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Effects of Substituents and Counterions on the Structures of Silenolates: A Computational Investigation
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.
2009 (English)In: Tetrahedron, ISSN 0040-4020, E-ISSN 1464-5416, Vol. 65, no 28, 5521-5526 p.Article in journal (Refereed) Published
Abstract [en]

The structures and charge distributions of substituted silenolates   [H2SiC(=O)X](-) (X-H, SiH3, Me, t-Bu, OMe, NMe2; group A),  [Y2SiC(=O)H](-) (Y=H, F, Me, Ph, SiH3, SiMe3; group B), and [Y2SiC(=O)X](-) (Y=Me, X=t-Bu, and Y=SiMe3; X=t-Bu, OMe, NMe2; group C)   were examined through density functional theory calculations. The effects of the solvated counterion (K+, Li+, or MgCl+) and coordination site (O or Si) on the properties of group C silenolates were also Studied. The variation in the degree of pi-conjugative reverse SiC bond   polarization, Sigma Phi(RP)(pi) calculated by natural resonance theory,   was determined. The Sigma Phi(RP)(pi) correlated with r(SiC) for both   group A and B silenolates, and the correlation between Sigma   Phi(RP)(pi) and the Sum of valence angles at Si, Sigma alpha(Si), was   good for group A but poor for group B due to strong influence of the   inductive effect. The SiC charge difference correlated well with Sigma   Phi(RP)(pi) for group A, but not for group B, again an effect of   inductive substituent effects. The group C silenolates were Coordinated   to Li(THF)(3)(+), MgCl(THF)(4)(+), and K(THF)(5)(+) either via the O or   Si atom. The coordination energies show that coordination to the hard O   is preferred for Li+ and MgCl+, but the K+ ion coordinated   simultaneously to Si and O. Coordination of the solvated metal ion to O  resulted in shorter SiC bond length, an increased Sigma alpha(Si)   value, and lower Delta q(SiC) when compared to the naked silenolate.  Choice Of counterion and substituent provides a means to extensively vary the properties of silenolates such as their reactivity.

Place, publisher, year, edition, pages
2009. Vol. 65, no 28, 5521-5526 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-97586DOI: 10.1016/j.tet.2009.02.088ISI: 000268128000010OAI: oai:DiVA.org:uu-97586DiVA: diva2:172586
Available from: 2008-10-03 Created: 2008-10-03 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Lowcoordinated Silicon and Hypercoordinated Carbon: Structure and Stability of Silicon Analogs of Alkenes and Carbon Analogs of Silicates
Open this publication in new window or tab >>Lowcoordinated Silicon and Hypercoordinated Carbon: Structure and Stability of Silicon Analogs of Alkenes and Carbon Analogs of Silicates
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Quantum chemical studies on lowcoordinated group 14-16 compounds have been performed. This thesis focuses particularly on silenes influenced by reverse Siδ-=Cδ+ bond polarization. Hypercoordinated carbon compounds are also studied.

The geometries from calculations with several common computationally inexpensive methods have been tested against high level CCSD/cc-pVTZ geometries for a series of substituted silenes. Hybrid HF/DFT methods performed best among the inexpensive methods tested for silenes.

Heavy alkenes strongly influenced by reverse polarization are found to have less exothermic dimerization energies for both head-to-head and head-to-tail dimerizations, and to have higher activation energies for water addition than naturally polarized heavy alkenes.

We also investigated solvated lithium, magnesium and potassium silenolates and found that lithium and magnesium ions coordinate preferably to O, giving their SiC bond some double bond character.

Reverse polarized 2-siloxy-, 2-thiosiloxy-, and 2-(N-sila-N-methyl)-silenes could according to calculations be formed thermolytically from the corresponding tetrasilanes as transient species. It was, however, found that silenes highly influenced by π-conjugative reverse polarization have low barriers for the back-reaction, and thus these silenes are more difficult to form as stable species than naturally polarized silenes.

It is also found that conjugated 1-siladienes, formed by electrocyclic ring-opening of 1-silacyclobut-2-enes, which are highly influenced by π-conjugative reverse polarization, have higher barriers for electrocyclization back to starting material than naturally polarized 1-siladienes.

It is found that CHe54+, CHe64+, CNe54+, and CNe64+ are the closest carbon analogs of SiH5-, SiH62-, SiF5- and SiF62-, respectively. However, due to their exothermic dissociation reaction, these very high-lying local minima will be impossible to reach experimentally.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2008. 85 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 557
Keyword
silenes, zwitterionic silenes, silenolates, reverse polarization, natural resonance theory, hypervalent carbon, hypercoordinated carbon, thermolysis, lowcoordinated silicon
National Category
Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-9298 (URN)978-91-554-7294-8 (ISBN)
Public defence
2008-10-25, C8:301, BMC, Husargatan 3 753 21, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2008-10-03 Created: 2008-10-03 Last updated: 2009-03-26Bibliographically approved

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