The photochemistry of polyhaloarenes .13. The photohydrodehalogenation of 3,4-dibromobiphenyl

Irradiation of 3,4-dibromobiphenyl(BpBr(2)) in acetonitrile resulted in the formation of 4-bromobiphenyl (BpBr) and 3-bromobiphenyl (3-BpBr) in a ratio of 7.6 +/- 0.1 up to 22% conversion of starting material. The dependence of the reciprocal of the quantum yield (1/Phi) upon 1/BpBr(2) is linear. Reduction of BpBr(2) with lithium di-tert-butylbiphenylide generates a 3.6:1.0 ratio of BpBr to 3-BpBr, while photohyrodebromination of BpBr(2) in the presence of triethylamine produces a similar ratio of 1.8:1.0. Product determining radical stability was assessed by radical debromination of BpBr(2) using Ph(3)SnH in THF, which resulted in a BpBr:3-BpBr ratio of 1.5:1.0. AM1 calculations on the product determining radicals provide the basis for rationalization of products via excimer and radical anion intermediates. (C) 1996 Elsevier Science Ltd