Organosilicon
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Organosilicon compounds are chemical compounds containing carbon silicon bonds. Organosilicon chemistry is the corresponding science exploring the properties and reactivity of organosilicon compounds [1]. Like carbon, organosilicon compounds are tetravalent and tetrahedral. Unlike carbon, silicon is not found in any biomolecule [2].
The first organosilicon compound, tetraethylsilane was discovered by Charles Friedel and James Crafts in 1863 by reaction of tetrachlorosilane with diethyl zinc. Discovered in 1893, the simplest marriage between silicon and carbon is silicon carbide which has many industrial applications.
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[edit] Organosilanes
Carbon silicon bonds compared to carbon carbon bonds are longer (186 pm vs. 154 pm) and weaker with bond dissociation energy 451 kJ/mol vs. 607 kJ/mol [3]. The C–Si is somewhat polarized towards carbon due to its higher electronegativity (C 2.55 vs Si 1.90). One manifestation of bond polarization in organosilanes is found in the Sakurai reaction.
Certain allyl silanes can be prepared from allylic ester such as 1 and monosilylcopper compounds such as 2 in [4] [5].
In this reaction type silicon polarity is reversed in a chemical bond with zinc and a formal allylic substitution on the benzoyloxy group takes place.
The chemistry of silanes such as tetramethylsilane is comparable to that of alkanes in many aspects such as thermal stability. The β-silicon effect describes the stabilizing effect of a β-silicon atom on a carbocation with many implications for reactivity.
[edit] Siloxides
More notably bonds of silicon to oxygen are much shorter and stronger (809 compared to 538 kJ/mol) than that of those of carbon to oxygen. The polarization in this bond increases towards oxygen. Examples are the siloxanes and the polymeric polysiloxanes. Silyl ethers are extensively used as protective groups for alcohols. Only silicon bonds to fluorine are stronger and that is why the fluorine source TASF (or more commonly TBAF) is useful in deprotection. The favorable formation of Si–O bonds drive many organic reaction such as the Brook rearrangement and Peterson olefination.
[edit] Silyl hydrides
The silicon to hydrogen bond is longer than the C–H bond (148 compared to 105 pm) and weaker (299 compared to 338 kJ/mol). Hydrogen is more electronegative than silicon hence the naming convention of silyl hydrides. Silyl hydrides are very reactive and used as reducing agents for example PMHS.
In one study triethylsilylhydride is used in the conversion of an phenyl azide to an aniline [6]:
In this reaction ACCN is a radical initiator and an aliphatic thiol transfers radical character to the silylhydride. The triethylsilyl free radical then reacts with the azide with expulsion of nitrogen to a N-silylarylaminyl radical which grabs a proton from a thiol completing the catalytic cycle:
Aqueous workup then gives aniline.
Silyl hydrides can even take up the reduction of robust molecules such as carbon dioxide (to methane) [7]:
Although it takes a very complex catalyst system.
[edit] Hydrosilylation
Silyl hydrides react with various unsaturated substrates such as alkenes, alkynes, imine and oximes to new organosilicon compounds in hydrosilylation. In the reaction of triphenylsilyl hydride with phenylacetylene the reaction product is a trans or cis or the geminal vinyl silane, for example [8]:
In the related silylmetalation, a metal replaces the hydrogen atom.
[edit] Silenes
Organosilicon compounds unlike their carbon counterparts do not have a rich double bond chemistry due to the large difference in electronegativity. Existing compounds with organosilene Si=C bonds are laboratory curiosities such as the silicon benzene analogue silabenzene, and Si=Si bond containing disilenes.
[edit] See also
- Compounds of carbon with period 3 elements: organosilicon compounds, organophosphorus compounds, organosulfur compounds,
- Compounds of carbon with other group 14 elements: organosilicon compounds, organogermanium compounds, organotin compounds, organolead compounds.
- silylenes, the carbene counterparts and silylenoids the carbenoid counterparts.
[edit] External links
- Magnus Walter's Selected Aspects of Organosilicon Chemistry
[edit] References
- ^ Silicon in Organic Synthesis Colvin, E. Butterworth: London 1981
- ^ Organosilicon Chemistry S. Pawlenko Walter de Gruyter New York 1986
- ^ Handbook of Chemistry and Physics, 81st Edition CRC Press ISBN 0-8493-0481-4
- ^ Mechanistic insight into copper-catalysed allylic substitutions with bis(triorganosilyl) zincs. Enantiospecific preparation of -chiral silanes Eric S. Schmidtmann and Martin Oestreich Chem. Commun., 2006, 3643 - 3645, DOI:10.1039/b606589a
- ^ By isotopic desymmetrisation on the substrate (replacing hydrogen by deuterium) it can be demonstated that the reaction proceeds not through the symmetrical π-allyl intermediate 5 which would give an equal mixture of 3a and 3b but through the Π-δ intermediate 4 resulting in 3a only, through an oxidative addition / reductive elimination step
- ^ Radical Reduction of Aromatic Azides to Amines with Triethylsilane Luisa Benati, Giorgio Bencivenni, Rino Leardini, Matteo Minozzi, Daniele Nanni, Rosanna Scialpi, Piero Spagnolo, and Giuseppe Zanardi J. Org. Chem.; 2006; 71(15) pp 5822 - 5825; (Note) DOI:10.1021/jo060824k
- ^ From Carbon Dioxide to Methane: Homogeneous Reduction of Carbon Dioxide with Hydrosilanes Catalyzed by Zirconium-Borane Complexes Tsukasa Matsuo and Hiroyuki Kawaguchi J. Am. Chem. Soc.; 2006; 128(38) pp 12362 - 12363; DOI:10.1021/ja0647250
- ^ Effect of the synthetic method of Pt/MgO in the hydrosilylation of phenylacetylene Eulalia Ramírez-Oliva, Alejandro Hernández, J. Merced Martínez-Rosales, Alfredo Aguilar-Elguezabal, Gabriel Herrera-Pérez, and Jorge Cervantesa Arkivoc 2006 (v) 126-136 Link
