Current Organic Synthesis - Volume 8, Issue 2, 2011

The application of the principals of Green Chemistry are becoming more and more common in organic synthesis, and the design of sustainable synthetic methods is more important today than ever. The volume of the industrial production of chemicals, pharmaceuticals, agrochemicals, or consumer goods directly related to the chemical industry (e.g. plastics, dyes etc.), makes the traditional ways obsolete and impossible to continue due to the strengthening environmental and safety concerns. In this special issue, our goal was to provide an overview, as broad as possible, about recent developments in the design of sustainable methods. While we certainly understand that green organic synthesis is much broader than what one can cover in this issue, nevertheless, several areas are discussed from catalysis, through fluorous chemistry to peptide synthesis. Interestingly, several articles in this issue deal with palladium-catalyzed coupling reactions, such as Heck, Suzuki, or Negishi couplings, for which their developers Heck, Suzuki and Negishi just have been awarded the Nobel Prize in Chemistry in 2010. The first part of the issue summarizes recent developments in the applications of heterogeneous catalytic methods in sustainable synthesis. The first paper is a review by B. C. Ranu and his colleagues that focuses on the application of supported metal catalysts in a broad variety of reactions including palladium, copper, ruthenium or rhodiummediated reactions. The next article by A. Molnar summarizes the new developments in sustainable Heck chemistry with palladium catalysts. The emphasis is placed on palladium catalysts of recyclable character. Continuing with metal-catalyzed reactions A. Kulkarni and B. Torok provide an update on heterogeneous metal-catalyzed hydrogenations, focusing on the application of commercially available metal catalysts. With a shift in the catalysis part, Y. Chang and C. Bae give an overview on the use of polymer supported catalysts in synthesis. The work summarizes numerous examples of Brønsted and Lewis acids immobilized on polymer surfaces. A variety of reactions from Friedel-Crafts chemistry to multicomponent reactions are covered in this review. The last account in the catalysis section by B. Torok and his coworkers reviews recent developments on microwave-assisted heterogeneous catalysis, including both solid acid and supported metal-catalyzed reactions. The other papers cover different aspects of sustainable synthesis design. As an example, M. Torok and her co-workers summarize and discuss steps that were made toward green peptide synthesis. With the broad use of peptides in many fields, the need is certainly pressing given the nature of these reactions. In the next work J. Han and coworkers describe the use of another strong contributor to green synthesis; biomimetic reactions, in particular, biomimetic transaminations. The work highlights the importance of these metal-free reactions in the synthesis of fluorinated amines and amino acids. W. Zhang and his coworkers provide a detailed account on the application of microwaveassisted fluorous multicomponent reactions; an approach that significantly contributes to combinatorial chemistry. Last but not least, V. A. Soloshonok and his coworkers summarize the developments in an exciting new area of optical purification. While this area is still in its infancy compared to the commonly applied methods of chiral purification, sublimation may become a mainstream green separation method for chiral compounds. We can conclude that these reviews while do not cover every aspect of green organic synthesis, provide a diverse picture of several areas; traditional ones such as catalytic methods, or microwave-assisted synthesis, and really unique works on possibilities of green peptide synthesis or optical purification by sublimation. I feel that this broad coverage will make this issue attractive for synthetic chemists working in industry or academia. Finally, as a guest editor for this special issue, I am very grateful for the valuable and excellent contributions from my colleagues and I am also highly appreciative of the referees for their expert comments. My sincere thanks are also due to Ms. Humaira Hashmi (Sr. Manager Publications, Current Organic Synthesis) and Ms. Maria Baig (Assistant Manager Publications, Current Organic Synthesis) for their help in the organization of this issue.

One of the major challenges confronted by modern chemists is the development of sustainable chemistry to maintain environmental chastity. With this perspective in mind, heterogeneous organometallic chemistry has been intensively investigated in recent times. The advantages offered by the heterogeneous catalysts are ease of separation of product, reusability of the catalyst, improved efficiency, eco-compatibility and economic feasibility. This review focuses on the application of heterogeneous catalysis in different synthetic approaches. The review consists of six units namely, palladium mediated reactions, copper mediated reactions, ruthenium mediated reactions, rhodium mediated reactions, multimetallic catalytic systems and supported catalysts for asymmetric transformations, which cover almost all sorts of latest developments in organic transformations promoted by immobilized transition metal catalysts. The discussion not only involves the illustration of efficiency and reusability but also highlights the scope and limitations associated with the supported catalytic systems from a green chemistry standpoint.

