Graphene Oxide as a Carbocatalyst for a Diels-Alder Reaction in an Aqueous Medium

Graphene oxide-mediated thermo-reversible bonds and in situ grown nano-rods trigger 'self-healable' interfaces in carbon fiber laminates

Carbon fiber reinforced epoxy (CFRE) laminate structures have emerged as futuristic materials having surpassed metals in strength and durability. The interfacial chemistry determines the mechanical performance of such laminates. In this study, a unique approach was adopted wherein the alternate layers of the carbon fiber (CF) mat were grown in situ with ZnO nano-rods and modified with bis-maleimide (BMI), and epoxy resin containing 0.2 or 0.5 wt% graphene oxide (GO) was infused using conventional VARTM technology to enhance the mechanical interlocking of epoxy with the fiber as well as to impart self-healing properties to the laminate. While ZnO rods offer surface roughness thereby facilitating better wetting of epoxy, the Diels-Alder thermo-reversible bonds between BMI and GO facilitate self-healing properties besides improving the interfacial adhesion between epoxy and CF. The rationale behind this work is to synergistically improve the interface-dominated mechanical properties like interlaminar shear strength (ILSS) while maintaining or even improving fiber-dominated properties like flexural strength (FS) as well as imparting considerable recovery in strength post the self-healing cycle. The laminates after this treatment (having 0.5 wt% GO) indeed exhibited 46% improvement in FS and 33% improvement in ILSS properties as well as an ILSS recovery of 70%. The surface analysis suggests that ZnO nano-rods offer surface roughness that helps in the wettability of the matrix on the fibers. In addition, the 2D and 3D representative volume analysis (RVE) model was established to identify the load transfer behaviour in the ZnO-CF-epoxy interface in the microscale reference region. The fractographic analysis confirmed that rigid ZnO nano-rods allowed better matrix adhesion resulting in improved mechanical performance.

Diels-Alder Cycloaddition Reactions in Sustainable Media

Diels–Alder cycloaddition reaction is one of the most powerful strategies for the construction of six-membered carbocyclic and heterocyclic systems, in most cases with high regio- and stereoselectivity. In this review, an insight into the most relevant advances on sustainable Diels–Alder reactions since 2010 is provided. Various environmentally benign solvent systems are discussed, namely bio-based derived solvents (such as glycerol and gluconic acid), polyethylene glycol, deep eutectic solvents, supercritical carbon dioxide, water and water-based aqueous systems. Issues such as method’s scope, efficiency, selectivity and reaction mechanism, as well as sustainability, advantages and limitations of these reaction media, are addressed.

One-pot bottom-up synthesis of a 2D graphene derivative: application in biomolecular recognition and nanozyme activity

The synthesis of two-dimensional (2D) nanosheets such as graphene and its derivatives through a bottom-up approach has many advantages such as growth control and functionalization, but it is always challenging to get the desired material. Herein, we have reported the synthesis of water soluble 2D-nanosheets through a bottom-up approach from 2,4,6-tribromo-3-hydroxybenzoic acid via a self-coupling pathway and characterized them using several techniques. AFM and TEM analyses reveal that the synthesized material has a layered structure with a thickness of ∼1.2 nm. Also, the prepared nanosheets are amorphous in nature with high negative charge (-38 ± 2.5 mV). The flexible nature of 2D-nanosheets and their functionality can be used in many related applications. Therefore, we have utilized the synthesized 2D-nanosheets in biomolecular recognition studies. It was found that the enzymatic activity of α-chymotrypsin can be controlled reversibly in the presence of the synthesized 2D-nanosheets. The kinetic study revealed that the nanosheet surface selectively binds to the active sites of the enzyme through a competitive pathway. Furthermore, we explored the nanozyme activity of the material in a peroxidase-like activity assay of two bio-active molecules: Nicotinamide Adenine Dinucleotide Phosphate (NADH) and dopamine. The results suggest that the prepared material efficiently catalyzed the oxidation of NADH to biological cofactor NAD⁺ and dopamine to aminochrome in the presence of H₂O₂. These synthesized graphene-like 2D-nanosheets with functional groups can be further tuned with other functionalities, which can open a new window for other related applications.

