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Carbon–carbon and carbon–heteroatom bond-formation reactions catalyzed by benign and inexpensive metals are of much interest in organic synthesis, as these reactions provide green and cost-effective routes. This account summarizes our recent contributions to the construction of carbon–carbon and carbon–heteroatom bonds by using benign-metal catalysts. A number of carbon–heteroatom bond formations, including C–N, C–O, C–S, C–Se, C–Te, and C–P bond formations, are discussed. Mechanistic insights into several reactions are also reported1 Introduction2 C–C Bond Formation3 C–N and C–O Bond Formation4 Carbon–Chalcogen (C–S, C–Se, C–Te) and C–P Bond Formation5 Conclusions

A green protocol has been developed for the synthesis of simple coumarins and linear pyrano[2,3-f] and [3,2-f]indoles by the reaction of phenol derivatives with β-ketoesters under ball milling at ambient temperature in the presence of methanesulfonic acid.

We have highlighted an example of a natural process like photosynthesis to introduce the subject of Green Chemistry. Photosynthesis can be an ideal example to illustrate a green process explaining all the features such as selection of raw materials, solvent, catalyst, energy, etc. for an environment-friendly reaction. From the same reaction, all the principles of Green Chemistry can be derived in a simpler way without the need of memorizing these in a set language. In this article, a few examples of green procedures for the synthesis of useful molecules have been illustrated in light of the knowledge of photosynthesis. The visible-light mediated reactions, organic reactions in water, and solvent-free organic reactions are discussed here for a practical illustration of Green synthesis.

A variety of procedures for the carbon–nitrogen, carbon–oxygen, carbon–sulfur and carbon–selenium bond formation using copper nanoparticles in greener conditions have been highlighted. The synthesis of several heterocyclic compounds of biological importance has also been reported using these protocols.

The carbon-chalcogen bond formation is of much importance as organochalcogenides scaffold, and in general, it shows by organochalcogenide scaffolds, in general, show promising biological activities and many compounds containing chalcogenide units are currently used as drugs, agrochemicals and useful materials. Thus, a plethora of methods has been developed for the formation of carbonchalcogen bonds. This review covers the recent developments on the formation of carbon-chalcogen bonds under microwave irradiation and synthesis of useful chalcogenides by employing this process.

Mechanochemical approaches in an organic synthesis have received increased interest because of their wide applications in green methodologies. Ball milling has been fruitfully utilized in the synthesis of various (elemental) organic scaffolds, including common drugs or drug candidates. However, the utilization of ball milling in asymmetric organic synthesis has not been adequately investigated. Thus, the aim of this review is to highlight the attractive features of the applications of mechanochemistry via ball milling in asymmetric organic syntheses in order to attract the attention of a wider section of the synthesis community.

In the last few decades, ball-milling has received tremendous attention as a "green tool" for conducting various challenging organic transformations under transition-metal-free and solvent-free conditions. Organosulfur and related heterocycles are ubiquitous in numerous biologically active molecules with potential applications, and those molecules could be synthesized from readily available starting materials under mechanochemical conditions without using any hazardous chemical or solvent. This synopsis highlights the green strategies developed in recent times to synthesize organosulfur and related heterocycles under ball-milling conditions.

The technological and scientific challenges coupled with environmental concern have prompted interest in simple, eco-friendly, and energy-efficient procedures for the synthesis of biologically active molecules and materials. During the last decade, ball milling has emerged as a powerful tool in effecting chemical transformations in a greener way. However, mechanochemical synthesis, which involves reactions conducted by grinding solid reactants with no or minimal solvent in a ball mill, is less explored in comparison to other techniques such as microwave and ultrasound. Remarkably, during the last 5 years the interest in this technique grew tremendously because of its several advantages toward development of sustainable methods. Thus ball mill has become a regular tool in academy and industry. On the other hand, heterocyclic compounds occupy a major space in the domain of modern medicines. Thus development of an efficient methodology for the synthesis of heterocyclic molecules is always a center of interest for synthetic organic chemists. This chapter gives an overview of the application of ball milling for the synthesis of biologically active heterocycles highlighting the recent developments achieved in this area.

The use of heterogeneous metal catalysts has received much interest for its primary advantages of ease of operation, easy recovery, and recyclability of catalysts. These features are of much significance in the context of green synthesis and applications in industry. This chapter highlights the recent progress in the synthesis of various five- and six-membered N-, O-, S-containing heterocycles utilizing a variety of heterogeneous-supported metal catalysts. These include pyrroles, pyrrolidines, indolizines, imidazoles, benzimidazoles, triazoles, pyridines and dihydropyridines, pyridinones, pyrimidines, pyrimidinones, benzofurans, furocoumarins, pyrans, chromenes, and a few fused heterocycles. The scopes and limitations of the procedures have been reported. The possible reaction pathways of several reactions are discussed for better understanding.

Diaryl ketones are of much importance in organic synthesis as versatile intermediates and in industry for their useful properties. A mild and efficient palladium-catalyzed traditional ligand-free decarboxylative coupling of aryl α-keto carboxylic acid with aryl diazonium fluoroborate has been developed. A series of unsymmetrical diaryl ketones has been synthesized in moderate to good yields using this procedure. A radical pathway involving the acyl radical has been suggested.

