Mimicking the HDS activity of ruthenium-based catalysts 2: The hydrogenation of benzo[b]thiophene to 2,3-dihydrobenzo [b]thiophene

The ruthenium(II) tris-acetonitrile complex [(triphos)Ru(MeCN)(3)]BPh4 (1) is an extremely efficient catalyst precursor for the regioselective hydroge nation of benzo[b]thiophene (BT) to 2,3-dihydrobenzo[b]thiophene (DHBT) in homogeneous phase under mild reaction conditions (THF, 40-100 degrees C, 1- 30 bar H-2) [triphos = MeC(CH2PPh2)(3)]. At 30 bar of H-2 and 100 degrees C , BT is converted to DHBT with an average rate of 500 mol of product (mol o f cat)(-1) h(-1). During the catalytic reactions with PH2 > 5 bar, the acet onitrile ligands in 1 are transformed into a mixture of NHEt2, NEt3, and NH 3, while the termination ruthenium products are the monohydrido complexes [ (triphos)Ru(H)(NH3)(2)]BPh4, [(triphos)Ru(H)(NH3)(NH2Et)]BPh4, and [(tripho s)Ru(H)(NH3)(eta(1)-SDHBT)]BP4. Below 5 bar of Hz, no hydrogenation of MeCN occurs and all of the ruthenium is recovered as [(triphos)Ru(H)(NCMe)(eta( 1)-S-DHBT)]BPh4 All of these Ru(II) hydride complexes catalyze the hydrogen ation of BT to DHBT as efficiently as 1. The substitution of D-2 for H2 in a catalytic reaction shows that BT is selectively cis-deuterated to DHBT-d( 2) with no deuterium enrichment in either the unreacted BT or the arene rin g of DHBT. Water in the reaction mixture decreases the hydrogenation rate o f BT due to the formation of the mu-OH and acetate Ru(II) complexes [(triph os)Ru(mu-OH)(3)Ru(triphos)]BPh4 and [(triphos)Ru(O2CCH3)(OH2)]BPh4, which a re catalytically inactive. The acetate complex is suggested to form via hyd ration of a MeCN ligand in the catalyst precursor. Catalytic runs at 30 and 2 bar of H were studied in situ by high-pressure NMR spectroscopy. The kin etics of the hydrogenation of BT in the presence of 1 were studied by gas a dsorption techniques at different catalyst, substrate, and dihydrogen conce ntrations and at different temperatures. The kinetic data together with all of the other evidence accumulated allowed us to deduce a catalytic cycle i n which the reversible dissociation of the thioether product from the metal center in the catalyst [(triphos)RuH](+) is a rate-limiting step. A compar ison of the hydrogenation reactions of BT catalyzed by either the Ru(II) 14 e(-) fragment [(triphos)RuH](+) or the Ru(0) 16e(-) fragment [(triphos)RuH] (-) has provided some clues to unravel a number of mechanistic aspects of t he HDS of thiophenes over single-component catalysts. In particular, the oc currence of either hydrogenation to thioether or hydrogenolysis to thiol ha s been related with the metal basicity.