Hydrogen shifts and benzene ring contractions in phenylenes

Mechanisms of benzene ring contractions in phenylenes were studied using density functional and coupled cluster methods. Rearrangement of biphenylene to benzopentalene can proceed via a carbene route, by initial 1,2‐carbon shift followed by a 1,2‐hydrogen shift, with a CCSD(T)/cc‐pVDZ//B3LYP/6‐31G* barrier of ~77 kcal/mol. An alternative carbene pathway consisting of an initial 1,2‐hydrogen shift followed by a 1,2‐carbon shift has a slightly higher computed barrier of 79 kcal/mol. The preferred carbene mechanism is computed to have a barrier at least 25 kcal/mol lower than competing diradical mechanisms at the BD(T)/cc‐pVDZ level. The various possible benzene ring contractions in angular [3]phenylene are predicted to have barriers of 79–82 kcal/mol, with little preference for one pathway over the others. Thus, mechanistic proposals to explain pyrolysis products of angular [3]phenylene can reasonably invoke any of the four possible initial reaction modes via carbene intermediates. Copyright © 2013 John Wiley & Sons, Ltd.