Current Green Chemistry

Lignocellulosic biomass, the most abundant and non-edible renewable source, is expected to be a promising alternative to fossil resources for the production of fine chemicals and biofuels. Typically, the utilization of biomass and its derivatives involves the selective cleavage or coupling of C-C and C-X (X = O, N) bonds, where toxic and environmentally unfriendly reagents should be avoided to meet the principles of green engineering and green chemistry.

This thematic issue intends to highlight current progress on the development of green catalytic systems and processes for selective synthesis of bioproducts, emphasizing the preparation of sustainable, functional materials and the use of correlated eco-friendly auxiliaries to boost the reaction rate and selectivity in the production process. Attention is also paid to the relevant reaction pathways and mechanisms. Five eminent research groups in the field of sustainable and green upgrading of biomass have kindly accepted our invitation to contribute to this special issue by providing an overview on the recent advances in selected topics briefly listed as follows:

Dr. Chang et al., in “A Review of Catalytic Upgrading of Biodiesel Waste Glycerol to Valuable Products” introduce the catalytic transformation of glycerol into various value-added chemicals such as propylene glycol, propanol, glyceraldehyde, dihydroxyacetone, hydrocarbons, and ethers via relevant conversion reactions including hydrogenation, oxidation, pyrolysis, gasification and etherification [1].

Dr. Zhang et al., in “Recent Progress on Heteropolyacids for Green Fuels Synthesis,” comprehensively summarize the studies on esterification/transesterification for the synthesis of biodiesel using various types of heteropolyacids (HPAs)-based catalysts including pristine HPAs, substituted HPAs, supported HPAs and encapsulated HPAs, and also discuss drawbacks and future challenges on HPAs-based catalytic systems for producing biodiesel [2]. \

Dr. Liu et al., in “Sustainable Conversion of Biomass-derived Carbohydrates into Lactic Acid using Heterogeneous Catalysts”, overview the state-of-the-art production of LA from triose, hexose, cellulose, and other biomass feedstocks catalyzed by heterogeneous acids and bases [3].

Dr. Sarvanamurugan et al., in “Rice Straw: A Major Renewable Lignocellulosic Biomass for Value-added Carbonaceous Materials”, describe the most recent advancement in the preparation method for carbonaceous materials derived from rice straw as well as their applications in various fields, with an emphasis on the influential reaction factors to improve the characteristic properties of the activated carbons [4]. \

Dr. Yang et al., in “Catalytic Transfer Hydrogenation of Biomass-derived Levulinates to γ-Valerolactone using Alcohols as Hdonors”, discuss representative examples on the catalytic production of γ-valerolactone (GVL) through the hydrogen transfer process with alcohols as hydrogen sources as well as various applications of GVL and its major feedstock ethyl levulinate (EL) [5]. As Guest Editors, we would like to appreciate all the authors for their valuable contributions and all the referees for their excellent work in reviewing the submitted manuscripts.

We are also grateful to the Editor-in-Chief Prof. György Keglevich and the Editorial Manager, Ms. Humaira Bilal, for their kind support in preparing this thematic issue.

