Silver-Catalyzed Decarboxylative Bromination of Aliphatic Carboxylic Acids

Transition metal-free decarboxylative olefination of carboxylic acid salts

The cost-effective and efficient synthesis of alkenes is highly significant due to their extensive applications in both synthetic and polymer industries. A transition metal-free approach has been devised for the chemoselective olefination of carboxylic acid salts. This modular approach provides direct access to valuable electron-deficient styrenes in moderate to good yields. Detailed mechanistic studies suggest anionic decarboxylation is followed by halogen ion transfer. This halogen transfer leads to an umpolung of reactant electronics, allowing for a rate-limiting rebound elimination.

Decarboxylative halogenation of aliphatic carboxylic acids catalyzed by iron salts under visible light

In this article, we report a general protocol for the direct decarboxylative chlorination, iodination, and bromination of aliphatic carboxylic acids catalyzed by iron salts under visible light. This method enjoys a broad substrate scope with good functional group compatibility, including complex natural products. Benzylic and allylic C(sp3)-H bonds can be retained under the oxidative halogenation conditions. This method also shows application potential for late-stage functionalization.

Photoredox-Catalyzed Decarboxylative Bromination, Chlorination and Thiocyanation Using Inorganic Salts

Decarboxylative halogenation reactions of alkyl carboxylic acids are highly valuable reactions for the synthesis of structurally diverse alkyl halides. However, many reported protocols rely on stoichiometric strong oxidants or highly electrophilic halogenating agents. Herein, we describe visible-light photoredox-catalyzed decarboxylative halogenation reactions of N-hydroxyphthalimide-activated carboxylic acids that avoid stoichiometric oxidants and use inexpensive inorganic halide salts as the halogenating agents. Bromination with lithium bromide proceeds under simple, transition-metal-free conditions using an organic photoredox catalyst and no other additives, whereas dual photoredox-copper catalysis is required for chlorination with lithium chloride. The mild conditions display excellent functional-group tolerance, which is demonstrated through the transformation of a diverse range of structurally complex carboxylic acid containing natural products into the corresponding alkyl bromides and chlorides. In addition, we show the generality of the dual photoredox-copper-catalyzed decarboxylative functionalization with inorganic salts by extension to thiocyanation with potassium thiocyanide, which was applied to the synthesis of complex alkyl thiocyanates.

Synthesis of Benzylic Alcohols by Decarboxylative Hydroxylation

Herein, we demonstrate an efficient method for the decarboxylative hydroxylation of carboxylic acids with silver(I) as the catalyst and cerium ammonium nitrate as the oxidant and its utility in chemoselective late-stage functionalization of natural products and drug molecules. The chemoselectivity of this protocol arises from a benzylic nitrate intermediate that retards further oxidation and is hydrolyzed to the final benzylic alcohol product. Mechanistic investigation reveals that the facile oxidation of silver carboxylate affords silver(II) species as an intermediate oxidant responsible for decarboxylation.

Silver-catalyzed direct selanylation of indoles: synthesis and mechanistic insights

Herein we describe the Ag(i)-catalyzed direct selanylation of indoles with diorganoyl diselenides. The reaction gave 3-selanylindoles with high regioselectivity and also allowed direct access to 2-selanylindoles when the C3 position of the indole ring was blocked via a process similar to Plancher rearrangement. Experimental analyses and density functional theory calculations were carried out in order to picture the reaction mechanism. Among the pathways considered (via concerted metalation-deprotonation, Ag(iii), radical, and electrophilic aromatic substitution), our findings support a classic electrophilic aromatic substitution via Lewis adducts between Ag(i) and diorganoyl diselenides. The results also afforded new insights into the interactions between Ag(i) and diorganoyl diselenides.

Epicoccanes A-D, Four Oxidative Dimers of Pyrogallol Analogues from Epicoccum nigrum

Epicoccanes A-D (1-4) are four novel metabolites of an endophytic fungus Epicoccum nigrum. Their distinct unprecedented structures are hypothesized as oxidative dimers of pyrogallol analogues. Compounds 1 and 2 possess a novel spirobicyclo[3.2.1]octane-6,1'-cyclopentane or -cyclohexane core skeleton. Compound 3 is of a unique cage-like pentacyclic system, which unusually contained three continuous spiro-carbons. Compound 4 is a highly rearranged dimer with five contiguous chiral centers. The absolute structures of 1 and 2 were deduced by electronic circular dichroism (ECD) calculations, and those of 3 and 4 were determined by X-ray crystallography. Compounds 1 and 4 showed potential antiliver fibrosis activity.

