Mixed zincate

C. Preparation of Mixed Zincates by Halogen-Zinc Exchange Reaction. . . 688... 

Mixed zincates R1 (R2)2ZnLi are prepared either from (R2)2Zn and R1 Li or by successive addition of R2Li (2 equiv) and R Li (1 equiv) to ZnX2. Sodium and potassium zincates are prepared from diorganozinc according to Wanklyn s procedure (equation 5)1. 

The chiral organocopper compound (186) adds diastereoselectively to 2-methyl-2-cyclopentenone, allowing the preparation of optically active steroid CD-ring building blocks (Scheme 68).202-204 A related method was applied to a synthesis of the steroid skeleton via an intramolecular (transannular) Diels-Alder reaction of a macrocyclic precursor.203 Chiral acetone anion equivalents based on copper azaeno-lates derived from acetone imines were shown to add to cyclic enones with good selectivity (60-80% ee, after hydrolysis).206-208 Even better ee values are obtained with the mixed zincate prepared from (187) and dimethylzinc (Scheme 69). Other highly diastereoselective but synthetically less important 1,4-additions of chiral cuprates to prochiral enones were reported.209-210... 

Zincates derived from the ketone ,/(-dianions react with enones, affording unsymmetrical 1,6-diketones (Table 5)29. As shown in the second example of Table 5, quenching with TMSC1 gave the corresponding dienol disilyl ether. A mixed zincate consisting of the dianion and methyllithium in a 2 1 ratio gave comparable results to the symmetrical zincate. 

The more difficult cross-coupling reaction between Csp -Csp can be accomplished by mixed zincate compounds, prepared by the reaction of diorganozinc derivatives with Me2CuMgCl-MgCl(CN) (equation 59). Substitution reaction at Csp centers are also efficiently catalyzed by Ni salts when the alkyl halide contains a double bond in remote position (Scheme 22). Changing the double bond for a triple bond leads to an intramolecular carbozincation reaction (equation 60). 

Reagents formed in situ by reaction of r-BUjZnI with organic halides add to carbonyl compounds, such mixed zincates do not transfer their f-butyl group. a-Diazo- 3-hydroxy esters are formed by treatment of diazoacetic esters with Et Zn and aldehydes at low temperature. Allylzinc species derived from allylic esters add to carbonyl compounds to give a- or y-adducts according to the nature of the latter. ... 

---