Competition of mechanisms in the photochemical cleavage of the C-X bond of aryl-substituted vinyl halides

The photolysis of aryl halides causes homolysis of the carbon-halogen bond and formation of aryl radicals. In contrast, photolysis of vinyl halides can induce both heterolysis of the C-X bond, thereby generating vinyl cations, and homolysis, giving vinyl radicals. Examples of this competition among pathways is reported here for three vinylic precursors, namely, 1,2,2-triphenylbromoethene (1), 1-phenyl-2,2-bis(o-methoxyphenyl)-1-bromoethene (11), and â-bromostyrene (19). Incursion of the photoinduced SRN1 process, through the intermediacy of the vinyl radical, is verified in the presence of reducing nucleophiles, such as the enolate ions of ketones, and in part with (EtO)2PO-. Conversely, incursion of the heterolytic path, and intermediacy of the vinyl cation, occurs in the presence of weak electron-donor anions, such as NO2 -, N3 -, and Cl-. The vinyl cation produced from 19, which is less stable than those derived from 1 and 11, gives phenylacetylene via an E1-type elimination. An estimate is provided for the intramolecular rate of interception of the vinyl cation derived from 11 by the ortho-methoxy groups of the â-o-anisyl substituents. Finally, evidence against a photoinduced electron transfer from RO- ions to vinyl halide 1 is presented.