Thermal Isomerisations, XXIV. Gas Phase Kinetics of the Pyrolysis of Some 3,3‐Dimethyl‐1‐alkyl‐cyclopropenes: Some Surprising Substituent Activation Effects and the Intramolecular Trapping of Vinylidene Intermediates

Abstract

AbstractThe gas phase pyrolyses of four 3,3‐dimethyl‐1‐alkylcyclopropenes were studied for which the 1‐alkyl substituents are ethyl‐ (6), isopropyl‐ (7), 1′‐butyl‐ (8) and 1′‐isoamyl‐ (9). Rate data over a 50°C temperature range were obtained and Arrhenius parameters are reported both for overall reactions and individual pathways for all compounds. Tests confirm all reactions to be unimolecular and homogeneous. For 8 and 9, noteworthy products are cyclopentenes, whose presence demonstrates the involvement of vinylidene intermediates. The substantial product yields of 2,4‐alkadienes together with dramatic rate enhancement effects of these 1‐alkyl substituents (relative to 1‐methyl) are explained by the involvement of vinylcarbene intermediates in this pathway. Minor, transient products from 6, 8 and 9 are probably isomeric cyclopropenes. This suggests further pathways for vinylidene intermediates, whose reactivity pattern is briefly reviewed.