An electron resonance study of some reactions involving silyl radicals

Abstract

Specific alkyl radicals for e.s.r. studies can be prepared conveniently in the cavity of an e.s.r. spectrometer by photolysis of a mixture of the appropriate alkyl bromide (or chloride) with triethylsilane and t-butyl peroxide. The triethylsilyl radical, produced as an intermediate, will abstract chlorine or bromine from alkyl halides, but abstraction of fluorine has not been observed. Although ketones and esters react with triethylsilyl radicals by addition to the oxygen atom of the carbonyl group, halogeno-ketones and halogeno-esters react preferentially by abstraction of the halogen atom. Bromobenzene, 1-bromohex-1-yne, and cyclopropyl bromide do not give e.s.r. spectra corresponding to the organic radicals: it appears that these reactive radicals react further with triethylsilane to give the observed Et₂SiHĊHCH₃ radical, but the cyclopropyl radical could be detected when trimethylsilane was used instead of triethylsilane in the reaction mixture. Competition experiments show that ease of removal of halogen atoms increases in the order primary alkyl < secondary < tertiary < acetonyl ≈·CH₂·CO₂Et < allyl ≈ propargyl ≈ benzyl < cyanomethyl.