Hydrogen activation, benzoin

The catalytic hydrogenation of optically active benzoin oxime resulted in the stereoselective formation of optically active erythro diphenylethanolconine in... 

Only a few other cobalt complexes of the type covered in this review (and therefore excluding, for example, the cobalt carbonyls) have been reported to act as catalysts for homogeneous hydrogenation. The complex Co(DMG)2 will catalyze the hydrogenation of benzil (PhCOCOPh) to benzoin (PhCHOHCOPh). When this reaction is carried out in the presence of quinine, the product shows optical activity. The degree of optical purity varies with the nature of the solvent and reaches a maximum of 61.5% in benzene. It was concluded that asymmetric synthesis occurred via the formation of an organocobalt complex in which quinine was coordinated in the trans position (133). Both Co(DMG)2 and cobalamin-cobalt(II) in methanol will catalyze the following reductive methylations ... 

The asymmetric hydrogenation of C—O bonds have now been achieved in optical yields up to 95%, rivalling the performance of alkenes. Here also, rhodium complexes have been used almost exclusively, but some success has been obtained with cobalt catalysts. Using [Co(HDMG)2] in presence of optically active bases, benzil could be reduced to benzoin (equation 54) in an optical yield of 78%. Quinine or quinidine were the chiral bases employed. The best optical yields were obtained with quinine (60). It was found that when benzylamine was also present, the rate of hydrogenation was greatly enhanced without any decrease in the optical yield.276... 

Oxidation of benzoin to benzil with Fe(III), in the presence of 2,2/-bipyridine or ferrozine is of first order in Fe(III) and benzoin. An inverse second-order dependence was observed with respect to hydrogen ions. For oxidation of substituted benzoins the reaction constant is p 1.2, indicating an electron-rich transition state and an inner sphere mechanism has been proposed.66 The order with respect to iodide, in the reaction of iodide ions with a diiron(III)-l,10-phenanthroline complex, is 2. The hydrolytic derivatives of the complex are not kinetically active. Both inner and outer sphere pathways are operative.67... 

When, in 1832, Wohler and Liebig first discovered the cyanide-catalyzed coupling of benzaldehyde that became known as the benzoin condensation , they laid the foundations for a wide field of growing organic chemistry [1]. In 1903, Lapworth proposed a mechanistical model with an intermediate carbanion formed in a hydrogen cyanide addition to the benzaldehyde substrate and subsequent deprotonation [2]. In the intermediate active aldehyde , the former carbonyl carbon atom exhibits an inverted, nucleophilic reactivity, which exemplifies the Umpo-lung concept of Seebach [3]. In 1943, Ukai et al. reported that thiazolium salts also surprisingly catalyze the benzoin condensation [4], an observation which attracted even more attention when Mizuhara et al. found, in 1954, that the thiazolium unit of the coenzyme thiamine (vitamin Bi) (1, Fig. 9.1) is essential for its activity in enzyme biocatalysis [5]. Subsequently, the biochemistry of thiamine-dependent enzymes has been extensively studied, and this has resulted in widespread applications of the enzymes as synthetic tools [6]. 

The addition of HCN to aldehydes or ketones produces cyanohydrins. This is an equilibrium reaction, and for aldehydes and aliphatic ketones the equilibrium lies to the right therefore the reaction is quite feasible, except with sterically hindered ketones such as diisopropyl ketone. However, ketones ArCOR give poor yields, and the reaction cannot be carried out with ArCOAr since the equilibrium lies too far to the left. With aromatic aldehydes the benzoin condensation (16-55) competes. With ot, 3-unsaturated aldehydes and ketones, 1,4-addition competes (15-38). The reaction has been carried out enantioselectively optically active cyanohydrins were prepared with the aid of optically active catalysts.Hydrogen cyanide adds to aldehydes in the presence of a lyase to give the cyanohydrin with good enantioselectivity. " Cyanohydrins have been formed using a lyase m an ionic liquid. 

Air-stable and easy-to-prepare imidazolium-2-carboxylates (122) and (benz)imidaz-olium hydrogen carbonates (123) have been evaluated for their potential as organic precatalysts for diverse NHC-mediated reactions.Both (122) and (123) proved efficient as NHC sources and have been shown to catalyse benzoin condensation as well as transesterification and cyanosilylation reactions. Interestingly, the catalytic activity of (122) has been found to be higher than for (123). 

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