Asymmetric ethylation of aromatic aldehyde by diethylzinc in the presence of chiral auxiliaries derived from (S)-[4-HOC6H4CH2CH(NH2)CPh2OH]

A comparative study has been carried out on enantioselective ethylation of aromatic aldehydes (ArCHO) by diethylzinc (Et₂Zn) in the presence of catalytic amounts of (S)-4-HOC₆H₄CH₂CH(NH₂)CPh₂OH] (1) or chiral auxiliaries derived from (1). Various procedures have been adopted; these involved (i) different sequences of addition of the reagents ArCHO, Et₂Zn and (1), and (ii) use of titanium(IV)isopropoxide Ti(OPrⁱ)₄] or the boranetetrahydrofuran addition compound (BH₃ · THF) as additional reagents. In all but one of the procedures adopted, (S)-ArCH(OH)Et was selectively formed; enantiomeric excess values up to 87% were obtained.