Organosilicon compound

Organosilicon compounds are organic compounds containing carbonsilicon bonds. Organosilicon chemistry is the corresponding science exploring their properties and reactivity. Most organosilicon compounds are similar to the ordinary organic compounds, being colourless, flammable, hydrophobic, and stable. The first organosilicon compound, tetraethylsilane, was discovered by Charles Friedel and James Crafts in 1863 by reaction of tetrachlorosilane with diethylzinc. The carbosilicon silicon carbide is an inorganic compound.

Occurrence and applications

Organosilicon compounds are widely encountered in commercial products. Most common are sealants, caulks, adhesives, and coatings made from silicones.


Biology and medicine

Carbon–silicon bonds are however generally absent in biochemical processes,[1] although their fleeting existence has been reported in a freshwater alga.[2] Silafluofen is an organosilicon compound that functions as a pyrethroid insecticide. Several organosilicon compounds are being investigated as pharmaceuticals.[3]

Properties of Si–C, Si–O, and Si–F bonds

In most organosilicon compounds, Si is tetravalent and tetrahedral. 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.[4] The C–Si bond is somewhat polarised towards carbon due to carbon's greater electronegativity (C 2.55 vs Si 1.90). The Si–C bond can be broken more readily than typical C–C bonds. One manifestation of bond polarization in organosilanes is found in the Sakurai reaction.[5] Certain alkyl silanes can be oxidized to an alcohol in the Fleming–Tamao oxidation.

Another manifestation is the β-silicon effect describes the stabilizing effect of a β-silicon atom on a carbocation with many implications for reactivity.

Si–O bonds are much stronger (809 kJ/mol compared to 538 kJ/mol) than a typical C–O single bond. The favorable formation of Si–O bonds drive many organic reactions such as the Brook rearrangement and Peterson olefination. Compared to the strong Si–O bond, the Si–F bond is even stronger.

Production

The bulk of organosilicon compounds derive from organosilicon chlorides (CH3)4-xSiClx. These chlorides produced by the "Direct process", which entails the reaction of methyl chloride with a silicon-copper alloy. The main and most sought-after product is dimethyldichlorosilane:

2 CH3Cl + Si → (CH3)2SiCl2

A variety of other products are obtained, including trimethylsilyl chloride and methyltrichlorosilane. About 1 million tons of organosilicon compounds are prepared annually by this route. The method can also be used for phenyl chlorosilanes.[6]

Hydrosilylation

Compounds with Si-H bonds add to unsaturated substrates in the process called hydrosilylation (also called hydrosilation).[7] Commercially, the main substrates are alkenes. Other unsaturated functional groups—alkynes, imines, ketones, and aldehydes—also participate, although these uses are rather specialized. An example is the hydrosilation of phenylacetylene:[8]

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

Functional groups

Silanols, siloxides, and siloxanes

Silanols are analogues of alcohols. They are generally prepared by hydrolysis of silyl chlorides and oxidation of silyl hydrides:[9]

R3SiCl + H2O → R3SiOH + HCl

Less frequently they are prepared by oxidation of silyl hydrides:

2 R3SiH + O2 → 2R3SiOH

The parent H3SiOH is too unstable for isolation, but the many organic derivatives are known including (CH3)3SiOH and (C6H5)3SiOH. They are about 500x more acidic than the corresponding alcohols. Siloxides (silanoates) are the deprotonated derivatives of silanols:[9]

R3SiOH + NaOH → R3SiONa + H2O

Silanols tend to dehydrate to give siloxanes:

2 R3SiOH → R3Si-O-SiR3 + H2O

Polymers with repeating siloxane linkages are called silicones.

Silyl ethers

Silyl ethers have the connectivity Si-O-C. They are typically prepared by the reaction of alcohols with silyl chlorides:

(CH3)3SiCl + ROH → (CH3)3Si-O-R + HCl

Silyl ethers are extensively used as protective groups for alcohols.

Exploiting the strength of the Si-F bond, fluoride sources such as tetra-n-butylammonium fluoride (TBAF) are used in deprotection of silyl ethers:

CH3)3Si-O-R + F- + H2O → (CH3)3Si-F + H-O-R + OH-

Silyl chlorides

Organosilyl chlorides are important commodity chemicals. They are mainly used to produce silicone polymers as described above. Especially important silyl chlorides are dimethyldichlorosilane (Me2SiCl2), methyltrichlorosilane (MeSiCl3), and trimethylsilyl chloride (Me3SiCl). More specialized derivatives that find commercial applications include dichloromethylphenylsilane, trichloro(chloromethyl)silane, trichloro(dichlorophenyl)silane, trichloroethylsilane, and phenyltrichlorosilane.