Copper-Mediated Cyclization of o-Hydroxyaryl Enaminones with 3-Indoleacetic Acids toward the Synthesis of 3-Indolmethyl-Chromones

Here, we describe a copper-mediated tandem decarboxylative coupling/annulation protocol of o-hydroxyaryl enaminones with 3-indoleacetic acids. A series of 3-indolmethyl-chromones were afforded in up to 97% yield. A one-pot method for 3-indolmethyl-chromones from o-hydroxy acetophenones, N, N-dimethylformamide dimethyl acetal, and 3-indoleacetic acids was also developed. Derivatization of the products was conducted to provide various indolmethyl-substituted pyrimidines. Moreover, a biological evaluation revealed that some compounds had anti-influenza viral activities.

Copper-Mediated Decarboxylative Coupling of 3-Indoleacetic Acids with Pyrazolones

A copper-mediated decarboxylative coupling reaction of 3-indoleacetic acids with pyrazolones was described. This protocol realized new functionalization of pyrazolones under simple reaction conditions and exhibited high functional group compatibility and broad substrate scope. Notably, the products displayed antiproliferative activity against cancer cells.

An in situ masking strategy enables radical monodecarboxylative C-C bond coupling of malonic acid derivatives

The utilization of malonic acids in radical decarboxylative functionalization is still underexploited, and the few existing examples are primarily limited to bisdecarboxylative functionalization. While radical monodecarboxylative functionalization is highly desirable, it is challenging because of the difficulty in suppressing the second radical decarboxylation step. Herein, we report the successful radical monodecarboxylative C–C bond coupling of malonic acids with ethynylbenziodoxolone (EBX) reagents enabled by an in situ masking strategy, affording synthetically useful 2(3H)-furanones in satisfactory yields. The keys to the success of this transformation include (1) the dual role of a silver catalyst as a single-electron transfer catalyst to drive the radical decarboxylative alkynylation and as a Lewis acid catalyst to promote the 5-endo-dig cyclization and (2) the dual function of the alkynyl reagent as a radical trapper and as an in situ masking group. Notably, the latent carboxylate group in the furanones could be readily released, which could serve as a versatile synthetic handle for further elaborations. Thus, both carboxylic acid groups in malonic acid derivatives have been well utilized for the rapid construction of molecular complexity.

An in situ masking strategy has been developed based upon the unique properties of silver catalysts to successfully achieve a radical monodecarboxylative C–C bond coupling of malonic acids with ethynylbenziodoxolone reagents.

Beetleane A and Epicoane A: Two Carbon Skeletons Produced by Epicoccum nigrum

Two novel natural products, beetleane A (1) and epicoane A (2), were obtained from the metabolites of an endophytic Epicoccum nigrum. Compound 1 has a unique beetlelike structure that is constructed by the fusion and further fold of an unusual [5.5.5.6]trioxafenestrane with a cycloheptane ring. Compound 2 possesses a compact cagelike structure with a unique 6/5/5/5/6/6/5 heptacyclic ring system. Both 1 and 2 showed strong antiliver fibrosis activity in vitro.

Decarboxylative Halogenation of Organic Compounds

Decarboxylative halogenation, or halodecarboxylation, represents one of the fundamental key methods for the synthesis of ubiquitous organic halides. The method is based on conversion of carboxylic acids to the corresponding organic halides via selective cleavage of a carbon-carbon bond between the skeleton of the molecule and the carboxylic group and the liberation of carbon dioxide. In this review, we discuss and analyze major approaches for the conversion of alkanoic, alkenoic, acetylenic, and (hetero)aromatic acids to the corresponding alkyl, alkenyl, alkynyl, and (hetero)aryl halides. These methods include the preparation of families of valuable organic iodides, bromides, chlorides, and fluorides. The historic and modern methods for halodecarboxylation reactions are broadly discussed, including analysis of their advantages and drawbacks. We critically address the features, reaction selectivity, substrate scopes, and limitations of the approaches. In the available cases, mechanistic details of the reactions are presented, and the generality and uniqueness of the different mechanistic pathways are highlighted. The challenges, opportunities, and future directions in the field of decarboxylative halogenation are provided.

