An electron spin resonance study of trialkylsilyl radical addition to alkyl isocyanates

Abstract

Addition of photochemically-generated trialkylsilyl radicals to alkyl isocyanates to give imidoyl radicals (A) has been studied in cyclopropane solution using e.s.r. spectroscopy. The rate of addition of trimethylsilyl radicals to R¹₃Si˙+ R²NCO [graphic omitted] alkyl isocyanates has been determined relative to that of addition to methyl trifluoroacetate or ethylene and k_add(R¹= Me) was found to decrease in the order R²= Me > Et > Prⁱ > Bu^t at 164 K. It is proposed that this trend is steric in origin and that the Me₃Si group is closer to the N-alkyl group in the transition state for addition than in the product imidoyl radical. The palladium chloride catalysed addition of triethylsilane to n-butyl isocyanate gives the imidate BuⁿNC(H)OSiEt₃ rather than the isomeric C-silylamide BuⁿN(H)C(O)SiEt₃, as previously reported.