Cationic rhodium catalysts hydrogenation

Based on these preliminary findings, related couplings to pyruvates and iminoacetates were explored as a means of accessing a-hydroxy acids and a-amino acids, respectively. It was found that hydrogenation of 1,3-enynes in the presence of pyruvates using chirally modified cationic rhodium catalysts delivers optically enriched a-hydroxy esters [102]. However, chemical yields were found to improve upon aging of the solvent 1,2-dichloroethane (DCE), which led to the hypothesis that adventitious HC1 may promote re-... 

In less-coordinating solvents such as dichloromethane or benzene, most of the cationic rhodium catalysts [Rh(nbd)(PR3)n]+A (19) are less effective as alkyne hydrogenation catalysts [21, 27]. However, in such solvents, a few related cationic and neutral rhodium complexes can efficiently hydrogenate 1-alkynes to the corresponding alkene [27-29]. A kinetic study revealed that a different mechanism operates in dichloromethane, since the rate law for the hydrogenation of phenyl acetylene by [Rh(nbd)(PPh3)2]+BF4 is given by r=k[catalyst][alkyne][pH2]2 [29]. 

Cationic iridium and rhodium catalysts are also effective for the hydrogenation of exocyclic olefmic alcohols (see Table 21.5), except for 2-exomethylenecy-clohexanol and 2-methylenecyclohexanemethanol (entries 2 and 3). In entry 4, a cationic rhodium catalyst gave a single product whilst a cationic iridium catalyst induced only modest selectivity (72 28). 

In entries 10-13 (Table 21.8) of trisubstituted alkenes, very high diastereo-selectivity is realized by the use of a cationic rhodium catalyst under high hydrogen pressure, and the 1,3-syn- or 1,3-anti-configuration naturally corresponds to the ( )- or (Z)-geometry of the trisubstituted olefin unit [48, 49]. The facial selectivity is rationalized to be controlled by the A(l,3)-allylic strain at the intermediary complex stage (Scheme 21.2) [48]. 

In the case of tri-substituted alkenes, the 1,3-syn products are formed in moderate to high diastereoselectivities (Table 21.10, entries 6—12). The stereochemistry of hydrogenation of homoallylic alcohols with a trisubstituted olefin unit is governed by the stereochemistry of the homoallylic hydroxy group, the stereogenic center at the allyl position, and the geometry of the double bond (Scheme 21.4). In entries 8 to 10 of Table 21.10, the product of 1,3-syn structure is formed in more than 90% d.e. with a cationic rhodium catalyst. The stereochemistry of the products in entries 10 to 12 shows that it is the stereogenic center at the allylic position which dictates the sense of asymmetric induction... 

The trianionic cobalt catalyst has been successfully employed in the hydrogenation of 1,3-butadiene in [bmim][BF4] [10], The product from this reaction is 1-butene which is formed with 100% selectivity. Unfortunately the catalyst undergoes a transformation to an inactive species during the course of the reaction and reuse is not possible. The cationic rhodium catalyst together with related derivatives have been used for numerous reductions, including the hydrogenation of 1,3-cyclohexadiene to cyclohexane in [bmim][SbF6] [11],... 

One last remark concerning the two catalysts we have discussed in more detail, cationic rhodium catalysts and the neutral chloride catalyst of Wilkinson. The difference of the catalytic system discussed above from that of the Wilkinson catalyst lies in the sequence of the oxidative addition and the alkene complexation. The hydrogenation of the cinnamic acid derivative involves a cationic catalyst that first forms the alkene complex the intermediate alkene (enamide) complex can be observed spectroscopically. 

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... 

Hydrogenation of a mixture of styrenes ArCH=CH2 (or reactive alkenes, such as norbornene or ethylene) and symmetric or mixed carboxylic anhydrides [(RC0)20 or (RCO)O(COR )] in the presence of cationic rhodium catalysts ligated by triphenylar-sine (Ph3As), generates hydroacylation products ArCH(Me)COR as single regioiso-mers in high yields.108... 

Introduction. Homogeneous catalytic hydrogenation with cationic rhodium catalysts has been extensively explored by Schrock and Osborn. Use of these complexes in stereoselective organic synthesis has been a topic of more recent interest, and has been recently reviewed. The reagent of choice for many of these directed hydrogenations has continued to be [Rh(nbd)(dppb)]BF4 (1). 

The ferrocenylbisphosphines 8f—h bearing the amino pendant side chain are unique ligands that effect the rhodium-catalyzed asymmetric hydrogenation of tetrasubstituted olefms 57 (Scheme 2-43) [61]. Thus, the hydrogenation of a-aryl-acrylic acid 57a in the presence of a cationic rhodium catalyst coordinated with 8g gives a quantitative yield of carboxylic acid 58a with 98.4% ee. Other tetra-... 

Table 2. Hydrogenation of methyl-acetamidecinnamate [MAC] with free and immobilized cationic rhodium catalysts...

Asymmetric hydrogenation. This diphosphinite has been incorporated into a cationic rhodium catalyst (2) by reaction of 1 with chloronorbornadienerho-dium(I) dimer, [(NBD)RhCl]2, and AgPFe in acetone. Use of 2 in the hydrogenation of a-acetamidoacrylic acids and esters results in amino acids with the natural... 

C—C bridge. This phosphine is a remarkable ligand for asymmetric hydrogenation of olefins with cationic rhodium catalysts and ketones with ruthenium (11) catalysts. 

Other protein-rhodium conjugates containing cationic rhodium catalysts have also been prepared using bis(diphenylphosphino-ethyl)amino derivatives 5-7 and solutions of these bis(phosphine) ligands in the presence of carbonic anhydrase, a-chymotrypsin and bovine serum albumin (47). However, the exact nature of the complexes formed has not been discerned in any of these cases, and these latter enzyme-transition metal complexes evidently do not exhibit enantioselectivity in hydrogenation of a-acetamidoacrylic acid. 

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