DESULFURIZATION OF BENZOTHIOPHENES AND DIBENZOTHIOPHENES WITH NICKEL BORIDE

Nickel boride, prepared from the reduction of nickel chloride hexahydrate with sodium borohydride in methanol-tetrahydrofuran, reduces benzothiophenes to alkylbenzenes and dibenzothiophenes to biphenyls. The reaction is rapid at or below room temperature and does not require protection from the atmosphere. Best results are obtained when the nickel boride is generated in situ in the presence of the sulfur compound. Hydroxyl, carboxyl, ester, and amino groups are unaffected while chloro, bromo, and nitro substituents are also reduced under these conditions. A short-lived intermediate, possibly a nickel hydride species, appears to be required in the desulfurization. Complexation of the substrate to the nickel boride surface, followed by stepwise reduction of the two C-S bonds, occurs. The faster disappearance of dibenzothiophene containing the lighter S-32 isotope compared to that with S-34 (k(S-32)/k(S-34) = 1.005 to 1.006) suggests that C-S bond cleavage is the rate-determining step.