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10 P. T. Anastas and E. S. Beach . Alternative driving forces for reaction, such as microwave irradiation, sonochemistry, and photo- chemical reactions. . Reduction of, or, ideally, elimination of solvents. Use of alternative, environmentally benign solvents. . Avoiding toxic materials and designing for reduced environmental exposure. . Switching from petroleum-based feedstocks to re- newable resource-based feedstocks. This paper does not aim to be a comprehensive review, but rather seeks to provide a useful sampling of the past successes associated with green chemistry research since the early 1990s as a foundation to consider the future potential of the field as it continues to emerge. Important advances for green chemistry Assessing synthetic efficiency Chemists have long admired elegance in synthetic pathways, but streamlining synthesis by reducing the number of steps and avoiding inherently wasteful chemical transformations has taken on a greater sense of urgency. Efficient synthetic pathways are at the heart of many green chemistry principles: reducing the use of non-renewable materials, eliminating waste, and reducing emissions. There are clear benefits for safety and sustainability. The concept of ‘‘atom economy’’ encourages chemists to seek out synthetic methods that incorporate all the atoms of a reactant into the product molecule (2) (Figure 1). A similar philosophy is described by ‘‘step economy,’’ achieving the complexity of the target molecule in the fewest possible steps, using carefully selected reactions (3). Explicit assessments of complexity can be devised, allowing for strategic design of molecules requiring multiple carboncarbon bond-forming steps (4). Efficiency may also be measured in terms of an ‘‘E-factor,’’ or how much waste is generated per kg of product (5). Reaction mass efficiency (RME) takes into account a calculation of atom economy as well as the stoichiometry and yield of each step (6). These metrics have been successfully applied in industry to devise greener processes, for example in pharmaceu- tical production (7). Atom economy and RME calculations typically do not account for use of hazardous reagents, or solvents, which may constitute the primary source of waste in a synthesis. In developing more sophisticated green chemistry me- Figure 1. A calculation of atom economy. trics, there are challenges in defining safety and waste in terms of scope and environmental impact. Life- cycle assessment (LCA) aspires to provide a more comprehensive analysis of environmental impact, taking into account not only the product but also cradle-to-grave aspects such as the raw materials needed and ultimate fate upon disposal. The scope of LCA may include potential for global warming, eutrophication, energy consumption, and human and ecological toxicity (8). Catalysis The 2005 Nobel Prize in Chemistry awarded to Chauvin, Grubbs, and Schrock for their olefin metathesis catalysts specifically highlighted the po- tential of their research to provide access to ‘‘greener’’ synthetic routes (9). The recognition of their work is just one indicator of the paramount role catalysis has played in expanding the range of reactions available to green chemists. Olefin metathesis is an example of a very atom- economical reaction, with potential to eliminate the waste associated with alternate multi-step synthetic routes. A versatile method for forming carbon carbon bonds in both organic and aqueous media, it has many applications in polymer chemistry (living polymerization, preparation of block copolymers, and synthesis of liquid crystal materials). It has been used to construct ring systems for total synthesis of natural products, and can cyclize or crosslink polypeptides (10). Olefin metathesis has also been useful in deriving useful materials from renewable resources. Unsaturated fatty acid esters and various natural oils and fats can be used to produce valuable chemical feedstocks and macrocyclic frameworks for natural product synthesis (11). Further ‘‘greening’’ of homogeneous olefin metathesis has been achieved by developing catalysts that can be easily separated from the products, which is particularly important in applications such as pharmaceutical synthesis (12). Homogeneous transition metal catalysts have also proven to be extremely useful in other carboncarbon bond-forming reactions. Cycloaddition and cyclo- isomerization can produce a variety of organic molecule structural skeletons, often replacing sequen- tial, stoichiometric reduction and oxidation steps with one-step, highly atom-economical reactions (13). Hydrogenation using H2 gas is another major atom-economical reaction of high utility in designing green synthetic pathways. There is a need for new synthetic methods based on H2, in order to phase out hazardous and wasteful hydride and borane reagents. Progress has been made in asymmetric hydrogenation. For example, highly enantioselective, high-yieldingPDF Image | Green chemistry: the emergence of a transformative framework
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