Hydrogenation of acetophenone

The enantioselective hydrogenation of acetophenone afforded an excess of the (S)-phenylethanol isomer. An enantiomeric excess (e.e.) of around 20% was obtained with all the chiral organotin compounds tested, which is a good result for acetophenone, a non-activated ketone, and especially because of the high selectivity to PE (over 97%). Previously published results with the classical Pt/cinchoni-dine system showed an e.e. of 17%, corresponding to a yield of 4.7% [133]. The similar e.e.s obtained for the three rather different modifiers employed in this work seems to indicate that chiral induction must be assigned to the presence of at least one menthyl group attached to the surface. 

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Ruthenium complexes of (129) and (130)336 were investigated for the asymmetric hydrogenation of prochiral 2-R-propenoic acids (Scheme 62a) rhodium complexes of these ligands were used for hydrogenation of acetoamido-cinnamic acid methyl ester (Scheme 62c) and hydrogenation of acetophenone-benzylamine (Scheme 62b). The results obtained with these... 

Asymmetric ECH with [Rh(L)2(Cl)2]+ complexes containing chiral polypyridyl ligands has been attempted, in homogeneous media (L = (7)-(12)) and at carbon electrodes coated with polymer films prepared by electropolymerization of [Rh(13)2(Cl)2]+ -61 62 The latter catalytic system gave the best results in terms of turnover number (up to 4,750) and enantiomeric excess, (ee) when applied to the hydrogenation of acetophenone (ee 18%) and 2-butanone (ee 10%).62 Polymeric materials derived from the complexes [RhI(bpy)(COD)]+ 36 and [Pd(bpy)2]2+33have also been applied to the ECH reaction. 

The cluster complex H4Ru4(CO)8(DIOP)2 has been used at 150°C with 100 atm H2 for asymmetric hydrogenation of acetophenone and methyl-... 

According to Chen et ah, alkali cation co-catalysis kinetics cannot be distinguished from classic ideas (proton instead of alkali) for the asymmetric hydrogenation of acetophenone with the Noyori-catalyst trans-RuC12[(S)-BINAP]... 

The metal carbonyls Cr(CO)6, Mo(CO)6, W(CO)6 and Fe(CO)5 have all been tested in the hydrogenation of acetophenone in the presence of a strong base [61, 62]. In reactions performed in either triethylamine of sodium methoxide in methanol using 5 mol% of catalyst, the Mo and Cr complexes proved to be superior. The different bases had an effect on the yield that was further demonstrated when Cr(CO)6 was used in the hydrogenation of a series of ketones under the same conditions. In most cases, the reactions were found to be better in the methoxide system, with over 98% yields obtained in reactions lasting 3 h at 120 °C. 

Fig. 32.30 Rapid hydrogenation of acetophenone catalyzed by the TolBINAP/DPEN-Ru complex.

Fig. 32.33 Schematic view of the RuH2 species and diastereo-meric transition states in the hydrogenation of acetophenone. The equatorially oriented phenyl groups in the DPEN are omitted in 33A.

Table 41.14 Rhodi um-catalyzed hydrogenation of acetophenone (A) and ethyl benzoylformate (B) [105].

Ionic liquids have also been applied in transfer hydrogenation. Ohta et al. [110] examined the transfer hydrogenation of acetophenone derivatives with a formic acid-triethylamine azeotropic mixture in the ionic liquids [BMIM][PF6] and [BMIM][BF4]. These authors compared the TsDPEN-coordinated Ru(II) complexes (9, Fig. 41.11) with the ionic catalyst synthesized with the task-specific ionic liquid (10, Fig. 41.11) as ligand in the presence of [RuCl2(benzene)]2. The enantioselectivities of the catalyst immobilized by the task-specific ionic liquid 10 in [BMIM][PF6] were comparable with those of the TsDPEN-coordinated Ru(II) catalyst 9, and the loss of activities occurred one cycle later than with catalyst 9. 

Table 41.15 Recycling of 9- and 10-Ru in the asymmetric transfer hydrogenation of acetophenone with the azeotrope in [BMIM][PF6] [110].

Fig. 41.11 R u-complex and task-specific ionic liquid for the hydrogenation of acetophenone derivatives.

Scheme 6-51. Asymmetric transfer hydrogenation of acetophenone in the presence of 119. Reprinted with permission by Am. Chem. Soc., Ref. 112.

Ruthenium-catalysed asymmetric transfer hydrogenation of acetophenone 133... 

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