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12 P. T. Anastas and E. S. Beach Figure 5. One-step, solvent-free synthesis of o-caprolactam from NH3, air, and cyclohexanone, using a bifunctional nanoporous solid catalyst (27). considered: for example, TiO2 production via inter- mediate TiCl4 is known to generate chlorinated dioxins and furans (30). Immobilization of catalysts on polymer supports also improves catalyst recycl- ability and performance. Progress has been made in using polymer supports based on natural resources; for example, chemically modified chitosan films have been used to support catalysts for Suzuki and Heck reactions (31), and materials made from corn starch show promise in heterogeneous base catalysis (32). Photocatalysis, the conversion of light energy into chemical energy, has proven to be extremely impor- tant in oxidation chemistry. TiO2 is a well known photocatalyst for decomposition of water pollutants by UV light (33). Sensitization of TiO2 to visible light presents opportunities for a wider range of indust- rially useful photocatalysis, such as hydroxylation of CH bonds and oxidation of water to evolve O2 or provide a source of oxygen atoms in alkene epoxida- tion (34). While new synthetic catalysts are often of biomi- metic design, another green chemistry strategy is to use enzymes to achieve transformations that are reagent- or synthetic-step intensive. An increasing amount of information about enzyme function is available to chemists; for example, an online database maintained by the University of Cologne compiles data on 3500 enzymes, including reaction types, products, and substrates (35). Challenges for success- ful biocatalytic systems include achieving high activ- ity toward xenobiotic substrates and avoiding inhibition by substrate or products (36). Nevertheless, numerous biotransformations have been reported for production of organic chemicals (37). One-pot ‘‘cas- cade’’ reactions have been developed, based purely on enzymatic processes, or with enzymatic and chemical reactions working in concert. As many as 12 distinct enzymatic transformations have been carried out in sequence without recovery of inter- mediates (Figure 6). Typically, low-value carbohy- drates are converted into chiral building blocks with very high enantioselectivity (38). A promising field of research aims to modify enzyme active sites to increase biocatalyst ‘‘promis- cuity’’: enzymes may be induced to accept a wider variety of substrates, to catalyze different reactions depending on the substrate introduced, or to produce different products from a single substrate. Potentially high regio- and stereospecificity are a driving force for applying these engineered enzymes in asymmetric synthesis. Further development of non-natural activ- ity should make biocatalysts more broadly useful (39). Directed evolution, or optimization of enzymes by mutation and screening, emerged in the 1990s as a powerful technique to produce broadly useful bioca- talysts (40). Recently, performance of a halohydrin dehydrogenase enzyme was increased 4,000-fold by introduction of 35 mutations; the new enzyme is used to produce a precursor of the pharmaceutical ator- vastatin (41). The enzymatic process reduces waste, phases out dangerous reagents, uses less solvent, and eliminates purification steps known to interfere with product yield (42). Catalysis by enzymes also pro- vides routes to both natural and non-natural poly- mers, in many cases with precise control of structure (43). An enzymatic polymerization process has been used to produce macromolecules with superior anti- oxidant properties, having potential applications in a variety of industries. Production of the antioxidants is environmentally benign and requires only one step (17). Figure 6. One-pot, 12-enzyme synthesis of a vitamin B12 precursor from d-aminolevulinic acid (44).PDF Image | Green chemistry: the emergence of a transformative framework
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