Through‐Space effects on vicinal proton spin–spin coupling constants mediated via hetero atoms: Nonequivalence of cis couplings in five‐membered rings

Abstract

Abstract Theoretical studies are presented, aimed at the elucidation of through‐space effects exerted by bridge‐head oxygen and nitrogen atoms on cisoidal proton spin–spin coupling constants in 7‐hetero‐substituted norbornanes. The finite perturbation theory intermediate neglect of differential overlap (INDO) self‐consistent field (SCF) molecular orbital (MO) method, modified according to the description given by Barfield [ J. Am. Chem. Soc. , 102 , 1 (1980)], was employed. It is predicted that the known nonequivalence between J endo–endo and J exo–exo in norbornanes, which is in part mediated by the bridgehead methylene group (Barfield transmission effect), also occurs in 7‐heteronorbornanes. A trend is recognized in which the nonequivalence induced by oxygen is rather smaller than that induced by the rear lobe of the NH bond or by the nitrogen lone pair. It is shown that the Barfield effect also explains the observed nonequivalence between cisoidal HC β C γ H and HC γ C δ H couplings in prolines. The calculations also predict similar, though smaller, effects on cisoidal couplings in the tetrahydrofuranyl ring system. Special attention was paid to the practical consequences of the existence of this effect with regard to the pseudorotation analysis of the five‐membered sugar ring in nucleic acids. It is found that, in addition to the recently introduced correction for electronegativity and orientation of substituents [Haasnoot, de Leeuw, and Altona, Tetrahedron 36 , 2783 (1980)], a cos 2 dependency of 3 J cis upon the phase angle of pseudorotation must be taken into account, especially in the case of deoxyribofuranoses.