Rhodium-ethyl-DuPhos

Cationic rhodium(I) complexes such as Rh[(5,5)-ethyl-DuPHOS](cod) +X- (X=OTf, PFe, BF4, SbFe) are usually employed as precatalysts for enantioselective hydrogenation " or hydrosilylation reactions. The precatalysts can be prepared from the chiral ligand and [Rh(cod)2] X -complexes by a standard method. The corresponding Rh[(5, 5)-ethyl-DuPHOS](nbd) complex can be accessed equally well by the method of Schrock and Osborne or by exchange of cod in the relevant rhodium complex by norbornadiene (nbd). ... 

Enol acetates and corresponding derivatives constitute another class of unsaturated compounds that can advantageously be hydrogenated with high enantiomeric excess. This reaction is related to the enantioselective reduction of ketones. Acylated enol carboxy-lates (as an equivalent of a-keto carboxylic acid) can likewise be successfully reduced with rhodium(I) catalysts based on (5,5)-ethyl-DuPHOS (eq 8). Subsequent deprotection of the hydroxyl group or reduction of the carboxylic acid derivatives so obtained deliver chiral a-hydroxy carboxylates and 1,2-diols, respectively. 

Finally, several examples of the enantioselective hydrogenation of unsaturated substrates without any heteroatom attached to the olefinic double bond are noteworthy. Of particular relevance to the production of pharmaceutics, agrochemicals, flavors and aroma stuffs is the formation of the chiral 2-substituted succinates based on relevant itaconic acid derivatives. Burk et al. demonstrated that a rhodium(I) catalyst derived from (5,5)-ethyl-DuPHOS is able to hydrogenate aryl- or alkyl-substituted itaconic... 

Chiral rhodium-DuPHOS complexes are highly efficient catalyst for the enantioselective hydrogenation of enamides. One drawback of these catalysts is that they are easily oxidised and inert conditions are required for optimal results. The methyl- and ethyl substituted Rh-DuPHOS compounds, 3a and 3b, have been successfully applied in the reduction of a-acetamidocinnamic acids in [C4Ciim][PF6], Scheme 3.7.[7,39] While activities and selectivities are slightly lower compared to the homogeneous reaction in 2-propanol, the ionic liquid-immobilised catalyst is less prone to oxidation and recycling is feasible at least three times. 

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