STERIC AND ELECTRONIC EFFECTS ON THE INSERTION OF A RHODIUM PHOSPHINECOMPLEX INTO THE C-S BOND OF SUBSTITUTED DIBENZOTHIOPHENES - HOMOGENEOUS MODEL FOR THE HYDRODESULFURIZATION PROCESS

The unsaturated 16e(-) fragment [(C(5)Me(5))Rh(PMe(3))] has been obser ved to insert into the sulfur-carbon bond in a variety of substituted dibenzothiophenes to give a six-membered metallacycle product. The reg ioselectivity of the C-S insertion was probed by varying the position and nature of substituents on dibenzothiophene, and selectivities were found to be directed by steric hindrance with a small electronic cont ribution. Insertion exclusively into the sterically less hindered C-S bond was found in 4-methyldibenzothiophene, 2,6-dimethyldibenzothiophe ne, and benzo[b]naphtho[2,1-d]thiophene. Insertion toward the unsubsti tuted ring was the major product of two C-S insertion isomers with 2-f luorodibenzothiophene and 2-methoxydibenzothiophene. Insertion toward the substituted ring was seen to be favored with 2-(trifluoromethyl)di benzothiophene and 2-cyanodibenzothiophene. Unselective insertion into either C-S bond was found in 2-methyldibenzothiophene, 3-methyldibenz othiophene, 2,8-dimethyldibenzothiophene, 1,3,7-trimethyldibenzothioph ene, and benzo[b]naphtho[1,2-d]thiophene. 2-Bromodibenzothiophene show ed C-Br activation as well as two C-S insertion products (2:1), while 2-iododibenzothiophene showed C-I activation only. 4,6-Dimethyldibenzo thiophene formed a labile S-bound complex which was observed to lose t hiophene by substitution with free trimethylphosphine with Delta G dou ble dagger = 24.5 kcal/mol at 23 degrees C. X-ray structure determinat ion of the products of reactions with 2-methyldibenzothiophene, 3-meth yldibenzothiophene, 4-methyldibenzothiophene, 2,6-dimethyldibenzothiop hene, 2,8-dimethyldibenzothiophene, benzo[b]naphtho[2,1-d]thiophene, 2 -fluorodibenzothiophene, and 2-methoxydibenzothiophene confirmed the f inal identification of insertion selectivities. H-1 COSY and NOE diffe rence experiments allowed assignment of regioselectivity with the prod ucts of 2-(trifluoromethyl)dibenzothiophene and 2-cyanodibenzothiophen e by comparison with known samples.