Although proportionately a minor outlet, organosilicon compounds are widely used in organic synthesis. Notably trimethylsilyl chloride Me3SiCl is the main silylating agent. One 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.[10]

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. Commonly the presence of the hydride is not mentioned in the name of the compound. Triethylsilane has the formula Et3SiH. Phenylsilane is PhSiH3. The parent compound SiH4 is called silane. Unlike tetraorganosilicon compounds, the hydrides are more susceptible to oxidation. For example, triethylsilane reduces phenyl azide to an aniline.:[11]

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 abstracts a proton from a thiol completing the catalytic cycle:

Silyl hydrides can even reduce carbon dioxide to methane.:[12]

Although this process requires a complex catalyst system and is not catalytic. The polymer PMHS is also used as reducing agents in organic synthesis.

Silanylidenes

Main article: Silanylidene group

Silanylidenes are compounds containing a silicon based chain, joined by a double bond to the main molecule, such as silylidenemethanol. Where it is the main functional group, the molecule is named after the parent silane, with the -ylidene- infix, such as methylidenesilane.

Silenes

Organosilicon compounds, unlike their carbon counterparts, do not have a rich double bond chemistry due to the large difference in electronegativity.[13] Existing compounds with silene Si=C bonds (also known as alkylidenesilanes) are laboratory curiosities such as the silicon benzene analogue silabenzene. In 1967, Gusel'nikov and Flowers provided the first evidence for silenes from pyrolysis of dimethylsilacyclobutane.[14] The first stable (kinetically shielded) silene was reported in 1981 by Brook [15] [16]

Disilenes have Si=Si double bonds and disilynes are silicon analogues of an alkyne. The first Silyne (with a silicon to carbon triple bond) was reported in 2010 [17]

Siloles

Siloles, also called silacyclopentadienes, are members of a larger class of compounds called metalloles. They are the silicon analogs of cyclopentadienes and are of current academic interest due to their electroluminescence and other electronic properties.[18][19] Siloles are efficient in electron transport. They owe their low lying LUMO to a favorable interaction between the antibonding sigma silicon orbital with an antibonding pi orbital of the butadiene fragment.

Hypercoordinated silicon

Unlike carbon, silicon compounds can be coordinated to five atoms as well in a group of compounds ranging from so-called silatranes, such as phenylsilatrane, to a uniquely stable pentaorganosilicate:[20]

The stability of hypervalent silicon is the basis of the Hiyama coupling, a coupling reaction used in certain specialized organic synthetic applications. The reaction begins with the activation of Si-C bond by fluoride:

R-SiR'3 + R"-X + F- → R-R" + R'3SiF + X-

Various reactions

Certain allyl silanes can be prepared from allylic ester such as 1 and monosilylcopper compounds such as 2 in.[21][22]

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.

See also

CH He
CLi CBe CB CC CN CO CF Ne
CNa CMg CAl CSi CP CS CCl CAr
CK CCa CSc CTi CV CCr CMn CFe CCo CNi CCu CZn CGa CGe CAs CSe CBr CKr
CRb CSr CY CZr CNb CMo CTc CRu CRh CPd CAg CCd CIn CSn CSb CTe CI CXe
CCs CBa CHf CTa CW CRe COs CIr CPt CAu CHg CTl CPb CBi CPo CAt Rn
Fr CRa Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo
CLa CCe CPr CNd CPm CSm CEu CGd CTb CDy CHo CEr CTm CYb CLu
Ac CTh CPa CU CNp CPu CAm CCm CBk CCf CEs Fm Md No Lr
Chemical bonds to carbon
Core organic chemistry Many uses in chemistry
Academic research, but no widespread use Bond unknown

References

External links

  • Magnus Walter's Selected Aspects of Organosilicon Chemistry
  • Silicon in organic synthesis
  • Safety data for methyltrichlorosilane from the Chemistry Department at Oxford University.
  • S. Marsden (Editor): Contemporary organosilicon chemistry. Thematic Series in the Open Access Beilstein Journal of Organic Chemistry.
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