Metal-free decarboxylative annulation of N-arylacrylamides with vinyl acids to synthesize benzo[b]azepin-2-ones

An efficient oxidative decarbonylative (3 + 2)/(5 + 2) annulation of N-arylacrylamides with vinyl acids for the synthesis of diverse seven-membered N-heterocycles under metal-free conditions has been developed.

Selective photoredox decarboxylation of α-ketoacids to allylic ketones and 1,4-dicarbonyl compounds dependent on cobaloxime catalysis

The selective synthesis of allylic ketones and 1,4-dicarbonyl compounds by photoredox/cobaloxime co-catalysis and photoredox catalysis, respectively, is described herein.

Synthesis of substituted 2-alkylquinolines by visible-light photoredox catalysis

A simple and efficient protocol for the synthesis of substituted 2-alkylquinoline scaffolds was developed by employing visible-light photoredox catalysis.

Visible-light-promoted acyl radical cascade reaction for accessing acylated isoquinoline-1,3(2H,4H)-dione derivatives

A visible-light-promoted radical acylation/cyclization cascade reaction of N-methacryloylbenzamides with α-keto acids was developed to construct acylated isoquinoline-dione derivatives.

Visible-light-promoted acridine red catalyzed aerobic oxidative decarboxylative acylation of α-oxo-carboxylic acids with quinoxalin-2(1H)-ones

Visible-light-mediated procedure has been developed for the synthesis of 3-acyl quinoxalin-2(1H)-ones through acridine red catalyzed decarboxylative acylation of α-oxo-carboxylic acids with quinoxalin-2(1H)-ones.

A Phenolic Acid Decarboxylase-Based All-Enzyme Hydrogel for Flow Reactor Technology

Carrier-free enzyme immobilization techniques are an important development in the field of efficient and streamlined continuous synthetic processes using microreactors. Here, the use of monolithic, self-assembling all-enzyme hydrogels is expanded to phenolic acid decarboxylases. This provides access to the continuous flow production of p-hydroxystyrene from p-coumaric acid for more than 10 h with conversions ≥98% and space time yields of 57.7 g·(d·L)-1. Furthermore, modulation of the degree of crosslinking in the hydrogels resulted in a defined variation of the rheological behavior in terms of elasticity and mesh size of the corresponding materials. This work is addressing the demand of sustainable strategies for defunctionalization of renewable feedstocks.

A Hydroperoxide-Mediated Decarboxylation of α-Ketoacids Enables the Chemoselective Acylation of Amines

Strategies for the formation of amide bonds, that is, one of the most basic and important transformations in organic synthesis, have so far focused predominantly on dehydration reactions. Herein, we report and demonstrate the practical utility of a novel decarboxylative amidation of α-ketoacids by using inexpensive tert-butyl hydroperoxide (TBHP), which is characterized by high yields, a broad substrate scope, mild reaction conditions, and a unique chemoselectivity. These features enable the synthesis of peptides from amino acid derived α-ketoacids under preservation of the stereochemical information.

Visible-Light-Mediated Direct Decarboxylative Acylation of Electron-Deficient Heteroarenes Using α-Ketoacids

Acylation of electron-deficient heteroaromatic compounds has been developed using visible light. α-Ketoacids have been used as an efficient source of acyl radicals under photoredox conditions. The in situ generated acyl radicals from α-ketoacids have been coupled to a wide variety of electron-deficient heteroaromatic compounds in a Minisci type reaction. This method would be attractive to access biologically attractive molecules.

Acyl Radical Smiles Rearrangement To Construct Hydroxybenzophenones by Photoredox Catalysis

The first visible-light-induced acyl radical Smiles rearrangement to transform biaryl ethers to hydroxybenzophenones under mild and metal-free conditions is reported. Using the dual catalysis of hypervalent iodine(III) reagents and organophotocatalysts, ketoacids readily generate acyl radicals and undergo 1,5- ipso addition. This method can construct electron-deficient and electron-rich hydroxybenzophenones with excellent chemoselectivity and on gram scale. The performance of the reaction in neutral aqueous conditions holds potential for future biomolecule applications.

Synthesis of indolo[2,1-a]isoquinoline derivatives via visible-light-induced radical cascade cyclization reactions

We describe a photocatalyzed transformation for the synthesis of the indolo[2,1-a]isoquinoline core structure.

