Photoorganocatalytic One-Pot Synthesis of Hydroxamic Acids from Aldehydes

Sequential Modifications of Metal-Organic Layer Nodes for Highly Efficient Photocatalyzed Hydrogen Atom Transfer

Herein, we report the synthesis of a bifunctional photocatalyst, Zr-OTf-EY, through sequential modifications of metal cluster nodes in a metal-organic layer (MOL). With eosin Y and strong Lewis acids on the nodes, Zr-OTf-EY catalyzes cross-coupling reactions between various C-H compounds and electron-deficient alkenes or azodicarboxylate to afford C-C and C-N coupling products, with turnover numbers of up to 1980. In Zr-OTf-EY-catalyzed reactions, Lewis acid sites bind the alkenes or azodicarboxylate to increase their local concentrations and electron deficiency for enhanced radical additions, while EY is stabilized by site isolation on the MOL to afford a long-lived catalyst for hydrogen atom transfer. The proximity between photostable EY sites and Lewis acids on the nodes of Zr-OTf-EY enhances the catalytic efficiency by approximately 400 times over the homogeneous counterpart in the cross-coupling reactions.

Light-accelerated "on-water" hydroacylation of dialkyl azodicarboxylates

The hydroacylation of dialkyl azodicarboxylates has received a lot of attention lately due to the great importance of acyl hydrazides in organic chemistry. Herein, we report an inexpensive and green photochemical approach, where light irradiation (390 nm) significantly accelerates the reaction between dialkyl azodicarboxylates and aldehydes, while water is employed as the solvent. A variety of aromatic and aliphatic aldehydes were converted into their corresponding acyl hydrazides in good to excellent yields in really short reaction times (15-210 min) and the reaction mechanism was also studied. Applications of this reaction in the syntheses of Vorinostat and Moclobemide were demonstrated.

Cyrene: a bio-based solvent for the Mizoroki-Heck reaction of aryl iodides

The development of greener and more sustainable methods, as well as the adaptation of already existing protocols to more environmentally friendly procedures, has become crucial for organic synthesis. The introduction and utilization of greener solvents is a very promising alternative, especially when they can replace toxic organic solvents in the known and widely used organic reactions. Cyrene has appeared to be an excellent alternative solvent for a number of organic reactions. In this work, the development of a new, greener and more economical protocol for the Mizoroki-Heck reaction is described, using Cyrene as the green solvent and Pd/C as the palladium catalyst source. A wide substrate scope for the coupling of aryl iodides with acrylamides, acrylates, acrylic acid, acrylonitrile and styrene was demonstrated. The recyclability of Cyrene and the leaching of palladium in the final product were examined in order to enhance the industrial applicability of this protocol. Furthermore, the synthesis of the natural product piperlotine A is reported.

A Unified Mechanism for the PhCOCOOH-mediated Photochemical Reactions: Revisiting its Action and Comparison to Known Photoinitiators

Since 2014, we have introduced in literature the use of phenylglyoxylic acid (PhCOCOOH), a small and commercially available organic molecule, as a potent promoter in a variety of photochemical processes. Although PhCOCOOH has a broad scope of photochemical reactions that can promote, the understanding of its mode of action in our early contributions was moderate. Herein, we are restudying and revisiting the mechanism of action of PhCOCOOH in most of these early contributions, providing a unified mechanism of action. Furthermore, the understanding of its action as a photoinitiator opened a new comparison study with known and commercially available photoinitiators.

Accelerated and Scalable C(sp3)-H Amination via Decatungstate Photocatalysis Using a Flow Photoreactor Equipped with High-Intensity LEDs

Carbon-nitrogen bonds are ubiquitous in biologically active compounds, prompting synthetic chemists to design various methodologies for their preparation. Arguably, the ideal synthetic approach is to be able to directly convert omnipresent C-H bonds in organic molecules, enabling even late-stage functionalization of complex organic scaffolds. While this approach has been thoroughly investigated for C(sp2)-H bonds, only few examples have been reported for the direct amination of aliphatic C(sp3)-H bonds. Herein, we report the use of a newly developed flow photoreactor equipped with high intensity chip-on-board LED technology (144 W optical power) to trigger the regioselective and scalable C(sp3)-H amination via decatungstate photocatalysis. This high-intensity reactor platform enables simultaneously fast results gathering and scalability in a single device, thus bridging the gap between academic discovery (mmol scale) and industrial production (>2 kg/day productivity). The photocatalytic transformation is amenable to the conversion of both activated and nonactivated hydrocarbons, leading to protected hydrazine products by reaction with azodicarboxylates. We further validated the robustness of our manifold by designing telescoped flow approaches for the synthesis of pyrazoles, phthalazinones and free amines.

