DDQ/tert-Butyl nitrite-catalyzed aerobic oxidation of diarylmethane sp3 C–H bonds

DDQ-catalyzed oxidative α-allylation of isochromans under aerobic conditions

The allylation of isochromans at the α-position via aerobic DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) catalysis is described. This process involves the DDQ oxidation of various isochromans under mild conditions to generate oxocarbenium intermediates, which are effectively stabilized in equilibration in the presence of acid before undergoing allylation. Molecular oxygen and tert-butyl nitrite are employed as an environmentally benign oxidant and mediator, respectively, in the catalytic cycle. The transformation of an allylated product to biologically active molecules has also been successfully demonstrated.

Aerobic Benzylic C(sp3)-H Bond Oxygenations Catalyzed by NBS under Visible Light Irradiation

An efficient photocatalytic oxidation of benzylic C(sp3)-H bonds to ketones, esters, and amides has been developed using NBS as a metal-free photocatalyst and O2 as an oxidant. A variety of synthetically and biologically valuable compounds are assembled in moderate to excellent yields. The synthetic utility of this approach has been demonstrated by gram-scale experiments. A possible free radical mechanism was proposed to rationalize the reaction procedure.

Stereoselective Synthesis of Benzo[a]quinolizidines via Aerobic DDQ-Catalyzed Allylation and Reductive Cyclization

Stereoselective synthesis of C₄-substituted benzo[a]quinolizidines via redox-controlled catalytic C-C-bond-forming reactions was carried out. Aerobic DDQ-catalyzed allylation of N-Cbz tetrahydroisoquinolines efficiently provided α-allylated products 5, which were transformed to enones 6 via cross-metathesis reactions using the second-generation Hoveyda-Grubbs catalyst. Palladium-catalyzed hydrogenation of 6 prompted alkene reduction, protecting group removal, and intramolecular reductive amination in one step to afford the desired benzo[a]quinolizidines 7 as single diastereomers.

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.

Nanostructured Manganese Oxides within a Ring-Shaped Polyoxometalate Exhibiting Unusual Oxidation Catalysis

Nanosized manganese oxides have recently received considerable attention for their synthesis, structures, and potential applications. Although various synthetic methods have been developed, precise synthesis of novel nanostructured manganese oxides are still challenging. In this study, using a structurally defined nanosized cavity inside a ring-shaped polyoxometalate, we succeeded in synthesizing two types of discrete 18 and 20 nuclear nanostructured manganese oxides, Mn18 and Mn20, respectively. In particular, Mn18 showed much higher catalytic activity than other manganese oxides for the oxygenation of alkylarenes including electron-deficient ones, and the reaction proceeded through a unique reaction mechanism due to its unusual manganese oxide structure.

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.

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.

Benzylic Methylene Functionalizations of Diarylmethanes

Diarylmethanes are cardinal scaffolds by virtue of their unique structural feature including the presence of a benzylic CH2 group that can be easily functionalized to generate a variety of fascinating molecules holding immense importance in pharmaceutical, agrochemical, and material sciences. While the originally developed protocols for benzylic C-H functionalization in diarylmethanes employing base-mediated and metal-catalyzed strategies are still actively used, they are joined by a new array of metal-free conditions, offering milder and benign conditions. With the recent surge of interest towards the synthesis of functionalized diarylmethanes, numerous choices are now available for a synthetic organic chemist to transform the benzylic C-H bond to C-C or C-X bond offering the synthesis of any molecule of choice. This review highlights benzylic methylene (CH2 ) functionalizations of diaryl/heteroarylmethanes utilizing various base-mediated, transition-metal-catalyzed, and transition-metal free approaches for the synthesis of structurally diverse important organic molecules, often with a high chemo-, regio- and enantio-selectivity. This review also attempts to provide analysis of the scope and limitations, mechanistic understanding, and sustainability of the transformations.

