Some aspects of homolytic arylation and silylation

Abstract

The effects of additives such as ferric benzoate, cupric benzoate, m-dinitrobenzene and nitrobenzene on the thermal decomposition of benzoyl peroxide in toluene, anisole and t-butylbenzene have been investigated over a wide range of additive concentrations. In the reactions of anisole, the effect of air, oxygen, t-butyl hydroperoxide and nitrosopentafluorobenzene were also examined. The additives exhibited different effects on the product distribution of these substrates. Explanations have been suggested to account for these observations.

The effects of these additives on the competitive reactions of toluene, anisole and t-butylbenzene with benzene as a standard solvent have been studied over a wide range of additive concentrations. Relative rates are significantly lowered in the presence of additives, and partial rate factors are also altered. These observations have been interpreted in terms of the formation of some weak complexes between phenyl radicals or their precursors with the substrate molecules in the absence of additives, but such complications are removed in the presence of additives. The mean values of the relative rate and the partial rate factors which are determined in this work in the presence of additives are considered to be the best definitive set now available. The competitive reactions containing various molar concentrations of anisole and benzene, toluene and benzene with or without the presence of additives were also studied. Finally, the cross-checked experiments involving the direct competitive reactions between toluene/anisole, toluene/t-butyibenzene and anisole/ t-butylbenzene were carried out. The results substantiated the order of reactivity determined for toluene, anisole and t-butylbenzene towards the phenylation reactions by benzoyl peroxide.

To study the silylation reaction on aromatic substrates, attempts were made to generate dimethylphenylsilyl radicals, thermally or photolytically, from dimethylphenylsilane in the presence of radical initiators such as t-butyl peroxide, azo-bisisobutyronitrile and benzoyl peroxide. The formation of dimethyldiphenylsilane anddiphenyltetramethyldisilane in the reaction of the dimethylphenylsilane in benzene indicated the formation of silyl radicals. Benzoyl peroxide was found to be the most efficient initiator in this reaction. Investigation was extended to the study of the reactions of dimethylphenylsilane with toluene, naphthalene and halobenzenes (chlorobenzene, bromobenzene, p-dichlorobenzene and iodobenzene) and anisole. In the reactions with toluene, naphthalene and anisole, nuclear silylated products were detected. However, in the reaction with halobenzene, the main reaction was the displacement or the abstraction of the halogen from the aromatic nucleus by silyl radicals. Mechanisms have been suggested to account for these observations. These results indicated the nucleophilic character of the dimethylphenylsilyl radical.