C25-Al1-C17 117.8(2), C24-Al1-C17 119.8(2); b) Synthesis of 1-Al(C6F5)2 by methylalumination of silirene A with methylbis(pentafluorophenyl)alane and X-ray structure of 1-Al(C6F5)2 with displacement ellipsoid plot at 50 % probability level, hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°]: Si1-C9 1.9050(17), Si1-C1 1.9217(17), Si1-C5 1.9081(17), Si1-C10 1.8939(15), Al-C24 1.9768(17), Al-C30 1.9806(17), Al-C17 1.9411(15), C10-C17 1.352(2); C9-Si1-C10 104.45(7), C9-Si1-C1 105.56(8), C10-Si1-C1 111.97(7), C9-Si1-C5 106.51(8), C10-Si1-C5 111.47(7), C1-Si1-C5 115.89(8), C30-Al1-C24 115.11(7), C30-Al1-C17 118.72(7), C24-Al1-C17 120.37(7).
<figure><img src="image_4.png" /></figure>

This particular feature suggests the existence of a weak intramolecular interaction involving the Si-CH₃ electron rich fragment and the electron deficient aluminum center. **1-AlMe₂** was further investigated by NMR spectroscopy³⁰ and the data compared to that of the aluminyl-free model compound **1-H**.³¹ In the ²⁹Si NMR spectrum of **1-AlMe₂**, the resonance signal appears significantly low-field shifted at δ 7.1 compared to **1-H** (δ 2.3). In the ¹H-coupled ¹³C NMR spectra, the ¹Jₚₚ coupling constant for the Si-CH₃ group shows a clear decrease of 4.5 Hz going from **1-H** (119.7 Hz) to **1-AlMe₂** (115.2 Hz) which is diagnostic of a weaker C-H bond. The ¹Jcsi coupling constant extracted from the ¹³C{¹H} NMR spectrum also display a clear decrease of 4.6 Hz going from **1-H** to **1-AlMe₂** (1-H, ¹Jcsi = 52.4 Hz, **1-AlMe₂**, ¹Jcsi = 47.8 Hz). The sum-up of the result of the XRD and NMR analyses is given in Table 1a.
To go further and assess the influence of the Lewis acidity of the aluminum center on the extent of the [Si-CH3···Al] interaction, the synthesis of the substituted bis-pentafluorophenyl analogue 1-Al(C6F5)2 was considered from H3C-Al(C6F5)2 generated in situ. The reaction with A leads to the selective methylalumination of the endocyclic Si-C bond of the silacyclopropene at room temperature as evidenced by NMR monitoring.32 The Z-1-silyl-2-aluminyl-disubstituted alkene 1-Al(C6F5)2 bearing a -Al(C6F5)2 group was isolated as clear yellow crystals in 32 % yield and fully characterized by NMR and X-Ray diffraction analysis (Figure 2b). In the solid-state, the structural conformation of 1-Al(C6F5)2 is similar to the one of its methylated congeners 1-AlMe2 with an even shorter C9Al distance of 2.536(2) Å, a more pronounced pyramidalization of aluminum toward the C9 carbon of the methyl group [Σα(C-Al-C) = 354.2°] as well as a pronounced bending of the C=C-Al angle toward C9 [∠C=C-Al; 1-AlMe2, 128.5(3)°, 1-Al(C6F5)2, 125.84(11)°], despite the larger steric demand of the defluorinated aryl groups. In solution, the 29Si NMR resonance signal at δ 8.9, deshielded compared with that of 1-AlMe2, also suggests an increase in the Si-CH3···Al interaction. The 1JCH coupling constant for the Si-CH3 group (112.5 Hz) follows the same trend with a decrease of 2.7 Hz compared to that of 1-AlMe2 and 7.2 Hz compared to that of the 1-H reference molecule.

**Figure 3.** a) Synthesis of 1-AlCl2 by transmetallation reaction between 1-AlMe2 and aluminum trichloride; b) Synthesis of 1-AlMes2 by arylation of 1-AlCl2 in presence of mesityl lithium and X-ray structure of 1-AlMes2 with displacement ellipsoid plot at 50 % probability level, hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°]: Si1-C9 1.8787(16), Si1-C1 1.9143(16), Si1-C5 1.9161(16), Si1-C10 1.8986(14), Al-C24 1.9797(15), Al-C33 1.9722(14), Al-C17 1.9797(14), C10-C17 1.3639(18); C9-Si1-C10 106.39(7), C9-Si1-C1 106.32(8), C10-Si1-C1
109.40(6), C9-Si1-C5 106.28(7), C10-Si1-C5 112.31(7), C1-Si1-C5 115.52(7), C33-Al1-C24 116.39(6), C33-Al1-C17 115.21(6), C24-Al1-C17 125.84(6).

<figure><img src="image_5.png" /></figure>

The influence of steric hindrance on the [Si–CH₃···Al] interaction was also assessed via the synthesis of **1-AlMes₂** and the incorporation of mesityl groups at the aluminum center. Starting from **1-AlMe₂**, **1-AlMes₂** was prepared in a two-step sequence (Figure 3). Firstly, a transmetallation reaction in the presence of AlCl₃ gave the dichloroaluminum derivative **1-AlCl₂** (Figure 3a) which was fully characterized after purification including by X-ray diffraction analysis (see S20 for details).³³ Notably, the molecule exhibited a dimeric arrangement due to chloride bridges typical of tricoordinated alkyl³⁴,³⁵ and aryl-³⁶ dichloroalanes containing a -AlCl₂ fragment. **1-AlCl₂** was then reacted at room temperature with mesityl lithium in diethylether leading to the clean substitution of chlorides by mesityl groups (Figure 3b). After crystallization from pentane in 39 % yield, **1-AlMes₂** was fully characterized by NMR, elemental analysis and its structure was determined by X-ray analysis at 150 K. In the solid state, the C9Al distance of 2.999(2) Å is significantly longer than that was observed in the case of **1-AlMe₂** and **1-Al(C₆F₅)₂**, along with a larger C=C-Al angle value large [134.43(11) °] (Table 1a). In this case, no interaction could be detected in the NMR signature of **1-AlMes₂** which display a ²⁹Si resonance signal chemical shift
at $\delta$ 3.6 and coupling constant values $^1J_{CH}$ (48.5 Hz), and $^1J_{CSi}$ (48.5 Hz) close to those of **1-H**
aluminium-free reference molecule.

**1-AlMe<sub>2</sub>, 1-Al(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> and 1-AlMes<sub>2</sub>** were then analyzed by theoretical calculations (Table 1b).
The structures of the three compounds were optimized by DFT at the PBE0/def2-TZVP level of
theory which reproduced the metrics of the solid state very accurately (see S96).<sup>37,38</sup> To ascertain
the description of the electronic structure and better define the [Si–CH<sub>3</sub>···Al] interaction, an Atoms
in Molecules (AIM) analysis of the topology of the electron density was carried out. In the case of
**1-AlMe<sub>2</sub>OPT** and **1-Al(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>OPT**, a bond critical point (BCP) is located on a path connecting the
Si–CH<sub>3</sub> carbon atom and the aluminum atom suggesting the existence of a bonding interaction
(Table 1b and 1c), which is not the case for **1-AlMes<sub>2</sub>OPT** in agreement with the experimental
data.<sup>39</sup> The electron density $\rho(r)$ and its Laplacian $\nabla^2\rho(r)$ at the BCP for both **1-AlMe<sub>2</sub>OPT** and **1-Al(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>OPT** are small and positive and, together with a negative value of the energy density $H(r)$,
indicate a weak closed shell interaction that increases in strength going from AlMe<sub>2</sub> to Al(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>
fragments. The NPA charges analysis for the three compounds is also informative with the net
positive charge of the aluminum atom decreasing gradually from **1-AlMes<sub>2</sub>OPT**, > **1-AlMe<sub>2</sub>OPT**, >
**1-Al(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>OPT** (Table 1b and 1d), which is opposite to the expected electronic effect of the
corresponding substituents. This is in line with a stronger [Si–CH<sub>3</sub>→Al] electron donation when
the Lewis acidity of the Al center increases.<sup>40</sup> Overall, the theoretical analysis argues in favor of a
[Si–CH<sub>3</sub>···Al] metal-methyl bond<sup>41</sup> of an electrostatic nature within **1-AlMe<sub>2</sub>** and **1-Al(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>**.
The situation is somewhat reminiscent of the Al–CH<sub>3</sub>–Al bonding in trimethylaluminum dimer<sup>42</sup>
but of much smaller magnitude due to the less polar Si–C bond in comparison with the Al–C bond.
While [Si–H···Al] interaction have already been identified in intermolecular<sup>43</sup> and
intramolecular<sup>44</sup> silane–alane complexes, **1-AlMe<sub>2</sub>** and **1-Al(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>** represent the first examples
where a [Si-CH3···Al] interaction can be observed. We then decided to assess the regioselectivity of the methylalumination reaction starting from unsymmetrical silirenes. It is well-known that the uncatalyzed carboalumination of terminal alkynes is poorly regioselective affording a mixture of compounds, zirconium catalyst being mandatory to regioselectivity as Negishi historically showed.⁴⁵

