Benzothiophene hydrogenation

Hydrogenation of benzothiophene (BT) to 2,3-dihydrobenzothiophene (DHBT) takes place much more readily than reduction of thiophene, since the C(2)=C(3) bond in this molecule has more of a localized double bond character, instead of the pseudoaromatic nature of an isolated thiophene ring or of a heterocycle in a fused-ring system like dibenzothiophene and its derivatives. As shown by the data in Table 3.2, several Ru, Os, Rh, and Ir complexes are known to be very efficient catalysts for the reduction of BT to DHBT under moderate reaction conditions [74-86]. 

The kinetics and other experimental findings disclosed in these two papers led to a generic catalytic cycle for both systems, which is schematically represented in Fig. 

The very large solvent effect observed in these reactions (rates of the order of 10 times faster in dichloroethane than in 2-methoxyethanol ) has been interpreted in terms of solvation effects, which in the case of 2-methoxyethanol can prevent the binding of the substrate to a considerable extent. This would not be the case for the much more poorly coordinating 1,2-dichloroethane, once again pointing to the importance of vacant or readily available coordination sites for efficient catalysis to occur. 

Some general key points, common to all the well understood catalysts, that merit further comment are the following  

Saturation of the benzene ring has never been detected in solution when using such precursors. In the case of Cp Rh(NCMe) some H-D exchange has been observed in the 7-position of dihydrobenzothiophene, and this has been taken as indicative that the hydrogenated product remains bonded to the metal in a ri -coordination mode. This latter point seems to be a characteristic of this particular complex and not a general trend that can be related to active sites in HDS catalysts. 

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The mechanism of benzothiophene hydrogenation by [(triphos)Ir(H)(NCMe)JBF4. 

Aqueous-biphasic and solid-supported catalysts for benzothiophene hydrogenation as a pretreatment for HDS... 

Subsequently, Paquette and Johnson used LAH reductions to convert strained thietane or thiolane derivatives to their respective sulphides, generally in good yields. Whitney and Cram described the LAH reduction of a chiral derivative of benzothiophene sulphone, as outlined in equation (24). The authors also noted the formation of hydrogen gas and suggested that their results were consistent with those of Bordwell as outlined in equation (22), namely that reduction takes place by the formation of an aluminium oxide and hydrogen gas. In this case, the reduction clearly cannot involve the formation of an a-sulphonyl carbanion and it is unlikely that any C—S bond cleavage and reformation could have occurred. 

In 2006, Berens et al. reported the synthesis of novel benzothiophene-based DuPHOS analogues, which gave excellent levels of enantioselectivity when applied as the ligands to the asymmetric rhodium-catalysed hydrogenation of various olefins, such as dehydroamino acid derivatives, enamides and itaco-nates (Scheme 8.10). ... 

Scheme 8.10 Hydrogenations of olefins with benzothiophene-based DuPHOS analogue ligands.

Thiophene is the typical model compound, which has been extensively studied for typifying gasoline HDS. Although, some results are not completely understood, a reaction network has been proposed by Van Parijs and Froment, to explain their own results, which were obtained in a comprehensive set of conditions. In this network, thiophene is hydrodesulfurized to give a mixture of -butenes, followed by further hydrogenation to butane. On the considered reaction conditions, tetrahydrothiophene and butadiene were not observed [43], The consistency between the functional forms of the rate equations for the HDS of benzothiophene and thiophene, based on the dissociative adsorption of hydrogen, were identical [43,44], suggesting equivalent mechanisms. 

Scheme 52 explains the [(Cp )Rh(MeCN)3]2+-assisted regioselective hydrogenation of pyridines, benzoquinolines, acridines as well as indoles and benzothiophene.258 The relative hydrogenation rates were attributed to both electronic and steric effects, the rate generally decreasing with increasing basicity and steric hindrance at the nitrogen atom. 

Oxidation of thiophene and its derivatives was studied using hydrogen peroxide (H2O2), t-butyl-hydroperoxide and Ti-Beta redox molecular sieve as selective oxidation catalysts. A new reaction pathway was discovered and investigated using C-13 NMR, GC, GC-MS, HPLC, ion chromatography, and XANES. The thiophene oxidized to thiophene-sesquioxide [3a,4,7,7a-tetrahydro-4,7-epithiobenzo[b]-thiophene 1,1.8-trioxide] and the sesquioxide oxidized mostly to sulfate. 2-Methyl-thiophene and 2,5 dimethylthiophene also oxidized to sulfate and sulfone products. The Benzothiophene oxidation product was sulfone. This proposed new reaction pathway is different from prior literature, which reported the formation of thiophene 1,1-dioxide (sulfone ) as a stable oxidation product... 

Lin, S.D., and C. Song, Noble metal catalysts for low-temperature naphthalene hydrogenation in the presence of benzothiophene. Catal. Today, 31, 93-104 (1996). 

Several heteroaromatic compounds can be hydrogenated by [Rh(COD) (PPh3)2]+ species. Thus, this cationic complex has been reported to be a catalyst precursor for the homogeneous hydrogenation of heteroaromatic compounds such as quinoline [32] or benzothiophene [33]. Detailed mechanistic cycles have been proposed by Sanchez-Delgado and coworkers. The mechanism of hydrogenation of benzothiophene by the cationic rhodium(III) complex, [Rh(C5Me5) (MeCN)3]2+, has been elucidated by Fish and coworkers [34]. 

Electron donation by potential carcinogens, such as 2-acetylaminodi-benzothiophene, has been estimated from the strength of their charge-transfer complexes with chloranil in acetonitrile. - In this context it should be noted that the hydrogen bonding of phenol to the 77-electrons of dibenzothiophene has been studied and that a thiourea adduct has proved useful in the removal of dibenzothiophene from oil. - ... 

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