Highly enantioselective arylation of aldehydes and ketones using AlArEt(2)(THF) as aryl sources

A series of AlArEt(2)(THF) (Ar = Ph (1a), 4-MeC(6)H(4) (1b), 4-MeOC(6)H(4) (1c), 4-Me(3)SiC(6)H(4) (1d), 2-naphthyl (1e)) were synthesized from reactions of AlEt(2)Br(THF) with ArMgBr. In CDCl(3) solution, the (1)H NMR spectra showed that AlArEt(2)(THF) compounds exist as a mixture of four species of formulas of AlAr(x)Et(3-x) (THF) (x = 0, 1, 2, or 3). AlArEt(2)(THF) compounds were found to be superior and atom-economic reagents for asymmetric aryl additions to organic carbonyls. Aryl additions of AlArEt(2)(THF) to aldehydes catalyzed by the titanium(IV) complex of (R)-H(8)-BINOL were efficient with a short reaction time of 1 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 AlArEt(2)(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 AlArEt(2)(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.