from first principles PP-I-1

OP-III-29Enantioselective Hydrogenation of Activated Ketones in the Presenceof Pt-Cinchona Catalysts. Is the Proton Transfer Concept Valid?Tálas E. 1 , Margitfalvi J.L. 21 Research Center for Natural Sciences, HAS H-1025 Pusztaszeri 59-67, Budapest, Hungary2 Combitech-Nanotech Kft., Magyar jakobinusok tere 7, 1122 Budapest, Hungaryjoemarg@t-online.huIn this contribution we express our view and supporting experimental data related to the proposedhydrogen (proton) transfer [1,2], in the catalytic system Pt-cinchona alkaloids used in theenantioselective hydrogenation of activated ketones (-ketoesters). We suggest that this scheme[1,2] does not work. Our doubts related to the scheme given in refs. [1,2] are based on variousexperimental facts. If we assume that the quinuclidine nitrogen in CD can perform the hydrogentransfer as described in refs. [1,2] in this case other tertiary amines have to accomplish similar job,i.e. they should be able to transfer hydrogen from the Pt site to the prochiral substrate.Consequently, the proton transfer should be an intrinsic property of tertiary amines. The rateacceleration observed both in the presence of cinchonaalkaloids and different achiral tertiary amines [3] can beconsidered as a basis for this assumption. Therefore, ifother tertiary amines can be involved in the abovehydrogen transfer, the addition of tertiary nitrogencompounds to the reaction mixture containing CD shoulddecrease the enantioselectivity, as in this case twocatalytic cycles will operate simultaneously as shown inthe scheme. However, we observed just an opposite effect.Our results clearly demonstrated that in the presence ofachiral tertiary amine additives, no decrease of theenantioselectivity was found upon using ethyl pyruvate,2,3-butanedione, 4,6-hexanedione and methylbenzoylformate. In all cases substantial increase of ee and reactionrates was observed. Among the tertiary aminesquinuclidine and 1,4-diazabicyclo-[2.2.2]octane provided the highest increase in the reaction rateand ee values. Accordingly, the lack of the negative effect of tertiary amine additives used in theenantioselective hydrogenation of four different substrates suggests that the proton transfermechanism is not adequate. This mechanism should be revised.[1] A. Vargas, D. Ferri, A. Baiker, J. Catal. 236 (2005) 1.[2] T. Mallat, E. Orglmesiter, A. Baiker, Chem. Rev. 107 (2007) 4863.[3] H.U. Blaser, H.P. Jalett, D.M. Monti, J.F. Reber, J.T. Wehrli, Stud. Surf. Sci. Catal. 41 (1988) 153.73