FORMATION AND ELECTROCYCLIZATION OF GASEOUS ALPHA-PHENYL-ORTHOQUINODIMETHANE RADICAL CATIONS

alpha-Aryl-ortho-quinodimethane radical cations are generated by the l oss of water via a two-step process (''ortho-effect'') from metastable molecular ions of 2-methylbenzhydrol derivatives. A competing fragmen tation 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 alpha- phenyl-ortho-quinodimethane radical cations. The reaction of these lat ter ions was studied with the aid of specifically labelled derivatives . The results can be interpreted by an electrocyclization of the alpha -phenyl-ortho-quinodimethane radical ions to form dihydroanthracene ra dical cations which decompose by loss of a hydrogen atom or a substitu ent from one of the phenyl rings in the case of substituted alpha-phen yl-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 alpha-phenyl-ortho-quinodimethane radical cations is observed which is the consequence of a cyclization via a f ive-membered transition state. This latter process is best depicted as an intramolecular aromatic substitution by a radical.