Enantioselectivity organozinc compound 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... 

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... 

The more recent development of NHC ligands, such as those derived from the imidazolium salt L30, has allowed the addition of dialkylzinc (Scheme 63) and diarylzinc reagent (Scheme 64) to simple unactivated p-substituted cyclic enones [98]. Very good yields and enantioselectivities are obtained with a wide variety of organozinc compounds only the less reactive MeaZn does not provide any conversion. 

An alternative process to the use of enantiodivergent ligands and applicable to add more functionalized organozinc species is the use of reagents 25a and 25b in a catalytic form [11]. This reaction is enantioselectively catalyzed in the addition process of organozinc compounds to generate 1,3-diols. 

TADDOL 104 and 126 afford 95-99% ee in the asymmetric addition of organozinc reagents to a variety of aldehydes. The best enantioselectivities are observed when a mixture of the chiral titanium TADDOL compound 127 and excess [Ti(OPr1)4] are employed (Scheme 2-49). The mechanism of the alkylzinc addition involves acceleration of the asymmetric catalytic process by the... 

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