Mechanisms of benzene in phenylenes were studied using density functional and coupled cluster methods. 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 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 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. [ABSTRACT FROM AUTHOR]