Alkenes transfer hydrogenation

Consequently, it is I ICo(CN)s3 that functions as a catalyst in hydrogenation processes. In the first step of the process shown in Figure 22.9, the alkene coordinates to HCo(CN)s3 as one hydrogen atom is added to the molecule so that only one double bond remains. The monoene is bonded to the cobalt in rf fashion. In the second step, another HCo(CN)53- transfers hydrogen to the alkene, which undergoes reductive elimination and leaves, having been converted to 1-butene. 

Directed intramolecular transfer hydrogenations are catalyzed by rhodium complexes with the pendant alkene acting as an internal sacrificial olefin (Equation (39)). 

A few remarkable, but rather uncommon, transfer hydrogenations also deserve mention within the context of this chapter namely, the reduction of alkynes to alkenes using a chromium catalyst, and the reduction of double bonds using diimines. 

Transfer Hydrogenation including the Meerwein-PonndotfVeriey Reduction Table 20.4 Reduction of alkynes to trans-alkenes by chromous sulfate. 

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

The TEAF system can be used to reduce ketones, certain alkenes and imines. With regard to the latter substrate, during our studies it was realized that 5 2 TEAF in some solvents was sufficiently acidic to protonate the imine (p K, ca. 6 in water). Iminium salts are much more reactive than imines due to inductive effects (cf. the Stacker reaction), and it was thus considered likely that an iminium salt was being reduced to an ammonium salt [54]. This explains why imines are not reduced in the IPA system which is neutral, and not acidic. When an iminium salt was pre-prepared by mixing equal amounts of an imine and acid, and used in the IPA system, the iminium was reduced, albeit with lower rate and moderate enantioselectivity. Quaternary iminium salts were also reduced to tertiary amines. Nevertheless, as other kinetic studies have indicated a pre-equilibrium with imine, it is possible that the proton formally sits on the catalyst and the iminium is formed during the catalytic cycle. It is, of course, possible that the mechanism of imine transfer hydrogenation is different to that of ketone reduction, and a metal-coordinated imine may be involved [55]. 

Hydrogen donors that function poorly with homogenous catalysts include hydrazine hydrate, alkenes (e.g., cyclohexene), and ascorbic acid. This is somewhat surprising as they can be very effective in heterogeneous transfer hydrogenation. 

A recent development is the transfer hydrogenation of heterocyclic systems such as pyrrole, pyridinium and quinoline systems. Whilst at present the yields and enantioselectivities are modest, further development may improve this situation. For example, 1-methyl-isoquinoline has been reduced to the tetrahydro species and 1-picoline has been reduced to 1-methylpiperidine [86]. Interestingly, these reductions involve alkene as well as imine reduction. 

Fig. 35.8 Optical activities achieved by enantioselective transfer hydrogenation of alkenes.

Brunner, Leitner and others have reported the enantioselective transfer hydrogenation of alpha-, beta-unsaturated alkenes of the acrylate type [50]. The catalysts are usually rhodium phosphine-based and the reductant is formic acid or salts. The rates of reduction of alkenes using rhodium and iridium diamine complexes is modest [87]. An example of this reaction is shown in Figure 35.8. Williams has shown the transfer hydrogenation of alkenes such as indene and styrene using IPA [88]. 

In total, over the past six years, the chelating P,N-ligands have shown considerable promise in a variety of enantioselective processes, including transfer-hydrogenation and hydrosilylation of ketones, hydroboration of alkenes, conjugate addition to enones and Lewis-acid catalysed Diels-Alder reactions, in addition to those described above.128,341 It is anticipated that this list will continue to grow, and... 

Keywords Alcohols Alkenes Asymmetric transfer hydrogenation C-alkylation Imines Ketones W-aUcylation Oxidation Reduction Transfer hydrogenation... 

A large number of reports have concerned transfer hydrogenation using isopropanol as donor, with imines, carbonyls-and occasionally alkenes-as substrate (Scheme 3.17). In some early studies conducted by Nolan and coworkers [36], NHC analogues of Crabtree catalysts, [Ir(cod)(py)(L)]PF,5 (L= Imes, Ipr, Icy) all proved to be active. The series of chelating iridium(III) carbene complexes shown in Scheme 3.5 (upper structure) proved to be accessible via a simple synthesis and catalytically active for hydrogen transfer from alcohols to ketones and imines. Unexpectedly, iridium was more active than the corresponding Rh complexes, but... 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

A unique example,80 the reduction via transfer hydrogenation of aryl-substituted alkenes, is facilitated by A1C13 ... 

As mentioned earlier (see Section 11.1.4), cycloalkenes (primarily cyclohexene) can be used to carry out transfer hydrogenations. Similar processes during which the alkene itself serves as hydride ion donor may be observed under ionic hydrogenation conditions 223... 

The hydrogenation step is stereospecific and transfers hydrogen in the suprafacial manner. For example, alkynes are converted to m-alkenes ... 

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