Homogeneous rhodium catalyzed alkene hydrogenations

Polymeric Cofactors for Homogeneous Rhodium(I) Catalyzed Alkene Hydrogenations... 

In 1968, Knowles et al. [1] and Horner et al. [2] independently reported the use of a chiral, enantiomerically enriched, monodentate phosphine ligand in the rhodium-catalyzed homogeneous hydrogenation of a prochiral alkene (Scheme 28.1). Although enantioselectivities were low, this demonstrated the transformation of Wilkinson s catalyst, Rh(PPh3)3Cl [3] into an enantioselective homogeneous hydrogenation catalyst [4]. 

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... 

Related catalyst. A related homogeneous rhodium catalyst, RhHCI[Tj -CsCCHslellPICeHsIa], has recently been reported to catalyze hydrogenation of benzene to cyclohexane, also without intermediate formation of cyclohexadienes or cyclohexene. However, this catalyst functions in the absence of an added base. It also has high catalytic activity for hydrogen transfer from secondary alcohols to alkenes and alkadienes and is recovered unchanged after the transfer. ... 

The complex has enjoyed relatively little use in organic synthesis. For iridium-catalyzed homogeneous hydrogenation of alkenes, Crabtree s iridium complex ((1,5-Cycloocta-diene)(tricyclohexylphosphine)(pyridine)iridium(I) Hexafluoro-phosphate) is generally preferred, although this readily prepared Ir complex is active. It is more reactive than its rhodium counterpart in the catalytic isomerization of butenyl- to allylsilanes. ... 

Whereas water-soluble rhodium complexes catalyze the biphasic hydrogenation of various unsaturated substrates but rapidly form colloids, the addition of yd-CD has been shown to lead to efficient homogeneous catalytic systems [40]. For instance, in the absence of transfer agent, as reported by Larpent et al. [41], maleic acid is completely transformed in 83 h instead of a reaction time of 17 h with yd-CD. Similarly, the hydrosilylation reaction of an alkene catalyzed by a platinum precursor can be notably accelerated by the presence of yd-CD the case of a Lamoreaux Pt Pl catalyst is more spectacular since the yield can reach 100% in 12 min, whereas no transformation of the substrate occurs in 24 h in the absence of yd-CD [42]. 

Table 15.2. A comparison of turnover frequencies for the homogeneous hydrogenation of different alkenes catalyzed by rhodium, ruthenium, and iridium catalysts.

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