An electron spin resonance study of radical addition to alkyl isocyanides

Abstract

Using e.s.r. spectroscopy, a variety of types of free radical (Z·) have been shown to add to the terminal carbon of alkyl isocyanides (RNC) to form imidoyl radical adducts (RNĊZ). At 253 K the rate constants for radical addition to ButNC are in the order k(Ph˙) > k(ButO˙) > k(Me˙), and k(ButO˙)= 6 × 105 l mol–1 s–1. The imidoyl adducts are σ-radicals in which the unpaired electron occupies an orbital orthogonal to the NC π-orbital. The magnitudes of a(13Cα) and a(Nβ) are markedly dependent upon the nature of Z, and a(13Cα) increases with the electronegativity of Z. The radical ButNĊSiEt3 appears to be close to linear at Cα, whilst ButNĊOR is bent. There appear to be two contributions, of opposite sign, to a(Nβ), a positive one resulting from direct overlap of the orbital of the unpaired electron with that of the lone pair on nitrogen, and a negative one resulting from spinpolarisation of the Nβ–Cασ-bonding orbital. The β-scission of some imidoyl radicals has also been studied using e.s.r. spectroscopy. Radicals of the type RNĊXBut(X = O or S) undergo X–C cleavage to form RNCX and But˙, whilst the radicals RNĊY (Y = MeS, Et3Si, or Ph) undergo cleavage to produce R˙ and NCY. At 243 K the rate constants for β-scission were ca. 1.4 × 105(ButNĊSiEt3), 3.1 × 105(ButNĊSBut), and 3.6 × 103 s–1(ButNĊOBut).