Evidence for competing syn and anti pathways in palladium-catalyzed eliminations of acyclic allylic carbonates

Syn- and anti-configured carbonates 6a and 6b, available from stereoselecti ve aldol additions and subsequent protection with methyl chloroformate, ser ve as probes for the elucidation of the stereochemistry of the P-Pd-H elimi nation. Upon treatment with [Pd(PPh3)(4)], the carbonates 6a and 6b give th e dienes 7a and 7b in different ratios; the latter stereoisomer 7b is forme d as a result of pi-sigma-pi conversions. Both syn and anti eliminations ar e shown to occur as competing reactions, the former one being the strongly preferred pathway. The highly reactive [Pd{P(nBu)(3)}(4)] catalyst, generat ed in situ from Pd(OAc)(2) and P(nBu)(3) causes thermodynamic control in th e elimination; thus, it serves as a smooth reagent for Z-E isomerization.