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 †, Dong Gyun Jwa †, Ha-Eun Lee †, 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. † 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

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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 to achieve The homocouplingselective cross couplingof terminal of twoalkynes more occurs hydrocarbons. easily under The most homocoupling oxidative conditions of terminal [16]. alkynes This occursreport showseasily under that Ag most2CO oxidative3 cancan bebe conditionsusedused asas aa [mediatormediator16]. This reporttoto induceinduce shows C–H/C–HC–H/C–H that Ag2 COfunctionalizationfunctionalization3 can be used as ofof a mediatorterminalterminal alkynesto induce and C–H 1,3-dicarbonyl/C–H functionalization compounds. of terminal This reaction alkynes affordsaffords and 1,3-dicarbonyl furansfurans withwith severalseveral compounds. functionalfunctional Thisreaction groupsgroups inaﬀ aords facile, furans one-step with manner. severalfunctional Terminal alkyne groups 1in cancan a facile,bebe convertedconverted one-step toto manner.thethe correspondingcorresponding Terminal alkyne silversilver acetylideacetylide1 can be intermediateconverted to theby correspondingAg2CO3,, andand furanfuran silver 3 acetylide isis synthesizedsynthesized intermediate byby alkylationalkylation by Ag2 CO withwith3, and1,3-dicarbonyl1,3-dicarbonyl furan 3 is synthesized compoundcompound by 2 followedalkylation by with oxidative 1,3-dicarbonyl radical cyclization. compound 2 followed by oxidative radical cyclization.

Scheme 2. 2. Silver-mediatedSilver-mediated oxidative oxidative C–H/C–H C–H functionalization/C–H functionalization process to process afford highly to aﬀ ordsubstituted highly furans.substituted furans.

In the same year, thethe LeiLei groupgroup reportedreported aa Ag-mediated,Ag-mediated, highlyhighly selectiveselective C–CC–C/N–H/N–H oxidative cross-coupling/cyclizationcross-coupling/cyclization to to construct construct imidazo[1,2- imidazo[1,2-a]pyridinesa]pyridines5 from 5 2-aminopyridinesfromfrom 2-aminopyridines2-aminopyridines4 and terminal 4 andand terminalalkynes 1alkynes(Scheme 1 (Scheme(Scheme3)[ 17]. 3)3) This [17].[17]. reactionThisThis reactionreaction is also isis alsoalso initiated initiatedinitiated by byby the thethe generation generationgeneration of ofof a aa silver silversilver acetylideacetylide intermediate fromfrom the the Ag Ag salt salt and and phenylacetylene; phenylacetylene; then, then, subsequent subsequent Ag-promoted Ag-promoted nucleophilic nucleophilic attack attackand oxidative and oxidative cyclization cyclization form imidazo[1,2- form imidazo[1,2-a]pyridinea]pyridine5 as the 5 as product.as thethe product.product. After theAfterAfter reaction, thethe reaction,reaction, the spent thethe spentsilver silver species species can be can recycled be recycled by ﬁltration by filtration and treatment and treatment with nitric with acidnitric and acid Na and2CO Na3. 2CO3..

Scheme 3. Ag-mediatedAg-mediated C–H/N–H C–H/N–H oxidative cross-coupling/cyclization. cross-coupling/cyclization.

As described above, oxidative C–H C–H functionalizat functionalizationion/cross-coupling/cross-coupling reactions are considered an attractive strategy for the synthesis of variousvarious heterocycles.heterocycles. Pyrroles, Pyrroles, as as 5-membered heterocycles, are also a basicbasic buildingbuilding blockblock ofof numerousnumerous biologicallybiologically andand pharmaceuticallypharmaceutically important natural compounds [12], [12], and the regiospeciﬁcregiospecific synthesissynthesis of pyrrolespyrroles isis anan attractiveattractive syntheticsynthetic tool.tool. The Lei 7 group reported that pyrroles 7 containingcontaining various functional grou groupsgroups can be eeffectivelyﬀectively synthesized by the C–H/C–H oxidative cross coupling/cyclization of terminal alkynes 1 andand β-enamino esters 6 using Ag2CO3 asas aa mediatormediator (Scheme(Scheme 4)4) [18].[18]. TheThe proposedproposed mechanismmechanism isis shownshown inin SchemeScheme 5.5. DueDue toto

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6 using Ag2CO3 as a mediator (Scheme4)[ 18]. The proposed mechanism is shown in Scheme5. 2 3 thethe effectseffects ofof AgAg2CO3,, terminalterminal alkynealkyne 1 isis primarilyprimarily convertedconverted toto silversilver acetylideacetylide 1-I;; then,then, thethe Duethe effects to the eofﬀ ectsAg2CO of Ag3, terminal2CO3, terminal alkyne alkyne1 is primarily1 is primarily converted converted to silver to silver acetylide acetylide 1-I; then,1-I; then, the nucleophilic addition of deprotonated β-enamino ester intermediate 6-I to silver acetylide 1-I delivers nucleophilicthe nucleophilic addition addition of deprotonated of deprotonated β-enamino-enaminoβ-enamino esterester intermediateintermediate ester intermediate 6-I toto silversilver6-I to acetylideacetylide silver acetylide 1-I deliversdelivers1-I oxidative cross-coupling intermediate 6-II via two single-electron oxidations. The coordination of oxidativedelivers oxidative cross-coupling cross-coupling intermediate intermediate 6-II viavia6-II twotwovia single-electronsingle-electron two single-electron oxidations.oxidations. oxidations. TheThe The coordinationcoordination coordination ofof Ag(I) generated internal alkyne 6-III, which is cyclized by an intramolecular nucleophilic addition to Ag(I)of Ag(I) generated generated internal internal alkyne alkyne 6-III6-III,, whichwhich, which isis cyclizedcyclized is cyclized byby byanan anintramolecularintramolecular intramolecular nucleophilicnucleophilic nucleophilic additionaddition addition toto provide cyclic intermediate 6-IV, and subsequent aromatization and protodemetallation give highly provideto provide cyclic cyclic intermediate intermediate 6-IV6-IV,, andand, and subsequentsubsequent subsequent aromatizationaromatization aromatization andand and protodemetallationprotodemetallation protodemetallation givegive give highlyhighly highly substituted pyrrole 7 as the product. substituted pyrrole 77 asas thethe product.product.

Scheme 4. Synthesis of polysubstituted pyrroles via Ag-mediated oxidative cyclization. Scheme 4. Synthesis of polysubstituted pyrroles via Ag-mediated oxidative cyclization.

Scheme 5. 5. ProposedProposed mechanism mechanism for for Ag-mediated Ag-mediated pyrro pyrrolelele synthesis.synthesis. synthesis. Chem.Chem. Chem. Commun.Commun. Commun. 2013,2013, 49,49, 2013, 7549–7549– 49, Scheme 5. Proposed mechanism for Ag-mediated pyrrole synthesis. Chem. Commun. 2013, 49, 7549– 7551,7549–7551, doi:10.1039/c3cc43682a doi:10.1039/c3cc43682a—Reproduced – Reproduced by perm byission permissionission ofof TheThe of RoyalRoyal The Royal SocietySociety Society ofof Chemistry.Chemistry. of Chemistry. 7551, doi:10.1039/c3cc43682a – Reproduced by permission of The Royal Society of Chemistry.

In 2014, Bi and co-workers reported an Ag2CO3 -catalyzed cross-coupling reaction of propargylic InIn 2014,2014, BiBi andand co-workersco-workers reportedreported anan AgAg2CO33-catalyzed-catalyzed cross-couplingcross-coupling reactionreaction ofof propargylicpropargylic In 2014, Bi and co-workers reported an Ag2CO3-catalyzed cross-coupling reaction of propargylic alcohols 8 withwith isocyanidesisocyanides 99 toto givegive 2,3-allenamides2,3-allenamides 1010 (Scheme(Scheme6 )[6) 19[19].]. Although the reactionreaction alcohols 8 withwith isocyanidesisocyanides 9 toto givegive 2,3-allenamides2,3-allenamides 10 (Scheme(Scheme 6)6) [19].[19]. AlthoughAlthough thethe reactionreaction mechanism is not clear, thisthis reactionreaction involvesinvolves aa uniqueunique ββ-C–C-C–C couplingcoupling and oxygenoxygen transpositiontransposition of mechanism is not clear, this reaction involves a unique β-C–C-C–C couplingcoupling andand oxygenoxygen transpositiontransposition ofof propargylic alcohols.alcohols. propargylic alcohols.

Scheme 6. Ag-catalyzed cross coupling of propargylic alcohols with isocyanides. Scheme 6. Ag-catalyzed cross coupling of propargylic alcohols with isocyanides. Scheme 6. Ag-catalyzed cross coupling of propargylic alcohols with isocyanides. The Agrawal group reported the synthesis of indolizines 12 viavia aa Ag-mediatedAg-mediated oxidativeoxidative C–H The Agrawal group reported the synthesis of indolizines 12 viavia aa Ag-mediatedAg-mediated oxidativeoxidative C–HC–H functionalization and cyclization of terminal alkynes alkynes 1 with pyridine derivatives 1111 (Scheme(Scheme 77))[ [20].20]. functionalizationfunctionalization andand cyclizationcyclization ofof terminalterminal alkynesalkynes 1 withwith pyridinepyridine derivativesderivatives 11 (Scheme(Scheme 7)7) [20].[20]. Indolizine is a basicbasic organicorganic buildingbuilding blockblock forfor manymany alkaloidsalkaloids andand bioactivebioactive molecules,molecules, so thethe IndolizineIndolizine isis aa basicbasic organicorganic buildingbuilding blockblock forfor manymany alkaloidsalkaloids andand bioactivebioactive molecules,molecules, soso thethe development of of regioselective regioselective synthetic synthetic pathways pathways is an is anattractive attractive topic topic in organic in organic synthesis synthesis [21]. This [21]. development of regioselective synthetic pathways is an attractive topic in organic synthesis [21]. This reaction allows the highly selective synthesis of indolizines from readily available starting materials reaction allows the highly selective synthesis of indolizines from readily available starting materials via a 5-endo-dig cyclization between the silver phenylacetylide generated from alkyne 1 and a chelated via a 5-endoendo--dig cyclizationcyclization betweenbetween thethe silversilver phphenylacetylide generated from alkyne 1 andand aa chelatedchelated

Catalysts 2019, 9, 1032 5 of 27 Catalysts 2019, 9, x FOR PEER REVIEW 5 of 28 Catalysts 2019, 9, x FOR PEER REVIEW 5 of 28 Catalysts 2019, 9, x FOR PEER REVIEW 5 of 28 This reaction allows the highly selective synthesis of indolizines from readily available starting materials via a 5-endo-dig cyclization between the silver phenylacetylide generated from alkyne 1 and a intermediate from the deprotonation of 2-pyridine derivative 11. After the reaction, Ag2CO3 can be intermediate from the deprotonation of 2-pyridine derivative 11. After the reaction, Ag2CO3 can be chelatedintermediate intermediate from the from deprotonation the deprotonation of 2-pyridine of 2-pyridine derivative derivative 11. After11 .the After reaction, the reaction, Ag2CO Ag3 can2CO be3 recycledrecycled andand reused.reused. canrecycled be recycled and reused. and reused.

Scheme 7. Ag-mediated oxidative C–H functionalization to synthesize indolizines. Scheme 7. Ag-mediated oxidative C–H functionalization to synthesize indolizines. In 2015, Lei and co-workers developed a Ag(I)-mediated selective oxidative cross coupling InIn 2015,2015, Lei Lei and and co-workers co-workers developed developed a Ag(I)-mediated a Ag(I)-mediated selective selective oxidative oxidative cross coupling cross coupling between betweenIn 2015, C–H Leiand and P–H co-workers bonds to afforddeveloped alkynyl a Ag(I)-mediated (diaryl) phosphine selective oxides oxidative 14 (Scheme cross 8) coupling[22]. To C–Hbetween and C–H P–H bondsand P–H to a ﬀbondsord alkynyl to afford (diaryl) alkynyl phosphine (diaryl) oxides phosphine14 (Scheme oxides8)[ 1422 (Scheme]. To determine 8) [22]. theTo determinebetween C–H the mechanismand P–H bonds of this to reaction, afford alkynyl the auth (diaryl)ors performed phosphine experiments oxides 14 with (Scheme silver complexes8) [22]. To mechanismdetermine the of thismechanism reaction, of the this authors reaction, performed the authors experiments performed with experiments silver complexes with silver as the complexes starting as the starting materials. Phenylacetylide was mixed with Ph2P(O)H and prepared silver; the reaction materials.as the starting Phenylacetylide materials. Phenylacetylide was mixed with was Ph 2mixedP(O)H with and Ph prepared2P(O)H silver;and prepared the reaction silver; only the provided reaction onlyas the provided starting materials.only 17% Phenylacetylideof the desired product, was mixed but 60%with ofPh the2P(O)H desired and product prepared was silver; obtained the reaction when only 17%provided of the only desired 17% product, of the desired but 60% product, of the desired but 60% product of the wasdesired obtained product when was an obtained additional when 1.0 an additional 1.0 equiv. of Ag2CO3 was added. When the reaction used Ph2P(O)Ag instead of an additional 1.0 equiv. of Ag2CO3 was added. When the reaction used Ph2P(O)Ag instead of equiv.an additional of Ag2CO 1.03 wasequiv. added. of Ag When2CO3 thewas reaction added. used When Ph 2theP(O)Ag reaction instead used of PhPh22P(O)H,P(O)Ag only instead a trace of Ph2P(O)H, only a trace amount of the product was obtained with phenylacetylene. In the presence of Ph2P(O)H, only a trace amount of the product was obtained with phenylacetylene. In the presence of amountPh2P(O)H, of theonly product a trace wasamount obtained of the with product phenylacetylene. was obtained Inwith the phenylacetylene. presence of an additional In the presence 1.0 equiv. of an additional 1.0 equiv. of Ag2CO3 in the same reaction, 37% of the product was obtained. With both an additional 1.0 equiv. of Ag2CO3 in the same reaction, 37% of the product was obtained. With both ofan Agadditional2CO3 in the1.0 sameequiv. reaction, of Ag2CO 37%3 in of the the same product reaction, was obtained. 37% of the With product both silver was obtained. phenylacetylide With both and silver phenylacetylide and Ph2P(O)Ag, the reaction provided 30% of the desired product, and 71% of silver phenylacetylide and Ph2P(O)Ag, the reaction provided 30% of the desired product, and 71% of Phsilver2P(O)Ag, phenylacetylide the reaction and provided Ph2P(O)Ag, 30% ofthe the reaction desired provided product, 30% and of 71% the of desired the product product, was and produced 71% of the product was produced when an additional 1.0 equiv. of Ag2CO3 was added the product was produced when an additional 1.0 equiv. of Ag2CO3 was added whenthe product an additional was produced 1.0 equiv. when of Ag an2 additionalCO3 was added. 1.0 equiv. of Ag2CO3 was added

