| Silabenzene | |
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Silabenzene |
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Siline |
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| Identifiers | |
| CAS number | 289-77-0 |
| PubChem | 136138 |
| ChemSpider | 119915 |
| Jmol-3D images | Image 1 Image 2 |
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| Properties | |
| Molecular formula | C5H6Si |
| Molar mass | 94.19 g mol−1 |
| Exact mass | 94.023876725 g mol-1 |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
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 benzenes 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.
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Several attempts to synthesize stable silabenzenes have been reported from the late 1970s using well-known bulky substituents such as a tert-butyl, 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,[1][2] 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. .[3] A 9-sila anthracene derivative has been reported in 2002,[4] a 1-silanaphtalene also in 2002 [5]
A 1,4-disilabenzene was reported in 2002.[6] In 2007, 1,2-disilabenzene was synthesized via formal [2+2+2] cyclotrimerization of a disilyne (Si-Si triple bonded species) and phenylacetylene.[7]
Some theoretical studies suggest that the symmetric 1,3,5-trisilabenzene may be more stable than 1,2-disilabenzene.[8]
Isolated silabenzene reacts with various reagents at 1,2- or 1,4-positions to give diene-type products, so the aromaticity of the silabenzene is destroyed. It is different from benzene, which reacts with electrophiles to give not dienes 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 shifts of silabenzenes, though, show that silabenzene is an aromatic compound in spite of the different reactivity from benzene and other classical aromatic compounds.
In calculations the all-silicon hexasilabenzene Si6H6 is predicted to have 6-fold symmetry [9] or a chair conformation.[10] A stable hexasilaprismane has been known since 1993 [11] A compound isomeric with hexasilabenzene was first reported in 2010.[12] This compound is reported as stable and with according to X-ray crystallography a chairlike tricyclic silicon frame.