One-pot synthesis of the new Hydroxamic acid and its complexes with metals

Abstract:

A one-pot conversion of 2-hydroxy-1-naphthoic aldehyde to hydroxamic acid was described. An efficient photoorganocatalytic method of synthesis was developed. The obtained hydroxamic acid was identified by various physicochemical methods such as IR, UV- and NMR-spectroscopy. Solid colored complexes of copper (II) and iron (II), respectively, green and brown colours with the obtained hydroxamic acid were synthesized in ethanol medium for the first time. The molar ratio of ligand and metal in the complex was 2:1. Their structures were established using IR, UV- spectroscopy and thermogravimetric analysis.

Photocatalytic cross-dehydrogenative coupling reaction toward the synthesis of N,N-disubstituted hydrazides and their bromides

An efficient strategy for the divergent synthesis of N,N-disubstituted hydrazides and their bromides is reported through photoredox-catalytic cross-dehydrogenative coupling of N,N-disubstituted hydrazines and aldehydes.

Catalyst-Free Visible-Light-Promoted Cyclization of Aldehydes: Access to 2,5-Disubstituted 1,3,4-Oxadiazole Derivatives

An efficient synthesis of a variety of 2,5-disubstituted 1,3,4-oxadiazole derivatives via a cyclization reaction by photoredox catalysis between aldehydes and hypervalent iodine(III) reagents is described. The reaction proceeds under mild conditions and affords various target compounds in excellent yields. The commercially available aldehydes without preactivation and a simple visible-light-promoted procedure without any catalysts make this strategy an alternative to the conventional methods.

Hydroxamic Acid Derivatives: From Synthetic Strategies to Medicinal Chemistry Applications

Since the approval of three hydroxamic acid-based HDAC inhibitors as anticancer drugs, such functional groups acquired even more notoriety in synthetic medicinal chemistry. The ability of hydroxamic acids (HAs) to chelate metal ions makes this moiety an attractive metal binding group-in particular, Fe(III) and Zn(II)-so that HA derivatives find wide applications as metalloenzymes inhibitors. In this minireview, we will discuss the most relevant features concerning hydroxamic acid derivatives. In a first instance, the physicochemical characteristics of HAs will be summarized; then, an exhaustive description of the most relevant methods for the introduction of such moiety into organic substrates and an overview of their uses in medicinal chemistry will be presented.

Decatungstate-Mediated C(sp3 )-H Heteroarylation via Radical-Polar Crossover in Batch and Flow

Photocatalytic hydrogen atom transfer is a very powerful strategy for the regioselective C(sp3 )-H functionalization of organic molecules. Herein, we report on the unprecedented combination of decatungstate hydrogen atom transfer photocatalysis with the oxidative radical-polar crossover concept to access the direct net-oxidative C(sp3 )-H heteroarylation. The present methodology demonstrates a high functional group tolerance (40 examples) and is scalable when using continuous-flow reactor technology. The developed protocol is also amenable to the late-stage functionalization of biologically relevant molecules such as stanozolol, (-)-ambroxide, podophyllotoxin, and dideoxyribose.

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

Photochemical Activation of Aromatic Aldehydes: Synthesis of Amides, Hydroxamic Acids and Esters

A cheap, facile and metal-free photochemical protocol for the activation of aromatic aldehydes has been developed. Utilizing thioxanthen-9-one as the photocatalyst and cheap household lamps as the light source, a variety of aromatic aldehydes have been activated and subsequently converted in a one-pot reaction into amides, hydroxamic acids and esters in good to high yields. The applicability of this method was highlighted in the synthesis of Moclobemide, a drug against depression and social anxiety. Extended and detailed mechanistic studies have been conducted, in order to determine a plausible mechanism for the reaction.

Generation of Mixed Anhydrides via Oxidative Fragmentation of Tertiary Cyclopropanols with Phenyliodine(III) Dicarboxylates

Oxidative fragmentation of tertiary cyclopropanols with phenyliodine(III) dicarboxylates in aprotic solvents (dichloromethane, chloroform, toluene) produces mixed anhydrides. The fragmentation reaction is especially facile with phenyliodine(III) reagents bearing electron-withdrawing carboxylate ligands (trifluoroacetyl, 2,4,6-trichlorobenzoyl, 3-nitrobenzoyl), and affords 95−98% yields of the corresponding mixed anhydride products. The latter can be straightforwardly applied for the acylation of various nitrogen, oxygen and sulfur-centered nucleophiles (primary and secondary amines, hydroxylamines, primary alcohols, phenols, thiols). Intramolecular acylation yielding macrocyclic lactones can also be performed. The developed transformation has bolstered the synthetic utility of cyclopropanols as pluripotent intermediates in diversity-oriented synthesis of bioactive natural products and their synthetic congeners. For example, it was successfully applied for the last-stage modification of a cyclic peptide to produce a precursor of a known histone deacetylase inhibitor.

