Homolytic cleavage benzylic cations

Early contributions by Cristol and Bindel [1] set the stage for benzyl chloride and benzyl bromide photochemistry. More recently, Bartl, Steenken, Mayr, and McClelland [34] have implicated both heterolytic and homolytic cleavage from nanosecond LFP of diphenylmethyl halides. Transient absorption spectroscopy in the 70-ns time frame allowed for observation of both cationic and radical intermediates. The ratio of the two pathways (heterolytic/ homolytic) was essentially the same for the chloride and the bromide. 

Homolytic bond cleavage as a result of a - a or a - ct photoexcitation produces a singlet cation-radical/radical pair that can recombine to produce starting material, molecular rearrangement, undergo intersystem crossing to the triplet pair or ET to form the benzyl carbocation. A possible description of the a/cr excited state is represented below. The rate of bond cleavage must be competitive with back ET. 

Step of the reaction apparently involved the formation of amine radical cations followed by homolytic fission of the doubly benzylic bonds. Hence, oxidative photolysis emerged as still another alternate to Na/NHs cleavage in the elucidation of structure of bisbenzylisoquinoline alkaloids by conversion to simpler products. 

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