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 w ide variety of thiophene C-S bonds. The structures of the thiophene, b enzothiophene and dibenzothiophene insertion complexes have been deter mined. While the thiophene complex adopts a planar six-membered ring s tructure the other metallacycles are bent and all molecules possess lo calized 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 bo nd, but then rearranges to cleave the sulfur-aryl bond. A number of su bstituted dibenzothiophenes were examined, showing little electronic e ffect of substituents, but showing a large steric effect of substituen ts at the 4 and 6 positions. 4,6-Dimethyldibenzothiophene does not und ergo cleavage, but instead forms an S-bound complex. Reactions of a co balt analog, (C5Me5)Co(C2H4)(2) with thiophenes also lead to C--S clea ved products and the use of a dinuclear iridium system produces a buta diene complex in which both C--S bonds have been cleaved. (C) 1997 Els evier Science Ltd.