**Table 1.** Analysis of the [Si-CH3···Al] interaction within compound **1-AlMes**2, **1-AlMe**2 and **1-Al(C6F5)2** based on a) Experimental descriptors; b) Theoretical descriptors; c) Graph of the Laplacian of the electron density and bond critical point between Si-CH3 carbon atom and Al. d) Selected NPA charges within **1-AlMe**2**OPT**.
<figure><img src="image_6.png" /></figure>

1-H

1-AlMes₂

1-AlMe₂

1-Al(C₆F₅)₂

a

Experimental data

Geometrical data

d(C9Al) (Å)

2.999(2)

2.657(4)

2.536(2)

Σα(C-Al-C) (°)

357.4

357.6

354.2

α(C=C-Al) (°)

134.43(11)

128.5(3)

125.84(11)

NMR data for
SiCH₃ fragment

¹J<sub>CH</sub> (Hz)

119.7

116.6

115.2

112.5

²⁹Si δ

2.3

3.6

7.1

8.9

¹J<sub>Si</sub> (Hz)

52.4

48.2

47.8

43.9

b

Theoretical data

AIM analysis
(C···Al)

ρ(r) (e.bohr⁻³)

0.017

0.022

0.057

∇ρ(r) (e.bohr⁻⁵)

0.038

0.002

0.002

H(r)

NPA charges

Si

+1.91

+1.91

+1.92

Al

+1.89

+1.80

+1.75

C

-1.22

-1.23

-1.25

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>
<figure><img src="image_9.png" /></figure>

**Scheme 1.** Synthesis of silacyclopropenes 2a-f by thermal decomposition of a silabicyclo[4.1.0]heptane in presence of terminal alkynes. [a]The product being unstable under vacuum, it was prepared and stored as a toluene solution. The yield of 2e was determined by integration of the resonance signal in the 31P NMR spectrum.

Thus, the obtention of a Z-1-silyl-2-aluminyl-disubstituted olefins with high regioselectivity
remains a subject of interest. For this end, silirenes **2a-f** were synthetized by thermal
decomposition of a silirane precursor in presence of terminal alkynes (Scheme 1). 46-48 Compounds
**2a-c** are already described (see Experimental Section for details) whereas **2d-f** are new compounds
and could be clearly authenticated by NMR. It is interesting to note here the tolerance of the
silacyclopropenation reaction to a phosphinyl and a boronate functional group enabling the
synthesis of **2e** and **2f**, respectively. Silacyclopropenes **2a-f** were then reacted with TMA. After
optimization of the reaction conditions, we were able to show that the outcome of the reaction
depended on the substitution pattern of the silirene (-R group). In the case of reasonably small
substituents (Ph, Si(CH3)3 and nBu), silirenes react with TMA in a 3:1 ratio affording selectively
the product of tri-methylalumination **3a-c** (Figure 4). In the case of sterically demanding mesityl
group, the di-methylalumination reaction took place affording the divinylmethylalane **3d**. Finally,
a mono methylalumination reaction was observed in the case of the phosphinyl derivative **2e**
leading to the corresponding phosphinylalane **3e**. Interestingly, the reaction proved tolerant to this
Lewis base despite the presence of organoalanes in the reaction medium. In contrast,
methylalumination did not tolerate the presence of a boryl moiety, and the targeted compound was
not observed, but an intractable mixture of products, certainly due to B / Al exchange reactions.⁴⁹-
⁵¹ Compounds **3a-e** could be isolated either by evaporation of the crude reaction mixture (**3a** and **3c**) or after crystallization (**3b**, **3d**, **3e**). They were fully characterized by multinuclear NMR spectroscopy. Single crystals of compounds **3a**, **3b**, **3d** and **3e** could be obtained and their structure was ascertained by X-ray diffraction analysis (Figure 4 and SI 22). In the case of **3e**, the crystal analysis revealed a head-to-tail dimeric structure displaying intermolecular P→Al coordination.⁵²,⁵³ The in solution ¹H and ³¹P NMR analysis of **3e** revealed the presence of two species in equilibrium attributed to the monomeric and dimeric species in exchange.⁵⁴

**Figure 4.** Synthesis of trivinylalane **3a-c**, methyldivinylalane **3d** and dimethylvinylalane **3e** by methylalumination of silirenes **2a-e** in presence of TMA; X-ray structure of **3a** with displacement ellipsoid plot at 50 % probability level, hydrogens and solvent molecules are omitted for clarity. Selected bond lengths [Å] and angles [°]: Al1-C17 1.9710(15), Al1-C18 1.9733(15), Al1-C35 1.9567(15); C17-Al1-C35 122.36(6), C17-Al1-C18 125.05(6), C18-Al1-C35 110.34(6); Si1-C10-C17-Al1 -2.2(2), Al1-C18-C19-Si2 4.2(2), Al1-C35-C36-Si3 17.7(3); X-ray structure of **3d** with displacement ellipsoid plot at 50 % probability level, hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°]: Al1-C21 1.9504(18), Al1-C20 1.9627(16), Al1-C22 1.9663(17); C20-Al1-C22 120.05(7), C20-Al1-C21 119.83(8), C21-Al1-C22 120.11(8); Si1-C10-C20-Al1 3.8(3), Al1-C22-C23-Si2 4.3(3); X-ray structure of **3e** with displacement ellipsoid plot at 50 % probability level, hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°]: Al1-P1 2.5595(9), P1-C10 1.842(2), C10-C11 1.357(3); Al1-P1-C10 112.46(7), P1-C10-Si1 126.94(12), P1-C10-C11 108.19(15).
<figure><img src="image_10.png" /></figure>

In all case, the methylalumination reaction proved regiospecific with the addition of the Al-CH3 bond on the less hindered side of the silirene, even in the case of the rather small -Ph substituent (2a).<sup>55</sup>

The mechanism of the methylalumination process in the case of silirenes **2a-c** was then studied by DFT calculations (Figure 5). It turned out that the substitution of the silacyclopropene has a small influence on the mechanistic profile from a quantitative point of view. Qualitatively, similar conclusions can be drawn from the three cases and, as a consequence, only the case of **2a** will be discussed in the following section (see S100 for details). The first step involves the formation of an adduct between the silirene and trimethylaluminum (**Int-A** and **Int-B**). It results from the coordination of one of the silirene olefinic carbon to the aluminum center which is coplanar with the heterocycle. The approach of TMA on the less hindered CH carbon (**Int-A**, ΔG = +9.4 kcal.mol⁻¹) is preferred over the approach on the carbon bearing the phenyl group (**Int-A**, ΔG = +14.6 kcal.mol⁻¹). The adduct then evolves to a distorted 4-membered transition state where the
formation of the =C–Al bond is advanced (dCAI = 2.077 Å) while the Al–CH₃ methyl carbon only
interacts with silicon (dCSi = 2.411 Å), the endocyclic C–Si bond cleavage being in its early stage
(dCSi 2.056 Å). The transition state is somewhat reminiscent of the alkyne case,⁵⁶ albeit less
symmetrical due to polarity of the Si–C bonds. **TS-A** is significantly lower in energy than **TS-B**
(ΔΔG = -4.4 kcal.mol⁻¹) and evolves to the less stable product (ΔΔG = +5.2 kcal.mol⁻¹). Thus,
according to this theoretical mechanism, the regioselectivity of the methylalumination of the
silirenes is the result of a kinetically controlled reaction.

