Stereoselectivity in the addition of hydrogen chloride to cyclohex-1-enecarbonitrile: possible role of chlorine lone pair–nitrile π-orbital pseudoallylic A1, 3 interaction

Abstract

The (overall trans) addition of hydrogen chloride to cyclohex-1-enecarbonitrile in anhydrous alcoholic media proceeds to give cis-2-chlorocyclohexanecarboxylate (together with some cis-2-chlorocyclohexanecarboxamide): no corresponding products with the trans-configuration are detectable. In anhydrous ether the addition proceeds to give a single isomer, presumably cis-, of 2-chlorocyclohexanecarbonitrile, indicating that the configuration of the products may not be equilibrium-controlled in alcoholic media. An examination of the steric factors indicates that the transition state for protonation of the presumed intermediate, 2-chlorocyclohexylidenemethylideneimine, leading to cis-product is favoured if interaction between the lateral π-orbital of the C–N double bond and the lone-pairs on the chlorine atom at the 2-position is large. Consideration of interactions in the transition states meets Zimmerman's criticism that invoking A^{1, 3} interaction existing in ground states to explain product configuration takes insufficient account of the Curtin–Hammett principle.