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.