Transmetallation to zinc

The key to the success of the synthesis was the development of a novel method for enantioselective formation of a-arylpyrrolidines. In this method, (-)-sparteine-mediated, enantioselective lithiation of N-Boc-pyrrolidine 19 was followed by an in situ transmetallation to zinc and Pd-catalyzed coupling reaction with aryl bromide 3, which afforded 2-arylpyrrolidine in 63% isolated yield and 92% ee. Notably, the acidic aniline NH2 group was tolerated under the coupling reaction conditions. 

It is well known that alkenylzirconocenes are transmetalated to zinc and conveniently undergo further palladium-catalyzed cross-coupling reactions [7k,8a,32]. Although the transmetalation of zirconacyclopentadienes to zinc is not yet well developed, some interesting reactions that take place in the presence of ZnCl2 have already been described (Eq. 2.30) [9]. 

On the other hand, hydrozirconation-transmetalation to zinc, when carried out in THF, can be used to prepare allylic amines. When carried out in CH2CI2, however, a different reaction pathway is followed (Scheme 24). Even more, with a different order of introduction of the reagents, it is possible to obtain diastereoselectively homoaUyhc amines instead of the expected aminocyclopropanes. ... 

Transmetallation of the organic group from zirconium to another metal opens up possibilities. The palladium-catalysed coupling reactions can be found in Section 2.4. Addition of dimethyl cuprate results In transmetallation to copper. The resulting cuprate then displays typical cuprate reactivity, such as addition to enones. More economically, small amounts of copper can catalytically activate the zirconium complex towards this kind of chemistry, although the precise mechanism is unclear. Additions to enones can also be achieved directly using nickel catalysis (Scheme 5.64). Transmetallation to zinc has also been demonstrated. ... 

Walsh developed an efficient protocol for the asymmetric catalytic vinylation of a variety of aldehydes and ketones, using the vinylzinc species generated in situ by treatment of terminal alkynes with Schwartz s reagent for hydrozirconation, followed by transmetallation to zinc via reaction with Me2Zn. Under the catalysis of titanium complex of the chiral dihydroxyl bis (sulfonamide) ligand, the reaction provides access to the corresponding secondary or tertiary allylic alcohols in high yields and excellent enantiomeric excesses [65] (Scheme 14.13). 

Preparation of the triester 300 via chemoselective hydroboration, transmetalation to zinc and conjugate addition ... 

The sequential metallation of a terminal alkyne, transmetalation to zinc and selective mono-coupling of l-bromo-2-iodoaIkene fiimish enynes that are prone to a subsequent cross-coupling reaction (Scheme 5-128). ... 

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