A MODEL FOR HOMOGENEOUS HYDRODESULFURIZATION - THE IMPORTANCE OF ETA-2-COORDINATION AND SULFUR COORDINATION IN C-H AND C-S BOND-CLEAVAGE REACTIONS OF THIOPHENE

The reaction of (C5Me5)Rh(PMe3)PhH with thiophene at 60-degrees-C produces the C-S cleavage adduct (C5Me5)-Rh(PMe3) (SCH = CHCH = CH) in high yield. Irradiation of (C5Me5)Rh(PMe3)H-2 at -20-degrees-C in the presence of thiophene results in the formation of both the C-S insertion product and the new C-H addition adduct (C5Me5)Rh(PMe3)(2-thienyl)H. The latter can be independently prepared by the reaction of (C5Me5)Rh(PMe3)Cl2 with 2-thienyllithium, followed by LiHBEt3. The 2-thienyl hydride rearranges intramolecularly to the C-S insertion adduct with activation parameters DELTA-H = 25.15 (45) kcal/mol and DELTA-S = 3.0 (2) eu. Preparation with LiDBEt3 generates (C5Me5)Rh(PMe3)(2-thienyl)D, which rearranges to the C-S insertion product in which the deuterium is scrambled over both carbons that were originally adjacent to the sulfur atom. Similarly, the complex (C5Me5)Rh(PMe3)(3-thienyl)H was prepared and observed to isomerize intramolecularly to the 2-thienyl derivative at 23-degrees-C with a rate constant of 1.2 (1) X 10(-3) s-1. The corresponding deuteride complex (C5Me5)Rh(PMe3)(3-thienyl)D was observed to rearrange selectively first to (C5Me5)Rh(PMe3)[2-(3-deuteriothienyl)]H and then to a mixture of the C-S insertion adducts with deuterium attached to either the 3 or 4 carbon. The rearrangements of several methyl derivatives have also been studied and found to occur with similar regiospecificity. Heating of (C5Me5)Rh-(PMe3) PhH in the presence of tetramethylthiophene results in the formation of the S-bound complex, (C5Me5)Rh(PMe3)(SC4Me4). The chloro derivative (C5Me5)Rh(PMe3)(2-thienyl)Cl was found to crystallize in the monoclinic space group P2(1)/c with a = 8.992 (7) angstrom, b = 11.324 (10) angstrom, c = 18.480 (8) angstrom, beta = 91.52 (6)-degrees, V = 1881.0 (2.3), and Z = 4.