Research Philosophy
Research Interests Gansäuer Group
As highlighted in the resolution adopted by the general assembly of the United Nations ‘Transforming our world: the 2030 Agenda for Sustainable Development’, sustainability is a global key-issue.1Catalysis is an enabling branch of science providing access to novel materials and processes. It is therefore ideally suited to deliver sustainable solutions for the demands of modern societies. As pointed out by Anastas2 this is so, because catalysis can be employed
- in the design of processes that maximize the amount of raw material ending up in the product
- in the use of renewable material feedstocks and energy sources
- in the use of safe, environmentally benign substances, including solvents
- in avoiding the production of waste.
Moreover, catalysis offers unique perspectives for the synthesis of substances that increase the quality of live, such as drugs.
To fully exploit the potential of catalysis in this context we are focusing on two broad research areas:
- We are interested in the use of radicals as key-intermediates for catalysis in single electron steps, a concept that has emerged from our investigations on sustainable catalytic radical reactions. It features high atom-economy and allows the exploitation of renewable energy sources.
- We employ epoxides as ‘spring-loaded’ substrates for the design of novel catalytic reactions.
We cooperate with many national and international groups on many aspects of our work, such as catalyst design, reaction design, synthesis of complex molecules, and mechanistic as well as electrochemical problems. Many of these projects involve joint grants and active exchange of students between the groups. Our international partners include the groups of Bob Flowers at Lehigh University (USA), Kim Daasbjerg (Aarhus, DK), Jack R. Norton (Columbia University, USA), Juan Manuel Cuerva and Enrique Oltra (Granada, Spain). Within the framework of the SFBs in Bonn we have actively interacted with the groups of Stefan Grimme and Olav Schiemann.
1. Catalysis in single electron steps:
Catalysis in single electron steps is a concept that allows conducting radical reactions under reagent (or catalyst) control and with high or complete atom-economy.3Catalysis in single electron steps is related to classical organometallic catalytic reactions, such as the Heck-reaction or Pd-catalyzed cross-coupling, because it is initiated by an oxidative addition and terminated by a reductive elimination. However, the steps occur in single electron steps4and it is therefore mandatory to identify metal complexes than can easily shuttle between neighboring oxidation states (and not in two electron steps). The radical translocation step that takes place between oxidative addition and reductive elimination can in principle be any elementary radical reaction, including C-C bond forming steps and C-H bond forming steps.
In our research, epoxides are predominantly used as radical precursors and the titanocene(IIII)/titanocene(IV) redox couple mediates catalysis. Examples of reactions proceeding by catalysis in singe electron steps are epoxide arylations,5epoxide hydrosilylations,6tetrahydrofuran forming reactions7, and most recently epoxide hydrogenation by dual catalysis8and the use of renewable energy sources for catalyst generation.9,10
a) Epoxide arylations:5
The catalytic cycle of the epoxide arylation is comprised of four steps. Radical generation is accomplished by single electron oxidative addition of the titanocene(III) catalyst to the epoxide. After radical translocation via addition to the arene, the single electron reductive elimination takes place by the ‘back electron transfer’ from the radical σ–complex to the pendant titanocene(IV) and subsequent protonation of the Ti-O bond. The last two steps may be concerted.
The substrates are readily available, the reaction can be carried out with low catalyst loading, catalyst performance can be fine-tuned by modulation of the redox-properties of the catalyst, and the reaction is an excellent and sustainable method for the preparation of N-heterocycles.
b) Epoxide hydrosilylation:6
Epoxide hydrosilylations are a class of virtually unexplored reactions. Our titanocene catalyzed reaction proceeds via catalysis in single electron steps and features an intramolecular HAT as key step. The reactions are amongst the most diastereoselective reductions of acyclic radicals. Our hydrosilylation is of high interest for large scale synthesis because they can be carried out with low catalyst loading (<1 mol%) and the less substituted alcohols can be obtained with very high regioselectivity. Catalyst regeneration is accomplished by a σ–bond metathesis reaction.
