2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)/tert-Butyl Nitrite/Oxygen: A Versatile Catalytic Oxidation System

DDQ catalyzed oxidative lactonization of indole-3-butyric acids

Benzylic C-H bonds next to electron rich aromatic rings are susceptible to 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) promoted functionalization. In this work benzylic carbocations formed in this manner are trapped by a pendant carboxylic acid to form a lactone. Indole-3-butyric acids are especially good substrates for the reaction. The lactonization functions well with catalytic amounts of DDQ combined with MnO2 as the stoichiometric oxidant. This cyclization proceeds with a number of indole-3-butyric acids, and yields were generally good as long as the indole was electron rich and free of bulky substituents near the reacting benzylic carbon. Indole-3-butyric acids functionalized with electron withdrawing groups tend to give more moderate yields. Other aromatic substrates also participate as long as the aromatic ring is functionalized with electron donating groups.

An organic transistor for detecting the oxidation of an organic sulfur compound at a solid-liquid interface and its chemical sensing applications

The development of chemical sensors has advanced due to an increase in demand; however, the potential of chemical sensors as devices to monitor organic reactions has not been revealed yet. Thus, we aim to propose a chemical sensor platform for facile monitoring of chemical reactions, especially at a solid-liquid interface. In this study, an extended-gate-type organic field-effect transistor (OFET) has been employed as a platform to detect chemical reactions at an interface between the extended-gate electrode and an aqueous solution. The OFET device functionalized with 4,4'-thiobisbenzenthiol has shown time- and concentration-dependent shifts in transistor characteristics upon adding H2O2. In a selectivity test using seven oxidant agents, the transistor responses depended on the oxidation of the organic sulfur compound (i.e., 4,4'-thiobisbenzenthiol) stemming from the ability of the oxidant agents. Therefore, the observed changes in the transistor characteristics have suggested the generation of sulfur-oxidized products at the interface. In this regard, the observed responses were caused by disulfide formation accompanied by changes in the charges under neutral pH conditions. Meanwhile, weak transistor responses derived from the generation of oxygen adducts have also been observed, which were caused by changes in the dipole moments. Indeed, the yields of the oxygen adducts have been revealed by X-ray photoelectron spectroscopy. The monitoring of gradual changes originating from the decrease in the disulfide formation and the increase in the oxygen adducts implied a novel aspect of the OFET device as a platform to simultaneously detect reversible and irreversible reactions at interfaces without using large-sized analytical instruments. Sulfur oxidation by H2O2 on the OFET device has been further applied to the indirect monitoring of an enzymatic reaction in solution. The OFET-based chemical sensor has shown continuous changes with an increase in a substance (i.e., lactate) in the presence of an enzyme (i.e., lactate oxidase), which indicates that the OFET response depends on the H2O2 generated through the enzymatic reaction in the solution. In this study, we have clarified the versatility of organic devices as platforms to monitor different chemical reactions using a single detection method.

Selective Oxidation of Cyclohexanone to Adipic Acid Using Molecular Oxygen in the Presence of Alkyl Nitrites and Transition Metals as Catalysts

This paper presents a not previously reported catalytic system consisting of transition metals Co2+ and Mn2+ and alkyl nitrites R-ONO for the oxidation of cyclohexanone with oxygen to adipic acid. The influence of type and amount of catalyst, temperature, time, and type of raw material on conversion and product composition were determined. In addition, the oxidation of selected cyclic ketones such as cyclopentanone, cyclohexanone, cyclooctanone, cyclododecanone, 2-methylcyclohexanone, 3-methylcyclohexanone, and 4-methylcyclohexanone in acetic acid as solvent was performed. The results showed that R-ONO systems, under established reaction conditions, form NO·radicals, which oxidize to NO2 under a strong oxidization reaction environment. The Co2+/Mn2+/NO2 system was shown to be highly active in the oxidation of cyclic ketones with oxygen.

Arene C-H Amination with N-Heteroarenes by Catalytic DDQ Photocatalysis

Arene C-H aminations using catalytic amounts of a 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) photocatalyst are described. Benzene, which has an oxidation potential of 2.48 V (vs SCE), was functionalized by pyrazoles, triazoles, tetrazoles, purines, and tert-butoxycarbonyl amine. Arenes underwent amination via a combination of ultraviolet (UV) light and a DDQ photocatalyst without a typical co-oxidant. Although the mechanism remains an open question, DDQH₂, which is generated from DDQ after oxidation, is reactivated to DDQ under UV light irradiation conditions, possibly with the assistance of adventitious O₂ and/or a solvent as the terminal oxidant(s) in this system.

