Unstable intermediates. Part 110.—Hyperconjugation by groups containing heavy atoms: electron spin resonance studies of carbon radicals having β-SnR3, –PR2, –PR +3, –AsR2, –AsR +3 and –AsO3H groups1

Abstract

Hyperfine coupling to β-atoms such as Sn, P or As in carbon π-radicals is found to be remarkably large, corresponding to as much as 6 % delocalization into the valence s orbital on the heavy atom. The radicals all have a preferred conformation in which the metal atom is constrained above the radical plane, so that overlap between the carbon 2p orbital nominally containing the unpaired electron, and the carbon-metal σ-bond orbital is a maximum. This magnetic interaction is shown to be about four times larger than that for similarly placed –CR₃ or –NR⁺₃ groups.

These observations are linked with those of others on rate enhancements by CH₂X groups in electrophilic aromatic substitution. They are also significant with respect to the stereochemical course of some olefin additions.