Photochemical Aromatic Cyanomethylation: Aromatic Substitution by Way of Radical Cations

Article abstract of DOI:10.1021/jo00189a016

Photolysis at 254 nm of chloroacetonitrile in the presence of aromatic hydrocarbons led to ring cyanomethylation.In addition radical coupling products were found, especially with toluene where 3-phenylpropionitrile, succinonitrile, and bibenzyl accompanied the tolylacetonitriles.These same byproducts were obtained from toluene and chloroacetonitrile upon thermolysis with peroxide initiators, but no nuclear cyanomethylation was observed.The mechanism for aromatic cyanomethylation involves initial excitation of the aromatic hydrocarbons, followed by an electron transfer (probably by way of an exiplex) to chloroacetonitrile, which was found to quench aromatic fluorescence at high rates.Direct spectral evidence for the resulting radical cation with p-dimethoxybenzen was obtained by using flash laser spectroscopy.Loss of a chloride ion from the resulting radical anion produces a cyanomethyl radical in close proximity to an aromatic radical cation.Coupling leads to the aromatic substitution products whereas radicals escaping from the cage account for the observed byproducts.