Bioinspired Reactions

Oxidative coupling reactions, reduction reactions and catalytic processes are described in other chapters of this book and in recent reviews [86-88]. In this section, attention is focused only on some recent examples of synthetically useful processes where new C-L bonds are formed, and where the formation of a metal-C02 complex as an intermediate has been suspected or demonstrated by spectroscopic evidence and/or calculations. The peculiar case of a few bioinspired reactions is also cited. 

Schenk S, Notni J, Kohn U, Wermann K, Anders E (2006) Carbon dioxide and related heterocumulenes at zinc and lithium cations bioinspired reactions and principles. Dalton Trans 4191 206... 

The selective oxidation of alcohols to the corresponding aldehydes and ketones is of prime importance for organic synthesis, and various types of reagents have been described that achieve this transformation selectively and efficiently [141,142]. However, the number of sub-stoichiometric, nontoxic, non-hazardous oxidation systems has been relatively Umited. As copper enzymes such as galactose oxidase are known to catalyze this oxidation reaction, bioinspired homogeneous catalysts based on copper species have also been developed in recent years. 

Day two of the workshop opened with remarks by Leonard Buckley, Naval Research Laboratory, followed by the academic perspective on bioinspired chemistry, Solar Fuels A Reaction Chemistry of Renewable Energy presented by Daniel Nocera, Massachusetts Institute of Technology. 

Bioinspired Supramolecular Device and Self-Assembly for Artificial Photosynthetic Reaction Center... 

Finally, the best building blocks identified can be coupled in a modular way to complete photocatalytic reaction cycles, which then should be able to mimic a certain biological process. If these bioinspired photocatalytic systems are performing rmder identical conditions as their native counterparts, a direct comparison of quantitative criteria such as turnover frequencies and the total number of catal5dic cycles is possible and should always be the finad goal to demonstrate the potential usefulness of the biomimetic process. 

To activate carbon dioxide for chemical reactions, it is advantageous to fix and destabilize this rather inert molecule. An important strategy to control and influence the reactivity of CO2 is its coordination to amines, metal-bound imido moieties, or metal centers (144), which leads to a decrease of the CO bond order, while the molecule in most cases becomes considerably bent. In biochemical pathways, the CO2 adduct of the coenzyme biotin (17) is involved in various carboxylation and transcarboxylation reactions (146). Similar structural motifs could become important functional building blocks for bioinspired photoreactions involving carbon dioxide activation steps (Fig. 18). 

Despite the obvious versatility of light-activated key steps and their numerous advantages for the biomimetic modeling of natural systems, up to now, only very few examples are known, where such types of photosensitized processes have been successfully combined to complete reaction cycles with reasonable catalytic turnovers 6). In the last section, we are therefore briefly presenting two case studies which describe some recent work performed in our own group focusing on bioinspired catalytic systems that can be controlled and driven by visible light. 

The Diels-Alder reactions of cyclopentadiene with vinyl ketones in water were favoured by high temperature at 1000 atm, owing to increasing entropy.A bioinspired triazacyclophane tridentate ligand (58) has been developed for the Cu(II)-catalysed Diels-Alder reaction in water. The new organocatalyst, 1-naphthyl-trani-a,a -(dimethyl-l,3-dioxolane, 5-diyl)bis(diphenyl methanol) (TADDOL)/Emim BF4,... 

A variety of catalysts work well in hydrolyses reactions, including acids, bases, and salts. Basic catalysts give precipitated phases that are generally well-defined particles, whereas the acidic catalysts give more poorly defined, diffuse particles. - The solvent can be of considerable importance. In some cases, particles are not formed at all, and bicontinuous (interpenetrating) phases result. - Another approach uses poly(allylamine hydrochloride) in a buffered solution to obtain what was called bioinspired silica. ... 

S. Santra, P. R. Andreana, Angew. Chem. Int. Ed. 2011, 50, 9418-9422. A bioinspired Ugi/Michael/aza-Michael cascade reaction in aqueous media natural-product-Uke molecular diversity. 

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