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Home / Green Solvents for Sustainable Organic Synthesis State of the Art

Green Solvents for Sustainable Organic Synthesis State of the Art

Introduction

The central target of dark-green or sustainable chemistry is making available to mankind useful compounds and materials, while causing no harm to the environment. This arroyo has acquired a cardinal office in present day's chemistry, although the first embryo has long been present in the literature. A century ago, when chemic industry was just outset its development on a large scale, a particularly clear-sighted scientist, K. Ciamician, observed that it was now possible to synthesize products identical to the natural ones. However, this was done in the laboratory by using harsh conditions and excess free energy. The actual advancement, he meant, would be obtained when men would acquire to run chemic reactions in the mild way nature does and would develop an environment-friendly chemistry i.

Such concepts were much in advance of their time and embodied ii most of the "12 Principles" of greenish chemical science every bit formulated past Anastas and Warner 3–5 at the end of the twentieth century. Present, the increasing concern for a sustainable development and the perception of chemic manufacture every bit 1 of the about harmful human being activity have created the conditions for the nativity of dark-green chemistry (based on the above principles) equally an independent field of study.

The development of green chemical science

As mentioned in a higher place, sustainable chemistry in its mature grade can be considered an established discipline since about 10 years. Thus, it could exist worthwhile analyzing its evolution and agreement in which management(s) information technology is moving, when entering the second decade of the twenty-outset century. Reviewing the state of art of green chemical science has become incommunicable, since this topic is too extensive. What is attempted beneath, is offering a review of which topics can be classified within the field of greenish chemistry and correspond to the key concepts of this subject area. The overview compares what is expected from green chemistry on ane mitt, and in which way chemists reply this expectation, on the other i. Equally for the kickoff issue, the principles by Anastas and Warner iii–5, which have been variously rephrased 6, express the master challenge in sustainable chemistry (and chemic engineering). These urge chemists to use alternative feedstocks, to avert hazardous reagents or products and to minimize waste.

Green chemistry involves an interdisciplinary effort, guided past the principle "benign by design" 7–x. This requires to introduce the desirable aspects as early on as possible when planning a procedure and to consider the whole lifecycle of the production, possibly by adopting a "cradle to cradle" approach. Information technology is no more acceptable that "a new molecule is designed or a new process is discovered" 11 and then in separate steps the optimization of the synthesis and application are carried out. Rather, the chemical and the engineering aspects must be fused from the beginning 11–16 and the environmental aspects quantitatively assessed likewise at an early on phase 17–xx, giving priority to new processes that couple efficiency and protection of the environs. The targets are those indicated past the above mentioned 12 principles 3–half-dozen or in related statements such every bit those due to Clark 21 and Sheldon 22. These require an enhanced incorporation of the reagent atoms in the product, the utilize of catalytic rather than stoichiometric reagents as well as devising new synthetic pathways, using less harmful solvents and new purification methods, introducing intensive processes that are less harmful.

On pinnacle of that, the procedure must be economically sound, otherwise the fact of existence environment-friendly is useless. Indeed, in many cases industry resists – or has resisted – the introduction of this arroyo fearing a cost increase. Even so, the awareness is growing that at that place is no alternative option because of the social force per unit area and regulation introduced. Moreover, several cases demonstrate that economically satisfactory solutions are possible by introducing the "light-green" arroyo 23–xxx.

Less easy is judging what is light-green, or how dark-green is a new process proposed. The "revolutionary" idea is refocusing research and evolution on a lower environmental impact and better sustainability, rather than merely on a higher yield. Arriving at solutions that are satisfactory under all of the aspects is clearly a demanding job, which is not always fulfilled past a unmarried newspaper or in a patent that claims to contribute to green chemistry. Indeed, in many cases the assertion is based on a single aspect. Sometimes it is claimed that a new catalyst improves the yield of a process nether mild conditions, simply no attention is paid to check whether such catalyst is harmful to human being or environment, too expensive, labile or hard to obtain. Too, maintaining mild conditions requires the utilise of expensive (economically or environmentally) reagents or solvents or of a large amount of energy, so that industrial implementation is not viable.