Given the mild conditions required for multi-methylalumination to occur, we reasoned that the
methodology could be transposed to the preparation of organoaluminum polymers incorporating
tricoordinated aluminum. To date, poly(organoaluminum) chemistry is restricted to alumoxane-
type oligomeric materials,⁵⁷ and to nitrogen-based moieties where the main-chain is hold by amino-
or imido-alane bonds.⁵⁸,⁵⁹

**Figure 5.** DFT computed mechanism of the first methylalumination process of silirene 2a<sup>OPT</sup> at the M06-2X/def2-TZVP level of theory.
<figure><img src="image_11.png" /></figure>

To this end, disilirene 2g, prepared from diethynylbenzene (see Experimental section and S11-S12), was reacted with TMA in a 2:3 ratio, at room temperature, leading to the gelification of the mixture after 1 hour (Figure 6). After work-up, polymer 4 was obtained as insoluble light yellow grains in 71 % isolated yield. 4 was analyzed by solid state NMR spectroscopy and compared to its monomer equivalent 3a. {¹H}¹³C and {¹H}²⁹Si spectra are shown on figure S93-94. We noticed that chemical shifts of ¹³C and ²⁹Si resonances are the same for the two samples and in agreement with that of 3a in solution NMR (see S72-75). Nevertheless, a broadening is observed on the solid-state NMR spectrum acquired on polymer 4, expressing a more disordered environment than in 3a. These results confirm that 3a structure remains unchanged in the polymer form 4. To go further, ²⁷Al spectra obtained on 4 and its monomeric form 3a are depicted on figure 6. Both spectra show a broad band from 200 kHz to -250 kHz meaning that the aluminum is in a similar environment in monomer 3a and polymer 4. By considering only one single position for aluminum in the structure, the ²⁷Al NMR signal can then be fitted using only a 1st order
quadrupolar lineshape with an isotropic position (δiso) at about 210 ppm, a quadrupolar constant (Cq) about 45 MHz and an asymmetric parameter (h) about 0.1. The high Cq value observed is about ten times higher than the Cq value expected for four-coordinated Al centers and is characteristic of three coordinated Al centers.⁶⁰,⁶¹

**Figure 6.** Synthesis of polymer **4** by methylalumination of disilirene **2g**; b) $^{27}$Al solid-state NMR spectra obtained for **3a** (top) and **4** (bottom). Dotted lines depict a fit of $^{27}$Al quadrupolar line assuming a single Al environment in the structure.
<figure><img src="image_12.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_13.png" /></figure>

**Conclusion**

Here, a methodology to access Z-1-silyl-2-aluminyl-disubstituted olefins from silacyclopropenes (silirenes) and trimethylaluminum (TMA) is developed. It relies on the ring-opening of the silicon heterocycle by methylalumination and operates in smooth and uncatalyzed
conditions. While the reaction results globally in the formation of Si-CH₃ and Al-C bonds, the
exact outcome depends strongly on the nature of the silirene starting material.

In the case of symmetrically substituted silicon heterocycles, a mono-methylalumination is
obtained and the proximity of the [Si-CH₃] electron rich fragment and the aluminyl group gives
rise to a peculiar [Si-CH₃···Al] interaction that could be analyzed by experimental (solid state
metrics, scalar *J* coupling constants in NMR spectroscopy) and theoretical (AIM calculations)
descriptors. In this context, a family of compounds bearing different substitution pattern, -
Al(CH₃)₂, -Al(C₆F₅)₂ and -AlMes₂, was successfully prepared. Our study shows an increase in the
strength of the [Si-CH₃···Al] interaction going from -Al(CH₃)₂ to -Al(C₆F₅)₂ groups whereas the
interaction is not present with the -AlMes₂ group due to steric repulsion.

In the case of monosubstituted silacyclopropenes precursors substituted by different -R groups,
the methylalumination reaction with AlMe₃ leads selectively to the mono-, di- or tri-
vinylaluminum due to mono-, di- or tri-methylalumination processes, depending on the nature of
the -R group. In all cases, the ring-opening reaction proceeds regiospecifically with the -R group
in trans-position of the aluminum atom in the product. The regioselectivity could be rationalize by
DFT calculations on the mechanism showing that it results from a kinetically controlled reaction.
Finally, the methodology was applied to polymer synthesis. A disilacyclopropene substrate was
reacted with TMA leading to the formation of a poly(organoalane) where aluminum is
tricoordinated. This unique type of polymer could be characterized by solid-state NMR
spectroscopy including <sup>27</sup>Al NMR, which displays an Al signal characteristic of a strong
quadrupolar coupling (Cq) confirming the presence of tricoordinated Al in the polymer main chain.
Experimental section

All reactions and manipulations were carried out under an atmosphere of dry argon using
standard Schlenk techniques or in a glove-box. Toluene ⁸d and C₆D₆ were dried over calcium
dihydride and distilled prior to use. All other solvents were purged with argon and dried using an
MBRAUN Solvent Purification System (SPS). ¹H, ¹³C, ¹¹B, ³¹P, ²⁹Si and ¹⁹F NMR spectra were
recorded on a Brucker Avance III 400 NMR spectrometer or a Brucker Avance III HD 500 NMR
spectrometer. Chemical shifts were expressed in positive sign, in parts per million, calibrated to
residual ¹H (2.08 ppm for Tol⁸d, 7.16 ppm for C₆D₆) and ¹³C (20.43 ppm for Tol⁸d, 128.06 ppm
for C₆D₆), 85 % H₃PO₄, and external tetramethylsilane solvent signals respectively. Mass spectra
were recorded on a MAXIS 4G or a Ultraflex III Mass spectrometers.

Elementary analyses were performed in Oberhausen, Germany, by the Mikroanalytisches
Laboratorium Kolbe (MIKROLAB, https://www.mikro-lab.de/?lang=en). The values for CH were
determined using an Elementar Model Vario Mikro CHNS analyzer. The elements Al and Si were
measured after microwave digestion on a MARS 6 from CEM on a Spectro Model Spectro Arcos
ICP. The sample was opened and weighed in a glovebox.

7,7-Bis(1,1-dimethylethyl)-7-silabicyclo[4.1.0]heptane was prepared as reported in our group¹⁶ based on a slightly modified literature procedure.⁶² 2,4,6-trimethylphenylacetylene,⁶³ tris(pentafluorophenyl)alane [Al(C₆F₅)₃·0.5Tol],⁶⁴ methyl-bis(pentafluorophenyl)alane,⁶⁵ and diphenylphosphinoacetylene⁶⁶ Ph₂P–C≡C–H were prepared according to literature procedures. Methyl-bis(pentafluorophenyl)borane was prepared in 3 steps according to literature procedures: 1) Me₂Sn(C₆F₅)₂;⁶⁷ 2) Cl-B(C₆F₅)₂;⁶⁸ 3) H₃C-B(C₆F₅)₂.⁶⁹ **2a-c** are reported compounds but have been prepared in a different way: **2a,c**⁷⁰ and **2b**.⁷¹
**Synthesis of 1-AlMe2.** Trimethylaluminum in solution in toluene (1.64 mL, 2M, 3.28 mmol, 1.05 eq.) was added to a solution of silirene A (1.00 g, 3.12 mmol) in toluene (10 mL) and the resulting solution was stirred at 50 °C for 24 hours. Then, the volatiles were removed under reduced pressure giving a white crystalline solid. The solid was solubilized in pentane (1 mL) and the pentane solution was placed at -30 °C giving white crystals. The crystals were solubilized again in pentane (1 mL) and a second crystallization at -30 °C gave pure **1-AlMe2** in 53 % yield (644 mg). Single crystals suitable for X-ray diffraction analysis were grown from a saturated toluene solution at -30 °C.

