Decomposition of Azocumene on Silica Surfaces

Article abstract of DOI:10.1021/ja00521a041

The decomposition of azocumene on silica surfaces in the absence of solvent is slower than in toluene solution and has a much lower radical efficiency.Radicals that escape geminate reaction can be trapped by adsorbed bisdiphenylenephenylallyl.The ratio of cumene to α,α-dicumyl is about five times that in toluene.Aromatized α-ortho and α-para cumyl dimers, found only in the reactions on silica, are formed in low yield.These products (10 and 11) are formed via the quinoidal α-ortho and α-para dimers, which have been reported as transient products in toluene.The aromatized dimers (10 and 11) are exclusively geminate recombination products, presumably because they are formed only if the site at which the azocumene decomposes is also a catalytic site for the aromatization of the quinoidal dimers.The rates and product ratios depend to some extent on the purity and degree of hydroxylation of the silica.They also depend on the fraction of a monolayer occupied by the azocumene.At low monolayer coverages the azocumene is preferentially adsorbed on sites that give lower rates, higher cumene to α,α-dicumyl ratios, and higher yields of the aromatized dimers 10 and 11.The silicas were characterized by their acidities, R_f values, catalytic properties, and the electron paramagnetic resonance spectra of adsorbed radicals.