HYDRODESULFURIZATION (HDS) MODEL SYSTEMS - OPENING, HYDROGENATION, AND HYDRODESULFURIZATION OF DIBENZOTHIOPHENE (DBT) AT IRIDIUM - FIRST CASE OF CATALYTIC HDS OF DBT IN HOMOGENEOUS PHASE

The kinetic selectivity for C-H vs C-S activation of dibenzothiophene (DBT) by the [(triphos)IrH] fragment has been observed upon either the rmolysis of(triphos)Ir(H)(2)(C2H5) in THF in the temperature range fro m 70 to 160 degrees C or dehydrohalogenation of (triphos)Ir(H)(2)Cl wi th t-BuLi at room temperature [triphos = MeC(CH(2)PPh(2))(3)]. C-H bon d cleavage already occurs at 20 degrees C to give as many as three iso meric DBTyl complexes of the formula (triphos)Ir(H)(2)(DBTyl). The kin etic preference follows the order 3-DBTyl > 4-DBTyl greater than or eq ual to 2-DBTyl, while the thermodynamic stability is in the order 4-DB Tyl > 3-DBTyl > 2-DBTyl. Both C-H insertion and C-S insertion occur in the temperature range from 120 to 160 degrees C. Above the latter tem perature, C-S insertion prevails over C-H insertion, and the complex ( triphos)IrH(eta(2)-C,S-DBT) (5) is generated quantitatively. By reacti on with H-2 (THF, 170 degrees C, 30 atm of H-2, 4 h), 5 is converted t o a 31:69 mixture of the 2-phenylthiophenolate dihydride (triphos)Ir(H )(2)(SC12H9) (7) and the trihydride (triphos)Ir(H)(3) (8) while free 2 -phenylthiophenol, DBT, and biphenyl + H2S are evolved in a relative r atio of 48:42:10. In the presence of an excess of DBT, the reaction is catalytic and converts 10 mol of DBT/mol of 5 in 24 h to both hydroge nation (60%) and desulfurization (40%) products. A rationale of the ca talysis cycle is discussed in the light of the results of a study invo lving the use of isolated compounds in a variety of independent reacti ons. In accord with previous studies, the thiolate complex 7 is propos ed as the intermediate species that undergoes desulfurization by actio n of H-2.