Homogeneous asymmetric catalysis provides a fundamental tool for the preparation of chiral building blocks for biologically important substances and advanced materials. Polymer-bound catalysts have inherent operational and economical advantages : immobilization on solid matrices facilitates the separation from reaction mixtures, recovery, and reuse of the expensive chiral compounds. Further, the solid-phase reaction offers a technical basis for diversity-oriented combinatorial synthesis. Numerous resin-supported (pre)catalysts have been developed along this line, though their practical application application is limited to only a few cases. Many of these are much less reactive than the homogeneous catalyst systems, and their stereoselectivity is often reduced, impeding practical use. We have found that RuCl_2(phosphine)_2(1,2-diamine) complexes, coupled with an alkaline base in 2-propanol, hydrogenate a C=0 function preferentially over coexisting conjugated or nonconjugated C=C linkage
s, halogen atoms, various hetrocycles, and many other functional groups. The use of appropriate chiral diphosphines and diamines results in rapid and productive asymmetric hydrogenation of a range of achiral and chiral ketones. Their high efficiency prompted us and other groups to immobilize the BINAP/diamine RuCl_2 complexes. This reaction exhibits a turnover number (TON) as high as 12,300/batch or a total of 33,000 by repeated use. The enantioselectivity, productivity, and rate of hydrogenation using the polymer-bound catalysts are comparable to those attained under homogeneous conditions.
Reaction of a chiral RuCl_2(diphosphine)(1,2-diamine) complex and NaBH_4 forms trans-RuH(η^1-BH_4)(diphosphine)(1,2-diamine)quantitatively. The TolBINAP/DPEN Ru complex has been characterized by single crystal X-ray analysis as well as NMR and IR spectra. The new Ru complexes allows for asymmetric hydrogenation of simple ketones in 2-propanol without an additional strong base. Various base-sensitive ketones are convertible to chiral alcohols in a high enantiomeric purity with a substrate/catalyst ratio of up to 100 000 under mild conditions. Configurationally unstable 2-isopropyl- and 2-methoxycyclohexanone can be kinetically resolved with a high enantiomer discrimination. This procedure overcomes the drawback of an earlier method using RuCl_2(diphosphine)(diamine) and an alkaline base, which sometimes causes undesired reactions such as ester exchange, epoxy-ring opening, (β-elimination, and polymerization of ketonic substrates. Less