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 (<i>E</i>)-[Pt­(PNP)­(CHCHAr)]­BF₄ (Ar = C₆Me₅, C₆H₂Me₃-2,4,6, C₆H₃Me₂-2,6, C₆H₃Me₂-2,4). A slow <i>E</i>–<i>Z</i> isomerization of the arylalkenyl complexes was observed. Single-crystal X-ray structure analyses were obtained for both <i>E</i> and <i>Z</i> isomers of the pentamethylbenzene derivative. The <i>E</i> 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 <i>Z</i> isomers did not undergo immediate protonolysis. Using (<i>E</i>)-[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