Mechanism, Raney nickel desulfurization

The subject of Raney nickel desulfurization has been exhaustively dealt with in CHEC-I. A few further interesting applications of this procedure will be presented in this section, along with the use of some nickel(O) complexes and nickel boride for this purpose. The major goal of understanding the mechanism of hydrodesulfurization by studying metal complexes as models is briefly dealt with in Section 2.10.6. 

Active Raney nickel induces desulfurization of many sulfur-containing heterocycles thiazoles are fairly labile toward this ring cleavage agent. The reaction occurs apparently by two competing mechanisms (481) in the first, favored by alkaline conditions, ring fission occurs before desul-, furization, whereas in the second, favored by the use of neutral catalyst, the initial desulfurization is followed by fission of a C-N bond and formation of carbonyl derivatives by hydrolysis (Scheme 95). 

In 1974, Gassman et al. reported a general method for the synthesis of indoles. For example, aniline 5 was reacted sequentially with r-BuOCl, methylthio-2-propanone 6 and triethylamine to yield methylthioindole 7 in 69% yield. The Raney-nickel mediated desulfurization of 7 then provided 2-methylindole 8 in 79% yield. The scope and mechanism of the process were discussed in the same report by Gassman and coworkers as well. 

That the desulfurizing action of Raney nickel is retained even in the absence of hydrogen has been demonstrated by Hauptmann, Wladislaw and Camargo16 who found that Raney nickel which had been deprived of its hydrogen through heating in vacuum at 200° converted, for example, benzaldehyde dibenzyl thioacetal into a mixture of stilbene and bibenzyl. That this type of reaction proceeds by a free-radical mechanism appears... 

Rearrangement of a pyridazine into a pyrrole has been found when 3-anilinopyridazine is heated in the presence of Raney nickel 2-anilino-2-pyrroline is formed in this reductive ring contraction. Similarly, 2,3,4-triphenylpyrrole is obtained from 4,5,6-triphenyl-3(2H)-pyridazinethione by reductive desulfurization with Raney nickel. A mechanism suggested for similar ring contractions of cinno-lines into indoles may be operating also in this case. Another... 

Raney Ni is a spongy form of nickel that has adsorbed hydrogen atoms. It is these hydrogen atoms that ultimately replace the sulfur atoms, although a discussion of the mechanism for desulfurization is beyond the scope of this text. 

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