Hydrogenation of thiophene

The chemical shifts of monosubstituted thiophenes relative to the a- and )8-hydrogens of thiophene at infinite dilution in cyclohexane are given in Table I and are discussed in the following. 

In the design of a homogeneous catalyst for the plain hydrogenation of thiophenes it is necessary to take into account that, unlike simple alkenes, (102) and (103) are polyfunctional ligands which can bind metal centers in a variety of bonding modes, often in a rapid equilibrium with each other.166-172,192 Among the possible coordination modes, the /-(S) and the 772-(C,C)... 

The use of water-soluble metal catalysts for the hydrogenation of thiophenes in aqueous biphasic systems has been primarily introduced by Sanchez-Delgado and coworkers at INTEVEP S.A. [61]. The precursors RuHC1(TPPTS)2(L2) (TPPTS=triphenylphosphine trisulfonate L=aniline, 1,2,3,4-tetrahydroquinoline) and RuHC1(TPPMS)2(L2) (TPPMS=triphenylphosphine monosulfonate) were... 

Studies on heterogeneous catalysts seem to invoke partial hydrogenation of thiophene prior to desulfurization [42] the catalysts are also active hydrogenation catalysts. Recently evidence for a facile and selective desulfurisation of partly hydrogenated thiophene has been reported, the reaction of 2,5-dihydrothiophene on (110) molybdenum surfaces (Figure 2.41) [43]. 

Catalytic hydrogenation of thiophene poses a problem since noble metal catalysts are poisoned, and Raney nickel causes desulfurization. Best catalysts proved to be cobalt polysulfide [425], dicobalt octacarbonyl [426], rhenium heptasulfide [5i] and rhenium heptaselenide [54]. The last two require high temperatures (230-260°, 250°) and high pressures (140, 322 atm) and give 70% and 100% of tetrahydrothiophene (thiophane, thiolene), respectively. 

Many of these studies utilized noble metals such as Ir, Os, Rh, Ru, or Re, whereas others used more conventional metals such as Mn, Fe, Mo, or Co. The particular metal on which the observations were made is not important at this point. What is important is that all of the important steps required for direct sulfur removal and hydrogenation of thiophene and more condensed derivatives have been shown to occur with soluble metal complexes. Thus, organometallic complex chemistry can be of great value in elucidating the mechanisms involved in conventional HDS processes and perhaps can point the way to improved catalyst formulations. 

The steric effect exerted by a substituent depends on whether it is attached to the a- or / -position of the thiophene ring. A substituent at the / -position, if it makes the same bond angles as the /3-hydrogen of thiophene (see 1), is flanked by two carbon atoms and their substituents and will make an angle of 124° with each of the adjacent ring carbon-carbon bonds. A substituent at an a-position is adjacent to only one ring carbon-carbon bond it... 

Figure 22.6 Hydrogenation of thiophene over W2C supported on zeolite Y.

Other examples of SIMS for surface analyses are studies of CU2S-CdS solar-cell samples (101) and the study of chemisorbed species on inorganic substrates such as methanol on Cu (100) and titania (102). De Pauw s studies of such adsorbed systems may prove to be valuable in determining the mechanism of catalytic reactions on surfaces (103). Winograd and co-workers (104-6) have studied chemisorption on metal surfaces, using SIMS. In a related study (107), Unger et al. have used molecular SIMS to study the reactions of thiophene on a silver surface. They observed the self-hydrogenation of thiophene on the sur-... 

Temperature Increase Dynamics after the First Cycle. As with the start up of the bed, subsequent temperature cycles resulted in the formation of a mild hot spot. The occurrence of this temperature fluctuation is undesirable since the past history of the catalyst may be altered. The adsorption of thiophene upon the active hydrogenation sites was assumed to be irreversible and therefore unaffected by temperature. However, as will become apparent later, the effect of temperature may have altered the poison coverage/or profile. Lyubarski, et. al [73 determined that, as a result of the hydrogenation of thiophene and subsequent hydrogenolysis to butane, the adsorption capacity of a suported Ni... 

Of the three possible substrates, thiophene, benzo[ ]thiophene, and dibenzo[, thiophene, benzo[ ]thiophene is the most easily hydrogenated to the dihydro derivative this is ascribable to the more pronounced olefinic character of the C(2)-C(3) double bond in benzo[, ]thiophene as compared to that in thiophene. There is no example in the literature of the hydrogenation of dibenzothiophene either to the tetrahydro or the hexahydro stage. The hydrogenation of benzol ]thiophene is catalyzed by transition metals such as Ru, Os, Rh, and Ir. An excellent overview of homogeneous catalytic hydrogenation of thiophenic substrates has been presented recently <2004JOM (689)4277>. 

Ionic hydrogenation of thiophenes and some other heterocycles... 

Thiophenium ions not only play a key role in electrophilic substitution reactions but they also allow some addition reactions to be possible, such as the ionic hydrogenation of thiophene derivatives proceeding by repeated protonations and additions of hydride ion (Scheme 16) (73IZV1918, 75T311, 78T1703, 79MI1). 

However, fi adsorption on catalysts is also supported by deuterium exchange studies of thiophene. When thiophene and D2 are passed over Mo-based HDS catalysts at temperatures below which substantial HDS occurs, the thiophene protons exchange with deuterium. This exchange occurs much more rapidly at the 2- and 5-positions than at the 3- and 4-sites. When CpRu(jj -T)+ (equation 9) is dissolved in CD3OD in the presence of OD at 23 °C, the 2,5-hydrogens of thiophene... 

More recently, several milder reducing conditions have been described (equations 19-21). Noteworthy is the cis hydrogenation of thiophene (10) performed by Confalone and coworkers in a synthesis of the vitamin biotin (equation 20). ... 

For this ionic hydrogenation of thiophenes, Kursanov and coworkers have proposed the pathway shown in Scheme 4, based on the labeling results obtained for the reduction of 2,5-dimethylthiophene. ... 

In fluid-solution systems, the plain hydrogenation of thiophenes to thioether products has been found to be catalyzed by various transition metal complexes surprisingly, none of these catalysts contains either molybdenum or tungsten, which are essential components of heterogeneous HDS catalysts [1 b]. Unpromoted M0S2, on the other hand, is quite active for the HDS of thioethers with no need of assistance by a late transition metal promoter [lb]. 

As a general trend, the homogeneous hydrogenation of thiophenes to thioethers is catalyzed by complexes that are not sterically demanding with relatively electrophilic metal centers (e. g., d metal ions such as Rh , Ir , Ru , or Os ). This is because moderate electron density and low steric hindrance at the metal favor the rf--C,C coordination mode of the thiophene (precursor to hydride migration) over rj -S binding (precursor to C-S bond cleavage) [7-10]. 

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