Copper compounds allylic displacement

While y-alkylations did not occur with saturated and benzylic halides, y-selectivities in the 62-99% range were observed in reactions of dicopper(I) dianions of a variety of a, -unsaturated acids with allylic halides. y-Unsubstituted allylic halides reacted by an Sn2 mechanism, y-disubstituted compounds underwent direct 5n2 displacement and y-monosubstituted systems reacted by both 5n2 and 5n2 pathways. Scheme 72 provides an example of the dramatic reversal in regioselectivity that was observed in the allylation of the dianion (147) of tiglic acid when the metal cation was changed from lithium to copper(I). The y-alkylation product from the latter species was exclusively the ( )-isomer. 

Allylic acetates or related compounds can also be displaced by organo-cuprates. A synthetic approach to prostaglandins (PC s) via cross-coupling of a vinylic copper reagent with an allylic electrophile has been elaborated (Corey and Mann, 1973). Notably the reaction of LXI and LXII proceeds exclusively by an Sn2 pathway. 

---