Transmetallation with CuCN-2LiCl

This method can be extended to the preparation of alkenylcopper compounds. Thus, treatment of the iodoalkenyl azide 10 with nBuLi at —100 °C (Scheme 2.3), followed by transmetalation with Znl2 in THF and then by a second transmetalation with CuCN-2LiCl, produces the copper species 11. This then effects a cis-selective carbocupration of ethyl propiolate to furnish the ( , ) diene 12 in 81% yield. 

When the Pd-catalyzed exchange protocol was applied to the 5-alkenyl iodides of type 51, the cyclized organozinc reagents 52 were directly obtained and functionalized by allylation with ethyl 2-bromomethylacrylate after transmetallation with CuCN 2LiCl. The five-membered ring products 53 were obtained with high trans diastereoselectivity, irrespective of the nature of the allylic substituent (R = Ph or OBz) at the allylic position (equation 21)34,39. By contrast, the zinc-induced carbocychzations were found to be moderately cis stereoselective when an acyloxy group was present at the allylic position (see Section II.B.2)29. 

TMEDA or sparteine) and transmetallation with CuCN 2LiCl, providing the adduct 51 with 90% yield. Enantioselective 1,4-additions of chiral, non-racemic a-aminoalkylcuprates (formed from the corresponding stannyl-carbamates by transmetallation, or by enantioselective deprotonation) have also been described recently.45,45a-45c... 

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]. 

Zinc and copper P-metallated alkylphosphonates, prepared by the insertion of zinc into dialkyl P-bromoalkylphosphonates followed by transmetallation with CuCN 2LiCl, react with ethyl propiolate and dimethyl acetylenedicarboxylate to give stereospecifically the i yn-adduct in 85% and 91% yields, respectively. ... 

Thienylzinc iodides can be prepared from iodothiophenes by treatment with activated zinc on graphite. These can be transmetalated with CuCN 2LiCl and reacted subsequently with an allylic halide, or acylated with acid chlorides <94TL1047>. 

This very active, chemoselective, sterically hindered base was employed to seleaively zincate 3,6-dibromopyridazine (42) in >95% yield (Scheme 19). Subsequent transmetalation with CuCN-2LiCl followed by benzoylation with benzoyl chloride gave (3,6-dibromopyridazin-4-yl)(phenyl)methanone (43) in 86% yield (Scheme 19). Treatment with other electrophiles, such as iodine and allylic bromides, furnished the respective dibromopyridazines in good yields. 

Copper salts are useful for 5-endo-dig furan formation and C3 functionalization. Treatment of 2-alkynylphenols 257 (Scheme 19.69) with BuLi and ZnCh produced the corresponding zinc phenoxides, which, when concentrated and heated in toluene, resulted in formation of the benzofurylzinc species. Transmetallation with CuCN/2LiCl afforded organocopper reagents 258, which were allowed to react with electrophiles to produce a variety of 2,3-disubstituted benzofurans 259 in good yields... 

The uncatalyzed reaction of acid chlorides with organozincs is sluggish and inefS-cient. It is often complicated by side reactions and leads usually to low yields of the desired acylation products. In contrast, the CuCN 2LiCl-mediated acylation of various zinc reagents affords ketones in excellent yields. The reaction of the zinc-copper reagent 328 obtained by the direct zinc insertion to the iodide 329 followed by a transmetallation with CuCN 2LiCl reacts with benzoyl chloride at 25 °C leading to the ketone 330 in 85% yield [52b]. The functionalized benzylic bromide 331... 

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