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14 P. T. Anastas and E. S. Beach Figure 8. Stereospecific, solid-state photodecarbonylation of an intermediate in the synthesis of ()-(a)-cuparenone (54). microreactors, with high attenuation of increases in temperature. Precise control of temperature resulting from the high surface area-to-volume ratio can elim- inate unwanted side reactions in temperature-sensitive pathways. In some cases this can improve regioselec- tivity, for example the ratio of E/Z isomers in a Wittig reaction. There are relatively few examples of enantio- selective reactions performed in microreactors, but in those cases the enantiomeric excess matches or exceeds that of conventional reactors. Rapid heat transfer can reduce solvent requirements, allowing reactions under more concentrated (or even solvent-free) conditions. Intermediates may be generated in situ and then used in subsequent reactions, providing a way to minimize risks associated with toxic reagents. Microreactors can be placed in series, and progress has been made toward multistep microreactions; a 5-step synthesis of cipro- floxacin has been reported that eliminated chromato- graphic purification, relying on only minimal workup steps. While potentially capital-intensive, scale-up of reactions is said to be safer with microreactors because only the number of reactors increases, not the size of each reactor (so-called ‘‘numbering up’’). Real-time, in-reactor monitoring to precisely control reaction conditions is an additional advantage of microreactor technology, another way to avoid waste and formation of hazardous materials (55,56). Alternative solvents Solvents can account for 85% of mass in pharma- ceutical synthesis, and coupled with low recovery rates, can be a major source of waste regardless of the atom economy of the reaction. Toxicity, flammabil- ity, corrosivity, and volatility are human safety and environmental concerns. These problems typically arise due to difficulties in containment, recovery, and reuse (57). When possible, ‘‘the best solvent is no solvent’’ (57). Many examples of solventless reactions have been reported, often in conjunction with new syn- thetic techniques such as microwave-assisted or microreactor chemistry, as discussed previously. Ad- vantages include increased reactivity and access to mechanisms dependent on weak interactions that would ordinarily be disrupted by solvent. Use of solid supports (such as clays, zeolites, and silica) is common, and the support material may behave as a Lewis acidic or basic catalyst. Specific interactions between the reagents and the support material can influence selectivity. As these reactions are activated by heating, they often benefit from microwave exposure. In reactions activated by anions, phase transfer catalysts are extremely useful (58). A selec- tion of solvent-free reactions reported in recent years: . ClaisenSchmidt condensation was achieved using a solid base (59). . Fatty acid esters for use in cosmetic applications were prepared by solid acid-catalyzed reaction of alcohols and carboxylic acids or alkylation of carboxylates with alkyl bromides using a phase- transfer catalyst (60). . Aldol condensations have been achieved by grind- ing the solid reagents with NaOH, resulting in better selectivity than when solvent was present (61). . Supramolecular ligandmetal complexes having two- or three-dimensional topography (including double helical structures) were formed much more rapidly under solventless conditions (62). Another innovation in solid-state chemistry is ‘‘non- covalent derivatization,’’ the use of hydrogen bond- ing, p-stacking, lipophiliclipophilic interactions, and electrostatic interactions to manipulate the physical properties of a molecule. By identifying auxiliary reagents having specific intermolecular interactions with the target compound, changes in melting point, solubility, vapor pressure, diffusivity, and other physical properties can be induced. There is excellent potential for eliminating pollution associated with the solvents, purification steps, and energy inputs needed for covalent chemistry. The use of supramolecular aggregates in Polaroid Instant Photography is an example of the viability of non-covalent chemistry on the industrial scale (63,64). Carbon dioxide (CO2), an abundant and cheap waste material, has been extensively studied as a replacement for conventional solvents. It is non-toxic, non-flammable, and having a relatively low critical temperature (318C) and critical pressure (73.8 bar), both liquid and supercritical phases are easily acces- sible. One of the first ‘‘green’’ applications of CO2 was decaffeination of coffee beans by extraction with supercritical CO2, replacing earlier technology which often relied on toxic solvents such as dichloromethane (65). CO2 has been used as a medium for a wide range of synthetic chemistry. Examples include hydroformylation (66,67), epoxida-PDF Image | Green chemistry: the emergence of a transformative framework
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