Enamides, Noyori catalytic asymmetric hydrogenation

Although the Rh-catalyzed asymmetric hydrogenations of prochiral enamides have been extensively studied and excellent results have been frequently achieved, the catalytic asymmetric hydrogenations of 2-arylacrylic acids have been less successful. Until recently most catalyst systems gave only moderate optical yields for the 2-arylpropionic acid products (77). An important breakthrough in the study of these reactions was reported by Noyori et al. By using Ru(BINAP)(OAc)2 as a catalyst precursor, these researchers obtained excellent optical yields in the asymmetric hydrogenation of 2-(6 -methoxy-2 -naphthyl)acrylic acid (72). 

Recent work by several research groups has shown that supercritical fluids can be superior to other solvents for several chemical processes. For example, DeSimone has demonstrated the ability of supercritical CO2 to replace Freons in the free radical polymerization of fluorinatkl acrylate monomers. 34) Noyori has shown that significant rate enhancements can be achieved in supercritical carbon dioxide relative to other solvents for the homogeneous catalytic hydrogenation of carbon dioxide to either formic acid or its derivatives in the presence of triethylamine or triethylamine/methanol respectively, (equation 1). (55-57) As discussed below, we have recently demonstrated that improved enantioselectivities can be achieved in supercritical carbon dioxide for the catalytic asymmetric hydrogenation of several enamides. 5 8)... 

In 1986, Noyori reported a hexa coordinate BINAP Ru(II) complex that can efficiently catalyze the asymmetric hydrogenation of (Z) N acyl 1 alkylidenete tra hydroisoquinolines (70) in the presence of 0.5 1 mol% of the catalyst loadings in a 5 1 mixture of ethanol and dichloromethane under 1 4 atm of hydrogen at 23 °C (Scheme 9.18) [63]. However, the corresponding E enamide substrates were inert to this catalytic system under the same hydrogenation conditions. 

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