Redox Condensation Research Articles

Iron‐Promoted Redox Access to 2‐Aminobenzoxazoles from Amines, Carbon Disulfide and 2‐Nitrophenols

AbstractA method to assemble 2‐aminobenzoxazoles via iron‐catalyzed redox condensation of amines, carbon disulfide and 2‐nitrophenols is reported. The reaction features advantages of environmentally friendly iron catalyzed conditions, readily available starting materials, and excellent atom‐, step‐ and redox‐ economy compared to known protocols. Experimental investigation suggests a mechanism encompassing iron‐catalyzed reduction of the nitro group of 2‐nitrophenols by the dithiocarbamate group from amine‐CS2 adduct.

Redox condensation of o-halonitrobenzenes, sulfur and isothiocyanates: access to 2-aminobenzothiazoles.

The elemental sulfur and isothiocyanate (S8/RNCS) couple was found to undergo redox condensation with o-halonitrobenzenes in the presence of N-methylpiperidine as a base in N-methylpyrrolidin-2-one to provide step- and redox-economical access to 2-aminobenzothiazoles. Alternatively, dithiocarbamate salts generated in situ from addition of amines and CS2 could be used in place of the S8/RNCS couple.

Sodium sulfide-promoted regiodefined redox condensation of o-nitroanilines with aryl ketones to benzo[a]phenazines and quinoxalines.

Inexpensive sodium sulfide trihydrate was found to promote unprecedented 6e-regio-predefined redox condensation of o-nitroanilines with α-tetralones to benzo[a]phenazines. The method was also successfully extended to acetophenones and higher homologs as reducing partners to provide 2-phenylquinoxalines. Compared to traditional approaches toward benzo[a]phenazine and quinoxaline cores starting with o-phenylenediamines, the present strategy could afford these heterocycles with well-defined regiochemistry based on the structure of starting o-nitroanilines.

Fe/S-Catalyzed Redox Condensation of o-Nitrophenols with Isothiocyanates to 2-Aminobenzoxazoles.

As frequently encountered byproducts of isocyanate chemistry, hydrogen sulfide and related sulfur containing compounds should be treated in a safe way to lower their adverse health and environmental effects, especially in large scale syntheses. As a proof of concept, we report herein an example of in situ recycling of sulfur byproduct to reductant in the synthesis of bioactive 2-aminobenzoxazoles 3. Using an Fe/S catalytic system, this heterocyclic scaffold could be obtained from o-nitrophenols 1 with isothiocyates 2 via direct redox condensation consisting of reduction of the nitro group of 1 by the sulfur moiety of 2.

Insights into the Structure-Property-Activity Relationship of Zeolitic Imidazolate Frameworks for Acid-Base Catalysis.

Zeolitic imidazolate frameworks (ZIFs) have been extensively examined for their potential in acid-base catalysis. Many studies have demonstrated that ZIFs possess unique structural and physicochemical properties that allow them to demonstrate high activity and yield products with high selectivity. Herein, we highlight the nature of ZIFs in terms of their chemical formulation and the textural, acid-base, and morphological properties that strongly affect their catalytic performance. Our primary focus is the application of spectroscopic methods as instruments for analyzing the nature of active sites because these methods can allow an understanding of unusual catalytic behavior from the perspective of the structure-property-activity relationship. We examine several reactions, such as condensation reactions (the Knoevenagel condensation and Friedländer reactions), the cycloaddition of CO2 to epoxides, the synthesis of propylene glycol methyl ether from propylene oxide and methanol, and the cascade redox condensation of 2-nitroanilines with benzylamines. These examples illustrate the broad range of potentially promising applications of Zn-ZIFs as heterogeneous catalysts.

T‐BuONa‐Mediated Redox Condensation between o‐Nitroanilines and Benzyl Alcohols towards 2‐Phenyl Benzimidazoles under Transition‐Metal‐Free Conditions

AbstractA novel and environmentally benign method was developed for the direct synthesis of 2‐substituted benzimidazoles via a transition metal‐free redox tandem annulation of benzyl alcohols and ortho‐nitroanilines in the presence of a strong base, e.g. alkaline alkoxides or hydroxides. A good tolerance of functional groups was observed, affording an array of 2‐aryl benzimidazole derivatives in high yields. Early mechanistic studies showed that sodium tert‐butoxide could initiate the hydrogen transfer from benzyl alcohol to the nitro group in ortho‐nitroaniline followed by imine condensation, cyclization, and oxidation. This protocol features a convenient tactic that does not require any external oxidants or solvents for the synthesis of bioactive benzimidazole‐involving compounds.

Synthesis of 2-Aminofurans and 2-Aminothiophenes through Elemental Sulfur-Promoted Switchable Redox Condensation Reaction of Enaminones with Methylene Nitriles.