An efficient method for the C(sp2) amidation of 8-aminoquinoline benzamide by acyl azide in the presence of copper acetate has been achieved via C–H activation. Interestingly, the loading of copper...

A new approach for the synthesis of aryl cyanides through CC cleavage of styrenyl gem-dibromide via an in situ cyanation process by sodium azide in the presence of CuO nanoparticles and simultaneous intermolecular cyanation of an iodoarene by capturing a released CN unit has been achieved. Several diversely substituted aryl/heteroaryl cyanides including a precursor to etravirine, a drug for the treatment of HIV, have been obtained by this procedure. This protocol is unique with regard to cyanation without a cyanide reagent and provides access to two different nitriles in one reaction involving full utilization of CN units.

Chalcogenophenes are of much importance in organic synthesis as well as in pharmaceutical industry. Thus synthesis of these molecules has received considerable interest. Although a number of methods are available many of them suffer from one or more of these drawbacks - harsh conditions, use of costly metals, general applicability, tedious purification process and low yield. To avoid these drawbacks a transition metal free iodine catalyzed reaction of aryl susbstituted 1,3-dienyl bromides with KSeCN/K2S leading to the corresponding selenophenes and thiophenes has been developed. Iodine is relatively benign, less expensive and readily available. A series of diversely substituted selenophenes and thiophenes have been obtained by this procedure in high yields. Using this procedure 2-(4-chlorophenyl)thiophene, a key intermediate for the synthesis of a melanin concentrating hormone receptor ligand involved in the treatment of eating disorders, weight gain, obesity, depression and anxiety has been synthesized. Although the reaction is one-pot essentially it proceeds in two steps involving a selenocyanate intermediate leading to selenophene. A possible mechanism for selenophene formation has been suggested. Similar mechanism is also suggested for thiophene formation via the intermediacy of potassium trans-1,3-dienyl thiolate. The simple operation, use of inexpensive reagent and a metal free process make this procedure more attractive for an easy access to substituted selenophenes and thiophenes.

A simple procedure for the synthesis of organo-chalcogenides has been developed by the reaction of aryl/heteroaryl amines with di-aryl/heteroaryl dichalcogenides in the presence of t BuONO and Fe(0) nanoparticles. The reaction proceeds via in situ diazotization followed by chalcogenation. A series of functionalized diaryl/aryl heteroaryl/diheteroaryl/aryl-alkyl selenides, sulfides and tellurides have been obtained by this procedure. Significantly, using this procedure 2,4-dinitroaniline is converted to (2,4-dinitrophenyl)(phenyl)selane which is known as thioredoxin reductase (TR) and glutathione reductase (GR) inhibitor. The reaction goes by a radical pathway and a plausible mechanism has been suggested.

A highly chemoselective reduction of aryl, heteroaryl, acyl and sulfonyl azides to the corresponding amines has been achieved by Fe(0) nanoparticles in water at room temperature in the absence of external hydride source. Several readily reducible functionalities including alkene, alkyne, S-S linkage, OTBDMS remain unaffected during reduction.

Traditionally, a metal-catalyzed cross-coupling reaction is governed by displacement of leaving groups such as halogens, tosylates, etc. by different nucleophiles leading to the formation of carbon-carbon and carbon-heteroatom bonds. Besides displacement of traditional leaving groups in coupling reactions decyanative cross-coupling has also received current attention. The objective of this work is to develop a decyanative cross-coupling through metal-assisted nucleophilic displacement, which is less explored so far. Thus, a decyanative cross-coupling of aryl selenocyanate with aryl-/alkylacetylenes, boronic acids and silanes has been accomplished by a copper-assisted nucleophilic displacement reaction for an easy access to a series of diaryl, aryl alkyl, aryl vinyl and aryl alkynyl selenides. The best yield of product was obtained using 5 mol% of Cu(OAc)2, Ag2CO3 (20 mol%) and Cs2CO3 (1 equiv.) at 100 °C for 8 h in N-methylpyrrolidinone (NMP). The advantages of simple operation, high yields and general applicability make this procedure more attractive. A mechanistic pathway has been proposed. Silver plays a key role in the decyanation process. A plausible mechanistic pathway of this decyanative carbon-selenium cross-coupling has been proposed based on UV, EPR, HR-MS and IR analytical data along with results obtained from control experiments.

A new calcium chloride mediated C–F bond cleavage in electron deficient fluoroarenes followed by functionalisation via C–Se/Te bond formation has been achieved in the absence of any additive/ligand/organometallic reagent. A series of functionalised organo-selenides and -tellurides were obtained in high yields by this procedure. The precise role of calcium and the possible mechanism have been proposed based on DFT analysis.

Silver(I) catalyzed phenylselenylation of terminal alkynes and organoboronic acids has been demonstrated using N-(phenylseleno)phthalimide as an electrophilic SePh donor. A wide variety of terminal alkynes and organoboronic acids are selenylated efficiently to produce the corresponding alkynyl and diaryl selenides, respectively, in good yields. Silver(I) acts as a Lewis acid in this process.

