Bergman cyclization of sterically hindered substrates and observation of phenyl-shifted products.
Heating 1,2-bis(phenylethynyl)benzene in the presence of 1,4-cyclohexadiene at temperatures ranging from 260 to 360 degrees C yielded the expected Bergman product, 2,3-diphenylnaphthalene, as only a minor product (<3%) under all reaction conditions studied. The major products, resulting from one or more phenyl shifts, were 1,3- and 1,4-diphenylnaphthalene which formed in up to 16% and 11% yield, respectively. Although somewhat less efficiently, 1-ethynyl-2-(phenylethynyl)benzene and (Z)-1,6-diphenylhex-3-ene-2,5-diyne also yielded products resulting from phenyl shifts. These results are explained through computations that point to the role of steric repulsion and benzyne stability in driving these isomerizations. The computed barriers for phenyl shifting are dramatically higher than those observed in the case of sp3 radicals. However, these transformations are relevant in solution chemistry as well as in more extreme environments such as those encountered during combustion, pyrolysis, and electric discharge heating.
Lewis
Kevin D
KD
Department of Chemistry and the Macromolecular Science and Engineering Program, The University of Michigan, Ann Arbor, Michigan 48109-1055, USA.
Matzger
Adam J
AJ