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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SILYCAT (Asymmetric Catalysis with Chiral Silylium Cations)

Teaser

Summary

The activation of thermally stable small molecules is one of the central tasks in catalysis and modern synthetic chemistry, closely related to some important issues for our society such as hydrogen economy or carbon dioxide conversion into organic materials. Such processes require new catalysts. Therefore, the access to new highly active species is the key to gain in efficiency and to find novel activation modes, which will potentially allow to develop new catalytic reactions.
In this context, we considered silyl cations, which present extraordinarily strong Lewis acid character, as catalysts having a high potential. However, the application of silylium ions in synthesis and catalysis is still quite limited due to their inherent exceptional reactivity. Moreover, chiral induction with these species has yet to be investigated.
We thus proposed in this research program, the development of new ligand systems which allow to finely control the stability and reactivity of silyl cations. These systems having the potential for chirality, another important objective of the project were the investigation of such chiral silylium ion complexes in an effort to combine their inherent high reactivity with enantioselective processes. The use of chiral silylium ions as efficient Lewis acid catalysts in organic transformations should have a significant impact on enantioselective organic synthesis.

Work performed

The project has achieved some of its objectives and milestones, including the development of new binaphthyl and carbazole based silylium ions and their behavior in chiral memory processes. We had unfortunately no time to study stable chiral anions, but were however able to carry out first catalysis tests. The preparation of optically pure or enriched organosilanes has required a lengthy sequence, such as the formation of diastereochemically distinct siloxanes, their separation and reduction into homochiral silanes. The positive point is that it also allowed us to develop a new straightforward method to prepare siloxanes from sterically hindered silanes and alcohols.
This work has been published in a high impact factor journal :
Pramanik, S.; Fernandes, A.; Liautard, V.; Pucheault, M.; Robert F., and Landais, Y.
Dehydrogenative Silylation of Alcohols Under Pd-Nanoparticle Catalysis. Chem. Eur. J. 2019, 25, 728-732.
It was also presented as a poster at the 1st Stable Carbene Symposium on carbene chemistry, Toulouse, France, November 30th 2018.

We also reported studies on the stabilization of silylium ion using internal as well as external coordination of Lewis bases. The extent of coordination has been revealed by the chirality studies. Loss of chirality at elevated temperature suggests an epimerization of the silylium ion. Thus, chirality of the silicon center in the silylium ion proved to be a useful tool to study the coordination of the silylium center with Lewis bases. Chiral memory was also clearly demonstrated at low temperature, depending on the nature of the substituents at silicon. When the silicon center was linked to a sp3 center, a certain freedom of the system allowed an efficient coordination of the highly Lewis acidic silylium center with a pyridine nitrogen and therefor a complete retention of the chirality at silicon. In contrast, when the silicon center was linked to a sp2 center, the coordination to nitrogen was weaker, then leading to a partial loss of the chiral memory. X-ray diffraction studies have been carried out recently which further support the stabilisation of the silylium cations species.
An article is under preparation and should be published during the first semester of 2019:
Pramanik, S.; Fernandes, A.; Laye, C.; Robert F., and Landais, Y.
Memory of Chirality with Silylium Ions. Manuscript under preparation, 2019.

Final results

Our strategy showed that chirality at silicon can be retained through the formation of silylium cations without the addition of any external chiral source. This investigation introduces silylium cations as potent catalysts in organic synthesis.
Further, we have shown that the extent of coordination of external Lewis base with silylium ion can be revealed by chirality studies. A stronger coordination with external Lewis base at lower temperature retains the stereospecificity at silicon, while a loss of chirality is observed at elevated temperature, suggesting an epimerization of the silylium ion. Thus, chirality of the silicon center in the silylium ion proved to be a useful tool to study the coordination of the silylium center with Lewis bases. Recent crystallization of the silylium cations and their X-ray diffraction studies further supported conclusions drawn from this research. This should be published during the first semester of 2019.
For the purpose of the synthesis of chiral silanes we have also developed a new dehydrogenative coupling of silanes in the presence of congested alcohols, a reaction for which very few effective methods have been described to date. The use of easily synthesized palladium nanoparticles opens a new access to siloxanes that has caught the attention of the editors of the Chemistry European Journal in which this work was published.