- Synthesis of the first polymer-supported tripodal triphosphine ligand and its application in the heterogeneous hydrogenolysis of benzo[b]thiophene by rhodium catalysis
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A p-styrenyl substituent attached to the ligand framework allows the tripodal triphosphine moiety -C(CH2PPh2)3 to be introduced as a pendant group in polystyrene matrices via free-radical copolymerisation; in conjunction w
- Bianchini,Frediani,Vizza
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p. 479 - 480
(2007/10/03)
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- First example of opening and hydrogenation of 2,3-dihydrobenzo[b]thiophene to 2-ethylthiophenol assisted by a soluble metal complex
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Displacement of THF by dihydrobenzo[b]thiophene (DHBT) in [(triphos)Ir(H)2(THF)]BPh4 yields the η1-S-DHBT adduct [(triphos)Ir(H)2(η1-DHBT)]BPh4 which reacts in THF at room temperature with KOBu(t) to give the neutral 2-vinylthiophenolate derivative (triphos)Ir(H)2{η1-o-S(C6H4)CH=CH2} via C2-S bond cleavage; the latter compound is hydrogenated under mild conditions (2 bar H2, 80°C) to the 2-ethylthiophenolate derivative (triphos)Ir(H)2{o-S(C6H4)Et} and under harsh conditions (30 bar H2, 160°C) to free 2-ethylthiophenol and (triphos)IrH3.
- Bianchini, Claudio,Meli, Andrea,Oberhauser, Werner,Vizza, Francesco
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p. 671 - 672
(2007/10/03)
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- Reductive desulfurization of organosulfur compounds with sodium in liquid ammonia
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Greater than 95% sulfur removal was observed when dialkyl mono or polysulfides were treated with Na in liquid ammonia. Polycyclic aromatic sulfur heterocycles were only moderately desulfurized under these conditions while phenylthio derivatives gave thiophenol as the major product and dithiophenols as the minor products.
- Yu, Zhengkun,Verkade, John G.
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- Liquid-biphase hydrogenolysis of benzo[b]thiophene by rhodium catalysis
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The catalytic activity of the zwitterionic complex [(sulphos)Rh(cod)] for the hydrogenation and hydrogenolysis reactions of benzo[b]thiophene (BT) has been studied in either methanol or liquid-biphase systems comprising MeOH or MeOH-H2O as the polar phase and n-heptane as the organic phase [sulphos = - O3S(C6H4)CH2C(CH2PPh2)3]. The catalyst activity is independent of the phase variation. Under neutral conditions, the slow but selective hydrogenation of BT to 2,3- dihydrobenzo[b]thiophene is observed. Conversely, in the presence of NaOH or other strong bases, the fast and selective hydrogenolysis of BT to 2-ethylthiophenol sodium salt occurs. In a typical liquid-biphase hydrogenolysis reaction [35 mg (0.035 mmol) of catalyst, 470 mg (3.5 mmol) of BT, 180 mg (4.5 mmol) of NaOH, 5 mL of MeOH, 5 mL of H2O, 10 mL of n-heptane. 30 bar of H2, 160 °C], all the substrate is practically consumed in ca. 5 h to give the 2-ethylthiophenolate product. The strong base plays a dual role in the hydrogenolysis reaction: it promotes the formation of the Rh-H species (which is necessary for the C-S insertion step) by heterolytic splitting of H2 and accelerates the conversion of BT by aiding the inductive elimination of the hydrogenolysis product. The effect of the H2 pressure and of the substrate, catalyst, and base concentrations on the conversion rate of BT has been studied under liquid-hiphase conditions. High- pressure NMR experiments in sapphire tubes have provided mechanistic information on the catalysis cycle for the hydrogenolysis of BT in MeOH. Under catalytic conditions, the phosphorus-containing rhodium compounds (visible on the NMR time scale) are the trihydride [(sulphos)RhH3]- and the dihydride 2- ethylthiophenolate complex [(sulphos)Rh(H)2(o-S(C6H4)C2H5)]- . Consistent with previous studies, the inductive elimination of the thiol from the metal center is suggested to be the rate- determining step of the hydrogenolysis reaction of BT catalyzed by the 16e- fragment [(sulphos)RhH]-.
- Bianchini, Claudio,Meli, Andrea,Patinec, Veronique,Sernau, Volker,Vizza, Francesco
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p. 4945 - 4954
(2007/10/03)
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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
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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.
