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4.4.7.2.2.3 Photochemical Radical Silylation

DOI: 10.1055/sos-SD-104-00395

Delvos, L. B.; Oestreich, M.Science of Synthesis Knowledge Updates, (2017196.

A single report describes the photochemical cleavage of Si—B bonds by irradiation with a mercury lamp.[‌34‌] A UV absorption test series with several silylboron reagents showed that only silylboron compounds with a diisopropylamino-substituted boron atom absorb at wavelengths higher than 300 nm to undergo photolysis to release the triorganosilyl and the corresponding boryl radical. The silyl radical generated from silylboron reagent 79 adds across terminal and internal alkenes 78 to furnish silanes 80 in acceptable yields (Scheme 37). In hexane or benzene as solvent an excess of alkene is needed to secure satisfactory yields, whereas diethyl ether allows the use of almost equimolar amounts of reagents with similar results. The transformation is regioselective for 1,1-disubstituted alkenes with exclusive Si—C bond formation at the less-substituted carbon atom. The reaction with methyl (E)-but-2-enoate (78, R1 = Me; R2 = H; R3 = CO2Me) leads to a mixture of regioisomers with 75:25 preference for β-silylation with respect to the ester functionality. If the substrates are used in large excess, polymerization is favored (not shown).

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M8 M8 M8 Meeeeee Meeee ee 88 Meeee (%) Mee
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M Meeee (eee/eeeee) Meeeee (%) Mee
M 88:88 88 [‌88‌]
MMe 88:88 88 [‌88‌]
M(MM8Me)8 88:88 88 [‌88‌]
M(MM8MMMMMM)8 88:88 88 [‌88‌]
MM8 88:88 88 [‌88‌]

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