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 Bu^tNC are in the order k(Ph˙) > k(Bu^tO˙) > k(Me˙), and k(Bu^tO˙)= 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(¹³C_α) and a(N_β) are markedly dependent upon the nature of Z, and a(¹³C_α) increases with the electronegativity of Z. The radical Bu^tNĊSiEt₃ appears to be close to linear at C_α, whilst Bu^tNĊOR is bent. There appear to be two contributions, of opposite sign, to a(N_β), 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 N_β–C_ασ-bonding orbital. The β-scission of some imidoyl radicals has also been studied using e.s.r. spectroscopy. Radicals of the type RNĊXBu^t(X = O or S) undergo X–C cleavage to form RNCX and Bu^t˙, 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(Bu^tNĊSiEt₃), 3.1 × 10⁵(Bu^tNĊSBu^t), and 3.6 × 10³ s^–1(Bu^tNĊOBu^t).