This review discusses the results of the application of efficient and recyclable palladium catalysts providing highly selective product formation in the Heck reaction. Such green approach promises to result in new catalysts with properties similar to those of homogeneous palladium complexes and environmentally benign protocols. The review focuses on new, exciting results disclosed during the last 5 years.

Recent advances in the development of heterogeneous catalytic hydrogenation reactions are reviewed. The work includes two chapters, highlighting the major types of commercially available solid hydrogenation catalysts and the discussion of recent results related to the hydrogenation of major functional group types. Efforts have also been made to summarize contemporary results in microwave- and ultrasound-assisted hydrogenations.

This paper reviews recent advances in polymer-supported acid catalysts and their applications in organic synthesis. The discussion is mainly divided into the polymer-supported Bronsted acids and Lewis acids and subdivided by the nature of the reactions. In the former section, organic reactions catalyzed by polymer Bronsted acids, such as polymers functionalized with sulfonic acid (e.g., Friedel- Craft reactions, etherification, esterification, aldol reaction, multicomponent reaction, isomerization/oligomerization of alkenes, hydration of alkene/dehydration of alcohols, protection/deprotection of alcohols, etc) are discussed. Specific emphasis is placed on esterification for biodiesel production. In the latter section, organic reactions catalyzed by polymer Lewis acid (e.g., Diels-Alder reaction, aldol reaction, Michael reaction, addition reaction to carbonyl group, etc) are discussed. Because of convenient access to diverse ranges of polymer support structures and importance of acid-catalyzed organic reactions in organic synthesis, these polymer-supported acid-catalyzed reactions will continue to play critical roles in environmentally friendly organic synthesis and combinatorial library development.

Recent advances in the development of environmentally benign heterogeneous catalytic microwave-assisted synthetic reactions are reviewed. The work includes two main chapters, highlighting acid and metal-catalyzed reactions. The literature data are summarized based on the chemical nature of reaction types, such as condensations, protection-deprotection, rearrangements, cyclizations, couplings etc.

Peptides have numerous application possibilities in chemistry, biology, medicine, and pharmaceutical and materials sciences. Their traditional synthesis, however, usually poses major environmental problems. The goal of this review is to evaluate classic and contemporary methods and highlight efforts toward environmentally benign synthesis of peptides.

The biomimetic transamination of fluorinated carbonyl compounds, represent the most methodologically concise and generalized approach to the family of fluorine-containing amines and amino acids. Such amino compounds are of critical importance in the medicinal, bioorganic, biochemistry research and pharmaceutical industry. This review summarizes all of the synthetically and methodologically important achievements in the field published to date. The review consists of two major parts summarizing the literature methods on biomimetic transamination of fluorine-free carbonyl compounds and the authors' own results on the application of this bio-inspired approach to fluorinated derivatives. Discussion on each particular method includes synthetic opportunities, limitations, practicality and efficiency of the synthetic procedures.

The development of combinatorial chemistry has significantly improved the efficiency of compound library synthesis for highthroughput screening. This technology could also be used to address the green chemistry concerns in organic synthesis. The combination of microwave reactions, multicomponent reactions, and fluorous separations to synthesize organic compounds described in this paper have demonstrated the advantages of energy efficiency, atom economy, easy purification, and reduced amount of waste solvents.

This Review article presents all available in the literature data on optical purification of enantiomerically enriched organic compounds via simple procedure of sublimation. The material is arranged in a chronological order from early publications to current research. Relatively small amount of the results reported in the literature suggest that this area of research is still in its infancy it terms of commonly accepted experimental procedures, generalizations, interpretation or prediction of the stereochemical outcome of sublimation. On the other hand, the available data clearly show the potential of sublimation as most economical and greener alternative to the conventional optical purification techniques such as chromatography on chiral stationary phase or fractional crystallization. Another important goal of this Review is to bring to the attention of scientific community a warning that special care should always be taken in evaluation of the stereochemical outcome of enantioselective reactions. Thus, due to the always different sublimation rates of racemic and enantiomerically pure crystals, such a routine procedure as drying reaction products in vacuum or even removal of solvents under reduced pressure can significantly alter the true enantiomeric composition (% ee) of a compound under study and lead to erroneous results.