TY - JOUR AU - Huck, W.-R. AU - Bürgi, T. AU - Mallat, T. AU - Baiker, A. TI - Asymmetric hydrogenation of 4-hydroxy-6-methyl-2-pyrone: Role of acid-base interactions in the mechanism of enantiodifferentiation PY - 2001 JF - Journal of Catalysis JA - J. Catal. SN - 0021-9517 VL - 200 IS - 1 SP - 171 EP - 180 L1 - http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6WHJ-45BCCDD-88-1&_cdi=6852&_user=9565874&_pii=S0021951701932051&_origin=search&_coverDate=05%2F15%2F2001&_sk=997999998&view=c&wchp=dGLbVzW-zSkWA&md5=64d4b4be04ceed617cb5801230d59ee4&ie=/sdarticle.pdf L3 - http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WHJ-45BCCDD-88&_user=9565874&_coverDate=05%2F15%2F2001&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000043220&_version=1&_urlVersion=0&_userid=9565874&md5=5017ef3596785f4ee1f8675e49b8b99b&searchtype=a M3 - 10.1006/jcat.2001.3205 UR - http://dx.doi.org/10.1006/jcat.2001.3205 N2 - Enantioselective hydrogenation of the pseudo-aromatic 4-hydroxy-6-methyl-2-pyrone to the corresponding 5,6-dihydropyrone has been studied over cinchonidine-modified Pd/Al2O3 and Pd/TiO2 catalysts. A mechanistic model for enantiodifferentiation is proposed, involving two H-bond interactions (N–H···O and O–H···O) between the deprotonated reactant and the protonated chiral modifier. The model can rationalize (i) the sense of enantiodifferentiation, i.e., the formation of (S)-product in the presence of cinchonidine as modifier; (ii) the complete loss of enantioselectivity when the acidic OH group of the reactant is deprotonated by a base stronger than the quinuclidine N of the alkaloid; and (iii) the poor enantiomeric excesses obtained in good H-bond donor or acceptor solvents. NMR and FTIR investigations, and ab initio calculations, of reactant–modifier interactions support the suggested model. Several factors, such as catalyst prereduction conditions, trace amounts of water, presence of strong bases and acids, and competing hydrogenation of acetonitrile to ethylamines, were found to affect the efficiency of this catalytic system. ID - 1150 ER -