Desulfurization of dibenzothiophene by nickel(0) complexes: evidence for electron transfer in oxidative additions

Two equivalents of the complex (2,2'-bipyridyl)(1,5-cyclooctadiene)nickel(0) cleave dibenzothiophene in homogeneous media to form nickel-substituted biphenyl derivatives and nickel sulfide. The cleavage proceeds more readily in tetrahydrofuran (THF) than in benzene solution, and excess amine (either bipyridyl or pyridine) retards the reaction. Neither biphenylene nor 2-phenylbenzenethiol was a detectable transient in the desulfurization process. The possible fleeting formation of biphenylene was ruled out by a study of the desulfurized products obtained from the 2,8-dimethyl and the 3,7-dimethyl derivatives of dibenzothiophene. A possible role of nickel hydride in some of the C-S bond cleavage is suspected from the observed effects of either acid or lithium aluminum hydride (LiAlH/sub 4/) used in the workup of the reaction mixture. The reaction is not catalytic in nickel when an excess of LiAlH/sub 4/ is used; indicating that Ni(0) cannot be regenerated from NiS under these conditions. The observed cleavage of approx. 50% of the dibenzothiophene by 2 equiv of (2,2'-bipyridyl)(1,5-cyclooctadiene) nickel ((C/sub 10/H/sub 8/N/sub 2/)Ni(C/sub 8/H/sub 12/)) is ascribed to the gradual formation of bis(cyclooctadiene) nickel ((C/sub 10/H/sub 8/N/sub 2/)/sub 2/Ni) which is ineffectual in cleaving dibenzothiophene. Finally, the relative reactivity of the dibenzothiophenes (dibenzothiophene > 2,8-dimethyldibenzothiophene >> 3,7-dimethyldibenzothiophene), the relative effectiveness of amine donors or solvents, and the observed hydrogen abstraction from the solvent are interpreted in terms of an electron-transfer mode of action by the nickel(0) desulfurizing agent.