Mechanistic Studies on Platinum(II) Catalyzed Hydroarylation of Alkynes

The dicationic acetylene platinum(II) complex [Pt(PNP)(C2H2)](BF4)(2) (PNP = 2,6-bis(diphenylphosphinomethyl)pyridine) was generated in situ by ligand substitution from the ethylene complex [Pt(PNP)(C2H4)](BF4)(2) and was reacted with a series of arenes at low temperature. Only electron-rich arenes added across the coordinated C-C triple bond and gave the corresponding arylalkenyl complexes (E)-[Pt(PNP)(CH=CHAr)]BF4 (Ar = C6Me5, C6H2Me3-2,4,6, C6H3Me2-2,6, C6H3Me2-2,4). A slow E-Z isomerization of the arylalkenyl complexes was observed. Single-crystal X-ray structure analyses were obtained for both E and Z isomers of the pentamethylbenzene derivative. The E isomers of [Pt(PNP)(CH=CHAr)]BF4 (Ar = C6Me5, C6H2Me3-2,4,6) reacted with excess HBF4 center dot Et2O to give the corresponding arylalkene complexes [Pt(PNP)(CH2=CHAr)](BF4)(2), whereas the Z isomers did not undergo immediate protonolysis. Using (E)-[Pt(PNP)(CD=CDC6Me5)]BF4 it was shown that the stereochemistry of the C-C double bond in the protonolysis product depends on the nature of the acid anion HX (X- = Cl-, BF4-). The catalytic hydroarylation was studied in solution by NMR spectroscopy. The reaction studies provide a more refined view of the individual steps proposed for the Friedel-Crafts type mechanism of the Pt-II-catalyzed intermolecular hydroarylation of alkynes.