Acrylic esters asymmetric hydrogenation

Asymmetric hydrogenation has been achieved with dissolved Wilkinson type catalysts (A. J. Birch, 1976 D. Valentine, Jr., 1978 H.B. Kagan, 1978). The (R)- and (S)-[l,l -binaph-thalene]-2,2 -diylblsCdiphenylphosphine] (= binap ) complexes of ruthenium (A. Miyashita, 1980) and rhodium (A. Miyashita, 1984 R. Noyori, 1987) have been prepared as pure atrop-isomers and used for the stereoselective Noyori hydrogenation of a-(acylamino) acrylic acids and, more significantly, -keto carboxylic esters. In the latter reaction enantiomeric excesses of more than 99% are often achieved (see also M. Nakatsuka, 1990, p. 5586). 

In the early 1990s, Burk introduced a new series of efficient chiral bisphospholane ligands BPE and DuPhos.55,55a-55c The invention of these ligands has expanded the scope of substrates in Rh-catalyzed enantioselective hydrogenation. For example, with Rh-DuPhos or Rh-BPE as catalysts, extremely high efficiencies have been observed in the asymmetric hydrogenation of a-(acylamino)acrylic acids, enamides, enol acetates, /3-keto esters, unsaturated carboxylic acids, and itaconic acids. 

Table 1 Asymmetric hydrogenation of (Z)-2-(acetamido) cinnamic acid, 2-(acetamido) acrylic acid and their methyl esters...

Table 12.4 Rh-Catalyzed asymmetric hydrogenation of a-(acylamino)acrylic acids and esters.

Furthermore FERRIPHOS ligands bearing alkyl groups instead of dimethy-lamino substituents proved to be excellent ligands in the asymmetric hydrogenation of a-acetamidoacrylic acids[34] and acetoxy acrylic esters[35l Their air stability and the easy modification of their structure make the FERRIPHOS ligands particularly useful tools for asymmetric catalysis. 

Although enol esters have a similar structure to enamides, they have proven more difficult substrates for asymmetric hydrogenation, which is evident from the significantly fewer number of examples. One possible explanation is the weaker coordinating ability of the enol ester to the metal center, as compared to the corresponding enamide. Some rhodium complexes associated with chiral phosphorous ligands such as DIPAMP [100, 101] and DuPhos [102] are effective for asymmetric hydrogenation of a-(acyloxy)acrylates. 

Rhodium-BisP and -MiniPHOS catalysts are capable of high enantioselective reductions of dehydroamino acids in 96-99.9% ee.109 A variety of aryl enamides give optically active amides with 96-99% ee with the exception of ort/jo-substituted substrates.111 Despite the high enantio-selectivity, the rate of reaction in this transformation is slow. Rhodium-BisP and -MiniPHOS catalysts perform excellently in the asymmetric reduction of ( >P-(acylamino)acrylates to the corresponding protected-P-amino esters in 95-99% ee.112 Within the family of BisP and MiniPHOS, the ligands that contain t-Bu groups were found to be the most effective in a variety of asymmetric hydrogenations. 

The asymmetric hydrogenation of nitrogen-free acrylic acids or acrylic acid esters proceeds in a similar enantioselective fashion. For these substrates Rh- and Ru-catalysts are used with the same frequency. 

Kitamura, M., Tsukamoto, M., Bessho, Y., Yoshimura, M., Kobs, LI., Widhaim, M., Noyori, R. Mechanism of asymmetric hydrogenation of a-(acylamino)acrylic esters catalyzed by BINAP-ruthenium(ll) diacetate. J. Am. Chem. Soc. 2002, 124, 6649-6667. 

Hydrogenation Easily prepared monodentate phosphoramidite ligands containing a BINOL residue are employed in conjunction with (cod)2RhBp4 in asymmetric hydrogenation of dehydroamino acid derivatives and a-substituted acrylic esters (ee > 99% reachable). 

Fig. 16 Rh-catalyzed asymmetric hydrogenation of a-acetamido-acrylic acid ester 37 using streptavidin mutants as hosts for complexing 36 [130]...

Asymmetric Hydrogenation of Dehydro a-Amino Acids [a-(Acylamino)acrylic Acids and Esters]... 

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