Phenylglyoxylic Acid: An Efficient Initiator for the Photochemical Hydrogen Atom Transfer C-H Functionalization of Heterocycles

C-H functionalization at the α-position of heterocycles has become a rapidly growing area of research. Herein, a cheap and efficient photochemical method was developed for the C-H functionalization of heterocycles. Phenylglyoxylic acid (PhCOCOOH) could behave as an alternative to metal-based catalysts and organic dyes and provided a very general and wide array of photochemical C-H alkylation, alkenylation, and alkynylation, as well as C-N bond forming reaction methodologies. This novel, mild, and metal-free protocol was successfully employed in the functionalization of a wide range of C-H bonds, utilizing not only O- or N-heterocycles, but also the less studied S-heterocycles.

Synthesis of Hydroxamic Acid Derivatives Using Blocked (Masked) O-Isocyanate Precursors

Hydroxamic acids are present in a several pharmaceuticals and agrochemicals. Synthetic strategies providing access to hydroxamic acid derivatives remain limited, typically requiring the use of nucleophilic hydroxylamine reagents. Herein, a synthesis of hydroxamates from unactivated carboxylic acids is reported making use of rare blocked (masked) O-substituted isocyanates. The applicability of this transformation was highlighted by targeting the synthesis of vorinostat and belinostat derivatives.

Synthetic Approaches to Acyl Hydrazides and Their Use as Synthons in Organic Synthesis

Acyl hydrazides constitute very important moieties in organic chemistry and have been employed as starting materials for different transformations to many classes of organic compounds. In this review, at first the approaches towards the synthesis of acyl hydrazides are presented. Furthermore, in the second part, the uses of this skeleton as an important intermediate for the synthesis of useful organic compounds are analyzed.

1 Introduction

2 Approaches towards the Synthesis of Acyl Hydrazides

2.1 Synthesis of Acyl Hydrazides from Aldehydes

2.2 Synthesis of Acyl Hydrazides from Carboxylic Acid Derivatives

3 Acyl Hydrazides as Useful Synthons in Synthetic Organic Transfor mations­

3.1 Acyl Hydrazides as Synthetic Precursors for the Synthesis of Car bonyl Compounds

3.2 Acyl Hydrazides as Synthetic Precursors for the Synthesis of Het erocyclic Rings

4 Conclusion

Formation of Synthetically Versatile 2-Aminobenzophenones from Readily Accessed Acyl Hydrazides

Herein, we report the transformation of readily accessed acyl hydrazides into protected 2-aminobenzophenones via a two-step process involving an aryne-based molecular rearrangement followed by a one-pot addition-elimination procedure. The assembly of the scaffold is tolerant of a wide variety of functional groups, and the carbamate group on the product can be facilely removed to afford highly valuable 2-aminobenzophenones. Application of the protocol was demonstrated in the synthesis of neurological medicine phenazepam.

Photochemical Hydroacylation of Michael Acceptors Utilizing an Aldehyde as Photoinitiator

The hydroacylation of Michael acceptors constitutes a useful tool for the formation of new C-C bonds. In this work, an environmentally friendly procedure was developed, utilizing 4cyanobenzaldehyde as the photoinitiator and household bulbs as the irradiation source. A great variety of substrates was well-tolerated, leading to good yields, and mechanistic experiments were performed to elucidate the catalyst's possible mechanistic pathway. Moreover, the inherent selectivity challenge regarding α,α-disubstituted aldehydes (decarbonylation problem) was studied and addressed.

The magnetic graphene oxide/NHC catalyzed aerobic direct amidation and cross-dehydrogenative coupling of aldehydes

The synthesis of a novel recoverable catalyst based on graphene and NHC for the cross dehydrogenative coupling of aldehydes is described, as well as the synthesis of amides and the construction of the C–C bonds.

Methods for Hydroxamic Acid Synthesis

Substituted hydroxamic acid is one of the most extensively studied pharmacophores because of their ability to chelate biologically important metal ions to modulate various enzymes, such as HDACs, urease, metallopeptidase, and carbonic anhydrase. Syntheses and biological studies of various classes of hydroxamic acid derivatives have been reported in numerous research articles in recent years but this is the first review article dedicated to their synthetic methods and their application for the synthesis of these novel molecules. In this review article, commercially available reagents and preparation of hydroxylamine donating reagents have also been described.

Recent advances in metal-free aerobic C–H activation

An overview of recent reactions based on the metal-free, dioxygen-induced, C–H activation of various radical precursors.

A facile route to 1H- and 2H-indazoles from readily accessible acyl hydrazides by exploiting a novel aryne-based molecular rearrangement

Herein we report the transformation of readily synthesised acyl hydrazides into 2-hydrazobenzophenones via a novel molecular rearrangement pathway using aryne chemistry. The developed reaction protocol is performed under relatively mild conditions and is tolerant of a wide variety of functional groups, and the 2-hydrazobenzophenone products provide access to both 1H- and 2H-indazoles from a single intermediate.

