Inductive heating and flow chemistry - a perfect synergy of emerging enabling technologies

Inductive heating has developed into a powerful and rapid indirect heating technique used in various fields of chemistry, but also in medicine. Traditionally, inductive heating is used in industry, e.g., for heating large metallic objects including bending, bonding, and welding pipes. In addition, inductive heating has emerged as a partner for flow chemistry, both of which are enabling technologies for organic synthesis. This report reviews the combination of flow chemistry and inductive heating in industrial settings as well as academic research and demonstrates that the two technologies ideally complement each other.

Mechanically strong multifunctional three-dimensional crosslinked aramid nanofiber/reduced holey graphene oxide and aramid nanofiber/reduced holey graphene oxide/polyaniline hydrogels and derived films

To endow high mechanical strength and thermal stability aramid nanofibers (ANF) with novel functionality will lead to great applications. Herein, a strategy to generate covalent bonds among components towards obtaining uniform ANF/reduced holey graphene oxide (ANF/rHGO) and ANF/rHGO/polyaniline (ANF/rHGO/PANI) hydrogels with high mechanical properties is proposed through solvent exchange gelation and subsequent hydrothermal treatment. The as-prepared ANF/rHGO and ANF/rHGO/PANI hydrogels demonstrate excellent recoverability at high compressive strength of 20.2 and 13.8 kPa with a strain of 34.4% and 30.6%, respectively, compared to a recoverability of 92.5% at a strain of ∼20% for ANF hydrogels. Moreover, ANF/rHGO and ANF/rHGO/PANI aerogels possess fast and high oil absorption capacity of 38.9-64.1 g g^-1 and 24.5-44.0 g g^-1, respectively. ANF/rHGO and ANF/rHGO/PANI films obtained after vacuum-drying exhibit a high tensile strength of 121.4 and 95.5 MPa, respectively. Additionally, ANF/rHGO/PANI thin films present good selective absorption of visible light by controlling the doping level of PANI. ANF/rHGO/PANI aerogel films prepared by freeze-drying are assembled into flexible solid-state symmetric supercapacitors and deliver a favorable specific capacitance of 200 F g^-1, a desirable capacitance retention of 98.9% after 2500 mechanical bending cycles and an approximately 100% capacitance retention even after keeping tensile force for 15 h. The as-prepared hydrogels, aerogels and derived films with such excellent performances are promising for applications in oil pollution removal, optical filters and flexible load-bearing energy storage devices.

Direct Amide Synthesis over Composite Magnetic Catalysts in a Continuous Flow Reactor

Composite magnetic catalysts containing different amounts of sulfated titania (33–50 wt %) have been prepared by means of high energy ball-milling between TiO2 and NiFe2O4. The catalysts have been characterized with N2 adsorption/desorption isotherms, XRD, temperature programmed oxidation (TPO) and vibrating sample magnetometer (VSM). The catalytic activity was measured in the reaction of aniline and 4-phenylbutyric acid in the continuous mode under conventional and inductive heating. The effect of catalyst loading in the reactor on reaction and deactivation has been studied, indicating the catalyst containing 50 wt % titania gave the highest reaction rate and least deactivation. The operation in a flow reactor under inductive heating increased the amide yield by 25% as compared to conventional heating. The initial reaction rate decreased by 30% after a period of 15 h on stream. The catalyst activity was fully restored after a treatment with an air flow at 400 °C.

Solvent-Free Iron(III) Chloride-Catalyzed Direct Amidation of Esters

Amide functional groups are prominent in a broad range of organic compounds with diverse beneficial applications. In this work, we report the synthesis of these functional groups via an iron(iii) chloride-catalyzed direct amidation of esters. The reactions are conducted under solvent-free conditions and found to be compatible with a range of amine and ester substrates generating the desired amides in short reaction times and good to excellent yields at a catalyst loading of 15 mol%.

Lewis acidity quantification and catalytic activity of Ti, Zr and Al-supported mesoporous silica

Water-tolerant metal supported Lewis acids were synthesized for the catalytic amidation of electron-poor and bulky amines.

Nonclassical Routes for Amide Bond Formation

The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .

Direct amide synthesis over core–shell TiO2@NiFe2O4 catalysts in a continuous flow radiofrequency-heated reactor

A core–shell TiO₂@NiFe₂O₄ catalyst showed high activity and stability in direct amide synthesis with easy regeneration from coke by a treatment with a 30 wt% hydrogen peroxide solution.

An azobenzene-containing metal–organic framework as an efficient heterogeneous catalyst for direct amidation of benzoic acids: synthesis of bioactive compounds

An Zr-MOFs was demonstrated to be an efficient heterogeneous catalyst for direct amidation.

Catalytic amide formation from non-activated carboxylic acids and amines

The amide functionality is found in a wide variety of biological and synthetic structures such as proteins, polymers, pesticides and pharmaceuticals. Due to the fact that synthetic amides are still mainly produced by the aid of coupling reagents with poor atom-economy, the direct catalytic formation of amides from carboxylic acids and amines has become a field of emerging importance. A general, efficient and selective catalytic method for this transformation would meet well with the increasing demands for green chemistry procedures. This review covers catalytic and synthetically relevant methods for direct condensation of carboxylic acids and amines. A comprehensive overview of homogeneous and heterogeneous catalytic methods is presented, covering biocatalysts, Lewis acid catalysts based on boron and metals as well an assortment of other types of catalysts.

Fast TiO2-catalyzed direct amidation of neat carboxylic acids under mild dielectric heating

The development of green protocols for amide bond formation is a major socioeconomic goal for chemical and pharmaceutical industries and an important challenge for academic research.

Direct amide formation using radiofrequency heating

We present a simple method for direct and solvent-free formation of amides from carboxylic acids and amines using radiofrequency heating. The direct energy coupling of the AC magnetic field via nickel ferrite magnetic nanoparticles enables fast and controllable heating, as well as enabling facile work-up via magnetic separation.

A facile, rapid, one-pot regio/stereoselective synthesis of 2-iminothiazolidin-4-ones under solvent/scavenger-free conditions

A rapid and efficient one pot solvent/scavenger-free protocol for the synthesis of 2-iminothiazolidin-4-ones has been developed. Interestingly, the regio/stereoselective synthesis affords the regioisomeric (Z)-3-alkyl/aryl-2-(2-phenylcyclohex-2-enylimino)thiazolidin-4-one as the sole product in good yield. The selectivities observed have been rationalized based on the relative magnitude of the allylic strains developed during the course of the reaction. This is the first report wherein the impact of allylic strains in directing the regiocyclization has been noted.