[(NHC)Au<sup>I</sup>]-Catalyzed Acid-Free Alkyne Hydration at Part-per-Million Catalyst Loadings

Marion, Nicolas; Ramón, Rubén S.; Nolan, Steven P.

doi:10.1021/ja809403e

Cited by 445 publications

(282 citation statements)

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“…1,2 Classical Kucherov procedures for alkynes' hydration into ketones employ mercuric salt as the catalyst in aqueous sulfuric acid. 3,4 Due to the toxicity of Hg salts, many other metal-catalyzed alkyne hydration procedures have been developed, including ones catalyzed by Ru, 5 Rh, 6 Pd, 7 Pt, 8,9 Sn−W, 10 Au, [11][12][13][14][15][16][17]18,19 Ir,20,21 Co, 22 Ag, 23,24 etc. Besides various metal catalysts, Brønsted-acidcatalyzed hydration reactions also exist in the literature.…”

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Metal-Free Markovnikov-Type Alkyne Hydration under Mild Conditions

2016

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A Markovnikov-type alkyne hydration protocol is presented using 20% CF 3 SO 3 H (TfOH) as the catalyst under unprecedented mild conditions applicable to various alkynes, including terminal arylalkynes, terminal nonfunctionalized aliphatic alkynes, and internal alkynes with excellent regioselectivity in good to excellent yields (average yields >85%). The reaction procedure operates under mild conditions (25−70°C), with broad functional group compatibility, and uses only slightly more than a stoichiometric amount of water in the absence of any transition metal. The success of this protocol hinges upon the utilization of trifluoroethanol as the solvent.C onverting alkynes into carbonyl compounds through hydration, especially Markovnikov-type hydration (Scheme 1a), is a fundamental transformation in organic synthesis with 100% atom economy. 1,2 Classical Kucherov procedures for alkynes' hydration into ketones employ mercuric salt as the catalyst in aqueous sulfuric acid. 3,4 Due to the toxicity of Hg salts, many other metal-catalyzed alkyne hydration procedures have been developed, including ones catalyzed by Ru, 5 Rh, 6 Pd, 7 Pt, 8,9 Sn−W, 10 Au, [11][12][13][14][15][16][17]18,19 Ir,20,21 Co, 22 Ag, 23,24 etc. Besides various metal catalysts, Brønsted-acidcatalyzed hydration reactions also exist in the literature. 25,26 However, most of the reactions reported so far have suffered from at least one drawback: (1) high temperature (>100°C); (2) expensive noble metals (Ru, Rh, Pd, Pt, Au, Ir, Ag, etc.); (3) narrow functional group compatibility; (4) large excess of water and acidic additive besides the metal catalysts if any (Scheme 1b). Therefore, a method that could operate under mild conditions (i.e., room temperature) with broad functional group tolerance at the cost of close to a stoichiometric amount of water and a catalytic amount of acid in the absence of any transitional metal 27 would be highly desirable. In this paper, we describe such a protocol that fulfills all of these requirements (Scheme 1c).Our work was inspired by the facts that (1) vinyl (phenyl) carbocation can be strongly stabilized in CF 3 CH 2 OH (TFE) as the solvent and (2) TFE cannot react with the carbocation due to its low nucleophilicity. 28−30 Given that alkyne's hydration is generally via a vinyl carbocation intermediate, 31−34 we hypothesized that TFE's ability to stabilize a vinyl carbocation could decrease the transition state energy to form the carbocation intermediate, which may cause the hydration reaction to proceed under conditions milder than those of reactions occurring in other solvents. Besides, low Lewis basicity of TFE would not compete with the triple bond to be activated by the acid catalysts (including metal and Brønsted catalysts). To quantitatively compare the difference between TFE and other solvents, we conducted DFT calculation on the protonation step in different solvents (Figure 1). TFE and ethanol were selected as the solvents to conduct the DFT analysis comparison because these two solvents bear nearl...

