Thiophenes homogeneous hydrogenation

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]. 

Furthermore, the unusually high stability of these Ir(in) hydrido-thiophene complexes allowed some interesting studies to be carried out, involving the transformation of the coordinated thiophene through intramolecular migration of a coordinated hydride to the C=C bond (Eq. 2.6). This led to a particularly rare example of the mild homogeneous hydrogenation of thiophene to tetrahydrothiophene described in detail in Chapter 3. 

Asymmetric homogeneous hydrogenation of a thiophene derivative. 2-(4-Methoxy-5-phenyl-3-tbienyl)acrylic acid (1) on hydrogenation with Kagan s catalyst (4,273) is converted into (+)-2-(4-methoxy-5-phenyl-3-thienyl)pro-pionic acid (2) in 97% yield and with an optical purity of 88%. 

Scheme 3 The mechanism of homogeneous hydrogenation of thiophene by [irH2(PPh3)2( S-T)2] -...

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 aqueous-biphase hydrogenation reactions of thiophenes to the corresponding cyclic thioethers have been shown to be mechanistically similar to those in truly homogenous phase. 

In the heterocyclic series several heteroarylalanines have been obtained by asymmetric hydrogenation using chiral homogeneous catalysts. For example, replacement of the phenyl ring by furan, thiophene, selenophene, pyridine, or indole yields fiirylalanines, thienyManines, 2- and 3-selenienyManines,... 

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>. 

An jj (S) equilibrium (equation 6) is observed in the benzo[fc]thiophene (BT) complexes Cp (CO)2Re(BT), where Cp is Cp or Cp. As for selenophene, the Cp ligand favors j -BT, while Cp favors the rj (S) form it should be noted that 2-MeBT and 3-MeBT form only the jj (S) complexes. The formation of the r] form from the is important for understanding the first step in the HDS of BT, which gives dihydrobenzothiophene (equation 7). This alkene hydrogenation reaction is catalyzed by homogeneous catalysts and may occur similarly on heterogeneous HDS catalysts. While there is no evidence that there is an r] (S) equilibrium in thiophene complexes, both thiophene species may exist on a catalyst surface. Low metal oxidation states would favor r] coordination, which could lead to the initially observed alkene hydrogenation products. 

Irrespective of the phase system (homogeneous, aqueous biphasic, heterogeneous single-site), the hydrogenation mechanisms of thiophene (T) or BT catalyzed by metal complexes comprise the usual steps of H2 oxidative addition, f-C,C coordination of the substrate, hydride transfer to form dihydrobenzothienyl, and elimination of DHBT by hydride/dihydrobenzothienyl reductive coupling (Scheme 2) [8],... 

However, in contrast to heterogeneous hydrogenation, homogeneous reduction has attracted considerable attention. Furthermore, the coordination and reactivity of thiophenes on transition metal centers continue to attract considerable attention as model chemistry for the species and reactions that occur during heterogeneous hydrodesulfurization (HDS) of petroleum and other fossil fuels. 

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