Nuclear magnetic resonance as a means of assessing structure versus reactivity relationships in alicyclic compounds

Abstract

Proton–carbon-13 coupling constants (J_13C,H) have been measured for three series of N-substituted azacycloalkanes. Structural features of the alicyclic compounds, as revealed by this and other n.m.r. techniques, reflect the strain present in the ring and can be used as a guide to predict ring-closure reactivity. Deviations from such structure versus reactivity relationships are found with highly strained small rings. Hence, it must be stressed that meaningful trends in the behaviour of ring compounds can only be revealed when large homologous series are examined. Literature X-ray structural studies are also exploited in order to comment on the situation in small rings and on their peculiar ring-closure tendency. It is suggested that, although some of the experimental features and behaviour can be accounted for in terms of strain, others, in the case of three-membered rings, are to be attributed to the presence of σ-aromaticity. Because of this, these three-membered ring compounds can efficiently gain stabilization by resonance interaction with exocyclic conjugative substituents: n.m.r. evidence is given on this point.