Enantioselective additions of diorganozincs to aldehydes using chiral catalysts

Enantioselective additions of diorganozincs to aldehydes using chiral catalysts 

As the nucleophilicity of dialkylzinc is accelerated by the presence of (3 amino-alcohol, diethylzinc adds to benzaldehyde to afford 1-phenylpropanol.2 Several duraTfS-aminoalcohols 1-7 have been proven to be highly enantioselective catalysts for the addition of primary dialkylzincs mostly to aromatic aldehydes. For example, 1-phenylpropanol is prepared with high ee from the enantioselective addition of diethylzinc to benzaldehyde using 1-7 (Fig. 12.1, the ee and configuration of prepared 1-phenylpropanol are shown in parentheses).3 8 

Asymmetric amplification, i.e. ee of the product is higher than that of the chiral catalyst, is observed with some chiral (3-aminoalcohols 73g and 3.3° Formation of the less reactive dimeric complex from (+) and (-) catalysts increases the ee of the higher reactive monomeric catalyst. The remaining monomeric chiral catalyst with higher ee than that of the total chiral catalyst affords sec-alcohols with higher ee.3° 

With regard to diorganozincs other than primary dialkylzincs, various diorganozincs such as divinylzinc ( 96% ee, addition to benzaldehyde),4 difurylzinc (72% ee, addition to benzaldehyde),5 diphenylzinc (78-82% ee, addition to various aldehydes),6 and diisopropylzinc (93% ee, addition to benzaldehyde)7 have been utilized in the chiral (3-aminoalcohol catalysed reactions. Alkenyl(alkyl)zincs4b,c and alkynyl(alkyl)zincs8 afford the corresponding chiral allylic and propargyl alcohols. 

The enantioselective addition of dialkylzinc to aldehydes catalysed by chiraPp-aminoalcohol has also excellent chemoselectivities which are not usually achieved with Grignard reagents and alkyllithiums both of which have excess reactivities. Thus enantio- and chemoselective addition of dialkylzincs 

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