Current Green Chemistry (v.3, #1)

Meet Our Regional Editor: by Rafael Luque (1-1).

Editorial: by György Keglevich (2-2).

Editorial (Thematic Issue: Organochalcogens in Green Chemistry) by Claudio Santi, Luca Sancineto (3-3).

KF/Al2O3 as a Green System for the Synthesis of Organochalcogen Compounds by Eder J. Lenardão, Liane K. Soares, Angelita M. Barcellos, Gelson Perin (4-17).
This mini-review describes the interest in synthetic methodologies using KF/Al2O3 in various organic reactions focusing on the preparation of organochalcogen compounds. The use of this environmentally friendly reagent as an efficient and versatile heterogeneous base to promote the addition of chalcogen nucleophiles to alkynes and α,β;-unsaturated compounds has increased in the last decade. Besides addition reactions, substitution, cross-coupling, condensation, cyclization, multicomponent and oxidation reactions and others are among the plethora of methods to prepare organic compounds containing selenium, tellurium or sulfur. In a general way, the reactions using KF/Al2O3 as the base are cleaner and faster than those using homogeneous basic catalysis. The solid supported KF is suitable to reactions under solvent-free conditions accelerated by irradiation with microwaves and can be easily recycled and reused in successive reactions, making it an excellent choice for the green synthesis of organochalcogen compounds.

Green Trends in Synthesis of Alkenyl and Alkynyl Chalcogenides by Barahman Movassagh, Elmira Mohammadi (18-35).
Background: Vinylic and acetylenic chalcogenides have received considerable attention, due to the significant role of these compounds in organic synthesis as versatile building blocks, ligand chemistry, materials science, and biologically relevant processes. Furthermore, these functional groups are found in some important drugs such as (Z)-tamoxifen, which is known as an estrogen antagonist as well as antitumor fredericamycin A. Methods: Therefore, several synthetic methods have been reported in the literature for the preparation of these valuable compounds. Efficeint synthetic protocols based on the green chemistry aspects for the preparation of alkenyl and alkynyl chalcogenides involving the use of benign reaction media and alternative energy resources have been discussed in this review. These processes include examples of coupling reactions, hydrochalcogenation reactions and a variety of reactions mediated by water or recyclable and reusable catalysts. Results: It has been demonstrated that various Csp2Y and Csp Y (Y = S, Se, Te) bond forming reactions can be successfully carried out with high selectivities and yields in an environmentally friendly fashion. Conclusion: This review focuses on the sustainable and ecofriendly synthetic processes toward alkenyl and alkynyl chalcogenides from readily available alkynes/alkenes and organochalcogens. Moreover, the potential synthetic applications and mechanistic aspects of these transformations are also discussed where appropriate.

Background: This review is focused on important and useful group of natural compounds called terpenes. From this versatile class of natural, optically active compounds various derivatives, bearing different functional groups can be chosen and easily transformed to “green” reagents and catalysts. Introduction of a chalcogen in the structure of a terpene provides additional interesting properties associated with their reactivity. This is the first review summarizing the utilization of terpenes in the synthesis of organoselenium and organotellurium compounds. Methods: Collected material was divided into two sections. First, including terpene derivatives containing selenium and second, tellurium atom. Results: The synthesis and application of all derivatives was presented. In most cases, presented organochalcogenides were built up from a camphane skeleton. Compounds described herein were consequently divided according to the location of the selenium or tellurium atom in the structure of the bornane scaffold, and as a separate section in other terpenyl derivatives from p-menthane, pinane and carane groups. Conclusion: In this paper we present the applicability of terpenes in constructing new, chiral selenium and tellurium containing compounds and their utilization in asymmetric synthesis. The pursuit of making chemistry more “green” is constantly increasing and terpenes with their unique properties and availability seem to fit this goal perfectly.

Synthesis of Biologically Active Selenium-Containing Molecules From Greener Perspectives by Juliano B. Azeredo, Ricardo S. Schwab, Antonio L. Braga (51-67).
Background: In this review we focus on the environmentally friendly processes used for the preparation of organoselenium compounds with biological and/or pharmacological properties. In this regard, the methods most commonly applied are alternative energy sources (microwave and ultrasound), reactions carried out under metal and solvent-free conditions, catalytic processes, and reactions that involve sustainable solvents (ionic liquids, glycerol, PEG-400, ethanol and water). The biological relevance of the compounds synthesized is also highlighted. Conclusions: In this review, we have highlighted the wide and growing interest in the development of new environmentally-friendly processes for the synthesis of biologically-active selenium-containing molecules. This interest is strongly correlated to the concept of sustainable chemistry, which encourages a reduction in the adverse effects of chemical synthesis through new approaches and careful planning. In this context, considerable progress has been made in relation to obtaining greener protocols for the synthesis of organoselenium compounds. Notable examples are the replacement of hazardous reagents and solvents with more sustainable chemicals, the use of renewable solvents obtained from biomass, the application of new catalytic processes, the introduction of new energy-saving sources and the use of metal- and solvent-free protocols. Although many important advances have been achieved in this field, further improvements are needed, particularly in relation to the increased use of bio-based solvents derived from renewable feedstock, due to the regulatory constraints that restrict the use of a large number of traditional organic solvents. This will lead to an increased number of new and sustainable methods for the synthesis of biologically-active organoselenium compounds.