Scheme 8. Ag-mediatedAg-mediated selective selective oxidative oxidative cross cross coup couplingling between between C–H C–H and P–H. Adapted with Scheme 8. Ag-mediated selective oxidative cross coupling between C–H and P–H. Adapted with permission from Org.Org. Lett. 2015, 2015, 17, 118–121, doi:10.1021doi:10.1021/ol503341t./ol503341t. Copyright Copyright (2019) American permission from Org. Lett. 2015, 17, 118–121, doi:10.1021/ol503341t. Copyright (2019) American Chemical Society. Chemical Society. Based on the experimental results, a mechanism was proposed as shown in SchemeScheme9 9.. SilverSilver Based on the experimental results, a mechanism was proposed as shown in Scheme 9. Silver acetylideacetylide 1-I1-I1-I andandand disubstituteddisubstituted disubstituted phosphorylphosphoryl phosphoryl silversilver silver speciesspecies species 13-I13-I13-I areare are formedformed formed fromfrom from silversilver silver carbonate,carbonate, carbonate, andand aandacetylide cascade a cascade 1-Iinvolving and involving disubstituted coordination coordination phosphoryl and and cross-couplin cross-coupling silver speciesg reactions reactions13-I are via formed via two two fromsingle-electron single-electron silver carbonate, oxidations and a cascade involving coordination and cross-coupling reactions via two single-electron oxidations provided alkynyl(diaryl)phosphine oxides 14.. provided alkynyl(diaryl)phosphine oxides 14.

Scheme 9. Proposed mechanism of the Ag-mediated oxidative cross coupling between C–H and P–H. SchemeScheme 9.9. ProposedProposed mechanismmechanism ofof thethe Ag-mediatedAg-mediated oxoxidativeidative crosscross couplingcoupling betweenbetween C–HC–H andand P–P– H.Scheme 9. Proposed mechanism of the Ag-mediated oxidative cross coupling between C–H and P– H.

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2.2. Terminal Alkynes: Cycloaddition Reactions The ideal method for the synthesis of substituted pyrroles is the cycloaddition of an alkyne and an isocyanide. Isocyanide-alkyne cycloaddition reactions are usually limited to use electron-deficient electron-deﬁcient alkynes, and the yields are low. In 2013, two interestinginteresting papers were published on the synthesis of pyrroles viavia AgAg22COCO33-catalyzed-catalyzed cycloadditionscycloadditions of of isocyanides isocyanides and and terminal terminal alkynes. alkynes. One One is fromis from the the Bi groupBi group (Scheme (Scheme 10 10,, Conditions Conditions A) A) [23 [23],], and and the the other other is is from from the the Lei Lei groupgroup (Scheme(Scheme 1010,, Conditions B) [[24].24]. The reaction reaction mechanism is unclear, so th theseese two groups explained the reaction mechanism in didifferentﬀerent ways.ways. Based on the results of a deuterium- deuterium-labelinglabeling experiment, Bi suggests that the reaction is initiatedinitiated byby thethe formation formation of of a a silver silver acetylide acetylide from from terminal terminal alkyne alkyne1 and 1 and the the Ag(I) Ag(I) catalyst, catalyst, and and this isthis followed is followed by the by 1,1-insertion the 1,1-insertion of isocyanide of isocyanide9 into the Ag–C9 into bond. the ProtonolysisAg–C bond. andProtonolysis intramolecular and cyclizationintramolecular provide cyclization a pyrrolenine provide intermediate, a pyrrolenine and aintermediate, 1,5-hydrogen and shift a gives 1,5-hydrogen the pyrrole. shift On gives the other the hand,pyrrole. Lei On suggests the other that hand, the mechanism Lei suggests involves that the a mechanism click reaction involves like a Cu(I)-catalyzed a click reaction azide-alkynelike a Cu(I)- cycloadditioncatalyzed azide-alkyne (CuAAC) [cycloaddition25], because they (CuAAC) observed [25], the because coordination they observed of the silver the to coordination the isocyano groupof the bysilver in situto the IR spectroscopyisocyano group of theby stoichiometricin situ IR spectr reactionoscopy betweenof the stoichiometr ethyl 2-isocyanoacetateic reaction between and Ag 2ethylCO3. Therefore,2-isocyanoacetate the Ag2 andCO3 Agcatalyst2CO3. generatesTherefore, both the Ag a silver2CO3 acetylidecatalyst generates and a silver both isocyanide a silver acetylide complex, and and a clicksilver cyclization isocyanide provides complex, substituted and click cyclization pyrroles 15 provides. substituted pyrroles 15.

Scheme 10. Ag-catalyzed cycloaddition of alkynes and isocyanides.

In 2014,2014, Bi Bi and and co-workers co-workers reported reported the synthesis the synt ofhesis pyrroles of andpyrroles indolizines and indolizines from the cyclization from the of 2-pyridylcyclization alkynyl of 2-pyridyl carbinols alkynyl with carbinols isocyanides with (Scheme isocyanides 11)[26 (Scheme]. The divergent 11) [26]. pathways The divergent are described pathways in Schemeare described 12. 2,4-Disubstituted in Scheme 12 pyrroles. 2,4-Disubstituted17a were synthesized pyrroles 17a from were secondary synthesized 2-pyridyl from alkynyl secondary carbinols 2- 16pyridylby a regioselectivealkynyl carbinols [3+2] 16 cycloaddition. by a regioselective Coordination [3+2] cycloaddit of the pyridineion. Coordination moiety affords of the coordinated pyridine silvermoiety acetylide affords16-I coordinated, and an additional silver acetylide coordination 16-I occurs, and withan additional isocyanide 9coordinationto form intermediate occurs 16-IIwith. [3isocyanide+2] Cycloaddition 9 to form aintermediateffords 2,4-disubstituted 16-II. [3+2] pyrroleCycloaddition16-III, andafford subsequents 2,4-disubstituted oxidation pyrrole gives pyrrole 16-III, 17aandas subsequent the product. oxidation The indolizines gives pyrrole are generated 17a as from the tertiaryproduct. propargyl The indolizines alcohols 16are. The generated Ag(I) catalyst from reactstertiary with propargyl terminal alcohols alkyne 16 16to. The provide Ag(I) silver catalyst acetylide reacts16-IV with, andterminal then a alkyne 1,1-insertion 16 to ofprovide isocyanide silver9 intoacetylide the Ag–C 16-IV bond, and a ffthenords aacetylenic 1,1-insertion imido of isocyanide intermediate 9 into16-V the. Intramolecular Ag–C bond affords rearrangement acetylenic converts imido 16-Vintermediateto 2,3-allenamide 16-V. Intramolecular16-VI, and subsequent rearrangement intramolecular converts cycloisomerization 16-V to 2,3-allenamide gives indolizine 16-VI, 17band. subsequent intramolecular cycloisomerization gives indolizine 17b.

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Scheme 11. Ag-catalyzed cyclization of 2-pyridyl alkynyl carbinols with isocyanides. Scheme 11. Ag-catalyzed cyclization of 2-pyridyl alkynyl carbinols with isocyanides. Scheme 11. Ag-catalyzed cyclization of 2-pyridyl alkynyl carbinols with isocyanides.

Scheme 12. Proposed mechanism for Ag-catalyzed cyclization of 2-pyridyl alkynyl carbinols with Scheme 12.12. Proposed mechanism for Ag-catalyzed cyclization of 2-pyridyl2-pyridyl alkynylalkynyl carbinolscarbinols withwith isocyanides.Scheme 12. ProposedChem. Commun. mechanism 2014, for Ag-catalyzed50, 11837–11839, cyclization doi:10.1039/c4cc04905e of 2-pyridyl alkynyl – Reproduced carbinols with by isocyanides.isocyanides. Chem. Chem. Commun. Commun. 2014, 2014, 50, 50, 11837–11839, 11837–11839, doi:10.1039/c4cc04905e doi:10.1039/c4cc04905e—Reproduced – Reproduced by permissionisocyanides. of Chem. The Royal Commun. Society of2014, Chemistry. 50, 11837–11839, doi:10.1039/c4cc04905e – Reproduced by permissionpermission ofof TheThe RoyalRoyal SocietySociety ofof Chemistry.Chemistry. permission of The Royal Society of Chemistry. 2.3. Terminal Alkynes: Reactions with Azide 2.3. Terminal Alkynes: Reactions with Azide 2.3. Terminal Alkynes: Reactions with Azide In 2013, 2013, the the Jiao Jiao group group reported reported an an Ag-catalyzed Ag-catalyzed nitrogenation nitrogenation of alkynes, of alkynes, representing representing the first the In 2013, the Jiao group reported an Ag-catalyzed nitrogenation of alkynes, representing the first ﬁrstdirect directIn conversion 2013, conversion the Jiao of groupalkynes of alkynes reported to nitriles to nitrilesan Ag-catalyzedvia C via≡C CbondC bondnitrogenation cleavage cleavage (Scheme of (Scheme alkynes, 13) [27].13 representing)[ 27The]. reaction The reactionthe firstwas direct conversion of alkynes to nitriles via C≡C bond≡ cleavage (Scheme 13) [27]. The reaction was wasperformeddirect performed conversion with withvarious of alkynes various alkynes to alkynes nitriles as the asvia starting the C≡ startingC bond material, material,cleavage including (Scheme including aryl-, 13 aryl-,) heteroaryl-,[27]. heteroaryl-,The reaction alkyl- alkyl- wasand performed with various alkynes as the starting material, including aryl-, heteroaryl-, alkyl- and andalkenyl-substitutedperformed alkenyl-substituted with various alkynes. alkynes.alkynes In theas In the proposed proposedstarting reactionmaterial, reaction mechan including mechanismism aryl-,(Scheme (Scheme heteroaryl-, 14), 14), Ag(I) Ag(I) alkyl- simply and alkenyl-substituted alkynes. In the proposed reaction mechanism (Scheme 14), Ag(I) simply coordinatesalkenyl-substituted toto thethe alkyne, alkyne, alkynes. and and noIn no silver thesilver proposed acetylide acetylide isreaction is generated. generated. mechan Then, Then,ism the the azide(Scheme azide nucleophile nucleophile14), Ag(I) adds addssimply to the to coordinates to the alkyne, and no silver acetylide is generated. Then, the azide nucleophile adds to activatedcoordinatesthe activated alkyne to alkyne the (1-II alkyne, )( to1-II form )and to alkenylformno silver alkenyl metal acetylide complexmetal is complex generated.1-III. The 1-III protodemetallationThen,. The the protodemetallation azide nucleophile of 1-III provides addsof 1-III to the activated alkyne (1-II) to form alkenyl metal complex 1-III. The protodemetallation of 1-III vinyltheprovides activated azide vinyl1-IV alkyne azide, and 1-IV ( unstable1-II, and) to unstableform intermediate alkenyl intermediate 1-Vmetalis formedcomplex 1-V is formed by 1-III the. clickbyThe the reactionprotodemetallation click reaction of vinyl of vinyl azide of azide1-IV1-III. provides vinyl azide 1-IV, and unstable intermediate 1-V is formed by the click reaction of vinyl azide Uponprovides1-IV. Upon the vinyl release the azide release of HN 1-IV of3 and ,HN and3 CH unstableand2N CH2, intermediate2 Nintermediate2, intermediate1-V 1-Vprovides 1-V is formed provides nitrile by thenitrile product click product reaction19. 19 of. vinyl azide 1-IV. Upon the release of HN3 and CH2N2, intermediate 1-V provides nitrile product 19. 1-IV. Upon the release of HN3 and CH2N2, intermediate 1-V provides nitrile product 19.

Scheme 13. Ag-catalyzed nitrogenation of alkynes. Scheme 13. Ag-catalyzed nitrogenation of alkynes. Scheme 13. Ag-catalyzed nitrogenation of alkynes.

