Hydrogenation, catalytic thiophene derivative

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>. 

When 111a is treated with CO or Ce(IV)salts, cleavage of the PRj group takes place with almost quantitative formation of the thiophene derivative 112 (Scheme 39). Under catalytic hydrogenation conditions with Raney nickel, ring contraction via selective sulfur extrusion to the bicy-clo[2.1.1]hexene 113 occurs. - ... 

Few examples of catalytic hydrogenation of partially saturated thiophene derivatives are known, because sulfur normally poisons the catalyst. Nevertheless, hydrogenations are sometimes possible when a larger amount of catalyst is used. One example, with a high degree of diastereoselectivity, is hydrogenation of the biotin precursor l18. 

There are several different ways in which a lactam precursor can be generated. An interesting but relatively specific synthesis is shown in reaction 8 and employs thiophene derivatives. Catalytic hydrogenation of bicyclic thiophene 2.69 gave 2-elhylhexahydroazepine-2-one [2.70) and acid hydrolysis gave 2-ethyl-7-amino-hexanoic acid, 2.77.33... 

Similar procedures to those used for the catalytic hydrogenation of furan and thiophene can usually be applied to the functional derivatives of these heterocycles. 

Tetraphenylthieno[3,4-c]thiophene (6) and the thieno[3,4-c]pyrrole (12) were reduced catalytically to the corresponding cis dihydro derivatives (24 X = S, NMe). The addition of hydrogen occurred in cis fashion across the 1,3-positions in the thiophene system (Scheme 46). Both thienopyrroles (12) and (13) were resistant to chemical reduction (75ACR139). 

Benzo[fr]thiophene 1,1-dioxide and its derivatives are reduced to the corresponding 2,3-dihydro derivatives by catalytic hydrogenation in good yields (Scheme 94). In this reduction, the sulfonyl moiety remains unchanged [13,189, 238, 239]. However, reduction of benzo[ ]thiophene 1,1-dioxide with LiAlH4 affords deoxygenated 2,3-dihydrobenzo[ ]thiophene in 79% yield [233]. Benzo[fr]thiophene 1,1-dioxide was also reduced to the 2,3-dihydro derivative by an electrochemical method (Scheme 95) [240]. 

Heterocyclic Compounds. Such materials undergo catalytic hydrogenation to yield the corresponding saturated derivatives. Thus, pyrrole is slowly converted to pyrrolidine at 200 C over either a nickel or copper-chromium oxide catalyst pyridine and pyridine derivatives behave similarly. Compounds such as furan and dihydropyran reduce rapidly and behave more like olefins in reactivity. Similarly, thiophene is converted to the tetrahydro derivative. 

It is tempting to speculate whether the role of promoter HDS metals (Co, Ni) is the C-S activation of the thiophenes whereas the catalytic metal (group 6, Mo, W) would be responsible for the transfer of hydrogen to the activated substrate. This is a very likely pathway to be envisaged on metal sulfide surfaces where sulfides or thiolates bonded to two or more metal atoms are plentiful. However, this type of interaction is obviously unavailable in single-site complexes, and this could explain why ring opening is frequent but complete desulfurization is rare on mononuclear derivatives. 

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