Organosilicon

Organosilicon compounds are chemical compounds containing carbon silicon bonds. Organosilicon chemistry is the corresponding science exploring the properties and reactivity of organosilicon compounds. Like carbon, organosilicon compounds are tetravalent and tetrahedral. Unlike carbon, silicon is not found in any biomolecule.

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.

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. 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. In oxidative couplings silicon is represented by the Hiyama coupling.

Certain allyl silanes can be prepared from allylic ester such as 1 and monosilylcopper compounds such as 2 in.


 * [[Image:AllylSilaneSynthesis.png|400px|Allylic substitution forming a allyl silane ]]

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.

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 silyl acetals RR'Si(OR)2, 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 reactions such as the Brook rearrangement and Peterson olefination.

Another manifestation is the highly explosive nature of the silicon pendant of pentaerytritol tetranitrate :


 * [[Image:SilaExplosives.png|300px|Sila explosive Klapötke 2007]]

A single crystal of this compound, first synthesized in 2007 even detonates when in contact with a teflon spatula and in fact made full characterization impossible. Another contributor to its exothermic decomposition (inferred from much safer in silico experimentation) is the ability of silicon in its crystal phase to coordinate to two oxygen nitrito groups in addition to regular coordination to the four carbon atoms. This additional coordination would make formation of silicon dioxide (one of the decomposition products) more facile.

Silyl halides
Organosilyl halides are important reagents in organic chemistry notably trimethylsilyl chloride Me3SiCl. A classic method called the Flood reaction for the synthesis of this compound class is by heating hexaalkyldisiloxanes R3SiOSiR3 with concentrated sulfuric acid and a sodium halide. Other relevant silyl halides are dichloromethylphenylsilane, dimethyldichlorosilane, methyltrichlorosilane, (4-aminobutyl)diethoxymethylsilane, trichloro(chloromethyl)silane,  trichloro(dichlorophenyl)silane,  trichloroethylsilane, trichlorophenylsilane and trimethylchlorosilane

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 :


 * [[Image:AzideReductionByTriethylsilylhydride.png|400px|Azide Reduction By Triethylsilylhydride]]

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:


 * [[Image:AzideReductionMechanism.png|400px|Azide Reduction By Triethylsilylhydride mechanism]]

Aqueous workup then gives aniline.

Silyl hydrides can even take up the reduction of robust molecules such as carbon dioxide (to methane) :


 * [[Image:Carbondioxidereduction.png|400px|Carbon dioxide reduction]]

Although it takes a very complex catalyst system.

Hydrosilylation
Silyl hydrides react with various unsaturated substrates such as alkenes, alkynes, imines, carbonyls 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 :


 * [[Image:HydrosilylationTriphenylsilylhydride.png|400px|Hydrosilylation with Triphenylsilyl hydride]]

In the related silylmetalation, a metal replaces the hydrogen atom.

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.

Hypercoordinated silicon
Unlike carbon, silicon compounds can be coordinated to five atoms as well in a group of compounds ranging from so-called silatranes to a uniquely stable pentaorganosilicate :
 * [[Image:Pentaorganosilicate.png|200px|Pentaorganosilicate]]