Rhodium catalysts alkene hydrogenation, chiral

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]. 

Asymmetric Hydrogenation. Asymmetric hydrogenation with good enantio-selectivity of unfunctionalized prochiral alkenes is difficult to achieve.144 145 Chiral rhodium complexes, which are excellent catalysts in the hydrogenation of activated multiple bonds (first, in the synthesis of a-amino acids by the reduction of ol-N-acylamino-a-acrylic acids), give products only with low optical yields.144 146-149 The best results ( 60% ee) were achieved in the reduction of a-ethylstyrene by a rhodium catalyst with a diphosphinite ligand.150 Metallocene complexes of titanium,151-155 zirconium,155-157 and lanthanides158 were used in recent studies to reduce the disubstituted C—C double bond with medium enantioselectivity. 

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... 

The cationic rhodium catalysts are useful for asymmetric hydrogenation.152 In this variant, the presence of a chiral phosphine leads to differences in the rates of H2 addition to the two faces of a prochiral alkene. Where the alkene has groups such as C02Me suitably placed to bind to the metal, the selectivity can become very great enantiomeric excesses of the product over its enantiomer can reach 95-98% (equation 67). The mechanism has recently been elucidated by Halpern.153... 

Asymmetric hydrogenation of alkenes is efficiently catalysed by rhodium complexes with chiral diphosphite and diphosphoramidite ligands derived from BINOL or diphenylprolinol. Choice of a proper achiral backbone is crucial.341 Highly enantioselective hydrogenation of A-protected indoles was successfully achieved by use of the rhodium catalyst generated in situ from [Rh(nbd)2]SbF6 (nbd = norborna-2,5-diene)... 

Potential difference in reactivity between two G-B bonds allowed the transformation of l,2-bis(boryl)-l-alkenes to 1-alkenylboranes via a cross-coupling with the aryl, 1-alkenyl, benzyl, and cinnamyl halides (Equation (23)).211-213 This tandem procedure synthetically equivalent to a yy/z-carboboration of alkynes was used for synthesizing Tamoxifen derivatives via stepwise double coupling with two of the G-B bonds.212,213 Hydrogenation of the resulting bisborylalk-enes with a chiral rhodium catalyst is synthetically equivalent to an asymmetric diboration of alkenes (Equation (24)).214... 

Hydrogenation of simple alkenes in high enantiomeric excesses has not been attained with the chiral rhodium catalysts, but a fused cyclopentadienyl ligand with C2 symmetry has been used in enantiose-lective titanocene-catalyzed hydrogenation of alkenes. Thus 2-phenyl-1-butene (103) was hydrogenated in 96% ee. 

The highly developed enantioselective hydrogenation of prochiral functionalized alkenes using chiral phosphine complexes of ruthenium or rhodium as catalysts has become very common in academic laboratories as well as in industry... 

When steric hindrance inhibits hydrogenation on one face of a double bond, addition will take place exclusively to the less hindered face. This principle has been used to develop enantioselective or so-called asymmetric hydrogenation. The process employs homogeneous (soluble) catalysts, consisting of a metal, such as rhodium, and an enantiopure chiral phosphine ligand, which binds to the metal. A typical example is the Rh complex of the diphosphine (R,R)-DIPAMP (margin). After coordination of the alkene double bond and a molecule of H2 to rhodium, hydrogenation occurs via syn addition, just as in the case of insoluble metal catalysts. 

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

Following Wilkinson s discovery of [RhCl(PPh3)3] as an homogeneous hydrogenation catalyst for unhindered alkenes [14b, 35], and the development of methods to prepare chiral phosphines by Mislow [36] and Horner [37], Knowles [38] and Horner [15, 39] each showed that, with the use of optically active tertiary phosphines as ligands in complexes of rhodium, the enantioselective asymmetric hydrogenation of prochiral C=C double bonds is possible (Scheme 1.8). 

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