Organosilicon chemistry

Polydimethylsiloxane (PDMS) is the principal component of silicones.

Organosilicon chemistry is the study of organometallic compounds containing carbon–silicon bonds, to which they are called organosilicon compounds. Most organosilicon compounds are similar to the ordinary organic compounds, being colourless, flammable, hydrophobic, and stable to air. Silicon carbide is an inorganic compound.

History

See also: Organometallic chemistry

In 1863 Charles Friedel and James Crafts made the first organochlorosilane compound.[1] The same year they also described a «polysilicic acid ether» in the preparation of ethyl- and methyl-o-silicic acid.[1] Extensive research in the field of organosilicon compounds was pioneered in the beginning of 20th century by Frederic S. Kipping.[2] He also had coined the term "silicone" (resembling ketones, this is erroneous though)[3][4]: 286  in relation to these materials in 1904. In recognition of Kipping's achievements the Dow Chemical Company had established an award in 1960s that is given for significant contributions into the silicon chemistry.[5] In his works Kipping was noted for using Grignard reagents to make alkylsilanes and arylsilanes and the preparation of silicone oligomers and polymers for the first time.[2]

In 1945 Eugene G. Rochow also made a significant contribution into the organosilicon chemistry by first describing Müller-Rochow process.[6]

Occurrence and applications

Silicone caulk, commercial sealants, are mainly composed of organosilicon compounds mixed with hardener.

Organosilicon compounds are widely encountered in commercial products. Most common are antifoamers, caulks (sealant), adhesives, and coatings made from silicones. Other important uses include agricultural and plant control adjuvants commonly used in conjunction with herbicides and fungicides.[7]

Biology and medicine

Carbon–silicon bonds are absent in biology, however enzymes have been used to artificially create carbon-silicon bonds in living microbes.[8][9][10] Silicates, on the other hand, have known existence in diatoms.[11] Silafluofen is an organosilicon compound that functions as a pyrethroid insecticide. Several organosilicon compounds have been investigated as pharmaceuticals.[12][13]

Bonding

Electronegativities Relevant to Organosilicon Chemistry
C Si H O
2.5 1.8 2.1 3.4
Properties Relevant to Organosilicon Chemistry
Bond Bond length (pm) Approx. bond
strength (kJ/mol)
C-C 154 334
Si-Si 234 196
C-Si 186 314
C-H 110 414
Si-H 146 314
C-O 145 355
Si-O 159 460
Dissociation energies of bonds to silicon[14]
Bond Energy (kJ/mol)
Si–Si 327(10)
Si–Br 343(50)
Si–C 435(21)
Si–Cl 456(42)
Si–F 540(13)
Si–H 298.49(46)
Si–I 339(84)
Si–N 439(38)
Si–O 798(8)
Si–S 619(13)
Si–Se 531(25)
H3Si–SiH3 339(17)
Me3Si–SiMe3 339
Ar3Si–SiAr3 368(31)
Si–Te 506(38)

In the great majority of organosilicon compounds, Si is tetravalent with tetrahedral molecular geometry. Compared to carbon–carbon bonds, carbon–silicon bonds are longer and weaker.[7][15]

The C–Si bond is somewhat polarised towards carbon due to carbon's greater electronegativity (C 2.55 vs Si 1.90), and single bonds from Si to electronegative elements are very strong.[14] Silicon is thus susceptible to nucleophilic attack by O−, Cl−, or F−; the energy of an Si–O bond in particular is strikingly high. This feature is exploited in many reactions such as the Sakurai reaction, the Brook rearrangement, the Fleming–Tamao oxidation, and the Peterson olefination.[16]

The Si–C bond (1.89 Å) is significantly longer than a typical C–C bond (1.54 Å), suggesting that silyl substitutents have less steric demand than their organyl analogues. When geometry allows, silicon exhibits negative hyperconjugation, reversing the usual polarization on neighboring atoms.[citation needed]

Preparation

The first organosilicon compound, tetraethylsilane, was prepared by Charles Friedel and James Crafts in 1863 by reaction of tetrachlorosilane with diethylzinc.

The bulk of organosilicon compounds derive from organosilicon chlorides (CH
3)
4-xSiCl
x. These chlorides are 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 CH
3Cl + Si → (CH
3)
2SiCl
2

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.[17]

Hydrosilylation

Main article: Hydrosilylation

Another major method for the formation of Si-C bonds is hydrosilylation (also called hydrosilation).[18] In this process, compounds with Si-H bonds (hydrosilanes) add to unsaturated substrates. Commercially, the main substrates are alkenes. Other unsaturated functional groups — alkynes, imines, ketones, and aldehydes — also participate, but these reactions are of little economic value.[19]

Idealized mechanism for metal-catalysed hydrosilylation of an alkene

Hydrosilylation requires metal catalysts, especially those based on platinum group metals.

