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The enantioselective hydrogenation of enol esters of formula CH2=C(OBz)R with rhodium\udcatalysts based on phosphine-phosphite ligands (P-OP) has been studied. The reaction has a\udbroad scope and it is suitable for the preparation of products possessing a wide variety of R\udsubstituents. For the cases where R is a primary alkyl high catalyst activity (S/C = 500) and\udenantioselectivities (95-99 % ee) were obtained with a catalyst characterized by an ethane\udbackbone and a PPh2 fragment. In contrast, for R = tBu a catalyst possessing a benzene backbone\udprovided the best results (97 % ee). Derivatives with a cycloalkyl R substituent were particularly\uddifficult substrates for this reaction. A broader catalyst screening was required for these\udsubstrates, which identified a catalyst possessing a P(m-xylyl)2 fragment as the most appropriate\udone, affording enantioselectivities between 90 and 95 % ee. Outstanding enantioselectivities (99\ud% ee) and high catalyst activity (S/C = 500-1000) were also obtained in the case of substrates\udbearing a Ph or a fluoroaryl R substituent. In addition, the system is also appropriate for the\udpreparation of other synthetically useful esters as those for R = benzyl, 2-phenylethyl or Nphthalimido\udalkyl chains. Likewise, the hydrogenation of divinyl dibenzoates proceeded with\udvery high diastero- and enantioselectivity, generating rather low amounts of the meso isomer (3-6\ud%). On the other hand, substrates with Br- and MeO- substituents at the phenyl benzoate ring,\udsuitable for further functionalization, have also been examined. The results obtained indicate no\uddetrimental effect of these substituents in the hydrogenation. Alternatively, the methodology has\udbeen applied to the highly enantioselective synthesis of deuterium isotopomers of 1-octyl\udbenzoate 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 high\udconcentrated solutions or even in the neat substrate, then minimizing both the amount of solvent\udadded and the volume of the reactionPeer reviewe

acs catalysis

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