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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.1 Catalysis 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.
  •   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 actively interact with the groups of Stefan Grimme and Olav Schiemann.


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.3 Catalysis 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 steps4 and it is therefore mandatory to identify metal complexes than can easily shuttle between neighboring oxidation states in single electron steps (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,5 epoxide hydrosilylations,6 and tetrahydrofuran forming reactions.7

 

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.

epoxide arylation

The substrates are readily available, the reaction can be carried out with low catalyst loading, catalyst performance 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.

 

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.

epoxide hydrosilation

Recent collaborative investigations have shown how the reaction can be carried out with very low catalyst loading (<0.25 mol%).

 

Tetrahydrofuran forming reactions:4

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 of the cyclopentadienyl ligands.

tetrahydrofuran synthesis

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):7

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.

REO

Currently, we are working on a sustainable version of the reaction and on applications in natural products synthesis.

 

Fluoride catalyzed hydrosilylations of β–hydroxy epoxides:8

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.

 

EPOXIDE F

 

Miscellaneous:

Supramolecular Catalyst Stabilization and Activation:9 In many cases the active species of catalysts 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.10 The compounds are highly interesting as catalysts in novel reactions, such as templated cyclizations11 and epoxide reductions.12 Other applications have emerged as well. They include the gelation of a variety of organic solvents,13 a highly selective activity against cancer cells as well as solid tumors,14 and electrodes with immobilized titanocenes.

 

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. accepted for publication. 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)   Gansäuer, A.; Klatte, M.; Brändle, G. M.; Friedrich, J. Angew. Chem. Int. Ed. 2012, 51, 8891-8894. DOI: 10.1002/anie.201202818

7)   Gansäuer, A.; Shi, L.; Otte, M. J. Am. Chem. Soc. 2010, 132, 11858-11859. DOI: 10.1021/ja105023y

8)   Zhang, Y.-Q.; Funken, N.; Winterscheid, P., Gansäuer, A. Angew. Chem. Int. Ed. 2015, 54, 6931-6934. DOI: 10.1002/anie.201501729

9)   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

10) Gansäuer, A.; Franke, D.; Lauterbach, T.; Nieger, M. J. Am. Chem. Soc. 2005, 127, 11622-11623. DOI: 10.1021/ja054185r

11) Gansäuer, A.; Worgull, D.; Knebel, K.; Huth, I.; Schnakenburg, G. Angew. Chem. Int. Ed. 2009, 48, 8882-8885. DOI: 10.1002/anie.200904428

12) Zhang, Y.-Q.; Jakoby, V.; Stainer, K.; Schmer, A.; Klare, S.; Bauer, M.; Grimme, S.; Cuerva J. M.; Gansäuer, A. Angew. Chem. Accepted for publication. DOI: 10.1002/anie.201509548

13) Klawonn, T.; Gansäuer, A.; Winkler, I.; Lauterbach, T.; Franke, D.; Nolte, R. J. M.; Feiters, M. C.; Börner, H.; Hentschel, J.; Dötz, K. H. Chem. Commun. 2007, 1894-1895. DOI: 10.1039/b701565h

14) Gansäuer, A.; Winkler, I.; Worgull, D.; Lauterbach, T.; Franke, D.; Selig, A.; Wagner, L.; Prokop, A. Chem. Eur. J. 2008, 14, 4160-4163. DOI: 10.1002/chem.200800407

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