As a matter of fact, the label "green chemistry" in its full meaning applies only to a fraction of the published literature reviewed under this heading (see further beneath), because the overall consideration of the process is missing. This even so, it may be worthwhile analyzing the papers published with this keyword, because these correspond to what chemists experience green chemistry is. This may exist compared with what the chemical manufacture expects from this kind of work. After all, the role that chemistry volition take in the future and the perspective that will be opened depend on the kind of research chemists will do, which remains in part curiosity driven.

Dark-green chemistry in the manufacture

Every bit for the demand from the industry, several accounts are available, some stressing out the utilise of a renewable feedstock 31, other ones process innovation 15 27 32. A notable contribution is a survey among industrial experts that has been carried out in 2004 33. These experts were interviewed on what could exist washed for enhancing the sustainability of chemical industry in Europe. To the question, which tool would they support, they listed kickoff of all Reckoner Aided Molecular Design (75% support) and then in rapidly decreasing preference, Integrated Product and Process Design 55, Process Synthesis 50, Industrial Symbiosis and Environmental 50, Lifetime Process Optimization (40), Procedure Integration (35), and others. The reasoning was clear, one had to choose the right product to brand offset. Then the integration betwixt the dissimilar aspects of the production's blueprint and the procedure leading to information technology should be ensured, instead of focusing on a single aspect (that is, a holistic arroyo had to exist used) 34.

The best technology to use was the side by side question. Hither the answers were more evenly distributed, with x items receiving between 65 and xc% back up. It appears that these suggestions could exist grouped in some large categories (run across Effigy 1). These were: a highly selective catalyzed synthesis (see category "green synthesis" in Figure 1, 90% mean back up); the use of green energy sources (80% mean support); the engineering aspects (both procedure intensification and economic small scale processes, 75% mean back up); the use of a greenish feedstock (encounter category "renewable" in Figure one, 75% mean support); the reasoned (computer assisted) choice of the production, by taking into account all of the environmental aspects relative to the product itself and its synthesis (encounter category "products" in Figure ane, 70% mean back up); and finally, the choice of the medium and the purification technique (lxx% mean back up).

Figure 1. Classification of the topics judged important for a dark-green evolution by a panel of industry experts. Elaborated from data in 33.

As for the classes of reactions that require almost urgently attending, a Circular Tabular array formed by experts from pharmaceutical industries 34 put frontwards 5 general processes that would have greatly beneficed from the introduction of green methods. These were: amide formation with good atom economic system; activation of the hydroxyl group; reduction of amides fugitive hydrides; oxidation and epoxidation reactions in non-chlorinated solvents; and Mitsunobu reactions with a amend cantlet economy. It does non appear for the moment that this statement has produced a significant redirection of the research toward such goals.

The literature on green chemistry

Since the 1990s, the increasing success of light-green chemical science has been indicated by the start of several series of well-attended international meetings, by the founding of interest groups or sections in most National Chemical Societies and past the introduction of journals aiming to encompass the various aspects of this subject area. Amongst them, one may mention Greenish Chemical science (Royal Chemical Society, 1999), Green Chemistry Letters and Reviews (Taylor & Francis, 2007), and ChemSusChem (diverse European Chemical Societies and Wiley, 2008). Other journals are devoted to environmental safety and in turn include both long established titles, such as Environmental Science and Engineering science (American Chemical Order, 1967), Clean (Wiley, new name of Acta Hydrochimica et Hydrobiologica, 1973), and Ecology Bear upon Assessment Reviews (Elsevier, 1980), as well as new ones, such as Energy & Evironmental Sciences (by the Royal Chemic Guild, 2008). Virtually chiefly, the number of papers retrieved by Chemic Abstracts ane under the heading "green chemistry" (that is where these words are nowadays in the abstract or as a keyword) has experienced a very rapid growth in the commencement few years of the new millennium and has now leveled well over 1000 publications per year (come across Figure 2).

Figure 2. Papers with the keyword "green chemistry" published in the period 1994–2008.

