Regioselective Transition-Metal-Free Allyl-Allyl Cross-Couplings

Readily prepared allylic zinc halides undergo S(N)2-type substitutions with allylic bromides in a 1:1 mixture of THF and DMPU providing 1,5-dienes regioselectively. The allylic zinc species reacts at the most branched end (gamma-position) of the allylic system furnishing exclusively gamma,alpha'-allyl-allyl cross-coupling products. Remarkably, the double bond stereochemistry of the allylic halide is maintained during the crosscoupling process. Also several functional groups (ester, nitrile) are tolerated. This cross-coupling of allylic zinc reagents can be extended to propargylic and benzylic halides. DFT calculations show the importance of lithium chloride in this substitution.