Wd. Jones et al., HOMOGENEOUS MODELS OF THIOPHENE HDS REACTIONS - SELECTIVITY IN THIOPHENE C-S CLEAVAGE AND THIOPHENE REACTIONS WITH DINUCLEAR METAL-COMPLEXES, Polyhedron, 16(18), 1997, pp. 3115-3128
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.