Reaction media, environmentally benign

In recent years, supercritical fluids such as scC02 were considered to be modern green solvents they were non-toxic, readily available, inexpensive, and environmentally benign. They are studied as a reaction medium for catalytic applications because of their interest in product separation and catalyst recovery, and... 

More recently, an environmentally benign method using air as oxidant has been developed for the oxidative cyclization of arylamine-substituted tricarbonyl-iron-cyclohexadiene complexes to carbazoles (Scheme 19). Reaction of methyl 4-aminosalicylate 45 with the complex salt 6a affords the iron complex 46, which on oxidation in acidic medium by air provides the tricarbonyliron-complexed 4a,9a-dihydrocarbazole 47. Aromatization with concomitant demetalation by treatment of the crude product with p-chloranil leads to mukonidine 48 [88]. The spectral data of this compound are in agreement with those reported by Wu[22j. 

Water is cheap, relatively abundant in many part of the world, safe, and, when pure, environmentally benign [40]. It is also true that some reactions show unusual selectivity and/or rate enhancements when run in, or more accurately, on water [41]. However, a closer examination of many reactions in water reveals that in fact one or more liquid reagents have been used in large excess, so they are in fact biphasic reactions. There is also a misguided perception that water, after use as a reaction medium, can be poured down the drain [42]. On an industrial scale, there can be a considerable cost and environmental burden associated with remediation of waste water streams contaminated with solvents and organic and metal residues-see Chapters 2 and 3. 

The desire for a sustainable development in chemistry lays the foundation for environmentally benign processes. From the view point of organic chemistry, the construction of carbon skeletons plays the pivotal role. The extraordinarily mild reaction conditions in addition to the non-toxic and non-bumable properties and ubiquitous availability of water as the reaction medium make enzyme-catalyzed C-C-bond formation the first choice even for industrial production. Thanks to subtle selectivity features of the corresponding enzymes a rather broad range in substrate specificity meets with a highly conserved stereospecificity at the newly connected carbon centers. In addition, these features and the availability of the respective biocatalysts are open to intervention by recombinant genetechnological techniques. 

The environmentally benign, nontoxic, and nonflammable fluids water and carbon dioxide (C02) are the two most abundant and inexpensive solvents on Earth. Water-in-C02 (w/c) or C02-in-water (c/w) dispersions in the form of microemulsions and emulsions offer new possibilities in waste minimization for the replacement of organic solvents in separations, reactions, and materials formation processes. Whereas the solvent strength of C02 is limited, these dispersions have the ability to function as a universal solvent medium by solubilizing high concentrations of polar, ionic, and nonpolar molecules within their dispersed and continuous phases. These emulsions may be phase-separated easily for product recovery (unlike the case for conventional emulsions) simply by depressurization. 

Leitner, W. Carbon dioxide as an environmentally benign reaction medium for chemical synthesis. Appl. Organomet. Chem. 2000, 14, 809-814. 

Development of environmentally benign industrial processes utilizing CO2, which is a cheap and safe Ci resource as well as a nontoxic reaction medium, has received much interest. One of the attractive reactions is the synthesis of dimethyl carbonate (DMC) from CO2 and methanol. The reaction is reversible and can be expressed by eqn. (3) ... 

Because of their high solubility in supercritical carbon dioxide (scCOj), similar fluorous catalyst systems have also been successfully used for enantioselective hydroformylation of olefins in this environmentally benign reaction medium [19] (Scheme 3.4). 

W. Leitner, Carbon Dioxide as an Environmentally Benign Reaction Medium for Chemical Synthesis, Appl Organomet Chem 14 809-814 2000. 

The use of water as a reaction medium in place of organic solvents is of great interest from the standpoint of environmentally benign synthesis of organics and polymers since the use of the former would significantly decrease harmful emissions, as well as cut costs associated with solvent recycling, ... 

The phosgene toxicity, separation of hydrochloric acid from excess phosgene, and the use of chlorinated solvents as a reaction medium are the major drawbacks of this reaction process. Extensive studies have suggested that carbamate and dicarbamate can serve as environmentally benign precursors for the synthesis of isocyanate and diisocyanate (1-5). Figure 1 illustrates nonphosgene routes for the synthesis of two important diisocyanates [i.e., 4,4 -diphenylmethane diisocyanate (MDI) and toluene diisocyanate(TDI)] via carbonate and carbamate. 

PTC in SCF solvents offers a plethora of opportunities. Certainly the technique will be both applicable and us ul for a wide variety of reactions. With opportunities to tune selectivity, enhance transport, and to remove solvents, all in an environmentally benign medium, this emerging area of research has great potential. With the recent surge of interest in developing technologies for a sustainable society, PTC/SCF systems can play an important role in industrial ecology. 

Microwave systems have promoted thermal preparation of heat-labile compounds, reactions that require high temperature, and they have aided optimization of established reactions known to require elevated temperature. Improved conditions obtained in comparison with literature methods have typically involved a combination of increased convenience, time savings, greater yields, greater selectivity, the need for less catalyst, or use of a more environmentally benign solvent or reaction medium [7, 8, 15]. 

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