Bioinspired o-Naphthoquinone-Catalyzed Aerobic Oxidation of Alcohols to Aldehydes and Ketones

A biomimetic alcohol dehydrogenase (ADH)-like oxidation protocol was developed using an ortho-naphthoquinone catalyst in the presence of a catalytic amount of base. The developed organocatalytic aerobic oxidation protocol proceeds through the intramolecular 1,5-hydrogen atom transfer of naphthalene alkoxide intermediates, a mechanistically distinctive feature from the previous alcohol dehydrogenase mimics that require metals in the active form of catalysts. The ADH-like aerobic oxidation protocol should provide green alternatives to the existing stoichiometric and metal-catalyzed alcohol oxidation reactions.

Synthetic applications of hydride abstraction reactions by organic oxidants

Carbon-hydrogen bond functionalizations provide an attractive method for streamlining organic synthesis, and many strategies have been developed for conducting these transformations. Hydride-abstracting reactions have emerged as extremely effective methods for oxidative bond-forming processes due to their mild reaction conditions and high chemoselectivity. This review will predominantly focus on the mechanism, reaction development, natural product synthesis applications, approaches to catalysis, and use in enantioselective processes for hydride abstractions by quinone, oxoammonium ion, and carbocation oxidants. These are the most commonly employed hydride-abstracting agents, but recent efforts illustrate the potential for weaker ketone and triaryl borane oxidants, which will be covered at the end of the review.

DDQ in mechanochemical C-N coupling reactions

2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is a commonly known oxidant. Herein, we report that DDQ can be used to synthesize 1,2-disubstituted benzimidazoles and quinazolin-4(3H)-ones via the intra- and intermolecular C-N coupling reaction under solvent-free mechanochemical (ball milling) conditions. In the presence of DDQ, the intramolecular C(sp2)-H amidation of N-(2-(arylideneamino)phenyl)-p-toluenesulfonamides leads to 1,2-disubstituted benzimidazoles and the one-pot coupling of 2-aminobenzamides with aryl/alkyl aldehydes resulted in substituted quinazolin-4(3H)-one derivatives in high yields.

Transition-metal-free synthesis of trifluoromethylated benzoxazines via a visible-light-promoted tandem difunctionalization of o-vinylanilides with trifluoromethylsulfinate

A Umemoto's reagent-free and cost-effective method for synthesis of trifluoromethylated benzoxazines by 9,10-phenanthrenedione visible-light photocatalysis is described in this article.

Visible-light-mediated aerobic Ritter-type C–H amination of diarylmethanes using DDQ/tert-butyl nitrite

A metal-free photocatalytic Ritter-type C–H amination of diarylmethanes using O2 as an oxidant has been developed using a co-catalytic system of DDQ and TBN and offers a low cost, sustainable way to synthesise secondary amides under mild conditions.

DDQ-Catalyzed Oxidative C(sp3)-H Functionalization of Aryltetralins and Subsequent Chemoselective Oxidative Demethylation to Access Dihydronaphthalenes and Dihydronaphthoquinones

A highly controlled DDQ-catalyzed oxidative C(sp³)-H functionalization of three contiguous carbon atoms in aryltetralins is reported for efficient access to diverse oxygenated dihydronaphthalene scaffolds. The first total synthesis of pachypostaudin B is realized. Further, a CAN-mediated chemoselective oxidative demethylation on the dihydronaphthalene scaffolds is demonstrated to arrive at the rarely observed dihydronaphthoquinone core in moderate to good yields. The present methodology enables quick access to a library of magnoshinin and merrilliaquinone analogs.

DDQ as a versatile and easily recyclable oxidant: a systematic review

2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is the most widely used quinone with a high reduction potential, and it commonly mediates hydride transfer reactions and shows three accessible oxidation states: quinone (oxidized), semiquinone (one-electron-reduced), and hydroquinone (two-electron-reduced). DDQ has found broad utility as a stoichiometric oxidant in the functionalization of activated C-H bonds and the dehydrogenation of saturated C-C, C-O, and C-N bonds. The cost and toxicity of DDQ triggered recent efforts to develop methods that employ catalytic quantities of DDQ in combination with alternative stoichiometric oxidants. The aerobic catalytic approach was established for the selective oxidation of non-sterically hindered electron-rich benzyl methyl ethers and benzylic alcohols, and effectively extended to the oxidative deprotection of p-methoxybenzyl ethers to generate the alcohols in high selectivity. A combination of DDQ and protic acid is known to oxidize several aromatic donors to the corresponding cation radicals. The excited-state DDQ converts benzyls, heteroarenes, fluoroarenes, benzene, and olefins into their radical cation forms as well as chloride and other anions into their respective radicals. These reactive intermediates have been employed for the generation of C-C and C-X (N, O, or Cl) bonds in the synthesis of valuable natural products and organic compounds. To the best of our knowledge, however, there is still no review article exclusively describing the applications of DDQ in organic synthesis. Therefore, in the present review, we provide an overview of DDQ-induced organic transformations with their scope, limitations and the proposed reaction mechanisms.

