The n.m.r. spectra and conformations of cyclic compounds part V. Mechanism of CH·CH coupling in heterocyclic compounds

The 1H spectra of some five-membered heterocyclic olefins and related compounds are reported and analysed. The observation of some weak, hitherto ignored transitions in the AA′BB′ spectrum of the CH2·CH2 groups removes the deceptive simplicity of the spectrum and gives well determined and non-equal values of Jcis and Jtrans.The mechanisms contributing to the CH·CH couplings in these systems are considered in detail. It is concluded that (a) ring buckle and the known dihedral angle dependence satisfactorily accounts for the values in carbocyclic rings; (b) these contributions plus angle deformations and the orientation of the electronegative substituents account for the values in saturated five-membered heterocyclics. (c) No reasonable combination of these mechanisms provides a quantitative explanation of the large couplings observed in the five-membered heterocyclic olefins investigated.Possible reasons for this are given.