Herein, we report an elemental sulfur-promoted switchable redox condensation reaction that can selectively prepare 2-aminofurans and 2-aminothiophenes from the corresponding enaminones and methylene nitriles, respectively. Mechanistic studies demonstrated that the enaminones, as dual nucleophiles, reacted with nitrile acetate to produce 2-aminofurans via 3,5-annulation under promotion by elemental sulfur. These reactions used readily available starting materials, transition metal-free, eco-friendly procedures, gram-scale syntheses, and wide functional group tolerance. The methodology may be useful for the construction of 2-aminofuran and 2-aminothiophene derivatives with potential biological activity.

Step- and atom-economical synthesis of 2-aryl benzimidazoles via the sulfur-mediated redox condensation between o-nitroanilines and aryl methanols

The synthesis of benzimidazoles usually requires use of expensive and sensivitve substrates such as phenylenediamine and aldehyde or metal catalysts. In this work, 2-nitroanilines and benzyl alcohols were alternatively applied for the condensation towards 2-phenyl benzimidazoles in the presence of elemental sulfur and an amine. Mechanistic results proved a dual role of sulfur as both oxidizing and reducing agents in this synthetic pathway. The study was expanded to the synthesis of benzimidazoles containing a variety of functionalities including halogens, methyl, trifluoromethyl, methoxy, cyano, and heterocycles groups in moderate-to-good yields. The utilization of more stable and available substrates in the absence of transition metals and other solvents offers a rapid and green synthesis of benzimidazoles.

Alkylphosphinites as Synthons for Stabilized Carbocations.

We present a new acid-free method for the generation of carbocations based on a redox condensation reaction that enables SN1 reactions with a variety of nucleophiles. We utilize readily synthesized phosphinites that are activated by diisopropyl azodicarboxylate to form betaine structures that collapse upon adding a pronucleophile, thereby yielding reactive carbocation intermediates. We also employ this approach for the alkylation of some bioactive molecules.

Heterometallic d8 -d10 Coupling of Rh(I) and M(0) (M=Pd, Pt) in a Sandwich Framework of π-Conjugated Ligands.

A heterometallic M-M' bond formation is a key to construct atomically precise bimetallic clusters and materials. However, it is sometimes not straightforward to construct a heterometallic M-M' bond through conventional methods including redox condensation. Here, we found that a sandwich framework of π-conjugated unsaturated hydrocarbon ligands provides a unique coordination environment that facilitates unusual coupling of d8 RhI and d10 M0 (M=Pd, Pt). The molecular orbital analysis showed that the electron-accepting ability of the sandwich framework through back-donation allows the formation of a dσ-type Rh-Pd bond in a (d-d)18 electron system.

Fe/S-Catalyzed synthesis of 2-benzoylbenzoxazoles and 2-quinolylbenzoxazoles via redox condensation of o-nitrophenols with acetophenones and methylquinolines.

An Fe/S catalyst generated in situ from FeCl2·4H2O and elemental sulfur S8 in the presence of a tertiary amine as a base was found to catalyze efficiently a 6e- redox condensation of o-nitrophenols with acetophenones and methylquinolines. The condensed products 2-benzoylbenzoxazoles and 2-quinolylbenzoxazoles were obtained in reasonable yields with water as the only byproduct at a temperature as low as 80 °C.

NaOH-Mediated Direct Synthesis of Quinoxalines from o-Nitroanilines and Alcohols via a Hydrogen-Transfer Strategy.

A NaOH-mediated sustainable synthesis of functionalized quinoxalines is disclosed via redox condensation of o-nitroamines with diols and α-hydroxy ketones. Under optimized conditions, various o-nitroamines and alcohols are well tolerated to generate the desired products in 44-99% yields without transition metals and external redox additives.

Iron(0)-Catalyzed Transfer Hydrogenative Condensation of Nitroarenes with Alcohols: A Straightforward Approach to Benzoxazoles, Benzothiazoles, and Benzimidazoles.

The iron-catalyzed hydrogen transfer strategy has been applied to the redox condensation of o-hydroxynitrobenzene with alcohol, leading to the formation of benzoxazole derivatives. A wide range of 2-substituted benzoxazoles were synthesized in good to excellent yields without the addition of an external redox agent. A series of control experiments provided a plausible mechanism. Furthermore, the reaction system was successfully extended to the synthesis of benzothiazoles and benzimidazoles.

Synthesis of 2‐Arylbenzoselenazoles from Se‐mediated Redox Condensation of 2‐Chloronitrobenzene and Arylmethyl Chloride

AbstractA selenium‐mediated redox condensation of 2‐chloronitrobenzene and arylmethyl chloride to obtain 2‐arylbenzoselenazoles has been realized under metal‐free condition. The reactions proceeded in moderate‐to‐good yields with good functional compatibility and gram‐scale achievement. Primarily mechanistic investigation suggested that key intermediate Bn(Se)nBn II, was first isolated and confirmed by NMR.

Fe/S‐Catalyzed Redox Condensation of o‐Nitrophenols/thiophenols/anilines with Dibenzyl Disulfide: Access to 2‐Arylbenzazoles

AbstractAn Fe/S‐catalyzed approach to 2‐arylbenzazoles from dibenzyl disulfides and o‐nitrophenols/thiophenols/anilines in the presence of base NMP has been developed. The reactions proceeded in moderate to excellent yields with good functional tolerance and gram‐scale application. Mechanistic research indicated that Fe/S clusters play an important role in the formation of products and DMSO is also important to oxidize benzyl mercaptans to give dibenzyl disulfides.

