Supercritical alcohol, reduction

There are still many developments in selective hydrogenation, both in terms of new catalysts and process operations. An example of the first is the discovery that Sn-substituted zeolite beta is the most active heterogeneous catalyst for the Meer-wein-Pondorff-Verley reduction of aldehydes and ketones to the corresponding alcohols, with high cis-selectivity (99-100%) in the reduction of 4-alkylcyclohexa-nones [301]. An example of process development is in the heterogeneous catalytic hydrogenation of organic compounds in supercritical fluids (SCFs) [302]. 

Reductions. Aldehydes are reduced to alcohols with HCOONa in supercritical water. The conditions can be controlled such that ketones are not affected. 

Enzymes are an increasingly available and important tool in the arsenal of the synthetic chemist. Enzymatic reductions are often straightforward and highly stereoselective. There are now many enzymatic transformations that are compatible with the use of organic solvents.6l5 Other solvents can be used as well, illustrated by the enzyme alcohol dehydrogenase from Geotrichum candidum, which is active in supercritical carbon dioxide.6i6 Prelog studied the reduction of ketones with several enzymatic systems. Reduction of... 

The couple S/S is selected with a specific and intense optical absorption of S or S , so that the electron-transfer reaction can be observed directly. In the early stages of atom coalescence, the redox potentials of the atom and of the smallest clusters are generally far below that of the donor and the transfer from S to the oligomer does not occur. The ion reduction is caused exclusively by solvated electrons and alcohol radicals (Eqs. 2, 8, and 9). The nucleation and coalescence dynamics are thus the same as in the absence of (Eqs. 10 and 11). Beyond a certain critical time, tc, that is large enough to enable the growth of clusters and the increase of their potential above the threshold imposed by the electron donor S , electron transfer from this monitor to the supercritical clusters is allowed (Eq. 32) and detected by the absorbance decay of S (Fig. 6). For n > ny. 

The utilization of biocatalysts other than hydrolytic enzymes has also been investigated. Alcohol dehydrogenase is used in the asymmetric reduction of ketones to yield optically active secondary alcohols (Scheme 51). " Although the productivity (substrate concentration) is low, high yields and excellent enantiomeric excess are obtained. On the other hand, carboxylation of pyrrole is efficient in SCCO2 (Scheme 52). The reaction under supercritical conditions is 12 times faster than that... 

Asymmetric reduction of ketones by crude alcohol dehydrogenase from G. candidum was carried out in aqueous-supercritical fluids biphasic systems (Figure 3.20) [30]. It was found that the lowered pH caused by scCO irreversibly inactivated the enzyme. In order to overcome this undesired side effect of scCO, basic salts were added to the reaction mixture. Sodium bicarbonate was found to be the best among several tested. For the reduction of acetophenone, adding sodium bicarbonate improved the )deld from 4% to 25%. The asymmetric reduction of o-fluoroacetophenone with the immobilized... 

---