Organozinc compounds enantioselective addition

The second structural motif, B (Figure 72), involves also a dimeric structure in which two alkoxy oxygen atoms bridge between two zinc atoms, but now an additional intramolecular coordinating substituent is present, resulting in the formation of four-coordinate zinc atoms. Most of these strucmral investigations are aimed at the elucidation of the mechanism of the enantioselective addition of organozinc compounds to aldehydes, a topic that will be discussed in Chapter 13. 

Nowadays, this chemistry includes a wide range of applications. The organozinc compounds employed in the enantioselective addition include dialkylzincs, dialkenylzincs, dialkynylzincs, diarylzincs and the related unsymmetrical diorganozincs. Electrophiles have been expanded to aldehydes, ketones and imines. Asymmetric amplification has been observed in the enantioselective addition of organozincs. Recently, asymmetric autocatalysis, i.e. automultiplication of chiral compounds, has been created in organozinc addition to aldehydes. 

In this chapter, we deal with these topics with an emphasis on the results from our group. However, it is beyond the scope of this chapter to include all of the papers regarding the enantioselective addition of organozinc compounds. Readers are encouraged to refer to the excellent reviews on these topics1 II.. 

Enantioselective Addition of Organozinc Compounds to Carbonyl Compounds Chiral Amplification... 

Even the very efficient enantioselective catalysts used in organozinc addition reactions to carbonyl compounds failed to catalyze the corresponding addition reactions to nonactivated imines such as A-silyl-, A-phenyl-, or iV-benzyl-imines. However, enantioselective additions of diaUcylzinc compounds to more activated imines, like iV-acyl- or iV-phosphinoyl-imines, in the presence of catalytic or stoichiometric amounts of chiral (see Chiral) aminoalcohols, have been recently reported. For example, in presence of 1 equiv of (A,A-dibutylnorephedrine) (DBNE) diethylzinc reacts with masked A-acyl imines like A-(amidobenzyl)benzotriazoles, to give chiral A-(l-phenylpropyl)amides with up to 76% e.e. (equation 68). 

Enantioselective addition of organozinc reagents to cai bonyl compounds furnishes chiral alcohols. ... 

Many other aminoalcohols and related compounds have been recommended as catalysts in enantioselective additions of organozinc reagents to aldehydes. Tri-carbonyl chromium complexation of the aryl ring of 1.14 (R = Me,n-Bu) can lead to improved enantioselectivities [1171], Other arenechromium carbonyl com-plexed aminoalcohols have also been proposed as Zn ligands [1172],... 

Similarly, in other enantioselective additions of organozinc compounds asymmetric autocatalyses were also demonstrated, [17] though they always gave a product with a much lower ee value than that of the catalyst used. Only at the end of 1995 was an important breakthrough achieved [3,4] when Soai et al. showed that the presence of 20 mol% of the pyrimidyl alcohol (S)-9a... 

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