Microwave-assisted protocol towards synthesis of heterocyclic molecules: a comparative analysis with conventional synthetic methodologies (years 2019-2023): a review

Microwave-assisted protocols have become extensively accepted across various scientific and technological domains because of their numerous advantages, shorter reaction times, higher yields, and often milder reaction conditions. In this review, we focus on the synthesis of N, O, and S-containing heterocyclic structural cores, crucial in the development of pharmaceuticals, agrochemicals, and materials science following through conventional and microwave method via eliminating the side products and enhances the product yield that is nowadays the biggest barrier for a synthetic chemist. The major findings emphasizes the substantial advantages of microwave-assisted techniques over conventional synthetic protocols. This comparative study underscores the potential of microwave-assisted techniques to revolutionize heterocyclic compound synthesis, providing insights into optimizing reaction conditions and expanding the scope of chemical synthesis in industrial applications.

Polyglycerol Sebacate Elastomer: A Critical Overview of Synthetic Methods and Characterisation Techniques

Poly (glycerol sebacate) is a widely studied elastomeric copolymer obtained from the polycondensation of two bioresorbable monomers, glycerol and sebacic acid. Due to its biocompatibility and the possibility to tailor its biodegradability rate and mechanical properties, PGS has gained lots of interest in the last two decades, especially in the soft tissue engineering field. Different synthetic approaches have been proposed, ranging from classic thermal polyesterification and curing to microwave-assisted organic synthesis, UV crosslinking and enzymatic catalysis. Each technique, characterized by its advantages and disadvantages, can be tailored by controlling the crosslinking density, which depends on specific synthetic parameters. In this work, classic and alternative synthetic methods, as well as characterisation and tailoring techniques, are critically reviewed with the aim to provide a valuable tool for the reproducible and customized production of PGS for tissue engineering applications.

Ultrafast process of microwave-assisted deep eutectic solvent to improve properties of bamboo dissolving pulp

Viscosity control and reactivity enhancement are critical to produce high-quality cellulose products, such as dissolving pulp, yet remain challenging. In this work, an ultrafast process, namely microwave-assisted deep eutectic solvent (MW-DES), is proposed for this purpose. It is based on the hypothesis that the MW-DES process can deliver an enhanced synergy: a simultaneous fiber swelling and cellulose depolymerization via hydrogen-bonding break-up and acid hydrolysis from the actions of polar and acidic DES further boosted under MW irradiation. Results showed that after the MW-DES (Choline chloride- oxalic acid, ChCl-OA) treatment for only 40 s, the pulp viscosity decreased from 715 to 453 mL/g, and the reactivity increased from 43.0 % to 84.6 %, which is ultrafast in comparison with those reported work. Furthermore, DES in the process shows a high reusability and chemical stability, thus offering a simple, sustainable and effective alternative for upgrading of dissolving pulp, particularly, using non-wood materials of bamboo.

Microflow chemistry and its electrification for sustainable chemical manufacturing

Sustainability is vital in solving global societal problems. Still, it requires a holistic view by considering renewable energy and carbon sources, recycling waste streams, environmentally friendly resource extraction and handling, and green manufacturing. Flow chemistry at the microscale can enable continuous sustainable manufacturing by opening up new operating windows, precise residence time control, enhanced mixing and transport, improved yield and productivity, and inherent safety. Furthermore, integrating microfluidic systems with alternative energy sources, such as microwaves and plasmas, offers tremendous promise for electrifying and intensifying modular and distributed chemical processing. This review provides an overview of microflow chemistry, electrification, their integration toward sustainable manufacturing, and their application to biomass upgrade (a select number of other processes are also touched upon). Finally, we identify critical areas for future research, such as matching technology to the scale of the application, techno-economic analysis, and life cycle assessment.

Semiflow Microwave Heating Reactor with Resonator Moving Mechanism Applied to Zeolite Synthesis

A semiflow microwave (MW) heating reactor similar to a flow reactor system was developed. Slurry raw materials in the reaction tube were heated continuously and cooled rapidly by moving a thin MW resonator instead of flowing slurry raw materials. From highly viscous mother slurries, Linde-type A (LTA) and faujasite (FAU)-type zeolite nanoparticles of small crystal grains were synthesized quickly. Results show that this heating system can synthesize hydroxy-sodalite (SOD)-type zeolite from coal fly ash particles including those larger than 50 μm. Numerical calculations using the COMSOL Multiphysics program revealed the thermal distribution of liquids of various viscosities using the semiflow MW heating reactor.

Revisiting aromatic diazotization and aryl diazonium salts in continuous flow: highlighted research during 2001–2021

Aryl diazonium salts play an important role in chemical transformations; however their explosive nature limits their applications in batch.

Impact of Microwaves on Organic Synthesis and Strategies toward Flow Processes and Scaling Up

Microwave-assisted organic synthesis has been widely studied and deliberated, opening up some controversial issues as well. Nowadays, microwave chemistry is a mature technology that has been well demonstrated in many cases with numerous advantages in terms of the reaction rate and yield. The strategies toward scaling up find an ally in continuous-flow reactor technology comparing dielectric and conductive heating.

