Highly Efficient AuI-Catalyzed Hydration of Alkynes

Abstract

Hydration of unsaturated carbon compounds is one of the most straightforward and environmentally benign methods to form the carbon±oxygen bond. Synthesis of carbonyl compounds by the hydration of alkynes is an important variation in this category, which has been extensively studied. [1] Acidcatalyzed hydration of alkynes is long known. [2, 3] However, only electron-rich acetylene compounds, such as alkynyl ethers, alkynyl thioethers and ynamines react satisfactorily. [1d, 4] The reaction of simple alkynes is usually sluggish and needs cocatalysts, typically toxic mercury(ii) salts, to enhance the reactivity. [5] More recent interest lies in the use of transition-metal-complex catalysts containing Ru II , [6] Ru III , [7] Rh, [8] Pt, [9] Au III , [10] and other metal centers. [11] However, the process catalyzed by these complexes is not efficient either. The highest turnover frequency (TOF) is 550 h À1 claimed as the initial TOF for the hydration of 3-pentyn-1-ol catalyzed by [cis-PtCl 2 (tppts) 2 ] (tppts ¼ P(m-C 6 H 4 SO 3 Na) 3 ), but its overall TOF is no more than approximately 100 h À1 . [9c] Recently Teles and co-workers reported the addition of methanol to alkynes catalyzed by Au I species in conjunction with acidic cocatalysts. [12] Hydration of propargyl alcohol was also briefly mentioned in their patent application, [13] although the yield was quite low. [14] Herein we report that the Au I ±acid systems in aqueous methanol serve as powerful catalysts, [15] which promote the hydration of alkynes [Eq. (1)] and have turnover frequencies of at least two orders of magnitude higher than [cis-PtCl 2 (tppts) 2 ].In a preliminary experiment, a mixture of 1-octyne (1 mmol), [(Ph 3 P)AuCH 3 ] (0.01 mmol, 1 mol %) and concentrated sulfuric acid (0.5 mmol, 50 mol %) in aqueous methanol (1.5 mL, methanol:H 2 O ¼ 2:1 v/v) was heated for 1 h at 70 8C affording the corresponding Markovnikov hydration product, 2-octanone, in 95 % yield without anti-Markovnikov hydration, or possible methanol addition. [12] The reaction did not proceed in the absence of either the Au catalyst or sulfuric acid.The following aspects about the catalytic system are worth noting. First, the nature of the reaction medium significantly affects the reaction. The reaction run without using solvent (in otherwise the same conditions as the preliminary experiment) did not furnish 2-octanone. On the other hand, the use of 2propanol (71 %), dioxane (56 %), acetonitrile (53 %), or THF (11 %) resulted in a low yield, and the yield obtained with dichloromethane, DMF, or toluene was even lower. Thus methanol was the solvent of choice for this particular transformation. [16] Second, the efficiency of the catalyst was significantly enhanced by addition of appropriate ligands, which enabled the quantity of the precious catalyst used to be minimized. For instance, the control experiment run without ligand addition under the conditions shown in Table 1 (only 0.01 mol % catalyst) gave 2-octanone in only 35 % yield (TOF ¼ 3500 h À1 , entry 1), while th...

Metal-Free Markovnikov-Type Alkyne Hydration under Mild Conditions

“…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.…”

An Alkyne Hydrosilylation-Oxidation Strategy for the Selective Installation of Oxygen Functionality

“…2b Hydroboration can be used to provide ketones after oxidation with good control of regiochemistry based on steric differentiation, 2 and recently the direct hydration of propargylic alcohols has been demonstrated with a gold catalyst. 3 In addition, intramolecular diol-alkyne bis-cyclization catalyzed by palladium can be used in the selective synthesis of spiroketals. 4 Despite these achievements, the use of alkynes as substrates for oxygenation in the context of functionalized molecules remains underutilized.…”

“…Recently, we disclosed preliminary reports of regio-and stereoselective hydrosilylation of internal alkynes-catalyzed by the ruthenium complex [Cp*Ru(MeCN) 3 ]PF 6 (1)-providing unique vinylsilanes previously available only by circuitous means (Scheme 2). 5-8 Of special interest to this work, propargylic, 7 homopropargylic, 5 and bishomopropargylic 5 alcohols all allow selective access to the (E)-vinylsilane with distal regioselectivity relative to the hydroxyl group by either intramolecular or intermolecular processes, as appropriate (Scheme 2).…”

Homogeneous Gold Catalysis: Mechanism and Relativistic Effects of the Addition of Water to Propyne

“…1), which is known for more than fifteen years now, [17,18] and is part of the standard toolkit of organic synthesis [19][20][21][22] which makes it appealing for consideration in a theoretical study (see computational section). Firstly, the high yield under mild conditions indicates a clear thermodynamical preference for the products which enables a discussion solely on the basis of the energy differences on the potential energy hypersurface.…”