Organosilicon
Organosilicon compounds are organic compounds containing carbon–silicon 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
Organosilicon compounds are widely encountered in commercial products. Most common are sealants, caulks, adhesives, and coatings made from silicones. Carbon–silicon bonds are however generally absent in biochemical processes,[1] although their fleeting existence has been reported in a freshwater alga.[2]
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.[3] 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.[4] 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.[5]
Hydrosilylation
Compounds with Si-H bonds add to unsaturated substrates in the process called hydrosilylation (also called hydrosilation).[6] 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:[7]
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:[8]
- 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:[8]
- 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.[9]
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.:[10]
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.:[11]
Although this process requires a complex catalyst system and is not catalytic. The polymer PMHS is also used as reducing agents in organic synthesis.
Silenes
Silenes 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.
In one study [12] a disilene is prepared by an intramolecular coupling of a 1,1-dibromosilane with potassium graphite. The silicon double bond in the resulting compound has a bond length of 227 picometer (second largest ever found) with trans-bent angles 33° and 31° (by X-ray diffraction).
In addition to this the substituents around the Si-Si bond are twisted by 43°. The disilene isomerizes to a tetracyclic compound by heating at 110°C in xylene thereby releasing its strain energy.
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.[13][14] 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:[15]
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.[16][17]
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
- Compounds of carbon with period 3 elements: organoaluminum compounds, organosilicon compounds, organophosphorus compounds, organosulfur compounds,
- Compounds of carbon with other group 14 elements: organosilicon compounds, organogermanium compounds, organotin compounds, organolead compounds.
| 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 | Ra | Rf | Db | Sg | Bh | Hs | Mt | Ds | Rg | Cn | Uut | Fl | Uup | Lv | Uus | Uuo | |
| ↓ | |||||||||||||||||
| CLa | CCe | CPr | CNd | Pm | CSm | CEu | CGd | CTb | CDy | CHo | CEr | CTm | CYb | CLu | |||
| Ac | CTh | CPa | CU | CNp | CPu | CAm | CCm | CBk | Cf | CEs | Fm | Md | No | Lr | |||
| Core organic chemistry | Many uses in chemistry |
| Academic research, but no widespread use | Bond unknown |
- silylenes, the carbene counterparts and silylenoids the carbenoid counterparts.
References
- ^ Organosilicon Chemistry S. Pawlenko Walter de Gruyter New York 1986
- ^ Stephen D. Kinrade, Ashley-M. E. Gillson and Christopher T. G. Knight (2002), Silicon-29 NMR evidence of a transient hexavalent silicon complex in the diatom Navicula pelliculosa. J. Chem. Soc., Dalton Trans., 307–309, doi:10.1039/b105379p
- ^ Handbook of Chemistry and Physics, 81st Edition CRC Press ISBN 0-8493-0481-4
- ^ Silicon in Organic Synthesis Colvin, E. Butterworth: London 1981
- ^ Röshe, L.; John, P.; Reitmeier, R. “Organic Silicon Compounds” Ullmann’s Encyclopedia of Industrial Chemistry, 2003, Wiley-VCH, Weinheim. doi:10.1002/14356007.a24_021.
- ^ B. Marciniec (ed.), Hydrosilylation, Advances in Silicon Science, DOI 10.1007/978-1-4020-8172-9 1, C Springer Science+Business Media B.V. 2009.
- ^ 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
- ^ a b Paul D. Lickiss "The Synthesis and Structure of Organosilanols" Advances in Inorganic Chemistry Volume 42, 1995, Pages 147–262 doi:10.1016/S0898-8838(08)60053-7
- ^ Preparation of Triethylsilicon Halides E. A. Flood J. Am. Chem. Soc.; 1933; 55(4) pp 1735 - 1736; doi:10.1021/ja01331a504
- ^ 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 Elumalai Palani., ''J. Org. Chem''.; 2006; volume 71, pp 5822 - 5825. 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
- ^ Fused Tricyclic Disilenes with Highly Strained Si-Si Double Bonds: Addition of a Si-Si Single Bond to a Si-Si Double Bond Ryoji Tanaka, Takeaki Iwamoto, and Mitsuo Kira Angewandte Chemie International Edition Volume 45, Issue 38 , Pages 6371 - 6373 2006 doi:10.1002/anie.200602214
- ^ Direct synthesis of 2,5-dihalosiloles Organic Syntheses 2008, 85, 53-63 http://www.orgsynth.org/orgsyn/pdfs/V85P0053.pdf
- ^ Synthesis of new dipyridylphenylaminosiloles for highly emissive organic electroluminescent devices Laurent Aubouy, Philippe Gerbier, Nolwenn Huby, Guillaume Wantz, Laurence Vignau, Lionel Hirsch and Jean-Marc Jano New J. Chem., 2004, 28, 1086 - 1090, doi:10.1039/b405238b
- ^ Tetraalkylammonium pentaorganosilicates: the first highly stable silicates with five hydrocarbon ligands Sirik Deerenberg, Marius Schakel, Adrianus H. J. F. de Keijzer, Mirko Kranenburg, Martin Lutz, Anthony L. Spek, Koop Lammertsma, Chem. Commun., 2002, (4),348-349 doi:10.1039/b109816k
- ^ 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 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 / reductive elimination step
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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