Novel organocatalysts and cooperative catalytic procedures for the utilization of CO2 as synthetic building block
Various studies suggest a close connection between global climate change and the emission of anthropogenic greenhouse gases. The by far largest part of this emission is accounted to carbon dioxide (CO2). Beside the reduction of CO2 emission, its use as synthetic building block is the central point of the overall CO2 management strategy. The atom economic and efficient utilization of CO2 as synthetic building block is closely connected to the effective activation of this very stable molecule.
The aim of our work is the development of novel metal free catalysts, so called organocatalysts, for the synthesis of industrially relevant products with CO2 as C1-building block. The combination of those catalysts with metal or enzyme catalyzed procedures in (sequential) one pot reactions shall lead to innovative and sustainable catalytic systems with high selectivity and energy efficiency, respectively. Those alternative methods with inclusion of up- and downstream steps shall afford a change and extension of the raw material base utilizing CO2. Objects of our studies are transformations and products of high industrial interest and large CO2-fixation potential.
Below a short outline about two selected subprojects will be given:
Synthesis of cyclic carbonates
So far we prepared a series of bifunctional phosphonium salts and evaluated their potential as catalysts under various reaction conditions in the synthesis of cyclic carbonates from epoxides and carbon dioxide. Based on this catalyst screening we determined structure activity relationships and identified promising candidates. Moreover, the reaction parameters were optimized for a model reaction. Even under mild conditions (90°C, p(CO2)= 10 bar) quantitative conversions were obtained after 2–3 h for 15 different epoxides. The reaction was also performed on a multi gram scale and monitored by in situ FTIR.
Synthesis of polycarbonates
Bis-urea derivatives are known as oxo-anion receptors. We prepared a series of those compounds and employed them as catalysts in the copolymerisation of CO2 with epoxides. The urea derivatives were envisioned to stabilize the anionic intermediates in polymer chain growth and, thus to support the incorporation of carbon dioxide in the polymer chain. This should lead to a polymer with well defined structure and material characteristics. Depending on the catalytic system and the reaction conditions we obtained copolymers with carbonate linkage >80% and molecular weights >35.000 g·mol–1 in excellent yields.
This project is funded by the Federal Ministry of Education and Research (BMBF) within the funding initiative "Technologies for Sustainability and Climate Protection – Chemical Processes and Use of CO2".
Novel organocatalyzed processes based on phosphorous containing catalysts
Phosphines are an important class of ligands in metal complexes. Due to their outstanding roll in homogenous catalysis numerous publications concerning the synthesis of achiral as well as chiral phosphine based ligands have been reported. In addition many of them are commercially available. However phosphines and their derivatives are not only employed as ligands but are also important reagents in organic synthesis. For example, they mediate the conversion of alcohols to halides (Appel reaction), the olefination of ketones and aldehydes (Wittig reaction) and the synthesis of imines via Staudinger reaction. In contrast to their tremendous importance as ligands and reagents the application of phosphines as organocatalysts is not very well established.
We are exploring the application of phosphorus based organic compounds as catalysts in a variety of reactions. Therefore we are utilizing their Lewis basic and Lewis acidic properties, respectively. On the one hand our aim is to develop asymmetric versions of the shown reactions by employing chiral phospines or derivatives. On the other hand we shall develop catalytic (asymmetric) variants of methods in which phosphines are employed in stoichiometric amounts up to now, e.g. Wittig Reactions.
A short outline about selected projects will be given below:
Asymmetric intramolecular γ‑addition to activated alkynes
A variety of O-, S-, and N-functionalized activated alkynes were prepared and cyclised in the phosphine catalyzed intramolecular γ-addition. For the first time we successfully converted N‑derivatives in this reaction. In an intense screening of chiral phosphine catalysts products with up to 84% ee were obtained.
Addition of Et2Zn to aldehydes
Beside the application of phosphines as Lewis basic catalysts we are interested in the utilization of phosphonium salts as potential Lewis acidic organocatalysts. In this context we studied the addition of diethyl zinc to aldehydes. It is known that this reaction can be catalyzed by Lewis acids. We showed that simple Bu4PCl is an efficient catalyst for this reaction. However, when chiral phosphonium salts were employed only racemic products were obtained and a strong influence of the anion was observed. Further investigations revealed that diethyl zinc is activated by the Lewis basic counter ion and the reaction can be catalyzed by a combination of simple alkaline metal salts and crown ethers.
Catalytic Wittig reaction
In our efforts to convert phosphine mediated processes into catalytic variants we investigated the Wittig reaction. So far we developed a microwave assisted catalytic variant employing commercially available tributyl phosphineoxide as well as a thermal method utilizing a simple accessible phospholane oxid as pre-catalyst. The right combination of reducing agent, base and solvent enabled us recently to convert over 20 substrates. In some cases yields and selectivities were significantly higher than previously reported.
Metallhydrid Mediated Tishchenko Reaction
During our studies concerning the catalytic Wittig reaction we observed the dimerization of benzaldehyde yielding benzyl benzoate (Tishchenko reaction) when sodium hydride was employed as the base in our model reaction. Based on this result we developed an efficient method for the dimerization of aromatic aldehydes. The reaction was also performed on large scale and easily monitored by in situ FT-IR spectroscopy. A mechanism was postulated and confirmed by labelling and capture experiments. Thus, we developed an efficient method for the dimerization of aromatic as well as hetero aromatic and aliphatic aldehydes.