Formation and electrocyclization of gaseous α-phenyl- ortho-quinodimethane radical cations

Abstract

α-Aryl- ortho-quinodimethane radical cations are generated by the loss of water via a two-step process (“ortho-effect”) from metastable molecular ions of 2-methylbenzhydrol derivatives. A competing fragmentation is the loss of the methyl substituent from metastable molecular ions which, however, is also observed for the corresponding meta and para isomers and which does not interfere with the formation of α-phenyl- ortho-quinodimethane radical cations. The reaction of these latter ions was studied with the aid of specifically labelled derivatives. The results can be interpreted by an electrocyclization of the α-phenyl- ortho-quinodimethane radical ions to form dihydroanthracene radical cations which decompose by loss of a hydrogen atom or a substituent from one of the phenyl rings in the case of substituted α-phenyl- ortho-quinodimethane radical cations. However, in competition with this cyclization process via a six-membered transition state the loss of a methyl radical from the α-phenyl- ortho-quinodimethane radical cations is observed which is the consequence of a cyclization via a five-membered transition state. This latter process is best depicted as an intramolecular aromatic substitution by a radical.