Recent developments in decarboxylative cross-coupling reactions between carboxylic acids and N–H compounds

Carboxylic acids and their derivatives are ubiquitous compounds in organic chemistry, and are widely commercially available in a large structural variety. Recently, carboxylic acids have been frequently used as non-toxic and environmentally benign alternatives to traditional organohalide coupling partners in various carbon–carbon and carbon–heteroatom cross-coupling reactions. Along this line, several methods have been reported for the synthesis of nitrogen-containing organic compounds through decarboxylative cross-coupling reactions between carboxylic acids and N–H compounds. This review focuses on recent advances and discoveries on these reactions with special attention on the mechanistic aspects of the reactions.

Carboxylic acids and their derivatives are ubiquitous compounds in organic chemistry, and are widely commercially available in a large structural variety.

Acyl Radical Chemistry via Visible-Light Photoredox Catalysis

Visible light photoredox catalysis has enabled easy access to acyl radicals under mild reaction conditions. Reactive acyl radicals, generated from various acyl precursors such as aldehydes, α-ketoacids, carboxylic acids, anhydrides, acyl thioesters, acyl chlorides, or acyl silanes, can undergo a diverse range of synthetically useful transformations, which were previously difficult or inaccessible. This review summarizes such recent progress of visible-light-driven acyl radical generation using transition metal photoredox catalysts, metallaphotocatalysts, hypervalent iodine catalysts or organic photocatalysts.

Decarboxylative sulfenylation of amino acids via metallaphotoredox catalysis

A nickel/visible light photoredox co-catalyzed decarboxylative thiolation reaction of carboxylic acids has been developed. This odorless sulfenylation reaction proceeded well via a nickel/photoredox cooperative catalysis pathway under very mild conditions.

Visible-light induced decarboxylative C2-alkylation of benzothiazoles with carboxylic acids under metal-free conditions

A photoredox catalyzed C2-alkylation of benzothiazoles with carboxylic acids was developed by using an acridinium salt as a photocatalyst and air as an oxidant.

Organic photoredox catalytic decarboxylative cross-coupling of gem-difluoroalkenes with unactivated carboxylic acids

The first redox neutral decarboxylative alkylation of gem-difluoroalkenes via C–F bond activation enabled by organic photoredox catalysis has been developed.

Dual nickel- and photoredox-catalyzed reductive cross-coupling of aryl vinyl halides and unactivated tertiary alkyl bromides

A novel reductive cross-coupling of aryl vinyl halides and unactivated tertiary alkyl bromides has been realized via photoredox/nickel dual catalysis to produce vinyl arene derivatives bearing all-carbon quaternary centers with excellent E-selectivity. A stoichiometric metal reductant could be avoided by employing commercially available N,N,N',N'-tetramethylethylenediamine (TMEDA) as the terminal reductant.

Generation of Phosphoranyl Radicals via Photoredox Catalysis Enables Voltage-Independent Activation of Strong C-O Bonds

Despite the prevalence of alcohols and carboxylic acids as functional groups in organic molecules and the potential to serve as radical precursors, C-O bonds remain difficult to activate. We report a synthetic strategy for direct access to both alkyl and acyl radicals from these ubiquitous functional groups via photoredox catalysis. This method exploits the unique reactivity of phosphoranyl radicals, generated from a polar/SET crossover between a phosphine radical cation and an oxygen-centered nucleophile. We show the desired reactivity in the reduction of benzylic alcohols to the corresponding benzyl radicals with terminal H atom trapping to afford the deoxygenated products. Using the same method, we demonstrate access to synthetically versatile acyl radicals, which enables the reduction of aromatic and aliphatic carboxylic acids to the corresponding aldehydes with exceptional chemoselectivity. This protocol also transforms carboxylic acids to heterocycles and cyclic ketones via intramolecular acyl radical cyclizations to forge C-O, C-N, and C-C bonds in a single step.

Ultrasound-enhanced Ag-catalyzed decarboxylative coupling between α-keto acids and disulfides for the synthesis of thioesters

Herein, we described the ultrasound-assisted synthesis of thioesters via the Ag-catalyzed radical oxidative decarboxylation of α-keto acids, in the presence of disulfides. This protocol takes advantage of the sonication to prepare the title compounds in moderate to very good yields, in only 20 min of reaction. The positive effect of ultrasonic irradiation is attributed to both, the high mass transfer efficiency and to the induced radical formation in the reaction medium.

Deoxygenative Deuteration of Carboxylic Acids with D2 O

We report a general, practical, and scalable means of preparing deuterated aldehydes from aromatic and aliphatic carboxylic acids with D₂ O as an inexpensive deuterium source. The use of Ph₃ P as an O-atom transfer reagent can facilitate the deoxygenation of aromatic acids, while Ph₂ POEt is a better O-atom transfer reagent for aliphatic acids. The highly precise deoxygenation of complex carboxylic acids makes this protocol promising for late-stage deoxygenative deuteration of natural product derivatives and pharmaceutical compounds.

