From Inorganic/Organic Hybrid Catalysis to Non-Innocent Ligand Catalysis: New Concepts for (Enantioselective) Group 2 Metal Catalysis
Final Report Abstract
Although cooperative catalysis with non-innocent ligands is well-established in transition metal chemistry, the first example of this concept in s-block metal catalysis was reported. This shows that the Ae metal catalysts not necessarily should have a classical build-up LAeR (L = spectator, R = reactive group). - The concept of hybrid catalysis, i.e. catalytic conversion with a combination of a Lewis acidic metal salt (e.g. CaI2) and a neutral organic base (e.g. a strong Schwesinger base) can be further extended but unfortunately there are very few alternatives for the Lewis acidic salt and no alternative for the Schwesinger base was found (the pKa value of the protonated base should > 39). - Using hybrid catalysis, the very first enantioselective alkene hydroamination has been achieved but ee values are generally low (< 30%). - Extending the concept to cooperative catalysis with non-innocent chiral ligands also gave generally poor ee values but one example with an ee value of 59% has been presented. Although we obtained a record ee of 59% in Ca catalyzed alkene hydroamination, the values are still too low to be of practical use. Therefore, enantioselective catalysis with heavier Ae metal catalysts (Ae = Ca, Sr, Ba) remains a challenge which likely is not going to be solved easily. - During the development of Lewis-acid/Brønsted-base hybrid catalysts a family of Ae metal amide complexes with fluorinated diphenylamide ligands, (C6F5)2N = NF, has been introduced. Crystal structures of homoleptic AeNF2∙(THF)2 and heteroleptic [NFAeN(SiMe3)2]2 complexes (Ae = Mg, Ca, Sr) are presented. The THF-free homoleptic complexes MgNF2 and CaNF2 have a Lewis acidity that is similar to that of B(C6F5)3. In hybrid catalysis, however, these complexes are inactive. The [NFAeN(SiMe3)2]2 complexes are exceptionally stable toward ligand exchange reactions. This is likely due to the favourable combination of electron-releasing and electron-withdrawing ligands which may be a new concept in group 2 metal chemistry for Schlenk equilibrium control.
Publications
- Dalton Transactions 2017, 46, 11192-11200. Bora-amidinate as a cooperative ligand in group 2 metal catalysis
B. Freitag, C. A. Fischer, J. Penafiel, G. Ballmann, H. Elsen, C. Färber, D. Piesik, S. Harder
(See online at https://doi.org/10.1039/c7dt02136d) - Dalton Transactions, 2019, 48, 6757-6766. Lewis Acidic Alkaline Earth Metal Complexes with a Perfluorinated Diphenylamide Ligand
C. A. Fischer, A. Rösch, H. Elsen, G. Ballmann, M. Wiesinger, J. Langer, C. Färber, S. Harder
(See online at https://doi.org/10.1039/c9dt01272a)
