Diorganozinc reactivity

The relatively low reactivity of diorganozinc compounds is demonstrated by the fact that even the addition of 2 or 3equiv. of ZnPh2 to [NbCl2 N(SiMe3)2 2] gave only the monophenyl product [NbCl N(SiMe3)2 2Ph].61... 

Since the early disclosure by Negishi that zinc halide salts accelerate Pd(0) -catalyzed crosscouplings between vinyl zirconocenes and various halides [78], several methods have been developed that extend the utility of this metathesis process from a zirconium chloride to a zinc chloride (79 Scheme 4.47). Alternatively, routes to more reactive diorganozinc intermediates, e. g., using Me2Zn, convert readily available zinc derivatives to mixed species 80, which selectively couple with various electrophiles [14]. 

In the case of primary organoboranes, the exchange reaction is best performed with Et2Zn, whereas less reactive secondary organoboranes require the use of iPr2Zn. Thus, a wide variety of terminal olefins have been converted into primary diorganozincs such as 87-89 (Scheme 2.32). 

The moderate intrinsic reactivity of zinc organometallics can be increased by transmetalation with various transition metal salts. Especially useful is the transmetalation of diorganozincs or organozinc halides with the THF-soluble complex of copper(I) cyanide... 

It was also reported that these reactions proceed equally well in pure THF when two equivalents of LiBr are added13. Alternatively, the diorganozinc can be prepared reacting ZnBr2 with two equivalents of RLi or RMgBr, thus producing LiBr or MgBr2, respectively in situ. Obviously, LiBr and polar cosolvents such as NMP increase the reactivity of intermediates like 2515,1 . 

Diorganozinc reagents display an enhanced chemical reactivity compared to alkylzinc halides and furthermore, they are better suited for applications in asymmetric synthesis.38,39... 

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