Transition-Metal-Free Coupling Reactions

Sun, Chang‐Liang; Shi, Zhang‐Jie

doi:10.1021/cr400274j

Cited by 967 publications

(440 citation statements)

References 620 publications

(312 reference statements)

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“…); (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).…”

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confidence: 99%

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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“…Although the past several decades have witnessed tremendous successes of transition-metal catalysis in organic chemistry, the difficulty associated with metal residues has led to concerns in, for example, the pharmaceutical and electronic industries. Transition-metal (depleting and precarious) 3,4 -free couplings in water (a greener solvent) 5 enabled by photoenergy (a cleaner energy input) 6 provide great opportunities towards more sustainable synthetic chemistry.…”

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Empowering a transition-metal-free coupling between alkyne and alkyl iodide with light in water

Liu

2015

Nat Commun

137

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Methods to assemble alkynes are essential for synthesizing fine chemicals, pharmaceuticals and polymeric photo-/electronic materials. Using light as a clean energy form and water as a green solvent has the potential to make synthetic chemistry more environmentally friendly. Here we present a transition-metal-free coupling protocol between aryl alkyne and alkyl iodide enabled by photoenergy in water. Under ultraviolet irradiation and in basic aqueous media, aryl alkynes efficiently couple with a wide range of alkyl iodides including primary, secondary and tertiary ones under mild conditions. A tentative mechanism for the coupling is also proposed.

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“…1,2 These reactions require a base (typically potassium tert-butoxide) and an additive. [3][4][5][6][7][8][9][10][11][12][13][14][15][16] Whilst organic additives are not always used, 10,11,15 they significantly enhance reaction efficiency in terms of yield and time.…”

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confidence: 99%