Preparation of Diorganozincs

Diorganozincs are usually more reactive totvard electrophiles than organozinc halides. A tvide range of methods is available for their preparation. The oldest being the direct insertion of zinc to an alkyl halide (usually an alkyl iodide) lead- 

Since the isopropyl group is also transferred in the reaction with an electrophile at a comparable rate as the second R group, an excess of electrophile has to be added and tedious separations may be required. A more straightforward approach is possible for diarylzincs. In this case, the I/Zn-exchange can be performed under very mild conditions [97]. 

The reaction of 4-bromotoluene with lithium in the presence of zinc bromide in ether affords the corresponding zinc reagent 118 that undergoes a smooth 1,4-addition to sterically hindered enones and leads to the cydopentanone 119 in 67% yield [99]. 

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The procednres nsed for the preparation of diorganozinc componnds mnst be modified in order to obtain organozinc... 

Vettel, S., Vaupel, A. and Knochel, P. 1995. A new preparation of diorganozincs from olefins via a nickel catalyzed hydrozincation. Tetrahedron Lett. 36 1023-1026. 

Scheme 7.42 Preparation of diorganozincs bearing a silyl ether using a boron-zinc exchange.

Scheme 11.23. Preparation of various functionally substituted aminocyclopro-panes from diorganozinc reagents 80 and N,N-dialkylcarboxamides [119],...

Scheme 2.30. Preparation of a diorganozinc compound by iodine-zinc exchange.

As outlined, electrophilic animation of diorganozincs with A-mono- and A,A-disub-stituted O-benzoylhydroxylamines is a general procedure and an easy route for the preparation of secondary and tertiary amines from diorganozincs. 

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. 

Triorganozincates are prepared by the reaction of diorganozincs with organolithium... 

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. 

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