Hydroacylation of α,β-unsaturated esters via aerobic C–H activation

[3 + 2] Annulation of Vinyl Azides with Aldehydes for the Synthesis of 3-Oxazolines via the [CO + CCN] Strategy

Despite the widespread utilizable value of 3-oxazolines, mild and efficient access to such a class of unique structures still remains, to date, a challenge. Herein, we present a [3 + 2] annulation strategy, guided by the retrosynthetic principle of [CO + CCN], that utilizes vinyl azides as the CCN module and aldehydes as the CO module. This approach enables the efficient construction of the 3-oxazoline framework with remarkable features, including operational simplicity, environmental friendliness, and high efficiency. Notably, it solely requires the addition of inexpensive and readily available N-hydroxyphthalimide (NHPI) and air oxygen to obtain the desired product. It also provides a new way to generate the hydroxyl radical, which is produced by the homolysis of peroxycarboxylic acid. In addition, control experiments, X-ray crystallographic analysis, high-resolution mass spectrometry (HRMS), and density functional theory (DFT) calculations afford evidence for the key intermediates (hydroxyl radical, carboxyl radical, imine radical, hydroxyl substituted amide derivatives), further confirming the path for realization of 3-oxazolines.

Silver-Catalyzed Radical Umpolung Cross-Coupling of Silyl Enol Ethers with Activated Methylene Compounds: Access to Diverse Tricarbonyl Derivatives

A silver-catalyzed protocol for the intermolecular radical umpolung cross-coupling protocol of silyl enol ethers with activated methylene compounds is disclosed. The protocol exhibits excellent functional group tolerance, enabling the expedient preparation of a variety of tricarbonyl compounds. Preliminary mechanistic investigations suggest that the reaction proceeds through a process involving free radicals in which silver oxide has a dual role, acting as both a catalyst and a base.

Cobalt(II)-Catalyzed Synthesis of γ-Diketones from Aryl Alkenes and Its Utilization in the Synthesis of Various Heterocyclic Compounds

An earth-abundant Co(II) salt-catalyzed mild and affordable synthetic route has been developed for the synthesis of industrially relevant 1,4-dicarbonyl compounds (or γ-diketones) via oxidative coupling between aryl alkenes and ketones (both cyclic and acyclic) using TBHP and DBU as the oxidant and base, respectively. 1,4-Dicarbonyl compounds are known to be synthesized using expensive metal catalysts, dual catalysts, or low-cost metal complexes combined with an additive or ligand template, which further needs to be synthesized. Herein, we report the synthesis of 1,4-dicarbonyl compounds using cobalt(II) acetate as a catalyst without any expensive co-catalyst or ligand templates. This methodology has a broad substrate scope with significant yields and good functional group tolerance. Generation of unsymmetrical 1,4-dicarbonyls at room temperature and its versatile synthetic expansion to produce synthetically and biologically valuable heterocyclic compounds are salient features of this novel methodology. In addition, various controlled experiments such as primary kinetic isotope effect study, Hammett analysis with variation of the nature of the substituents on the styrene ring, and theoretical calculations (density functional theory) unravel the mechanistic intricacies involved in this new, simple, and atom-economic methodology.

Amide-Directed, Rhodium-Catalyzed Regio- and Enantioselective Hydroacylation of Internal Alkenes with Unfunctionalized Aldehydes

Despite the current progress achieved in asymmetric hydroacylations, highly enantioselective catalytic addition of unfunctionalized aldehydes to internal alkenes remains an unsolved challenge. Here, using a coordination-assisted strategy, we developed a rhodium-catalyzed regio- and enantioselective addition of unfunctionalized aldehydes to internal alkenes such as enamides and β,γ-unsaturated amides. Valuable α-amino ketones and 1,4-dicarbonyl compounds were directly obtained with high enantioselectivity from readily available materials.