¹H NMR (C₆D₆, 400 MHz): δ= -0.26 (br. s, 6H, Al-CH₃), 0.49 (s, 3H, Si-CH₃), 1.06 (s with ²⁹Si sat., 18H, ³JHSi ≈ 5 Hz, CH₃tBu), 6.72-6.82 (m, 2H, Hₚ-Ph), 6.84-6.88 (m, 2H, Hₐ-Ph), 6.90-6.99 (m, 4H, Hₘ-Ph), 7.03-7.08 (m, 2H, Hₐ-Ph); ¹³C {¹H} NMR (C₆D₆, 100 MHz): δ = -7.0 (br. s, Al-CH₃), -5.6 (s, ¹JCSi = 47.9 Hz, Si-CH₃), 20.3 (s, C tBu), 29.6 (s, CH₃ tBu), 125.1 (s, CHₚ-Ph), 125.4 (s, CHₚ-Ph), 127.4 (s, CHₐ-Ph), 127.7 (s, CHₘ-Ph), 128.6 (s, CHₘ-Ph), 129.7 (s, CHₐ-Ph), 145.4 (s, Cipso-Ph), 145.9 (s, Cipso-Ph), 152.5 (s, Si-CH₃ vinyl), 175.6 (s, Al-CH₃ vinyl); ²⁹Si NMR (C₆D₆, 79 MHz): δ = 7.1.

¹J<sub>CH</sub> (H<sub>3</sub>C-Si) = 115.2 Hz (determined from ¹H and ¹³C NMR).

¹J<sub>CSi</sub>(H<sub>3</sub>C-Si) = 47.8 Hz (determined from ¹³C {¹H} NMR).

Anal. Calcd. For [C<sub>25</sub>H<sub>37</sub>SiAl]; C, 76.48; H, 9.50; Al, 6.87; Si, 7.15. Found: C, 76.37; H, 9.48; Al, 6.85; Si, 7.12.

HRMS (APCI): exact mass (monoisotopic) calcd for [C<sub>25</sub>H<sub>37</sub>AlSi-CH<sub>3</sub>]<sup>+</sup>, 377.22397; found 377.2241 (0 ppm).

**Synthesis of 1Et-AlEt2.** To a solution of silirene A (30.0 mg, 9.4 x 10⁻² mmol) in C₆D₆ (0.6 mL) was added triethylaluminum 1M in hexanes (103 µL, 0.100 mmol, 1.1 equiv.) and the mixture was heated at 70 °C for 48 hours showing the complete consumption of the starting material and the
selective formation of the expected compound (yield determined by NMR with mesitylene as an internal standard, 92 %). Then, volatiles were removed under reduced pressure affording the expected compound as a white solid. The compound was characterized by multinuclear NMR. Single crystals suitable for X-ray diffraction analysis were grown from a saturated pentane solution at -30 °C.

¹H NMR (C₆D₆, 500 MHz): δ = 0.39 (q, 4H, ³JHH = 8.1 Hz, CH₂-Al), 1.14 (s, 18H, CH₃ tBu), 1.22-1.29 (m, 9H, CH₃-CH₂-Al and CH₃-CH₂-Si), 1.32 (br., 2H, CH₂-Si), 6.74 (t br., 1H, ³JHH = 7.4 Hz, Hₚ-Ph), 6.79 (t br., 1H, ³JHH = 7.4 Hz, Hₚ-Ph), 6.83-6.89 (m, 2H, Hₐ-Ph), 6.91-6.99 (m, 4H, Hₘ-Ph), 7.02-7.07 (m, 2H, Hₐ-Ph); ¹³C{¹H} NMR (C₆D₆, 125 MHz): δ = 2.0 (s, CH₂-Al), 7.2 (s, CH₂-Si), 9.4 (s, CH₃), 10.2 (s, CH₃), 21.3 (s, C'Bu), 30.1 (s, CH₃'Bu), 125.0 (s, CHₚ-Ph), 125.3 (s, CHₚ-Ph), 127.1 (s, CHₐ-Ph), 127.6 (s, CHₘ-Ph), 128.6 (s, CHₚ-m-Ph), 129.9 (s, CHₐ-m-Ph), 146.3 (s, Cipso-Ph), 146.5 (s, Cipso-Ph), 154.2 (s, Si-CH₃), 174.9 (s, Al-CH₃); ²⁹Si NMR (C₆D₆, 99 MHz): δ = -0.9.

**Synthesis of 1-H.** Water (10.1 µL, 0.560 mmol, 1.1 equiv.) was added to a solution of **1-AlMe₂** (200 mg, 0.509 mmol) in toluene (1 mL) under stirring and the resulting colorless reaction mixture was stirred for another 10 minutes. Water and chloroform were then added and the fractions were separated. The organic layer was dried over MgSO₄, filtered and volatiles were removed under vacuum. The residue was purified by flash chromatography on silica gel (SiO₂ 15 g, L x 185 x 25 mm, eluant hexane, R<sub>f</sub> = 0.65) giving the expected compound as a colorless oil in 68 % yield.

¹H NMR (C₆D₆, 400 MHz): δ = -0.11 (s, 3H, Si-CH₃), 1.15 (s with ²⁹Si sat., 18H, ³JHSi ≈ 5 Hz, CH₃ tBu), 6.85-6.90 (m, 1H, Hₚ), 6.90-6.96 (m, 2H, Hₚ), 6.98-7.08 (m, 5H, Hₚ), 7.09-7.15 (m, 2H, Hₚ), 7.15 (s, 1H, H₃vinyl); ¹³C{¹H} NMR (C₆D₆, 100 MHz): δ = -7.1 (s with ²⁹Si sat., ¹JCSi = 52.4 Hz, Si-CH₃), 20.8 (s, C' tBu), 20.9 (s with ²⁹Si sat., ¹JCSi = 53.6 Hz, C' tBu), 29.5 (s, CH₃ tBu), 125.8 (s, CHₚ-Ph), 127.4 (s, CHₚ-Ph), 128.3 (s, CHₐ/m-Ph), 128.3 (s, CHₐ/m-Ph), 128.9 (s, CHₐ/m-Ph),
130.1 (s, CH<sub>o/m-Ph</sub>), 137.9 (s, C<sub>ipso-Ph</sub>), 141.5 (s, CH<sub>vinyl</sub>), 144.3 (s, Si-CH<sub>vinyl</sub>), 144.6 (s, C<sub>ipso-Ph</sub>); <sup>29</sup>Si NMR (C<sub>6</sub>D<sub>6</sub>, 79 MHz): δ = 2.3.

The two CH<sub>o/m-Ph</sub> resonance signals at δ 128.3 overlap with the signal of the solvent and where
assigned thanks to a <sup>13</sup>C–<sup>1</sup>H 2D HSQC experiment.

<sup>1</sup>J<sub>CH</sub> (H<sub>3</sub>C-Si) = 119.7 Hz (determined from <sup>1</sup>H and <sup>13</sup>C NMR).

<sup>1</sup>J<sub>CSi</sub> (H<sub>3</sub>C-Si) = 52.4 Hz (determined from <sup>13</sup>C{'H} NMR).

