Hydrogenation of enol esters

Catalyst R R1 Geometry Reaction conditions Percent ee of product (confign.) References 

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The rhodium-catalyzed enantioselective hydrogenation of enol esters is an alternative to the asymmetric reduction of ketones. Although enol esters are accessible both from ketones and alkynes, the number of studies reporting successful asymmettic hydrogenation has been limited. It appears that, compared... 

Asymmetric Hydrogenation of Enol Esters. Prochiral ketones represent an important class of substrates. A broadly effective and highly enantioselective method for the asymmetric hydrogenation of ketones can produce many useful chiral alcohols. Alternatively, the asymmetric hydrogenation of enol esters to yield a-hydroxyl compounds provides another route to these important compounds. 

The asymmetric hydrogenation of enol esters can also be catalyzed by chiral amidophosphine phosphinite catalysts derived from chiral amino acids, but the enantioselectivity of these reactions has thus far been only moderate.35... 

The asymmetric hydrogenation of enol esters is an alternative to asymmetric ketone hydrogenation. The precursors can be prepared from the ketones but also via ruthenium-catalyzed addition of the carboxylic acids to the 2-postion of terminal alkynes. This latter method allows the study of the effect of the carboxylate on the enantioselectivity of the asymmetric hydrogenation. A remarkable study by Reetz and colleagues established that it is possible to hydrogenate enolate... 

Hydrogenation of Enol Esters. In addition to the high activity and excellent enantioselectivity being obtained in the asymmetric... 

Table 3 Et-DuPHOS-Rh-Catalyzed Hydrogenation of Enol Esters 26 (R, R = Me)...

Scheme 7.3 Rh-catalysed hydrogenation of enol esters and enamides.

Frequently used substrates include a-acetoxystyrene (AXS) and a-acet-amidostyrene (AS). Hydrogenation of enol esters leads to chiral acetates, which can be easily hydrolysed to the parent alcohols. Hydrogenation of enamides is equally interesting because the obtained amines are extremely important synthetic precursors. Some results with those and related substrates are listed in Table 7.3. 

There is a relatively small amount of results about the hydrogenation of enol esters (entries 1-6). For AXS and related substrates, very high enantioselec-tivities have been obtained with bulkier DiPAMP analogues (entry 1) and with phosphines based on the phospholane backbone (entries 2-5). 

Rubio M, Suarez A, Alvarez E et al (2005) Highly enantioselective hydrogenation of enol ester phosphonates catalyzed by rhodium phosphine-phosphite complexes. Chem Commun... 

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