Reactive solvent, selection

Water has physical hemical properties that are very different from those of other solvents [1] and its role in enhancing the reactivity and selectivity of some organic reactions is still a debated question. Recent experimental studies [3e, 9] and computer simulations [10] seem to indicate, at least with respect to the rate enhancement of aqueous Diels Alder reactions, that the main effects are due to the enforced hydrophobic interactions and hydrogen bond interactions. 

Chloroaluminate ionic liquids (typically a mixture of a quaternary ammonium salt with aluminum chloride see Table 6.9) exhibit at room temperature variable Lewis acidity and have been successfully used as solvent/catalyst for Diels-Alder reactions [57]. The composition of chloroaluminate ionic liquids can vary from basic ([FMIM]C1 or [BP]C1 in excess) to acidic (AICI3 in excess) and this fact can be used to affect the reactivity and selectivity of the reaction. The reaction of cyclopentadiene with methyl acrylate is an example (Scheme 6.31). 

This concept meshes with another important environmental issue solvents for organic reactions. The use of chlorinated hydrocarbon solvents, traditionally the solvent of choice for a wide variety of organic reactions, has been severely curtailed. In fact, so many of the solvents favoured by organic chemists have been blacklisted that the whole question of solvents requires rethinking. The best solvent is no solvent and if a solvent (diluent) is needed then water is preferred. Water is non-toxic, non-inflammable, abundantly available, and inexpensive. Moreover, owing to its highly polar character, one can expect novel reactivities and selectivities for organometallic catalysis in water. 

Choices of precursor(s) may be dictated by solubility, reactivity, or other property. For multicomponent systems, mutual solubility is another factor that must be considered. For such solutions, the solvent selected must facilitate dissolution of all precursors. 

The Diels-Alder reaction is among the most useful tools in organic chemistry. It has been the object of a great number of theoretical studies95-131 dealing with almost every one of the experimental aspects reactivity, mechanism, selectivity, solvent effects, catalysis and so on. 

In the early 1960s it became evident that the reaction environment had an important role in dictating the course of photochemical conversions acting on the course of the relaxation processes and stabilizing photoproducts.17 A constrained medium such as that of a porous matrix or a micelle provides the restricted environment to stop any bimolecular processes that could lead to degradation of products. These effects, however, are subtle. For instance, confinement of a molecule within a host instead of leading to inhibition of reactions of the trapped substrate often results in enhanced reactivity and selectivity because confinement does not mean steric inhibition of all motions of the entrapped host molecule which may eventually enjoy less restriction of some motions than in common solvents. 

In this chapter we wish to review the collected evidence for the astonishing effects of water on reactivities and selectivities as exemplified by the Diels-Alder reactions of dienes. Examples of Lewis acid and micellar catalysis in aqueous media are also presented. Finally, the newest computational investigations including solvent effects on Diels-Alder reactions are put forward in order to rationalize some of the remarkable observations. 

The reachons of hydroxamic acid derivatives required carefully optimized reactions conditions that is, 3-arylallyl phosphates were used as substrates in conjunction with PhCFs/water 2 1 as solvent (Scheme 9.35) [69]. The reactivity and selectivity were influenced by base, and the best results were obtained with Ba(OH)2. 

Recently, it has been shown that the reactivity and selectivity of [ F]p2 in the electrophilic fluorination of l-DOPA can be modulated by using different acidic solvents (Scheme 7). These conditions have been exploited to produce clinically useful quantities of the regioisomer [5- F]fluoro-L-DOPA [69]. 

The use of water instead of organic solvents is key to attaining the goal of environmentally benign chemical synthesis. In this context, organic reactions in water are now of great interest and much research effort has been devoted to pursuing efficient reactions in water [1-5]. Unique reactivity and selectivity have been often observed in aqueous media, but one of the big issues is the stability of catalysts in water. Many active catalysts are not stable in water but decompose in the presence of even a small amount of water. To overcome this, we searched for efficient catalysts that are stable and can work well in aqueous media. 

There is an obvious and urgent need for more systematic kinetic evidence and isotope effect studies, especially for the less strai tforward processes, and even purely qualitative observations of relative reactivities, solvent and spectator ligand effects, selectivity, and the nature of minor products would be of the greatest value. 

Table 1.3 provides rate constants for the decay of selected carbocations and oxocar-bocations in H2O, TFE, and HFIP. As a general comment, water, methanol, and ethanol are highly reactive solvents where many carbocations that are written as free cations in standard textbooks have very short lifetimes. The diphenylmethyl cation, with two conjugating phenyl groups, has a lifetime in water of only 1 ns. Cations such as the benzyl cation, simple tertiary alkyl cations such as tert-butyl, and oxocarbocations derived from aldehydes and simple glycosides, if they exist at all, have aqueous lifetimes in the picosecond range, and do not form and react in water as free ions. This topic is discussed in more detail in Chapter 2 in this volume. 

As will happen, for example, with perfluorotriethylamine, a substance that is a good alternative reaction medium may be found for reactions and it can be easily recycled by simple extractive workup (Nakano and Kitazume, 1999). One of the most important issues in twenty-first century scientific society is the environmental problem. Thus far, there have been increasing demands for substitutes for toxic and/or harmful solvents like CH2CI2. Recently, inert and less toxic fluorous (perfluorinated) fluids have been recognized and are attracting attention as new alternative solvents (Fish, 1999). This fact shows the need to survey the scope and limitations of the utility of these liquids as alternative reaction media for reactions, which are of great current interest due to their unique reactivities and selectivities. 

The addition of certain ionic promoters to ruthenium catalytic solutions has been found to dramatically affect the rate and selectivity of CO hydrogenation. Whereas ruthenium solutions do not otherwise produce ethylene glycol as a significant product (except as its derivatives in in reactive solvents),... 

Selective Absorption in the Sweetening Process. In recent years removal of the acid gas (H2S, CO2) components from a gas stream has increasingly been by absorption in a solvent system containing amines. While non-reactive solvent sweetening processes are in use, the ability of the basic amine to react chemically with the acid gas to yield water soluble salts has favored the chemical sweetening system. Thus... 

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