Cyclocarbopalladation involving an unusual 1,5-palladium vinyl to aryl shift as termination step: Theoretical study of the mechanism

A DFT/B3LYP model study has been carded out on the cyclocarbopallaclation and on an unusual 1,5 vinyl to aryl palladium shift which are the two first steps of a cyclocarbopalladation-Stille coupling tandem reaction of various gamma-bromopropargylic-1,2 diols with alkenyls or alkynyl stannanes catalyzed by Pd(PPh3)(4). From the calculations, the active intermediates in the catalytic process appear to bear a single phosphine ligand, the palladium(11) center keeping in all cases a square-planar coordination pattern either through intramolecular binding of the triple bond or via an intramolecular Pd center dot center dot center dot C-phenyl interaction. The computation of the various transition states and intermediates for the 1,5 vinyl to aryl palladium shift reveals that the intimate mechanism of this pathway corresponds to a one-step hydrogen transfer between the two negatively charged carbon atoms of the vinyl and phenyl groups. A two-step pathway involving a Pd(IV) intermediate is not likely to occur. This conclusion may apply to other 1,n-palladium shifts which have been experimentally observed in various organometallic transformations.