Catalysis in Biological and Biomimetic Reactions

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. 

The whole catalytic cycle of this system, including the mononuclear Mo, Mo, and Mo species as well as the detrimental binuclear p-oxido species and relevant transition states for OAT, have also been viewed from a theoretical perspective [89]. This system hence represents a rare example in which only the heterogenized version is competent to perform the biomimetic catalysis while the homogeneous system is not and, furthermore, for which detailed information concerning the elementary reaction steps is available, being fully congruent with the biological prototype. 

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