Highly Enantioselective Arylation of Aldehydes and Ketones Using AlArEt_2(THF) as Aryl Sources

摘要

A series of A1ArEt_2(THF) (Ar = Ph(1a), 4-MeC_6H_4(1b), MeOC_1c), Me_3SiC_1d), 2-naphthyl(le)) were synthesized from reactions of AlEt_2Br(THF) with ArMgBr. In CDC1_3solution, the ~1H NMRspectra showed that AIArEt_2(THF) compounds exist as a mixture of four species formulas AlAr_x5Et_(3-x)(THF) (x = 0, 1, 2, or 3). AlArEt_2(THF) compounds were found to be superior and atom-economicreagents for asymmetric aryl additions to organic carbonyls. Aryl additions of AlArEt_2(THF) to aldehydescatalyzed by the titanium(IV) complex of (R)-H8—BINOL were efficient with a short reaction time of1 h, affording aryl addition products as exclusive or main products in high yields and excellentenantioselectivities 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 1.6 equivsignificantly improved the aryl addition products of up to >99%. On the other hand, asymmetric arylationsof AlArEt_2(THF) to ketones employing a titanium(IV) catalyst of (S)-BINOL produced optically activetertiary alcohols exclusively in excellent enantioselectivities of up to 94% ee.