Direct organocatalytic transfer hydrogenation and C-H oxidation: high-yielding synthesis of 3-hydroxy-3-alkyloxindoles

Readily available 3-alkylideneoxindoles were effectively reduced to 3-alkyloxindoles through transfer hydrogenation using Hantzsch ester as a reducing agent at ambient temperature and the greenness/sustainability of this protocol was assessed by correlation with Pd/C-mediated hydrogenation with hydrogen gas. Furthermore, an organocatalytic method was developed to access drug-like 3-alkyl-3-hydroxyoxindoles by C-H oxidation of 3-alkyl-indolin-2-one, using a catalytic amount of 1,1,3,3-tetramethylguanidine (TMG) as an organic base and dissolved oxygen in THF as an oxidant at room temperature. Key reaction intermediates were observed by controlled on-line ESI-HRMS experiments and identified by their corresponding mass (m/z) analysis. This two-step high-yielding transfer hydrogenation/C-H oxidation protocol was used for the total synthesis of medicinally important 3-cyanomethyl-3-hydroxyoxindole and formal total synthesis of (±)-alline and (±)-CPC-I in very good overall yields compared to previous methods.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

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.

Formyl Radical Generation from α-Chloro N-Methoxyphthalimides Enables Selective Aldehyde Synthesis

The aldehydes installation by radical formylation constitutes an attractive synthetic strategy. However, the generation of formyl radicals for organic synthesis applications remains unknown. Herein we report the first formyl radical generation from α-chloro N-methoxyphthalimides, which selectively synthesize aldehydes by alkene hydroformylation under mild photoredox conditions. The aldehydes can be installed on acrylates, acrylamides, vinyl sulfones, vinyl ketones, and complex steroids by radical hydroformylation in excellent chemoselectivity and regioselectivity. The concerted hydrochloride elimination for the formyl radical generation from α-chloro methoxy radicals is established by experimental and computational approaches.

Photoredox-Catalyzed Acyl Lactonization of Alkenes with Aldehydes: Synthesis of Acyl Lactones

An acyl lactonization of alkenes with aldehydes under visible-light photoredox catalysis is described. With the protocol, a broad scope of alkenoic acids and aldehydes could be compatible and good functional group tolerance is obtained. A series of acyl lactones are obtained with isolated yields ranging from 50-95%. Mechanistic studies revealed that the transformation should proceed via a radical chain process.

Silver-Catalyzed Controlled Intermolecular Cross-Coupling of Silyl Enol Ethers: Scalable Access to 1,4-Diketones

A protocol for silver-catalyzed controlled intermolecular cross-coupling of silyl enolates is disclosed. The protocol displays good functional group tolerance and allows efficient preparation of a series of synthetically useful 1,4-diketones. Preliminary mechanistic investigations suggest that the reaction proceeds through a one-electron process involving free radical species in which PhBr acts as the oxidant.

Oxidase‐Type C−H/C−H Coupling Using an Isoquinoline‐Derived Organic Photocatalyst

Oxidase‐type oxidation is an attractive strategy in organic synthesis due to the use of O₂ as the terminal oxidant. Organic photocatalysis can effect metal‐free oxidase chemistry. Nevertheless, current methods are limited in reaction scope, possibly due to the lack of suitable photocatalysts. Here we report an isoquinoline‐derived diaryl ketone‐type photocatalyst, which has much enhanced absorption of blue and visible light compared to conventional diaryl ketones. This photocatalyst enables dehydrogenative cross‐coupling of heteroarenes with unactivated and activated alkanes as well as aldehydes using air as the oxidant. A wide range of heterocycles with various functional groups are suitable substrates. Transient absorption and excited‐state quenching experiments point to an unconventional mechanism that involves an excited state “self‐quenching” process to generate the N‐radical cation form of the sensitizer, which subsequently abstracts a hydrogen atom from the alkane substrate to yield a reactive alkyl radical.

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.

Solvent-mediated switching between oxidative addition and addition–oxidation: access to β-hydroxysulfides and β-arylsulfones by the addition of thiols to olefins in the presence of Oxone

Solvent-switching allows formation of either β-hydroxy-2-arylethyl aryl sulfides or 2-arylethyl aryl sulfones exclusively in thiol–ene ‘click’ reactions conducted with Oxone.

Modular synthesis of 3-substituted isocoumarins via silver-catalyzed aerobic oxidation/6-endo heterocyclization of ortho-alkynylbenzaldehydes

A method involving silver-catalyzed aerobic oxidation/6-endo heterocyclization of ortho-alkynylbenzaldehydes to yield 3-substituted isocoumarins is described. The developed protocol allows convenient access to a range of synthetically useful 3-substituted isocoumarins and related fused heterocyclolactones in good to high yields, using silver tetrafluoroborate as the catalyst, and atmospheric oxygen as the terminal oxidant and the source of endocyclic oxygen. Mechanistic studies suggest the involvement of a free-radical pathway.

