Reduction via inclusion complexes

Reduction via inclusion complexes has received considerable attention because it allows both the diastereo- and enantioselectivity of the reaction to be controlled. Cyclodextrins and water-soluble cyclophanes are the most frequently used hosts in aqueous medium. P-CD/ketone (1 1) complexes suspended in NaBH4 aqueous alkaline solution give the corresponding alcohols quantitatively but with modest ee [75]. An improvement is obtained by using crystalline a-, P- and y-CD inclusion complexes of achiral amine-boranes at 0°C in heterogeneous aqueous medium [76]. Under these conditions, 1-phenylethanone and 4-phenyl-2-butanone give the (5) and (R) alcohol with 91% and 89% ee, respectively. 

Aqueous suspensions of ferrocenyl ketone p-CD inclusion complexes are reduced with NaBH, [77], The presence of salting-out agents (LiCl, NaCl, KCl) increases the enantioselectivity of the reaction (Table 6.10). 

Enantioface differentiating reduction of arylglyoxylic acids can be performed with NaBH4 in aqueous buffer media using a modified CD, 6-deoxy-6-amino-p-cyclodextrin [78]. The reaction proceeds with high yields but the arylglycolic acids obtained had low ee. The use of water-soluble chiral paracyclophanes does not increase the enantioselectivity of the reaction. 

The ring-opening reaction rate of epoxides with NaBH, is very slow in aqueous media, but in the presence of CDs the reaction proceeds smoothly with high regioselectivity, and racemic epoxides are kinetically resolved [79]  

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