Skeletal Rearrangement Processes in Organometallic Cations of Group 14 Elements

Despite the highly interesting electronic properties and the technological potential of polysilanes the chemist`s repertoire for their synthesis or structural manipulation is mostly limited to the Wurtz type coupling reaction. In this connection Lewis acid catalyzed rearrangement reactions represent an interesting method to access molecular structures, which are otherwise difficult to obtain. Closely related to the well known Wagner-Meerwein rearrangement, it is a much more facile process for polysilanes. Although first examples of polysilane rearrangements have been reported almost 40 years ago not much is known about the governing principles of this reaction. The current project, to be carried out by the groups in Oldenburg and Graz, proposes a joint study comprising new synthetic approaches, investigations of intermediates and reaction mechanisms, both supported by the computational methods. Preliminary studies have already shown that the reaction of isomeric polysilanes with aluminium chloride can result in the formation of rather complex structures such as an adamantine type tricyclic polysilane. In addition it was found that germanium containing polysilanes rearrange with the germanium atoms selectively moved to trisilylated positions of the product. Other preliminary results suggest the Lewis acid catalyzed reaction can also be used for the synthesis of polysilanes containing alkylene units and even further for the preparation of complex carbosilanes. The synthetic work in this project, conducted by the Austrian group, will benefit from the theoretical and basic studies (of the German group) which will provide insights into the underlying principles of the chemistry. On the other hand will the synthetic work allow formulating new questions which will be addressed by computational methods. The unique combined synthetic and theoretical expertise of the two groups will allow the best possible treatment of the complex matter of the Lewis acid catalyzed rearrangement reactions.

In organic chemistry carbocations were postulated as important reaction intermediates more than 100 years ago and stable trivalent examples of these species in solution are known for more than 50 years. Analogous compounds with silicon bearing a positive charge should be even more stable given its diminished electronegativity. Though, the route to the first free silyl cations proved to be a difficult one and only in the early 1990s a first example was reported. Over the last years, however, the chemistry of silyl cations has developed into a flourishing field. The extreme Lewis acidity of these compounds renders them interesting as catalysts for a number of interesting reactions. In particular C-F bond activation chemistry and the acceleration and selectivity of challenging Diels-Alder reactions are in the focus of research. The current project was concerned with a sparsely investigated class of silyl cations, namely those with silyl substituents. The latter had been postulated as intermediates for the AlCl3 catalyzed rearrangement of polysilanes for a long time. Although we have substantiated this before using theoretical methods, we could now prove this experimentally by NMR spectroscopic characterization. The deliberate synthesis of silyl substituted silyl and germyl cations was accomplished by clean hydride abstraction using a stable carbocation. As expected the obtained cations rearranged to the most stable isomers. Strikingly, these compounds act as much better catalysts than the previously used aluminum chloride in the polysilane rearrangement reaction. In the course of attempted formation of silyl dications by abstraction of two hydride equivalents, hydrogen bridged silyl cations were obtained, where the positive charge is delocalized between two silyl groups and a hydrogen atom. The extremely high reactivity of the obtained silyl cations poses a problem. As the compounds decompose at temperatures above -20C so far crystal structure analysis was not possible. By addition of trialkylphosphanes adduct formation occurs which diminishes the reactivity but still allows reactions with substrates that are able to displace the phosphane.One of the original driving forces behind the project was the study of Lewis acid mediated alkyl migration reactions of oligosilanes. While so far the research focus was on silyl migration it was clear for a long time that silyl cation formation occurs via abstraction of alkyl groups, which in the course of the reaction can be shifted in and even between silane molecules. The alkyl shift reactions can be exploited by initial introduction of a very bulky trialkysilyl group and a subsequent rearrangement reaction which distributes the bulky alkyl groups evenly. The thus obtained molecules can serve as sterically enhanced starting materials for further oligosilanes chemistry.