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Catalysts 2019, 9, x FOR PEER REVIEW 8 of 28 Catalysts 2019, 9, 1032 8 of 27 H H H H N - Ag N3- H H Ag 3 - R R N3 R R Ag + 1 Ag+ 1-II 1R Ag R 1-II 1 Ag+ 1-II N3 H N3 H NR AgH R3 Ag 1-III N H H2O R 1-III Ag N3 H H2O 3 1-IV 1-IV 1-III H2O NR3 HH R H 1-IV HN3 R HNH3

N HNN3 N N + HN N R N N+ HN N R N N - N N- N +NHNR N HN3 R 19 N N R HN3 19 1-VN- CH2N2 CH2N2 1-V R HN3 19 CH N Scheme 14. Proposed1-V mechanism of the2 Ag-catalyzed2 nitrogenation. Scheme 14. Proposed mechanism of the Ag-catalyzed nitrogenation. Scheme 14. Proposed mechanism of the Ag-catalyzed nitrogenation. In 2014, the Bi groupScheme reported 14. Proposed a Ag(I)-catalyzed mechanism of thehydroazidation Ag-catalyzed nitrogenation.to provide 2-azidoallyl alcohols In 2014, the Bi group reported a Ag(I)-catalyzed hydroazidation to provide 2-azidoallyl alcohols 21 from ethynyl carbinols 20 (Scheme 15) [28]. Hydroazidation is one of the most attractive routes for 21 fromIn 2014, ethynyl the carbinols Bi group reported20 (Scheme a Ag(I)-catalyzed 15) [28]. Hydroazidation hydroazidation is one toof providethe most 2-azidoallyl attractive routes alcohols for synthesizingIn 2014, thevinyl Bi azidesgroup reported[29]. In a a studyAg(I)-catalyzed of the effects hydroazidation of residual waterto provide in DMSO 2-azidoallyl as the alcoholssolvent 21synthesizingfrom ethynyl vinyl carbinols azides [29].20 (Scheme In a study 15)[ of28 ].the Hydroazidation effects of residual is one water of the in mostDMSO attractive as the solvent routes (Scheme21 from ethynyl16), dry carbinols DMSO provided 20 (Scheme a mixture 15) [28]. of Hydroazidation product 23 and isTMS-protected one of the most product attractive 23-TMS routes in for a for(Schemesynthesizing synthesizing 16), dryvinyl vinylDMSO azides azides provided [29]. [29 In]. Inaa mixturestudy a study of of ofthe product the effects eﬀects 23 of ofand residual residual TMS-protected water water in in DMSO product as 23-TMS thethe solventsolvent in a 1:3 ratio. When 1.0 equivalents of H2O was added, only the TMS-free product was obtained. When a (Scheme1:3 ratio. 16 When), dry 1.0 DMSO equivalents provided of aH mixture2O was ofadded, product only23 theand TMS-free TMS-protected product product was obtained.23-TMS Whenin a 1:3 a large(Scheme amount 16), dryof water DMSO was provided added, aa mixturesubstantial of product amount 23of andunreacted TMS-protected starting material product 2223-TMS remained in a ratio.large amount When 1.0 of equivalents water was ofadded, H2O wasa substantial added, only amount the TMS-free of unreacted product starting was obtained. material When22 remained a large in1:3 the ratio. reaction. When 1.0Based equivalents on the results of H2O of was these added, reactions, only the water TMS-free must productplay an wasimportant obtained. role When in the a amountin the reaction. of water Based was added, on the a results substantial of these amount reactions, of unreacted water must starting play material an important22 remained role in the reaction.large amount of water was added, a substantial amount of unreacted starting material 22 remained reaction.in the reaction. Based onBased the resultson the of results these reactions,of these reactions, water must water play must an important play an roleimportant in the reaction.role in the reaction.

Scheme 15. Ag-catalyzed hydroazidation of ethynyl carbinols. Scheme 15. Ag-catalyzed hydroazidation of ethynyl carbinols. Scheme 15. Ag-catalyzed hydroazidation of ethynyl carbinols.

Scheme 16. EffectEﬀect of water as an additive in the Ag-catalyzed hydroazidation. Scheme 16. Effect of water as an additive in the Ag-catalyzed hydroazidation. Based on aScheme deuterium-labeling 16. Effect of water experiment, as an additive the in Ag the catalystAg-catalyzed generates hydroazidation. a silveracetylide from ethylnyl carbinol 20 and HN3 by the Ag(I)-catalyzed hydrolysis of TMS-N3 18. HN3 adds to the silver

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Catalysts 2019, 9, 1032 9 of 27 Based on a deuterium-labeling experiment, the Ag catalyst generates a silver acetylide from ethylnyl carbinol 20 andand HNHN33 by the Ag(I)-catalyzed hydrolysis of TMS-N33 18.. HNHN33 addsadds toto thethe silversilver acetylideacetylide toto givegive aa vinylvinyl silversilver intermediate,intermediate, andand subsequentsubsequent protodemetallationprotodemetallationtion givesgivesgives 2-azidoallyl2-azidoallyl2-azidoallyl alcoholalcohol 2121 asas thethe product.product. UnactivatedUnactivated terminalterminal alkynesalkynes alsoalso provideprovide vinylvinyl azidesazides viavia aa Ag-catalyzed hydroazidationhydroazidation withwith TMS-N (Scheme 17)[30]. Thus, various terminal alkynes with or without hydroxyl groups at the TMS-N33 (Scheme(Scheme 17)17) [30].[30]. Thus,Thus, variousvarious terminalterminal alkynalkynes with or without hydroxyl groups at the propargylpropargyl positionposition cancan bebe successfullysuccessfullyfully converted convertedconverted into intointo vinyl vinylvinyl azides azideazides inin thethe presencepresence of of water. water.

SchemeScheme 17.17. Ag-catalyzedAg-catalyzed hydroazidationhydroazidation ofof propargylpropargyl compounds.compounds. Adapted withwith permissionpermission fromfrom Org.Org. Lett. 2014, 2014, 16, 3668–3671, doi:10.1021/ol501661k. doi:10.1021/ol501661k. Co Copyrightpyright (2019) American Chemical Chemical Society. Society.

WithWith diynes, the generated generated vinyl vinyl azides azides can can go go on on toto to reactreact react withwith with thethe the secondsecond second alkynealkyne alkyne moietymoiety moiety viavia via anan anintramolecularintramolecular intramolecular pathway.pathway. pathway. TheThe The BiBi Bigroupgroup group reportedreported reported aaa Ag(I)-catalyzedAg(I)-catalyzed Ag(I)-catalyzed tandemtandem tandem hydroazidation hydroazidation andand 26 alkyne-azidealkyne-azide cycloaddition of diynes 26 toto give give 1,5-fused 1,5-fused 1,2,3-triazole 1,2,3-triazole frameworksframeworks (Scheme(Scheme 18)18)18)[ [31].[31].31]. AsAs discusseddiscussed above,above, terminalterminal alkynes readily prov provideideide vinyl vinyl azides azides via via Ag-catalyzedAg-catalyzed hydroazidation.hydroazidation. TheThe remainingremaining unreactedunreacted alkynealkyne undergoesundergoes anan intramolecularintramolecular alkyne-azide 1,3-dipolar cycloaddition toto givegive bicyclicbicyclicbicyclic 1,2,3-triazoles1,2,3-triazoles1,2,3-triazoles 2727...

SchemeScheme 18.18. Ag-catalyzedAg-catalyzed tandem hydroazidation/alkyne- hydroazidation/alkyne-azideazide cycloadditioncycloaddition ofof diynes with TMS-N 33.. AdaptedAdapted withwith permissionpermission fromfrom Org.Org. Lett.Lett. 2015, 2015, 17, 2198–2201,2198–2201, doi:10.1021doi:10.1021/acs.orglett.5b00784./acs.orglett.5b00784. CopyrightCopyright (2019)(2019) AmericanAmerican ChemicalChemical Society.Society.

In 2015, Reddy and co-workers developed an eﬃcient pathway for accessing 3-amino quinoline InIn 2015,2015, ReddyReddy andand co-workersco-workers developeddeveloped anan efefficientficient pathwaypathway forfor accessingaccessing 3-amino3-amino quinolinequinoline derivatives 30 from readily available 1-(2-aminophenyl) propargyl alcohols 28 (Scheme 19)[32]. derivatives 30 fromfrom readilyreadily availableavailable 1-(2-aminophenyl)1-(2-aminophenyl) propargylpropargyl alcoholsalcohols 28 (Scheme(Scheme 19)19) [32].[32]. TheThe The Ag(I) catalyst activates the terminal alkynes to promote the 5-exo-dig cyclization with the amine Ag(I) catalyst activates the terminal alkynes to promote the 5-exoexo--dig cyclizationcyclization withwith thethe amineamine group.group. group. [2+3] Cycloaddition with azide 29 and a C–N bond migration with N2 expulsion results in ring [2+3][2+3] CycloadditionCycloaddition withwith azideazide 29 andand aa C–NC–N bondbond migrationmigration withwith NN22 expulsionexpulsion resultsresults inin ringring expansion and 6-endo cyclization. Subsequent tautomerization and dehydration aﬀord 4-substituted expansion and 6-endoendo cyclization.cyclization. SubsequentSubsequent tautomerizationtautomerization andand dehydrationdehydration affordafford 4-substituted4-substituted 3-tosylaminoquinolines 30. This is a rare example of a 6-endo-dig reaction of a terminal alkyne, which 3-tosylaminoquinolines 30.. ThisThis isis aa rarerare exampleexample ofof aa 6-6-endoendo--dig reactionreaction ofof aa terminalterminal alkyne,alkyne, whichwhich tend to undergo 5-exo-dig cyclizations. tendtend toto undergoundergo 5-5-exoexo--dig cyclizations.cyclizations.

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Scheme 19. Ag-catalyzed hydroamination, [2+3] cycloaddition and ring expansion. Scheme 19.19. Ag-catalyzed hydroamination, [2[2+3]+3] cycloaddition and ring expansion. 2.4. Internal Alkynes: Synthesis of Heterocyclic Compounds 2.4. Internal Alkynes: Synthesis of Heterocyclic Compounds 2.4. InternalAlthough Alkynes: internal Synthe alkynessis of Heterocycliccannot form Compounds reactive silver acetylides with Ag(I), where terminal alkynesAlthough easily can, internal internal alkynes alkynes cannot can formbe activa reactiveted by silver coordination. acetylides In with 2005, Ag(I), the Ding where group terminalterminal used internalalkynes easilyalkynes, can, (Z internal)-2-alken-4-ynylphosphonic alkynes can be activatedactiva monoestersted by coordination.coordination. 31, as the starting In 2005, material the Ding to group access used 2H- Z 31 1,2-oxaphosphorininternal alkynes, alkynes, ( (Z )-2-alken-4-ynylphosphonic2-oxides)-2-alken-4-ynylphosphonic 32 by a Ag(I)-catalyzed monoesters monoesters cyclization 31, as, (Scheme asthe the starting starting 20) material[33]. material Compared to access to access with 2H- H 32 using21,2-oxaphosphorin-1,2-oxaphosphorin (Z)-2-en-4-ynoic 2-oxides 2-oxidesacid to32 preparebyby a Ag(I)-catalyzed a Ag(I)-catalyzedthe five- or six- cyclization cyclizationmembered (Scheme (Schemering products 20) 20)[ [33].33 ].[34], Compared Compared this reaction with providesusing (Z(Z)-2-en-4-ynoic)-2-en-4-ynoic high 6-endo-dig acid acid regioselectivity. to to prepare prepare the ﬁve-the The five- or phosphonyl six-membered or six-membered oxyge ringn productsregioselectively ring products [34], this [34],attacks reaction this the providesreaction Ag(I)- activatedhighprovides 6-endo highC-≡digC 6-bondregioselectivity.endo -indig an regioselectivity. endo manner The phosphonyl to The give phosphonyl a oxygen vinyl silver regioselectively oxyge intermediate,n regioselectively attacks and thesubsequent attacks Ag(I)-activated the proton Ag(I)- transferC C bond and in protodemetalla an endo mannertion to giveprovided a vinyl 2-ethoxy-2 silver intermediate,H-1,2-oxaphosphorin and subsequent 2-oxides proton under transfer mild activated≡ C≡C bond in an endo manner to give a vinyl silver intermediate, and subsequent proton conditions.andtransfer protodemetallation and The protodemetalla cyclization provided oftion P–OH 2-ethoxy-2provided onto an 2-ethoxy-2internalH-1,2-oxaphosphorin alkyneH-1,2-oxaphosphorin is a unique 2-oxides reaction, under 2-oxides and mild thisunder conditions. platform mild Theprovidesconditions. cyclization an The effective ofcyclization P–OH synthetic onto of anP–OH internalroute onto for alkynean accessing internal is a unique alkynepotentially reaction,is a uniquebioactive and reaction, this phosphorus-containing platform and this provides platform an heterocycles.eprovidesﬀective synthetican effective route synthetic for accessing route potentially for accessing bioactive potentially phosphorus-containing bioactive phosphorus-containing heterocycles. heterocycles.

