Nitrile-group transfer from solvents to aryl halides. Novel carbon-carbon bond formation and cleavage mediated by palladium and zinc species

The palladium phosphine complexes Pd(PPh3)(2)Cl-2/PPh3, Pd(PBu3)(2)Cl-2, and Pd(dppp)(2)Cl-2 (dppp = 1,2-bis(diphenylphosphino)propane) in the presence of zinc metal powder catalyze nitrile-group transfer from organic nitriles RCN (R = Me, Et, n-Pr, Bn, Ph) to bromoarenes to give the corresponding ArCN compounds. Only ortho-disubstituted bromoarenes (1-bromo-2-methylnaphthalene (1), 2-bromo-m-xylene, 2-bromomesitylene (3), 1-bromo-2-methoxynaphthalene, and 9-bromoanthracene) afforded good yields of the cyanation products. Analysis of the reaction mixture of 3 with benzyl cyanide using Pd(PBu3)(2)Cl-2 as the catalyst showed the presence of dibenzyl ketone and a cyclic product, 2,4,6-tribenzyl-5-phenylpyrimidine, in addition to 2,4,6-trimethylbenzonitrile. To understand the mechanistic features of these catalytic reactions, a number of experiments were performed and the results are summarized as follows: (i) the palladium intermediate Pd(PPh3)(2)(2-Me-Np)Br (6; 8-Me-Np = 2-methylnaphthyl), which can be prepared separately from the oxidative addition of 1 to Pd(PPh3)(4), was detected from H-1 NMR spectra in the catalytic cyanation of 1 in acetonitrile; (ii) heating 6 in acetonitrile in the presence of Zn powder or ZnBr2 at 160 degrees C afforded 1-cyano-2-methylnaphthalene (2) in 76 and 45% yields, respectively; (iii) heating 6 in acetonitrile alone gave no cyanation product but the imine product CH3(2-Me-Np)C=NH (7) in 35% yield; (iv) treatment of 7 by ZnBr2 at 160 degrees C afforded 2 in 44% yield. On the basis of these results, a mechanism involving the cooperation of two metals, palladium and zinc, with the former responsible for C-C bond formation and the latter for C-C bond cleavage, is proposed to account for the observed catalytic reactions.