Quinine copolymerization

These reactions, performed many times, show, in addition to the reversal of the absolute configuration of the product with the change in the configuration at C-8 and C-9 of the alkaloids, a small but reproducible difference in the e.e. of the product. It is evident that the diastereomeric nature of quinine vs. quinidine and cinchonidine vs. cinchonine expresses itself via small but important energy differences in the best fits of the transition states. Noteworthy in this respect is the fine work of Kobayashi (20), who observed larger differences (in the e.e. s of products) when the diastereomeric cinchona alkaloids were used as catalysts after having been copolymerized with acrylonitrile (presumably via the vinyl side chain of the alkaloids). 

Much greater success with chiral polymer catalysts was obtained by Norio Kobayashi (20). The Japanese researcher copolymerized quinine and acrylonitrile, using the vinyl group of the cinchona alkaloid as the connecting site. Enantiomeric yields of nearly 50% were realized with this polymer. 

Quinine-acrylonitrile copolymer. Cinchona alkaloids can be copolymerized with another vinyl monomer such as acrylonitrile with AIBN as initiator. The highest yield of polymer in the case of quinine is obtained when the quinine/acrylonitrile ratio is 1 20. This method was used to obtain a polymeric form of the alkaloid in which the crucial part of the molecule for asymmetric reactions—the amino alcohol unit—is free. The polymers are stable, light yellow solids, soluble in polar aprotic solvents (DMF and DMSO), but insoluble in common organic solvents. 

Recently, Wang [64] prepared by radieal copolymerization a cinchona alkaloid copolymer the methyl acrylate-co-quinine (PMA-QN (71)) (Scheme 34). Complexed with palladium(II), its catalytic activity in the heterogeneous catalytic reduction of aromatic ketones by sodium borohydride was studied. High yields in their corresponding alcohols are obtained but it is found that the efficiency of the catalyst depended on the nature of the solvent and the ketone which related to the accessibility of the catalytic active site. The optical yields in methanol and ethanol 95% were lower than in ethanol. This ability was attributed to a bad coordination between PMA-QN-PdCl2 and sodium borohydride and a reaction rate which was very rapid. The stability of the chiral copolymer catalyst was studied... 

In order to improve the enantioselectivity obtained with their supported alkaloid for the AD, Salvadori has prepared polymer 269 which contained a spacer between the alkaloid and the polymer backbone [167]. This one was isolated after copolymerization of quinine-based monomer 268 with styrene and DVB in a 1/7/2 molar ratio (Scheme 109). This polymer-supported quinine was then tested in the AD of trans stilbene, styrene and (A)-3-methylstyrene. The enantioselectivities were higher than those obtained with polymers 263-264 with a shorter reaction time at rt (4-6h) or 0°C (7h). At 0°C, the enantioselectivity reached 87% for the dihydroxylation of trans stilbene. This polymer was also reused after recovering without significant loss of the activity. 

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