Scheme 20.20. Ag-catalyzed cyclization ofof ((ZZ)-2-alken-4-ynyl-phosphonic)-2-alken-4-ynyl-phosphonic monoesters.monoesters. Adapted with permission from Org.Org. Lett.Lett. 2005, 7,7, 3299–3301,3299–3301, doi:10.1021doi:10.1021/OL051126+./OL051126+. Copyright (2019) AmericanAmerican Scheme 20. Ag-catalyzed cyclization of (Z)-2-alken-4-ynyl-phosphonic monoesters. Adapted with Chemical Society. permission from Org. Lett. 2005, 7, 3299–3301, doi:10.1021/OL051126+. Copyright (2019) American Chemical Society. In 2014, 2014, Pan Pan and and co-workers co-workers reported reported the theconstruction construction of substituted of substituted indolizines indolizines via a Ag(I)- via a Ag(I)-mediated C–H bond functionalization of 2-alkylazaarenes with alkynes (Scheme 21)[35]. mediatedIn 2014, C–H Pan bond and functionalization co-workers reported of 2-alkylazaa the constructionrenes with of alkynes substituted (Scheme indolizines 21) [35]. viaWe aalready Ag(I)- Wediscussed already a discussedAg(I)-mediated a Ag(I)-mediated synthesis of synthesis indolizines of indolizines12 from terminal12 from alkynes terminal 1 alkynesand pyridine1 and pyridinemediated derivatives C–H bond 11functionalization(Scheme7), and of the 2-alkylazaa reaction wasrenes proposed with alkynes to involve (Scheme a 5- 21)endo [35].-dig Wecyclization already derivativesdiscussed a 11 Ag(I)-mediated (Scheme 7), and synthesis the reaction of wasindolizines proposed 12 to from involve terminal a 5-endo alkynes-dig cyclization 1 and pyridinebetween silverbetween phenylacetylide silver phenylacetylide and a andchelated a chelated intermediate. intermediate. Here, Here, the theauthors authors suggested suggested a a reaction mechanismderivatives 11 based (Scheme on the 7), results and the of reaction a deuterium-labeling was proposed experiment. to involve a In 5- theendo case-dig ofcyclization a terminal between alkyne mechanismsilver phenylacetylide based on the and results a ofchelated a deuterium-labeli intermediate.ng experiment.Here, the authorsIn the case suggested of a terminal a reaction alkyne (Scheme 2222a), (a)), silver silver acetylide acetylide1-I was 1-I initially was initially generated generated from alkyne from1 andalkyne Ag(I), 1 andand2-alkylazaarene Ag(I), and 2- 11mechanismwas separately based deprotonatedon the results byof KOAc.a deuterium-labeli The nucleophilicng experiment. attack of anion In the11-I caseby of silver a terminal acetylide alkyne1-I alkylazaarene(Scheme 22 (a)), 11 wassilver separately acetylide deprotonated 1-I was initially by KOAc. generated The nucleophilicfrom alkyne attack 1 and of Ag(I),anion 11-Iand by2- providessilver acetylide oxidative 1-I cross-coupledprovides oxidative intermediate cross-coupled11-II. Subsequentintermediate Ag(I)-assisted 11-II. Subsequent cycloisomerization Ag(I)-assisted andalkylazaarene protodemetallation 11 was separately give indolizine deprotonated product by12 KOAc.. In cases The withnucleophilic internal attack alkynes of (Schemeanion 11-I 22 byb), cycloisomerizationsilver acetylide 1-I andprovides protodemetal oxidativelation cross-coupled give indolizine intermediate product 1211-II. In. casesSubsequent with internal Ag(I)-assisted alkynes (Schemeno silver 22 acetylide (b)), no intermediate silver acetylide is formed. intermediate 2-Alkylazaarene is formed. 112-Alkylazaareneundergoes single-electron 11 undergoes oxidation single- tocycloisomerization give radical intermediate and protodemetal11-VI. Then,lation single-electron give indolizine insertion product into12. In internal cases with alkyne internal33 provides alkynes electron(Scheme oxidation22 (b)), no to silver give acetylideradical intermediate intermediate 11-VI is formed.. Then, 2-Alkylazaarenesingle-electron insertion 11 undergoes into internal single- alkyneintermediate 33 provides11-VII intermediate. Further oxidation 11-VII of. Further intermediate oxidation11-VII of intermediateby Ag(I) generates 11-VII carbocation by Ag(I) generates11-VIII, andelectron indolizine oxidation34 is to produced give radical by intramolecular intermediate condensation.11-VI. Then, single-electron insertion into internal carbocationalkyne 33 provides 11-VIII ,intermediate and indolizine 11-VII 34 is. Furtherproduced oxidation by intramolecular of intermediate condensation. 11-VII by Ag(I) generates carbocation 11-VIII, and indolizine 34 is produced by intramolecular condensation.

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Scheme 21. Ag-mediated one-pot synthesis of indolizines from alkynes and 2-alkylazaarenes. Scheme 21. Ag-mediated one-pot synthesis of indolizinesindolizines from alkynes and 2-alkylazaarenes.

Scheme 22. 22. ProposedProposed mechanisms mechanisms for Ag-mediated for Ag-mediated indolizine indolizine synthesis synthesis with terminal with terminal and internal and Scheme 22. Proposed mechanisms for Ag-mediated indolizine synthesis with terminal and internal alkynes.internal alkynes. alkynes. A Ag(I)-mediated synthesis of alkynyl (diaryl) phosphine oxides from terminal alkyne with AA Ag(I)-mediatedAg(I)-mediated synthesissynthesis ofof alkynylalkynyl (diaryl)(diaryl) phosphinephosphine oxidesoxides fromfrom terminalterminal alkynealkyne withwith Ph2P(O)H was previously described in Scheme8. The reaction involves the generation of silver Ph2P(O)H was previously described in Scheme 8. The reaction involves the generation of silver acetylidePh2P(O)H andwas anpreviously oxidative described cross coupling. in Scheme The 8. Wu The group reaction developed involves a versionthe generation of this of reaction silver acetylide and an oxidative cross coupling. The Wu group developed a version of this reaction with withacetylide an internal and an alkyne,oxidative namely, cross coupling. a Ag(I)-catalyzed The Wu group synthesis developed of 3-phosphorated a version of coumarinsthis reaction36 withvia an internal alkyne, namely, a Ag(I)-catalyzed synthesis of 3-phosphorated coumarins 36 via the thean internal radical cyclizationalkyne, namely, of alkynoates a Ag(I)-catalyzed35 and dialkyl synthesisH-phosphonates of 3-phosphorated13 in a coumarins highly regioselective 36 via the radicalradical cyclizationcyclization ofof alkynoatesalkynoates 3535 andand dialkyldialkyl HH-phosphonates-phosphonates 1313 inin aa highlyhighly regioselectiveregioselective mannermanner

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(Schememanner(Scheme 23) [36]. 23The)[ 36reaction]. The reactionwas completely was completely suppressed suppressed by the presence by the presence of stoichiometric of stoichiometric TEMPO TEMPO((2,2,6,6,-tetramethylpiperidin-1-yl)oxyl), ((2,2,6,6,-tetramethylpiperidin-1-yl)oxyl), which implies which that implies the reaction that the proceeds reaction proceedsthrough a through radical pathway.a radical pathway.With this Withexperimental this experimental result in hand, result a in reaction hand, amechanism reaction mechanism was proposed, was proposed,as shown asin Schemeshown in 24. Scheme Phosphorus 24. Phosphorus radical 13-I radical isis generatedgenerated13-I is generated fromfrom diethyldiethyl from diethyl H-phosphonateH-phosphonate 13 byby 13Ag(I),Ag(I),by Ag(I), andand subsequentand subsequent selective selective addition addition of the of P-radical the P-radical to the to α the-positionα-position of the of C=O the bond C=O bondin the inalkynoate the alkynoate gives vinylgives vinylradical radical 35-I.. CyclizationCyclization35-I. Cyclization ofof intermediateintermediate of intermediate 35-I generatesgenerates35-I generates intermediateintermediate intermediate 35-II,35-II, andand, subsequentsubsequent and subsequent SETSET fromSET from 35-II35-II toto Ag(I)Ag(I)to Ag(I) releasesreleases releases 3-phosphorated3-phosphorated 3-phosphorated coumarincoumarin coumarin 36.. 36.

Scheme 23. Ag-catalyzedAg-catalyzed phosphorus carbocyclization of alkynoates. Adapted with permission from Org. Lett. 2014, 2014, 16, 16, 3356–3359, 3356–3359, doi:10.1021/ol5013839. doi:10.1021/ol5013839. Co Copyrightpyright (2019) (2019) American American ChemicalChemical Society.Society.

Scheme 24. Proposed mechanism for the Ag-catalyzed phosphorus carbocyclization. Scheme 24. Proposed mechanism for the Ag-catalyzed phosphorus carbocyclization. In 2014, the Fillion group reported a Ag(I)-catalyzed 5-exo-dig cyclization of propargylic Meldrum’s In 2014, the Fillion group reported a Ag(I)-catalyzed 5-exo-dig cyclization of propargylic acidIn derivatives 2014, the37 Fillionto give groupγ-alkylidene reported butyrolactones a Ag(I)-catalyzed38 (Scheme 5-exo- dig25)[ cyclization37]. The formation of propargylic of the Meldrum’s acid derivatives 37 to give γ-alkylidene butyrolactones 38 (Scheme 25) [37]. The formation EMeldrum’s/Z-isomers acid of thederivatives product 37 is to controlled give γ-alkylidene by the coordination butyrolactones of 38 Ag(I). (Scheme The 25) alkyne-Ag(I) [37]. The formation complex of the E/Z-isomers of the product is controlled by the coordination of Ag(I). The alkyne-Ag(I) complex ofdetermines the E/Z-isomers the form of ofthe an productE- or Z is-isomer controlled based by on the the coordination 5-exo-dig cyclization of Ag(I). The through alkyne-Ag(I) the attack complex of the determines the form of an E- or Z-isomer based on the 5-exo-dig cyclization through the attack of the determinescarbonyl-O. the Syn-attack form of of an the E- carbonyl- or Z-isomerO and based Ag(I)-activated on the 5-exo alkyne-dig cyclization provides thethroughE-intermediate; the attack onof the carbonyl-O. Syn-attack of the carbonyl-O and Ag(I)-activated alkyne provides the E-intermediate; on carbonyl-other hand,O. anti-attackSyn-attack ofof the carbonyl-O ontoand Ag(I)-activated the Ag(I)-coordinated alkyne alkyneprovides gives the theE-intermediate;Z-intermediate. on the other hand, anti-attack of the carbonyl-O onto the Ag(I)-coordinated alkyne gives the Z- theBoth other intermediates hand, anti-attack undergo thermally of the carbonyl- induced cycloreversionO onto the Ag(I)-coordinated to provide the corresponding alkyne gives acylketene the Z- intermediate. Both intermediates undergo thermally induced cycloreversion to provide the intermediate.intermediates; then,Both γintermediates-alkylidene butyrolactones undergo ther38mallyare obtained induced by nucleophiliccycloreversion attack to ofprovide the solvent. the corresponding acylketene intermediates; then, γ-alkylidene butyrolactones 38 areare obtainedobtained byby nucleophilic attack of the solvent.

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Scheme 25. Ag-catalyzed formation of alkylidene γ-butyrolactones.

IndoloquinolinesIndoloquinolines are are among among the the most most importan importantt skeletons skeletons in the in thedesign, design, development, development, and andsynthesis synthesis of drugs of drugs currently currently on onthe the market market [38]. [38 Most]. Most syntheticsynthetic synthetic approachesapproaches approaches relyrely rely onon on thethe the useuse use ofof of indole indole or itsits derivativesderivatives as as a a coupling coupling partner, partner, and and a new a new synthetic synthetic pathway pathway for indolo[2,3- for indolo[2,3-b]quinoloneb]quinolone has been has developedbeen developed via a cascadevia a cascade radical radical annulation annulation of o-alkynylthiourea of o-alkynylthiourea [39]. In 2017,[39]. In the 2017, Patel the group Patel reported group areported microwave-assisted a microwave-assisted Ag(I)-catalyzed Ag(I)-catalyzed cascade reaction cascade toward reaction indolo[2,3- toward indolo[2,3-b]quinolinesb]quinolines41 via the 41 in situvia the generation in situ ofgenerationo-alkynylthioureas of o-alkynylthioureas from 2-(phenylethynyl)anilines from 2-(phenylethynyl)anilines39 with phenyl 39 isothiocyanate withwith phenylphenyl 40isothiocyanate(Scheme 26 )[4040 (Scheme].(Scheme The reaction 26)26) [40].[40]. of 2-(phenylethynyl)anilinesTheThe reactionreaction ofof 2-(phenylethynyl)anilines2-(phenylethynyl)anilines39 and phenyl isothiocyanate 39 andand phenylphenyl40 providesisothiocyanate thiourea, 40 providesprovides followed thiourea,thiourea, by desulfurization followedfollowed byby in desulfurizationdesulfurization the presence of inin Ag thethe2CO presencepresence3, to give ofof AgAg a carbodiimide2CO3,, toto givegive intermediate.a carbodiimide An intermediate. intramolecular An thermalintramolecularintramolecular cyclization thermalthermal drives cyclizationcyclization the carbodiimide drivesdrives intermediatethethe carbodiimidecarbodiimide to a carbene-typeintermediate intermediate;to a carbene-type then, intermediate; indoloquinoline then,41 indoloquinolineis produced by carbene41 is produced C H insertion by carbene followed C−H intermediate to a carbene-type intermediate; then, indoloquinoline 41 is produced− by carbene C−H byinsertion aromatization. followed by aromatization.

