No abstract

The chemical reduction of aqueous solutions of platinum salts in the presence of the alkaloid dihydrocinchonidine yields chirally coated platinum colloids. The particle sizes can be varied in a controlled fashion from 1.5 to 4 nm by changing the alkaloid concentration. These chirally coated platinum colloids function in quasi‐homogenous phases as enantioselective hydrogenation catalysts for ethylpyruvate (76% ee), and no structure sensitivity was observed.

Metal colloids protected with chiral molecules can lead to a new type of enantioselcctive catalyst combining good selectivity control with an extraordinarily high activity in hydrogenation reactions. This concept has been applied for the first time in thc form of platinum sols stabilized by the alkaloid dihydrocinchonidine. Keywordsasymmetric synthesis * catalysis . colloids * hydrogenations * platinum Mono-and multimetal colloids of 1-10 nanometer size have been shown to be efficient catalysts for various chemical reactions such as hydrogenation, ['] oxidation, [" hydrosilylation,131 and C-C bond formation.[41 In addition, colloidal metal precursors have been used for the formation of high-performance fuel-cell catalysts.[51 In colloids the metal particles are protected sterically or eletrostatically to prevent agglomeration. The protecting shell present at the surface of the nanoparticle has been demonstrated to have a decisive influence on the regioselectivityr61 and stere~selectivity [~] in the hydrogenation of olefins and substituted aromatics. We have recently reported for the first time that metal colloids protected by a chiral stabilizer can induce enantioselectivity in catalytic hydrogenation.[*] The concept leading to this new type of catalyst is a n extension to colloidal mctals of the well-known heterogeneous enantioselective Supported platinum particles modified by cinchona alkaloids are known to catalyze thc hydrogenation of 2 -0 x o e s t e r s [~~-~~ and -acids ''dl to the corresponding 2-hydroxy compounds in optical yields of up to 95 % ec.'""I This reaction has been investigated systematically with respect to the catalyst preparation,['O1 reaction conditions," 'I kinetics," 21 and mass transfer phen~rnena."~] Further, the structural elements of the modifier required to induce enantioselectivity have been studied in detail by Pfaltz et. al.""' The interaction between the modifier and the substrate on the surface can be rationalized with the help of coniputcr modeling,['51 and the reaction rate and enantioselectivity have both been monitored as a function of conversion. ['h1 catalyst.Max-Plenck-I nhtitiit fiir Kohlcnforschung Kaiser-WilIirlm-Plat7 I , 45470 Mulhciin a n der Ruhr (Geriiiany) -(208) c-nxiil . hociincm;iii~i~ir mpi-muclhcim.mpg.dc Platinum sols stabilized by dihydrocinchonidine ( I , Figure 1) can be synthesized in different particle sizes by the reduction of platinum tetrachloride with formic acid in the presence of different amounts of alkaloid. The resulting nanoparticles induce enantioselectivity in the hydrogenation of ethyl 2-oxopropionate (ethyl pyruvate) with optical yields of 75-80% ec (Scheme Turnover frequencies (ca. 1 s-') and enantiomeric excess were both found to be independent of the particle size.In order to evaluate the catalytic characteristics of colloidal platinum, we compared the efficiency in the reaction shown in Scheme 1 of Pt nanoparticles in "quasi-homogeneous" phase with that of supported colloids of the same charge and of a [a] R is the m...

No abstract