Spin-spin coupling between side-chain and ring protons and the effect of electron delocalization on allylic coupling

Abstract

The side-chain-ring coupling constants3 in the three isomeric tetrachlorotoluenes are Jortho 0.63�0.02 c/s, Jmeta 0.36�0.02 c/s, and Jpara 0.63�0.01 c/s. The magnitudes of side-chain coupling constants in benzene derivatives are discussed. In symmetrical six-membered aromatic and heterocyclic compounds the magnitude of the ortho side-chain coupling constants is approximately 0.6 c/s but varies from less than 0.3 cis to 1.1 c/s in molecules where some bond localization occurs. Ortho side-chain coupling constants in five-membered heterocyclic compounds also appear to be affected by electron localization. It is therefore likely that the magnitude of the ortho side-chain coupling constants may be a direct measure of bond order. Long-range coupling data on indole derivatives are reported. In two β,β-dimethylstyrenes the stronger side-chain to ring coupling is to the ortho position. The relation between allylic coupling and electron delocalization is explored and a new criterion for electron delocalization proposed.