Scheme 26.26.Ag-catalyzed Ag-catalyzed cascade cascade cyclization cyclization by the reactionby the ofreaction 2-(phenylethynyl)aniline of 2-(phenylethynyl)aniline and isothiocyanate. and Adaptedisothiocyanate.isothiocyanate. with permission AdaptedAdapted from withwith J. Org. permissipermissi Chem. 2017,on 82,from 2089–2096, J. Org. doi:10.1021 Chem./ acs.joc.6b02912.2017, 82, 2089–2096,Copyright (2019)doi:10.1021/acs.joc.6b02912. American Chemical Society. Copyright (2019) American Chemical Society.

3. Functionalization of Carboxylic Acid Using Ag2CO3 3. Functionalization of Carboxylic Acid Using Ag2CO3 3.1. Decarboxylation of Carboxylic Acids 3.1. Decarboxylation of Carboxylic Acids In 2009, the Larrosa group reported a Ag(I)-catalyzed protodecarboxylation of ortho-substituted benzoicIn 2009, acids the42 Larrosa(Scheme group 27)[41 reported]. This reaction a Ag(I)-catalyzed only proceeds protodecarboxylation with ortho-substituted of ortho benzoic-substituted acids benzoic acids 42 (Scheme(Scheme 27)27) [41].[41]. ThisThis reactionreaction onlyonly proceedsproceeds withwith ortho-substituted benzoic acids although they can have a range of functionalities, such as halogens, NO2, alkoxides, and even unprotectedalthough they phenols can have and amines,a range regardless of functionalities, of the presence such ofas anyhalogens, functional NO group2,, alkoxides,alkoxides, at positions andand othereveneven thanunprotected the ortho phenols-position and of the amines, benzoic re acid.gardless A proposed of the presence mechanism of forany the functional reaction involvesgroup at conversion positions ofother the than Ag(I) the species ortho- toposition the silver of carboxylatethe benzoic withacid. benzoic A proposed acid 42 mechanism. After decarboxylation for the reaction of theinvolves silver carboxylate,conversion of the the silver Ag(I) arene species intermediate to the silver gives carboxylate corresponding with benzoic arene 43acidby 42 protodemetallation.. AfterAfter decarboxylationdecarboxylation with concomitantof the silver generation carboxylate, of a newthe silversilver carboxylate arene intermediate to turn over gives the catalytic corresponding cycle (Scheme arene 28 ).43 byby protodemetallation with concomitant generation of a new silver carboxylate to turn over the catalytic cycle (Scheme 28).

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Scheme 27. Ag-catalyzed decarboxylation of ortho-substituted benzoic acids. Scheme 27. Ag-catalyzed decarboxylation of ortho-substituted benzoic acids.acids.

Scheme 28. Proposed mechanism for decarboxylation of ortho-substituted-substituted benzoicbenzoic acids.acids. Scheme 28. Proposed mechanism for decarboxylation of ortho-substituted benzoic acids. InIn thethe samesame year,year, thethe LarrosaLarrosa groupgroup reportedreported anotheranother Ag(I)-catalyzedAg(I)-catalyzed protodecarboxylationprotodecarboxylation of In the same year, the Larrosa group reported another Ag(I)-catalyzed protodecarboxylation of variousvarious heteroaromaticheteroaromatic carboxylic carboxylic acids acids44 (Scheme44 (Scheme(Scheme 29)[ 4229)29)]. [42].[42]. This reactionThisThis reactionreaction uses acetic usesuses acid aceticacetic as anacidacid additive. asas anan various heteroaromatic carboxylic acids 44 (Scheme 29) [42]. This reaction uses acetic acid as an Notadditive. only αNot-heteroaromatic only α-heteroaromatic carboxylic acidscarboxylic but also acidsortho but-substituted also ortho-substituted benzoic acids benzoic are converted acids are to additive. Not only α-heteroaromatic carboxylic acids but also ortho-substituted benzoic acids are silverconverted carboxylates to silver carboxylates by Ag(I), and by the Ag(I), silver and carboxylates the silver carboxylates then undergo then decarboxylation. undergo decarboxylation. The details converted to silver carboxylates by Ag(I), and the silver carboxylates then undergo decarboxylation. ofThe the details role of of the the heteroatoms role of the in heteroatoms the decarboxylation in the decarboxylation are difficult to predict,are difficult but heteroatomsto predict, but are The details of the role of the heteroatoms in the decarboxylation are difficult to predict, but expectedheteroatoms to serve are expected as activators to serve in this as reaction. activators Diacids in this were reaction. also subjected Diacids were to these also conditions, subjected andto these they heteroatoms are expected to serve as activators in this reaction. Diacids were also subjected to these providedconditions, the and corresponding they provided monoacids the corresponding with complete monoacids regioselectivity. with complete In the regioselectivity. case of heteroaromatic In the conditions, and they provided the corresponding monoacids with complete regioselectivity. In the compounds,case of heteroaromatic the reactivity compounds, of the carboxylic the reactivity acid at the ofα -positionthe carboxylic is still acid controlled at the by α-position the heteroatoms. is still case of heteroaromatic compounds, the reactivity of the carboxylic acid at the α-position is still Incontrolled addition by to thethe eheteroatoms.ffect of the α -heteroatom,In addition to the the conversion effect of tothe the α-heteroatom, monoacid proceeds the conversion well even to if the an controlled by the heteroatoms. In addition to the effect of the α-heteroatom, the conversion to the electron-withdrawingmonoacid proceeds well group even is if located an electron-withdrawing at the ortho-position. group Since is both located acids, at 1,2-diacid the ortho-position. and 1,4-diacid, Since monoacid proceeds well even if an electron-withdrawing group is located at the ortho-position. Since underwentboth acids, decarboxylation1,2-diacid and 1,4-diacid, with high underwent yields, the numberdecarboxylation of adjacent with carboxylic high yields, acids the seems number to have of both acids, 1,2-diacid and 1,4-diacid, underwent decarboxylation with high yields, the number of noadjacent effect oncarboxylic the decarboxylation. acids seems to have nono effecteffect onon thethe decarboxylation.decarboxylation. adjacent carboxylic acids seems to have no effect on the decarboxylation.

Scheme 29. Protodecarboxylation of hetero heteroaromaticaromatic carboxylic acids.acids. Scheme 29. Protodecarboxylation of heteroaromatic carboxylic acids. The Jafapour group reported a Ag(I)-catalyzed decarboxylation of heteroaromatic compounds The Jafapour group reported a Ag(I)-catalyzed decarboxylation of heteroaromatic compounds (SchemeThe 30 Jafapour)[43]. In group the case reported of coumarins, a Ag(I)-catalyzed the decarboxylation decarboxylation is restricted of toheteroaromatic carboxylic acids compounds containing (Scheme 30) [43]. In the case of coumarins, the decarboxylation is restricted to carboxylic acids α(Scheme-heteroatoms. 30) [43]. After In tuningthe case the of solvent, coumarins, even unactivatedthe decarboxylation coumarin-3-carboxylic is restricted to acids carboxylic46 produced acids containing α-heteroatoms. After tuning the solvent, even unactivated coumarin-3-carboxylic acids 46 correspondingcontaining α-heteroatoms. decarboxylated After products tuning the47 undersolvent, mild even conditions unactivated with coumarin-3-carboxylic wide substrate compatibility. acids 46 produced corresponding decarboxylated products 47 underunder mildmild conditionsconditions withwith widewide substratesubstrate produced corresponding decarboxylated products 47 under mild conditions with wide substrate compatibility. compatibility.

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Scheme 30. Ag-catalyzed protodecarboxylation of coumarin-3-carboxylic acid. Scheme 30. Ag-catalyzed protodecarboxylation of coumarin-3-carboxylic acid. Scheme 30. Ag-catalyzed protodecarboxylation of coumarin-3-carboxylic acid. 3.2. Decarboxylative Functionalization and Carboxylations 3.2. Decarboxylative Function Functionalizationalization and Carboxylations 3.2. Decarboxylative Functionalization and Carboxylations Aryl halides are useful synthetic intermediates and can be used for a variety of reactions [44]. In Aryl halides are are useful useful synthetic synthetic intermediates intermediates and and can can be be used used for for a variety a variety of ofreactions reactions [44]. [44 In]. general,Aryl boron–halogen halides are useful exchange synthetic reactions intermediates have attractedand can be attention used for asa variety a synthetic of reactions pathway [44]. for In Ingeneral, general, boron–halogen boron–halogen exchange exchange reactions reactions have have attracted attracted attention attention as as a a synthetic synthetic pathway for accessinggeneral, boron–halogen aryl halides. Previous exchange common reactions decarboxylative have attracted halogenation attention asreactions a synthetic use very pathway insoluble for accessing aryl halides. Previous common decarboxylative halogenation reactions use very insoluble NaClaccessing and arylLiCl halides.as chloride Previous sources. common In 2010, decarboxylative the Wu group reported halogenation a Ag(I)-catalyzed reactions use decarboxylative very insoluble NaCl and LiCl as chloride sources. In 2010, the the Wu Wu group reported a Ag(I)-catalyzed decarboxylative halogenationNaCl and LiCl of as benzoic chloride acids sources. in the In 2010,presence the Wu of Cu(II)group halidesreported to a provideAg(I)-catalyzed the corresponding decarboxylative aryl halogenation of benzoic acids in the presence of Cu(II) halides to provide thethe correspondingcorresponding aryl halideshalogenation (Scheme of 31)benzoic [45]. Cu(II)acids inha lidesthe presence are inexpensive of Cu(II) organometallic halides to provide reagents the and corresponding readily available aryl halides (Scheme 3131))[ [45].45]. Cu(II) Cu(II) ha halideslides are are inexpensive inexpensive organometallic organometallic reagents and readily available halidehalides sources (Scheme with 31) superior[45]. Cu(II) solubility halides arein organic inexpensive solvents. organometallic reagents and readily available halide sources with superior solubilitysolubility inin organicorganic solvents.solvents. halide sources with superior solubility in organic solvents.

Scheme 31. Ag-catalyzed decarboxylative halogenation of carboxylic acids. Scheme 31. Ag-catalyzed decarboxylative halogenation of carboxylic acids. Scheme 31. Ag-catalyzed decarboxylative halogenation of carboxylic acids. Decarboxylation can can lead lead to to changes changes in in the the functional functional groups groups on on the the starting starting material material as aswell well as Decarboxylation can lead to changes in the functional groups on the starting material as well as asthe the formationDecarboxylation formation of ofnew new can C C− leadC Cbonds. bonds.to changes Muthusubramanian Muthusubramanian in the functional and andgroups co-w co-workers onorkers the starting reported reported material a Ag-catalyzedAg-catalyzed as well as the formation of new C−C− bonds. Muthusubramanian and co-workers reported a Ag-catalyzed acylationthe formation of pyridine of new N -oxidesC−C bonds. 49 by Muthusubramanianα-oxocarboxylic acids and 50 ininco-w 20142014orkers (Scheme(Scheme reported 3232))[ [46].46 a]. Ag-catalyzedThrough this acylation of pyridine N-oxides 49 by α-oxocarboxylic acids 50 in 2014 (Scheme 32) [46]. Through this reaction,acylation variousof pyridine aryl N ketones-oxides can 49 by be α synthesized-oxocarboxylic by acids eeffectivelyﬀectively 50 in 2014 promoting (Scheme the 32) acylation [46]. Through at the this C-2 reaction, various aryl ketones can be synthesized by effectively promoting the acylation at the C-2 positionreaction, ofvarious the pyridine. aryl ketones When can this be reaction synthesized was conducted by effectively in the promoting presence ofthe TEMPO, acylation the at acylated the C-2 position of the pyridine. When this reaction was conducted in the presence of TEMPO, the acylated TEMPOposition adductofadduct the pyridine. was was formed, formed, When indicating thisindicating reaction that the thatwas reaction conductedthe reaction proceeds in theproceeds through presence anthrough acylof TEMPO, radical an intermediate.acylthe acylated radical TEMPO adduct was formed, indicating that the reaction proceeds through an acyl radical Basedintermediate.TEMPO on thisadduct result,Based wasthe on formed, reaction this result, mayindicating startthe reaction by that generating themay reaction astart silver by dicarboxylateproceeds generating through a intermediate, silver an dicarboxylateacyl and radical then intermediate. Based on this result, the reaction may start by generating a silver dicarboxylate theintermediate,intermediate. acyl radical andBased intermediate then on the this acyl isresult, generated radical the inte byreactionrmediate the release may is generated ofstart CO 2byand generatingby a Ag(I)the release carboxylate. a silver of CO dicarboxylate2 Aand new a Ag(I) C C intermediate, and then the acyl radical intermediate is generated by the release of CO2 and a Ag(I)− bondcarboxylate.intermediate, between A and thenew acylthen C−C radicalthe bond acyl andbetween radical pyridine theinte acylrmediateN-oxide radical49 is wasandgenerated generatedpyridine by Nthe by-oxide release a radical 49 wasof addition CO generated2 and reaction, a Ag(I) by a carboxylate. A new C−C bond between the acyl radical and pyridine N-oxide 49 was generated by a aradicalcarboxylate.ﬀording addition acylated A new reaction, pyridine C−C bond affordingN-oxide between acylated51 asthe the acyl pyridine product. radical N -oxideand pyridine 51 as the N -oxideproduct. 49 was generated by a radical addition reaction, affording acylated pyridine N-oxide 51 as the product. radical addition reaction, affording acylated pyridine N-oxide 51 as the product.

