Chiral ligands derived from abrine.: Part 7:: Effect of O, S, N in aromatic ring substituents at C-1 on enantioselectivity induced by tetrahydro-β-carboline ligands in diethylzinc addition to aldehydes

The effect or O, S and N atoms in aromatic ring substituents at C-1 position of tetrahydro-B-carboline ligands on the enantioselectivity of diethylzinc additions to benzaldehyde was studied when esters or tertiary alcohol functions were present at C-3. A mechanism is proposed to explain why the ester ligands 2c and 2d, in which the pyridyl N atom is at C'-2 in 2c and at C'-3 in 2d, catalyzed the addition of diethylzinc to benzaldehyde to form the (R)- and (S)-enantiomers of 1-phenyl-1-propanol, respectively. An explanation was also proposed for the moderate enantioselectivity induced by tert-alcohol 3c versus the very small enantioselectivity induced by 3d. containing a 3-pyridyl function at C-1, during diethylzine additions. A -CH2-t-Bu substituent at C-1 leads to very high enantio selectivities. (C) 2001 Elsevier Science Ltd. All rights reserved.