From Inorganic/Organic Hybrid Catalysis to Non-Innocent Ligand Catalysis: New Concepts for (Enantioselective) Group 2 Metal Catalysis

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There is an increasing interest in catalysts of more earth-abundant metals that are non-poisonous, cheap and price-stable. Our group is among the pioneers in organocalcium catalysis, a fast-growing area that has been extended to general group 2 metal catalysis (Mg, Ca, Sr, Ba). In contrast to transition metals, the group 2 metals cannot easily switch between oxidation states and do not posses d-orbitals for substrate activation. Catalysis with such metals is purely based on polar reaction mechanisms and Lewis-acid activation. We recently discovered that Lewis-acid activation is crucial for conversion of unactivated substrates and developed inorganic/organic hybrid catalysts that consist of a metal salt (CaI2) and a strong neutral organic base (Schwesinger-base P4) for intramolecular alkene hydroamination. Preliminary investigations on catalyst systems in which CaI2 is replaced for LCa (L = unreactive chelating dianionic ligand) showed that in some cases P4 is not needed. The ligand L showed a higher reactivity as expected and likely takes actively part in the catalytic cycle as a non-innocent ligand. Although the concept of cooperative catalysis is well-known in transition metal catalysis, it is new to group 2 metal catalysis.The research project will be carried out by two PhD students that work closely together and aims to tackle the two main shortcomings of group 2 catalysis: (i) limited functional group tolerance (ii) limited enantioselectivity control due to Schlenk equilibria L*MR <=> L*2M + MR2 (L* = chiral ligand). We propose extensive investigations on inorganic/organic hybrid catalysts in the intramolecular hydroamination of alkenes (variation of the metal salt, the organic neutral base, substrates) and an in-depth study towards the mechanism by experimental methods (isolation of intermediates, NMR, kinetics) as well as by ab initio theory. We are also interested in catalyst mixtures like LM/P4 (L = unreactive, dianionic spectator ligand, M = group 2 metal, Zn). Increasing the Brönsted basicity of L will result in a switch from hybrid catalysis to cooperative catalysis. The border between these different catalyst types will be probed by making systems of increasing Brönsted basicity. As both types of catalysts are not subject to Schlenk equilibria, enantioselective catalysis is an interesting application. We propose syntheses of a large number of chiral catalysts (L*M and L*M/P4). Both concepts, inorganic/organic hybrid catalysis and cooperative catalysis, will also be tested for their scope in other transformations (alkene hydrophosphination and hydrosilylation) and for functional group tolerance. Mechanisms will be studied by ab initio calculations.These new catalytic systems may result in important breakthroughs in group 2 metal catalysis.