Scheme 32. Ag-catalyzed decarboxylative acylation of pyridine N-oxides. Scheme 32. Ag-catalyzed decarboxylative acylation of pyridine N-oxides. Scheme 32. Ag-catalyzed decarboxylative acylation of pyridine N-oxides. In 2014, Qi and co-workers reported Ag-catalyzed decarboxylative acylation of arylglyoxylic acids 50 with arylboronic acids 52 (Scheme 33)[47]. This Ag2CO3-catlayzed decarboxylation enables

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Catalysts 2019, 9, 1032 16 of 27 In 2014, Qi and co-workers reported Ag-catalyzed decarboxylative acylation of arylglyoxylic acids 50 with arylboronic acids 52 (Scheme 33) [47]. This Ag2CO3-catlayzed decarboxylation enables the synthesis of ketonesketones 53 with aromaticaromatic ringsrings bearingbearing variousvarious functionalfunctional groups. The most useful feature ofof thisthis reactionreaction is is not not only only the the fact fact that that the the arylglyoxylic arylglyoxylic acid acid bears bears a carboxylic a carboxylic acid acid but but also also the thedecarboxylative decarboxylative acylation acylation that occursthat occurs when when a phenylboronic a phenylboronic acid has acid a carboxylic has a carboxylic acid. The acid. reaction The pathwayreaction pathway may be may initiated be initiated by a free by radicala free radical because because no product no product was was generated generated in the in the presence presence of TEMPO.of TEMPO. Acyl Acyl radicals radicals can can be be prepared prepared by by decarboxylation decarboxylation of of silver silver carboxylate,carboxylate, andand subsequent radical transformation can provideprovide cross-coupledcross-coupled diaryldiaryl ketoneketone 5353..

Scheme 33. Ag-catalyzedAg-catalyzed cross cross coupling of arylboronic acids with arylglyoxylic acids.

Among the the various various C1 C1 insertion insertion reactions, reactions, CO CO is the is the most most common common acceptor acceptor for generating for generating new acylnew acylradicals radicals through through carbonylation. carbonylation. Isocyanide Isocyanide is also is also a aradical radical acceptor acceptor for for imidoyl imidoyl radical formation. InIn recent recent years, years, decarboxylations decarboxylations of carboxylic of carboxylic acids using acids transition using transition metals have metals contributed have contributedsigniﬁcantly significantly to the formation to the of formation various C–C of va bondsrious C–C and C–heteroatombonds and C–heteroatom bonds [48]. bonds The Lei [48]. group The Leireported group an Ag-mediatedreported an oxidative Ag-mediated radical decarboxylation-cyclizationoxidative radical decarboxylation-cyclization of α-oxocarboxylates 55of andα- oxocarboxylatesisocyanides 54 to 55 give and6-acyl isocyanides phenanthridines 54 to give 6-acyl56 (Scheme phenanthridines 34)[49]. In 56 the (Scheme proposed 34) mechanism[49]. In the (Schemeproposed 35 mechanism), acyl radical (Scheme55-I is formed35), acyl by radical oxidative 55-I radicalis formed decarboxylation by oxidative radical in the presence decarboxylation of Ag2CO in3 as a catalyst. Then, radical addition to isocyanide 54 provided imidoyl radical 55-II. The intramolecular the presence of Ag2CO3 as a catalyst. Then, radical addition to isocyanide 54 provided imidoyl radical 55-IIcyclization. The intramolecular of imidoyl radical cyclization55-II then of imidoyl gave cyclohexadienyl radical 55-II then radical gave cyclohexadienyl55-III, and SET fromradical Ag(I) 55- providesIII, and SET 6-acyl from phenanthridine Ag(I) provides56 6-acylas the phenanthridine product. 56 as the product.

Scheme 34. 34.Oxidative Oxidative radical radical decarboxylation decarboxylation/cyclization/cyclization of α-oxocarboxylates of α-oxocarboxylates and isocyanides. and isocyanides. Chem. Chem.Commun. Commun. 2014, 2014, 50, 2145–2147,50, 2145–2147, doi:10.1039 doi:10.1039/c3/c3cc49026b—Reproducedcc49026b – Reproduced by permission by permission of The Royal of The Society of Chemistry. Royal Society of Chemistry. In 2016, the Kim group developed an Ag-mediated temperature-controlled acyloxylations and subsequent hydrolysis for hydroxylation (Scheme 36)[50]. At lower temperature (100 ◦C), various esters 59 were synthesized from 10-bromobenzo[h]quinoline 57. Interestingly, at higher temperature (150 ◦C), ester products 59 were hydrolyzed to 10-hydroxybenzo[h]quinoline 60. 2-Methylbenzoic acid showed the best conversion in this hydroxylation. The silver species activated the carboxylic acid through silver benzoate formation, and the two-step acyloxylation and hydrolysis were conﬁrmed by NMR spectroscopy.

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Scheme 35. Proposed mechanism of the Ag-catalyzed oxidative radical decarboxylation/cyclization. Chem. Commun. 2014, 50, 2145–2147, doi:10.1039/c3cc49026b – Reproduced by permission of The Royal Society of Chemistry.

In 2016, the Kim group developed an Ag-mediated temperature-controlled acyloxylations and subsequent hydrolysis for hydroxylation (Scheme 36) [50]. At lower temperature (100 °C), various esters 59 were synthesized from 10-bromobenzo[h]quinoline 57. Interestingly, at higher temperature (150 °C), ester products 59 were hydrolyzed to 10-hydroxybenzo[h]quinoline 60. 2-Methylbenzoic acid showed the best conversion in this hydroxylation. The silver species activated the carboxylic SchemeScheme 35.35. ProposedProposed mechanism of the Ag-catalyzedAg-catalyzed oxidative radical radical decarboxylation/cyclization. decarboxylation/cyclization. acid through silver benzoate formation, and the two-step acyloxylation and hydrolysis were Chem.Chem. Commun. Commun. 2014,2014, 50, 2145–2147, doi:10.1039/c3cc doi:10.1039/c3cc49026b—Reproduced49026b – Reproduced by by permission permission of of The The confirmed by NMR spectroscopy. Royal Society of Chemistry. Royal Society of Chemistry.

Scheme 36. Temperature-controlledTemperature-controlled acyloxylation and hydroxylation of bromoarene. 4. Miscellaneous 4. Miscellaneous Leblanc’s deamination of hydrazides is a useful reaction using Zn and acetic acid [51]. In general, Leblanc’s deamination of hydrazides is a useful reaction using Zn and acetic acid [51]. In general, hydrazides are converted into hydrazines instead of generating the corresponding amines from hydrazides are converted into hydrazines instead of generating the corresponding amines from transition metal-catalyzed hydrogenolytic N–N bond cleavage [52]. In 2002, the Cho group reported a transition metal-catalyzed hydrogenolytic N–N bond cleavage [52]. In 2002, the Cho group reported Ag(I)-mediated deamination of N-Boc aryl hydrazines 61 to aﬀord N-Boc aryl amines 62 (Scheme 37)[53]. a Ag(I)-mediated deamination of N-Boc aryl hydrazines 61 to afford N-Boc aryl amines 62 (Scheme When an electron-donating group was present on the substrate, the corresponding products were obtained in high yields within relatively short reaction time (2 h). When an electron-withdrawing Scheme 36. Temperature-controlled acyloxylation and hydroxylation of bromoarene. group was present on the substrate, the required reaction time increased to over 48 h (NO2) or 72 h (CO4. Miscellaneous2Me) and the yields were relatively low. Leblanc’s deamination of hydrazides is a useful reaction using Zn and acetic acid [51]. In general, hydrazides are converted into hydrazines instead of generating the corresponding amines from transition metal-catalyzed hydrogenolytic N–N bond cleavage [52]. In 2002, the Cho group reported a Ag(I)-mediated deamination of N-Boc aryl hydrazines 61 to afford N-Boc aryl amines 62 (Scheme

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37) [53]. When an electron-donating group was present on the substrate, the corresponding products 37) [53]. When an electron-donating group was present on the substrate, the corresponding products were37) [53]. obtained When an in electron-donating high yields within group relatively was present short on reactionthe substrate, time the(2 correspondingh). When an productselectron- were obtained in high yields within relatively short reaction time (2 h). When an electron- withdrawingwere obtained group in high was presentyields withinon the substrate,relatively theshort required reaction reaction time time(2 h). increased When anto overelectron- 48 h withdrawing group was present on the substrate, the required reaction time increased to over 48 h Catalysts(NOwithdrawing2) or2019 72, 9h, 1032group(CO2Me) was and present the yields on the were substrate, relatively the low. required reaction time increased to over18 48 of 27h (NO2) or 72 h (CO2Me) and the yields were relatively low. (NO2) or 72 h (CO2Me) and the yields were relatively low.

Scheme 37. Ag-mediated deamination of N-Boc aryl hydrazines. Scheme 37. Ag-mediated deamination of N-Boc aryl hydrazines. Scheme 37. Ag-mediated deamination of N-Boc aryl hydrazines. The construction of C−N bonds in heteroaromatic compounds is important in the fields of The construction of C−N bonds in heteroaromatic compounds is important in the fieldsfields of biological,The construction pharmaceutical, of C −andN bondsmaterials in heteroaromaticscience [54]. Although compounds hydroaminati is importanton [55] in and the oxidativefields of biological, pharmaceutical, and materials science [[5544].]. Although hydroaminationhydroamination [[55]55] and oxidative aminationbiological, [56],pharmaceutical, for the formation and materials of C−N bonds,science have [54]. beenAlthough studied hydroaminati in great detailon [55]in recent and oxidative decades, amination [[56],56], for the formation of C−NN bonds, have been studied in great detail in recent decades, theamination direct [56],installation for the formationof amino groupsof C−N bonds,or their have surrogates been studied into aryl in great or alkyl detail C–H in recent bonds decades, is still the direct installationinstallation of amino groups or their surrogates into aryl or alkyl C–H bonds is still challenging.the direct installation In 2009, the of Chang amino group groups reported or their a Ag-mediatedsurrogates into amination aryl or ofalkyl benzoxazoles C–H bonds 63 is using still challenging. In 2009, the Chang group reported a Ag-mediated amination of benzoxazoles 63 using formamideschallenging. 64In 2009,or their the parent Chang amines group (Schemereported 38)a Ag-mediated [57]. The reaction amination gives of2-aminobenzoxazoles benzoxazoles 63 using 65 formamides 64 oror their their parent parent amines amines (Scheme (Scheme 38) 38 )[[57]57.]. The The reaction reaction gives gives 2-aminobenzoxazoles 2-aminobenzoxazoles 65 formamideswith various 64 functional or their parent groups amines by the (Scheme regioselective 38) [57] formation. The reaction of C–N gives bonds 2-aminobenzoxazoles through the direct 65 65withwith various various functional functional groups groups by by the the regioselective regioselective formation formation of of C–N C–N bonds bonds through through the direct C(spwith 2)–Hvarious functionalization functional groups of heteroarenes. by the regioselective formation of C–N bonds through the direct C(sp2)–H)–H functionalization of heteroarenes. C(sp2)–H functionalization of heteroarenes.

Scheme 38. Ag-mediated direct amination of benzoxazoles. Scheme 38. Ag-mediated direct amination of benzoxazoles. Scheme 38. Ag-mediated direct amination of benzoxazoles. A possiblepossible reaction reaction mechanism mechanism is shown is shown in Scheme in 39Scheme. First, the39. acid First, promotes the acid the decarbonylationpromotes the A possible reaction mechanism is shown in Scheme 39. First, the acid promotes the decarbonylationof formamideA possible64 ofreactionto formamide give correspondingmechanism 64 to give is corresponding secondary shown in amine Scheme secondary64-I .39. Secondary amineFirst, 64-Ithe amine. Secondaryacid64-I promotesreacts amine with the64- decarbonylation of formamide 64 to give corresponding secondary amine 64-I. Secondary amine 64- protonatedIdecarbonylation reacts with benzoxazole protonated of formamide benzoxazole63-I to 64 form to give 63-I 2-amino correspondingto form benzoxazoline 2-amino secondary benzoxazoline intermediate amine intermediate 64-I63-II. Secondary. Finally, 63-II amine. Ag Finally,CO 64- I reacts with protonated benzoxazole 63-I to form 2-amino benzoxazoline intermediate 63-II. Finally,2 3 IAgfacilitates reacts2CO3 with facilitates rearomatization protonated rearomatization benzoxazole to provide to 2-aminated63-Iprovide to form 2-aminated benzoxazole2-amino benzoxazole benzoxazoline65. 65 .intermediate 63-II. Finally, Ag2CO3 facilitates rearomatization to provide 2-aminated benzoxazole 65. Ag2CO3 facilitates rearomatization to provide 2-aminated benzoxazole 65.

Scheme 39. Proposed mechanism for the Ag-mediated direct amination of benzoxazoles.

