Concerning the conformational preferences of the 2-cyano derivatives of oxane, thiane, and selenane

Abstract

This paper investigates the origin of the anomalous anomeric effect in merosinigrin, a 2-cyanothiane in which the cyano group is axial as expected for the anomeric effect, but in which bond distances are opposite to that expected from the nS→[Formula: see text] orbital interaction, which underlies the classical anomeric effect. The model compounds, 2-cyanooxane, 2-cyanothiane, and 2-cyanoselenane, were synthesized and studied both experimentally and computationally. Both the thia and selena systems displayed an even higher preference for the axial conformation than the oxa system but also exhibited the bond length anomalies found previously in merosinigren. Natural bond order (NBO) analysis of the B3LYP/6–311+G(3df,2p) wave functions of the axial and equatorial forms of the three systems confirmed a weakening of the n→σ* orbital interaction in the O, S, and Se series, and a strengthening of a σ–π*(CN) interaction that explains the bond length reversals observed in the S and Se systems. It also revealed a new mechanism, n→π*, namely, a through-space interaction between the nonbonded lone pair electrons of the heteroatom and the π* orbital of the cyano group, which selects for the axial conformation.