Towards catalyst compartimentation in combined chemo- and biocatalytic processes: immobilization of alcohol dehydrogenases for the diastereoselective reduction of a ß-hydroxy ketone obtained from an organocatalytic aldol reaction.

The alcohol dehydrogenases (ADHs) from Lactobacillus kefir and Rhodococcus sp., which earlier turned out to be suitable for a chemoenzymatic one-pot synthesis with organocatalysts, were immobilized with their cofactors on a commercially available superabsorber based on a literature known protocol. The use of the immobilized ADH from L. kefir in the reduction of acetophenone as a model substrate led to high conversion (>95%) in the first reaction cycle, followed by a slight decrease of conversion in the second reaction cycle. A comparable result was obtained when no cofactor was added although a water rich reaction media was used. The immobilized ADHs also turned out to be suitable catalysts for the diastereoselective reduction of an organocatalytically prepared enantiomerically enriched aldol adduct, leading to high conversion, diastereomeric ratio and enantioselectivity for the resulting 1,3-diols. However, at a lower catalyst and cofactor amount being still sufficient for biotransformations with "free" enzymes the immobilized ADH only showed high conversion and >99% ee for the first reaction cycle whereas a strong decrease of conversion was observed already in the second reaction cycle, thus indicating a significant leaching effect of catalyst and/or cofactor.

Highly enantioselective organocatalytic trifluoromethyl carbinol synthesis--a caveat on reaction times and product isolation.

Aldol reactions with trifluoroacetophenones as acceptors yield chiral α-aryl, α-trifluoromethyl tertiary alcohols, valuable intermediates in organic synthesis. Of the various organocatalysts examined, Singh's catalyst [(2S)-N-[(1S)-1-hydroxydiphenylmethyl-3-methylbutyl]-2-pyrrolidinecarboxamide] was found to efficiently promote this organocatalytic transformation in a highly enantioselective manner. Detailed reaction monitoring ((19)F-NMR, HPLC) showed that, up to full conversion, the catalytic transformation proceeds under kinetic control and affords up to 95% ee in a time-independent manner. At longer reaction times, the catalyst effects racemization. For the product aldols, even weak acids (such as ammonium chloride) or protic solvents, can induce racemization, too. Thus, acid-free workup, at carefully chosen reaction time, is crucial for the isolation of the aldols in high (and stable) enantiomeric purity. As evidenced by (19)F-NMR, X-ray structural analysis, and independent synthesis of a stable intramolecular variant, Singh's catalyst reversibly forms a catalytically inactive ("parasitic") intermediate, namely a N,O-hemiacetal with trifluoroacetophenones. X-ray crystallography also allowed the determination of the product aldols' absolute configuration (S).