Redox active Ni-Pd carbonyl alloy nanoclusters: syntheses, molecular structures and electrochemistry of [Ni22-xPd20+x(CO)48]6- (x = 0.62), [Ni29-xPd6+x(CO)42]6- (x = 0.09) and [Ni29+xPd6-x(CO)42]6- (x = 0.27).

A redox active Ni-Pd alloy nanocluster [Ni22-xPd20+x(CO)48]6- (x = 0.62) ([1]6-) was obtained from the redox condensation of [NBu4]2[Ni6(CO)12] with 0.7-0.8 equivalents of Pd(Et2S)2Cl2 in CH2Cl2. Conversely, [Ni29-xPd6+x(CO)42]6- (x = 0.09) ([2]6-) and [Ni29+xPd6-x(CO)42]6- (x = 0.27) ([3]6-) were obtained by employing [NEt4]2[Ni6(CO)12] and 0.6-0.7 equivalents of Pd(Et2S)2Cl2 in CH3CN. The molecular structures of these high nuclearity Ni-Pd carbonyl clusters were determined by single-crystal X-ray diffraction (SC-XRD). [1]6- adopted an M40ccp structure comprising five close-packed ABCAB layers capped by two additional Ni atoms. Conversely, [2]6- and [3]6- displayed an hcp M35 metal core composed of three compact ABA layers. [1]6-, [2]6- and [3]6- showed nanometric sizes, with the maximum lengths of their metal cores being 1.3 nm ([1]6-) and 1.0 nm ([2]6- and [3]6-), which increased up to 1.9 and 1.5 nm, after including also the CO ligands. Ni-Pd distribution within their metal cores was achieved by avoiding terminal Pd-CO bonding and minimizing Pd-CO coordination. As a consequence, site preference and partial metal segregation were observed, as well as some substitutional and compositional disorders. Electrochemical and spectroelectrochemical studies revealed that [1]6- and [2]6- were redox active and displayed four and three stable oxidation states, respectively. Even though several redox active high nuclearity metal carbonyl clusters have been previously reported, the nanoclusters described herein represent the first examples of redox active Ni-Pd carbonyl alloy nanoclusters.

Elemental Sulfur/DMSO‐Promoted Multicomponent One‐pot Synthesis of Malonic Acid Derivatives from Maleic Anhydride and Amines

AbstractMalonic acid derivatives could be conveniently prepared with high degree of functional flexibility via redox condensation reactions between anhydride maleic, amines, elemental sulfur and DMSO as oxidant. This multicomponent decarboxylative transformation consists in a cascade of ring opening, decarboxylative oxidative thioamidation at temperature as low as 50 °C.magnified image

Unexpected Decarboxylation‐Triggered o‐Hydroxyl‐Controlled Redox Condensation of Phenylglycines with 2‐Nitrophenols in Aqueous Media

AbstractA decarboxylation‐triggered and o‐hydroxyl‐controlled hydrogen‐transfer strategy for the one‐pot synthesis of benzoxazoles from readily available amino acids and 2‐nitrophenols is reported. On the basis of this autoredox reaction, the C−N bond can be efficiently constructed to afford the desired products in moderate to good yields under transition‐metal‐free conditions in aqueous media.magnified image

Iron‐Catalyzed Sulfur‐Promoted Decyanative Redox Condensation of o‐Nitrophenols and Arylacetonitriles: An Unprecedented Route to 2‐Arylbenzoxazoles

Elemental sulfur in the presence of a catalytic amount of FeCl2·4H2O was found to be highly efficient for the promotion of decyanative redox condensation reactions of o‐nitrophenols with arylacetonitriles, to give a wide range of 2‐arylbenzoxazoles. The utility of elemental sulfur was highlighted by its role as cyanide scavenger and external reducing agent.

Zeolite imidazolate frameworks in catalysis: Synthesis of benzimidazoles via cascade redox condensation using Co-ZIF-67 as an efficient heterogeneous catalyst

Zeolite imidazolate framework Co-ZIF-67 was synthesized and utilized as a recyclable catalyst for the synthesis of benzimidazoles via the cascade redox condensation of 2-nitroanilines with benzylamines without added oxidizing or reducing agents. The Co-ZIF-67 catalyst expressed better performance in the generation of benzimidazoles than many MOFs and many conventional homogeneous catalysts. The transformation progressed via genuinely heterogeneous catalysis, and the formation of benzimidazoles via homogeneous catalysis due to the leaching phenomenon, if any, was not considerable. The cobalt-based framework catalyst could be reutilized many times for the reaction without losing its efficiency. The fact that the cascade redox condensation could proceed readily with a recyclable catalyst while additional reducing or oxidizing agents were not required would outperform previous synthetic protocols. To the best of our knowledge, this is the first heterogeneous catalytic approach to achieve benzimidazoles via the cascade redox condensation of 2-nitroanilines with benzylamines.