Allylic bromides interconversions

The stereospecificity of the allylation in Scheme 5.12 is rationalized by identifying the most favorable transition state of the retro-allylation (Scheme 5.14). Oxidative addition of Np-Br followed by bromide-alkoxide exchange with 4 generates intermediate 6 or 7. In the case of threo-4,6a should be the more favorable conformation than 6b according to the routine conformational analysis of chairlike transition states. The retro-allylation thus predominantly proceeds from 6a to yield ( )-8. Thanks to bulky tricyclohexylphosphine, the following reductive elimination should take place rapidly prior to o-jt interconversion to retain the E-stereochemistry. A similar picture is applicable to the reaction of erythro-4, where 7a is more favorable than 7b to afford the corresponding Z-product selectively. 

With a racemic mixture of the secondary Grignard reagent, asymmetric cross-coupling with chiral catalysts creates a stereogenic center on the nucleophile. Using (iS,S)-chiraphos as ligand, the facile interconversion between the two enantiomers of a-phenethylmagnesium bromide allows the formation of the allylated product in 87% yield and 58% ee by a dynamic kinetic asymmetric transformation (Eq. 8E.28) [207]. 

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