Water Addition to Alkynes Promoted by a Dicationic Platinum(II) Complex

Dicationic platinum alkyne complexes were generated in situ by substitution of ethylene in [Pt­(PNP)­(C₂H₄)]­(BF₄)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)­(CH₂CHO)]­BF₄, which was analyzed by X-ray diffraction. ¹H and ³¹P NMR studies were performed to elucidate the mechanism of formation of [Pt­(PNP)­(CH₂CHO)]­BF₄. A reversible acid–base equilibrium between the platina-acetaldehyde and the corresponding η²-vinyl alcohol complex [Pt­(PNP)­(CH₂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)­{CH₂C­(O)­R¹}]­BF₄ and [Pt­(PNP)­{C­(O)­CH₂R¹}]­BF₄ (R¹ = 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CR²)]­BF₄ (R² = t-Bu, Ph). Complexes with internal alkynes R³CCR⁴ (R³ = Me; R⁴ = Me, n-Pr) react with water and form the corresponding β-ketonyl complexes [Pt­(PNP)­{CHR³C­(O)­R⁴}]­BF₄. Moderate regioselectivity was observed for 2-hexyne.