Facile one-pot synthesis of ketones from primary alcohols under mild conditions

One-pot transformation of primary alcohols to ketones was achieved by using dibromoisocyanuric acid and Fe₂O₃. This synthetic method provides a variety of ketones in high yield under mild reaction conditions.

Photocatalytic dual decarboxylative alkenylation mediated by triphenylphosphine and sodium iodide

An efficient photocatalytic dual decarboxylative alkenylation of α,β-unsaturated carboxylic acids and alkyl N-hydroxyphthalimide (NHP) esters mediated by triphenylphosphine and sodium iodide has been developed. This protocol proceeds under 456-nanometer irradiation by visible blue light in the absence of transition metals or organic dye based photoredox catalysts. The reaction is successfully applied to a wide range of redox-active esters derived from aliphatic carboxylic acids (1°, 2° and 3°) and α-amino acids, enabling transformations of diverse α,β-unsaturated carboxylic acids to α,β-alkylated styrenes with high efficiency and excellent selectivity under mild conditions.

Mechanistic Study on AgI-Catalyzed Oxidative Cross-Coupling/Cyclization between Terminal Alkynes and β-Enamino Esters under Base Conditions

A combined computational and experimental study was performed to elucidate the mechanism of the Ag^I-catalyzed oxidative cross-coupling/cyclization of terminal alkynes with β-enamino esters. The results indicated a more favorable Ag^I/Ag⁰-catalyzed radical mechanism (than cationic mechanism) which involves three key stages: (i) the initiation of radical species, (ii) the cyclization, and (iii) the formal 1,2-H shift. Meanwhile, the Ag^I species was found to be the active initiator for the delocalized nitrogen radical species generation, and Ag₂CO₃ acts as an effective oxidant to initiate the β-enamino ester radical formation. Furthermore, it was shown that the silver acetylide is the key intermediate in the title reaction and that the coordination of solvent dimethyl sulfoxide (DMSO) regulates the electronic properties of the Ag center better as compared with base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), thereby enhancing the negative charge of the reaction sites and promoting the cyclization process. Finally, the DBU was revealed to play a key role in the final 1,2-H shift process through the formation of [DBU-H]⁺, acting as a proton shuttle to assist the proton migration process. The theoretical results provide key insights into the Ag^I/Ag⁰-catalyzed radical mechanism and guidelines for further development of Ag-catalyzed synthetic methods.

Rhodium(III)-Catalyzed Decarboxylative Aminomethylation of Glycine Derivatives with Indoles via C-H Activation

A rhodium(III)-catalyzed decarboxylative aminomethylation of glycine derivatives with indoles bearing a pyrimidine directing group was developed via C-H activation, exclusively affording the corresponding aminomethylated products with C3 selectivity. In this synthetic protocol, easily available and resourceful arylglycines were utilized as aminomethyl functional moieties.

Direct Decarboxylative Functionalization of Carboxylic Acids via O-H Hydrogen Atom Transfer

Decarboxylative functionalization via hydrogen atom transfer offers an attractive alternative to standard redox approaches to this important class of transformations. Herein, we report a direct decarboxylative functionalization of aliphatic carboxylic acids using N-xanthylamides. The unique reactivity of amidyl radicals in hydrogen atom transfer enables decarboxylative xanthylation under redox-neutral conditions. This platform provides expedient access to a range of derivatives through subsequent elaboration of the xanthate group.

Silver-Catalyzed Decarboxylative Alkylfluorination of Alkenes

A decarboxylation of alkyl carboxylic acids for alkylfluorination of alkene was developed, with the catalysis of silver(I) and Selectfluor as both the oxidant and fluorine source. This reaction is highly chemoselective, producing the decarboxylative alkylfluorination products rather than the competitive fluorination of aliphatic carboxylic acids. This practical transformation proceeds efficiently in aqueous media at room temperature and exhibits a large range of functional-group tolerance in various primary and secondary aliphatic carboxylates and alkenes.

Metal complex catalysis in the chemistry of lower diamondoids

The review presents the first survey of published data on the use of compounds, complexes and nanoparticles of transition metals (Fe, Co, Ni, Mn, V, Mo, Cu, Pd, Pt, Rh, Ru, Os, Au, Re and Th) in the catalytic transformations of lower diamondoids — adamantane, diamantane and their derivatives. Catalytic halogenation, oxidation, alkylation and cross-coupling reactions are considered, and the formation pathways of C–N, C–S and C–Se bonds in the series of adamantanoids are discussed. Reaction conditions, appropriate catalytic systems and the structures of products are presented.

