Determination of subtle differences in π-acceptor ability of ethynyl, cyano, and phenylethynyl groups using the cycloheptatriene–norcaradiene equilibrium in 7-substituted 2.5-di-t-butylcyclohepta-1,3,5-trienes

Abstract

The equilibrium constant {K=[norcaradiene]/[cycloheptatriene]} determined by ¹H n.m.r. spectroscopy of the title system with the substituent in the exo or equatorial position are 0.213 ± 0.002, 0.258 ± 0.002, and 0.374 ± 0.004 for ethynyl, cyano, and phenylethynyl, respectively, in CS₂–CD₂Cl₂(3 : 1 v/v) at –120 °C, which indicate an increasing π-acceptor ability towards the cyclopropane ring which follows this order of the substituents.