Homogeneous models of thiophene HDS reactions. Selectivity in thiophene C-S cleavage and thiophene reactions with dinuclear metal complexes

The reactive 16 e(-) metal fragment [(C5Me5)Rh(PMe3)] inserts into a wide variety of thiophene C-S bonds. The structures of the thiophene, benzothiophene and dibenzothiophene insertion complexes have been determined. While the thiophene complex adopts a planar six-membered ring structure the other metallacycles are bent and all molecules possess localized diene structures. The mechanism of C--S cleavage was found to proceed by way of initial sulfur coordination. 2-Methylbenzothiophene gives a kinetic product resulting from cleavage of the sulfur-vinyl bond, but then rearranges to cleave the sulfur-aryl bond. A number of substituted dibenzothiophenes were examined, showing little electronic effect of substituents, but showing a large steric effect of substituents at the 4 and 6 positions. 4,6-Dimethyldibenzothiophene does not undergo cleavage, but instead forms an S-bound complex. Reactions of a cobalt analog, (C5Me5)Co(C2H4)(2) with thiophenes also lead to C--S cleaved products and the use of a dinuclear iridium system produces a butadiene complex in which both C--S bonds have been cleaved. (C) 1997 Elsevier Science Ltd.