Water is the most precious and essential element for the sustainability of life. It has emerged as a versatile solvent for various chemical transformations in recent times. It is a naturally abundant, cheap, non-toxic, inexhaustible, and non-flammable green solvent that possesses several unique physiochemical properties like hydrogen bonding, stays in a liquid state at a high range of temperatures, high dielectric constant, large surface tension, and heat capacity. These features make water a powerful domain for the eco-friendly and green synthesis of heterocycles via both in-water and on-water methods. Diversified heterocyclic moieties are formed in an aqueous phase through various organic reactions like multi-component reactions (MCRs), pericyclic, Wittig, Michael, Mannich, Aldol, Suzuki, Sonogashira, hydroformylation, and other organocatalyzed reactions with high atom economy, stereo-selectivity, and sustainability. This article gives a systematic, comprehensive, and authoritative study of a range of reactions in which water is used as a solvent for the synthesis of heterocycles. This article endows an impetus to explore the synthetic and mechanistic aspects of “on” and “in” water reactions and gives insights into the divergence between on-water and inwater synthesis.

Functionalized organic-inorganic hybrids of ionic liquids (ILs) with Keggin polyoxometalate (POM) anions of heteropolyacids have received considerable attention in the catalytic study as homogeneous/heterogeneous catalysts owing to their combined unique structural flexibility of the POM anions and tunable behavior imparted by the ionic liquids to the organic-inorganic hybrid of POMs (IL-POM). This includes enhancement of thermal and chemical stability, acidity, surface activity, redox properties, etc. The attachment of Brønsted acidic sites such as -COOH or –SO3H on organic cations of the hybrid materials provides a strong acidic character that can make them appropriate for the replacement of non-recyclable liquid Brønsted acid catalysts in organic reactions. Hydrophobicity, reusability, recyclability, and productivity of these materials certainly satisfy the need for advanced organic synthesis. A large number of designable target-oriented hybrids have been studied as recyclable homogeneous/heterogeneous catalytic systems to explore their industrial values for oxidation and acid-catalyzed organic reactions. This chapter gives a brief introduction to the development of acidic ionic liquid-based POMs hybrid material with varied organic cations tethered with Brønsted acidic sites and their beneficial effects in catalytic organic reactions. The content of this report may provide enormous scope for the development of industrial-scale organic processes based on the IL-POMs hybrid catalysts.

Most of the heterocyclic compounds are pharmaceutically active and have numerous applications in various industries. Rigorous effects have been done and still going on in the search for more dynamic and advantageous compounds. Many researchers have focused their work on the synthesis of lower member heterocyclic compounds. However, not many efforts have been made for the higher membered compounds, though having equipotency. The aim of the current study is to collect and document the data available in the seven-membered heterocyclic compounds i.e. oxepin and thiepine compounds. The present study includes natural sources, drugs, synthetic methods, reactions, and biological activities of these compounds and their derivatives. Various seven-membered heterocyclic compounds have been synthesized via Friedel-craft Cyclization, Ullmann cross-coupling, Sonogashira coupling, and etc. However, biological potencies have still not been explored much by the researchers. This encouraged us to write this book chapter, as it has too much scope for future research.

Nitriles are organic compounds bearing the -C≡N functional group and thus derivatives of hydrocyanic acid. Besides their application in polymers, nitriles are also present in more than 30 pharmaceuticals, including antidepressants, antidiabetics, and anticancer drugs, since they can act as hydrogen acceptors, increase water solubility and shield the drug from oxidative metabolism. Moreover, nitrile derivatives are usually stable and inexpensive and can be converted into many functional groups and heterocycles, and, therefore, are very interesting precursors in the synthesis of robust molecular libraries in medicinal chemistry. It is also important to mention that 5- membered azaheterocycles are of the utmost importance in the medicinal chemistry field, being present in many marketed top-selling drugs. In this context, this chapter will provide an update on the synthesis of prominent 5-membered azaheterocycles from nitriles, focusing on the most interesting reactions and methodologies reported from 2014-2020.

In synthetic organic chemistry, heterogeneous catalysis has opened a new era for progressive green and eco-benign synthesis. Fundamentally, it is interminably interesting and perennially novel. Carbon materials are widely used for renewable energy and environmental studies. Significant advancements have been achieved in modern organic chemistry by replacing conventional acid catalysts with pollution-free, recyclable, and eco-benign solid acid catalysts to reduce toxicity and increase efficiency. Solid acid catalysts play a profound role in organic synthesis as they are heterogeneous, recyclable, ease of workup, corrosion-free, immiscible in routine organic solvents, easy recovery from the products, selective synthesis, etc. Furthermore, carbon-based solid acid catalysts have attracted scientists because of its simple preparation method, increased acidity, selectivity, high stability, easy separation of products, no corrosion of equipment, easy recovery and recyclability, etc. They exhibit remarkable catalytic activity for various acid-catalyzed reactions, such as biofuel production, hydration, hydrolysis, and esterification of higher fatty acids. This article profoundly discusses the synthesis, properties, and applications of carbon-based solid acid catalysts in organic synthesis.