Add time:08/30/2019         Source:sciencedirect.com

A systematic study of the asymmetric reduction of prochiral α-acetylenic α′-fluoroalkyl ketones with (−)-B-chlorodiisopinocampheylborane [(−)-DIP-Chloride, 1] and (−)-B-isopinocampheyl-9-borabicyclo[3.3.1]nonane (R-Alpine-Borane, 2) reveals that perfluoroalkyl acetylenic ketones can be reduced in very high ee (92->-99%) with both of these reagents. For example, 1,1,1-trifluoro-4-phenyl-3-butyn-2-one, 1,1,1,2,2-pentafluoro-5-phenyl-4-pentyn-3-one, and 4,4,5,5,6,6,6-heptafluoro-1-phenyl-1-hexyn-3-one are all reduced with 1 in EE at −25 °C within 0.25–2 h in 98%, 96%, and 94% ee, respectively. The same ketones are reduced with 2 under neat condition, within 1–4 h at rt in 98%, 97%, and 96% ee, respectively. Similarly, 1,1,1-trifluoro-3-octyn-2-one, 1,1,1,2,2-pentafluoro-4-nonyn-3-one, and 1,1,1,2,2,3,3-heptafluoro-5-decyn-4-one are reduced with both 1 and 2 in ≥97% ee. Difluoromethyl and monofluoromethyl acetylenic ketones are reduced with 2 in relatively high ee (78–88% ee) whereas 1 is ineffective for these types of ketones. In all of the above reductions, the fluoroalkyl group acts as the enantiocontrolling group with one exception. A remarkable inversion in selectivity in the reduction of monofluoromethyl acetylenic ketones with 1 is observed as compared to the reduction with 2, indicating that in the transition state the acetylenic moiety acts as the enantiocontrolling group instead of the anticipated monofluoromethyl group. These results highlight the combined effects of both the electronic and steric influences of the fluorine in controlling both the rate and the enantioselectivity in the asymmetric reduction of prochiral fluorinated acetylenic ketones.

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