Current Green Chemistry - Volume 8, Issue 1, 2021

The environmental pollution and hazards have become the prime topic of concern in the twenty-first-century world and a warning to existing mankind. The researchers and chemists are also putting their way forward to combat the alarming situation across the world. In this context, reactions that are being carried out in water have attracted attention from the past decade. The conventional organic synthesis relies profoundly on organic solvents for a huge number of reactions, mainly for dissolving the compounds and assisting chemical reactions, because a number of reagents and catalysts are either unsuited or immiscible in water. In spite of the constraints, substantial efforts have been dedicated to overcome the problems because water not only reduces the environmental hazards of organic synthesis but also may benefit chemical reactions by delivering unforeseen selectivities and reactivities. The development of water-compatible/water-stable catalysts has emerged as a troubleshooter, eliciting a quantum leap in organic synthesis in water. Use of water has simplified the workup procedure too, as organic compounds are typically insoluble in water, extraction with suitable organic solvents can separate the compounds from catalysts, and the catalysts can also be separated and reused by centrifugation and filtration. This review deals with advances in catalytic methods under aqueous medium over the past few years.

Nowadays, it is of utmost importance to design synthetic methods, which can be utilized for the generation of substances that will minimize toxicity to the health of humans and the environment. The utilization of acid catalysts generates lots of corrosive and harmful wastes, which have to be treated with appropriate alkalis. Hence, it generates lots of sludge and alarms environmental issues of its storage and disposal. Zeolites and Zeotypes, by virtue of their peculiar properties, such as specific morphology, porosity and residing acidity, are gaining enormous attention as they replace harmful acid catalysts efficiently and also reduce chemical waste in the industrial process; hence, emerged as a new plethora in the field of “Green Chemistry”.

Nowadays, the use of carbocatalyst has attracted much attention due to its sustainability and affordability. Graphene oxide (GO) and sulfonated graphene oxide (SGO) have emerged as environmental friendly, metal free, inexpensive carbocatalyst for the synthesis of organic compounds. Herein, the profound role of GO and SGO in different organic transformation has been established. The GO and SGO being heterogeneous in nature, have high thermal stability over homogeneous catalyst in successive runs and can easily be recovered. The presence of oxygen containing acidic groups in GO and SGO mainly catalyzes the organic reactions. This mini review is emphasized on the catalytic role of GO and SGO as green and reusable solid acid catalyst in different organic syntheses.

Organic synthesis, under environment-friendly conditions, has a great impact on sustainable development. In this context, visible light photocatalysis has emerged as a green model, as this offers an energy-efficient pathway towards the organic transformation. Different transition-metal catalysts (Ir-, Ru-, Cu- etc.) and organic dyes (eosin Y, rose bengal, methylene blue, etc.) are wellknown photocatalysts in organic synthesis. Apart from the well-known organophotoredox catalysts, rhodamines (Rhodamine B and Rhodamine 6G) have also been employed as efficient photocatalysts for different organic transformations. In this review, we will focus on the photocatalysis by rhodamines in organic synthesis. The mechanistic pathway of the methodologies will also be discussed. We believe this review will stimulate the employment of rhodamines in the visible light photocatalysis for efficient organic transformations in the future.

Green chemistry methods are important modern synthetic chemistry pathways with main advantages such as milder reaction conditions, shorter time, and higher yields. A series of quinazolinone based Schiff bases (1-27) have been successfully synthesized by using green synthetic methods, including microwave-assisted, ultrasound-assisted and mechanochemical synthesis, respectively. Desired compounds were prepared from 3-amino-2-methylquinazolin-4(3H)-one and substituted benzaldehydes with moderate to high yields (11-90%). Benzoxazinone and 3-amino-2- methylquinazolin-4(3H)-one, the precursors of Schiff bases, were prepared by microwave-induced synthesis. The structures of all Schiff bases are confirmed by ¹H and ¹³C NMR spectroscopy and mass spectrometry as well. A novelty of this research is the application and comparison of green chemistry methods in the synthesis of desired compounds which are listed above. The simplicity of synthesis includes ethanol as a solvent and no need for further purification.

A natural driving force is always working behind the synthetic organic chemists towards developing ‘green’ synthetic methodologies for the synthesis of useful classes of organic molecules having potential applications. The majority of the essential classes of organic transformations, including C-C and C-X (X = heteroatom) bond-forming cross-coupling reactions, and cross dehydrogenative- coupling (CDC) mostly rely on the requirement of transition-metal catalysts and hazardous organic solvents. Hence, the scope of developing green synthetic strategies by avoiding the use of transition-metal catalysts and hazardous organic solvents for these important and useful classes of organic transformations is very high. Consequently, several attempts have been made so far. Water is the most abundant, cheap, and green solvent globally. Therefore, numerous synthetic methods have been developed in an aqueous medium. In this review, the development of transition-metalfree green synthetic strategies for various important classes of organic transformations such as C-C and C-X bond-forming cross-coupling, cross dehydrogenative-coupling, and oxidative-coupling in an aqueous media is discussed.