A General Approach to Intermolecular Olefin Hydroacylation through Light-Induced HAT Initiation: An Efficient Synthesis of Long-Chain Aliphatic Ketones and Functionalized Fatty Acids

Herein, an operationally simple, environmentally benign and effective method for intermolecular radical hydroacylation of unactivated substrates by employing photo-induced hydrogen atom transfer (HAT) initiation is described. The use of commercially available and inexpensive photoinitiators (Ph₂ CO and NHPI) makes the process attractive. The olefin hydroacylation protocol applies to a wide array of substrates bearing numerous functional groups and many complex structural units. The reaction proves to be scalable (up to 5 g). Different functionalized fatty acids, petrochemicals and naturally occurring alkanes can be synthesized with this protocol. A radical chain mechanism is implicated in the process.

Catalyst-free and additive-free reactions enabling C-C bond formation: a journey towards a sustainable future

This review focuses on the catalyst- and additive-free C-C bond forming reactions reported mostly from the year 2005 to date. C-C bond forming reactions are highly important as large and complex organic molecules can be derived from simpler ones via these reactions. On the other hand, catalyst- and additive-free reactions are economical, environmentally friendly and less sensitive to air/moisture, allow easy separation of products and are operationally simple. Hence, a large number of research articles have been published in this area. Though a few reviews are available on the catalyst-free organic reactions, most of them were published a few years ago. The current review excludes catalysts as well as additives and is specific to only C-C bond formation. Besides many organic name reactions, catalyst/additive-free C-H functionalizations, coupling reactions and UV-visible-light-promoted reactions are also discussed. Undoubtedly, the contents of this review will motivate readers to do more novel work in this area which will accelerate the journey towards a sustainable future.

Reagent-free aerobic oxidative synthesis of amides from aldehydes and isothiocyanates

A reagent-free autoxidative reaction of aldehydes with isothiocyanates is developed to readily access amides, involving capture of carboxylic acids in situ generated from aldehydes by isothiocyanates as both coupling mediators and amine surrogates.

Radical Hydroalkylation and Hydroacylation of Alkenes by the Use of Benzothiazoline under Thermal Conditions

The hydroalkylation and hydroacylation of electron-deficient alkenes proceeded smoothly by using benzothiazoline derivatives as radical-transfer reagents under thermal conditions without light irradiation or any additive. Both benzyl and benzoyl moieties were transferred efficiently.

Photografting and Patterning of Poly(ethylene glycol) Methacrylate Hydrogel on Glass for Biochip Applications

An efficient procedure for chemical initiator-free, in situ synthesis of a functional polyethylene glycol methacrylate (PEG MA) hydrogel on regular glass substrates is reported. It is demonstrated that self-initiated photografting and photopolymerization driven by UV irradiation can yield tens of nanometer-thick coatings of carboxy-functionalized PEG MA on the aldehyde-terminated borosilicate glass surface. The most efficient formulation for hydrogel synthesis contained methyl methacrylic acid (MAA), 2-hydroxyethyl methacrylate (HEMA), and PEG methacrylate (PEG₁₀MA) monomers (1:1:1). The resulting HEMA/PEG₁₀MA/MAA (HPMAA) coatings had a defined thickness in the range from 11 to 50 nm. The physicochemical properties of the synthesized HPMAA coatings were analyzed by combining water contact angle measurements, stylus profilometry, imaging null ellipsometry, and atomic force microscopy (AFM). The latter technique was employed in the quantitative imaging mode not only for direct probing of the surface topography but also for swelling behavior characterization in the pH range from 4.5 to 8.0. The estimated high swelling ratios of the HPMAA hydrogel (up to 3.2) together with its good stability and resistance to nonspecific protein binding were advantageous in extracellular matrix mimetics via patterning of fibronectin (FN) at a resolution close to 200 nm. It was shown that the fabricated FN micropatterns on HPMAA were equally suitable for single-cell arraying, as well as controlled cell culture lasting at least for 96 h.