A Two-Step Microwave Annealing Process for PAN Pre-Oxidation through a TM-Mode Cavity

A novel microwave annealing system and a specific processing condition are proposed for the pre-oxidation of carbon fiber. The microwave annealing system consists of a TM-mode resonant cavity and a silicon carbide (SiC) susceptor. The TM-mode cavity enhances the electric field at the center. The SiC susceptor absorbs part of the microwave energy and converts it to heat. The enhanced fields and the SiC susceptor provide both nonthermal and thermal treatments for fibrous materials with various dielectric properties. Furthermore, a two-step microwave annealing process is used to oxidize polyacrylonitrile (PAN) fiber. The scanning electron microscopy (SEM) images, differential scanning calorimetry (DSC), and X-ray diffraction (XRD) results support the theory that the microwave annealing can achieve a high aromatic index of 66.39% in just 13 min, 9 times faster than the traditional processing time. The results of the Raman spectra also illustrate that the sheath-core factor of the microwave-heated specimen is closer to one than that of the conventional furnace-heated type, which agree with the images of the cross-section area.

Process intensification using immobilized enzymes for the development of white biotechnology

Process intensification of biocatalysed reactions using different techniques such as microwaves, ultrasound, hydrodynamic cavitation, ionic liquids, microreactors and flow chemistry in various industries is critically analysed and future directions provided.

Emerging Concepts in Carbon Nitride Organic Photocatalysis

Carbon nitrides encompass a class of transition-metal-free materials possessing numerous advantages such as low cost (few Euros per gram), high chemical stability, broad tunability of redox potentials and optical bandgap, recyclability, and a high absorption coefficient (>10⁵  cm^-1 ), which make them highly attractive for application in photoredox catalysis. In this Review, we classify carbon nitrides based on their unique properties, structure, and redox potentials. We summarize recently emerging concepts in heterogeneous carbon nitride photocatalysis, with an emphasis on the synthesis of organic compounds: 1) Illumination-Driven Electron Accumulation in Semiconductors and Exploitation (IDEASE); 2) singlet-triplet intersystem crossing in carbon nitride excited states and related energy transfer; 3) architectures of flow photoreactors; and 4) dual metal/carbon nitride photocatalysis. The objective of this Review is to provide a detailed overview regarding innovative research in carbon nitride photocatalysis focusing on these topics.

C-Alkylation of alkali metal carbanions with olefins

C-Alkylations of alkali metal carbanions with olefins, first reported five decades ago, is a class of reaction undergoing a resurgence in organic synthesis in recent years. As opposed to expectations from classical chemistry and transition metal-catalysis, here olefins behave as closed-shell electrophiles. Reactions range from highly reactive alkyllithiums giving rise to anionic polymerization, to moderately reactive alkylpotassium or alkylsodium compounds that give rise to defined, controlled and bimolecular chemistry. This review presents a brief historical overview on C-alkylation of alkali metal carbanions with olefins (typically mediated by KOtBu and KHMDS), highlights contemporary applications and features developing mechanistic understanding, thereby serving as a platform for future studies and the widespread use of this class of reaction in organic synthesis.

A Novel Combined Microstrip Resonator/Nanospray Ionization Source for Microwave-Assisted Trypsin Digestion of Proteins

Enzymatic digestion of proteins is a critical step in bottom-up and middle-down proteomics. Here, we demonstrate a method for decreasing the time required for proteolytic digestion of proteins from multiple hours to minutes by using an in-line microstrip cavity for programmed microwave heating. When a nanospray emitter tip, containing a digestion sample, is exposed to a region of highly focused microwave field, the rate of proteolytic digestion is enhanced and the time required for digestion greatly decreased. The design is advantageous for mass spectrometry because the solution-based digestion can then be directly sprayed from a nanoelectrospray tip emitter, decreasing sample transfer loss and allowing the system to be used in a flow-through proteolytic workflow. Microwave-assisted digestion using this method is evaluated against standard overnight digestion protocols using a variety of proteins, evaluating sequence coverage and observed peptide location, digestion rate, and overall efficacy. The influence of applied microwave power is investigated, and enzymatic kinetic parameters are evaluated to estimate temperature within the microreactor. Finally, the modulation of the proteolytic digestion of proteins based upon the modulation of applied microwave power is demonstrated on a time scale of seconds in a flow-through system.

Continuous Flow Esterification of a H-Phosphinic Acid, and Transesterification of H-phosphinates and H-Phosphonates under Microwave Conditions

The microwave (MW)-assisted direct esterification of phenyl-H-phosphinic acid, transesterification of the alkyl phenyl-H-phosphinates so obtained, and the similar reaction of dibenzyl phosphite (DBP) were investigated in detail, and the batch accomplishments were translated into a continuous flow operation that, after optimization of the parameters, such as temperature and flow rate, proved to be more productive. Alcoholysis of DBP is a two-step process involving an intermediate phosphite with two different alkoxy groups. The latter species are of synthetic interest, as precursors for optically active reagents.

Microwave Heating Outperforms Conventional Heating for a Thermal Reaction that Produces a Thermally Labile Product: Observations Consistent with Selective Microwave Heating

Microwave (MW) heating is more effective than conventional (CONV) heating for promoting a high-temperature oxidative cycloisomerization reaction that was previously reported as a key step in a total synthesis of the natural product illudinine. The thermal reaction pathway as envisioned is an inverse electron-demand dehydro-Diels-Alder reaction with in situ oxidation to generate a substituted isoquinoline, which itself is unstable to the reaction conditions. Observed reaction yields were higher at a measured bulk temperature of 200 °C than at 180 °C or 220 °C; at 24 hours than at earlier or later time points; and when the reaction solution was heated using MW energy as opposed to CONV heating with a metal heat block. Selective MW heating of polar solute aggregates is postulated to explain these observations.