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[q3-S(C6H4)CH=CH2] (l), 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 (~~~~~os)R~(H)~[o-S(C~H~)CZH~] (2) and [(q2-triphos)-Rh@-o-S(C&&2H5}]2 (3); removal of H2 in the presence of ETSH leads to the quantitative formation of (triphos)-R~H [ o -S ( C~H~) C~H~]~ (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 (~~~~~os)R~H(CO)[O-S(C~H~)C~H~] (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 Hf to yield [(triphos)Rh{q4-S(C6H4)CH(CH3)}]BF4 (7). Reaction of the latter complex with H-produces (triphos)RhH[v2-S(C&)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(C6&)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 7'-S and q2-2,3-BT isomers probably exist in equilibrium, but the 7'4 intermediate prevails over the q2-2,3 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.
The q4-benzene complex [(triphOS)h(C6H6)] BPh4 reacts with thiophene to give the iridathiabenzene complex [(triphos)Ir(qW,S-C4H4S)]BPh4 (1) [triphos = MeC(CH2PPh2)J. Compound 1 is selectively converted to the butadienethiolatecomplex [ (triphos)Ir(q3-SCH=CHCH=CH2)] (2) by reaction with LiHBEt3 via the (thiapentadieny1)-hydride kinetic intermediate [(triphas)IrH(q2-C,S-C4H4S)] (3). Compound 2 is straightforwardly obtained by reaction of [(tripho~)Ir(H)~(C2Hs)l with thiophene. This reaction produces also the (2-thieny1)dihydride [(triphos)Ir(H)z-(2-C4H3S)] (4) through a parallel C-H bond activation path. The thienyl complex is not a kinetic intermediate for the opening reaction of thiophene. Compound 2 reacts with HBFcOEt2 in the presence of CO yielding the thiocrotonaldehyde complex [(triphos)Ir(CO)(q4-S=CHCH=CH(Me))]BF4 (5) and with PhSH to give the allylthioaldehyde derivative [(triphos)Ir(SPh)(q4-S=CHCH2CH=CH2)] (6). The latter compound is stable in the solid state, but re-forms 2 and PhSH in solution unless an excess of thiophenol is added. Stirring 2 with a 5-fold excess of HCl produces the trichloride [(triphos)IrC13], H2S, CH2=CHCH=CH2, C H~= C H C H Z C H~S H , and CH3CH2-CH=CHSH. Methylation of 2 with MeI, followed by NaBPh4 addition, gives the methyl buta-1,3-dienyl thioether complex [ (triphos)Ir(q3-S(Me)CH=CHCH=CH2)]BPh4.0.5EtOH (8) which has been characterized by a singlecrystal X-ray analysis. Compound 8 crystallizes in the space group P 2 ] / n . The coordination geometry around the iridium center is a distorted octahedron. The phosphorus atoms of triphos occupy threefuc positions of the coordination polyhedron. The coordination of the metal fragment is completed by the thioether ligand which uses the sulfur atom and the two carbon atoms of the distal olefinic moiety to bind the metal. Compound 8 reacts with gaseous HC1 converting to [(triphos)IrC13] and evolving CH*=CHCH2CH2SMe and CH3CH2CH=CHSMe. Treatment of 8 with THF-BHj, followed by R O H addition (R = Me, Et), gives [(triphos)IrH(SMe)(ROH)]BPh4 (R = Me, 9; Et, 10) and buta-1,3-diene. All reactions have been carried out in tetrahydrofuran.
The ruthenium(II) tris-acetonitrile complex [(triphos)Ru(MeCN)3]BPh4 (1) is an extremely efficient catalyst precursor for the regioselective hydrogenation of benzo[b]thiophene (BT) to 2,3-dihydrobenzo[b]thiophene (DHBT) in homogeneous phase under mild reaction conditions (THF, 40−100 °C, 1−30 bar H2) [triphos = MeC(CH2PPh2)3]. At 30 bar of H2 and 100 °C, BT is converted to DHBT with an average rate of 500 mol of product (mol of cat)-1 h-1. During the catalytic reactions with PH2 > 5 bar, the acetonitrile ligands in 1 are transformed into a mixture of NHEt2, NEt3, and NH3, while the termination ruthenium products are the monohydrido complexes [(triphos)Ru(H)(NH3)2]BPh4, [(triphos)Ru(H)(NH3)(NH2Et)]BPh4, and [(triphos)Ru(H)(NH3)(η1-S-DHBT)]BPh4. Below 5 bar of H2, no hydrogenation of MeCN occurs and all of the ruthenium is recovered as [(triphos)Ru(H)(NCMe)(η1-S-DHBT)]BPh4. All of these Ru(II) hydrido complexes catalyze the hydrogenation of BT to DHBT as efficiently as 1. The substitution of D2 for H2 in a catalytic reaction shows that BT is selectively cis-deuterated to DHBT-d 2 with no deuterium enrichment in either the unreacted BT or the arene ring of DHBT. Water in the reaction mixture decreases the hydrogenation rate of BT due to the formation of the μ-OH and acetate Ru(II) complexes [(triphos)Ru(μ-OH)3Ru(triphos)]BPh4 and [(triphos)Ru(O2CCH3)(OH2)]BPh4, which are catalytically inactive. The acetate complex is suggested to form via hydration of a MeCN ligand in the catalyst precursor. Catalytic runs at 30 and 2 bar of H2 were studied in situ by high-pressure NMR spectroscopy. The kinetics of the hydrogenation of BT in the presence of 1 were studied by gas adsorption techniques at different catalyst, substrate, and dihydrogen concentrations and at different temperatures. The kinetic data together with all of the other evidence accumulated allowed us to deduce a catalytic cycle in which the reversible dissociation of the thioether product from the metal center in the catalyst [(triphos)RuH]+ is a rate-limiting step. A comparison of the hydrogenation reactions of BT catalyzed by either the Ru(II) 14e- fragment [(triphos)RuH]+ or the Ru(0) 16e- fragment [(triphos)RuH]- has provided some clues to unravel a number of mechanistic aspects of the HDS of thiophenes over single-component catalysts. In particular, the occurrence of either hydrogenation to thioether or hydrogenolysis to thiol has been related with the metal basicity.
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