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

Abstract

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.

C-C and C-O Coupling Reactions of Terminal Alkynes by a Water Soluble Organoiridium Electron-transfer Mediator in Thin Layer of Water on Gold Electrode

“…It has been known that terminal alkynes react with water in the presence of organometallic compounds to produce aldehyde, ketone and carboxylic acids [6][7][8][9][10]. Hahn and Wakatsuki recently carried out detailed mechanistic studies for the alkyne hydration mediated by metal compounds in homogeneous solutions [6][7][8][9][10]39].…”

“…Hahn and Wakatsuki recently carried out detailed mechanistic studies for the alkyne hydration mediated by metal compounds in homogeneous solutions [6][7][8][9][10]39]. The formation of aldehyde and ketone seems to be initiated by the cathodic reduction of the water-soluble iridium compound 1, because compound 1 did not demonstrate any reactivity for the hydration of alkynes unless it is activated by cathodic reduction process.…”

“…Terminal alkynes are known to coordinate toward metal center to give stable π-alkyne or σ-alkynyl compound [6][7][8][9][10]. In fact, it has been well-documented that metal vinylidene is obtained directly from the rearrangement of π-coordinated terminal alkynes [6][7][8][9][10][11][12][13][14][15][16][17][18][19]39,40]. It seems reasonable to include the direct attack of H 2 O on the carbon of the Ir-(HC≡CR) to produce ketones and on the α-carbon of the vinylidene moiety (Ir=C=CHR) to yield a hydroxyl carbene compounds followed by reductive elimination to give aldehydes.…”

“…This resulting anionic radical compound could produce highly reactive species which will be used to catalyze the activation of organic substrates such as alkynes. Metal-catalyzed hydration and oligomerization of alkynes provide important processes in organic chemistry to aldehydes, ketones, and conjugated olefin compounds since they are the carbon-oxygen [6][7][8][9][10] and -carbon [11][12][13][14][15][16][17][18][19] bond forming reactions which produce a variety of new inexpensive starting materials.…”

Tropylium Ion Catalyzes Hydration Reactions of Alkynes

“…These developments have focused on two distinct angles of the reaction: activating alkyne substrate[1c] or stimulating the “hydrative” reaction medium . The first research direction typically involves activation of alkynes with transition‐metal catalysts containing Au, Ag, Ru, Pd, Fe, Cu, Co and other metals . More recently, there has been a number of interesting hydration processes promoted by Brønsted acid catalytic systems .…”