Highly Enantioselective Arylation of Aldehydes and Ketones Using AlArEt2(THF) as Aryl Sources

A series of AlArEt2(THF) (Ar = Ph (1a), 4-MeC6H4 (1b), 4-MeOC6H4 (1c), 4-Me3SiC6H4 (1d), 2-naphthyl (1e)) were synthesized from reactions of AlEt2Br(THF) with ArMgBr. In CDCl3 solution, the H-1 NMR spectra showed that AlArEt2(THF) compounds exist as a mixture of four species of formulas of AlArxEt3-x (THF) (x = 0, 1, 2, or 3). AlArEt2(THF) compounds were found to be superior and atom-economic reagents for asymmetric aryl additions to organic carbonyls. Aryl additions of AlArEt2(THF) to aldehydes catalyzed by the titanium(IV) complex of (R)-H-8-BINOL were efficient with a short reaction time of I h, affording aryl addition products as exclusive or main products in high yields and excellent enantioselectivities of up to 98% ee. Although ethyl additions to aldehydes occurred in minor extent, this study demonstrates that increasing the amount of AlArEt2(THF) from 1.2 to 1.4 or to 1.6 equiv significantly improved the aryl addition products of up to >99%. On the other hand, asymmetric arylations of AlArEt2(THF) to ketones employing a titanium(IV) catalyst of (S)-BINOL produced optically active tertiary alcohols exclusively in excellent enantioselectivities of up to 94% ee.