Enantioselective enamide reduction

Zhang Catalytic Enantioselective N-acyl-a-Aryl-Enamide Reductions... 

The MonoPhos family of ligands for the reduction of C-C double bonds, including enamides, is based on the 2,2-bisnaphthol (BINOL) backbone. These phosphoramidite ligands are comparatively inexpensive to prepare compared to bisphosphine ligands. For enamide reductions with MonoPhos (20) as ligand, it was found that the reaction is strongly solvent dependent. Very good enantioselectivities were obtained in nonprotic solvents. - ... 

The reduction of a-(aryloxy)acrylates has been reported with the Rh/DUPHOS (2.05) catalyst. Interestingly, the substrate (2.48) was used as a 3 1 ( )/(Z) mixture, but the enantioselectivity is still very high in the product (2.49). When this reaction was attempted using benzene as solvent, no product was formed, even though benzene is a suitable solvent for enamide reduction. This was attributed to coordination of the benzene to the cationic rhodium complex. The enol ester is not able to displace the benzene, whilst an enamide can do so. 

BINAP (40a) was first reported as a ligand in an enantioselective hydrogenation in 1980 [172], and provides good selectivity for the reductions of dehydroamino acid derivatives [173], enamides, allylic alcohols and amines, and a,p-unsaturated acids [4, 9, 11, 12, 174, 175]. The fame of the ligand system really came with the reduction of carbonyl groups with ruthenium as the metal [11, 176]. The Rh-BINAP systems is best known for the enantioselective isomerizations... 

Alkyl enamides, such as N-(1 -tert-bulyl-viriyl)-acelamide 88 and N-(l-adaman-tyl-vinyl)-acetamide 89, can also be hydrogenated in high enantioselectivity (>99% ee) and activity (TON 5000 TOF >625 h ) with Rh-Me-DuPhos [19]. Remarkably, these bulky alkyl enamides are reduced with the opposite sense of induction, a phenomenon also observed when the bisphospholane Di Square P 63 was applied [87 b]. A computational modeling study by Landis and Feldgus suggested that the reduction of a-alkyl and a-arylenamides involves different coordination pathways [93, 158]. 

Homogeneous enantioselective hydrogenation constitutes one of the most versatile and effective methods to convert prochiral substrates to valuable optically active products. Recent progress makes it possible to synthesize a variety of chiral compounds with outstanding levels of efficiency and enantioselectivity through the reduction of the C=C, C=N, and C=0 bonds. The asymmetric hydrogenation of functionalized C=C bonds, such as enamide substrates, provides access to various valuable products such as amino acids, pharmaceuticals, and... 

Asymmetric catalysis undertook a quantum leap with the discovery of ruthenium and rhodium catalysts based on the atropisomeric bisphosphine, BINAP (3a). These catalysts have displayed remarkable versatility and enantioselectivity in the asymmetric reduction and isomerization of a,P- and y-keto esters functionalized ketones allylic alcohols and amines oc,P-unsaturated carboxylic acids and enamides. Asymmetric transformation with these catalysts has been extensively studied and reviewed.81315 3536 The key feature of BINAP is the rigidity of the ligand during coordination on a transition metal center, which is critical during enantiofacial selection of the substrate by the catalyst. Several industrial processes currently use these technologies, whereas a number of other opportunities show potential for scale up. 

Rhodium and palladium catalysts that contain 4 display high enantioselectivities for the asymmetric hydrogenation of enamides, itaconates, P-keto esters, asymmetric hydroboration, and asymmetric allylic alkylation,80 82 but this ligand system distinguishes itself from other chiral bisphos-phines in the asymmetric reduction of tetrahydropyrazines and tetrasubstituted olefins (see also Chapter 15). The reduction of tetrahydropyrazines produces the piperazine-2-carboxylate core,... 

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. 

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