**Synthesis of 1-Al(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>.** A solution of MeAl(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> was prepared from a literature procedure: a solution of (C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>Al•0.5 toluene (40.0 mg, 6.97 x 10<sup>-2</sup> mmol) and trimethylaluminum (17.4 µL, 2M in hexane, 3.48 x 10<sup>-2</sup> mmol, 0.50 eq.) in toluene (0.5 mL) was stirred for 2 hours at room temperature.

Silirene A (22.3 mg, 6.97 x 10⁻² mmol) was then added as a powder and the resulting solution was stirred for 24 hours at room temperature. Volatiles were then removed under reduced pressure and the residue was extracted with heptane (0.7 mL). Placing the heptane solution at -30 °C for a night resulted in the formation of clear yellow crystals with a yield of 32 % (15.5 mg). Single crystals suitable for X-ray diffraction analysis were obtained from a saturated heptane solution at room temperature.

<sup>1</sup>H NMR (Tol<sup>8</sup>d, 400 MHz): δ= 0.93 (s, 18H, C<sub>H3</sub> tBu), 1.05 (s, 3H, Si-C<sub>H3</sub>), 6.69-6.75 (m, 1H, H<sub>p-Ph</sub>), 6.75-6.81 (m, 1H, H<sub>p-Ph</sub>), 6.87-6.94 (m, 6H, H<sub>Ph</sub>), 6.97 -7.02 (m, 2H, H<sub>Ph</sub>); <sup>13</sup>C{'H} NMR (Tol<sup>8</sup>d, 100 MHz): δ = -5.6 (s, Si-CH<sub>3</sub>), 20.2 (s, C<sub>tBu</sub>), 29.1 (s, CH<sub>3</sub> tBu), 111.7 (br. t, <sup>2</sup>J<sub>CF</sub> = 44.9 Hz, C<sub>ipso-C6F5</sub>), 126.1 (s, CH<sub>p-Ph</sub>), 126.2 (s, CH<sub>p-Ph</sub>), 128.1 (s, CH<sub>Ph</sub>), 128.2 (s, CH<sub>Ph</sub>), 128.5 (s, CH<sub>Ph</sub>), 129.1 (s, CH<sub>Ph</sub>), 129.2 (s, CH<sub>Ph</sub>), 137.5 (d m, J<sub>CF</sub> = 255.0 Hz, CF), 143.1 (s), 143.1 (d m, J<sub>CF</sub> = 255.3 Hz, CF), 144.4 (s), 150.1 (d m, J<sub>CF</sub> = 235.1 Hz, CF), 159.3 (s, C<sub>quat.</sub>), 166.8 (br.,
Cquat.); ¹⁹F>{¹H} NMR (Tol⁸d, 376 MHz): δ = -159.8--159.5 (m, 2F), -148.9--148.7 (m, 1F), -120.4--120.2 (m, 2F); ²⁹Si NMR (Tol⁸d, 79 MHz): δ = 8.9.

¹J<sub>CH</sub> (<u>H</u><sub>3</sub>C-Si) = 112.5 Hz (determined from ¹H and ¹³C NMR).

¹J<sub>CSi</sub> (<u>H</u><sub>3</sub>C-Si) = 43.9 Hz (determined from ¹³C>{¹H} NMR, 125 MHz).

**Synthesis of 1-AlCl<sub>2</sub>.** Aluminum trichloride (333 mg, 2.50 mmol, 2.00 equiv.) was added as a powder to a solution of compound **1-AlMe<sub>2</sub>** (491 mg, 1.25 mmol) in toluene (15 mL) and the resulting mixture was stirred for 3 h at room temperature leading to the slow solubilization of AlCl<sub>3</sub> and a yellow solution. This solution was placed at -30 °C giving colorless crystals over two days. The mother liquor was placed in a new vial at -30 °C affording crystals again. The two crystal fractions were washed with pentane (combined yield of 332 mg, 61 % yield). The compound degrades slowly under vacuum and under light. Single crystals suitable for X-ray diffraction analysis were obtained from a saturated toluene solution at -30 °C.

Anal. Calcd. For [C<sub>23</sub>H<sub>31</sub>SiAlCl<sub>2</sub>]; C, 63.73; H, 7.21; Al, 6.22; Si, 6.48. Found: C, 63.70; H, 7.20; Al, 6.20; Si, 6.45.

¹H NMR (C<sub>6</sub>D<sub>6</sub>, 400 MHz): δ = 0.74 (s, 3H, Si-CH<sub>3</sub>), 0.97 (s, 18H, CH<sub>3</sub> tBu), 6.71-6.78 (m, 2H, H<sub>Ph</sub>), 6.83-6.90 (m, 2H, H<sub>Ph</sub>), 6.90-6.97 (m, 2H, H<sub>Ph</sub>); ¹³C>{¹H} NMR (C<sub>6</sub>D<sub>6</sub>, 100 MHz): δ = -3.77 (s, Si-CH<sub>3</sub>), 20.3 (s, C tBu), 29.5 (s, CH<sub>3</sub> tBu), 125.9 (s, CH<sub>p-Ph</sub>), 125.9 (s, CH<sub>p-Ph</sub>), 128.7 (s, CH<sub>o/m-Ph</sub>), 129.1 (s, CH<sub>o/m-Ph</sub>), 144.5 (s, Cquat.) 145.0 (s, Cquat.), 162.1 (s, Cquat.), 162.4 (br., Cquat.); ²⁹Si NMR (C<sub>6</sub>D<sub>6</sub>, 79 MHz): δ = 5.6.

**Synthesis of 1-AlMes<sub>2</sub>.** To a solution of **1-AlCl<sub>2</sub>** (93.0 mg, 1.08 x 10<sup>-1</sup> mmol) in diethylether (9 mL) was added mesityl lithium as a powder (51.4 mg, 4.08 x 10<sup>-1</sup> mmol, 1.90 equiv.) and the
resulting suspension was stirred for 3 hours at room temperature giving a cloudy mixture. Lithium chloride was allowed to decant and the mother liquor was transferred in a new schlenk. The salts were washed with diethylether (6 mL) and the organic fractions were combined. The volatiles were then eliminated under reduced pressure giving a fluffy white solid. The solid was then solubilized in pentane (2 mL) and filtered. The resulting colorless solution was concentrated at room temperature leading to the cristallization of compound **1-AlMes2** as colorless blocks in 39 % yield.

Single crystals suitable for X-ray diffraction analysis were obtained in the same way.

¹H NMR (C₆D₆, 400 MHz): δ = 0.93 (s, 3H, Si-CH₃), 1.01 (s, 18H, CH₃ tBu), 2.16 (s, 3H, CH₃ pMes), 2.45 (s, 6H, CH₃ oMes), 6.64-6.70 (m, 1H, Hₚ), 6.75-6.82 (m, 7H, Hₘₐ₈ & Hₚ), 6.82-6.84 (m, 2H, Hₚ), 6.89-6.97 (m, 2H, Hₚ), 7.03-7.09 (m, 2H, Hₚ); ¹³C {¹H} NMR (C₆D₆, 100 MHz): δ = -2.75 (s, Si-CH₃), 20.9 (s, C tBu), 21.4 (s, CH₃ pMes), 26.2 (s, CH₃ oMes), 29.7 (s, CH₃ tBu), 124.9 (s, CHₚ), 125.3 (s, CHₚ), 127.4 (s, CHₚ), 127.8 (s, CHₘₐ₈), 129.7 (s, CHₚ), 130.2 (s, CHₚ), 138.9, 144.2, 144.5, 144.9, 146.8, 158.1, 172.0 (s, Cquat.); ²⁹Si NMR (C₆D₆, 79 MHz): δ = 3.6.

Anal. Calcd. For [C₄₁H₅₃SiAl]; C, 81.95; H, 8.89; Al, 4.49; Si, 4.67. Found: C, 81.84; H, 8.92; Al, 4.44; Si, 4.61.

¹J<sub>CH</sub> (H₃C-Si) = 116.2 Hz (determined from ¹³C NMR).

