Asymmetric Hydrogenation of 1-Alkyl and 1-Aryl Vinyl Benzoates: A Broad Scope Procedure for the Highly Enantioselective Synthesis of 1-Substituted Ethyl Benzoates

The enantioselective hydrogenation of enol esters of formula CH2=C(OBz)R with rhodium catalysts based on phosphine phosphite ligands (P-OP) has been studied. The reaction has a broad scope, and it is suitable for the preparation of products possessing a wide variety of R substituents. For the cases where R is a primary alkyl, high catalyst activity (S/C = 500) and enantioselectivities (95-99% ee) were obtained with a catalyst characterized by an ethane backbone and a PPh2 fragment. In contrast, for R = t-Bu, a catalyst possessing a benzene backbone provided the best results (97% ee). Derivatives with a cycloalkyl R substituent were particularly difficult substrates for this reaction. A broader catalyst screening was required for these substrates, which identified a catalyst possessing a P(m-xylyl)(2) fragment as the most appropriate one, affording enantioselectivities between 90 and 95% ee. Outstanding enantioselectivities (99% ee) and high catalyst activity (S/C = 500-1000) were also obtained in the case of substrates bearing a Ph or a fluoroaryl R substituent. In addition, the system is also appropriate for the preparation of other synthetically useful esters as those for R = benzyl, 2-phenylethyl or N-phthalimido alkyl chains. Likewise, the hydrogenation of divinyl dibenzoates proceeded with very high diastero- and enantioselectivity, generating rather low amounts of the meso isomer (3-6%). On the other hand, substrates with Br and MeO substituents at the phenyl benzoate ring, suitable for further functionalization, have also been examined. The results obtained indicate no detrimental effect of these substituents in the hydrogenation. Alternatively, the methodology has been applied to the highly enantioselective synthesis of deuterium isotopomers of 1-octyl benzoate bearing CDH2, CD2H, or CD3 fragments. Finally, as a practical advantage of the present system, it has been observed that the high performance of the catalysts is retained in highly concentrated solutions or even in the neat substrate, minimizing both the amount of solvent added and the volume of the reaction.