Organic Reactions in Water

In order to be able to provide answers to these questions, a Diels-Alder reaction is required that is subject to Lewis-acid catalysis in aqueous media. Finding such a reaction was not an easy task. Fortunately the literature on other Lewis-acid catalysed organic reactions in water was helpful to some extent... 

Bcamples of metal-ion catalysed organic reactions in water where the catalyst acts exclusively as Lewis acid are the hromination of diketones" " and the decarboxylation of oxaloacetate. The latter reaction has been studied in detail. In 1941 it was demonstrated that magnesium(II) ions catalyse this reaction" Later also catalysis by other multivalent metal ions, such as Zn(II), Mn(II), Cu(II), Cd(ir), Fe(II), Pb(II), Fe(III)... 

To our knowledge, the results presented in this chapter provide the first example of enantioselective Lewis-acid catalysis of an organic reaction in water. This discovery opens the possibility of employing the knowledge and techniques from aqueous coordination chemistry in enantioselective catalysis. This work represents an interface of two disciplines hitherto not strongly connected. 

Catalyzed organic reactions in water, and the aqueous Diels Alder reactions in particular, are currently a topic of great interest [3f h]. 

Breslow R. Hydrophobic Effects on Simple Organic Reactions in Water Acc. Chem. Res. 1991 24 159-164... 

Aqueous mixtures, kinetics of organic reactions in water and, 14, 203... 

Kinetics, and mechanisms of reactions of organic cation radicals in solution, 20, 55 Kinetics, of organic reactions in water and aqueous mixtures, 14, 203 Kinetics, reaction, polarography and, 5, 1... 

For example, consider organic reactions in water. Beyond a general methodology for carrying out catalytic conversions in H20 mediated by doped ORMOSIL in the presence of a modest amount of surfactant,19 another recent method for the waste-free oxidation of alcohol affords high yields of commercially valued carbonyl compounds in water with complete selectivity and remarkable stability.20... 

This aggregation of organic molecules in water actually leads to accelerated rates for some organic reactions in water. Diels-Alder and other sigmatropic reactions work very well in water, despite not being soluble or miscible in that solvent. This effect is discussed further in Chapter 7. 

Another example of the use of Lewis acids in organic reactions in water is the lan-thanide(III) triflate catalysed aza-Diels-Alder reaction, exemplified in Scheme 14. In this reaction the hetero-dienophile is formed in situ from a primary ammonium hydrochloride and a carbonyl compound followed by the actual Diels-Alder reaction288,289. This type of reaction proceeds readily in aqueous media290-296, and a dramatic increase in the yield upon addition of lanthanide triflates was observed288,289. The exact role of the catalyst, however, is not entirely clear. Although it was suggested that the catalyst binds to the dienophile, other mechanisms, such as simple proton catalysis, are also plausible. Moreover, these reactions are further complicated since they are often heterogeneous. 

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. 

Organic reactions in water without the use of any organic cosolvents are among the ideal reactions for green chemistry. However, successful examples are still... 

Polymerization in P-cyclodextrin (CD) complexes with monomer offers a route to polymerization, as well as other organic reactions, in water without the need for organic solvents [Ritter and Tabatabai, 2002]. P-Cyclodextrins are toms-shaped, cyclic oligosaccharides obtained by degradation of starch. The hydroxyl groups of the glucose repeat unit of CD are located on the outer surface. This makes the outer surface hydrophilic, whereas the inner surface and cavity are hydrophobic. Water-insoluble monomers become solubilized in water when mixed with CD or CD derivatives because the monomers are absorbed into the cavity. This allows polymerization in aqueous, not organic media, with water-soluble initiators. 

Lindstrom U, editor. Organic reactions in water principles, strategies and applications. Oxford Blackwell Publishing 2007. 

Engbats, J.B.F.N. Blandama, M.J. (2001) Undastanding organic reactions in water from hydrophobic encounters to surfactant aggregates. Chan. Commun. 1701-8. 

Lindstrom, U.M. (2007) Organic Reactions in Water, Blackwell, Oxford. 

Classical organic reactions that have been carried out in water include, among others, the Diels-Alder reaction, the Claisen rearrangement, aldol condensations, Michael additions, and nucleophilic substitutions. In the Diels-Alder reaction, for example, water has been found to increase the reaction rate and to enhance the endoselectivity 120). Two reviews summarize the results for organic reactions in water 121). 

Breslow, R., Hydrophobic effects on simple organic reactions in water, Acc. Chem. Res., 1991,24, 159. 

---