The ?-? Interaction between Nitrile and Ethylene Functionalities

Article abstract of DOI:10.1021/jo01291a015

The axial/equatorial free-energy difference for 3-cyano-1-methylenecyclohexane-2,2,4,4,6,6-d6 is 0.26 kcal/mol in CF2Cl2 and 0.07 kcal/mol in CHFCl2 at -102 deg C in favor of the equatorial conformer.The decreased equatorial preference in the polar, hydrogen-bonding solvent results from a less repulsive interaction between the 3-substituent and the 1-exo-methylene group.For the low-polarity solvent (CF2Cl2), the equatorial preference of cyano in the exo-methylene system is about the same as that in cyanocyclohexane (0.24 kcal/mol), which lacks the ?-? interactionin the axial conformation.These results contrast with polar, lone-pair-containing substituents such as methoxyl, which have dramatically increased equatorial preferences in the exo-methylene systems compared to their cyclohexyl counterparts.The ?-? interaction between the cyano group and the C=CH2 group appears to offer very little repulsion in comparison with the strong electrostatic interactions between methoxyl, hydroxyl, or thiomethyl and the same C=CH2 group.