Liquid-Biphase Hydrogenolysis of Benzo[b]thiophene by Rhodium Catalysis

Abstract

The catalytic activity of the zwitterionic complex [(sulphos)Rh(cod)] for the hydrogenation and hydrogenolysis reactions of benzo[b]thiophene (BT) has been studied in either methanol or liquid-biphase systems comprising MeOH or MeOH−H2O as the polar phase and n-heptane as the organic phase [sulphos = -O3S(C6H4)CH2C(CH2PPh2)3]. The catalyst activity is independent of the phase variation. Under neutral conditions, the slow but selective hydrogenation of BT to 2,3-dihydrobenzo[b]thiophene is observed. Conversely, in the presence of NaOH or other strong bases, the fast and selective hydrogenolysis of BT to 2-ethylthiophenol sodium salt occurs. In a typical liquid-biphase hydrogenolysis reaction [35 mg (0.035 mmol) of catalyst, 470 mg (3.5 mmol) of BT, 180 mg (4.5 mmol) of NaOH, 5 mL of MeOH, 5 mL of H2O, 10 mL of n-heptane, 30 bar of H2, 160 °C], all the substrate is practically consumed in ca. 5 h to give the 2-ethylthiophenolate product. The strong base plays a dual role in the hydrogenolysis reaction: it promotes the formation of the Rh−H species (which is necessary for the C−S insertion step) by heterolytic splitting of H2 and accelerates the conversion of BT by aiding the reductive elimination of the hydrogenolysis product. The effect of the H2 pressure and of the substrate, catalyst, and base concentrations on the conversion rate of BT has been studied under liquid-biphase conditions. High-pressure NMR experiments in sapphire tubes have provided mechanistic information on the catalysis cycle for the hydrogenolysis of BT in MeOH. Under catalytic conditions, the phosphorus-containing rhodium compounds (visible on the NMR time scale) are the trihydride [(sulphos)RhH3]- and the dihydride 2-ethylthiophenolate complex [(sulphos)Rh(H)2(o-S(C6H4)C2H5)]-. Consistent with previous studies, the reductive elimination of the thiol from the metal center is suggested to be the rate-determining step of the hydrogenolysis reaction of BT catalyzed by the 16e- fragment [(sulphos)RhH]-.