The big bulk of the authors of such papers were from the academia. Taking into consideration those published in the decade 1999–2008, it has been examined in which periodical this kind of research has been published. As shown in Table ane, past adding the first 18 bibliographic sources, one arrives at ca. 32% of the total number of published reports. Those published in Dark-green Chemistry, a journal devoted to this topic that has been published over the entire decade 1999–2008, make by far the largest contribution (6.44%, twenty% of the 18 most popular journals in the field). About a half of the papers in the other 17 sources have been published in journals of organic chemistry, ca. fourteen% in those of full general chemistry and ca. 14% in those of catalysis. Notably, 1.21% of the total number of the references in this group are international patents and x.46% Chinese patents. These are remarkable figures for this topic, sometimes considered of purely academic interest.

Table 1. Papers with the keyword "dark-green chemistry" published in the decade 1999–2008. Journals nigh frequently chosen.

Thus, from the bespeak of view of publications, green chemistry is essentially a sub-bailiwick of organic chemistry, with particular regard to catalytic reactions. However, in that location is likewise a clear identification of light-green chemical science every bit a subject on its own, as indicated by the considerable fraction that is submitted to journals directly devoted to the topic (see for case, Green. Chem. and Int. J. Life Bike Assess. in Table 1).2 Another bespeak worth notice is that almost a one-half of the considered reports (42%) is on journals reserved to rapid communications. This fact, forth with the high charge per unit of communications at major meetings, is an indication that this topic is highly regarded by the chemical community and advancement in this field is considered equally a high priority.

The topics investigated were examined in more item for the year 2008 in the above series of papers besides equally in two further series including the papers published in that year in Green Chemical science and ChemSusChem (see Figure 3(A)–(C); Green Chemistry Letters and Reviews is fully devoted to green synthesis).

Figure 3. Classification of the papers published in 2008: (a) in Green Chemistry; (b) in ChemSusChem; (c) classed as green chemistry, from any journal.

The topics are grouped in broad categories. It is credible from the histograms that greenish synthesis is largely dominating, viz. about a half of the papers in the specialized journals and more three-fourths in the full general literature. In this category papers reporting "new" reactions or "new" conditions for a reaction have been grouped. This appears to be the main pregnant that chemists (mostly from the academia) attach to the name "green chemical science," whereas for the industry this represents only ane of the questions, even though a quite important i (compare the share of "dark-green synthesis" in Figure 3 and the support given to this topic in Effigy 1). The experts from the industry attach a great importance to the optimization of the reaction grade by taking intendance of the engineering science aspects and of the products purification, topics less ofttimes addressed in the literature.

The use of renewable feedstocks receives a comparable attention both by the industrial panel and in specialized journals, less in the "full general" literature. The same holds for the use of green sources of energy and related problems, such equally solar calorie-free conversion and the generation and utilise of hydrogen, and for other environmental bug, including the COii cycle. These are more frequently addressed in Green Chemistry and ChemSusChem (and are accounted equally quite important past the industry) than in papers with the keyword "green chemistry" published in journals of full general interest.

Analysis of the investigated topics

A further refinement of the analysis reveals which innovative elements characterized the reactions assigned to the "green synthesis" category in the "general" 2008 literature (see Figure iii©). Three main fields tin be recognized viz. "catalysis," "reaction medium," and "physical techniques" (in lodge of importance, see Figure four). Some of the main directions of development within each of such fields are indicated below through some examples. The very limited choice is neither exhaustive nor even consistently representative. Further notice that quite oftentimes ii labels are appropriate and in this example the paper has been considered in both groups.

Effigy 4. Topics investigated in the 2008 literature on green chemistry.

Catalysis has been defined the "foundation pillar" of green chemistry. The better yields/increased selectivity/milder weather condition that by definition characterize catalytic processes are of course equally qualifying for greenish chemical science 35, and so that one may think that light-green chemical science is a section of catalytic chemistry. Homogeneous, heterogeneous, and supported (nano)catalysts 36 accept all been applied for processes classified every bit "greenish." In papers begetting this label usually the attention to the environmental characteristics is specially loftier, with regard both to the catalyst operation (high turn-over number, reasonable stability, and express cost) and to the reaction conditions (safety aspects).

