Radical Decarboxylation Research Articles

Visible Light‐Mediated Radical Decarboxylative Strategies for the Synthesis of Heterocyclic Compounds

AbstractHeterocycles are an important structural motif found in many bioactive compounds, natural products and pharmaceuticals. Heterocycles have been synthesised at large employing conventional methods which are usually associated with limitations such as the need for an external oxidant or reductant, harsh reaction conditions, lower selectivity and its unsustainable nature. To circumvent these drawbacks, visible light photocatalysis has emerged as a milder, energetically efficient and environmentally benign tool in organic synthesis. Visible light‐mediated radical decarboxylation of carboxylic acid and its derivatives has been widely explored to affect a wide range of chemical transformation reactions. Redox‐active NHPI derivatives are easily prepared from carboxylic acids which are inexpensive, non‐toxic and stable finding wide applications across various fields. Further, carboxylic acids are one of the most plentiful and renewable feedstock of our planet. Hence, this review brings forth the current developments in the area of visible light‐mediated decarboxylative strategies employing carboxylic acid and its derivatives for the synthesis of heterocyclic compounds.

Ring Expansion from 16‐Membered Macrocyclic Lactones Using Yamaguchi and Photoinduced Decarboxylative Radical Macrolactonization

This study describes a new strategy for synthesizing 31‐, 33‐, 46‐, and 48‐membered macrocyclic lactones from commercially available 16‐membered macrocyclic lactones using the Yamaguchi reaction and photoinduced decarboxylative radical macrolactonization. Despite challenges with the low solubility of substrates and the slow rate of photochemical radical cyclization, the preparation of enlarged macrocyclic lactones from 16‐membered macrocyclic lactones was successfully accomplished. Additionally, the effect of the Lewis acidity of the counter cation in the carboxylate salt provided mechanistic insights into the photoinduced decarboxylative radical macrolactonization.

Synthesis of Alkyl Bis(trifluoromethyl)carbinols via Fe-LMCT-Enabled Hydrobis(trifluoromethyl)carbinolation of Alkenes.

Bis(trifluoromethyl)carbinols are valuable pharmacophores, yet their synthesis is challenging, largely due to a scarcity of safe and effective bis(trifluoromethyl)carbinolation reagents. Here, we realized the hydrobis(trifluoromethyl)carbinolation of alkenes, utilizing stable and readily accessible 2,2-bis(trifluoromethyl)glycolic acid as a source of both the bis(trifluoromethyl)carbinol unit and a hydrogen atom. This process leverages a photoinduced Fe-LMCT-enabled radical decarboxylation process that generates a key bis(trifluoromethyl)carbinol radical intermediate. Our mild protocol facilitates the synthesis of a diverse range of alkyl bis(trifluoromethyl)carbinols, including structurally complex molecules with pharmaceutical relevance.

Enhanced Yield of Methyl Ethyl Ketone through Levulinic Acid Decarboxylation in the AgNO3/K2S2O8 System: Mechanistic Insights and Characterization of Metallic Species.

Methyl ethyl ketone (MEK) is among the most extensively utilized solvents in various industrial applications. In this study, we present a highly efficient synthesis route for MEK via the decarboxylation of biomass-derived levulinic acid, using potassium persulfate (K2S2O8) and silver nitrate (AgNO3) as key reagents. The specific roles of AgNO3 and K2S2O8 were thoroughly investigated. Additional silver species, such as Ag2O and AgO, were also detected during the reaction. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses provided evidence of the evolution of solid phases throughout the reaction. Based on these findings, we propose a radical decarboxylation mechanism initiated by the generation of sulfate radicals (SO4•⁻) through the catalytic breakdown of K2S2O8 by AgNO3. This mechanistic understanding, combined with a parametric study, enabled us to achieve an unprecedented level of levulinic acid conversion (97.9%) and MEK yield (86.6%) with this system, surpassing all previously reported results in the literature.

