Enantioselective synthesis of a chiral fluoropiperidine via asymmetric hydrogenation of a vinyl fluoride

Asymmetric hydrogenation of a vinyl fluoride derivative gives efficient access to enantioenriched 1,3,4-trisubstituted piperidine 1 with a stereogenic alkyl fluoride center. Extensive catalyst screening across transition metals and chiral ligands identified only one catalyst, a Rh/Walphos complex, that gives high conversion, enantioselectivity and chemoselectivity for olefin reduction over defluorination. The presence of acid additives in the hydrogenation exerts a profound effect on reaction outcome. The results of deuterium labeling studies demonstrate that significant olefin isomerization accompanies the undesired defluorination side-reaction. (C) 2009 Elsevier Ltd. All rights reserved.