The catalytic transformation of benzo[b]thiophene to 2-ethylthiophenol by a soluble rhodium complex: The reaction mechanism involves ring opening prior to hydrogenation

Article abstract of DOI:10.1021/ja00138a011

The thermally generated 16-electron fragment [(triphos)RhH] reacts with benzo[b]thiophene (BT) by C-S bond scission to ultimately yield the 2-vinylthiophenolate complex (triphos)Rh[η³-S(C₆H₄)CH=CH₂] (1), which is an efficient catalyst precursor for the hydrogenation of BT into 2-ethylthiophenol (ETSH) and, to a lesser extent, into 2,3-dihydrobenzo[b]thiophene (DHBT) at 160 °C and 30 atm H₂ [triphos = MeC(CH₂PPh₂)₃]. The mechanism of this unusual catalytic transformation has been established by high pressure NMR spectroscopic (HPNMR) studies combined with the isolation and characterization of key species related to the catalysis. Under catalytic conditions 1 was shown by HPNMR to be completely transformed into (triphos)Rh(H)₂[o-S(C₆H₄)C₂H ₅] (2) and [η²-triphos)-Rh{μ-o-S(C₆H₄)C ₂H₅}]₂ (3); removal of H₂ in the presence of ETSH leads to the quantitative formation of (triphos)-RhH[o-S(C₆H₄)C₂H₅] ₂ (4), which is also the terminal state of the catalytic system in all experiments carried out in a high pressure reactor under various reaction conditions. The dimer 3 was prepared in a pure form by reaction of (triphos)RhH₃ with 1 equiv of ETSH in THF and reacted with excess ETSH to produce 4, with H₂ to give 2, and with CO to yield (triphos)RhH(CO)[o-S(C₆H₄)C₂H₅] (6). Conversely, 3 could be obtained by thermally induced reduction elimination of H₂ from 2 even under 30 atm of H₂ or of ETSH from 4. The formation of the dihydride 2 from the vinylthiophenolate derivative 1 under H₂ (>15 atm) was also observed by HPNMR; this reaction was mimicked by the stepwise addition of H⁺ to yield [(triphos)Rh{η⁴-S(C₆H₄)CH(CH ₃)}]BF₄ (7). Reaction of the latter complex with H^- produces (triphos)RhH[η²-S(C₆H₄)CH(CH₃)] (8), which converts to the dimer 3 by reductive coupling of the terminal hydride ligand with the metalated alkyl substituent in the thioligand, via the unsaturated fragment [(triphos)Rh{o-S(C₆H₄)C₂H₅}]. In the mechanistic picture proposed, the catalytically active species for both reactions is [(triphos)RhH] generated from 2 by the rate-determining reductive elimination of ETSH. The hydrogenation of BT to ETSH occurs after the substrate has been C-S inserted, although hydrogenation to DHBT also takes place as a minor, parallel path. Then η¹-S and η²-2,3-BT isomers probably exist in equilibrium, but the η¹-S intermediate prevails over the η²-2,3 isomer for steric reasons, thus determining the chemoselectivity of the reaction. The chemistry herein described provides further insight into the mechanistic aspects of HDS reactions on solid catalysts.