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Silaphenolates and Silaphenylthiolates: Two Unexplored Unsaturated Silicon Compound Classes Influenced by Aromaticity
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Biochemistry and Organic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Biochemistry and Organic Chemistry.
2012 (English)In: Molecules, ISSN 1420-3049, Vol. 17, no 1, 369-389 p.Article in journal (Refereed) Published
Abstract [en]

Monosilicon analogs of phenolates and phenylthiolates are studied by quantum chemical calculations. Three different silaphenolates and three different silaphenylthiolates are possible; the ortho-, meta-, and para-isomers. For the silaphenolates, the meta- isomer is the thermodynamically most stable, regardless if the substituent R at Si is H, t-Bu or SiMe3. However, with R = H and SiMe3 the energy differences between the three isomers are small, whereas with R = t-Bu the meta- isomer is similar to 5 kcal/mol more stable than the ortho- isomer. For the silaphenylthiolates the ortho- isomer is of lowest energy, although with R = H the ortho- and meta- isomers are isoenergetic. The calculated nucleus independent chemical shifts (NICS) indicate that the silaphenolates and silaphenylthiolates are influenced by aromaticity, but they are less aromatic than the parent silabenzene. The geometries and charge distributions suggest that all silaphenolates and silaphenylthiolates to substantial degrees are described by resonance structures with an exocyclic C=O double bond and a silapentadienyl anionic segment. Indeed, they resemble the all-carbon phenolate and phenylthiolate. Silaphenylthiolates are less bond alternate and have slightly more negative NICS values than analogous silaphenolates, suggesting that this compound class is a bit more aromatic. Dimerization of the silaphenolates and silaphenylthiolates is hampered due to intramolecular Coulomb repulsion in the dimers, and silaphenolates with a moderately bulky SiMe3 group as substituent at Si should prefer the monomeric form.

Place, publisher, year, edition, pages
2012. Vol. 17, no 1, 369-389 p.
Keyword [en]
silicon, aromaticity, quantum chemical calculations
National Category
Organic Chemistry
URN: urn:nbn:se:uu:diva-166896DOI: 10.3390/molecules17010369ISI: 000299535700025OAI: oai:DiVA.org:uu-166896DiVA: diva2:478653
Available from: 2012-01-16 Created: 2012-01-16 Last updated: 2012-04-19Bibliographically approved
In thesis
1. Low-coordinate Organosilicon Chemistry: Fundamentals, Excursions Outside the Field, and Potential Applications
Open this publication in new window or tab >>Low-coordinate Organosilicon Chemistry: Fundamentals, Excursions Outside the Field, and Potential Applications
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis reports on unsaturated silicon compounds, as well as excursions from these into germanium chemistry, single molecule electronics, and silyl protective group chemistry. Both experimental and computational investigations were performed.

Potassium germenolates were synthesized through reactions of tris(timethylsilyl) substituted acyl- and carbamylgermanes with potassium tert-butoxide. The potassium germenolates calculated by density functional theory have pyramidal structures at the Ge atoms, similar to the Si in the corresponding potassium silenolates, indicating negative charge on germanium rather than on oxygen. Germenolates also display germyl anion-like reactivity instead of germene-like reactivity as they are alkylated at Ge and initiate anionic polymerization of dienes rather than form [4+2] cycloadducts. The NMR chemical shifts reveal more negative charge at Ge in germenolates than at Si in analogous silenolates.

Computations indicate that silabenzenes and silapyridines are reachable via [1,3]-silyl shifts from cyclic conjugated acylsilanes. Differently sized substituents were considered to prevent dimerizations, and 1-triisopropylsilyl-2-triisopropylsiloxy-6-tert-butylsilabenzene is a good synthetic target. Computations also show that silaphenolates are species with negative charge primarily localized at oxygen atom. Their planar structures, bond lengths, and NICS values reveal significant influence of aromaticity. Electrostatic repulsion should increase their stability, however, steric bulk is also important.

Furthermore, it was found computationally that [1,3]-silyl shift from an acylsilane to a silene can function as a molecular switch reaction. Conductance calculations support this proposition.  

Finally, tris(trimethylsilyl)silylmethaneamide (hypersilylamide) together with catalytic amounts of triflic acid were found to be efficient for protection of a range of alkyl and aryl alcohols and thiols in good to excellent yields. The protocol can be used to protect the less hindered OH group of a diol and has a broad functional group tolerance. A catalytic cycle is proposed. Hypersilyl protected alcohols and thiols are deprotected efficiently under photolytic conditions.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. 75 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 907
organosilicon, silene, silaaromatics, silenolate, hypersilyl group, alcohol protection, molecular switch
National Category
Organic Chemistry
Research subject
Chemistry with specialization in Organic Chemistry
urn:nbn:se:uu:diva-169796 (URN)978-91-554-8296-1 (ISBN)
Public defence
2012-04-21, B7:101a, BMC, Husargatan 3, Uppsala, 13:30 (English)
Available from: 2012-03-30 Created: 2012-03-06 Last updated: 2012-04-19Bibliographically approved

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