Mechanistic Investigations of an Enantioselective Hydrogenation Catalyzed by a Ruthenium−BINAP Complex. 1. Stoichiometric and Catalytic Labeling Studies

• Published in: Organometallics Vol. 17; no. 11; pp. 2228 - 2240
• Main Authors: Wiles, Jason A., Bergens, Steven H.
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 25.05.1998; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Inorganic & Nuclear; Chemistry, Organic; Physical Sciences; Science & Technology; UNSATURATED CARBOXYLIC-ACIDS; REDUCTION; INTRAMOLECULAR HYDROSILATION; KINETICS; CHIRAL METAL-COMPLEXES; OLEFINS; DERIVATIVES; ASYMMETRIC HOMOGENEOUS HYDROGENATION; RHODIUM(I)
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om980113f

The enantioselective hydrogenation of (Z)-methyl α-acetamidocinnamate ((Z)-MAC) is catalyzed by [Ru((R)-BINAP)(H)(MeCN) n (sol)3 - n ](BF4) (2, n = 0−3, sol = acetone or methanol, (R)-BINAP = (R)-2,2‘-bis(diphenylphosphino)-1,1‘-binaphthyl) in up to 94% ee (R). The mechanism of this catalytic hydrogenation was investigated using in situ NMR spectroscopy, and by comparing the stereochemistry and regiochemistry of deuterium-labeling studies carried out on the catalytic reaction to those carried out on an isolated, possible catalytic intermediate. The isolated species is the olefin−hydride insertion product, [Ru((R)-BINAP)((S)-MACH)(MeCN)](BF4) ((S C α )-1). Compound (S C α )-1 is the only species present in detectable amounts, by 1H and 31P NMR spectroscopy, during the catalytic hydrogenation at room temperature. The absolute configuration at the stereogenic α-carbon of MACH is the same (assuming stereospecific replacement of ruthenium with hydrogen) as that of the major enantiomer of the catalytic hydrogenation ((R)-N-acetylphenylalanine methyl ester ((R)-MACH2)). Results obtained from the stoichiometric hydrogenolysis and deuteriolysis of (S C α )-1, from the catalytic deuteration of (E)-MAC and (Z)-MAC, and from the reaction of (S C α )-1 with excess (Z)-MAC-Me-d 3 all indicate that formation of (S C α )-1 is rapid and reversible prior to the irreversible hydrogenolysis of the ruthenium−carbon bond. The sum of the stereoselectivities and regioselectivities of the formation and hydrogenolysis of (S C α )-1 equals the stereoselectivity and regioselectivity of the catalytic hydrogenation. Solvolysis of the ruthenium−carbon bond occurs to less than 4% during the catalytic hydrogenation carried out in methanol. Removal of MeCN from the catalyst system has no effect on the enantioselection of the catalytic hydrogenation.