Steric and Electronic Effects on the Insertion of a Rhodium Phosphine Complex into the C−S Bond of Substituted Dibenzothiophenes. Homogeneous Model for the Hydrodesulfurization Process

The unsaturated 16e- fragment [(C5Me5)Rh(PMe3)] has been observed to insert into the sulfur−carbon bond in a variety of substituted dibenzothiophenes to give a six-membered metallacycle product. The regioselectivity 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 contribution. Insertion exclusively into the sterically less hindered C−S bond was found in 4-methyldibenzothiophene, 2,6-dimethyldibenzothiophene, and benzo[b]naphtho[2,1-d]thiophene. Insertion toward the unsubstituted ring was the major product of two C−S insertion isomers with 2-fluorodibenzothiophene and 2-methoxydibenzothiophene. Insertion toward the substituted ring was seen to be favored with 2-(trifluoromethyl)dibenzothiophene and 2-cyanodibenzothiophene. Unselective insertion into either C−S bond was found in 2-methyldibenzothiophene, 3-methyldibenzothiophene, 2,8-dimethyldibenzothiophene, 1,3,7-trimethyldibenzothiophene, and benzo[b]naphtho[1,2-d]thiophene. 2-Bromodibenzothiophene showed C−Br activation as well as two C−S insertion products (2:1), while 2-iododibenzothiophene showed C−I activation only. 4,6-Dimethyldibenzothiophene formed a labile S-bound complex which was observed to lose thiophene by substitution with free trimethylphosphine with ΔG ⧧ = 24.5 kcal/mol at 23 °C. X-ray structure determination of the products of reactions with 2-methyldibenzothiophene, 3-methyldibenzothiophene, 4-methyldibenzothiophene, 2,6-dimethyldibenzothiophene, 2,8-dimethyldibenzothiophene, benzo[b]naphtho[2,1-d]thiophene, 2-fluorodibenzothiophene, and 2-methoxydibenzothiophene confirmed the final identification of insertion selectivities. 1H COSY and NOE difference experiments allowed assignment of regioselectivity with the products of 2-(trifluoromethyl)dibenzothiophene and 2-cyanodibenzothiophene by comparison with known samples.