Aryl organozinc reagents

Even if hundreds of chiral catalysts have been developed to promote the enantioselective addition of alkylzinc reagents to aldehydes with enantioselectivities over 90% ee, the addition of organozinc reagents to aldehydes is not a solved problem. For example, only very few studies on the addition of vinyl groups or acetylides and even arylzinc reagents to aldehydes have been published, in spite of the fact that the products of these reactions, chiral allylic, propargylic and aryl alcohols, are valuable chiral building blocks. 

Organozinc reagents have been used in conjunction with a-bromovinylboranes in a tandem route to Z-trisubstituted allylic alcohols. After preparation of the vinylborane, reaction with diethylzinc effects migration of a boron substituent with inversion of configuration and exchange of zinc for boron.176 Addition of an aldehyde then gives the allylic alcohol. The reaction is applicable to formaldehyde alkyl and aryl aldehydes and to methyl, primary, and secondary boranes. 

Reactions of Organozinc Reagents with Alkyl, Aryl, and Alkenyl Halides 405... 

Scheme 6.43 Palladium-catalyzed a-arylation of esters and amides with organozinc reagents.

Crucially, this allows organozinc reagents to be prepared from less reactive aryl bromides and secondary or tertiary alkyl bromides. Alternatively, organozinc iodides can be prepared by means of a palladium(0)-catalyzed reaction between alkyl iodides and Et2Zn (Scheme 2.25) [53-56]. 

Aryl-substituted enones (chalcones in particular) have been used as model substrates in studies of catalytic 1,4-additions vith organozinc reagents. Fig. 7.7 summarizes typical enantioselectivities achieved vith various chiral ligands. 

Aryl and alkyl organozincate reagents, generated in situ by reaction of Grignard reagents and sub-stoichiometric amounts of ZnCl2, cross-couple smoothly in refluxing THF with functionalized aryl and alkenyl as well as primary and secondary alkyl chlorides in the presence of Pd(dppf)Cl2. ... 

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