Many of the reported catalyzed processes involve transition metals and their complexes. Every bit an case, Ru complexes have been found to catalyze the hydrogenation of nitriles to master amines with remarkable chemoselectivity. An constructive catalyst is formed in situ from inexpensive starting materials, such as Ru(COD)(methylallylii) and PPh3 37. In some other case, amides have been prepared from alcohols and amines past extrusion of dihydrogen in the presence of Ru(COD)Cl2, a Due north-heterocyclic carbene forerunner and a phosphine 38. Some other oxidation process leads from primary amines to amides in the presence of a supported ruthenium hydroxide catalyst 39 (come across Scheme 1).

Scheme 1. Example of greenish synthesis: metallic catalysis.

Indeed, an of import motility toward dark-green reactions is substituting arable and cheap iron for scarce and not e'er surroundings-friendly noble metals in catalysis. This has been recently obtaining increasing involvement and has been practical, e.g. for redox processes and for coupling reactions forty 41. In the field of carbon–carbon bond forming reactions, iron catalyzed processes are becoming competitive, if compared to those using more expensive and toxic palladium- and nickel-based catalysts, with a significant economic and environmental advantage 42. Another quite active line of development involves the physical country of the active fabric, since catalysis by noble and transition metals is profoundly improved when (supported) nanoparticles are used 36.

In another large group of reactions, the catalyst is a (Lewis) acid. Thus, several acid catalyzed condensation reactions that occur in loftier yields and short time authorize for the category of "click" chemical science (see Scheme 2). An example is the condensation of 2-aminobenzamide with various alkyl, aryl, and alicyclic aldehydes or ketones to give 2,3-dihydroquinazolin-4(1H)-i derivatives (see Scheme 2(a)) in the presence of a catalytic amount of ammonium chloride in ethanol 43. Likewise, pyrazoles, diazepines, enaminones, and enamino esters take been smoothly obtained past the condensation of hydrazines (see Scheme two(b)) or hydrazides, diamines or primary amines with i,3-dicarbonyls. In the terminal example, 12-tungstophosphoric acid has been employed as a reusable catalyst in water 44. N-fused 2- and iii-aminoimidazoles take been prepared past means of a Ugi-blazon multi-component reaction mediated by zirconium(Four) chloride in polyethylene glycol-400 45 (run across Scheme ii(c)).

Scheme 2. Examples of green synthesis: acid catalysis.

Nucleophilic substitution at acyl groups has been conveniently carried out in the presence of enzymes. A "green" application in the field of polymers is the transesterification of vinyl methacrylate by hydroxyl-terminated polyisobutylenes in the presence of Candida antarctica lipase B 46 (come across Scheme 3).

Scheme 3. Examples of light-green synthesis: enzyme catalysis PIB=Poly-(isobutylene) chain.

The use of chiral auxiliaries is another topic considered in this group, particularly when these are effectively recyclable. As an example, a ion-tagged prolinol has been plant to induce the enantioselective alkylation of aldehydes by organometallic derivatives in ionic liquids and to exist recyclable for more than than 10 times 47 (run across Scheme 4(a)). In a similar line, it was demonstrated that several 4-acyloxyproline derivatives are able to catalyze the direct asymmetric aldol condensation between circadian ketones and benzaldehyde in h2o and can likewise be recycled 48 (see Scheme 4(b)). Analogously, an enantiomerically pure diketone (which tin be used for the synthesis of diterpenes) has been prepared from cheap starting materials on a large scale (>125 grand) 49 (encounter Scheme 4(c)).

Scheme 4. Examples of light-green synthesis: enantioselectivity.

Reactions carried out nether solvent-gratis weather or in an environment-friendly solvent class a big grouping. As an example, a tetranuclear zinc cluster, which is effective for the transesterification of a variety of methyl esters, in solution equally well as under solvent-free conditions, has been prepared and tested up to the 100 g scale. In this instance, the Due east cistron (that is the mass ratio of waste to desired product) was depression, 0.66 kg waste/kg product 50. Many processes affording highly stabilized products, such equally condensation reactions leading to heteroaromatics, proceed more efficiently and accept a wider scope when carried out in the absenteeism of any solvent, particularly when induced past microwaves 51.

In a combined arroyo a condensation (Hanztsch) synthesis (with a strong acid every bit the catalyst) has been coupled with dehydrogenation (catalyzed past Pd dehydrogenase) for obtaining pyridines rather than dihydropyridines in a one-pot process (see Scheme five) 52.

