Silabenzene
Encyclopedia
A silabenzene is a heteroaromatic compound containing one or more silicon
atoms instead of carbon
atom in benzene
. A single substitution gives silabenzene proper; additional substitutions give a disilabenzene (3 theoretical isomers), trisilabenzene (4 isomers), etc.
Silabenzenes have been the targets of many theoretical and synthetic studies by organic chemists
interested in the question of whether heavy benzene
s exhibit aromaticity.
Although several heteroaromatic compounds bearing nitrogen
, oxygen
, and sulfur
atoms have been known since the early stages of organic chemistry
, silabenzene had been considered to be a transient, un-isolable compound and was detected only in low-temperature matrices for a long time. In recent years, however, a kinetically stabilized silabenzene and other heavy aromatic compounds with silicon
or germanium
atoms have been reported.
, a (1,1-dimethylethyl) group or a TMS (trimethylsilyl
) group, but such silabenzenes readily react with themselves to give the corresponding dimer even at low temperature (below -100°C) due to the high reactivity of silicon
-carbon
π bonds.
Following the synthesis of the naphthalene
analog 2-silanaphthalene, the first sila-aromatic compound, by Norihiro Tokitoh and Renji Okazaki in 1997, the same group reported thermally stable silabenzene in 2000 taking advantage of a new steric
protective group.
. A 9-sila anthracene
derivative has been reported in 2002, a 1-silanaphtalene also in 2002
A 1,4-disilabenzene was reported in 2002. In 2007, 1,2-disilabenzene was synthesized via formal [2+2+2] cyclotrimerization
of a disilyne
(Si-Si triple bonded species) and phenylacetylene
.
Some theoretical studies suggest that the symmetric 1,3,5-trisilabenzene may be more stable than 1,2-disilabenzene.
-type products, so the aromaticity of the silabenzene is destroyed. It is different from benzene
, which reacts with electrophile
s to give not diene
s but substituted benzenes, so benzene sustains its aromaticity. Silicon
is a semi-metal
element
, so the Si-C π bond in the silabenzene is highly polarized
and easily broken. The silabenzene is also light-sensitive; UV
irradiation gives the valence isomer
, a silabenzvalene. The theoretical calculations and the NMR
chemical shift
s of silabenzenes, though, show that silabenzene is an aromatic compound in spite of the different reactivity from benzene
and other classical aromatic compounds.
a chairlike tricyclic silicon frame.
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
atoms instead of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
atom in benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
. A single substitution gives silabenzene proper; additional substitutions give a disilabenzene (3 theoretical isomers), trisilabenzene (4 isomers), etc.
Silabenzenes have been the targets of many theoretical and synthetic studies by organic chemists
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
interested in the question of whether heavy benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
s exhibit aromaticity.
Although several heteroaromatic compounds bearing nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
atoms have been known since the early stages of organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, silabenzene had been considered to be a transient, un-isolable compound and was detected only in low-temperature matrices for a long time. In recent years, however, a kinetically stabilized silabenzene and other heavy aromatic compounds with silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
or germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
atoms have been reported.
Synthesis
Several attempts to synthesize stable silabenzenes have been reported from the late 1970s using well-known bulky substituents such as a tert-butylButyl
In organic chemistry, butyl is a four-carbon alkyl radical or substituent group with general chemical formula -C4H9, derived from either of the two isomers of butane....
, a (1,1-dimethylethyl) group or a TMS (trimethylsilyl
Trimethylsilyl
A trimethylsilyl group is a functional group in organic chemistry. This group consists of three methyl groups bonded to a silicon atom [−Si3], which is in turn bonded to the rest of a molecule...
) group, but such silabenzenes readily react with themselves to give the corresponding dimer even at low temperature (below -100°C) due to the high reactivity of silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
-carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
π bonds.
Following the synthesis of the naphthalene
Naphthalene
Naphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings...
analog 2-silanaphthalene, the first sila-aromatic compound, by Norihiro Tokitoh and Renji Okazaki in 1997, the same group reported thermally stable silabenzene in 2000 taking advantage of a new steric
Steric effects
Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds , and this may affect the molecule's preferred shape and reactivity.-Steric...
protective group.
