Catalysis plays a vital role in chemical transformation. The
discovery of the Wilkinson catalyst, Rh(Ph3P)3Cl, for hydrogenation
stimulated a lot of attempts to develop the enantioselective version of these
reactions by using optically active transition metal complexes [1]. In the
beginning, only very low enantioselectivity was obtained in the
hydrogenation of 2-phenylacrylic acid and 2-phenyl-1-propene with certain
chiral tertiary phosphine rhodium complexes as catalysts [2]. However, the
situation was dramatically changed by the invention of well-designed Rh
complexes. This chemistry later became the standard method for the synthesis
of optically active amino acids. In particular, the discovery of Ru-BINAP
complexes in the mid 80’s significantly expanded the scope of asymmetric
hydrogenation [3]. High rates of reduction of C=X functional groups were
attainable only by the coordination of structurally well-designed catalysts
and suitable reaction conditions. The use of appropriate chiral diphosphine
ligands, particularly the BINAP class compounds, and chiral diamines
resulted in rapid and productive asymmetric hydrogenation of a range of
aromatic and heteroaromatic ketones and gave consistently high
enantioselectivity. Certain amino and alkoxy ketones were also used as the
substrates. In fact, this asymmetric transformation tolerated many
substituents including F, Cl, Br, I, CF3, OMe, OBn, COOCH(Me)2, NO2, NH2,
and NRCOR as well as various electron-rich and -deficient aromatics. In
addition, cyclic and acyclic a,b-unsaturated ketones were converted to chiral
allyl alcohols of high enantiomeric purity. Particularly noteworthy was that
the hydrogenation of configurationally labile ketones allowed for the
dynamic kinetic discrimination of diastereomers, epimers and enantiomers.
This important method showed promise in the synthesis of a wide variety of
chiral alcohols from achiral and chiral carbonyl compounds. Its versatility
was further manifested by the asymmetric synthesis of many biologically
active chiral compounds. These scientifically interesting and industrially
attractive processes have been recently applied to the synthesis of various
natural and unnatural chiral compounds in a practical manner.
