The dicationic acetylene platinum(II) complex [ P t ( P N P ) ( C 2 H 2 ) ] ( B F 4 ) 2 (PNP = 2,6-bis-(diphenylphosphinomethyl)pyridine) was generated in situ by ligand substitution from the ethylene complex [Pt(PNP)-(C 2 H 4 )](BF 4 ) 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)]BF 4 (Ar = C 6 Me 5 , C 6 H 2 Me 3 -2,4,6, C 6 H 3 Me 2 -2,6, C 6 H 3 Me 2 -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)]BF 4 (Ar = C 6 Me 5 , C 6 H 2 Me 3 -2,4,6) reacted with excess HBF 4 •Et 2 O to give the corresponding arylalkene complexes [Pt(PNP)(CH 2 CHAr)](BF 4 ) 2 , whereas the Z isomers did not undergo immediate protonolysis. Using (E)-[Pt(PNP)(CDCDC 6 Me 5 )]BF 4 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 − , BF 4 − ). 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.

Dicationic platinum alkyne complexes were generated in situ by substitution of ethylene in [Pt(PNP)(C 2 H 4 )]-(BF 4 ) 2 (PNP = 2,6-bis(diphenylphosphinomethyl)pyridine) with alkynes at low temperature. The dicationic acetylene complex readily adds water to form the platina-acetaldehyde complex [Pt(PNP)(CH 2 CHO)]BF 4 , which was analyzed by X-ray diffraction. 1 H and 31 P NMR studies were performed to elucidate the mechanism of formation of [Pt(PNP)(CH 2 CHO)]BF 4 . A reversible acid−base equilibrium between the platina-acetaldehyde and the corresponding η 2 -vinyl alcohol complex [Pt(PNP)(CH 2  CHOH)] 2+ was observed. The complexes with terminal alkynes (propyne and 1-hexyne) gave with water a mixture of Markovnikov and anti-Markovnikov addition products [Pt(PNP){CH 2 C(O)R 1 }]BF 4 and [Pt(PNP){C(O)CH 2 R 1 }]BF 4 (R 1 = Me, n-Bu) in a ratio of 1:4. However, with tert -butyl-and phenylacetylene C−H bond activation occurred, yielding the σ-alkynyl complexes [Pt(PNP)(CCR 2 )]BF 4 (R 2 = t-Bu, Ph). Complexes with internal alkynes R 3 CCR 4 (R 3 = Me; R 4 = Me, n-Pr) react with water and form the corresponding β-ketonyl complexes [Pt(PNP){CHR 3 C(O)R 4 }]BF 4 . Moderate regioselectivity was observed for 2-hexyne.

The synthesis of the flavones (VIII) is improved by using photo‐Fries rearrangement of the acetals (IV) instead of the ketones (V) as the key step.