The first part of the special Current Green Chemistry issue which you are keeping just in your hands contains 3 papers whose content is related to the title of the issue. The first manuscript co-authored by Prof. Michaela Gulea and Dr. Morgan Donnard presents progress in the synthesis of organic thiocyanates (OTC) over the past five years (from the end of 2015 to mid-2019). It is an update to the tutorial review published in the Chemical Society Reviews in 2016. Both reviews show that this topic, despite that the thiocyanate functional group has been known for decades, is still developing dynamically. This results in regular and often appearing new synthetic methodologies. The authors attempt to show sustainable aspects of the presented synthetic protocols related with thiocyanation or sulfur-cyanation. On 39 schemes present in the content, you will find very well balanced information regarding new synthetic protocols devoted to the reactions involving thiocyanation and reactions involving S-cyanation. The first part is subsequently divided into two subsections, which describe reactions with preservation of the SCN group and the procedures in which transformations of the SCN group take place. The second part devoted to reactions based on S cyanation presents three protocols used to convert disulphide or thiol into the corresponding SCN derivatives. The second paper co-authored by Prof. Lenardao and his colleagues presents Deep Eutectic Solvents (DES) as an alternative medium for the preparation of organosulfur compounds. This manuscript constitutes the first, comprehensive collection of synthetic procedures in which DES serve as a “green alternative” for the Volatile Organic Compounds (VOCs) and traditional ionic liquids (ILs), more or less commonly used as the reaction medium. A brief presentation of the twenty years old story of this solvent and its basic physicochemical properties which allow their use as a new reaction medium open this manuscript. This introduction is followed by sequential discussions on the use of DES as a reaction medium in procedures leading to a rich family of organosulfur derivatives based on the six general reactions (condensation, cyclization, ring-opening, C-S bond formation, one-pot reactions, C-H bond activation). For a few procedures mechanistic proposals complement experimental details. The third manuscript, co-authored by Prof. Uchiyama and his students, is an original article describing their successful attempts to develop procedures leading to a fluorescence fluoride ion sensor. The ability to detect fluoride ion based on this procedure is due to the colour change of N,N-bis(trimethylsilyl)-9-aminoanthracene, which under ultraviolet showed blue fluorescence originated from the anthracene skeleton due to orthogonal relation between anthracene skeleton and bis(trimethylsilyl)amino group, and 9-aminoanthracne, which shows green fluorescence due to conjugation that exists between the anthracene and the amino group.

The ever increasing chemical pollution associated with organic transformations is an alarming issue. To get rid of the hazardous chemical substances, numerous efforts have been made to modify chemical processes and make them environmentally sustainable. It has been well established that the adaptation of green credentials is the only way to attain this goal. As a result, the last decade has shown a tremendous outburst to adopt green techniques in organic transformations. This thematic issue intends to highlight some of the recent developments in this direction. After successful completion of the first part i.e. Part 1(A) with five important review articles [1-5], this part i.e. ‘Organic transformations by following green credentials- Part 1 (B)’ highlights some of the significant contributions towards the development of various organic transformations with special emphasis on greener aspects. The present issue consists of four review articles and two original research articles contributed by the renowned scientists working in this field. Nowadays, task specific ionic liquids have gained significant attention due to their environment-friendliness. These are being used as efficient catalysts as well as reaction media for various organic transformations. Metathesis reactions between carbonyls and olefins or alkynes are becoming the most popular tools for carbon-carbon bondforming reactions. In the first contribution, Das et al., [6] describe various strategies of carbonyl-alkyne and carbonyl-olefin metathesis reactions for the synthesis of complex molecules, natural products and pharmaceuticals with high atom-economy under environmentally benign reaction conditions. In the second contribution, Biswas et al., [7] demonstrate the catalytic applicability of the polymeric ion-exchange resin and polypeptide organic polymers as recoverable solid catalysts and the role of some organic polymers assisting catalyst recovery by accelerating the actual process of catalyst separation. In the third contribution, Saha et al., [8] summarize various catalytic applications of crystalline nano-ZnO towards the synthesis of diverse biologically promising heterocycles in aqueous media. The fourth contribution of this thematic issue by Padvi et al., [9] provides an up to date literature related to the preparation, characterization, physical properties and various applications of task-specific ionic liquids as catalysts or solvents in organic transformations. In the fifth contribution, Kumar et al., [10] and his group describe the synthesis of conformationally locked four different 3'- azido-C-4'-spirooxetano-xylonucleosides i.e., 3'-azido-3′-deoxy-C-4'-spirooxetano-xylothymidine, 3'-azido-3′-deoxy-C-4'- spirooxetano-xylouridine, 3'-azido-3′-deoxy-C-4'-spirooxetano-xylocytidine and 3'-azido-3′-deoxy-C-4'-spirooxetano-xylo adenosine with excellent yields via chemo-enzymatic convergent route. The last contribution, Singh et al., [11] describes a simple, straightforward and eco-friendly protocol developed by our research group for the efficient synthesis of pharmaceutically promising various indole derivatives viz, 3,3-bis(indol-3-yl)indolin- 2-ones, bis(indol-3-yl)(aryl)methanes and tris(indol-3-yl)methanes using a catalytic amount of mandelic acid as an efficient, naturally occurring, organo-catalyst in aqueous ethanol at room temperature. I am feeling honored and really grateful to all the contributors for their efforts to design this special thematic issue for Current Green Chemistry (CGC). I am very much thankful to the senior journal manager and the entire editorial team of Current Green Chemistry, especially Ms. Humaira Bilal, for her continuous support. No words are sufficient to express my gratitude to Prof. (Dr.) György Keglevich (Editor-in-Chief) for his unconditional help and encouragement in each step. I am also thankful to the respective reviewers for their valuable comments and suggestions to improve the quality of this special issue.