Scheme 5. Examples of green synthesis: combination of unlike steps in a one-pot procedure. In the presence of Pd dehydrogenase dehydrogenation accompanies condensation.

Successful reactions under solvent-complimentary atmospheric condition have often been obtained in the presence of supported catalysts, in nigh cases on (mesoporous) silica 53 54.

Alternatively, shifting to water as the solvent is a positive move, as shown in many instances, including a method for the solid-phase synthesis of peptides that uses h2o-dispersible protected aminoacids 55. Ionic liquids are used fairly often 56–58, while but a limited attending has been devoted to purification methods, reasonably because this is a minor problem in pocket-size scale reactions.

As far as physical techniques are concerned, the use of "culling" reaction conditions has been classified under this issue. Amongst the unlike approaches considered, microwaves are clearly the nearly largely used technique. Representative examples are the generation of ammonia from formamide (meet Scheme 6(a), for the synthesis of N-heterocycles) 59, the nucleophilic substitution in aromatics 60, and the Wolff rearrangement of diazoketones (see Scheme 6(b)) 61.

Scheme vi Examples of green synthesis: microwave (MW) and ultrasound (U.s.) assisted reactions.

Ultrasounds have been less ofttimes used, but a number of reports are available ranging from the acceleration of a Knøvenhagel condensation 62 to the training of lanthanide complexes able to promote nucleophilic addition reactions (encounter Scheme 6c) 63. Different physical techniques such as photochemistry and electrochemistry accept a minor role, although these are distinguished by the very balmy conditions and, particularly in the former case, open reaction paths that have no thermal counterpart. Representative examples are the synthesis of five- and six-membered ring compounds by environmentally friendly radical cyclizations using Kolbe electrolysis (see Scheme 7(a)) 64 and the photochemical formation of cyclobutanes (see Scheme seven(b)) 65.

Scheme vii. Examples of light-green synthesis: electrochemistry and photochemistry.

Other aspects are less extensively covered, as information technology appears in Effigy 3, but include important contributions on topics ranging from process intensification in chemic engineering 12 66 to appraise the role of wastewater treatment 3–5 67 and of the reuse of biosolids 68. Nigh important is the cess of the environmental quality both of chemical syntheses 69–71 and of the use of alternative sources of materials (east.g. solvents) 72 73 and energy 74, also equally the role that regulations accept on industrial chemistry 24.

Notice that a less frequent appearance of a topic in the present context does not hateful that it is less investigated in absolute terms. This rather indicates that work in that area is not recognized as pertaining to green chemistry by the authors themselves, who have non inserted that characterization as keyword, or in the title or abstract.

Determination

Green chemistry is a multi-faceted subject field that has been created equally a contribution of chemistry to sustainable development, avoiding harm to the environment. Apparently, the business organization is greater in the industry, since social alert and legislative limitation take a more than direct impact in this example. Various panels from the industry have conspicuously outlined the directions that should exist followed. Thus, of import themes are non limited to the diverse aspects of synthesis and purification, but involve a more complete survey. The terminal aim is putting a new product on the marketplace only after that this has been proven the (environmentally) correct decision. In club to open new perspectives, the use of renewable materials and energy must progress at the aforementioned fourth dimension. The bookish community appears to consider synthesis equally the most important theme, more example than the ecology behavior of the synthesized material. For this reason, the postulates of green chemistry have permeated catalysis and, in part, organic chemistry. Reports of "new" syntheses are past far the master component of the literature identified equally green chemistry. Nevertheless, many of the papers considered involve merely partial results. These usually lack the early consideration of all the environmentally relevant aspects that should exist peculiar of this discipline, likewise as whatever attention to the engineering perspective of scaling upwards. This even so, i can certainly recognize a tendency imparted by the popularity of green chemistry postulates. As a consequence, chemists pay a greater attention to the optimization of catalysis, to the elimination of toxic reagents and to the limitation or elimination of solvents (much less to the use of "alternative" physical methods). In improver, many preparative papers include an environmental cess of the considered processes. If not immediately, in the long range this piece of work will certainly contribute also to the industrial development.

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Source: https://www.tandfonline.com/doi/full/10.1080/17518250903583698