Photochemical Decarboxylative Radical Alkylation/Cyclization Reaction to Fused Nitrogen Heterocycles by LiI/PPh3 Catalysis

Comprehensive SummaryA visible‐light‐induced decarboxylative radical cascade cyclization reaction between N‐(2‐cyanoaryl)‐acrylamides and alkyl N‐(acyloxy)phthalimide (NHPI esters) for the construction of phenanthridine derivatives has been developed. This approach utilizes lithium iodide (LiI) and triphenylphosphine (PPh3) as the redox catalysts and the alkyl radical is produced through the photoactivation of the electron donor‐acceptor (EDA) complex. A series of primary, secondary, and tertiary alkyl‐substituted phenanthridines are prepared in up to 82% yield without transition‐metal catalysts, chemical oxidants, or metal‐/organic dye‐based photocatalysts.

Copper-Catalyzed Intramolecular Decarboxylative C(sp2)-Heteroatom Cross-Couplings: Mechanism Insights and Synthetic Applications.

Decarboxylative C(sp2)-heteroatom cross-coupling reactions hold extraordinary potential for the sustainable preparation of biologically active scaffolds. Herein, we report a copper sulfate/1,10-phenathroline catalytic system for the decarboxylative intramolecular C(sp2)-O, C(sp2)-S, and C(sp2)-N coupling reactions leading to the construction of a series of benzo[b]furans, benzo[b]thiophenes, and indole derivatives from the corresponding coumarins, thiocoumarins, or quinolones, respectively. Our mechanistic study based on benzo[b]furan formation suggests a three-step process of the transformations, which consists of (i) base-mediated hydrolytic ring opening of coumarin, (ii) copper-oxygen co-initiated radical decarboxylation, and (iii) copper-catalyzed C-heteroatom cross coupling. Application of this method in the total synthesis of egonol, a bioactive natural product, was demonstrated successfully, with an overall yield of 51.7%.

Synthesis of Modified Nucleosides Using 4'-Carbon Radicals

Nucleosides with a substituent at the 4'-position have received much attention as antiviral drugs and as raw materials for oligonucleotide therapeutics. 4'-Modified nucleosides are generally synthesized using ionic reactions through the introduction of electrophilic or nucleophilic substituents at the 4'-position. However, their synthetic methods have some drawbacks; e.g., (i) it is difficult to control stereoselectivity at the 4'-position; (ii) complex protection-deprotection processes are required; (iii) the range of electrophiles and nucleophiles is limited. With this background, we considered that a carbon radical generated at the 4'-position would be a useful intermediate for the synthesis of 4'-modified nucleosides. In this review, two novel methods for the generation of 4'-carbon radicals are summarized. The first utilizes radical deformylation involving β-fragmentation of a hydroxymethyl group at the 4'-position. The other utilizes radical decarboxylation and 1,5-hydrogen atom transfer (1,5-HAT), which enables the generation of 4'-carbon radicals while retaining the hydroxymethyl group at the 4'-position. These methods enable the rapid and facile generation of 4'-carbon radicals and provide various 4'-modified nucleosides including 2',4'-bridged structures.

Silver‐Catalyzed Decarboxylative Radical Cyclizations: Developments and Insights

AbstractTransition‐metal‐catalyzed decarboxylative cross‐coupling reaction provides a powerful method for the construction C−C, C−N, C−O bonds, which have great impact on synthetic chemistry. The use of silver salts as catalyst in decarboxylative reaction gradually increases because of their easy availability, low toxicity, biocompatibility and adequate stability. In fact, various radical cyclization reactions have been accomplished by exploiting single electron transfer (SET) ability of silver salts. The present review highlights recent advances (2015‐mid 2023) in the silver‐catalyzed decarboxylative radical cyclizations for the construction of cyclic molecular scaffolds. The content of the review is organized on the basis of decarboxylative reagents, such as, α‐keto acids, aliphatic/aromatic carboxylic acids, α,α‐difluoroacetic acids, amino acids, alkynyl carboxylic acids, oxamic acids and miscellaneous acids. Additionally, mechanistic insights of most of the reactions are highlighted for well understanding of the reaction mode.

