Hydrophobically directed selective reduction of ketones.

Three hydrophobic borohydrides carrying phenyl, pentafluorophenyl, and beta-naphthyl groups were used to reduce ketones in water and in methanol. With ketones carrying phenyl, naphthyl, or biphenyl substituents, there was preferential reduction in methanol of competing acetyl groups, either intermolecular or intramolecular, but preferential reduction of the aryl ketones in water. In the most extreme case, there was a 40-fold selectivity reversal. Lithium borohydride showed no such change in selectivity and favored acetyl reduction in both solvents. Salt and cosolvent effects indicate that hydrophobic packing is involved in the reaction of hydrophobic reagents with the aryl ketones. Some special interaction of the pentafluorophenyl group with aryl rings was also detected.