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 6- 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.