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Electrochemical reduction of benzonitrile in the presence of bromobenzene and tetra-N-butylammonium benzenethiolate in dimethyl sulfoxide (Me2SO) forms diphenyl sulfide (67%) and benzene (38%). The reaction consumes 0.37 faraday per mol of bromobenzene, indicating that an SRNl chain reaction is occurring. Reaction in Me2SO-d6 gives a decreased yield of benzene (17%), 57% of which was monodeuterated, which along with coulometric data indicates that a major termination pathway is abstraction of hydrogen atoms from Me2S0 by phenyl radicals. Photoinitiated reactions in the presence and absence of tetra-N-butylammonium ions indicate that they are also a significant source of hydrogen atoms in termination. Evidence from reactions of 4-bromotoluene with benzenethiolate ion indicates that fragmentation of the diary1 sulfide radical anion intermediate is an important reaction in these systems. The presence of benzonitrile suppresses that cleavage. Flora Hewlett Foundation G r a n t of Research Corporation for partial support of this work and the National Science Foundation for equipment grants allowing t h e purchase of t h e HP 5992 GCMS a n d electrochemical instrumentation. T h e authors also t h a n k t h e Hewlett-Packard Corporation for t h e gift of t h e HP 5880 gas chromatograph. Registry No.Bromobenzene, 108-86-1; tetrabutylammonium benzenethiolate, 4670-62-6; benzonitrile, 100-47-0; p-bromotoluene, 106-38-7; potassium benzenethiolate, 31 11-52-2.

The reaction of m-or p-iodoor -bromotoluene with potassium diphenylphosphide in refluxing ammonia or dimethyl sulfoxide at 25 °C gives the corresponding diphenyltolylphosphine in good yield. Reaction occurs in the dark, but is stimulated by irradiation with broad spectrum ultraviolet light. There is no evidence for the formation of an aryne intermediate. Electron acceptors such as m-dinitrobenzene and azobenzene inhibit the reaction and suggest a mechanism with electron transfer steps. Entrainment studies indicate a chain mechanism. All data are consistent with the SrnI mechanism of nucleophilic substitution with both thermal and photochemical initiation steps.The radical chain SrnI mechanism of aromatic nucleophilic substitution was first recognized in 1970.3 It is now understood as a mechanism of rather wide scope, both as to the nucleophiles and the substrates that participate.4Most aromatic SrnI reactions require stimulation by photons, by solvated electrons, or by electrons from a cathode. However, the initial examples3 concerning aryl iodides in reaction with KNH2 involved no intentional stimulation. More recently, it was observed that the enolate ions of acetone and pinacolone react spontaneously with iodobenzene in dimethyl sulfoxide in the dark.5These instances of thermally activated SrnI reactions suggest that there may be more in the same category. In the present study, we consider the reactions of potassium diphenylphosphide with m-and p-bromo-and -iodotoluenes (eq 1 and 2). These reactions were first reported in 1963 by Aguiar, Greenberg, and Rubenstein,6 who conducted their studies in tetrahydrofuran (THF) solution.

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