Inherent Reactivity of Spiro‐Activated Electrophilic Cyclopropanes

Abstract

Abstract The kinetics of the ring‐opening reactions of thiophenolates with geminal bis(acceptor)‐substituted cyclopropanes in DMSO at 20 °C was monitored by photometric methods. The determined second‐order rate constants of the S N 2 reactions followed linear relationships with Mayr nucleophilicity parameters ( N / s N ) and Brønsted basicities (p K aH ) of the thiophenolates as well as with Hammett substituent parameters ( σ ) for groups attached to the thiophenolates. Phenyl‐substituted cyclopropanes reacted by up to a factor of 15 faster than their unsubstituted analogues, in accord with the known activating effect of adjacent π‐systems in S N 2 reactions. Variation of the electronic properties of substituents at the phenyl groups of the cyclopropanes gave rise to parabolic Hammett relationships. Thus, the inherent S N 2 reactivity of electrophilic cyclopropanes is activated by electron‐rich π‐systems because of the more advanced C1−C2 bond polarization in the transition state. On the other hand, electron‐poor π‐systems also lower the energetic barriers for the attack of anionic nucleophiles owing to attractive electrostatic interactions.