Mimicking the HDS Activity of Ruthenium-Based Catalysts. Homogeneous Hydrogenolysis of Benzo[b]thiophene

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Abstract

The reaction of [(triphos)RuH(BH4)] (1) in THF with KOBut yields the novel trihydride complex K[(triphos)RuH3] (2) and BH2OBut (triphos = MeC(CH2PPh2)3). The ruthenate complex 2 can also be synthesized by hydrogenation (30 bar of H2) in THF of the tris(acetonitrile) complex [(triphos)Ru(NCMe)3](BPh4)2 (3) in the presence of a 5-fold excess of BH2OBut at 40 °C. This reaction produces a mixture of NH2Et, NHEt2, NEt3, and NH3 as a result of MeCN hydrogenation, followed by amine redistribution reactions. Compound 2 is isolated in analytically pure form as [K(C12H24O6)][(triphos)RuH3] (2a) by recrystallization from THF/n-hexane in the presence of 18-crown-6 ether. In the presence of a strong base such as KOBut, both 1 and 3 are effective catalyst precursors for the homogeneous hydrogenolysis of benzo[b]thiophene (BT) to 2-ethylthiophenol (ETP) in THF under mild reaction conditions (≥70 °C, 30 bar of H2). The hydrogenolysis rate increases with the concentration of the base, which, depending on the catalyst precursor, may play up to three distinct roles in the catalytic reactions. It promotes the formation and stabilization of the catalytically active species (i.e. the 16e- fragment [(triphos)RuH]-) and speeds up the hydrogenolysis rate, delivering the ETP product into the solution as 2-ethylthiophenolate potassium salt. High-pressure 31P{1H} and 1H NMR experiments (HPNMR) in sapphire tubes sealed by titanium-alloy valves show that the interaction of 2 with BT at ≥70 °C in THF-d8 selectively yields the dihydride thiolate complex K[(triphos)Ru(H)2(o-S(C6H4)C2H5)] (5) under H2 and the vinylthiophenolate complex K[(triphos)Ru(η3-S(C6H6)CHCH2)] (6) under N2. Compound 6 in THF transforms into 5 by treatment with H2 even at room temperature. Under catalytic conditions at 70 °C, 5 and 2 are the only NMR-detectable species in equilibrium concentrations that depend on the temperature and on the base concentration. The hydrogenolysis mechanism is proposed to involve C−S insertion of ruthenium into the C2−S bond of BT to give a 2-vinylthiophenolate ligand, followed by hydrogenation of the vinyl moiety and reductive elimination of the thiol. This latter step is accelerated by the strong Brønsted base. The possible similarity in the hydrogenolysis reactions catalyzed by the present soluble complexes to those occurring in the hydrodesulfurization of fossil fuels over Ru-promoted heterogeneous catalysts is discussed.

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