Oxidative Decarboxylation Enables Chemoselective, Racemization-Free Esterification: Coupling of α-Ketoacids and Alcohols Mediated by Hypervalent Iodine(III)

An α-ketoacid could be converted into a reactive acylating agent by treatment with hypervalent iodine(III) species, and in so doing, we discovered a novel decarboxylative acylation of alcohols that affords a variety of esters in excellent yields. The esterification has been applied to a sterol bearing a free carboxylic acid and shows unique chemoselectivity. The procedure is racemization-free and operates under mild conditions.

Palladium-Catalyzed Serendipitous Synthesis of Arylglyoxylic Amides from Arylglyoxylates and N,N-Dialkylamides in the Presence of Halopyridines

A palladium-catalyzed synthesis of arylglyoxylic amides by the reaction of arylglyoxylates and N,N-dialkylamides in the presence of a 2,3-dihalopyridine has been realized for the first time. An anticipated 2,3-diaroylpyridine did not form in this reaction. The current study unveils an unprecedented role of 2,3-dihalopyridine toward this amidation. Our mechanistic study reveals that the arylglyoxylate could react with halopyridine to form a traceless activated pyridyl ester of arylglyoxylic acid, which upon subsequent reaction with amino surrogate, N,N-dialkylamides could form the arylglyoxylic amides.

A general deoxygenation approach for synthesis of ketones from aromatic carboxylic acids and alkenes

The construction of an aryl ketone structural unit by means of catalytic carbon–carbon coupling reactions represents the state-of-the-art in organic chemistry. Herein we achieved the direct deoxygenative ketone synthesis in aqueous solution from readily available aromatic carboxylic acids and alkenes, affording structurally diverse ketones in moderate to good yields. Visible-light photoredox catalysis enables the direct deoxygenation of acids as acyl sources with triphenylphosphine and represents a distinct perspective on activation. The synthetic robustness is supported by the late-stage modification of several pharmaceutical compounds and complex molecules. This ketone synthetic strategy is further applied to the synthesis of the drug zolpidem in three steps with 50% total yield and a concise construction of cyclophane-braced 18–20 membered macrocycloketones. It represents not only the advancement for the streamlined synthesis of aromatic ketones from feedstock chemicals, but also a photoredox radical activation mode beyond the redox potential of carboxylic acids.

Synthesis of aryl ketones can be generally achieved via C–C coupling strategies. Here, the authors show a deoxygenative coupling of carboxylic acids and alkenes enabled by photoredox catalysis and report a wide scope of products, including bioactive and macrocyclic ketones.

Photocatalytic decarboxylations

During the last years, the field of photocatalytic decarboxylations has emerged rapidly. Carboxylic acids are inexpensive, non-toxic and renewable starting materials for the synthesis of pharmaceuticals or platform chemicals. The traceless extrusion of CO2 gives radical intermediates, which react in diverse cross-coupling reactions. Merging photocatalysis with metal catalysis enables even broader substrate scopes or enantioselective reactions. An overview of photocatalytic decarboxylative reactions of different classes of carboxylic acids is given within this chapter.

Photoredox Catalysis for Building C-C Bonds from C(sp2)-H Bonds

Transition metal-catalyzed C-H bond functionalizations have been the focus of intensive research over the last decades for the formation of C-C bonds from unfunctionalized arenes, heteroarenes, alkenes. These direct transformations provide new approaches in synthesis with high atom- and step-economy compared to the traditional catalytic cross-coupling reactions. However, such methods still suffer from several limitations including functional group tolerance and the lack of regioselectivity. In addition, they often require harsh reaction conditions and some of them need the use of strong oxidant, in a stoichiometric amount, avoiding these processes to be truly eco-friendly. The use of photoredox catalysis has contributed to a significant expansion of the scope of C(sp²)-H bond functionalizations which include the direct arylations, (perfluoro)alkylations, acylations, and even cyanations. Most of these transformations involve the photochemical induced generation of a radical followed by its regioselective addition to arenes, heteroarenes, or alkenes, leading to the building of a new C(sp²)-C bond. The use of photoredox catalysis plays crucial roles in these reactions promoting electron transfer, enabling the generation of radical species and single electron either oxidation or reduction. Such reactions operating at room temperature allow the building of C-C bonds with high chemo-, regio-, or stereoselectivity. This review surveys the formation of C(sp²)-C bonds initiated by photoredox catalysis which involves a C(sp²)-H bond functionalization step, describes the advantages compared to traditional C(sp²)-H bond functionalizations, and presents mechanistic insights into the role played by the photoredox catalysts.