Recent collaborative investigations have shown how the reaction can be carried out with very low catalyst loading (0.5 mol%).
c) Tetrahydrofuran forming reactions:7
We have developed a convenient synthesis of polycyclic tetrahydrofurans based on catalysis in single electron steps. As above, the single electron oxidative addition is constituted by epoxide opening. After radical translocation via 5-exo cyclization, the single electron reductive elimination takes place via tetrahydrofuran formation. This step exploits the difference in ring-strain between the three and five membered rings and can be considered as organometallic oxygen rebound. The efficiency of the overall process is critically dependent on the substitution pattern the titanocene catalyst. The modification of the cyclopentadienyl ligand and the inorganic ligands lead to more active catalysts that also display a broader substrate scope.
d) Epoxide hydrogenation:8
The hydrogenation of epoxides to the less-substituted epoxide is a large unexplored area of research. However, it is very attractive for a two-step synthesis of anti-Markovnikov alcohols consisting of olefin epoxidation and epoxide hydrogenation. Together with the group of Norton (Columbia University, USA), we have developed a system featuring Ti- and Cr-catalysts for a unique H2-activation and epoxide hydrogenation. Our work was highlighted in the ‘Frankfurter Allgemeine Zeitung (FAZ)’.
e) Use of renewable energy sources:9,10
A key-step of all of our reactions featuring catalysis in single electron steps is the initial reduction of the titanocene(IV) precatalyst to a catalytically active titanocene(III) complex. We have recently shown how this step can be realized using electrolysis in the presence of thiourea additives and with visible light.
2. Catalytic reactions of epoxides:
Opening reactions of suitably substituted epoxides can lead to important classes of compounds provided that the regioselectivity of ring opening can be controlled efficiently. We have recently focused on the synthesis of 1,3– and 1,4–difunctionalized building blocks from α– or β–functionalized epoxides.
Regiodivergent epoxide opening (REO):11
Enantiomerically pure cis-1,2-disubstituted epoxides can be opened with high regioselectivity through ET from enantiomerically pure titanocene(III) complexes. Reductive radical trapping and catalyst regeneration results in the liberation of the desired products typically in high yield and selectivity. Since our process is catalyst controlled the enantiomer of the catalyst provides the regioisomer of the product in high yield and almost equally high regioselectivity. Therefore, two products can be prepared with high selectivity and yield from a single substrate by judicious choice of the catalyst. Our regiodivergent process was amongst the first efficient examples in the field of epoxide opening and provides access to numerous (poly-) functional building blocks for the synthesis of complex products.11
More recently, we have shown how the use of regiodivergent catalysis can employed in a highly sustainable synthesis of either enantiomerically pure indolines or tetrahydroquinolines from the same substrates.12
Fluoride catalyzed hydrosilylations of β–hydroxy epoxides:13
β–Hydroxy epoxides are a readily accessible compounds that can be prepared enantiomerically pure on reasonably large scale. In the presence of a catalytic amount of a fluoride (such as TBAF) and a stoichiometric amount of a silane (such as PhSiH3) these epoxides are opened with high regioselectivity to yield 1,4-diols after work-up. Our reaction is much milder than related LAH-reductions and can even be performed under air. In the context of 1,3-diol or 1,3-amino alcohol synthesis, the reactions allows unique SN2 reactions at tertiary carbon atoms.
Miscellaneous:
Reactions of Aziridines:14 Recently, we have demonstrated that substituted aziridines are also excellent substrates for radical generation. However, for titanocene catalysis, it is mandatory to use N-acylated aziridines as substrates to enforce binding to the catalyst.
Supramolecular Catalyst Stabilization and Activation:15 In many cases the active species of catalytic cycles are inactivated by the formation of dimers. For titanocene(III) complexes, we have found a way of resolving this problem through the addition of hydrochloride additives that form supramolecular complexes with the catalysts. In this manner, the monomer-dimer equilibrium is disrupted and the concentration of the active monomer substantially increased. Moreover, the stability of the catalyst is increased. We are currently investigating the general applicability of this concept and strategies for enantioselective catalyst activation.
Functional Organometallic Compounds: We have devised a novel modular synthesis of structurally and functionally diverse cationic titanocenes.16The compounds are highly interesting as catalysts in novel reactions, such as templated cyclizations17and epoxide reductions.18Other applications have emerged as well.
References:
1) https://sustainabledevelopment.un.org/post2015/transformingourworld
2) Anastas, P. T.; Kirchhoff, M. M.; Acc. Chem. Res. 2002, 35, 686-694.
