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·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 PtII-catalyzed intermolecular hydroarylation of alkynes.