One-Pot Multicomponent Coupling Reaction of Catechols, Benzyl Alcohols/Benzyl Methyl Ethers, and Ammonium Acetate toward Synthesis of Benzoxazoles

The multicomponent coupling reaction of catechol, ammonium acetate, and benzyl alcohol/benzyl methyl ether in the presence of a Fe(III) catalyst precursor afforded benzoxazole derivatives in good to excellent yields. The notable features of this protocol are abundant availability of the catalyst system, large-scale synthesis, high diversity, and high yields of products.

Efficient Aerobic Oxidation of Organic Molecules by Multistep Electron Transfer

This Minireview presents recent important homogenous aerobic oxidative reactions which are assisted by electron transfer mediators (ETMs). Compared with direct oxidation by molecular oxygen (O2 ), the use of a coupled catalyst system with ETMs leads to a lower overall energy barrier via stepwise electron transfer. This cooperative catalytic process significantly facilitates the transport of electrons from the reduced form of the substrate-selective redox catalyst (SSRCred ) to O2 , thereby increasing the efficiency of the aerobic oxidation. In this Minireview, we have summarized the advances accomplished in recent years in transition-metal-catalyzed as well as metal-free aerobic oxidations of organic molecules in the presence of ETMs. In addition, the recent progress of photochemical and electrochemical oxidative functionalization using ETMs and O2 as the terminal oxidant is also highlighted. Furthermore, the mechanisms of these transformations are showcased.

Assessing Methodologies to Synthesize α-Sulfenylated Carbonyl Compounds by Green Chemistry Metrics

α-Sulfenylated carbonyl compounds are important both as active pharmaceutical ingredients and as intermediates in organic synthesis. Owing to their relevance in synthetic organic chemistry, this Minireview focuses on assessing the most relevant synthetic procedures based on green chemistry metrics. The Minireview starts with the traditional routes and then focuses on more recently developed methodologies. These routes include sulfenylating reagents using organocatalysis, cross-dehydrogenative couplings using in situ halogenations to prevent reactive intermediates in high concentrations, oxidative couplings using terminal oxidants such as DDQ or TEMPO, and redox-neutral couplings using transition metal catalysis. These methodologies have been evaluated on the basis of atom economy, E factor, and the safety and toxicity of the transformations and the solvents used. Besides using green metrics to evaluate these novel methodologies, the synthetic utility is also assessed with regard to the availability of starting materials and the generality of the reactions. This Minireview aims to inspire researchers to apply green assessments to other methodologies and also for them to take measures to increase the greenness of their developed transformations.

Visible-Light-Mediated Oxidative Debenzylation Enables the Use of Benzyl Ethers as Temporary Protecting Groups

The cleavage of benzyl ethers by catalytic hydrogenolysis or Birch reduction suffers from poor functional group compatibility and limits their use as a protecting group. The visible-light-mediated debenzylation disclosed here renders benzyl ethers temporary protective groups, enabling new orthogonal protection strategies. Using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as a stoichiometric or catalytic photooxidant, benzyl ethers can be cleaved in the presence of azides, alkenes, and alkynes. The reaction time can be reduced from hours to minutes in continuous flow.

Chemoselective Oxidation of p-Methoxybenzyl Ethers by an Electronically Tuned Nitroxyl Radical Catalyst

The oxidation of p-methoxy benzyl (PMB) ethers was achieved using nitroxyl radical catalyst 1, which contains electron-withdrawing ester groups adjacent to the nitroxyl group. The oxidative deprotection of the PMB moieties on the hydroxy groups was observed upon treatment of 1 with 1 equiv of the co-oxidant phenyl iodonium bis(trifluoroacetate) (PIFA). The corresponding carbonyl compounds were obtained by treating the PMB-protected alcohols with 1 and an excess of PIFA.

Redox-Active Guanidines in Proton-Coupled Electron-Transfer Reactions: Real Alternatives to Benzoquinones?

