Organic chemistry of subvalent transition metal complexes. Part 7. Desulfurization of dibenzothiophene by nickel(0) complexes: evidence for electron transfer in oxidative additions

Abstract

Two equivalents of the complex (2,2/-bipyridyl)(l,5-cyclooctadiene)nickel(0) cleave dibenzothiophene in homogeneous media to form nickel-substituted biphenyl derivatives and nickel sulfide. The cleavage proceeds more readily in 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 LiAlH4 used in the workup of the reaction mixture. The reaction is not catalytic in nickel when an excess of LiAlH4 is used; this, indicates that Ni(0) cannot be regenerated from NiS under these conditions. The observed cleavage of ~50% of the dibenzothiophene by 2 equiv of (C10H8N2)Ni(C8H12) is ascribed to the gradual formation of (C10H8N2)2Ni, 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(O) desulfurizing agent.

Electrode initiated proton-coupled electron transfer to promote degradation of a nickel(ii) coordination complex

“…Ni compounds have been found to insert into C–S bonds, occasionally releasing the alkyl group if a hydride equivalent is available, including the example presented in Scheme 2 . 59 – 63 …”

“…Ni compounds have been found to insert into C-S bonds, occasionally releasing the alkyl group if a hydride equivalent is available, including the example presented in Scheme 2. [59][60][61][62][63] While nickel hydrides are not necessary for desulfurization, 60,61 they are implicated in some cases of Ni-mediated C-S cleavage. 59 In the case of compound 1, it is plausible that initial formation of a nickel hydride then initiates C-S cleavage in the 1,3-propanedithiolate ligand.…”

“…[59][60][61][62][63] While nickel hydrides are not necessary for desulfurization, 60,61 they are implicated in some cases of Ni-mediated C-S cleavage. 59 In the case of compound 1, it is plausible that initial formation of a nickel hydride then initiates C-S cleavage in the 1,3-propanedithiolate ligand. Scheme 3 presents two possible pathways by which 1-propanethiol or propane fragments would be released.…”

Desulfurization of Benzonaphthothiophenes and Dibenzothiophene with a Raney Nickel Catalyst

“…Therefore, the preferential fission of a C-S bond can offset the effect of steric hindrance, and thus, the interaction of the ττ-electrons of the sulfur atom with the catalyst surface is more important than the molecular size. Eisch et al (24) performed a mechanistic study of the desulfurization of dibenzothiophene by a nickel(0)-bipyridyl complex and reported that a radical anion of the thiophene nucleus was formed and underwent C-S bond cleavage into S " and an aromatic radical. In addition, they suggested that the oxidative reaction of the nickel(0)-bipyridyl complex toward dibenzo thiophene had the characteristics of stepwise electron transfer rather than nucleophilic attack.…”

Mechanistic and synthetic aspects of organometallic oxidative additions

“…Thus, both ESR and reduction potential measurements show that the radical-anions should decrease in stability in the same order as the observed reactivity. The greater energy necessary to add an electron to the dibenzothiophene system is reflected in a lesser reactivity in desulfurization (26). (28,30).…”