Ruthenium-Catalyzed Hydration of 1-Alkynes to Give Aldehydes:  Insight into anti-Markovnikov Regiochemistry

Abstract

The mechanism of the selective conversion of 1-alkynes to aldehydes by hydration was investigated by isolating organic and organometallic byproducts, deuterium-labeling experiments, and DFT calculations. The D-labeled acetylenic hydrogen of 1-alkyne was found exclusively in the formyl group of the resulting aldehydes. After the reaction, the presence of metal-coordinated CO was confirmed. All of the experimental results strongly suggest the involvement of a metal-acyl intermediate with the original acetylenic hydrogen also bound to the metal center as a hydride, with the next step being release of aldehyde by reductive elimination. Theoretical analyses suggest that the first step of the catalytic cycle is not oxidative addition of acetylene C [bond] H or tautomerization of eta(2)-alkyne to a vinylidene complex, but rather protonation of the coordinated 1-alkyne at the substituted carbon to form a metal-vinyl intermediate. This cationic intermediate then isomerizes to Ru(IV)-hydride-vinylidene via alpha-hydride migration of the vinyl group to the metal center, followed by attack of the vinylidene alpha-carbon by OH(-) to give the metal-hydride-acyl intermediate.

Metal Vinylidenes and Allenylidenes in Catalysis: Applications in Anti‐Markovnikov Additions to Terminal Alkynes and Alkene Metathesis

“…[74] The best catalyst precursors were [Ru(methallyl) 2 (dppb)] (I; dppb = diphenylphosphanylbutane) and [Ru(methallyl) 2 -(dppe)] (II; dppe = diphenylphosphanylethane), which also led to a remarkable stereoselectivity in favor of the Z isomer; the overall reaction thus corresponds to a formal trans addition of RC(O)O-H to the triple bond. The formation of a vinylidene intermediate according to a mechanism proposed by Wakatsuki by initial protonation of the ruthenium center [75] cannot be excluded. The choice of the appropriate catalyst precursor I or II depends on the steric demand of both the alkyne and the carboxylic acid.…”

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

“…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.…”

Ruthenium piano-stool complexes bearing imidazole-based PN ligands

“…14,15,16 The first ruthenium(II) complex to be known to catalyze the anti-Markovnikov hydration was a phosphine complex described by Tokunaga and Wakatsuki. 17,18 The catalytic activity as well as the selectivity of ruthenium(II) depends on the nature of the phosphine ligands. 19 Using PPh 2 (C 6 F 5 ) and P(3-C 6 H 4 SO 3 Na) 3 as ligands leads to selective anti-Markovnikov addition of water and satisfactory yields of the aldehyde.…”