Guanidino-functionalized aromatics (GFAs) are readily available, stable organic redox-active compounds. In this work we apply one particular GFA compound, 1,2,4,5-tetrakis(tetramethylguanidino)benzene, in its oxidized form in a variety of oxidation/oxidative coupling reactions to demonstrate the scope of its proton-coupled electron transfer (PCET) reactivity. Addition of an excess of acid boosts its oxidation power, enabling the oxidative coupling of substrates with redox potentials of at least +0.77 V vs. Fc⁺ /Fc. The green recyclability by catalytic re-oxidation with dioxygen is also shown. Finally, a direct comparison indicates that GFAs are real alternatives to toxic halo- or cyano-substituted benzoquinones.

p-Methylbenzyl Group: Oxidative Removal and Orthogonal Alcohol Deprotection

We describe the practical removal of p-methylbenzyl (MBn) protections of alcohols by treatment with 2,3-dichloro-5,6-dicyano-p-benzoquinone. When a molecule bears benzyl and MBn groups, the oxidant selectively removes the latter groups. Further, the MBn groups tolerate ceric ammonium nitrate, resulting in chemoselective removal of the p-methoxybenzyl group in the presence of the MBn groups. These orthogonal alcohol deprotections would provide novel synthetic strategies of organic compounds.

Replacement of Stoichiometric DDQ with a Low Potential o-Quinone Catalyst Enabling Aerobic Dehydrogenation of Tertiary Indolines in Pharmaceutical Intermediates

A transition-metal/quinone complex, [Ru(phd)₃]²⁺ (phd = 1,10-phenanthroline-5,6-dione), is shown to be effective for aerobic dehydrogenation of 3° indolines to the corresponding indoles. The results show how low potential quinones may be tailored to provide a catalytic alternative to stoichiometric DDQ, due to their ability to mediate efficient substrate dehydrogenation while also being compatible with facile reoxidation by O₂. The utility of the method is demonstrated in the synthesis of key intermediates to pharmaceutically important molecules.

Solvent-controlled chemoselective N-dealkylation-N-nitrosation or C-nitration of N-alkyl anilines with tert-butyl nitrite

A metal-free, acid-free and chemoselective N-dealkylation-N-nitrosation or C-nitration of N-alkyl anilines has been developed.

Photocatalysis in organic and polymer synthesis

This review, with over 600 references, summarizes the recent applications of photoredox catalysis for organic transformation and polymer synthesis. Photoredox catalysts are metallo- or organo-compounds capable of absorbing visible light, resulting in an excited state species. This excited state species can donate or accept an electron from other substrates to mediate redox reactions at ambient temperature with high atom efficiency. These catalysts have been successfully implemented for the discovery of novel organic reactions and synthesis of added-value chemicals with an excellent control of selectivity and stereo-regularity. More recently, such catalysts have been implemented by polymer chemists to post-modify polymers in high yields, as well as to effectively catalyze reversible deactivation radical polymerizations and living polymerizations. These catalysts create new approaches for advanced organic transformation and polymer synthesis. The objective of this review is to give an overview of this emerging field to organic and polymer chemists as well as materials scientists.

Toward the oxidative deconstruction of lignin: oxidation of β-1 and β-5 linkages

Production of monomers and other products from the oxidation of β-1 and β-5 lignin models.

Teaching Old Compounds New Tricks: DDQ-Photocatalyzed C-H Amination of Arenes with Carbamates, Urea, and N-Heterocycles

The C-H amination of benzene derivatives was achieved using DDQ as photocatalyst and BocNH2 as the amine source under aerobic conditions and visible light irradiation. Electron-deficient and electron-rich benzenes react as substrates with moderate to good product yields. The amine scope of the reaction comprises Boc-amine, carbamates, pyrazoles, sulfonimides and urea. Preliminary mechanistic investigations indicate arene oxidation by the triplet of DDQ to radical cations with different electrophilicity and a charge transfer complex between the amine and DDQ as intermediate of the reaction.

Preparation and isolation of isobenzofuran

The synthesis, isolation and characterization of isobenzofuran are described in this publication. Isobenzofuran is of general interest in synthetic and physical organic chemistry because it is one of the most reactive dienes known. A number of synthetic pathways have been published which all suffer from disadvantages such as low yields and difficult purification. We present a synthetic pathway to prepare isobenzofuran in laboratory scale with high yields, from affordable, commercially available starting materials.

Visible-light-mediated C2-amination of thiophenes by using DDQ as an organophotocatalyst

A direct C–H amination of thiophenes was presented via an oxidation pathway under visible-light irradiation, in which the thiophene radical cation serves as the key intermediate. Various thiophenes and azoles could be transformed into the corresponding amination products well, and H₂O was the only byproduct, which is environmentally benign.

AIBN/NaBr-promoted aerobic oxidation of benzylic alcohols via a radical process

An economic and practical AIBN-initiated aerobic oxidation system of benzylic alcohols, hetero aryl alcohols and allyl alcohols was developed for the first time. Moderate to excellent yields were obtained with a broad substrate scope. Moreover, a proposed mechanism of a radical process was assumed and confirmed by the key intermediate detected.