Gas-Phase Study of Phenylacetylene and Norbornadiene on a Palladium(II) Phosphinous Acid Complex: Importance of the Order of Introduction of the Organic Partners

Abstract

Palladium(II) acetate complexes with phosphinous acids such as tert-butyl(phenyl)phosphinous acid (PAPd) have been successfully applied in a new [2 + 1] cycloaddition reaction between norbornadiene (NBD) and phenylacetylene (PhCCH) to produce exo-phenylmethylidenecyclopropane in the solution phase. In the present study, collision-activated reaction (CAR)/collision-induced dissociation (CID) mass spectral experiments were performed on a cationic Pd(II) complex (Pd(OPPhtBu)2H ion, [a]+). These experiments were useful for the identification of the reactive intermediates in the gas phase. On the basis of the results of the collision-activated reactions on this particular Pd(II) complex ion with phenylacetylene and norbornadiene, we demonstrated the essential role of the order of addition of organic partners on the efficiency to form the diadduct ion [a + PhCCH + NBD]+. Theoretical calculations combined with mass spectrometry experiments support first a change in coordination mode of alkyne (η2 into η1) prior to the coordination of NBD, yielding an insertion product. However, in this gas-phase reaction procedure, the role of the acetate ligand in the reactivity of the neutral complex is not addressed.