Graphene Oxide as a Mediator in Organic Synthesis: a Mechanistic Focus

Graphene oxide (GO) is experiencing growing interest by synthetic organic chemists as a promoter of chemical transformations. The synergistic role of the multiple functionalities featuring the nanostructured carbon materials and their π-domains enables the interplay of specific activation modes towards organic compounds that can explore unprecedented chemical modifications. A detailed comprehension of the mechanistic details that govern the transformations guided by GO is a not fully solved task in the field. In this direction, more sophisticated and diversified techniques are employed, providing insights towards intriguing activation modes exerted by the π-matrix and the oxygenated/sulfonate groups decorating the functionalized nano-carbon material. The present Minireview accounts for a critical survey of the most recent developments in the area of GO-mediated organic transformations with a specific focus on mechanist aspects.

Allylic and Allenylic Dearomatization of Indoles Promoted by Graphene Oxide by Covalent Grafting Activation Mode

The site-selective allylative and allenylative dearomatization of indoles with alcohols was performed under carbocatalytic regime in the presence of graphene oxide (GO, 10 wt % loading) as the promoter. Metal-free conditions, absence of stoichiometric additive, environmentally friendly conditions (H₂ O/CH₃ CN, 55 °C, 6 h), broad substrate scope (33 examples, yield up to 92 %) and excellent site- and stereoselectivity characterize the present methodology. Moreover, a covalent activation model exerted by GO functionalities was corroborated by spectroscopic, experimental and computational evidences. Recovering and regeneration of the GO catalyst through simple acidic treatment was also documented.

Bio-reduction of Graphene Oxide: Catalytic Applications of (Reduced) GO in Organic Synthesis

This work is based on various bio-reduction of graphene oxide into reduced graphene oxide and their applications in organic synthesis and group transformations. Graphene oxide, with abundant oxygencontaining functional groups on its basal plane, provides potential advantages, including excellent dispersibility in solvents and the good heterogeneous catalyst. This manuscript reviews various methods of synthesis of graphene and graphene oxide and a comparative study on their advantages and disadvantages, how to overcome disadvantages and covers extensive relevant literature review. In the last few years, investigation based on replacing the chemical reduction methods by some bio-compatible, chemical/impurity-free rGO including flash photo reductions, hydrothermal dehydration, solvothermal reduction, electrochemical approach, microwave-assisted reductions, light and radiation-induced reductions has been reported. Particularly, plant extracts have been applied significantly as an efficient reducing agent due to their huge bioavailability and low cost for bio-reduction of graphene oxide. These plant extracts mainly contain polyphenolic compounds, which readily get oxidized to the corresponding unreactive quinone form, which are the driving force for choosing them as bio-compatible catalyst. Currently, efforts are being made to develop biocompatible methods for the reduction of graphene oxide. The reduction abilities of such phytochemicals have been reported in the synthesis and stabilization of various nanoparticles viz. Ag, Au, Fe and Pd. Various part of plant extract has been applied for the green reduction of graphene oxide. Furthermore, the manuscript describes the catalytic applications of graphene oxide and reduced graphene oxide nanosheets as efficient carbo-catalysts for valuable organic transformations. Herein, important works dedicated to exploring graphene-based materials as carbocatalysts, including GO and rGO for organic synthesis including various functional group transformations, oxidation, reduction, coupling reaction and a wide number of multicomponent reactions have been highlighted. Finally, the aim of this study is to provide an outlook on future trends and perspectives for graphene-based materials in metal-free carbo-catalysis in green synthesis of various pharmaceutically important moieties.

2D-MoS2 photocatalyzed cross dehydrogenative coupling reaction synchronized with hydrogen evolution reaction

The chemically exfoliated 2D-MoS₂ has a mixture of 1T (metallic) and 2H (semiconducting) phases.

Heterogeneous graphene oxide as recyclable catalyst for azomethine ylide mediated 1,3 dipolar cycloaddition reaction in aqueous medium

Graphene oxide (GO) catalyzed regio and diastereoselective synthesis of spiro-indenoquinoxaline pyrrolizidines and spiro-oxindoles pyrrolizidines is described with good substrate scope and yield using azomethine ylide under aq. EtOH condition at RT.