The bibliography includes 242 references.

Catalytic enantioselective addition of organometallics to unprotected carboxylic acids

Conjugate addition of organometallics to carbonyl based Michael acceptors is a widely used method that allows the building of new carbon-carbon (C-C) bonds and the introduction of chirality in a single step. However, conjugate additions to the simplest Michael acceptors, namely unprotected, unsaturated carboxylic acids, are considered to be prohibited by the fact that acid-base reactions overpower any other type of reactivity, including nucleophilic addition. Here we describe a transient protecting group strategy that allows efficient catalytic asymmetric additions of organomagnesium reagents to unprotected α,β-unsaturated carboxylic acids. This unorthodox pathway is achieved by preventing the formation of unreactive carboxylate salts by means of a reactive intermediate, allowing modifications of the carbon chain to proceed unhindered, while the stereochemistry is controlled with a chiral copper catalyst. A wide variety of β-chiral carboxylic acids, obtained with excellent enantioselectivities and yields, can be further transformed into valuable molecules through for instance catalytic decarboxylative cross-coupling reactions.

Synthesis of Alkyl Halides from Aldehydes via Deformylative Halogenation

An unprecedented deformylative halogenation of aldehydes to alkyl halides is presented. Under oxidative conditions, 1,4-dihydropyridine (DHP), derived from an aldehyde, generated a C(sp3)- radical that coupled with a halogen radical that was generated from inexpensive and atom-economical halogen sources (NaBr, NaI, or HCl), to yield an alkyl halide. Because of the mild conditions, a wide range of functional groups were tolerated, and excellent site selectivity was achieved.

Total Synthesis and Stereochemical Confirmation of Heliolactone

Until now, the relative stereochemistry of the noncanonical strigolactone, heliolactone, has remained ambiguous. The total synthesis of heliolactone is described, with the key bond-forming event being a Stille cross-coupling that relied upon a reversal of the nucleophile-electrophile coupling partners. Spectroscopic analysis of synthetic heliolactone (and other stereoisomers) and comparisons with the isolated material enabled the absolute and relative stereochemistry of heliolactone to be secured.

Silver-catalyzed oxidative 1,2-alkyletherification of unactivated alkenes with α-bromoalkyl carbonyls: facile access to highly substituted 2,3-dihydrofurans

A silver-catalysed C–Br oxidative functionalization/annulative oxygenation process for producing valuable quaternary-carbon-possessing 2,3-dihydrofuran is presented.

Deconstructive diversification of cyclic amines

Deconstructive functionalization involves C–C bond cleavage followed by bond construction on one or more of the constituent carbons. For example, ozonolysis¹ and olefin metathesis^{2, 3} have allowed each carbon in C–C double bonds to be viewed as a functional group. Despite the significant advances in deconstructive functionalizations involving scission of C–C double bonds, there are very few methods that achieve C(sp³)–C(sp³) single bond cleavage/functionalization, especially in relatively unstrained cyclic systems. Here, we report a deconstructive strategy to transform saturated nitrogen heterocycles such as piperidines and pyrrolidines, important moities in bioactive molecules, into halogen-containing acyclic amine derivatives through sequential C(sp³)–N/C(sp³)–C(sp³) single bond cleavage followed by C(sp³)–halogen bond formation. The resulting acyclic haloamines serve as versatile intermediates that are transformed into a variety of structural motifs through substitution reactions. In this way, skeletal remodeling of cyclic amines, which constitutes a scaffold hop, can be achieved. The value of this deconstructive strategy has been demonstrated through the late-stage diversification of proline-containing peptides.

Mechanistic Insight into Asymmetric Hetero-Michael Addition of α,β-Unsaturated Carboxylic Acids Catalyzed by Multifunctional Thioureas

