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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - TAML-ArM (Development of an artificial alkane monooxygenase: A leap in bioinspired oxidation catalysis)

Teaser

Summary

Chemistry plays a central role in many aspects of modern society. Chemical products are ubiquitous in our daily life, ranging from pharmaceuticals prodcts, plastics, agriculture, etc. One of the main challenges that chemists face nowadays is the development of new processes to fulfil the societyâ€™s needs by manufacturing new products, compounds and materials in an economical and environmentally friendly way.

Catalysis, the process of accelerating a chemical reaction by adding certain chemical compounds (the latter being typically metal-based), has undoubtedly had a crucial impact in the development of new synthetic protocols during the last century. Catalytic methodologies have many advantages compared to traditional stoichiometric processes: they offer undisputable economy in reagent use by carrying out a transformation multiple times per catalyst molecule, and they proceed under mild reaction conditions, thus saving energy and reducing the amount of raw materials consumed. From a chemical point of view, catalytic reagents are used for improving the selectivity of a given chemical reaction, and they allow the development of unprecedented chemical reactions, the introduction of new features to organic molecules, etc.

The selective functionalization of strong Carbon-Hydrogen (C-H) bonds is one of the Holy Grail reactions of our times. Nature has developed chemical processes for inserting oxygen atoms in these inert bonds. These reactions are mainly performed by iron-dependent enzymes that are able to generate highly reactive iron-oxygen compounds which can hydroxylate hydrocarbons with exquisite selectivity. These enzymes have served as a source of inspiration to bioinorganic chemists, who have been striving to develop methodologies that allow performing similar oxidation reactions in a lab, in a so-called bioinspired approach. Undoubtedly, developing new ways of transforming hydrocarbon substrates into functionalized high-value-added products (i.e. alcohols, or epoxides) in a more environmentally friendly way and larger scale is of critical importance.

The field of bioinspired homogeneous catalysis has achieved significant milestones. Selected iron complexes bearing nitrogen-based ligands (that resemble the structure of the active site of enzymes) have been developed and their combination with hydrogen peroxide (as an alternative to oxygen acting as oxidant) elicits site-selective C-H bond oxidation. Even though some of the reported systems exhibit truly remarkable selectivities in the oxidation of strong C-H bonds in complex molecules, they display limited catalytic activity compared to the natural enzymes, possibly because of side reactions that lead to catalyst deactivation.

The assembly of Artificial Metalloenzymes (ArMS), that result from anchoring a metal catalyst to a protein and thus resemble some natural enzymes, has emerged as an attractive to homogenous catalysts during the last decade: in a sense, these systems provide a bridge between homogeneous catalysts and enzymes. As a result, ArMs exhibit some remarkable features that make them promising alternatives to traditional catalysts. In a biomimetic spirit, the well-defined secondary sphere coordination around the metal cofactor provided upon incorporation within the host offers fascinating perspectives to optimize metal-catalyzed transformations to exquisite levels of activity and of selectivity.

Work performed

During the development of the TAML-ArM project, we have aimed at developing a family of novel artificial metalloenzymes with the final aim of producing a functional monooxygenase. Our approach was based on the anchoring of the metal cofactor within streptavidin, a well-known protein in the Ward group. After assembling the hybrid catalyst, we tested its catalytic abilities towards the oxidation of hydrocarbons, such as ethylbenzene. In order to approach physiological reaction conditions, we used hydrogen peroxide as an oxidant in an aqueous medium.

Final results

In the long-term, one could envision directed evolution of the synthesized hybrid catalysts will afford an artificial methane monooxygenase with properties approaching that of natural methane monooxygenases. The knowledge that has been gained during the TAML-ArM project opens the door to the functionalization of complex molecules using mild methodologies (i.e. earth-abundant metals, environmentally friendly oxidants and mild reaction conditions). Indeed, developing more sustainable approaches towards chemical synthesis is one of the main challenges for the next decade. The development of artificial monooxygenases could be applied, for instance, at a given synthesis step during the production of pharmaceutical high-value added intermediates.