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 wit h benzo[b]thiophene (BT) by C-S bond scission to ultimately yield the 2-vinylthiophenolate complex (triphos)Rh[eta(3)-S(C6H4)CH=CH2] (1), wh ich is an efficient catalyst precursor for the hydrogenation of BT int o 2-ethylthiophenol (ETSH) and, to a lesser extent, into 2,3-dihydrobe nzo[b]thiophene (DHBT) at 160 degrees C and 30 atm H-2 [triphos = MeC( CH(2)PPh(2))(3)]. The mechanism of this unusual catalytic transformati on has been established by high pressure NMR spectroscopic (HPNMR) stu dies combined with the isolation and characterization of key species r elated to the catalysis. Under catalytic conditions 1 was shown by HPN MR to be completely transformed into (triphos)Rh(H)(2)[o-S(C6H4)C2H5] (2) and [(eta(2)-triphos)Rh{mu-o-S(C6H4)C2H5}](2) (3); removal of H-2 in the presence of ETSH leads to the quantitative formation of (tripho s)RhH[o-S(C6H4)C2H5](2) (4), which is also the terminal state of the c atalytic system in all experiments carried out in a high pressure reac tor under various reaction conditions. The dimer 3 was prepared in a p ure form by reaction of (triphos)RhH3 with 1 equiv of ETSH in THF and reacted with excess ETSH to produce 4, with H-2 to give 2, and with CO to yield (triphos)RhH(CO)[o-S(C6H4)C2H5] (6) Conversely, 3 could be o btained by thermally induced reduction elimination of H-2 from 2 even under 30 atm of H-2 or of ETSH from 4. The formation of the dihydride 2 from the vinylthiophenolate derivative 1 under H-2 (>15 atm) was als o observed by HPNMR; this reaction was mimicked by the stepwise additi on of H+ to yield [(triphos)Rh{eta(4)-S(C6H4)CH(CH3)}]BF4 (7) Reaction of the latter complex with H- produces (triphos)RhH[eta(2)-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 th ioligand, via the unsaturated fragment [(triphos)Rh{o-S(C6H4)C2H5}]. I n the mechanistic picture proposed, the catalytically active species f or both reactions is [(triphos)RhH] generated from 2 by the rate-deter mining reductive elimination of ETSH. The hydrogenation of BT to ETSH occurs after the substrate has been C-S inserted, although hydrogenati on to DHBT also takes place as a minor, parallel path. Then eta(1)-S a nd eta(2)-2,3-BT isomers probably exist in equilibrium, but the eta(1) -S intermediate prevails over the eta(2)-2,3 isomer for steric reasons , thus determining the chemoselectivity of the reaction. The chemistry herein described provides further insight into the mechanistic aspect s of HDS reactions on solid catalysts.