Carboxylic acids and their corresponding carboxylate anions are generally utilized as Brønsted acids/bases and oxygen nucleophiles in organic synthesis. However, a few asymmetric reactions have used carboxylic acids as electrophiles. Although chiral thioureas bearing both arylboronic acid and tertiary amine were found to promote the aza-Michael addition of BnONH₂ to α,β-unsaturated carboxylic acids with moderate to good enantioselectivities, the reaction mechanism remains to be clarified. Detailed investigation of the reaction using spectroscopic analysis and kinetic studies identified tetrahedral borate complexes, comprising two carboxylate anions, as reaction intermediates. We realized a dramatic improvement in product enantioselectivity with the addition of 1 equiv of benzoic acid. In this aza-Michael reaction, the boronic acid not only activates the carboxylate ligand as a Lewis acid, together with the thiourea NH-protons, but also functions as a Brønsted base through a benzoyloxy anion to activate the nucleophile. Moreover, molecular sieves were found to play an important role in generating the ternary borate complexes, which were crucial for obtaining high enantioselectivity as demonstrated by DFT calculations. We also designed a new thiourea catalyst for the intramolecular oxa-Michael addition to suppress another catalytic pathway via a binary borate complex using steric hindrance between the catalyst and substrate. Finally, to demonstrate the synthetic versatility of both hetero-Michael additions, we used them to accomplish the asymmetric synthesis of key intermediates in pharmaceutically important molecules, including sitagliptin and α-tocopherol.

W(OTf)6 -Catalyzed Synthesis of γ-Lactones by Ring Contraction of Macrolides or Ring Closing of Terminal Hydroxyfatty Acids in Ionic Liquid

γ-Lactones are an important class of fine chemical products and are widely used in perfumes, medicines, pesticides, dyes, and other fields. Herein, a new method for γ-lactones preparation based on ring contraction was developed. Starting from macrolides, W(OTf)₆ was used to catalyze the ring-opening polymerization then depolymerization. The depolymerization step was not a common ring-closing process, and the carbon number of the ring was reduced one by one by rearrangement to form the most thermodynamically stable five-membered ring compounds. γ-Caprolactone (180 °C for 10 h) was obtained in a yield of 94 % when [EMIM]OTf was used as the solvent, and the yield of isolated product was up to 85 %. The interaction of various components and the reaction mechanism were studied by FTIR spectroscopy and ¹ H NMR spectroscopy, respectively. Furthermore, γ-lactones could be produced when the substrate was extended to terminal hydroxyfatty acids. Unexpectedly, the catalyst was poisoned by 1 equivalent of H₂ O added during the process and thus the yield decreased greatly. The reaction is green and simple, and proceeds in one pot with high atom economy (100 % for macrolides and with water as the only byproduct for terminal hydroxyfatty acid), which provides a promising approach to synthesizing γ-lactones.

Transition-metal-free decarboxylative bromination of aromatic carboxylic acids

Aromatic acids are converted into aryl bromides simply and efficiently via decarboxylation providing new depth and insight into Hunsdiecker-type reactivity.

Methods for the conversion of aliphatic acids to alkyl halides have progressed significantly over the past century, however, the analogous decarboxylative bromination of aromatic acids has remained a longstanding challenge. The development of efficient methods for the synthesis of aryl bromides is of great importance as they are versatile reagents in synthesis and are present in many functional molecules. Herein we report a transition metal-free decarboxylative bromination of aromatic acids. The reaction is applicable to many electron-rich aromatic and heteroaromatic acids which have previously proved poor substrates for Hunsdiecker-type reactions. In addition, our preliminary mechanistic study suggests that radical intermediates are not involved in this reaction, which is in contrast to classical Hunsdiecker-type reactivity. Overall, the process demonstrates a useful method for producing valuable reagents from inexpensive and abundant starting materials.

Silver-Catalyzed Decarboxylative Trifluoromethylation of Aliphatic Carboxylic Acids

The silver-catalyzed decarboxylative trifluoromethylation of aliphatic carboxylic acids is described. With AgNO₃ as the catalyst and K₂S₂O₈ as the oxidant, the reactions of aliphatic carboxylic acids with (bpy)Cu(CF₃)₃ (bpy = 2,2'-bipyridine) and ZnMe₂ in aqueous acetonitrile at 40 °C afford the corresponding decarboxylative trifluoromethylation products in good yield. The protocol is applicable to various primary and secondary alkyl carboxylic acids and exhibits wide functional group compatibility. Mechanistic studies reveal the intermediacy of ^-Cu(CF₃)₃Me, which undergoes reductive elimination and subsequent oxidation to give Cu(CF₃)₂ as the active species responsible for the trifluoromethylation of alkyl radicals.

Diastereoselective building up polycyclic tetrahydrofurans via tandem annulation of 1,n-enynes with aliphatic acids

A silver-catalyzed tandem annulation of aniline-linked 1,7-enynes with aliphatic acids has been reported. These synergistic tandem annulations allowed for the straightforward and efficient synthesis of various highly diastereoselective polycyclic tetrahydrofurans.