A Sequential Umpolung/Enzymatic Dynamic Kinetic Resolution Strategy for the Synthesis of γ-Lactones

Combining biological and small-molecule catalysts under a chemoenzymatic manifold presents a series of significant advantages to the synthetic community. We report herein the successful development of a two-step/single flask synthesis of γ-lactones through the merger of Umpolung catalysis with a ketoreductase-catalyzed dynamic kinetic resolution, reduction, and cyclization. This combined approach delivers highly enantio- and diastereoenriched heterocycles and demonstrates the feasibility of integrating NHC catalysis with enzymatic processes.

Additions of Aldehyde-Derived Radicals and Nucleophilic N-Alkylindoles to Styrenes by Photoredox Catalysis

The consecutive addition of acyl radicals and N-alkylindole nucleophiles to styrenes was established, as well as some additional radical–nucleophile combinations. Both aryl and aliphatic aldehydes give reasonable yields. The reaction proceeds best for α-substituted styrenes, effectively creating a quaternary all-carbon center. Some iridium-based photoredox systems are catalytically active; furthermore, a base is needed in this transformation. Radicals are formed by reductive perester cleavage and hydrogen atom transfer.

Selective Dehydrogenative Acylation of Enamides with Aldehydes Leading to Valuable β-Ketoenamides

We have presented a unique example of dehydrogenative acylation of enamides with aldehydes enabled by an earth-abundant iron catalyst. The protocol provides the straightforward access to valuable β-ketoenamides with ample substrate scope and excellent functional group tolerance. Notably, distinct C-H acylation of enamide rather than at N-H moiety site occurs with absolute Z-selectivity was observed. Late-stage modifications of complex molecules and versatile synthetic utility of β-ketoenamides further highlight the practicability of this transformation.

Aerobically-initiated C(sp3)–H bond amination through the use of activated azodicarboxylates

In this article we report a procedure for α-C(sp³)–H amination of ethereal compounds through use of azodicarboxylates as the nitrogen source and freely-available atmospheric oxygen to access ethereal radical intermediates via aerobic C–H activation.

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.

Copper-Catalyzed 1,2-Methoxy Methoxycarbonylation of Alkenes with Methyl Formate

Reported here is a copper-catalyzed 1,2-methoxy methoxycarbonylation of alkenes by an unprecedented use of methyl formate as a source of both the methoxy and the methoxycarbonyl groups. This reaction transforms styrene and its derivatives into value-added β-methoxy alkanoates and cinnamates, as well as medicinally important five-membered heterocycles, such as functionalized tetrahydrofurans, γ-lactones, and pyrrolidines. A ternary β-diketiminato-Cu^I -styrene complex, fully characterized by NMR spectroscopy and X-ray crystallographic analysis, is capable of catalyzing the same transformation. These findings suggest that pre-coordination of electron-rich alkenes to copper might play an important role in accelerating the addition of nucleophilic radicals to electron-rich alkenes, and could have general implications in the design of novel radical-based transformations.

Cobalt-catalyzed cyclization with the introduction of cyano, acyl and aminoalkyl groups

An efficient synthesis of carbo- and heterocycles using C[double bond, length as m-dash]C, C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds under cobalt catalysis is described. The substituents on olefins are key for controlling the regio- and chemoselectivity in the initial hydrogen atom transfer step and quaternary carbons are efficiently constructed under mild conditions. Cyclopropane cleavage and tandem cyclization give highly functionalized bicyclic skeletons in a single operation.

Benzothiazolines as radical transfer reagents: hydroalkylation and hydroacylation of alkenes by radical generation under photoirradiation conditions

Novel radical transfer reagents under photoirradiation conditions were developed by the use of benzothiazoline derivatives.

Aerobic conversion of benzylic sp3 C–H in diphenylmethanes and benzyl ethers to CO bonds under catalyst-, additive- and light-free conditions

Catalyst-, additive- and light-free aerobic conversion of benzylic C–H to CO bonds is, for the first time, reported.

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.

Molecular Oxygen-Promoted General and Site-Specific Alkylation with Organoboronic Acid

A general alkylating method using organoboronic acid under 1 atm of oxygen is developed. It allows a facile access to a wide range of functionalized molecules with privileged scaffolds in drugs and natural products such as oxindoles, quinolinones, chromones, naphthoquinones, coumarins, and quinolones. In contrast to previous alkylation approaches that generally requiring transition-metal catalysis and a stoichiometric chemical oxidant, the present strategy features metal-free, molecular oxygen as the terminal oxidant and site specificity.

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.