Hydrogenation of Alkenes, Ketones, and Imines

In the hydrogenation of alkenes, rhodium-, ruthenium- and iridium-phosphine catalysts are typically used [2-4]. Rhodium-phosphine complexes, such as Wilkinson s catalyst, are effective for obtaining alkanes under atmospheric pres- 

Copyright 2007 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31161-3 

Major diastereomer Catalyst mol% PHj Solvent Temp. Diastereo- TOF Reference 

Catalyst mol% Ph2 Solvent Temp. [C] Diastereo- TOF meric ratio Reference 

---

The catalytic, asymmetric hydrogenations of alkenes, ketones and imines are important transformations for the synthesis of chiral substrates. Organic dihydropyridine cofactors such as dihydronicotinamide adenine dinucleotide (NADH) are responsible for the enzyme-mediated asymmetric reductions of imines in living systems [86]. A biomimetic alternative to NADH is the Hantzsch dihydropyridine, 97. This simple compound has been an effective hydrogen source for the reductions of ketones and alkenes. A suitable catalyst is required to activate the substrate to hydride addition [87-89]. Recently, two groups have reported, independently, the use of 97 in the presence of a chiral phosphoric acid (68 or 98) catalyst for the asymmetric transfer hydrogenation of imines. 

The catalytic hydrogenation of alkenes, ketones, and imines is arguably one of the most important transformations in chemistry. Powerful asymmetric versions have been realized that require metal catalysts or the... 

This chapter describes the results on hydrogenation of several families of substrates including alkenes, ketones and imines with Rh, Ru, Ir and Pd complexes bearing P-stereogenic ligands. In addition, a section with a brief... 

Asymmetric hydrogenation (of alkenes, ketones, imines, amides, aromatics and heteroaromatics) ... 

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... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

Imines can be saturated in preference to the hydrogenation of aldehydes, ketones or nitriles and the hydrogenolysis of benzyl ethers and amines. Alkenes, alkynes and nitro groups are, however, usually hydrogenated in preference to an imine. 

Ligand-metal bifunctional catalysis provides an efficient method for the hydrogenation of various unsaturated organic compounds. Shvo-type [83-85] Ru-H/OH and Noyori-type [3-7] Ru-H/NH catalysts have demonstrated bifimctionality with excellent chemo- and enantioselectivities in transfer hydrogenations and hydrogenations of alkenes, aldehydes, ketones, and imines. Based on the isoelectronic analogy of H-Ru-CO and H-Re-NO units, it was anticipated that rhenium nitrosyl-based bifunctional complexes could exhibit catalytic activities comparable to the ruthenium carbonyl ones (Scheme 29) [86]. 

Neutral diphosphine complexes, usually with chloride as coordinating anion, are preferentially used for the hydrogenation of various ketones but are also applied for the hydrogenation of a-dehydroamino acid derivatives as well as some imines. Since most of the alkene hydrogenations are carried out in MeOH, it is likely that the chloride dissociates, and that also here, the cationic species is the active catalyst. [cp Rh Uiy2l2 2 is applied for the transfer hydrogenation of C=0 and C=N moieties. 

Since the discovery of the Wilkinson catalyst, most of the work on hydrogenation has been carried out with functionalised alkenes as substrates and Rh(I) complexes as catalytic precursors. These hydrogenations are discussed in the next sections. There are also a few results on hydrogenation of ketones and imines, described in Section 7.2.3. 

In summary, the most popular hydrogen donors for the reduction of ketones, aldehydes and imines are alcohols and amines, while cyclic ethers or hydroaromatic compounds are the best choice for the reduction of alkenes and alkynes. 

---