Asymmetric carbohydroxylation of alkenes via sequential photocatalytic oxo-alkylation and enzymatic reduction

Asymmetric carbohydroxylation of alkenes offers an efficient approach to access chiral alcohols, a class of versatile building blocks in pharmaceutical and agrochemical industries, from abundant simple alkenes. Herein we reported a sequential photo-biocatalysis protocol for the asymmetric carbohydroxylation of alkenes, involving a stepwise photocatalytic decarboxylative radical addition/Kornblum oxidation and enzymatic reduction. A series of chiral alcohols with bulky structures were synthesized in up to 75 % isolated yields and 99 % ee from N-hydroxyphthalimide esters and aryl alkenes.

Visible-light-induced decarboxylative radical polymerization in two-molecule photoredox system

Radical polymerization via the photoinduced decarboxylation of carboxylic acids was achieved in a two-molecule photoredox system using visible light. In this photoreaction, unique polymers with chain ends derived from amino acids, peptides, sugars, and steroids were obtained under mild conditions. In addition, a two-molecule photoredox system for replacing both electron donor and acceptor, led to selective decarboxylation, facilitating the preparation of block copolymers with different segments.

Electrochemical ruthenium‐catalyzed C8‐regioselective aroylation of quinoline N‐oxides with aryl α‐ketoacids

An electrochemical method with high faradaic efficiency was developed for the ruthenium‐catalyzed C8‐regioselective aroylation of quinoline N‐oxides under oxidant‐free conditions by using aryl α‐ketoacids as the aroylation reagent. A series of 8‐aroylquinoline N‐oxides were effectively synthesized in moderate‐to‐excellent yields. Furthermore, the proposed method was successfully applied to the gram‐scale synthesis of a tubulin polymerization inhibitor. Additionally, a plausible reaction mechanism for the decarboxylation radical acylation was proposed on the basis of the results of cyclic voltammetry and control experiments. Overall, this method provides a novel solution for the C8‐regioselective aroylation of quinoline N‐oxides.

Radical Decarboxylation-Initiated SH2' Reaction of β,β-Difluoroenol Sulfonates: Access to α,α-Difluoroketones.

Reported herein is a novel radical decarboxylation-initiated SH2' reaction of β,β-difluoroenol sulfonates. This transformation is characterized by mild reaction conditions, a broad substrate scope, and late-stage modification of drug molecules, providing general and mechanistically distinct access to bioactive and synthetically versatile α,α-difluoroketones. Preliminary mechanistic studies demonstrate that this reaction proceeds through a succession of silver-mediated decarboxylative radical generation and radical-addition-induced β-elimination of the sulfonyl radical.

Access to Reverse Glycosyl Azides and Rare Sugar-Based Glycosyl Azides via Radical Decarboxylative Azidation: Divergent Synthesis of 4'-C-Azidonucleosides as Potential Antiviral Agents.

The radical decarboxylative azidation of structurally diverse uronic acids has been established as an efficient approach to reverse glycosyl azides and rare sugar-derived glycosyl azides under the action of Ag2CO3, 3-pyridinesulfonyl azide, and K2S2O8. The power of this method has been highlighted by the divergent synthesis of 4'-C-azidonucleosides using Vorbrüggen glycosylation of nucleobases with 4-C-azidofuranosyl acetates. The antiviral assessment of the resulting nucleosides revealed one compound as a potential inhibitor of covalently closed circular DNA.

Stereodivergent photobiocatalytic radical cyclization through the repurposing and directed evolution of fatty acid photodecarboxylases.

Despite their intriguing photophysical and photochemical activities, naturally occurring photoenzymes have not yet been repurposed for new-to-nature activities. Here we engineered fatty acid photodecarboxylases to catalyse unnatural photoredox radical C-C bond formation by leveraging the strongly oxidizing excited-state flavoquinone cofactor. Through genome mining, rational engineering and directed evolution, we developed a panel of radical photocyclases to facilitate decarboxylative radical cyclization with excellent chemo-, enantio- and diastereoselectivities. Our high-throughput experimental workflow allowed for the directed evolution of fatty acid photodecarboxylases. An orthogonal set of radical photocyclases was engineered to access all four possible stereoisomers of the stereochemical dyad, affording fully diastereo- and enantiodivergent biotransformations in asymmetric radical biocatalysis. Molecular dynamics simulations show that our evolved radical photocyclases allow near-attack conformations to be easily accessed, enabling chemoselective radical cyclization. The development of stereoselective radical photocyclases provides unnatural C-C-bond-forming activities in natural photoenzyme families, which can be used to tame the stereochemistry of free-radical-mediated reactions.