Photoorganocatalysis, small organic molecules and light in the service of organic synthesis: the awakening of a sleeping giant

Photocatalysis, the use of light to promote organic transformations, is a field of catalysis that has received limited attention despite existing for over 100 years. With the revolution of photoredox catalysis in 2008, the rebirth or awakening of the field of photoorganocatalysis has brought new ideas and reactions to organic synthesis. This review will focus on the sudden outburst of literature regarding the use of small organic molecules as photocatalysts after 2013. In particular, it will focus on acridinium salts, benzophenones, pyrylium salts, thioxanthone derivatives, phenylglyoxylic acid, BODIPYs, flavin derivatives, and classes of organic molecules as catalysts for the photocatalytic generation of C-C and C-X bonds.

Synthesis of Hydrazines via Radical Generation and Addition of Azocarboxylic tert-Butyl Esters

A new chemistry of azo compounds that is a radical generation and addition in situ of azocarboxylic tert-butyl esters to synthesize hydrazines has been described. The protocol provides a novel strategy for the synthesis of various hydrazines. The advantages of the transformation include broad substrate scope, benign conditions, and convenient operation.

Photochemistry and photophysics of MOFs: steps towards MOF-based sensing enhancements

In combination with porosity and tunability, light harvesting, energy transfer, and photocatalysis, are facets crucial for engineering of MOF-based sensors.

Metal-free photocatalyzed aerobic oxidative Csp3–H functionalization of glycine derivatives: one-step generation of quinoline-fused lactones

This reaction provides facile access to a series of valuable quinoline-fused lactones at room temperature under an air atmosphere.

Combining organocatalysis with photoorganocatalysis: photocatalytic hydroacylation of asymmetric organocatalytic Michael addition products

Organocatalysis and photoorganocatalysis are two areas of synthetic methodology that have been successfully combined.

Photocatalytic Synthesis of γ-Lactones from Alkenes: High-Resolution Mass Spectrometry as a Tool To Study Photoredox Reactions

A mild photocatalytic manifold for the synthesis of γ-lactones has been developed. Utilizing Ru(bpy)₃Cl₂ as the photocatalyst, a cheap and reproducible synthetic protocol for γ-lactones has been introduced. Mechanistic studies revealed the successful monitoring of photocatalytic reactions and radical intermediates via high-resolution mass spectrometry.

Visible-Light-Activated Asymmetric β-C-H Functionalization of Acceptor-Substituted Ketones with 1,2-Dicarbonyl Compounds

We report a visible-light-activated asymmetric β-C(sp3)-H functionalization of 2-acyl imidazoles and 2-acylpyridines with 1,2-dicarbonyl compounds (typically α-ketoesters) catalyzed by a tailored stereogenic-at-rhodium Lewis acid catalyst. The C-C bond formation products are obtained in high yields (up to 99%) and with excellent stereoselectivities (up to >20:1 dr and up to >99% ee). Experimental and computational studies support a mechanism in which a photoactivated Rh-enolate transfers a single electron to the 1,2-dicarbonyl compound followed by proton transfer and a subsequent stereocontrolled radical-radical recombination.

Alkynylation of Csp2 (O)-H Bonds Enabled by Photoredox-Mediated Hydrogen-Atom Transfer

The development of new hydrogen-atom transfer (HAT) strategies within the framework of photoredox catalysis is highly appealing for its power to activate a desired C-H bond in the substrate leading to its selective functionalization. Reported here is the first photoredox-mediated hydrogen-atom transfer method for the efficient synthesis of ynones, ynamides, and ynoates with high regio- and chemoselectivity by direct functionalization of Csp2 (O)-H bonds. The broad synthetic application of this method has been demonstrated by the selective functionalization of C(O)-H bonds within complex molecular scaffolds.

Hydrogen Atom Transfer (HAT): A Versatile Strategy for Substrate Activation in Photocatalyzed Organic Synthesis

The adoption of hydrogen atom transfer (HAT) in a photocatalytic approach, in which an excited catalyst is responsible for substrate activation, offers unique opportunities in organic synthesis, enabling the straightforward activation of R-H (R = C, Si, S) bonds in desired reagents. Either a direct strategy, based on the intrinsic reactivity of a limited number of photocatalysts in the excited state, or an indirect one, in which a photocatalytic cycle is used for the generation of a thermal hydrogen abstractor, can be exploited. This microreview summarizes the most recent advances (mainly from the last two years) in this rapidly developing area of research, collecting the selected examples according to the nature of the species promoting the HAT process. From the synthetic point of view, this area has led to the development of a plethora of strategies for C-C, C-Si, C-N, C-S, and C-halogen (particularly, fluorine) bond formation, as well as for oxidation reactions.