CHAPTER 8. Selective Hydrogenation in Carbon Dioxide-dissolved Expanded Liquid Phases

The principal goal of basic research in chemical synthesis is the development of efficient tools for functional group transformations and for the assembly of building blocks during the construction of molecules with increasing complexity. Traditionally, new approaches in this area have focused on the quest for new reaction pathways, reagents, or catalysts. Comparably less effort has been devoted to utilize the reaction medium as a strategic parameter, although the use of solvents is often crucial in synthetically useful transformations. The first choice for a solvent during the development of a synthetic procedure is usually an organic liquid, which is selected on the basis of its protic or aprotic nature, its polarity, and the temperature range in which the reaction is expected to proceed. Once the desired transformation is achieved, yield and selectivity are further optimized in the given medium by variation of temperature, concentration, and related process parameters. At the end of the reaction, the solvent must be removed quantitatively from the product using conventional workup techniques like aqueous extraction, distillation, or chromatography. If the synthetic procedure becomes part of a large-scale application, the solvent can sometimes be recycled, but at least parts of it will ultimately end up in the waste stream of the process. Increasing efforts to develop chemical processes with minimized ecological impact and to reduce the emission of potentially hazardous or toxic organic chemicals have stimulated a rapidly growing interest to provide alternatives to this classical approach of synthesis in solution. At the same time, researchers have started to realize that the design and utilization of multifunctional reaction media can add a new dimension to the development of synthetic chemistry. In particular, efficient protocols for phase separations and recovery of reagents and catalysts are urgently required to provide innovative flow schemes for environmentally benign processes or for high-throughput screening procedures. Fluorous liquid phases and supercritical carbon dioxide (sc CO2) have received particular attention among the various reaction media that are discussed as alternatives to classical organic solvents. The aim of this chapter is to compare these two media directly and to critically evaluate their potential for synthetic organic chemistry.

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Free-Radical Chemistry in Supercritical Carbon Dioxide

Green Chemistry Using Liquid and Supercritical Carbon Dioxide ◽

10.1093/oso/9780195154832.003.0008 ◽

2004 ◽

Author(s):

J. M. Tanko

Keyword(s):

Carbon Dioxide ◽

Free Radical ◽

Supercritical Carbon Dioxide ◽

Green Chemistry ◽

Chemical Reactions ◽

Environmentally Benign ◽

Supercritical State ◽

Liquid Phases ◽

Temperature And Pressure ◽

Supercritical Carbon

During the 1990s, the chemical industry has focused on ways to reduce and prevent pollution caused by chemical synthesis and manufacturing. The goal of this approach is to modify existing reaction conditions and/or to develop new chemistries that do not require the use of toxic reagents or solvents, or that do not produce toxic by-products. The terms “environmentally benign synthesis and processing” and “green chemistry” have been coined to describe this approach where the environmental impact of a process is as important an issue as reaction yield, efficiency, or cost. Most chemical reactions require the use of a solvent that may serve several functions in a reaction: for example, ensuring homogeneity of the reactants, facilitating heat transfer, extraction of a product (or by-product), or product purification via chromatography. However, because the solvent is only indirectly involved in a reaction (i.e., it is not consumed), its disposal becomes an important issue. Thus, one obvious approach to “green chemistry” is to identify alternative solvents that are nontoxic and/or environmentally benign. Supercritical carbon dioxide (sc CO2) has been identified as a solvent that may be a viable alternative to solvents such as CCl4, benzene, and chloroflurocarbons (CFCs), which are either toxic or damaging to the environment. The critical state is achieved when a substance is taken above its critical temperature and pressure (Tc, Pc). Above this point on a phase diagram, the gas and liquid phases become indistinguishable. The physical properties of the supercritical state (e.g., density, viscosity, solubility parameter, etc.) are intermediate between those of a gas and a liquid, and vary considerably as a function of temperature and pressure. The interest in sc CO2 specifically is related to the fact that CO2 is nontoxic and naturally occurring. The critical parameters of CO2 are moderate (Tc = 31 °C, Pc = 74 bar), which means that the supercritical state can be achieved without a disproportionate expenditure of energy. For these two reasons, there is a great deal of interest in sc CO2 as a solvent for chemical reactions. This chapter reviews the literature pertaining to free-radical reactions in sc CO2 solvent.

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