Organozinc halides transmetalation reactions

Preparation of organozinc halides using transmetalation reactions. . . 296... 

In summary, the direct insertion of zinc dust to organic halides is an excellent method for preparing a broad range of polyfunctional organozinc halides bearing various functional groups like an ester" , an ether, an acetate" , a ketone, cyano", halide" , N,N-bis(trimethylsilyl)amino °, primary and secondary amino, amide, phthalimide , sulfide, sulfoxide and sulfone , boronic ester , enone " or a phosphonate . An alternative method is based on transmetalation reactions. 

The electrochemical preparation of organozinc compounds obtained from the corresponding aromatic halides and with the use of a nickel complex as catalyst is only efficient in dimethylformamide as solvent. Moreover, in most cases and as described previously, the reaction requires the presence of excess 2,2 -bipyridine (five molar equivalents with respect to nickel) to achieve the transmetallation reaction leading to the organozinc compound and to avoid the formation of biaryl, Ar-Ar (equation 53). 

Organozinc halides, RZnI, undergo transmetallation to RCu(CN)ZnI on reaction with CuCN-2LiCl, prepared by reaction of CuCN with anhydrous LiCl in THF. The... 

Some interest in group 12 phosphide and arsenide complexes has come from their possible applications as precursors for IFV semiconductors (see Semiconductors). They can be prepared by two different ways, namely, by metaUa-tion reaction of phosphines and arsines with diorganozinc compounds (equation 32), and by transmetallation reaction between organozinc halides and lithium salt of the corresponding phosphines and arsines (equation 33). 

These reactions turned out to be preparatively very useful since they allow the performance of radical cyclization reactions but produce an organozinc halide as a final product (Scheme 9-33) [65-70]. The treatment of an unsaturated alkyl halide 36 (X = Br, I) with a palladium(O) or nickel(O) complex produces, via a one-electron transfer [72], a paramagnetic nickel(I) or palladium(I) salt MLj,(X) (M = Ni, Pd) and a radical 37 which undergoes a smooth cyclization reaction and produces, after recombination with the transition-metal moiety, the nickel(ll) or palladium(II) species 38. After transmetallation with a zinc(II) salt, a stable organozinc cyclopentylmethyl derivative of type 39 is produced. 

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... 

When organozinc halides are synthesized in situ by the reaction of organic halides with zinc metal or the zinc-copper couple, DME or benzene-DMA can be used as the solvent. " In the case of organozinc halides prepared by transmetallation of... 

Since transmetalation of organozinc halides in the presence of palladium or nickel compounds easily proceeds, the coupling reaction is carried out by the catalytic reaction via formation reaction of these organometallic compounds. For example, an aromatic /i-ester is prepared in a high yield by a reaction with a palladium catalyst as shown in eq. (5.31) [78]. 

Preparation of Organozinc Halides using Transmetallation Reactions... 

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