Hydrogenation chiral complexes

Another possibility for asymmetric reduction is the use of chiral complex hydrides derived from LiAlH. and chiral alcohols, e.g. N-methylephedrine (I. Jacquet, 1974), or 1,4-bis(dimethylamino)butanediol (D. Seebach, 1974). But stereoselectivities are mostly below 50%. At the present time attempts to form chiral alcohols from ketones are less successful than the asymmetric reduction of C = C double bonds via hydroboration or hydrogenation with Wilkinson type catalysts (G. Zweifel, 1963 H.B. Kagan, 1978 see p. 102f.). 

Platinum-thiourea complexes have been extensively studied because of their biological activity [54], but few have been used in catalysis. Neutral thioureas are able to coordinate to metal centres through their sulfur atom (Scheme 9) [55,56] monomeric (I) and oligomeric (II) species are known for Rh [57], and an X-ray structure has also been determined for the chiral complex III [58]. In many complexes hydrogen bonding has been observed... 

Hydrogenation of tiglic acid in scC02 catalyzed by a chiral complex such as (136) (Scheme 68) proceeds cleanly with cis stereochemistry to afford 2-methylbutanoic acid in up to 89% ee and over 99% yield.367 These studies revealed a different influence of H2 pressure on the selectivity between liquid solvents and scC02. 

M. Mons, E Piuzzi, I. Dimicoli, A. Zehnacker, and F. Lahmani, Binding energy of hydrogen bonded complexes of the chiral molecule 1 phenylethanol, as studied by 2C R2PI Comparison between diastereoisomeric complexes with butan 2 ol and the singly hydrated complex. Phys. Chem. Chem. Phys. 2, 5065 5070 (2000). 

An interesting intramolecular radical cyclization followed by enantioselective hydrogen atom transfer has recently been reported (Scheme 11) [40]. This reaction is carried out in the presence of a chiral complexing agent 38, which... 

Amino acidate N,0-coordinated chiral complexes of Ru, Rh and Ir have found applications in the catalytic hydrogenations of ketones and unsaturated aldehydes... 

Asymmetric hydrogenation of imines derived from trifluoropyruvate, in the presence of a chiral complex of palladium (ligand = (R)-BINAP), affords ethyl (/f)-trifluoroalaninate with ca. 90% The ee values strongly depend on the solvent,... 

Scheme 5.2-9 Hydrogenation of 2-phenylacrylic acid to (S)-2-phenylpropionic acid with the chiral complex [RuCl2(S)-BINAP]2NEt3 as catalyst in [BMIMHBFJ.

Ramies, O., Net, G., Ruiz, A. and Claver, C. (2000) Chiral diphosphites as ligands for the rhodium- and iridium-catalysed asymmetric hydrogenation Precatalyst complexes, intermediates and kinetics of the reaction. Eur. J. Inorg. Chem., 1287-1294. 

Remarkable advances have been achieved in the homogeneous asymmetric hydrogenation of prochiral alkenes by using chiral complexes of Rh and Ru, which... 

I. Alkorta, J. Elguero, Discrimination of hydrogen-bonded complexes with axial chirality. J. Chem. Phys. 117, 6463-6468 (2002)... 

O. Picazo et al., Large chiral recognition in hydrogen-bonded complexes and proton transfer in pyrrolo[2, 3-b]pyrrole dimers as model compounds. J. Org. Chem. 68, 7485-7489 (2003)... 

N. Seurre et al., Structural study of hydrogen-bonded complexes between 2-aminoethanol derivatives and a chiral aromatic alcohol. J. Mol. Struct. 692, 127-137 (2004)... 

S. Portmann et al., Chiral discimination in hydrogen-bonded complexes. J. Chem. Phys. 113, 9577-9585(2000)... 

Weak acid-base chiral complex formation represents hydrogen bond catalysis (see Chapter 9) and deprotonation followed by cation/anion association under homogeneous, and also under phase-transfer conditions (see Chapter 4) [14, 65],... 

Fig. 21 Schematic illustration of a predominantly one-handed helix induction in 64 by hydrogen bonded complex formation with a chiral saccharide 65...

A highly diastereoselective oxetane formation was identified in the PB reaction of dihydropyridone with a m-hydroxybenzaldehyde derivative (Scheme 7.33). The chiral auxiliary, when bound to the aldehyde, offered a binding site to which the reaction partner could attach by two hydrogen bonds. In the hydrogen-bonded complex that was produced, the two enantiotopic faces of the alkene could be differentiated [52]... 

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