Cinchona homogeneous catalysts

The scope of cinchona-based chiral auxiliary or chirality transmitters for enantioselective reductions is at the moment restricted to heterogeneous Pt and Pd catalysts and primarily to the reduction of a-functionalized ketones and to a lesser degree of activated C=C bonds. Up to now, very few homogeneous catalysts have been described, and with the exception of a transfer hydrogenation system, none shows any promise. 

For the hydrogenation of a-keto esters and a-keto acetals, the performance of the heterogeneous cinchona-modified Pt catalysts is equal to and in some cases superior to the best homogenous catalysts. Indeed, several industrial applications have been described that underline this statement. For most other substrates, the performance of the cinchona-modified Pd or Pt catalysts is not (yet) on a level where the application to real-world substrates has been demonstrated. 

In order to evaluate the catalytic characteristics of colloidal platinum, a comparison of the efficiency of Pt nanoparticles in the quasi-homogeneous reaction shown in Equation 3.7, with that of supported colloids of the same charge and of a conventional heterogeneous platinum catalyst was performed. The quasi-homogeneous colloidal system surpassed the conventional catalyst in turnover frequency by a factor of 3 [157], Enantioselectivity of the reaction (Equation 3.7) in the presence of polyvinyl-pyrrolidone as stabilizer has been studied by Bradley et al. [158,159], who observed that the presence of HC1 in as-prepared cinchona alkaloids modified Pt sols had a marked effect on the rate and reproducibility [158], Removal of HC1 by dialysis improved the performance of the catalysts in both rate and reproducibility. These purified colloidal catalysts can serve as reliable... 

The catalyst is a combination of a chemo-catalyst and a natural product taken from the cinchona alkaloids giving amazing results. In phosphine catalysed asymmetric catalysis these types of structures are lacking, as nature does not produce phosphines ( ) and the phosphines used in the early years of development of asymmetric homogeneous catalysis lacked the complexity of... 

Janda, Bolm and Zhang generated soluble polymer-bound catalysts for the asymmetric dihydroxylation by attaching cinchona alkaloid derivatives to polyethylene glycol monomethyl ether (MeO-PEG) [84—87]. Since these polymeric catalysts like (24) are soluble in many common solvents they are often as effective as their small homogenous counterparts. Janda et al. prepared catalyst (24) in which two dihydroquinidine (DHQD) units were linked together by phthalazine and finally were attached to MeO-PEG via one of the bicyclic ring system moieties (Scheme... 

Aldol and Related Condensations As an elegant extension of the PTC-alkylation reaction, quaternary ammonium catalysts have been efficiently utilized in asymmetric aldol (Scheme 11.17a)" and nitroaldol reactions (Scheme ll.lTb) for the constmction of optically active p-hydroxy-a-amino acids. In most cases, Mukaiyama-aldol-type reactions were performed, in which the coupling of sUyl enol ethers with aldehydes was catalyzed by chiral ammonium fluoride salts, thus avoiding the need of additional bases, and allowing the reaction to be performed under homogeneous conditions. " It is important to note that salts derived from cinchona alkaloids provided preferentially iyw-diastereomers, while Maruoka s catalysts afforded awh-diastereomers. 

Enantioselective synthesis is a topic of undisputable importance in current chemical research and there is a steady flow of articles, reviews and books on almost every aspect involved. The present overview will concentrate on the application of solid chiral catalysts for the enantioselective synthesis of chiral molecules which are a special class of fine chemicals. Included is an account on our own work with the cinchona-modified Pt catalysts. Excluded is the wide field of immobilized versions of active homogeneous complexes or of bio-catalysts. During the preparation of this survey, several reviews have been found to be very informative [1-14]. 

In this chapter, we do not attempt to give a comprehensive overview of the field, but we would rather concentrate on results where both enantioselectivity and catalyst activity are relevant to preparative application. In the first section, results obtained with cinchona-mediated homogeneous systems for the reduction of ketones are briefly reviewed. Then, heterogeneous cinchona-modified Pt catalysts applied to the hydrogenation of a-functionalized ketones and cinchona-modified Pd catalysts for the hydrogenation of activated C=C bonds are discussed from a synthetic point... 

Salen palladium(II) complexes have been reported to be effective homogeneous, or when immobilized, to be active heterogeneous catalysts for the hydrogenation of alkenes [1,2]. It is also known that platinum(O) catalysts modified with cinchona alkaloids catalyze the enantioselective hydrogenation of a-ketoesters to the corresponding a-hydroxyesters[3]. The platinum(O) catalyst is attached to AI2O3 [4] or to zeolites [5,6]. 

---