Increased enantioselectivity in the addition of diethylzinc to benzaldehyde by the use of chiral ligands containing the alpha-phenylethylamino group in combination with achiral ligands.
Chiral ligands (S,S)-1, (S,S)-2, (S,S)-3, (S)-4, (S)-5, (S,S)-6, (S,S)-7, and (S,S)-8 turned out to be effective promoters in the enantioselective addition of diethylzinc to benzaldehyde. Interestingly, diamine (S,S)-3 and amino alcohols (S)-5 and (S,S)-7 induce the preferential formation of carbinol (R)-10 (unlike stereoinduction) whereas amido analogues (S,S)-2, (S)-4, and (S,S)-6 favor (S)-10 (like stereoinduction). Molecular modeling at the semiempirical PM3 level provided a reasonable interpretation based on conformational effects in the corresponding transition structures. Combinations of chiral ligands 1-8 with an achiral, flexible ligand (9) gave rise to an activated catalytic system that resulted in faster and higher yielding reactions. Furthermore, substantial increases in the observed enantiomeric excesses of product 10 confirmed the relevant role of achiral bis(sulfonamide) 9 as activator and "chiral environment amplifier".
Departamento de Química, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Apartado Postal 14-740, 07000 México, D.F., México.
Omar
O
Muñoz-Muñiz
Eusebio
E
Juaristi