Zinc Chalcogenolates As Green Reagents by Bonifacio Monti, Claudio Santi, Luana Bagnoli, Francesca Marini, Luca Sancineto (68-75).
Background: Organoselenolates and thiolates are generally weak bases and soft nucleophiles because of the high polarizability of the chalcogen atoms. They are commonly used to introduce a chalcogen functionality through substitution or addition reactions. Among all the methods developed for their preparation, the reduction of the corresponding diselenides and disulfides represents the most versatile procedure. Using these protocols selenolates and thiolates were usually prepared in situ using atom expensive reducing agents such as NaBH4, LiAlH4, or metals (Na, Li) or metal hydride (NaH). Methods: We synthetized the first bench stable selenolates, PhSeZnX (X = Cl, Br) using a completely atom economy oxidative insertion of the elemental zinc on the commercially available PhSeX. More recently, the same procedure has been used for the synthesis of the corresponding sulfur derivatives. Results: These reagents showed a characteristic nucleophilic reactivity, strongly accelerated when the reactions were performed in water suspension. In this review we will describe alkyl, aryl, vinyl and acyl substitutions, ring-opening reactions of epoxides and aziridines and Michael type addition including the role of the zinc in the control of the chemoselectivity and stereoselectivity. PhSeZnCl has been also described in the catalytic oxidation of thiols using oxygen or hydrogen peroxide. Some examples of synthetic application as well as the possibility to effect the reaction in ionic liquids will be discussed. Conclusions: The green aspect is connected not only with the use of water as reaction medium but also with the possibility to reuse the same medium for several times without loss in yields and selectivity.

Background: Benzeneseleninic acid and anhydride are useful oxidants with numerous unique synthetic applications. They can be generated in situ from the oxidation of diphenyl diselenide with reagents such as hydogen peroxide, t-butyl hydroperoxide and ozone. In early work, stoichiometric amounts of the selenium reagent were typically employed, resulting in higher cost, more waste byproducts and occasionally complications from the further reactions of desired products with reduced selenium species. Methods: More recently, it has been demonstrated that catalytic amounts of the selenium compounds, often as low as 1 mol %, suffice for the required oxidations, provided that stoichiometric amounts of a co-oxidant such as hydrogen peroxide are present to regenerate the active selenium catalyst. Hydrogen peroxide is an environmentally benign oxidant that produces only water as a waste byproduct and can be employed in aqueous media. Results: Furthermore, appropriately substituted seleninic acids, anhydrides and their precursor diselenides afford improved catalytic activity and facilitate separation of the catalysts from the desired products. Conclusions: This review briefly summarizes the types of classical transformations effected by stoichiometric amounts of seleninic acid oxidants and their congeners, and then provides a survey of more modern catalytic variations, including the use of designer compounds that provide the means for greener reactions with improved efficiency and less waste.

Background: Homolytic substitution at higher order chalcogens is no doubt a useful tool for radical based synthetic methodologies. Intramolecular variants of these reactions have received considerable attention in recent years and are a reliable route to prepare interesting molecules exhibiting a diverse range of biological activities. Methods: This review highlights tin-fee and photo-initiated routes to homolytic substitution at chalcogens. The use of thiohydroximate esters as precursors for radicals makes intramolecular homolytic substitution reactions easily initiated using near UV light. Intermolecular variants of these reactions offer to the synthetic practitioner a convenient route to a number of different radicals whose reactivity can be utilized to form interesting, non-chalcogen containing molecules. Evolving from the latter method are unique tellurium based radical precursors that do not undergo homolytic substitution but rather produce synthetically useful radicals by using near UV light. Conclusions: Employing these reactions in synthesis provides a greener alternative to some traditional radical based methods eliminating the requirement for chemical initiators and toxic chain carriers. These versatile methods afford atom and chemical efficient processes that can be performed using low cost white light sources.

A variety of reactions can now be conducted in microwave ovens. Generally, equal or better yields are obtained when microwave heating is being utilized for a reaction compared to the use of conventional heating. One area of organic synthesis where microwave conditions is being used more is in the formation of secondary and tertiary amines. This review will highlight the use of microwave heating in one-pot formation of primarily secondary aryl amines with a few examples of formation of primary and tertiary amines being descussed.

Background: Solvents are required for most of the chemical transformations to increase the contact between the reagents and catalysts. Solvent also ensures a suitable isolation process of products from the reaction mixture. Ethyl lactate is a bio-based, environmentally benign and economically viable solvent with effectiveness comparable to petroleum-based solvents. This review focuses on the background of this bio-based solvent and the recent application for green chemistry development in this unconventional media. Application of ethyl lactate in organic synthesis: Recently synthetic organic chemists are paying attention to use ethyl lactate as reaction medium as it has some useful advantages. Ethyl lactate has been used for asymmetric induction, multi-component reactions, ligand free coupling reactions, light induced synthesis, stereoselective synthesis, cycloaddtion reaction etc. The advantages and limitations of this solvent are also demonstrated. In this article, we demonstrated some representative examples of organic reactions in ethyl lactate media. Conclusion: Being a promising green media this bio-based solvent ethyl lactate has an important contribution in synthesis and medicinal chemistry as well.