¹J<sub>CSi</sub> (H₃C-Si) = 48.5 Hz (determined from ¹³C {¹H} NMR).

**Synthesis of 2a.** 7,7-Bis(1,1-dimethylethyl)-7-silabicyclo[4.1.0]heptane (1.00 g, 4.46 mmol) and phenylacetylene (910 mg, 979 µL, 8.91 mmol, 2.00 eq.) in solution in toluene (1 mL) were heated during 24 hours at 100 °C under stirring. The resulting colorless solution was then allowed to cool down to room temperature and the volatiles were removed under reduced pressure. The
residue thus obtained was distilled under reduced pressure (8 x 10⁻³ mbar, 90 °C) affording the expected compound as a colorless liquid in 73 % yield.

**Synthesis of 2b.** 7,7-Bis(1,1-dimethylethyl)-7-silabicyclo[4.1.0]heptane (632 mg, 2.82 mmol) and trimethylsilylacetylene (468 µL, 332 mg, 3.38 mmol, 1.20 eq.) in solution in toluene (6 mL) were heated during 30 hours at 100 °C under stirring. The resulting colorless solution was then allowed to cool down to room temperature and the volatiles were removed under reduced pressure. The residue thus obtained was distilled under reduced pressure (2 x 10⁻² mbar, 80 °C) affording the expected compound as a colorless liquid in 70 % yield.

**Synthesis of 2c.** 7,7-Bis(1,1-dimethylethyl)-7-silabicyclo[4.1.0]heptane (515 mg, 2.29 mmol) and 1-hexyne (316 µL, 226 mg, 2.75 mmol, 1.20 eq.) in solution in toluene (5 mL) were heated during 30 hours at 100 °C under stirring. The resulting colorless solution was then allowed to cool down to room temperature and the volatiles were removed under reduced pressure (2 x 10⁻² mbar) affording the expected compound as a colorless liquid in 92 % yield.

**Synthesis of 2d.** A solution of 7,7-Bis(1,1-dimethylethyl)-7-silabicyclo[4.1.0]heptane (286 mg, 1.27 mmol) and 1,3,5-(trimethylphenyl)acetylene (239 µL, 220 mg, 1.52 mmol, 1.20 eq.) in toluene (4 mL) was heated for 30 hours at 100 °C under stirring giving a dark red oil. The volatiles were then removed under reduced pressure and the residue thus obtained was distilled under reduced pressure (2 x 10⁻² mbar, 100 °C) affording the expected compound as a colorless liquid in 83 % yield.
¹H NMR (C₆D₆, 400 MHz): δ = 1.10 (s, 18H, CH₃ tBu), 2.16 (s, 3H, CH₃p-Mes), 2.56 (s, 6H, CH₃o-Mes), 6.84 (s, 2H, Hm-Mes), 8.16 (s, 1H, Hvinyl); ¹³C {¹H} NMR (C₆D₆, 100 MHz): δ = 21.0 (s, CH₃p-Mes), 21.1 (s, C tBu), 22.4 (s, CH₃o-Mes), 30.2 (s, CH₃ tBu), 129.0 (s, Cipso-Mes), 129.9 (s, CHm-Mes), 137.6 (s, Cp-Mes), 138.3 (s, Co-Mes), 140.2 (s, CHvinyl), 158.3 (s, Si-<u>C</u> vinyl); ²⁹Si NMR (C₆D₆, 79 MHz): δ = -75.6 (s).

**Synthesis of 2e.** A solution of 7,7-Bis(1,1-dimethylethyl)-7-silabicyclo[4.1.0]heptane (500 mg, 2.23 mmol) and ethynyldiphenylphosphine (468 mg, 2.23 mmol) in toluene (20 mL) was heated during 36 hours at 100 °C under stirring. Compound **2e** is unstable under vacuum and was used in the next step as a toluene solution. The yield of **2e** was estimated by integration of the ³¹P NMR spectrum (92 %). To characterize the compound, it was also prepared in an NMR tube in C₆D₆ and characterize in the crude mixture.

¹H NMR (C₆D₆, 400 MHz): δ = 1.00 (s, 18H, tBu), 7.03-7.06 (m, 2H, *H* Ph), 7.10-7.18 (m, 4H, *H* Ph), 7.62-7.69 (m, 4H, *H* Ph), 8.56 (d, ³J<sub>HP</sub> = 8.7 Hz, 1H, CH=C); ¹³C{¹H} (C₆D₆, 100 MHz): δ = 21.2 (s, *C* tBu), 30.1 (s, CH₃ tBu), 128.7 (d, J<sub>CP</sub> = 1.9 Hz, *CH* mPh), 128.8 (d, J<sub>CP</sub> = 5.2 Hz, *CH* pPh), 133.6 (d, J<sub>CP</sub> = 21.1 Hz, *CH* oPh), 138.5 (d, J<sub>CP</sub> = 9.7 Hz, *Cquat*), 151.5 (d, J<sub>CP</sub> = 11.0 Hz, *CH=C*), 165.2 (d, J<sub>CP</sub> = 56.2 Hz, CH=*C*); ³¹P{¹H} (C₆D₆, 161 MHz): δ = -7.7 (s); ²⁹Si{¹H} (C₆D₆, 79 MHz): δ = -67.9 (d, J<sub>SiP</sub> = 1.5 Hz).

**Synthesis of 2f.** A solution of 7,7-Bis(1,1-dimethylethyl)-7-silabicyclo[4.1.0]heptane (336 mg, 1.49 mmol) and pinacolborylacetylene (314 mg, 1.49 mmol, 1.00 eq.) in toluene (4 mL) was heated during 30 hours at 100 °C under stirring. The volatiles were then removed under reduced pressure affording the expected compound as a colorless oil in quantitative yield.
¹H NMR (C₆D₆, 400 MHz): δ = 1.09 (s, 18H, CH₃ tBu), 1.12 (s, 12H, CH₃ Pin), 9.26 (s, 1H, H₃vinyl); ¹³C {¹H} NMR (C₆D₆, 100 MHz): δ = 20.7 (s, C tBu), 25.0 (s, CH₃ Pin), 30.3 (s, CH₃ tBu), 83.2 (s, C Pin), 162.3 (s, CH₃vinyl); ²⁹Si NMR (C₆D₆, 79 MHz): δ = -78.7 (s); ¹¹B NMR (C₆D₆, 128 MHz): δ = -31.0 (s).

HRMS (ASAP), 30°C: exact mass calcd for [C₁₆H₃₁O₂]¹¹BSi+H]⁺, 295.22591; found 295.2261 (1 ppm).

**Synthesis of 2g.** A solution of 7,7-Bis(1,1-dimethylethyl)-7-silabicyclo[4.1.0]heptane (532 mg, 2.37 mmol) and 1,4-diethynylbenzene (150 mg, 1.18 mmol, 0.50 eq.) in toluene (4 mL) was heated during 30 hours at 100 °C under stirring. The volatiles were then removed under reduced pressure. The residue was solubilized in pentane (3 mL) and placed at -30 °C giving clear yellow crystals of the expected compound in 61 % yield. Single crystals suitable for X-ray diffraction analysis were obtained in the same way.

¹H NMR (C₆D₆, 400 MHz): δ= 1.13 (s, 36H, CH₃ tBu), 7.63 (s, 4H, CH C₆H₄), 8.36 (s, 2H, H₃vinyl);
¹³C {¹H} NMR (C₆D₆, 100 MHz): δ= 21.2 (s, C tBu), 30.2 (s, CH₃ tBu), 129.6 (s, CH C₆H₄), 134.9 (s, CH C₆H₄), 139.4 (s, CH₃vinyl), 161.5 (s, Si-C₆vinyl); ²⁹Si NMR (C₆D₆, 79 MHz): δ = -65.2 (s).