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

Cleavage of Si-Si bonds

Hexamethyldisilane reacts with methyl lithium to give trimethylsilyl lithium:[20]

(CH3)6Si2 + CH3Li → (CH3)3SiLi + (CH3)4Si

Similarly, tris(trimethylsilyl)silyl lithium is derived from tetrakis(trimethylsilyl)silane:[21]

((CH3)3Si)4Si + CH3Li → ((CH3)3Si)3SiLi + (CH3)4Si

Functional groups

Silicon is a component of many functional groups. Most of these are analogous to organic compounds. The overarching exception is the rarity of multiple bonds to silicon, as reflected in the double bond rule.

Silanols, siloxides, and siloxanes

Silanols are analogues of alcohols. They are generally prepared by hydrolysis of silyl chlorides:[22]

R
3SiCl + H2O → R
3SiOH + HCl

Less frequently silanols are prepared by oxidation of silyl hydrides, a reaction that uses a metal catalyst:

2 R
3SiH + O
2 → 2 R
3SiOH

Many silanols have been isolated including (CH
3)
3SiOH and (C
6H
5)
3SiOH. They are about 500x more acidic than the corresponding alcohols. Siloxides are the deprotonated derivatives of silanols:[22]

R
3SiOH + NaOH → R
3SiONa + H2O

Silanols tend to dehydrate to give siloxanes:

2 R
3SiOH → R
3Si-O-SiR
3 + H2O

Polymers with repeating siloxane linkages are called silicones. Compounds with an Si=O double bond called silanones are extremely unstable.

Silyl ethers

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

(CH
3)
3SiCl + ROH → (CH
3)
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:

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

Silyl chlorides

Main article: Chlorosilane

Organosilyl chlorides are important commodity chemicals. They are mainly used to produce silicone polymers as described above. Especially important silyl chlorides are dimethyldichlorosilane (Me
2SiCl
2), methyltrichlorosilane (MeSiCl
3), and trimethylsilyl chloride (Me
3SiCl) are all produced by direct process. 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 Me
3SiCl is the main silylating agent. One classic method called the Flood reaction for the synthesis of this compound class is by heating hexaalkyldisiloxanes R
3SiOSiR
3 with concentrated sulfuric acid and a sodium halide.[23]

Silyl hydrides

Main article: Hydrosilane
Tris(trimethylsilyl)silane is a well-investigated hydrosilane.[24]

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 Et
3SiH. Phenylsilane is PhSiH
3. The parent compound SiH
4 is called silane.

Silenes

Organosilicon compounds, unlike their carbon counterparts, do not have a rich double bond chemistry.[25] 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.[26] The first stable (kinetically shielded) silene was reported in 1981 by Brook.[27][28]

Silenes Gusel'nikov 1967 Brook 1981

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.[29]

Siloles

Chemical structure of silole

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.[30][31] 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.

Pentacoordinated silicon

See also: Hypervalent molecule § Pentacoordinated 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:[32]

Pentaorganosilicate

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

In general, almost any silicon-heteroatom bond is water-sensitive, and will spontaneously hydrolyze.[33] Unstrained silicon-carbon bonds, however, are very strong, and cleave only in a small number of extreme conditions. Strong acids will protodesilate arylsilanes and, in the presence of a Lewis acid catalyst, alkylsilanes. Most nucleophiles are too weak to displace carbon from silicon: the exceptions are fluoride ions and alkoxides, although the latter often deprotonate the organosilane to a silicon ylide instead.[34]

As a covalent hydride source, hydrosilanes are good reductants.

Certain allyl silanes can be prepared from allylic esters such as 1 and monosilylcopper compounds, which are formed in situ by the reaction of the disilylzinc compound 2, with Copper Iodide, in:[35][36]

Allylic substitution forming an 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.

Unsaturated silanes like the above are susceptible to electrophilic substitution.

Environmental effects

Organosilicon compounds affect bee (and other insect) immune expression, making them more susceptible to viral infection.[13][37]

See also

References

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  36. ^ By isotopic desymmetrisation on the substrate (replacing hydrogen by deuterium) it can be demonstrated 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 or reductive elimination step
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