Preparation methods of diorganozincs

PAUL KNOCHEL, PHILIP JONES, and FALK LANGER 

Alternatively, it is possible to prepare almost salt-free diorganozincs in solution by the precipitation of the magnesium salts with 1,4-dioxane.7 However, the preparation of functionalized diorganozincs is generally not possible using these methods and accordingly new methods have recently been developed for such preparations, utilizing  

The addition of diorganozinc reagents to a-alkoxyaldehydes furnishes selectively protected 1,2-diols.19 Applications toward the synthesis of pheromones like (-)-exo- and (-)-endo-brevicomin 2 and 3 exploits the catalytic nature of the stereochemical induction, e.g. the newly formed chiral centre depends only on the configuration of the chiral catalyst 1. 

The solvents used (THF, ether, toluene) were dried and distilled over Na/K under argon. Diethylzinc was purchased from Witco (Bergkamen, Germany). CuCN was used as received (Degussa, Germany). All lithium and zinc halides 

Preparation of b/s(3-cyanopropyl)zinc by iodine-zinc exchange and its copper-mediated coupling with an electrophile preparation of 3-C3-cyanopropyD-cyclohexanone8 

---

Asymmetric addition of diorganozincs to aldehydes catalyzed by chiral -amino alcohols provides a general method for the preparation of chiral secondary alcohols. Oguni, Noyori, and co-workers found that the aminoalcohol, (2S)-3-exo-(dimethylamino)isobornenol ((2S)-DAIB), acts as a particularly efficient promoter for this asymmetric reaction [9, 10]. Reaction of benzalde-hyde with diethylzinc in the presence of 2 mol% of (2S)-DAIB gives, after aqueous workup, (S)-l-phenylpropanol in high yield with 99% ee as shown in Scheme 8. Detailed mechanistic and theoretical studies of the (2S)-DAIB-pro-moted asymmetric addition have been reported [11]. 

A major part of this chapter will show the synthetic applications of these transmetallation reactions for the perfonnancc of cross-coupling reactions. After a short description of the methods of preparation of organozinc halides (RZnX) and diorganozincs (R2Zn), the utility of these reagents for forming new carbon-carbon bonds will be presented. Emphasis will be... 

Diorganozinc reagents 55 require different methods of preparation, snch as iodine-zinc and boron-zinc exchange to form 56 (Langer et al. 1993,1996). These methods are applicable to the formation of primary and secondary diorganozinc species 55 (Scheme 7.20). 

The hydroboration of dienic silyl enol ethers, such as 124, with LItBH leads to organob-oranes which can be converted to new diorganozincs, such as 125 (Scheme 42) . More importantly, this method allows the preparation of chiral secondary alkylzinc reagents. 

---