Diels-Alder reactions, water chemistry

Abbott, A.P., G. Capper, D.L. Davies, R.K. Rasheed and V. Tambyrajah, Quaternary Ammonium Zinc- or Tin-Containing Ionic Liquids Water Insensitive, Recyclable Catalysts for Diels-Alder Reactions, Green Chemistry, 4,24—26 (2002). 

This thesis contributes to the knowledge of catalysis in water, us it describes an explorative journey in the, at the start of the research, unh odded field of catalysis of Diels-Alder reactions in aqueous media. The discussion will touch on organic chemistry, coordination chemistry and colloid chemistry, largely depending upon the physical-organic approach of structural variation for the elucidation of the underlying mechanisms and principles of the observed phenomena. 

Of all the work described in this thesis, this discovery is probably the most significant. Given the fact that the arene - arene interactions underlying the observed enantioselectivity of ftie Diels-Alder reactions described in Chapter 3 are also encountered in other organic reactions, we infer that, in the near future, the beneficial influence of water on enantioselectivity can also be extended to these transformations. Moreover, the fact that water can now be used as a solvent for enantioselective Lewis-add catalysed reactions facilitates mechanistic studies of these processes, because the number of equilibria that need to be considered is reduced Furthermore, knowledge and techniques from aqueous coordination chemistry can now be used directly in enantioselective catalysis. 

The kinetic effects of water and of cyclodextrins on Diels-Alder reactions. Host-guest chemistry, part 18 [65c]... 

The activation of various reactions by Lewis acids is now an everyday practice in synthetic organic chemistry. In contrast, solvent effects on Lewis acid catalysed Diels-Alder reactions have received much less attention. A change in the solvent can affect the association step leading to the transition structure. Ab initio calculations on the Diels-Alder reaction of cyclopentadiene and methyl vinyl ketone in aqueous media showed that there is a complex of the reactants which also involves one water molecule119. In an extreme case solvents can even impede catalysis120. The use of inert solvents such as dichloromethane and chloroform for synthetic applications of Lewis acid catalysed Diels-Alder reactions is thus well justified. General solvent effects, in particular those of water, will be discussed in the following section. 

The controversy on the existence of in vivo Diels-Alder reactions cannot be put to rest here, but the numerous examples of natural products containing cyclohexene groups and the catalytic effectivity of biological surroundings support the idea of in vivo Diels-Alder reactions. Apart from cell-free extracts, RNA-based mixtures of metals also show catalytic activity and it was demonstrated that this catalyst system can be quite effective as an artificial Diels-Alder-ase . We will show that water, the prime solvent of biosynthesis, also catalyses [4 -+- 2]-cycloadditions. Considering that biosyntheses are often of exceptional selectivity, it is clear that understanding biomimetic transfonna-tions in water as the solvent is an important goal of modem chemistry. The possibilities offered by and the reasons for Diels-Alder catalysis in water will be the main topic of this chapter. 

Reusability is a characteristic of the sensitizers prepared by stirring Ceo-fullerene with aminomethylated poly(styrene/vinylbenzene). They have been used to promote the standard O2 oxidation processes such as ene and Diels-Alder reactions (Scheme 7), and catalysts suitable for photoxidations in water have been prepared from them by reaction with poly(allylamine). The same reactions have been carried out using a novel solvent-free procedure which involves loading a porphyrin into solvent-swollen polystyrene beads and carrying out the photo-oxidation in neat liquid substrate. The formation of the allylic hydroperoxide (89) from p-pinene, with complete conversion and in 84% yield, is particularly noteworthy, as the standard liquid-phase reaction can be problematic. It is suggested that the possibility of using this approach under solar conditions is further evidence of the sustainable, green chemistry potential of synthetic photochemistry. 

Wittkopp, A. and Schreiner, PR. (2003) Metal-free, noncovalent catalysis of Diels-Alder reactions by neutral hydrogen bond donors in organic solvents and in water. Chemistry - A European Journal, 9, 407 14. 

N. K. Sangwan, H.-J. Schneider, The kinetic effects of water and of cyclodextrins on Diels-Alder reactions. Host-guest chemistry. Part 18, J. Chem. Soc., Perkin Trans. II, 1989, 1223-1227. 

When talking about organic and chemical synthesis in water, probably the first question that many people might ask is "why water " Perhaps a better question is "why not water " This question was asked by Professor Breslow in the early 1980 s during his study of Diels-Alder reactions and is an area of considerable research interest in the chemical community today. After all, aqueous chemistry is one of the oldest forces of change in the solar system (i), and water has been the solvent for all the biological transformations in nature. 

This experiment demonstrates Green Chemistry through the Diels-Alder reaction, which is an important reaction in organic chemistry because it is an important method of ring formation. The "green" components of this experiment include attention to atom economy and waste reduction, but the most important "green" aspect is the use of water as the solvent. Not only is water an environmentally benign solvent, but it also actually improves other aspects of this reaction due to hydrophobic solvent effects. 

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