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Recent Organic Transformations with Silver Carbonate As a Key External Base and Oxidant

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Recent Organic Transformations with Silver Carbonate As a Key External Base and Oxidant catalysts Review Recent Organic Transformations with Silver Carbonate as a Key External Base and Oxidant 1, 1, 1, 2,3, 1, Kwangho Yoo y, Dong Gyun Jwa y, Ha-Eun Lee y, Hyun Jin Kim *, Cheoljae Kim * and Min Kim 1,* 1 Department of Chemistry, Chungbuk National University, Cheongju 28644, Korea; [email protected] (K.Y.); [email protected] (D.G.J.); [email protected] (H.-E.L.) 2 Innovative Therapeutic Research Center, Therapeutics and Biotechnology Division, Korea Research Institute of Chemical Technology, Daejeon 34114, Korea 3 Department of New Drug Discovery and Development, Chungnam National University, Daejeon 34134, Korea * Correspondence: [email protected] (H.J.K.); [email protected] (C.K.); [email protected] (M.K.); Tel.: +82-42-860-7066 (H.J.K.); +82-43-261-2305 (C.K.); +82-43-261-2283 (M.K.) These authors equally contributed to this work. y Received: 11 November 2019; Accepted: 4 December 2019; Published: 6 December 2019 Abstract: Silver carbonate (Ag2CO3), a common transition metal-based inorganic carbonate, is widely utilized in palladium-catalyzed C–H activations as an oxidant in the redox cycle. Silver carbonate can also act as an external base in the reaction medium, especially in organic solvents with acidic protons. Its superior alkynophilicity and basicity make silver carbonate an ideal catalyst for organic reactions with alkynes, carboxylic acids, and related compounds. This review describes recent reports of silver carbonate-catalyzed and silver carbonate-mediated organic transformations, including cyclizations, cross-couplings, and decarboxylations. Keywords: silver; silver carbonate; Lewis acid; basicity; alkynophilicity 1. Introduction Organic transformations using transition metal catalysts are essential synthetic techniques in total synthesis, medicinal chemistry, industrial chemistry, and chemical engineering [1]. A variety of transition metal catalysts have been developed and studied for various organic reactions, and they are generally used for selective bond formation and the interconversion of specific organic functional groups. Transition metal-catalyzed reactions can generally be classified into three types based on the role of the metal catalyst. The first class includes those in which the catalytic reaction is based on the oxidation/reduction cycle of the transition metal. As many transition metals can have more than two relatively stable oxidation states, the interconversion between oxidation states of a metal can be utilized to oxidize or reduce target molecules or functionalities. The second class of catalytic reactions are those in which the transition metal serves as a Lewis acid. The final group of organic transformations are those catalyzed by coinage metals [2]. As both Cu(I) and Cu(II) are generally stable states for copper, copper-catalyzed organic transformations have been widely studied. However, silver- and gold-promoted organic reactions are relatively rare. Silver catalysts are generally less reactive than other transition metal catalysts; therefore, in organic reactions, silver compounds are generally used as cocatalysts or bond activators due to their Lewis acidity [3,4]. In addition, Ag(I) is one of the softest Lewis-acidic metal cations; thus, Ag(I) forms organometallic complexes with π-donor functionalities, such as alkenes, alkynes, allenes, and aromatic Catalysts 2019, 9, 1032; doi:10.3390/catal9121032 www.mdpi.com/journal/catalysts Catalysts 2019, 9, x FOR PEER REVIEW 2 of 28 aromatic rings. Concurrently, Ag(I) generates relatively stable organometallic complexes with n- donor type molecules, for example, ethers, amines, and phosphines. In the last decade, various silver-catalyzed organic transformations, such as carboxylations, Catalystshalogenations,2019, 9, 1032 C–H bond functionalizations, cycloadditions, and oxidative couplings, have 2been of 27 reported [5–8]. The reported silver-catalyzed reactions were performed under relatively milder reaction conditions than the reactions by other methodologies, and silver complexes are less expensiverings. Concurrently, than other Ag(I)rare transition generates metals, relatively such stable as palladium, organometallic rhodium, complexes ruthenium, with and n-donor platinum. type molecules,Among for the example, various ethers,silver spec amines,ies, silver and phosphines. carbonate (Ag2CO3) is not typically used as a catalyst or mediatorIn the of lastorganic decade, reactions, various but silver-catalyzedit is known as a good organic oxidant transformations, (for example in such the Fetizon as carboxylations, oxidation) andhalogenations, inorganic base C–H (for bond example functionalizations, in the