| Popis: |
(−)-B-Chlorodiisopinocampheylborane [(−)-DIP-ChlorideTM, 1], introduced by us several years ago, has been shown to reduce prochiral aryl and alkyl perfluorinated ketones to the corresponding optically active alcohols in very high ee. For example, 2,2,2-trifluoroacetophenone, trifluoroacetyl-1-naphthalene, and trifluoroacetyl-2-naphthalene are all reduced with 1 within 1–3 d at rt in 90% ee, 78% ee and 91% ee, respectively. The optical purity of 1-phenyl-2,2,2-trifluoroethanol is upgraded to ≥99% ee by crystallizing the initially formed product from pentane. 1,1,2,2,2-Pentafluoropropiophenone and 1,1,2,2,3,3,3,-heptafluorobutyrophenone are reduced in 3 d with 1 to the corresponding alcohols in 92% ee and 87% ee, respectively. The reagent reduces alkyl trifluoromethyl ketones at a rate faster than that of the aryl derivatives, while still providing the product alcohols in very high ee. Thus, 1,1,1-trifluoroacetone, 1,1,1-trifluorononan-2-one, and 1,1,1-trifluorodecan-2-one are all reduced within 4–8 h in 89% ee, 92% ee, and 91% ee, respectively. Even α-sec-alkyl trifluoromethyl ketones are handled by 1 very efficiently. Thus cyclohexyl trifluoromethyl ketone is reduced by 1 at rt in 12 h to the product alcohol in 87% ee. In all of these cases the trifluoromethyl group acts as the enantiocontrolling larger group as compared to the aryl or alkyl group. This produces alcohol products with stereochemistry opposite to those obtained for the corresponding hydrogen analogs. The steric and electronic influence of the trifluoromethyl group in achieving enantiocontrol in asymmetric reductions is discussed. |
