Transmetallation zinc organometallic

As well as alkenylstannanes [106-108], other classes such as a-heteroatom-substituted alkyltributylstannanes [109] and, more importantly, allylic stannanes [110, 111] also undergo these Sn-Cu transmetalations. Otherwise difficult to prepare, allylic copper reagents may, however, be obtained by treatment of allylic stannanes (produced in turn from organolithium, magnesium, or zinc organometallics) with Me2CuLi LiCN. They enter into cross-coupling reactions with alkyl bromides [110] or vinyl triflates (Scheme 2.52) [111]. 

A number of transmetalation procedures leading to zinc organometallics can be performed. Many organometallics having a polar C—Met bond are readily transmetalated by the reaction with a zinc salt to the more covalent organozinc compounds. The synthetic... 

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... 

The overall reaction makes it possible to perform intramolecular caibozincations [65-71] via a radical cyclization. This useful preparation of cyclopentylmethylzinc derivatives proceeds with excellent stereoselectivity and allows preparation of quaternary centers. After cyclization, the zinc organometallic can be transmetallated with CuCN-2LiCl and made to react with a broad range of electrophiles such as acid chlorides, allylic and alkynyl halides, ethyl propiolate, 3-iodo-2-cyclohexen-l-one, and nitroolefins such as nitrostyrene, leading to products of type 40 (see Section 9.6.9 Scheme 9-34) [65,70]. 

Whereas the direct cross-coupling reactions of zinc organometallics with organic electrophiles is of limited utility, the scope of these reagents after transmetallation... 

Transmetallation of zinc organometallics with manganese(II) salts does not occur and cannot be used to produce functionalized organomanganese compounds. The reaction of unsaturated alkyl bromides furnishes, in the presence of a mixed metal-salt system composed of copper(I) chloride and manganese(Il) bromide [32], cyclization products in satisfactory yields (see Section 9.6.17 Scheme 9-55) [94]. 

Far milder reaction conditions are possible if a transmetallation of the zinc organometallic (21) to the mixed c< >per-zinc derivative (22) is first performed and if the reaction is carried out in the presence of 2 equiv. of BFa-OEb. wide range of fimctional groups are tolerated in ctanpounds (21) and (22), and high yields are usually obtained (68-93% see Scheme 8), the reaction showing a good chemoselecdvity. The treatment of a 1 1 mixture of boizaldehyde and acetophenone with the organozinc iodide (23 2 h at... 

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