Bromosilanes synthesis

Fraser-Reid and co-workers have examined serial radical cyclization of pyranose-derivatives [95AJC333] in the stereocontrolled synthesis of Woodward s reserpine precursor [95JOC3859]. Treatment of the bromosilane 188 under reductive conditions resulted in a 5-exo followed by a 6-exo cyclization. The intermediate radical eliminates phenylsulfinyl radical to provide the alkene 189 as the product. The intermediate has been converted to the reserpine precursor 190. The temporary silicon method has been utilized for the synthesis of brassinolide side chain [95SL850]. 

Methylbromosilanes can be obtained by the direct synthesis from MeBr and silicon, but are normally obtained on a small scale by exchange reactions. Siloxanes can be converted into bromosilanes by reaction with phosphorus tribromide91 (equation 55). 

Silyl cyclopentadienes1,2 3 have been formed through the interaction of a cyclopentadienide prepared in situ with an appropriate silicon halide however, purification of the desired product required filtration, removal of solvent, and vacuum distillation at elevated temperatures. The synthesis of the silicon compounds, C5H5(SiH3) and C5H4(CH3)(SiH3), described here utilizes a low-temperature reaction between liquid bromosilane and the salt K[RC5H4] (where R = H, CH3), in a 1 2 mole ratio, respectively.4 5... 

A large number of different a,(o-bis[(trifluoromethyl)sulfonyloxy]-substituted organosilicon compounds can be obtained by relatively simple methods from the corresponding amino-, allyl-, or phenylsilanes. Moreover, it is remarkable that these silyl triflate derivatives are often easily formed, when the synthesis of the corresponding chloro- or bromosilanes is difficult or does not appear to have been attempted. Eq. 2 and Eq. 3 show selected examples of this synthesis [10-12]. The products were prepared in high purities and yields. The resulting triflates should be used for the polycondensation without further purification, because they often cannot be destilled without decomposition. 

Both reactions can be explained by the formation of a carbocation and rearrangement to the bromosilane 9. The silacycle 7 was formed from 9 through a second carbocation and C-C bond formation with HBr elimination. The reaction of 9 with H2O gives the silacycle 10. The existence of 9 was proved by ist synthesis by another synthetic route. Bromination of 8 with NBS produces the silacycle 7. 

For the synthesis of organylsilanethiols R3SiSH (725) a common approach is to employ chloro- or bromosilanes (724) with hydrogen sulphide in the presence of tertiary amines as hydrogen halide acceptor407-411 (equation 368). Without an HX acceptor no reaction occurs even SiCl4 reacts with H2S only above 500 °C to yield trichlorosilane-thiol412 (726) (equation 369). 

Synthesis of Bromosilanes from Hydrosilanes. In the presence of a catalytic amount of Cul, CuBr2 (2 equiv) converts hydrosilanes into monobromosilanes at room tenperature in good yield. Substrates include monohydrosilanes, dihydrosilanes, trihydrosilanes, and 1,2-dihydrodisilane. However, with di- and trihydrosilanes the second bromination employing 4 equiv of CuBr2 is much slower. 

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