Highly efficient iridium-catalyzed asymmetric hydrogenations of simple 2-substituted pyridinium salts gives the chiral 2-substituted piperidines with up to 93 % ee (see picture; cod=1,5-cyclooctadiene; synphos=(5,6),(5′,6′)-bis(ethylenedioxy)-2,2′-bis(diphenylphosphino)-1,1′-biphenyl). The key feature of this strategy is the activation of simple pyridines as the pyridinium salts, thus eliminating substrate inhibition and enhancing the reactivity.
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