Mukhopadhyay reported a Ag(I)-mediated synthesis of diverse 1,2,4,5-tetrasubstituted imidazoles 3 68 via the tandem Cα(sp )–H bond functionalization of primary amines 67 and oxidative C–N cross-coupling reaction (Scheme 40)[58]. Although the reaction required stoichiometric Ag2CO3, the recycled silver reagent provided the product in good yield without any loss of activity. Catalysts 2019,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 19 of 28

Scheme 39. Proposed mechanism for the Ag-mediated direct amination of benzoxazoles.

Mukhopadhyay reported a Ag(I)-mediated synthesis of diverse 1,2,4,5-tetrasubstituted 3 imidazoles 68 viavia thethe tandemtandem CCα(sp3)–H bond functionalization of primary amines 67 andand oxidativeoxidative C–N cross-coupling reaction (Scheme 40) [58]. Although the reaction required stoichiometric Ag2CO3, CatalystsC–N cross-coupling2019, 9, 1032 reaction (Scheme 40) [58]. Although the reaction required stoichiometric Ag192CO of 273, the recycled silver reagent provided the product in good yield without any loss of activity.

SchemeScheme 40.40. Ag-mediated C–N coupling forfor thethe synthesissynthesis ofof imidazoles.imidazoles.

AccordingAccording to the proposed proposed mechanism mechanism (Scheme (Scheme 41), 41), the the reaction reaction is is initiated initiated by by the the generation generation of 66-I 66 67 ofimino imino ketone ketone 66-I fromfromfrom diketonediketone diketone 66 andandand primaryprimary primary amineamine amine 67.. Ag(I)-mediatedAg(I)-mediated C–H C–H functionalizationfunctionalizationfunctionalization 66-II 67 thenthen givesgives intermediate intermediate 66-II, and,, andand the subsequentthethe subsequentsubsequent addition additionaddition of another ofof anotheranother equivalent equivalentequivalent of amine ofof amineamineprovides 67 66-III 66-III diamineprovides intermediate diamine intermediate. Finally, 66-III the.. silver-mediatedFinally, the silver-mediated oxidative cyclization oxidative of cyclizationgives imidazole of 66-III 68 productgives imidazole. product 68..

Scheme 41. Proposed mechanism for the Ag-mediated C-N coupling to aﬀord imidazoles. Scheme 41. Proposed mechanism for the Ag-mediated C-N coupling to afford imidazoles. In 2015, the Youn group reported the silver(I)-mediated synthesis of diverse substituted indoles via In 2015, the Youn group reported the silver(I)-mediated synthesis of diverse substituted indoles the C–H amination of 2-alkenylanilines 69 (Scheme 42)[59]. Interestingly, the products are dependent via the C–H amination of 2-alkenylanilines 69 (Scheme(Scheme 42)42) [59].[59]. Interestingly,Interestingly, thethe productsproducts areare on the reaction solvents. In DMF, indoles 70 with R1 at the 2-position are selectively obtained; however, dependent on the reaction solvents. In DMF, indoles 70 withwith RR1 atat thethe 2-position2-position areare selectivelyselectively mixtures of 2- and 3-substituted indoles are observed when heptane is used as the solvent. To elucidate obtained; however, mixtures of 2- and 3-substituted indoles are observed when heptane is used as the reaction mechanism, the reaction was performed in the presence of TEMPO and BHT (butylated the solvent. To elucidate the reaction mechanism, thethe reactionreaction waswas performedperformed inin thethe presencepresence ofof hydroxytoluene). In the absence of radical scavengers, the reaction was completed in 30 min, but it TEMPO and BHT (butylated hydroxytoluene). In the absence of radical scavengers, the reaction was took 3 h for the reaction to reach completion with 1.0 equiv. of TEMPO. With 1.0 equivalent of BHT, completed in 30 min, but it took 3 h for the reaction to reach completion with 1.0 equiv. of TEMPO. 80% of the starting material remained unreacted after 24 h. Based on the results of the reactions with With 1.0 equivalent of BHT, 80% of the starting material remained unreacted after 24 h. Based on the radical scavengers, the reaction should proceed through a radical pathway. According to the proposed results of the reactions with radical scavengers,, thethe reactionreaction shouldshould proceedproceed throughthrough aa radicalradical mechanism, a nitrogen radical cation is generated from substrate 69 with Ag(I) salt by a SET; then, pathway. According to the proposed mechanism, a nitrogen radical cation is generated from the nitrogen radical undergoes an intramolecular electrophilic addition to the alkene to provide a substrate 69 withwith Ag(I)Ag(I) saltsalt byby aa SET;SET; then,then, thethe nitrogennitrogen radicalradical undergoes an intramolecular benzylic radical with the loss of a proton. A benzylic carbocation is generated by a SET from the electrophilic addition to the alkene to provide a benzylic radical with the loss of a proton. A benzylic benzylic radical to Ag(I), and subsequent deprotonation aﬀords desired indole product 70.

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carbocation is generated by a SET from the benzylic radical to Ag(I), and subsequent deprotonation Catalystscarbocation2019, 9is, 1032 generated by a SET from the benzylic radical to Ag(I), and subsequent deprotonation20 of 27 affords desired indole product 70.

Scheme 42. Ag-mediated oxidative C–H amination of 2-alkenylanilines.

The isocyanide group group has has the the similar similar reactivity reactivity as ascarbenes, carbenes, making making them them useful useful reagents reagents for forgenerating generating new newC–C bonds, C–C bonds, and they and are they used are as building used as buildingblocks in organic blocks insynthesis organic [60]. synthesis In addition, [60]. Inisocyanides addition, have isocyanides reactivities have similar reactivities to multicompo similar tonent multicomponent reactions, and reactions, they are often and theyused are for oftenC–H usedfunctionalization for C–H functionalization reactions. In 2015, reactions. the Bi In group 2015, thereported Bi group a silver-catalyzed reported a silver-catalyzed cross coupling cross of β couplingisocyanides of isocyanides72 and active72 and methylene active methylene compounds compounds 73 to access73 to accessvarious various β-aminoenones-aminoenones 74 and74 andtricarbonylmethanes tricarbonylmethanes 75 by75 a byradical a radical process process (Scheme (Scheme 43) [61].43)[ 61].

1 2 Ag CO (30 mol%) EWG1 EWG2 EWG1 EWG2 1 2 Ag2CO3 (30 mol%) EWG EWG path A EWG EWG + EWG1 EWG2 path A R-NC + EWG EWG o 1,4-dioxane, 80 oC, 4-6 h R=Ar 72 73 RN ArHN 74 28 examples 46-99% 2 1 R2 OR1 path B R=t-Bu O HO O R=t-Bu O2 O NHt-Bu 75 8examples 42-98% 42-98% Scheme 43. Ag-catalyzed cross coupling of isocyanides and activated methylene compounds. Scheme 43. Ag-catalyzed cross coupling of isocyanides and activated methylene compounds. To determine the reaction mechanism, both isotope-labeling studies and control experiments To determine the reaction mechanism, both isotope-labeling studies and control experiments with radical scavengers were performed, and the reaction did not proceed in the presence of radical with radical scavengers were performed, and the reaction did not proceed in the presence of radical scavengers. Based on the deuterium-labeling experiments, hydrogen atom transfer occurred from scavengers. Based on the deuterium-labeling experiments, hydrogen atom transfer occurred from the the α-hydrogen of methylene compound 72 to the hydrogen on the oleﬁn of product 74. In the α-hydrogen of methylene compound 72 to the hydrogen on the olefin of product 74. In the 18O- 18α-hydrogen of methylene18 compound 72 to the hydrogen18 on the olefin of product 74. In the18 O- O-labeling experiments,18 O2 provided a high degree18 of O-incorporated product 75, but18 H2 O did labeling experiments, 18O2 provided a high degree of 18O-incorporated product 75, but H218O did not not produce any 18O-incorporated 75. Based on these experimental results, a plausible mechanism produce any 18O-incorporated 75. Based on these experimental results, a plausible mechanism was was proposed as described in Scheme 44. Ag2CO3 abstracts a proton from 73 due to its basicity, proposed as described in Scheme 44. Ag2CO3 abstracts a proton from 73 due to its basicity, and and subsequent oxidation by Ag(I) generates radical intermediate 73-I. Ag2CO3 has a dual role as subsequent oxidation by Ag(I) generates radical intermediate 73-I. Ag2CO3 has a dual role as a base a base and a one-electron oxidant. At the same time, Ag2CO3 coordinates to isocyanide 72 to form and a one-electron oxidant. At the same time, Ag2CO3 coordinates to isocyanide 72 to form 72-I. The 72-I. The coupling reaction between radical 73-I and silver complex 72-I produces imidoyl radical coupling reaction between radical 73-I and silver complex 72-I produces imidoyl radical 72-II. With 72-II. With an aromatic isocyanide, imidoyl radical 72-II can provide imine intermediate 72-IV by an aromatic isocyanide, imidoyl radical 72-II can provide imine intermediate 72-IV by abstracting a abstracting a H atom from the generated AgHCO3 during the one-electron oxidation or by a 1,2-H H atom from the generated AgHCO3 during the one-electron oxidation or by a 1,2-H migration to migration to form stable tricarbonylmethenyl radical 72-III followed by abstraction of an H atom from form stable tricarbonylmethenyl radical 72-III followed by abstraction of an H atom from AgHCO3. AgHCO3. Imine 72-IV gives β-aminoenone 74 by tautomerization. With tert-butyl isocyanide, radical 72-II is converted to hydroperoxide 72-V by oxygenation. Hydroperoxide 72-V is reduced by Ag(s) to form oxyanion intermediate 72-VI, and the protonation gives tricarbonylmethane 75 as the product. CatalystsCatalysts 2019 2019, ,9 9, ,x x FOR FOR PEER PEER REVIEW REVIEW 2121 ofof 2828

ImineImine 72-IV72-IV givesgives ββ-aminoenone-aminoenone 7474 byby tautomerization.tautomerization. WithWith terttert-butyl-butyl isocyanide,isocyanide, radicalradical 72-II72-II isis converted to hydroperoxide 72-V by oxygenation. Hydroperoxide 72-V is reduced by Ag(s) to form Catalystsconverted2019 ,to9, hydroperoxide 1032 72-V by oxygenation. Hydroperoxide 72-V is reduced by Ag(s) to21 form of 27 oxyanionoxyanion intermediate intermediate 72-VI 72-VI, ,and and the the protonation protonation gives gives tricarbonylmethane tricarbonylmethane 75 75 as as the the product. product.

SchemeScheme 44. 44.44. Proposed Proposed mechanism mechanism for for Ag-catalyzed Ag-catalyzed cross-couplings cross-couplings with withwith isocyanide. isocyanide.isocyanide.

sp2 2 ZouZouZou andand co-workersco-workers reported reportedreported a aa silver-catalyzed silver-catalyzedsilver-catalyzed direct directdirect C( C(C(spsp)–H2)–H)–H phosphorylation phosphorylationphosphorylation of indolesofof indolesindoles in the inin thepresence presence of Mg(NO of Mg(NO3)2 leading3)2 leading to phosphoindoles to phosphoindoles (Scheme (Scheme 45)[ 45)62]. [62]. Ag2 COAg23COis used3 is used as an as oxidant an oxidant and the presence of Mg(NO3)2 leading to phosphoindoles (Scheme 45) [62]. Ag2CO3 is used as an oxidant andMg(NO Mg(NO3)2 is3) used2 is used as an as additivean additive to form to form a new a new C–P C–P bond. bond. The The reaction reaction occurs occurs by theby the addition addition of theof and Mg(NO3)2 is used as an additive to form a new C–P bond. The reaction occurs by the addition of sp2 2 theelectrophilicthe electrophilic electrophilic phosphorous phosphorous phosphorous radical radical radical to the to to availablethethe available available C( C( C()–Hspsp2)–H)–H position position position of the of of indole,thethe indole, indole, followed followed followed by a by by SET a a SETbySET Ag(I) by by Ag(I) Ag(I) and and aromatizationand aromatization aromatization by deprotonation by by deproton deprotonationation to give to to give thegive phosphoindolethe the phosphoindole phosphoindole product. product. product.

Scheme 45. Ag-catalyzed direct Csp2–H phosphorylation of 2- and 3-substituted indoles. Catalysts 2019, 9, x FOR PEER REVIEW 22 of 28

Catalysts 2019, 9, 1032 22 of 27 Scheme 45. Ag-catalyzed direct Csp2–H phosphorylation of 2- and 3-substituted indoles.

Recently, thethe Bi Bi group group reported reported the the Ag(I)-catalyzed Ag(I)-catalyzed synthesis synthesis of of 1,2,4,5-tetrasubstituted 1,2,4,5-tetrasubstituted pyrroles pyrroles80 via80 thevia cascadethe cascade reaction reaction of β-enaminones of β-enaminones78 and isocyanoacetates78 and isocyanoacetates79 (Scheme 79 46 (Scheme)[63]. β-Enaminones 46) [63]. β- areEnaminones used as building are used blocks as building in a variety blocks of in reactions a variety and of arereactions easily obtained.and are easily One ofobtained. the key One features of the of βkey-enaminones features of is β their-enaminones tautomeric is their equilibrium tautomeric with equilibriumβ-ketoimines. with As ββ-ketoimines.-ketoimines areAs β a-ketoimines minor product, are iminesa minor are product, present imines in this are equilibrium present in system. this equilibrium system.

Scheme 46. Construction of pyrroles via the cascade reaction ofof ββ-enaminones-enaminones andand isocyanoacetates.isocyanoacetates.

