Synthesis of 1‐silacyclopent‐2‐ene derivatives using 1,2‐hydroboration, 1,1‐organoboration and protodeborylation

Abstract

The reaction of di(alkyn-1-yl)vinylsilanes R 1 (H 2 C CH)Si(C≡C-R) 2 (R 1 = Me (1), Ph (2); R = Bu (a), Ph (b), Me 2 HSi (c)) at 25°C with 1 equiv. of 9-borabicyclo[3.3.1]nonane (9-BBN) affords 1-silacyclopent-2-ene derivatives (3a-c, 4a,b), bearing one Si-C≡C-R function readily available for further transformations. These compounds are formed by consecutive 1,2-hydroboration followed by intramolecular 1,1-carboboration. Treated with a further equivalent of 9-BBN in benzene they are converted at relatively high temperature (80-100°C) into 1-alkenyl-1-silacyclopent-2-ene derivatives (5a,b, 6a,b) as a result of 1,2-hydroboration of the Si-C≡C-R function. Protodeborylation of the 9-BBN-substituted 1-silacyclopent-2-ene derivatives 3-6, using acetic acid in excess, proceeds smoothly to give the novel 1-silacyclopent-2-ene (7-10). The solution-state structural assignment of all new compounds, i.e. di(alkyn-1-yl)vinylsilanes and 1-silacyclopent-2-ene derivatives, was carried out using multinuclear magnetic resonance techniques ( 1 H, 13 C, 11 B, 29 Si NMR). The gas phase structures of some examples were calculated and optimized by density functional theory methods (B3LYP/6-311+G/(d,p) level of theory), and 29 Si NMR parameters were calculated (chemical shifts δ 29 Si and coupling constants n J( 29 Si, 13 C)).

Synthesis and Characterization of Spirosilanes – 1,2‐Hydroboration and 1,1‐Carboboration

“…The data obtained for the five-membered 1-silacyclopent-2-ene ring are within the expected range. [5,8,21] The silicon atom has two different ring structures at either side, and its surroundings correspond to a distorted tetrahedron, as expected (bond angles: N1-Si1-C9 104.4°, C1-Si1-C4 95.0°, C1-Si1-N1 108.3°, and C4-Si1-C9 123.8°). The molecule bears two boron centers, one is tricoordinate, and the other is tetracoordinate.…”

“…[14] By contrast, the intramolecular 1,1-organoboration proceeds quickly and more smoothly, as studied for vinylsilanes. [7,8,21] The results of the present study lead us to conclude that the consecutive 1,2-hydroboration and 1,1-vinylboration reactions proceed smoothly and selectively independent of the R group. All of the spirosilanes bearing boryl groups at the 2-and 7-positions were colorless oily or crystalline materials, and their structures could be deduced from a consistent set of solution-state NMR spectroscopic data ( Figure 2 and Table 1) and also by X-ray diffraction (Figures 3 and 4).…”

“…[21] In the present study, it is shown that different substituents R do not have an appreciable influence on the course of the reactions, which consist of twofold intermolecular 1,2-hydroborations and twofold intramolecular 1,1-vinylborations. All of the reactions were performed by following the literature procedure, [7,8,21] and the required products were obtained in reasonable quantity and with sufficient purity of Ͼ98 % (Scheme 2).…”

“…[1][2][3] With silicon, heterocycles such as 1-silacyclobutenes, [4][5][6] 1-silacyclopent-2-enes, [7,8] 1-silacyclohex-2-enes, [7,9] and silole derivatives are particularly noteworthy. [10][11][12] Addressing 1-silacyclopent-2-enes, numerous derivatives bearing functionalities such as Si-organyl, Si-Cl, and Si-H at a chiral silicon atom have become available (Figure 1, example A).…”

1,1-Carboboration to tellurium–boron intramolecular frustrated Lewis pairs

“…13 More recently, the use of the highly electrophilic Lewis acid, B(C 6 F 5 ) 3 , has allowed for 1,1-carboboration reactions to proceed cleanly under much milder conditions. 24 Despite the variety of alkynes that have been subjected to 1,1-carboboration reactions, group 16-substituted alkynes in this context has only been reported recently. [16][17][18] 1,1-Carboboration has also led to the facile synthesis of a variety of heterocycles and aromatic compounds, 19 including boroles, 20,21 stannoles, 22 1,4-stannaboracyclohexa-2,5-dienes 23 and 1-silacyclopent-2-ene.…”