Imprinted rhodium catalyst

Development of molecularly imprinted enantioselective hydrogenation catalysts based on immobilised rhodium complexes was reported by Gamez et al. [29]. The imprinted catalysts were prepared by polymerising Rh(I)-(A,A -dimethyl-l,2-diphe-nylethanediamine) with di- and tri-isocyanates, using a chiral alkoxide as the template (9). The imprinted polymer, after removal of the template, was tested for the reduction of ketones to alcohols. An enhanced enantioselectivity was observed in the presence of the imprinted polymeric catalyst, in comparison to the control polymer. 

Locatelli et al. [13] investigated the hydride transfer reduction of prochiral ketones using a rhodium based catalyst on a polyurea support. The homogeneous reduction of acetophenone using a rhodium catalyst with two equivalents of (1 S, 2 5 )-iV,iV -dimethyl-l,2-diphenylethane diamine was conducted to establish an appropriate comparison for the imprinting studies. This control reaction resulted in formation of 1-(J ) -phenyl ethanol with 67% ee (Scheme 6). The low enantioselectivity was attributed to a poor coordination sphere surrounding the metal center. The selectivity from the hydride transfer is proposed to arise from the approach of the substrate to the metal center, as shown in Scheme 7. The metal... 

This series of experiments represents a thorough study employing control experiments to evaluate the role of the imprinted polymer. The homogeneous catalyst was used to establish a baseline comparison to the heterogeneous systems. A comparison between deposited rhodium and polymerized rhodium catalyst demonstrated that the catalyst with a poorly defined coordination sphere was not capable of... 

We wish to use the "molecular imprinting effect" to obtain highly enantioselective catalytic reaction The new catalyst preparation allows us to polymerise the chiral rhodium complex in presence of optically pure l-(s)-phenylethanol as a template. 

Polymerised preformed [(N,N -dimethyl-l,2-diphenylethane diamine)2Rh] complex allows us to obtain enantioselective material. We have then shown that it is possible to imprint an optically pure template into the rhodium-organic matrix and to use the heterogeneous catalyst in asymmetric catalysis with an obvious template effect. The study of yield versus conversion graphs has shown that the mechanism occurs via two parallel reactions on the same site without any inter-conversion of the final products. Adjusting the cross-linker ratio at 50/50 allows us to find a compromise between activity and selectivity. Phenyl ethyl ketone (propiophenone) was reduced quantitatively in 2 days to (R)-l-phenyl propanol with 7tf% enantiomeric excess We have then shown that the imprinting effect is obvious for molecules related in structure to the template (propiophenone, 4 -trifluoromethyl acetophenone). It is not efficient if the structure of the substrate is too different to that of the template. 

Similarly, starting from the bridged Rh-dimer precursor Rh2Cl2(CO)4, Rh-dimer molecular imprinting catalyst was attached at Si02 surface for hydrogenation of alkenes, and exposed to P(OCH3)3 to make rhodium-phosphite... 

Gamez P.B., Dunjic B., Pinel C. and Lemaire M. (1995) "Molecular imprinting effect" in the synthesis of immobilized Rhodium complex catalyst. Tetrahedron Lett. 36, 8779-8782. 

Catalysis by imprinted surfaces has been extended to transition metal catalyst hydrolysis and hydrogenation [70-73]. For the catalytic hydrogenation of alkenes, the dimeric and monomeric rhodium complexes were attached to silica surfaces as shown in Fig. 23. 

Gamez, P. Dunjic, B. Pinel, C. Lemaire, M. Molecular imprinting effect in the synthesis of immobilized rhodium complex catalyst (IRC cat). Tetrahedron Lett. 1995, 36, 8779-8782. 

Polborn and Severin [23] recently reported ruthenium- and rhodium-based TSAs for the transfer hydrogenation reaction. These complexes were used as catalyst precursors in combination with molecular imprinting techniques. Phosphinato complexes were prepared as analogs for the ketone-associated complex. They demonstrated that the results obtained in catalysis were better in terms of selectivity and activity when these TSAs were imprinted in the polymer. This shows that organometallic complexes can indeed serve as stable TSAs (Figure 4.9). 

In 1995, Lemaire [32] reported the immobilized transfer hydrogenation catalyst using a tetramine-rhodium complex 35 in order to study the effect of molecular imprinting. The upper rhodium complex was formed by copolymeiization with a diisocyanide in the presence of sodium (S)-phenylethanolate (PM) leading to the imprinted polymers 36a and 36b (Scheme 21). The PM is then removed by adding 2-propanol. The resulting polymer as well as the non-templated polymer were tested in the transfer hydrogenation of acetophenone and phenylethylketone in the presence of 5 mol % of Rh-polymer with KOH/[Rh] ratio of 4 at 60°C. Results are presented in Table 3. 

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