- Bianchini, Claudio,Herrera, Verónica,Jimenez, M. Victoria,Meli, Andrea,Sánchez-Delgado, Roberto,Vizza, Francesco
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p. 8567 - 8575
(2007/10/03)
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- Oxidative Cleavage of S-Arylmercaptoacetic Acids by Sodium Perborate: Kinetic and Correlation Study
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Kinetics of oxidation of twenty six S-arylmercaptoacetic acids (SAMA) (I) by sodium perborate (PB) have been studied in acid medium.The product of oxidation is the corresponding thiophenol.The rate data of meta- and para-substituted acids have been correlated with DSP equations.While the para-compounds correlate well with ?I and ?0R values, the meta-compounds correlate well with ?I and ?-R values.The reaction constants are negative and of smaller magnitudes.Further, the ortho-substituted acids show a good correlation with a triparametric equation involving Taft's ?I and ?0R and Charton's steric parameter ν.There is a considerable steric contribution to the total ortho-substituent effect.Based on these observations, mechanism involving the formation of protonated arylsulfinylacetic acid intermediate, followed by an intramolecular rearrangement leading to the product thiophenol has been proposed.
- Kabilan, S.,Pandiarajan, K.,Krishnasamy, K.,Sankar, P.
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p. 443 - 452
(2007/10/02)
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- Process for phenol alkylthiolation and its application to the synthesis of 4-acyl-2-alkylthiophenols
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The invention relates to the preparation of alkylthiophenols by reaction of a dialkyl disulphide with a phenol. In the process according to the invention, the reaction is carried out in the presence of aluminum chloride or of ferric chloride in a solvent of the alkylbenzene type or, soley in the case of methylthiolation, in an excess of dimethyl disulphide. This process makes it possible, in particular to obtain, with a selectivity and in a yield which are excellent, 2-alkylthiophenols which may then be converted into 4-acyl-2-alkylthiophenols by means of a reaction at a temperature ranging from 40° to 100° C. with a complex BF3 :2RCOOH where R denotes an alkyl or propenyl radical, in a proportion of 10 to 15 moles of complex per mole of 2-alkylthiophenol.
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- Rapid and Selective Reduction of Functionalized Aromatic Disulfides with Lithium Tri-tert-butoxyaluminohydride. A Remarkable Steric and Electronic Control. Comparison of Various Hydride Reagents
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Lithium tri-tert-butoxyaluminohydride (LTBA), an exceptionally mild reducing agent in organic synthesis, reduces functionalized aromatic disulfides to the corresponding thiols in quantitative yield.The reaction is rapid (for example, o-tolyl disulfide is reduced to completion in 60 min at 25 deg C) and can tolerate a wide variety of functional groups, such as halogen, nitro, carboxylic acid, and their derivatives.The presence of electron-withdrawing substituents dramatically enhances the rate of reduction (p-chlorophenyl disulfide is quantitatively reduced in 30 s) and electron-releasing substituents diminishes the rate of cleavage.The reaction is sensitive to steric effects (2,4-di-tert-pentylphenyl disulfide underwent 25percent reduction in 24 h).However, such hindered disulfides can be rapidly and quantitatively reduced in refluxing THF.The reaction of LTBA with alkyl disulfides is extremely sluggish.The reaction provides a useful and simple means for the facile and selective reduction of aromatic disulfides where this is required in synthetic operations.
- Krishnamurthy, S.,Aimino, D.
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p. 4458 - 4462
(2007/10/02)
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- ACTIVATION AND DESULFURIZATION OF THIOPHENE AND BENZOTHIOPHENE BY IRON CARBONYLS.
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The synthesis, reactivity, and structure of thiaferroles Fe//2(R//2C//4H//2S)(CO)//6 and benzothiaferroles Fe//2(C//8H//6S)(CO)//6(1) are surveyed. These compounds are prepared by the reaction of Fe//3(CO)//1//2 with thiophenes and benzothiophenes. The be
- Ogilvy,Draganjac,Rauchfuss,Wilson
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p. 1171 - 1177
(2008/10/08)
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- Birch-Type Reductions in Aqueous Media: Benzothiophene and Diphenyl Ether
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The preparative cathodic reductions of benzothiophene and diphenyl ether were investigated.Aqueous and mixed organic-aqueous solutions of tetrabutylammonium salts served as electrolytes, and the cathode was a mercury pool.Experiments were carried out at a constant current and the products were isolated and identified.It was found that benzothiophene is initially reduced to 2,3-dihydrobenzothiophene, which is subsequently reductively cleaved to 2-ethylbenzenethiol.If the amount of charge transferred is controlled, it is possible to form the dihydro derivative or the cleaved benzenethiol as major product.The cathodic product of diphenyl ether was found to be a mixture of bis(2,5-dihydrophenyl) ether (ca.50percent) and phenol (ca.30percent).The first product is of particular interest since phenol and benzene are the only products of diphenyl ether obtained by reductions involving solvated electrons.The mechanism of reduction of the two reactants is discussed, and possible pathways for the formation of the various products are proposed.
- Pacut, Ryszard I.,Kariv-Miller, Essie
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p. 3468 - 3470
(2007/10/02)
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