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 β-bromostyrene ( 19 ). Incursion of the photoinduced S RN 1 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) 2 PO - . Conversely, incursion of the heterolytic path, and intermediacy of the vinyl cation, occurs in the presence of weak electron-donor anions, such as NO 2 -, N 3 -, 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 β- o -anisyl substituents. Finally, evidence against a photoinduced electron transfer from RO - ions to vinyl halide 1 is presented.