School of Chemical Sciences, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700 032, India E-mail: ocbcr@iacs.res.in Manuscript received online 12 October 2018, accepted 18 November 2018 The practice of green chemistry in chemical synthesis has received wide attention because of the general awareness of ill effects of chemical pollution in the environment. A chemical reaction is primarily guided by its design, use of solvent, catalyst and energy. The organic solvents are more or less toxic and thus use of alternate green solvents such as water, ionic liquids and supercritical fluids is appreciated. Catalysts play an important role in chemical synthesis. In the context of Green Chemistry catalysis has received intense interest both in academia and industry. Thus, development of both homogeneous and heterogeneous catalysts with high efficiency (TON) has become a major goal. However, the heterogeneous supported catalysts are of much current interest because of their primary advantages such as, ease of separation, reusability, improved efficiency due to stable active site and better steric control of a reaction intermediate, compared to their homogeneous counterparts. These qualities made them very promising for applications in industry. Although use of thermal energy at the expense of electricity is common for a reaction the application of other devices which consume less electrical energy such as microwave, ultrasound, ball milling, visible light (sunlight/LED bulb) is getting momentum. We have developed several organic solvent free reactions, a number heterogeneous supported catalysts which showed improved efficiency, ball milling and visible light induced reactions. We have also reported C-H functionalization via C-H activation, which controls generation of waste.

A convenient and efficient metal free borylation of aromatic amines has been achieved using tertiary butyl nitrite and B2Pin2 (bis(pinacolato)diborane) under irradiation with blue LED light at room temperature. This protocol has been successfully extended to subsequent Suzuki coupling in the same pot. Thus a series of functionalized aryl boronates and biaryls are obtained in high yields in a shorter reaction period starting from relatively cheap aryl amines in one-pot avoiding isolation of potentially unstable and hazardous intermediates.

The benzoxazine scaffolds are of much interest as they are found in a large array of natural products and pharmaceutical drugs with diverse activities. We have developed a palladium-catalyzed decarboxylative selective mono- and bis-acylation of 4H-benzo[d][1,3]oxazin-4-one derivatives with α-oxo carboxylic acids via preferential cyclic imine-N-directed CH activation. 2-Aryl-4H-benzo[d][1,3]oxazin-4-one was acylated with a variety of substituted phenylglyoxylic acids to produce the corresponding products. It was observed that electron-donating groups (CH3, OCH3) at any position of the aromatic ring of phenylglyoxylic acid provided good to excellent yields, whereas phenylglyoxylic acids containing electron-withdrawing groups (COCH3, CN, NO2) gave the products in moderate yields. Interestingly when the reaction was performed with silver triflate (AgOTf) in place of silver nitrate (AgNO3) in the presence of 4 equivalents of glyoxylic acid, the bis-acylated product was obtained together with a small amount of mono-acylated product. This is the first report of acylation of 2-aryl-4H-benzo[d][1,3]oxazin-4-ones via CH activation. The notable features of this reaction are acylation with more challenging heteroarene-oxo carboxylic acids and alkyl oxo carboxylic acids. This new protocol provides an easy and efficient access to a variety of o-acyl-4H-benzo[d][1,3]oxazin-4-one derivatives which are of pharmaceutical importance.

An efficient protocol for the cross coupling of styrenyl/aryl halides and H-phosphinate has been developed using a unique Co/Cu catalytic system in the absence of any additional ligand for the first time. A library of diversely functionalised styrenyl/aryl phosphonates has been synthesized using this procedure. The advantages offered using this procedure are operational simplicity, the use of inexpensive and relatively benign metals as a catalyst, wide tolerance to functionality including electron withdrawing and releasing substituents, excellent stereoselectivity and high yields. The mechanism has also been established. The unique feature of this Co/Cu system is that, it primarily involves Co in the catalytic cycle and Cu in the transmetalation process.

A new procedure for the oxidation of sulfides to sulfones utilizing heterogeneous ruthenium reagent has been developed. A small amount of ruthenium trichloride (RuCl 3 ) supported on alumina and excess sodium metaperiodate (NaIO 4 ) was used to produce the ruthenium oxidizing catalyst in the reaction mixture. Sodium metaperiodate oxidized the pre-catalyst RuCl 3 to RuO 4 and also maintained a constant supply of RuO 4 by oxidizing lower valent ruthenium ions during the course of the reaction. An environmentally friendly solvent mixture of dimethyl carbonate (DMC) and water was employed. A wide variety of aromatic sulfides were oxidized to sulfones in good to excellent yields by utilizing this procedure.

Bicyclic arenyl selenides are of much importance because of their pharmaceutical applications. A simple method for their synthesis has been developed by a reaction of 2-naphthol and styrenyl selenocyanate/diaryl diselenide in the presence of a base at room temperature. The selenation occurs exclusively at the 1-position of 2-naphthol unit. The reactions are relatively fast (2–4 h) and high yielding. A library of substituted naphthyl styrenyl and naphthyl aryl selenides are obtained by this procedure.

An efficient method for the C(sp