Wittig cycloadditions, reaction) [9,10]. and Scheme oxidative 1 describes couplings, the have general been reactionsreported [involving5–8]. The reportedAg2CO3. silver-catalyzedAg2CO3 enables reactions the use of were Lewis performed acids with under internal relatively alkynes, milder and reaction readily activatesconditions alkynes than the to reactions nucleophilic by other attack, methodologies, providing the and corresponding silver complexes addition are less products. expensive With than terminalother rare alkynes, transition Ag metals,2CO3 generates such as palladium, silver acetylide rhodium, intermed ruthenium,iates through and platinum. C–H functionalization, and thenAmong various the various reactions silver (for species,example, silver cross-couplings, carbonate (Ag and2CO cycloadditions)3) is not typically can used be used as a catalystto convert or thesemediator intermediates of organic reactions,into useful but organic it is known frameworks, as a good such oxidant as furans (for example [11], pyrroles in the Fetizon [12], and oxidation) nitriles [13].and inorganicAdditionally, base (forAg2 exampleCO3 is used in the as Wittig an oxidant reaction) for [9 ,10single-electron]. Scheme1 describes transfer the (SET) general processes reactions to generateinvolving reactive Ag2CO 3radical. Ag2CO species3 enables for thevarious use of organi Lewisc acidstransformations, with internal for alkynes, example, and the readily generation activates of acylalkynes radicals to nucleophilic from carboxylic attack, acids. providing In addition, the corresponding the basicity additionof the carbonate products. moiety With can terminal help prepare alkynes, Agreactive2CO3 partnersgenerates from silver acidic acetylide organic intermediates molecules such through as carboxylic C–H functionalization, acids, terminal andalkynes, then and various 1,3- dicarbonylreactions (for compounds. example, cross-couplings, and cycloadditions) can be used to convert these intermediates into usefulIn this organicreview, frameworks,we summarize such the as recently furans reported [11], pyrroles Ag2CO [123-catalyzed], and nitriles and [13Ag].2CO Additionally,3-mediated Agorganic2CO3 transformationsis used as an oxidant and small for single-electron molecule syntheses transfer from (SET) 2005 processes to 2019 to based generate on the reactive reactivity radical of Ag(I)species and for the various basicity organic of Ag transformations,2CO3. The focus for will example, be on their the generation synthesis ofand acyl substrate radicals scope from carboxylicas well as theacids. principle In addition, reactivity the basicity of silver of the carbonate, carbonate and moiety the cansubstrates help prepare will also reactive be discussed partners from based acidic on mechanisticorganic molecules clues. such as carboxylic acids, terminal alkynes, and 1,3-dicarbonyl compounds. Scheme 1. The role of Ag(I) in transformation transformationss of alkynes and carboxylic acids. In this review, we summarize the recently reported Ag2CO3-catalyzed and Ag2CO3-mediated organic transformations and small molecule syntheses from 2005 to 2019 based on the reactivity of Ag(I) and the basicity of Ag2CO3. The focus will be on their synthesis and substrate scope as well Catalysts 2019, 9, 1032 3 of 27 Catalysts 2019,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 3 of 28 as the principle reactivity of silver carbonate, and the substrates will also be discussed based on mechanistic clues. 2. Activation of Alkynes Using Silver Carbonate 2. Activation of Alkynes Using Silver Carbonate 2.1. Terminal Alkynes: Cross-Coupling Reactions 2.1. Terminal Alkynes: Cross-Coupling Reactions Oxidative double C–H functionalization/cross-coupling reactions are considered an attractive strategyOxidative for the double synthesis C–H of functionalization various heterocycles./cross-coupling The Ag2CO reactions3-mediated are considered activation anof attractiveterminal alkynesstrategy foris a the good synthesis starting of variouspoint for heterocycles. this heteroaromatic The Ag2CO cyclization.3-mediated In activation 2012, Lei of terminaland co-workers alkynes reportedis a good the starting direct point C–H for functionalization this heteroaromatic and cyclization. oxidative coupling In 2012, Leiof two and co-workersC–H bonds reported as an ideal the syntheticdirect C–H strategy functionalization for accessing and oxidativehighly substitu couplingted of furans two C–H (Scheme bonds as2) an[14]. ideal Although synthetic synthetic strategy methodsfor accessing for highlypreparing substituted substitute furansd furans (Scheme have2)[ been14]. Althoughreported syntheticpreviously methods [15], new for preparingsynthetic strategiessubstituted are furans required have been to reportedachieve previouslyselective cross [15], newcoupling synthetic of strategiestwo more are hydrocarbons. required
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