**Synthesis of 3a.** To a solution of silirene 2a (1.70 g, 6.93 mmol) in toluene (1 mL) was added trimethylaluminum (2M in toluene, 2.20 mmol, 1.10 mL, 0.32 equiv) under stirring and the resulting mixture was stirred for 3 hours at room temperature giving a white precipitate and a colorless mother liquor. The volatiles were then removed under vacuum giving the expected compound as a white solid in 98 % yield. The compound is contaminated with 18 % of toluene
based on ¹H NMR analysis. This remaining toluene could not be removed under prolonged vacuum.

Single crystals suitable for X-ray diffraction analysis were obtained by slow evaporation of a toluene solution at room temperature.

¹H NMR (C₆D₆, 400 MHz): δ= 0.54 (s, 9H, Si-CH₃), 1.03 (s, 48H, CH₃ tBu), 7.01 (m, 3H, Hvinyl), 7.14 (m, 6H, Hm-Ph), 7.16 (s, 3H, Hvinyl), 7.35 (m, 6H, Ho-Ph); ¹³C>{¹H} NMR (C₆D₆, 100 MHz): δ = -3.2 (s, Si-CH₃), 20.0 (s, C tBu), 29.8 (s, CH₃ tBu), 126.1 (s, CHp-Ph), 127.3 (s, CHo-Ph), 128.2 (s, CHm-Ph), 153.2 (s, Cipso-Ph), 160.9 (s, CHvinyl), 165.0 (s, Si-CHvinyl); ²⁹Si NMR (C₆D₆, 79 MHz): δ= 4.8.

**Synthesis of 3b.** To a solution of **2b** (821 mg, 3.28 mmol) in toluene (7 ml), was added trimethylaluminum 2.0 M in toluene (546 µl, 1.09 mmol, 0.33 eq) via syringe. The mixture was stirred for 12 hours at 50 °C. Then, volatiles were removed under vacuum (2.10⁻² mbar) and the resulting white solid was solubilized in pentane. The pentane solution was placed at -30°C affording colorless crystals. The mother liquor was placed at -30 °C giving a second batch of crystals (combined yield 41 % yield). Single crystals suitable for X-ray diffraction analysis were obtained in the same way.

¹H NMR (C₆D₆, 400 MHz): δ= 0.34 (s, 27H, Si(CH₃)₃), 0.55 (s, 9H, Si-CH₃), 1.07 (s, 54H, CH₃ tBu), 8.43 (s, 3H, Hvinyl); ¹³C>{¹H} NMR (C₆D₆, 100 MHz): δ= -3.5 (s, Si-CH₃), 2.5 (s, Si(CH₃)₃), 19.4 (s, C tBu), 29.9 (s, CH₃ tBu), 168.4 (s, Al-CH=C), 178.9 (s, Al-CH=C); ²⁹Si NMR (C₆D₆, 99 MHz): δ= -8.0 (br.), 10.4 (br.).
**Synthesis of 3c.** To a solution of silirene **2c** (430 mg, 1.91 mmol) in toluene (4 ml) was added trimethylaluminum 2.0 M in toluene (302 µl, 0.605 mmol, 0.316 eq) via syringe. The mixture was stirred for 4 hours at room temperature. Then, volatiles were removed under vacuum (2 x 10⁻² mbar) giving the expected compound as a yellow wax in quantitative yield.

¹H NMR (C₆D₆, 500 MHz): δ= 0.52 (s, 9H, Si-CH₃), 0.94 (t, 9H, ³JHH = 7.3 Hz, CH₂-CH₃), 1.12 (s, 54H, CH₃ tBu), 1.37 (pseudo-sext, 6H, ³JHH = 7.5 Hz, CH₂-CH₃), 1.64-1.74 (m, 6H, CH₂-CH₂-CH₃), 2.37-2.34 (m, 6H, =C-CH₂), 7.16 (s, 3H, *H*ₗvinyl); ¹³C{¹H} NMR (C₆D₆, 125 MHz): δ= -4.8 (s, Si-CH₃), 14.4 (s, CH₂-CH₃), 19.7 (s, *C* tBu), 23.5 (s, CH₂-CH₃), 29.9 (s, CH₃ tBu), 32.1 (s, CH₂-CH₂-CH₃), 40.2 (s, =C-CH₂), 152.7 (s, Al-CH=C), 161.5 (s, Al-CH=*C*); ²⁹Si NMR (C₆D₆, 99 MHz): δ= 4.5 (s).

**Synthesis of 3d.** To a solution of silirene **2d** (297.6 mg, 1.04 mmol) in toluene (3 ml) was added trimethylaluminum 2M in toluene (249 µl, 0.498 mmol, 0.48 eq) via syringe. The mixture was stirred for 5 hours at room temperature. Then, volatiles were removed under vacuum (2 x 10⁻² mbar) and the resulting white solid was solublized in pentane and placed at -30°C affording the expected compound as colorless crystals in 35% yield. Single crystals suitable for X-ray diffraction analysis were obtained in the same way.

¹H NMR (C₆D₆, 400 MHz): δ= -0.1 (br., 3H, Al-CH₃), 0.36 (s, 6H, Si-CH₃), 1.05 (s, 36H, CH₃ tBu), 2.17 (s, 6H, CH₃p-Mes), 2.35 (s, 12H, CH₃p-Mes), 6.84 (s, 4H, *H*ₘ-Mes), 6.91 (s, 2H, *H*ₗvinyl); ¹³C{¹H} NMR (C₆D₆, 100 MHz): δ= -5.9 (br., Al-CH₃), -2.4 (s, Si-CH₃), 20.5 (s, *C* tBu), 21.0 (s, CH₃p-Mes), 22.7 (s, CH₃o-Mes), 30.2 (s, CH₃ tBu), 128.9 (s, CHₘ-Mes), 133.7 (s, *C*o-Mes), 134.7 (s, *C*ₚ-Mes), 149.2 (s, *C*ipso-Mes), 161.1 (s, Al-CH=C), 164.4 (s, Al-CH-*C*); ²⁹Si NMR (C₆D₆, 79 MHz): δ = -1.3.
**Synthesis of 3e.** To a solution of silirene 2e in toluene (20 mL) was added trimethylaluminum 2M in toluene (1.0 mL, 2.05 mmol, 1 equiv.) and the mixture was heated at 50 °C for 1 hour. Then, volatiles were removed under reduced pressure and the resulting residue was washed with pentane (3 x 5 mL) giving the expected compound as a white solid (358 mg, 0.42 mmol, 56 %).

Single crystals suitable for X-ray diffraction analysis were obtained from a pentane solution at -30 °C. In benzene, an equilibrium between the monomer and the dimer is observed with broadened signals. As a consequence, the characterization revealed difficult (for ¹H and ³¹P {¹H} NMR spectra in C₆D₆, see SI 81-86). Thus, the compound was characterized in THF⁸d giving only the monomer-THF adduct.

¹H NMR (THF-d⁸, 500 MHz): δ = -0.79 (s, 6H, Al-CH₃), 0.21 (s, 3H, Si-CH₃), 1.04 (s, 18H, CH₃ tBu), 7.18-7.31 (m, 10H, HPh), 7.48 (d, ³JHP = 31.8 Hz, 1H, Hvinyl); ¹³C {¹H} (THF-d⁸, 125 MHz): δ = -8.1 (s, Al-CH₃), -5.3 (d, JCP = 8.2 Hz, Si-CH₃), 20.7 (d, JCP = 1.1 Hz, C tBu), 30.4 (d, JCP = 4.9 Hz, CH₃ tBu), 128.5 (s, CHp-Ph) 128.8 (d, JCP = 6.1 Hz, CHm-Ph), 135.3 (d, JCP = 18.5 Hz, CHO-Ph), 140.9 (d, JCP = 19.5 Hz, Cq-P), 154.8 (d, JCP = 56.3 Hz, Cvinyl) 189.8 (s, CHvinyl); ³¹P {¹H} (THF-d⁸, 161 MHz): δ = 5.3; ²⁹Si {¹H} (THF-d⁸, 99 MHz): δ = 6.5 (d, ²JSiP = 42.9 Hz).

