Asymmetric alkylation of aldehydes catalyzed by novel dinuclear bis-BINOLate titanium(IV) complexes
摘要:
Bis-BINOLs 1a,b in which two BINOL units are tethered by o- and m-phenylenebis(ethynyl) groups form stable dinuclear bis-BINOLate titanium(IV) complexes 2a,b by treatment with titanium tetraisopropoxide. In the presence of excess titanium tetraisopropoxide, 2a and 2b (2-20mol%) catalyze diethylzinc addition to aromatic and aliphatic aldehydes in an efficient manner to give the ethylation products with high enantioselectivities. While more than 1 equiv of titanium tetraisopropoxide (with respect to a substrate aldehyde) is generally employed for obtaining high turnover frequency and selectivity in reactions catalyzed by a parent (BINOLate)Ti(O'Pr)(2), the amount can be reduced as low as 0.2 equiv in the reactions catalyzed by 2a,b. (C) 2004 Elsevier Ltd. All rights reserved.