Environmentally acceptable reactions

Crabtree, E. W., and M. M. El-Halwagi (1994). Synthesis of Environmentally Acceptable Reactions. Pollution Prevention Via Process and Product Modifications, ed. M. El-Halwagi, and D. P. Petrides, 117-27. AIChE Symposium Series, 303. New York American Institute of Chemical Engineers. 

In order to address these challenges, a hierarchical approach may be adopted. This approach focuses on the big picture first, then adds details to promising solutions. Therefore, preliminary screening ought to be conducted first to identify overall reaction alternatives that meet process requirements in terms of desired product, cost effectiveness, environmental acceptability, and thermodynamic feasibility. At this stage, minimum details are to be invoked. The problem of synthesizing environmentally acceptable reactions EARs has been introduced by Crabtree and El-Halwagi (1994) and can be stated as follows ... 

With SC-CO2 high solubilities can be attained by increasing the pressure, and reactions can be carried out over a wide range of temperatures, pressures and densities. SC-CO2 is readily available, nontoxic, nonflammable, chemically inert under many conditions, inexpensive, environmentally acceptable and easy to remove and recycle. It has received considerable attention as a reaction medium for organic synthesis [77d, 80] as well as in some large-scale extraction processes in food chemistry [81], The Diels-Alder reaction in SC-CO2 has been investigated quite thoroughly. 

This reaction serves for removal of carbon monoxide from gas mixtures and is usually carried out over supported metal catalysts. In reforming techniques, carbon monoxide, poisonous for the catalyst in fuel cells, is removed in such a way. It is also applied in automobiles for reducing the exhaust gas carbon monoxide to an environmentally acceptable level. 

Silica-supported reagents have been exploited as nontoxic, inexpensive, reusable, and environmentally acceptable catalysts for developing stoichiometric reaction methods in organic chemistry and specifically in carbohydrate chemistry. Apart from being easy to handle and to store, these systems allow facile workup, the catalyst being removed by simple filtration, and the reaction products isolated by chromatographic purification, if necessary. 

The development phase invokes many instances where reactions must be optimised quickly because of changes in the route to or specification of the drug substance. The final challenge is to converge upon the most appropriate and environmentally acceptable methods for manufacture. In identifying optimal synthesis conditions, real-time tuning of a mi-... 

Although the Heck reaction is synthetically very useful, it requires quite high molar quantities of palladium catalyst to be effective. As such, one of the main goals is to find a solvent that helps to increase the lifetime of the catalyst and consequently reduce the amount of catalyst required. In this respect, ionic liquids show considerable promise. Another key goal in this area is to be able to replace iodo- and bromoarenes, usually used as substrates in these reactions, with chloroarenes, which are more environmentally acceptable. Again, ionic liquids show some promise in this respect. Scheme 10.2 shows the Heck reaction between styrene and chlorobenzene that has been investigated in a number of ionic liquids. 

An economical, practical, and environmentally acceptable procedure was elaborated for oxidative deprotection of trimethylsilyl ethers to their corresponding carbonyl compounds. The reaction proceeded in a solventless system, within a short period of time, and yields were good. On irradiation in a conventional microwave for 30 s, trimethylsilyl ether of benzyl alcohol in the presence of mont-morilonite KIO and finely grounded Fe(N03)3 9H2O gave rise to benzaldehyde in 95% yield. The applicability of this method was tested with several aromatic, alicyclic, and aliphatic trimethylsilyl ethers. Duration did not exceed 1 min, and yields were not lower than 80% (Mojtahedi et al. 1999). 

If a chemical process were to be measnred instead by what is minimally required for success or more generally by what is ideal, progress in the field wonld be greatly accelerated. In synthesis, for instance, one wonld seek to design a chemistry in which the target molecnle is made from readily available starting materials in one simple, safe, environmentally acceptable and resonrce-effective operation that proceeds quickly and in quantitative yield. The product should separate from the reaction in 100% purity (Wender and Miller, 1993). 

Although the initial process, route 1, has only four stages and on paper looks a reasonable synthesis, the process had several chemical issues and could not be considered environmentally acceptable. For example, bromination of 3 -chloropro-piophenone, although straightforward, did necessitate several base and aqueous washes to remove the hydrogen bromide by-product during the work-up and a change of solvent for the displacement reaction with 2-methyl-2-aminopropan-l-ol. 

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