To investigate the the reaction reaction pathway, pathway, a adeuterium-labeling deuterium-labeling experiment experiment was was performed performed using using 2.0 2.0 equiv. of D O. As shown in Scheme 47, deuterium-labeled pyrrole 80-D was obtained, and the equiv. of D2O.2 As shown in Scheme 47, deuterium-labeled pyrrole 80-D was obtained, and the reaction should involve a proton transfer between the imine-metal complex and D O. Based on this reaction should involve a proton transfer between the imine-metal complex and D22O. Based on this experimentalexperimental result,result, thethe reactionreaction maymay bebe initiatedinitiated by the deprotonation of isocyanoacetate 7979 by basic Ag CO to form an α-metalated isocyanoacetate. Then, [3+2] dipolar cycloaddition with β-ketoimine Ag22CO3 to form an α-metalated isocyanoacetate. Then, [3+2] dipolar cycloaddition with β-ketoimine follows,follows, andand thethe ββ-ketoimine-ketoimine waswas derivedderived fromfrom ββ-enaminone-enaminone 7878.. TheThe retro-hetero-Michaelretro-hetero-Michael additionaddition ring-opensring-opens thethe 2-imidazoline2-imidazoline ((78-I78-I)) toto givegive thethe imidamideimidamide intermediate,intermediate, andand thenthen aa cascade involving nucleophilic addition, addition, proton proton transfer, transfer, and and de dehydrationhydration provides provides high highlyly substituted substituted pyrrole pyrrole 80 as80 theas theproduct. product.

Scheme 47. Mechanistic investigation of the Ag-mediated pyrrole synthesis. Adapted with permission Scheme 47. Mechanistic investigation of the Ag-mediated pyrrole synthesis. Adapted with from Org. Lett. 2017, 19, 1346–1349, doi:10.1021/acs.orglett.7b00201. Copyright (2019) American permission from Org. Lett. 2017, 19, 1346–1349, doi:10.1021/acs.orglett.7b00201. Copyright (2019) Chemical Society. American Chemical Society. In 2017, Xu and co-workers developed a silver-catalyzed chemoselective [4+2] annulation of aryl In 2017, Xu and co-workers developed a silver-catalyzed chemoselective [4+2] annulation of aryl and heteroaryl isocyanides 72 with α-substituted isocyanoacetamides 81 to aﬀord pyridone-fused carbo- and heteroaryl isocyanides 72 with α-substituted isocyanoacetamides 81 to afford pyridone-fused and heterocycles 82 (Scheme 48)[64]. To determine the reaction mechanism, the reaction was performed carbo- and heterocycles 82 (Scheme 48) [64]. To determine the reaction mechanism, the reaction was with a 13C labeled isocyanide group in 72, and the NMR spectrum showed the benzo[h]quinolone performed with a 13C labeled isocyanide group in 72, and the NMR spectrum showed the product 82 with 13C at the 2-position. This result indicated that an α-amidoketenimine is a likely benzo[h]quinolone product 82 with 13C at the 2-position. This result indicated that an α- reaction intermediate. amidoketenimine is a likely reaction intermediate.

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Scheme 48. Ag-catalyzed cycloaddition reactions of isocyanoacetamides. Scheme 48. Ag-catalyzed cycloaddition reac reactionstions of isocyanoacetamides.

13 Based on the 13C-labelingC-labeling experiments, experiments, a a mechanism mechanism was was proposed proposed as as described described in in Scheme Scheme 49.49. Based on the 13C-labeling experiments, a mechanism was proposed as described in Scheme 49. Ag2CO3 abstracts a proton from isocyanoactamide 81 and coordinates to the isocyanide moiety. Ag2CO33 abstractsabstracts a a proton proton from from isocyanoactamide isocyanoactamide 8181 andand coordinates coordinates to to the the isocyanide isocyanide moiety. moiety. Ag2CO3 abstracts a proton from isocyanoactamide 81 and coordinates to the isocyanide moiety. Another equivalent of Ag2CO3 coordinates to isocyanide 72, and the subsequent C–C coupling occurs Another equivalent ofof AgAg22CO3 coordinatescoordinates to isocyanide isocyanide 72, and and the the subsequent subsequent C–C C–C coupling coupling occurs Another equivalent of Ag2CO3 coordinates to isocyanide 72, and the subsequent C–C coupling occurs via a nucleophilic attack and elimination to generate amidoketenimine 72-VII. The rearrangement of via a nucleophilic attack and elimination to generate amidoketenimine 72-VII. The rearrangement of α-amidoketenimine 72-VII to α-imidoylketene 72-VIII occurs via a 1,3-amino migration. Subsequent α-amidoketenimine 72-VII to α-imidoylketene 72-VIII occurs via a 1,3-amino migration. Subsequent 6π electrocyclization and 1,3-proton shift convert 72-VIII to quinolone 82. In this reaction, a catalytic 6π electrocyclization andand 1,3-proton1,3-proton shiftshift convertconvert 72-VIII72-VIII toto quinolonequinolone 8282.. In this reaction, a catalytic amount of Li2CO3 was used as an additive to promote the regeneration of the Ag2CO3 catalyst. amount of Li2CO33 waswas used used as as an an additive additive to to promote promote the the regeneration regeneration of the Ag 22COCO33 catalyst.catalyst. amount of Li2CO3 was used as an additive to promote the regeneration of the Ag2CO3 catalyst.

Scheme 49. Proposed mechanism for the cycloadditioncycloaddition of isocyanoacetamide. Scheme 49. Proposed mechanism for the cycloaddition of isocyanoacetamide. In 2017, the Xu group developed a formal [3+2] cycloaddition of α-trifluoromethylated methyl In 2017, the Xu group developed a formal [3+2] cycloaddition of α-trifluoromethylated methyl isocyanidesIn 2017,84 theand Xu aldehydes group developed83 for the a formal silver-catalyzed [3+2] cycloaddition divergent synthesisof α-trifluoromethylated of trifluoromethylated methyl isocyanides 84 and aldehydes 83 for the silver-catalyzed divergent synthesis of trifluoromethylated heterocyclesisocyanides 84 (Scheme and aldehydes 50)[65]. With 83 for imines the silver-catalyzed or acrylonitriles insteaddivergent of aldehyde synthesis83 of, the trifluoromethylated reaction produces heterocycles (Scheme 50) [65]. With imines or acrylonitriles instead of aldehyde 83, the reaction theheterocycles corresponding (Scheme imidazoles 50) [65]. or With pyrrolines imines as or the acrylonitriles major products. instead Ag(I) of coordinates aldehyde to83 isocyanide, the reaction84, produces the corresponding imidazoles or pyrrolines as the major products. Ag(I) coordinates to thenproducesisocyanide DBU the (1,8-diazabicyclo[5.4.0]undec-7-ene) 84 corresponding, then DBU (1,8-diazabicyclo[5.4.0]undec-imidazoles or pyrroli abstractsnes as a protonthe7-ene) major toabstracts formproducts. the a carbanion protonAg(I) coordinates to nucleophile. form the to isocyanide 84, then DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) abstracts a proton to form the Acarbanion formal [3 nucleophile.+2] cycloaddition, A formal which [3+2] consists cycloaddition, of the nucleophilic which consists addition of the of nucleophilic the carbanion addition from the of carbanion nucleophile. A formal [3+2] cycloaddition, which consists of the nucleophilic addition of isocyanidethe carbanion to aldehyde from the 83isocyanideand cyclization, to aldehyde affords 83 an and oxazoline cyclization, silver affords complex; an oxazoline then, protodemetallation silver complex; the carbanion from the isocyanide to aldehyde 83 and cyclization, affords an oxazoline silver complex; givesthen, protodemetallation oxazoline 85. gives oxazoline 85. then, protodemetallation gives oxazoline 85.

Scheme 50. Ag-catalyzed cycloaddition of α-trifluoromethylated-triﬂuoromethylated methyl isocyanides. Scheme 50. Ag-catalyzed cycloaddition of α-trifluoromethylated methyl isocyanides.

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In 2017, Wang Wang and and co-workers co-workers reported reported the the synthesis synthesis of ofpyrroles pyrroles via via a tandem a tandem Ag-catalyzed Ag-catalyzed 1,3- 1,3-dipolardipolar cycloaddition cycloaddition and and BPO BPO (benzoyl (benzoyl pero peroxide)-mediatedxide)-mediated oxidative dehydrogenativedehydrogenative aromatization (Scheme 5151))[ [66].66]. AgAg22CO3 facilitates the [3[3+2]+2] cycloaddition of the 1,3-dipolar1,3-dipolar intermediate generated generated from from compound compound 8686 byby deprotonation deprotonation of the of theα-positionα-position with with alkene alkene 87 to 87formto forma tetrasubstituted a tetrasubstituted pyrrolidine. pyrrolidine. Peroxide Peroxide abstracts abstracts a H a atom H atom from from the the α-positionα-position of of the the pyrrolidine, pyrrolidine, then two oxidations,oxidations, which each consistsconsists ofof aa SETSET andand tautomerization,tautomerization, provideprovide highlyhighly substitutedsubstituted pyrrole 8888..

Scheme 51.51. SynthesisSynthesis of of pyrrole via a Ag-catalyzedAg-catalyzed tandemtandem 1,3-dipolar1,3-dipolar cycloadditioncycloaddition/oxidative/oxidative dehydrogenative aromatization aromatization reac reaction.tion. Adapted Adapted with with permission permission from from J. Org. J. Org. Chem. Chem. 2017, 2017,82, 4194- 82, 4194-4202,4202, doi:10.1021/acs.joc.7b00180. doi:10.1021/acs.joc.7b00180. Copyrigh Copyrightt (2019) (2019) American American Chemical Chemical Society. Society.

5. Conclusions 5. Conclusion

In this review, we summarized the recent studies and developments in Ag2CO3-catalyzed/mediated In this review, we summarized the recent studies and developments in Ag2CO3- organic transformations. Silver carbonates were employed as either external bases for activating acidic catalyzed/mediated organic transformations. Silver carbonates were employed as either external molecules or an external oxidant for turning over catalytic cycles. Notably, heteroaromatic compounds bases for activating acidic molecules or an external oxidant for turning over catalytic cycles. Notably, can be elegantly synthesized using silver carbonate to combine heteroatom-containing fragments heteroaromatic compounds can be elegantly synthesized using silver carbonate to combine and more than two carbon skeletons through alkyne activation. At the same time, silver carboxylate heteroatom-containing fragments and more than two carbon skeletons through alkyne activation. At can be generated from the simple reaction between silver carbonates and various carboxylic acids. the same time, silver carboxylate can be generated from the simple reaction between silver carbonates The following cross-couplings and decarboxylative reactions were investigated. and various carboxylic acids. The following cross-couplings and decarboxylative reactions were Despite the substantial progress and important advances described in these reports, there is investigated. still significant room for improvement at the present level. For example, alkene-related substrates Despite the substantial progress and important advances described in these reports, there is still cannot be efficiently activated by silver carbonate. The utilization of an alkene could expand the significant room for improvement at the present level. For example, alkene-related substrates cannot possiblebe efficiently synthetic activated targets by of silver these silvercarbonate. carbonate-catalyzed The utilization reactions.of an alkene Further could detailed expand studies the possible on the transmetallationsynthetic targets from of these silver silver to other carbonate-cataly transition/mainzed group reactions. metals shouldFurther be de conducted.tailed studies In addition, on the detailedtransmetallation mechanism from studies, silver includingto other transition/mai the structuraln determination group metals ofshould various be intermediatesconducted. In shouldaddition, be performed.detailed mechanism We hope thatstudies, this reviewincluding will the inspire structural the development determination of various of various silver intermediates carbonate-catalyzed should organicbe performed. reactions We and hope other that related this review methodologies. will inspire the development of various silver carbonate- Authorcatalyzed Contributions: organic reactionsK.Y., D.G.J., and andother H.-E.L. related contributed methodologies. equally to this work. K.Y., D.G.J. and M.K. designed and conceptualized the original project. K.Y., D.G.J., and H.J.K. categorized target reactions. H.-E.L. and C.K. built Author Contributions: K.Y., D.J., and H.-E.L. contributed equally to this work. K.Y., D.J. and M.K. designed and the mechanism discussion. All authors wrote the manuscript together. conceptualized the original project. K.Y., D.J., and H.K. categorized target reactions. H.-E.L. and C.K. built the Funding: This project was supported by the Basic Science Research Program (2019R1A2C4070584) and the mechanism discussion. All authors wrote the manuscript together. Science Research Center (2016R1A5A1009405) through the National Research Foundation of Korea (NRF) fundedFunding: by This the Ministryproject was of Science supported and by ICT. the K. Basic Yoo was Science supported Research by NRFProgram Global (2019R1A2C4070584) Ph.D. Fellowship programand the (NRF-2017H1A2A1045495). Science Research Center (2016R1A5A1009405) through the National Research Foundation of Korea (NRF) Acknowledgments:funded by the MinistrySung ofMin Science Kang and isgratefully ICT. K. Yoo acknowledged was supported for discussion.by NRF Global Ph.D. Fellowship program Conflicts(NRF-2017H1A2A1045495). of Interest: The authors declare no conflicts of interest.

Acknowledgments: Prof. Sung Min Kang is gratefully acknowledged for discussion.

Conflicts of Interest: The authors declare no conflicts of interest.

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