Testing the limits of radical-anionic CH-amination: a 10-million-fold decrease in basicity opens a new path to hydroxyisoindolines via a mixed C-N/C-O-forming cascade

An intramolecular C(sp3)-H amidation proceeds in the presence of t-BuOK, molecular oxygen, and DMF. This transformation is initiated by the deprotonation of an acidic N-H bond and selective radical activation of a benzylic C-H bond towards hydrogen atom transfer (HAT). Cyclization of this radical-anion intermediate en route to a two-centered/three-electron (2c,3e) C-N bond removes electron density from nitrogen. As this electronegative element resists such an "oxidation", making nitrogen more electron rich is key to overcoming this problem. This work dramatically expands the range of N-anions that can participate in this process by using amides instead of anilines. The resulting 107-fold decrease in the N-component basicity (and nucleophilicity) doubles the activation barrier for C-N bond formation and makes this process nearly thermoneutral. Remarkably, this reaction also converts a weak reductant into a much stronger reductant. Such "reductant upconversion" allows mild oxidants like molecular oxygen to complete the first part of the cascade. In contrast, the second stage of NH/CH activation forms a highly stabilized radical-anion intermediate incapable of undergoing electron transfer to oxygen. Because the oxidation is unfavored, an alternative reaction path opens via coupling between the radical anion intermediate and either superoxide or hydroperoxide radical. The hydroperoxide intermediate transforms into the final hydroxyisoindoline products under basic conditions. The use of TEMPO as an additive was found to activate less reactive amides. The combination of experimental and computational data outlines a conceptually new mechanism for conversion of unprotected amides into hydroxyisoindolines proceeding as a sequence of C-H amidation and C-H oxidation.

Selective Oxidation of Benzylic C-H Bonds Catalyzed by Cu(II)/{PMo12}

Precise catalytic regulation of carbon radical generation by a highly active oxygen radical to abstract the H atom in a C-H bond is an effective method for the selective activation of C-H synthetic chemistry. Herein, we report a facile catalyst system with commercially available copper(II)/{PMo₁₂} to form a tert-butanol radical intermediate for the selective oxidation of benzylic C-H bonds. The reaction shows a broad range of substrates (benzyl methylene, benzyl alcohols) with good functional group tolerance and chemical selectivity. The corresponding carbonyl compounds were synthesized with good yields under mild conditions. DFT calculations and experimental analysis further demonstrated a reasonable carbon radical mechanism for this type of organic transformation reaction.

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.

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.

Potassium-Zincate-Catalyzed Benzylic C-H Bond Addition of Diarylmethanes to Styrenes

Direct functionalization of the benzylic C-H bond of diarylmethanes is an important strategy for the synthesis of diarylmethine-containing compounds. However, the methods developed to date for this purpose require a stoichiometric amount (usually more) of either a strong base or an oxidant. Reported here is the first catalytic benzylic C-H bond addition of diarylmethanes to styrenes and conjugated dienes. A potassium zincate complex, generated from potassium benzyl and zinc amide, acts as a catalyst and displays good activity and chemoselectivity. Considering the atom economy of the reaction and the ready availability of the catalyst, this reaction constitutes a practical, efficient method for diarylalkane synthesis.

TBAI-catalyzed oxidative C–H functionalization: a new route to benzo[b]phosphole oxides

The first metal-free, efficient TBAI-catalyzed radical addition/cyclization of diaryl(arylethynyl)phosphine oxides with toluene derivatives has been developed, affording a general, one-step approach to structurally sophisticated benzo[b]phosphole oxides via sequential C–H functionalization along with the formation of two new C–C bonds.

2,3-Dichloro-5,6-dicyano-1,4-benzoquinone-catalyzed aerobic oxidation reactions via multistep electron transfers with iron(ii) phthalocyanine as an electron-transfer mediator

A new biomimetic catalytic oxidation system was developed for oxidative deprotection of PMB ethers, alcohol oxidation, aromatization and α,β-unsaturated aldehyde formation.