General Approach to Amides through Decarboxylative Radical Cross-Coupling of Carboxylic Acids and Isocyanides.

Herein, we report a silver-catalyzed protocol for decarboxylative cross-coupling between carboxylic acids and isocyanides, leading to linear amide products through a free-radical mechanism. The disclosed approach provides a general entry to a variety of decorated amides, accommodating a diverse array of radical precursors, including aryl, heteroaryl, alkynyl, alkenyl, and alkyl carboxylic acids. Notably, the protocol proved to be efficient for decarboxylative late-stage functionalization of several elaborate pharmaceuticals, demonstrating its potential applications.

Dual Acridine/Decatungstate Photocatalysis for the Decarboxylative Radical Addition of Carboxylic Acids to Azomethines.

A concept for the dual use of acridine and tetrabutylammonium decatungstate photocatalysts in the reactions of carboxylic acids is proposed. Imines generated in situ from aldehydes and p-methoxyaniline, as well as other azomethines, were used as radical acceptors. The role of the decatungstate is believed to facilitate the turnover of the acridine photocatalyst by means of hydrogen atom transfer.

Synthesis of Quinazolinones and Benzothiazoles Using α-Keto Acids under Ball Milling.

Mechanochemistry refers to the initiation of chemical reactions via mechanical forces such as milling, grinding, or shearing to achieve the chemical transformations. As a manifestation of mechanocatalysis, herein, an oxidant-free and solvent-free approach for the synthesis of quinazolinones (23 derivatives) and benzothiazoles (23 derivatives) has been developed through stainless-steel-driven decarboxylative acyl radical generation from α-keto acids. A library of 2-arylquinazolinones and 2-arylbenzothiazoles has been prepared in moderate to good yields at room temperature. Moreover, control experiments and XPS studies supported the reduction (by zerovalent iron) of molecular oxygen through the moderate abrasion of balls, which promoted the generation of a superoxide radical anion via a SET process.

Photocatalytic Radical Decarboxylation [4 + 3] Annulation Reactions of Lactones via Dienoic Acid EDA Complexes

Photocatalytic Radical Decarboxylation [4 + 3] Annulation Reactions of Lactones via Dienoic Acid EDA Complexes

Radical-Mediated Decarboxylative C-C and C-S Couplings of Carboxylic Acids via Iron Photocatalysis.

Despite the significant success of decarboxylative radical reactions, the catalytic systems vary considerably upon different radical acceptors, requiring renewed case-by-case reaction optimization. Herein, we developed an iron catalytic condition that enables the highly efficient decarboxylation of various carboxylic acids for a range of radical transformations. This operationally simple protocol was amenable to a wide array of radical acceptors, delivering structurally diverse oxime ethers, alkenylation, alkynylation, thiolation, and amidation products in useful to excellent yields (>40 examples, up to 95% yield).

Photoinduced Decarboxylative Radical Phosphinylation.

Reported herein is an unprecedented protocol for C(sp3 )-phosphinylation. With 1 mol % 4CzIPN (1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene) as the catalyst, the visible light induced reaction of redox-active esters of aliphatic carboxylic acids with dimethyl arylphosphonites or diethyl alkylphosphonites at room temperature provides the corresponding decarboxylative phosphinylation products in satisfactory yields. The protocol exhibits broad substrate scope and wide functional-group compatibility, enabling the late-stage modification of complex molecules and rapid synthesis of bioactive phosphinic acids such as glutamine synthetase phosphinothricin and a kynureninase inhibitor. A radical-polar crossover mechanism involving the formation and subsequent oxidation of phosphoranyl radicals followed by nucleophilic demethylation (or deethylation) is proposed.