ARYLDISILANE PHOTOCHEMISTRY - THE ROLE OF SILYL FREE-RADICALS IN SILENE FORMATION FROM PHOTOLYSIS OF METHYLPENTAPHENYLDISILANE

Direct irradiation of methylpentaphenyldisilane in hydrocarbon solution containing acetone yields triphenylsilane and products derived from acetone trapping of 1,1-diphenylsilene and a 1,3,5-(l-sila)hexatriene reactive intermediate, in chemical yields of 29, 29, and 42%, respectively, relative to consumed disilane. Photolysis of the disilane in the presence of acetone and small amounts of chloroform (0.05 M) results in the formation of the same products in the same chemical yields, along with triphenylsilyl and methyldiphenylsilyl chloride in 18 and 17% yields due to halocarbon trapping of the corresponding silyl radicals. Triplet quenching experiments indicate that silyl radicals are triplet-derived, while the silene and silatriene are derived from the lowest excited singlet state of the disilane. Transient UV absorption spectra of the individual reactive intermediates formed in the photolysis of the disilane have been measured by nanosecond laser flash photolysis, employing selective quenchers to suppress the spectra of the others. Photolysis of the disilane in the presence of high concentrations of chloroform or carbon tetrachloride yields mainly the corresponding silyl chlorides and is thought to proceed by an electron-transfer mechanism under these conditions. The same products are formed upon electron-transfer photosensitization using chloranil as the photosensitizer. The photochemistry of methylpentaphenyldisilane in cyclohexane containing methanol has been reinvestigated.