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Metal-Free Markovnikov-Type Alkyne Hydration under Mild Conditions

2016

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“…33 As typical benchmark reaction, the hydration of alkynes -more specifically of diphenylacetylene -was chosen. 34 Quickly it became clear that only precatalyst DiPP-3 was considerably active in this reaction showing full conversion at 2.5 mol% catalyst loading after 24 h (Table 3, Entry 5). The reactions involving the other precatalysts indicated rapid decomposition of the complexes by the formation of black precipitates within the first minutes.…”

Section: Scheme 5 Formation Of Cationic Gold Complexes R-4mentioning

confidence: 99%

“…0.3 ml, filtered through a pad of silica (glass pipette, ½ filled) which was subsequently rinsed with DCM (3 × 1 ml). After evaporating the filtrate to dryness, the residue was triturated with pentane (3 × 2 ml) and dried in vacuo to give the product as a white powder (34 …”

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Synthesis and Characterization of Gold(I) Complexes of Dibenzotropylidene-Functionalized NHC Ligands (Trop-NHCs)

et al. 2014

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Gold(I) complexes of dibenzotropylidene-functionalized N-heterocyclic carbene ligands (Trop-NHCs) have been prepared in order to investigate their structural features and to reveal possible interaction of the olefin-unit with the metal center. The precursor imidazolium chloride salts (R-1) were generated in a single step using N-substituted imida-zoles (R = H, Me, DiPP, Ad) and one or two equivalents of Trop-Cl generating unsymmetrically and symmetrically NHC-olefin hybrids. The structural parameters of the ligands were determined by synthesis and X-ray diffraction analysis of their corresponding gold(I)-chloride complexes revealing highly flexible steric demands of the Trop-unit. Conversion of these complexes with halide abstracting reagents like AgNTf 2 and NaBArF 24 cleanly gave the neutral, NTf 2 -coordinated complexes of the type [(NHC)Au(NTf 2 )] (R-3) and the cationic bis-NHC-coordinated complexes [(NHC) 2 Au]BArF 24 (R-4), respectively. The Gagosz-type complexes R-3 were further tested in the hydration of diphenylacetylene showing a clear trend in activity depending on the ligands sensitivity to hydrolysis.

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“…11 Nolan has reported a gold(I) NHC cation that catalyzes the efficient hydration of alkynes to give ketones. 12 The success of the reaction is limited to the IPr ligand and the weakly coordinating solvent dioxane with a non-coordinating anion. A variety of carbonyl complexes were prepared with catalyst loadings as low as 50 ppm.…”

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Diverse Chemical Applications of N-Heterocyclic Carbenes

2009

J. Am. Chem. Soc.

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[(NHC)Au^I]-Catalyzed Acid-Free Alkyne Hydration at Part-per-Million Catalyst Loadings

Cited by 445 publications

References 22 publications

Metal-Free Markovnikov-Type Alkyne Hydration under Mild Conditions

Metal-Free Markovnikov-Type Alkyne Hydration under Mild Conditions

Synthesis and Characterization of Gold(I) Complexes of Dibenzotropylidene-Functionalized NHC Ligands (Trop-NHCs)

Diverse Chemical Applications of N-Heterocyclic Carbenes

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[(NHC)AuI]-Catalyzed Acid-Free Alkyne Hydration at Part-per-Million Catalyst Loadings

Cited by 445 publications

References 22 publications

Metal-Free Markovnikov-Type Alkyne Hydration under Mild Conditions

Metal-Free Markovnikov-Type Alkyne Hydration under Mild Conditions

Synthesis and Characterization of Gold(I) Complexes of Dibenzotropylidene-Functionalized NHC Ligands (Trop-NHCs)

Diverse Chemical Applications of N-Heterocyclic Carbenes

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[(NHC)Au^I]-Catalyzed Acid-Free Alkyne Hydration at Part-per-Million Catalyst Loadings