Permanent URL to this publication: http://dx.doi.org/10.5167/uzh-57172
Romanato, P. Intramolecular stabilization of 2,6-diarylphenylsilylium ions by π-arene and lone pair-halogen coordination. 2011, University of Zurich, Faculty of Science.
Silylium ions are group 14 analogs of carbocation of the general structure R3Si+. Their most striking feature is their extreme electrophilicity. The generation of longlived
silyl cationic species has therefore necessitated the development of novel synthetic approaches and weakly nucleophilic conditions. It was only in 2002 that the first crystal structure of a triarylsilylium ion dispelled any doubt about the existence of tricoordinated silyl cations in the condensed phase. In the recent years, several research groups have succeeded in applying silylium ion chemistry to the preparation of other reactive intermediates and to the field of Lewis acid catalysis. The aim of this thesis is to expand the family of terphenylsilylium ions previously developed in the Siegel group. The terphenyl skeleton provides steric shielding of the positively charged cavity as well as an overall thermodynamic stabilization by internal !-coordination. A systematic study was performed in order to tune the Lewis acidity at silicon by reducing the electron density of the flanking rings. At first, halogen atoms were introduced at the ortho positions of the flanking rings with the effect of quenching !-arene coordination in favor of lone pair–halogen coordination to silicon. This coordination mode was confirmed via NMR, X-ray and computational studies.
A second generation of terphenylsilylium ions, featuring halogen atoms in the para position of the flanking rings, was synthesized with the aim of avoiding both lateral ring and halogen coordination, in favor of a very deshielded silylium ion. As a result, the intramolecular coordination of the positively charged cavity by the flanking rings was effectively reduced but the intermolecular coordination by the counterion became competitive, as shown by X-ray crystallography. We became interested in probing the energetic details of !-arene coordination compared to lone pair–halogen coordination to silicon, and therefore a series of cations bearing 2,6-difluoro- and 2,6-dimethyl-substituted rings were synthesized. Remarkably, a competition, rather than a cooperation, is established between the two modes of stabilization: with methylated rings of lower basicity the preferred interaction is F"Si, while with duryl and pentamethylphenyl substituents !-coordination is favored. In the search of silylium ions as active as the truly tricoordinated triarylsilylium ion, but sterically more accessible, a new family of phenylsilylium ions featuring aliphatic chains in place of the flanking aromatic rings was synthesized and studied.
These compounds exhibited very low field-shifted resonances in 29Si NMR spectroscopy, sign of a very deshielded cationic center. At first, the stability of these “naked” silylium ions, which did not display significant solvent or anion coordination, was attributed to agostic interactions with the nearby aliphatic groups. Recently, a new hypothesis has been formulated regarding a rearrangement of the initial cations into more stable triarylsilyl cationic structures, which are responsible for the low field-shifted resonances. Further investigations are now ongoing to elucidate the mechanism of this rearrangement.
|Referees:||Siegel J S, Baldridge K K|
|Communities & Collections:||07 Faculty of Science > Institute of Organic Chemistry|
|Date:||07 March 2011|
|Deposited On:||05 Mar 2012 11:29|
|Last Modified:||24 Jul 2012 18:26|
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