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Hydrogen shifts and benzene ring contractions in phenylenes
Author(s) -
Pastor Michael B.,
Kuhn Ariel J.,
Nguyen Phuong T.,
Santander Mitchell V.,
Castro Claire,
Karney William L.
Publication year - 2013
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.3126
Subject(s) - carbene , chemistry , biphenylene , diradical , ring (chemistry) , benzene , density functional theory , hydrogen , phenylene , carbon fibers , activation barrier , computational chemistry , medicinal chemistry , photochemistry , singlet state , catalysis , organic chemistry , excited state , polymer , physics , materials science , composite material , composite number , nuclear physics
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.