To protect our loving ‘Mother Nature’ from the ever increasing chemical pollution associated with the organic transformations, scientists are continuously trying to modify their chemical processes to make them environmentally benign and sustainable. As a result, the last decade has shown tremendous progress in green credentials in organic transformations. The special thematic issue ‘Organic transformations by following green credentials- Part 1 (A)’ intends to highlight the current progress on the development of various organic transformations with special emphasis on greener aspects. The present issue consists a total of five review articles contributed by the leading scientists working in this field. Nowadays, photo-irradiated technique is treated as the most sustainable strategy. Sometimes organic transformations are not possible to carry out at room temperature by using conventional strategies, but in many occasions, those reactions were accomplished easily under photochemical conditions. As a result, photo-induced strategy has become a valuable tool for the synthesis of diverse organic scaffolds. The first contribution of this thematic issue by Kaur et al. deals with an up-to-date literature on the photo-induced synthesis of various fused five-membered oxygen containing heterocycles. Recently, spirooxindoles, specially of 3,3'-spirooxindoles, have drawn considerable attention as many of these scaffolds possess significant biological activities. As a result, a large number of methods are being reported for the synthesis 3,3'- spirooxindoles under various reaction conditions. In the second contribution, Deepthi et al. summarizes the recent literature related to the synthesis of 3,3'-spirooxindoles by using green credentials that include aqueous mediated reactions, multicomponent strategies and photocatalysis. Aziridines are being used as versatile precursors of various naturally occurring bioactive compounds including amino sugars, β-lactams, alkaloids etc. They are also being used as a versatile intermediate in many organic syntheses due to their susceptibility to ring-opening by facile C-N bond cleavage. In the third contribution, Santra and his group summarized recent developments related to the synthesis of various aziridines scaffolds under different reaction conditions. During the last few years, constant efforts have been made to carry out C-H functionalization reaction under greener conditions. A number of modifications were reported for the C-H functionalization reactions to make them eco-friendly and sustainable. The fourth contribution by Sarkar and Mukhopadhyay describes the current development of C-H functionalization carried out in aqueous medium and its synthetic utilities according to carbon-carbon and carbon-heteroatom bond formations under green conditions. Pyrazole and its related derivatives are very common in naturally occurring bioactive molecules. Many drug molecules contain pyrazole as the core structural unit. As a result, a large number of methods were reported for the efficient synthesis of these significant scaffolds. The last contribution by Singh and Nath offers an up-to-date literature on the synthesis of various substituted and fused pyrazole scaffolds under greener conditions. Though today organic chemists are paying much attention to green chemistry practices, we need to adopt green credentials as much as possible in our protocols to fulfill the challenges of sustainability. I feel honored and really grateful to all the contributors for their efforts to design this special thematic issue for Current Green Chemistry (CGC). I am very much thankful to the senior journal manager and the entire editorial team of Current Green Chemistry specially Ms. Humaira Bilal madam for her continued support. No words are sufficient to express my gratitude to Prof. (Dr.) György Keglevich (Editor-in-Chief) Sir for his unconditional help and encouragement in each step. I am also thankful to the respective reviewers for their valuable comments and suggestions to improve the quality of this special issue.

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