Asymmetric Hydrogenation of 3-Substituted Pyridinium Salts.
The use of an equivalent amount of an organic base leads to high enantiomeric excess in the asymmetric hydrogenation of N-benzylated 3-substituted pyridinium salts into the corresponding piperidines. Indeed, in the presence of Et3 N, a Rh-JosiPhos catalyst reduced a range of pyridinium salts with ee values up to 90 %. The role of the base was elucidated with a mechanistic study involving the isolation of the various reaction intermediates and isotopic labeling experiments. Additionally, this study provided some evidence for an enantiodetermining step involving a dihydropyridine intermediate.
['Renom-Carrasco|Marc|M|', 'Gajewski|Piotr|P|', 'Pignataro|Luca|L|', 'de Vries|Johannes G|JG|http://orcid.org/0000-0001-5245-7748', 'Piarulli|Umberto|U|', 'Gennari|Cesare|C|', 'Lefort|Laurent|L|http://orcid.org/0000-0003-2973-6540']
hydrogenation asymmetric catalysis homogeneous catalysis pyridines reaction mechanisms