JEAN LESSARD, PHAN VIET MINH TAA, ROBERT M A R T I~O ,and J o~h K. SAUYDERS. Can. J. Chem. 55, 1015 (1977).The analysis of the l3C and 'H nuclear magnetic resonance data of 2-substituted methylenecyclohexanes and 3-substituted cyclohexenes demonstrates that a double bond stabilizes the axial conformer for an electronegative substituent. Introduction of a methoxy group on the double bond further increases the relative stability of the axial conformer. These results are interpreted in terms of the 'double bond -no bond' resonance.JEAN LESSARD, PHAK VIET MINH TAN, ROBERT M A R T I~O et JOH\ K. SAUNDERS. Can. J. Chem. 55, 1015Chem. 55, (1977.L'analyse des spectres de resonance magnttique nucleaire du I3C et dl1 IH de methylenecyclohexanes substitues a la position 2 et de cyclohexenes substitues a la position 3 demontre qu'une double liaison stabilise le conformere axial lorsque le substituant est electronegatif. L'introduction d'un groupement mtthoxy silr la double liaison augmente d'avantage la stabilitt relative du conformere axial. Les rksultats sont interpret& en termes de resonance 'double bond -no bond'.The effect that causes electronegative groups to occupy sterically unfavoured conformations when attached to a carbon bearing an electronegative atom is known as the anomeric effect (1). A number of hypotheses have been advanced in order to explain this effect. These include the dipole-dipole interaction (2) which would favour the axial conformation relative to the equatorial since in the former the two dipoles are virtually perpendicular, whereas in the latter the two dipoles have the same direction and are in much closer proximity. A secoild explanation (31, labelled the rabbit-ear effect, states that the interaction lone pair -lone pair is significantly larger than either the interactions lone pairbonding pair or bonding pair -bonding pair. In the example where both X and Y are oxygens, a conformation can be defined for the axial conformer in which there is no lone pair -lone pair interaction whereas when V is equatorial all possible conformations have at least one such interaction. Thus again, the conforlnation in which Y is equatorial is destabilized. A third possible explanation is known as double bondno bond resonance (4). In this hypothesis, charged resonance forms such ascontribute. This can also be intcrpretcd in term of an overlap between the lone-pair electrons on X and the o antibonding orbital of the C-Y bond. This should be a inaximurn when the lone pair on X and the G-Y bond are antiperiplanar and thus this theory predicts that the form with Y axial would be stabilized.In an attempt to obtain experimental evidence, other than crystallographic data (5), of the importance of the latter postulate, we have 'Based on the M.Sc. dissertation of Phan Viet Minh studied a namber of 2-substituted methylene- Tan, Universitt. de Sherbrooke, 1976. 'NRCC Predoctorate Fellow 1974-1975 cyclohexanes and 3-substituted cyclohexenes. If
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