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

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−H 2 O as the polar phase and n -heptane as the organic phase [sulphos = - O 3 S(C 6 H 4 )CH 2 C(CH 2 PPh 2 ) 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 H 2 O, 10 mL of n -heptane, 30 bar of H 2, 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 H 2 and accelerates the conversion of BT by aiding the reductive elimination of the hydrogenolysis product. The effect of the H 2 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)RhH 3 ] - and the dihydride 2-ethylthiophenolate complex [(sulphos)Rh(H) 2 ( o -S(C 6 H 4 )C 2 H 5 )] - . 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] - .