**Synthesis of 3a-D.** To a solution of silirene 2a (215 mg, 0.879 mmol) in toluene (4 mL) was added trimethylaluminum (2M in toluene, 0.293 mmol, 440 µL, 1/3 equiv) under stirring and the resulting mixture was stirred for 3 hours at room temperature. Then, deuterium oxide (32.0 mL, 35.2 mg, 1.76 mmol, 2 equiv) was added and the resulting cloudy mixture was stirred for 1 hour at room temperature.

Water was then added, the phases were separated and the aqueous phase was washed with dichloromethane. The organic fractions were combined, dried over magnesium sulphate and
filtered. The volatiles were then removed under rotavapor vacuum (around 150 mbar) giving a colorless oil. The compound was then purified by flash chromatography on silica gel [m(SiO₂) 20 g, length 110 x width 25 mm, eluent hexane, R<sub>f</sub> = 0.70] and obtained as a colorless oil with a yield of 83 %.

¹H NMR (C₆D₆, 400 MHz): δ = -0.02 (s, 3H, Si-CH₃), 1.06 (s, 18H, CH₃ tBu), 5.79 (s, 1H, Hvinyl), 7.03-7.09 (m, 1H, Hp-Ph), 7.10-7.19 (m, 4H, Ho-Ph and Hm-Ph); ¹³C>{¹H} NMR (C₆D₆, 100 MHz): δ = -6.74 (s, Si-CH₃), 20.0 (s, CtBu), 29.2 (s, CH₃tBu), 126.1 (s, CHp-Ph), 128.2 (s, CHo-Ph or CHm-Ph), 130.7 (t111, ¹JCD = 23.9 Hz, CHD), 147.9 (s, Cipso-Ph), 151.8 (s, Si-Cvinyl); ²⁹Si NMR (C₆D₆, 79 MHz): δ = 2.4; ²H NMR (C₆D₆, 61 MHz): δ = 5.70 (s br).

¹J<sub>CH</sub> (H<sub>3</sub>C-Si) = 119.3 Hz (determined from <sup>13</sup>C NMR).

¹J<sub>CSi</sub> (H<sub>3</sub>C-Si) = 51.6 Hz (determined from <sup>13</sup>C>{<sup>1</sup>H} NMR).

**Synthesis of 3a-D<sup>isom</sup>.** This isomer 3a-D with deuterium in *trans* of the silyl group was prepared as 3a-D but from the *in situ* generated deuterated isomer of 2a prepared from Ph-C≡C-D instead of Ph-C≡C-H.

¹H NMR (C₆D₆, 400 MHz): δ = -0.02 (s, 3H, Si-CH₃), 1.06 (s, 18H, CH₃ tBu), 5.68 (s, 1H, Hvinyl), 7.03-7.10 (m, 1H, Hp-Ph), 7.10-7.19 (m, 4H, Ho-Ph and Hm-Ph); ¹³C>{¹H} NMR (C₆D₆, 100 MHz): δ = -6.73 (s, Si-CH₃), 20.0 (s, CtBu), 29.2 (s, CH₃tBu), 126.1 (s, CHp-Ph), 128.2 (s, CHo-Ph or CHm-Ph), 130.7 (t111, ¹JCD = 23.8 Hz, CHD), 147.9 (s, Cipso-Ph), 151.8 (s, Si-Cvinyl); ²⁹Si NMR (C₆D₆, 79 MHz): δ = 2.3; ²H NMR (C₆D₆, 61 MHz): δ = 5.82 (s br).

¹J<sub>CH</sub> (H<sub>3</sub>C-Si) = 119.5 Hz (determined from <sup>13</sup>C NMR).

¹J<sub>CSi</sub> (H<sub>3</sub>C-Si) = 51.8 Hz (determined from <sup>13</sup>C>{<sup>1</sup>H} NMR).
**Synthesis of 4.** To a solution of silirene 2g (250 mg, 0.609 mmol) in toluene (5 mL) was added trimethylaluminum (2M in toluene, 0.398 mmol, 199 µL, 2/3 equiv) under stirring and the resulting mixture was stirred for 1 hour at room temperature leading to the gelification of the solvent. The gel was then broken with a spatula and pentane was added (10 mL). After a vigorous shaking of the suspension, the mother liquor was eliminated via syringe. The remaining jelly solid was washed two times with pentane (2 x 10 mL) and dried under vacuum giving the expected polymer as clear yellow grains in 71 % yield (298 mg).

# ASSOCIATED CONTENT

## Supporting Information

The Supporting Information includes experimental details and characterization data (PDF), Cartesian coordinates for the calculated structure obtained by DFT calculations (XYZ).

This material is available free of charge via the Internet at http://pubs.acs.org.

## Accession Codes

The following crystal structures have been deposited in the Cambridge Crystallographic Data Center: CCDC 2383051 (1-AlMe2), 2394542 (1Et-AlEt2), 2383052 (1-Al(C6F5)2), 2383053 (1-AlCl2), 2383054 (1-AlMes2), 2383055 (2g), 2383056 (3a), 2383057 (3b), 2383058 (3d) and
2383059 (3e) contain the supplementary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk.

AUTHOR INFORMATION

Corresponding Author

**Marc Devillard** - ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, Université de Rennes, CNRS, 263 av. Du général Leclerc, F-35042 Rennes (France), https://orcid.org/0000-0002-3821-0885; E-mail: marc.devillard@univ-rennes.fr

Authors

**Akshitha Selvaraj** - ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, Université de Rennes, CNRS, 263 av. Du général Leclerc, F-35042 Rennes (France).

**Alejandra Acuna** - ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, Université de Rennes, CNRS, 263 av. Du général Leclerc, F-35042 Rennes (France), https://orcid.org/0000-0002-8976-1804

**Floriane Leboucher** - ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, Université de Rennes, CNRS, 263 av. Du général Leclerc, F-35042 Rennes (France).

**Marie Cordier** - ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, Université de Rennes, CNRS, 263 av. Du général Leclerc, F-35042 Rennes (France), https://orcid.org/0000-0003-0917-2725
**Claire Roiland** - ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, Université de Rennes, CNRS, 263 av. Du général Leclerc, F-35042 Rennes (France), https://orcid.org/0000-0001-9587-1340

**Gilles Alcaraz** - ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, Université de Rennes, CNRS, 263 av. Du général Leclerc, F-35042 Rennes (France), https://orcid.org/0000-0001-8705-5917

## Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENT

We thank Dr. P. Jéhan (CRMPO, Rennes) and F. Lambert (CRMPO, Rennes) for the HR-MS analyses, the Mikroanalytisches Laboratorium Kolbe (MIKROLAB, Oberhausen, Germany) for the elementary analyses. We also thank Remi Marchal and the CTI group of the ISCR for the computing resources. This work was supported by CNRS Chimie (Emergence, Project SILVA).

## ABBREVIATIONS

DFT, density functional theory; NMR, nuclear magnetic resonance; HR-MS, high resolution mass spectrometry.

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BRIEFS (Word Style “BH_Briefs”). Synthesis and study of Z-1-Silyl-2-Aluminyl-Disubstituted Olefins

SYNOPSIS (Word Style "SN_Synopsis_TOC"). The synthesis of olefins that combine silicon and aluminum in *cis* position is described by the ring-opening reaction of silacyclopropenes in presence of trimethylaluminum. Different aspects of the chemistry of these heterosubstituted olefins have been studied including the existence of a peculiar intramolecular [Si-CH3···Al] interaction that
could be described precisely, the regiospecificity of the methylalumination reaction that could be
rationalized by DFT calculations and the synthesis of a poly(organoalane) that incorporate
tricoordinated aluminum centers thanks to the same strategy.

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