Anions: the key to synthesis19/06/17
The ERC-funded project FRICatANIONS focuses on the use of ubiquitous anionic species for the development of innovative catalytic synthetic technologies. The ERC Consolidator Grant project FRICatANIONS – Frontiers in Catalytic Anion-Binding Chemistry – (project no. 724695) has been funded since June 2017 and is currently located at the Institute for Organic Chemistry at the University of Regensburg, Germany. With a duration of five years, the project investigates the interactions and role of anions in the different types of catalytic chemical reactions, which proceed via ionic intermediates. Within this framework, the development of new principles in the broad field of catalysis that enable innovative and efficient synthesis of organic compo unds was targeted, having as an ultimate objective the innovation, development and establishment of catalytic anion-binding processes as a general synthetic tool (Fig. 1).
Background to the project
Catalysis represents a key approach among the existing, most relevant and practical synthetic methods, since it is devoted to achieving efficient syntheses by contributing to lower energy consumption, optimising the use of natural resources, and minimising environmental impact. Consequently, catalysis has become a worldwide key research area to provide solutions to important societal demands. However, there is a continuous demand of new, more general and efficient methodologies to overcome emerging and current unsolved synthetic challenges. In this context, the so-called ‘anion-binding catalysis’ has very recently emerged as a new type of activation mode, opening new frontiers and enabling the discovery of unprecedented catalytic synthetic transformations.
Since the beginning of Life, Nature has exploited the ability of anions to tune the tridimensional structure and activity of macro-biomolecules such as enzymes. Anions play crucial roles in biological processes, as most of the cofactors’ and enzymes’ substrates are anionic species. Thus, interactions between enzymes and anionic species have a fundamental effect on molecular recognition, the induction of structural changes and of function activation/deactivation. This interaction of biomolecules with anions, normally referred to as an anion-binding process, relies on multiple non-covalent interactions such as electrostatic, hydrogen bonding or anion-π interactions. Important examples of anion-binding processes in Nature include membrane chloride channels (ClC), responsible for the regulation of chloride anion concentrations in the cells, and numerous enzymes such as the phosphate binding protein (PBP), which shows an important ATPase activity (Fig. 2, top).
Inspired by how efficiently Nature utilises anion-binding processes, early interest arose in the late 1960s in the area of supramolecular chemistry. In the decades since, different receptor structures for extraction and separation applications, anion-sensing devices and the development of responsive materials have been intensively investigated. Considering that all chemical processes comprise ionic species or the polarisation of at least one of the involved reactants, another interesting application was envisioned in non-covalent organic catalysis, leading to the emerging area of anion-binding catalysis in 2006. This type of catalysis implies the activation of an electrophilic ionic substrate, reactant or an in situ generated intermediate (R+) by binding to its counter-anion (A-) and the formation of a reactive close ion-pair, which can then react selectively with a reagent to form the desired product (Fig. 2, bottom).
Despite the huge potential of this counter-anion activation mode to solve stimulating synthetic problems, the fact that anion-binding processes can be essential for the design of new and more efficient catalytic transformations has only recently been recognised.
Aims and potential
In the past few years, Professor Dr Olga García Mancheño and her research team have made an important contribution to the field of anion-binding organic catalysis by introducing neutral C-H-bond-based oligo-triazole structures as a new class of catalysts, which have already shown a unique reactivity in the enantioselective dearomatisation of arenes to form valuable chiral N-heterocycles (Fig. 3).1,2,3 However, aiming at overcoming the existing limitations, García’s group started a research programme dedicated to broadening the nature of the catalysts that can be used in anion-binding driven reactions. Thus, in order to be able to effectively exploit the phenomenon of catalytic anion-binding processes in synthesis, there is an urgent need for i) more and structurally different catalysts, and ii) new approaches and combinations, which will allow the improvement of efficiency in relevant known catalytic reactions and the development of new transformations that are difficult or not possible to accomplish to date.
Based on the current limitations of the field and the above considerations, the aims of this project are:
- The design of novel anion-binding catalysts: Development of alternative catalysts exhibiting specific binding sites and interactions with differently shaped anions;
- To provide new concepts in anion-binding organocatalysis: Development of novel transformations using anion-binding activation of the nucleophilic reaction-partner;
- The development of catalytic anion-binding oxidative C-H functionalisation: Study of the effects of anion-binding in C-H functionalisation and the activation/modulation of the oxidants;
- To extend the application of anion-binding concepts to metal-catalysis: Study of the effects of anion-binding in cross-coupling reactions;
- To merge anion-binding and photocatalysis: Modulation of the activity of known photosensitisers and the development of novel chemical technologies for asymmetric photocatalysis; and
- To provide insights into the nature of the key interactions: Mechanistic investigations towards a basic understanding of the nature of the active catalytic species and complexes with anions, as well as the factors that determine the efficiency and selectivity of the targeted study-case reactions.
To sum up, diverse aspects in modern catalytic synthetic chemistry, including enantioselective organocatalysis, C-H bond functionalisation, metal-catalysed coupling reactions and photocatalysis, will be addressed in order to develop new transformations and enhance the efficiency and understanding of reactions involving anions. The proposed groundbreaking approaches have the potential to change the catalytic methods, as we know them nowadays; providing more general, efficient and easier-to-tune synthetic technologies.
1 M. Zurro, S. Asmus, S. Beckendorf, C. Mück-Lichtenfeld, O. García Mancheño, J. Am. Chem. Soc. 2014, 136, 13999;
2 O. García Mancheño, S. Asmus, M. Zurro, T. Fischer. Chem. Int. Ed. 2015, 54, 8823;
3 T. Fischer, Q.-N. Duong, O. García Mancheño, Chem. Eur. J. 2017, 23, 5983
Professor Dr Olga García Mancheño
Institute for Organic Chemistry
University of Regensburg & Research