PHOTOLYSIS OF (ARYLMETHYL)TRIPHENYLPHOSPHONIUM SALTS - SUBSTITUENT, COUNTERION, AND SOLVENT EFFECTS ON REACTION-PRODUCTS

Quaternary (arylmethyl)phosphonium salts of the general formula ArCH2- PR3+Y-(Ar = substituted phenyl or 1-naphthyl; R = phenyl, ferrocenyl, or butyl; Y- = BF4- or halide) have been photolyzed in acetonitrile or in methanol. Photolysis involved the cleavage of the P-CH2 bond and t he products derived from both, the arylmethyl radical and the carbocat ion, were formed. The proportion of the radical- and carbocation-deriv ed products was determined as a function of substituents in group Ar, of groups R, counterions Y-, and the solvent. For the nonoxidizable co unterion (BF4-), the proposed mechanism of the reaction involves initi al homolysis, followed by the escape of the radical products from a so lvent cage, or by the electron transfer from carbon to phosphorus, yie lding the corresponding arylmethyl carbocation. The latter can either react with the solvent to form the observed carbocation-derived produc t or can undergo recombination with the tertiary phosphine formed to y ield the starting phosphonium ion. Some indication of the ''inverted s ubstituent effect'' resulting from the inhibition of single electron t ransfer from an easily oxidized radical was obtained. For the oxidizab le counterions (halides), an additional pathway is suggested, that inv olves electron transfer from the anion, yielding the arylmethyl radica l and the phosphine, thus decreasing the ionic/radical products ratio.