Me-PennPhos

A Rh complex with (/f.S. /f.S -Me-PennPhos efficiently catalyzes asymmetric hydrogenation of simple ketones (Figure 1.22). Addition of catalytic amounts of 2,6-lutidine is crucial to achieve high enantioselectivity. This catalyst is also... 

Aromatic Ketones The DIOP-Rh [116] and DBPP-Rh [117] complexes, in conjunction with a tertiary amine, have been employed in the asymmetric hydrogenation of acetophenone, albeit with moderate enantioselectivity (80 and 82% respectively Tab. 1.10). The asymmetric hydrogenation of aromatic ketones was significantly improved by using the Me-PennPhos-Rh complex, with which enantioselectivities of up to 96% ee were achieved [36]. Interestingly, the additives 2,6-lutidine and potassium bromide were again found to be crucial for optimum selectivity, although their specific role has not been determined. 

Among the various Rh-phosphine catalysts used to perform enantioselective hydrogenation of lV-(3,4-dihydro-l-naphthyl)ac-etamide, Rh-(R,5,R,S)-Me-PennPhos afforded the desired hydrogenation product in highest optical yield. Thus, the use of Rh-L catalysts, where L = (S,5)-(-e)-DIOP (1), (R)-(-e)-BINAP (2), and (R,R)-(—)-Me-DuPhos (3), afforded lV-(l,2,3,4-tetrahydro-l-naphthyl)acetamide in only 10% ee(S), 24% ee (R), and 1% ee (R, with 57% conversion). ... 

Longer reaction times are required to achieve maximum conversion when aliphatic ketones are hydrogenated in the presence of Rh-(R,S,R,S)-Me-PennPhos catalyst. Thus, 2-hexanone is reduced by H2 [30 atm, Rh-(R,S,R,S)-Me-PennPhos catalyst in the presence of KBr (1 equiv)] at room temperature. After 48 h, the chemical yield of (S)-2-hexanol is 96% (optical yield 75% ee). ... 

In general, it was observed that introduction of bulky (i.e., branched) substituents into aliphatic methyl ketones of the type RC(0)CH3 dramatically reduced the reactivity of the C = 0 group in these substrates toward H2-Rh-(R,S,R,5)-Me-PennPhos... 

Table 1 Asymmetric hydrogenations of prochiral alkenes catalyzed by a (/ ,S,/f,S)-Me-PennPhos-Rh(I) complex...

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