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

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.