Stereoselective Stille Coupling of Enantiopure Haloallenes and Alkenylstannanes for the Synthesis of Allenyl Carotenoids. Experimental and Computational Studies

The stereoselectivity of the Stille cross-coupling of chiral enantiopure haloallenes and alkenylstannanes en route to allenic carotenoids is shown to depend on the nature of the halogen and palladium catalyst as well as on the presence of DMF as coordinating ligand and solvent. The results are consistent with DFT studies (B3LYP/6-31G*) on haloallene model systems, which compare the energetics of the competing oxidative addition to the allene−halogen bond (with retention of configuration) and the SN2′ displacement of the haloallene (with inversion of configuration) by the palladium complex.