3) Gansäuer, A. Hildebrandt, S.; Vogelsang, E; Flowers, R. A. II Dalton Trans. 2016, 45, 448-452. DOI: 10.1039/C5DT03891J
4) Gansäuer A.; Fleckhaus, A.; Alejandre Lafont, M.; Okkel, A.; Kotsis, K; Anoop, A.; Neese, F. J. Am. Chem. Soc. 2009, 131, 16989-16999. DOI: 10.1021/ja907817y
5) Gansäuer, A.; , Hildebrandt, S.; Michelmann, A.; Dahmen, T.; von Laufenberg, D.; Kube, C.; Fianu, G. D.; Flowers, R. A. II Angew. Chem. Int. Ed. 2015,54, 7003-7006. DOI: 10.1002/anie.201501955
6) a) Gansäuer, A.; Klatte, M.; Brändle, G. M.; Friedrich, J. Angew. Chem. Int. Ed. 2012, 51, 8891-8894. DOI: 10.1002/anie.201202818. b) Schwarz G. Henriques, D.; Zimmer, K.; Klare, S.; Meyer, A.; Rojo-Wiechel, E.; Bauer, M.; Sure, R.; Grimme, S.; Schiemann, O.; Flowers, R. A. II; Gansäuer, A. Angew. Chem. Int. Ed. 2016, 55, 7671-7675. DOI: 10.1002/anie.201601242 and 10.1002/ange.201601242
7) Cp2TiX Complexes for Sustainable Catalysis in Single Electron Steps, Richrath, R. B.; Olyschläger, T.; Hildebrandt, S.; Enny, D. G.; Fianu, G. D.; Flowers, R. A. II; Gansäuer, A. Chem. Eur. J. 2018, 24, 6371-6379. DOI: 10.1002/chem.201705707
8) Yao, C.; Dahmen, T.; Gansäuer, A.; Norton, J. Science 2019, 364, 764-767. DOI: 10.1126/science.aaw3913
9) Liedtke, T.; Spannring, P.; Riccardi, L.; Gansäuer, A. Angew. Chem. Int. Ed. 2018, 57, 5006-5010. DOI: 10.1002/anie.201800731
10) Zhang, Z.; Richrath, R. B.; Gansäuer, A.; ACS Catal. 2019, 9, 3208-3212. DOI: 10.1021/acscatal.9b00787
11) Funken, N.; Mühlhaus, F.; Gansäuer, A. Angew. Chem. Int. Ed. 2016, 55, 12030-12034. DOI: 10.1002/anie.201606064
12) Mühlhaus, F.; Weißbarth, H.; Dahmen, T.; Schnakenburg, G.; Gansäuer. A. Angew. Chem. Int. Ed. 2019, 58, 14208-14212. DOI: 10.1002/anie.201908860
13) a) Zhang, Y.-Q.; Funken, N.; Winterscheid, P., Gansäuer, A. Angew. Chem. Int. Ed. 2015, 54, 6931-6934. DOI: 10.1002/anie.201501729. b) Zhang, Y.-Q.; Poppel, C.; Panfilova, A.; Bohle, F.; Grimme, S.; Gansäuer, A. Angew. Chem. Int. Ed. 2017, 56, 9719-9722. DOI: 10.1002/anie.201702882
14) Zhang, Y.-Q.; Vogelsang, E.; Qu, Z.-W.; Grimme, S.; Gansäuer, A. Angew. Chem. Int. Ed. 2017, 56, 12654-12657. DOI: 10.1002/anie.201707673
15) Gansäuer, A.; Kube, C.; Daasbjerg, K.; Sure, R.; Grimme, S.; Fianu, G.; Sadasivam, D. V.; Flowers, R. A. II J. Am. Chem. Soc. 2014, 136, 1663-1671. DOI: 10.1021/ja4121567
16) Gansäuer, A.; Franke, D.; Lauterbach, T.; Nieger, M. J. Am. Chem. Soc. 2005, 127, 11622-11623. DOI: 10.1021/ja054185r
17) Gansäuer, A.; Worgull, D.; Knebel, K.; Huth, I.; Schnakenburg, G. Angew. Chem. Int. Ed. 2009, 48, 8882-8885. DOI: 10.1002/anie.200904428
18) Zhang, Y.-Q.; Jakoby, V.; Stainer, K.; Schmer, A.; Klare, S.; Bauer, M.; Grimme, S.; Cuerva J. M.; Gansäuer, A. Angew Chem. Int. Ed. 2016, 55, 1523-1526. 10.1002/anie.201509548