. A 9-sila anthracene
Anthracene
Anthracene is a solid polycyclic aromatic hydrocarbon consisting of three fused benzene rings. It is a component of coal-tar. Anthracene is used in the production of the red dye alizarin and other dyes...
derivative has been reported in 2002, a 1-silanaphtalene also in 2002
A 1,4-disilabenzene was reported in 2002. In 2007, 1,2-disilabenzene was synthesized via formal [2+2+2] cyclotrimerization
Cycloaddition
A cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity." The resulting reaction is a cyclization reaction.Cycloadditions are usually described by the...
of a disilyne
Disilyne
A disilyne is a chemical compound that contains a formal silicon - silicon triple bond and as such is formulated R2Si2 and is the silicon analogue of an alkyne. The hypothetical parent hydride, Si2H2 does not actually exist, all attempts to synthesize it results in a diradical disilene, with the...
(Si-Si triple bonded species) and phenylacetylene
Phenylacetylene
Phenylacetylene is an alkyne hydrocarbon containing a phenyl group. It exists as a colorless, viscous liquid. In research, it is sometimes used as an analog for acetylene; being a liquid, it is easier to handle than acetylene gas.-Preparation:...
.
Some theoretical studies suggest that the symmetric 1,3,5-trisilabenzene may be more stable than 1,2-disilabenzene.
Properties and reactions
Isolated silabenzene reacts with various reagents at 1,2- or 1,4-positions to give dieneDiene
In organic chemistry a diene or diolefin is a hydrocarbon that contains two carbon double bonds.Conjugated dienes are functional groups, with a general formula of CnH2n-2. Dienes and alkynes are functional isomers...
-type products, so the aromaticity of the silabenzene is destroyed. It is different from benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
, which reacts with electrophile
Electrophile
In general electrophiles are positively charged species that are attracted to an electron rich centre. In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile...
s to give not diene
Diene
In organic chemistry a diene or diolefin is a hydrocarbon that contains two carbon double bonds.Conjugated dienes are functional groups, with a general formula of CnH2n-2. Dienes and alkynes are functional isomers...
s but substituted benzenes, so benzene sustains its aromaticity. Silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
is a semi-metal
Metalloid
Metalloid is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, each element can usually be classified as a metal or a nonmetal. However, some elements with intermediate or mixed properties can be harder to characterize...
element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
, so the Si-C π bond in the silabenzene is highly polarized
Polar bond
In chemistry, a polar bond is a type of covalent bond between two atoms or more in which electrons are shared unequally. Because of this, one end of the molecule has a slight, relative negative charge and the other a slight, relative positive charge...
and easily broken. The silabenzene is also light-sensitive; UV
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
irradiation gives the valence isomer
Valence isomer
In organic chemistry, two molecules are valence isomers when they are constitutional isomers that can interconvert through pericyclic reactions.-Benzene:...
, a silabenzvalene. The theoretical calculations and the NMR
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
chemical shift
Chemical shift
In nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
s of silabenzenes, though, show that silabenzene is an aromatic compound in spite of the different reactivity from benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
and other classical aromatic compounds.
Hexasilabenzene
In calculations the all-silicon hexasilabenzene Si6H6 is predicted to have 6-fold symmetry or a chair conformation. A stable hexasilaprismane has been known since 1993 A compound isomeric with hexasilabenzene was first reported in 2010. This compound is reported as stable and with according to X-ray crystallographyX-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
a chairlike tricyclic silicon frame.
See also
- 6-membered aromatic rings with one carbon replaced by another group: borabenzeneBorabenzeneA borabenzene is a heteroaromatic compound that has a boron atom instead of the carbon atom of a benzene molecule. A free borabenzene, which has no donor ligand on the boron atom, has not yet been isolated despite its simple structure and the chemical robustness of boron-carbon bonds...
, silabenzene, germanabenzeneGermanabenzeneGermabenzene is the parent representative of a group of chemical compounds containing in their molecular structure a benzene ring with a carbon atom replaced by a germanium atom. Germabenzene itself has been studied theoretically, but has not been synthesized...
, stannabenzeneStannabenzeneStannabenzene is the parent representative of a group of organotin compounds that are related to benzene with a carbon atom replaced by a tin atom. Stannabenzene itself has been studied by computational chemistry, but has not been isolated....
, pyridinePyridinePyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
, phosphorinePhosphorinePhosphorine is a heavier element analog of pyridine, containing a phosphorus atom instead of an aza- moiety. It is also called phosphabenzene and belongs to the phosphaalkene class. Phosphorine is a planar aromatic compound with 88% of the aromaticity of that of benzene...
, pyrylium saltPyrylium saltA pyrylium salt is a salt containing a pyrylium cation or a derivative of it. The pyrylium cation is a conjugated 6-membered carbon ring system with one carbon atom replaced by a positively charged oxygen atom. It is, like benzene, an aromatic compound...