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| Names | |||
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| Preferred IUPAC name Tricyclo[1.1.1.01,3]pentane | |||
| Identifiers | |||
3D model (JSmol) | |||
| ChemSpider |
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| Properties | |||
| C5H6 | |||
| Molar mass | 66.103 g·mol−1 | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
[1.1.1]Propellane is anorganic compound, the simplest member of thepropellane family. It is ahydrocarbon with formulaC5H6 orC2(CH2)3. The molecular structure consists of three rings of threecarbon atoms each, sharing oneC–C bond.
[1.1.1]Propellane is ahighly strained molecule. The bonds of the two central carbon atoms have aninverted tetrahedral geometry, and thelength of the central bond is 160 pm. Thestrength of that bond is disputed; estimates vary from 59–65 kcal/mol to no strength at all. The energy of thebiradical state (with no central bond at all) is calculated to be 80 kcal/mol higher. At 114 °C it will spontaneouslyisomerize to3-methylidenecyclobutene (5 below) with ahalf-life of 5 minutes. Itsstrain energy is estimated to be 102 kcal/mol (427 kJ/mol). Surprisingly, [1.1.1]propellane is persistent at room temperature and is somewhat less susceptible to thermal decomposition than the less strained (90 kcal/mol) [2.2.2]propellane system, which has an estimated half-life of only about 1 h at 25 °C.[1] This unusual stability is attributed to delocalisation of electron density from the bond between the central carbon atoms onto the bridging carbon atoms.[2]
The type of bonding in this molecule has been explained in terms ofcharge-shift bonding.[3]
[1.1.1]Propellane was first reported byKenneth B. Wiberg and F. Walker in 1982. The synthesis commences withcyclopropanation of1,1-bis(chloromethyl)ethylene,[4] according to the following scheme:
Synthesis begins with conversion of the 1,3-di-carboxylic acid ofbicyclo[1.1.1]pentane1 in aHunsdiecker reaction to the corresponding dibromide2 followed by acoupling reaction withn-butyllithium. The final product3 was isolated bycolumn chromatography at −30 °C.
However, a much simplified synthesis was published by Szeimies.[5] It starts withdibromocarbene addition to thealkene bond of 3-chloro-2-(chloromethyl)propene6 followed bydeprotonation bymethyllithium andnucleophilic displacements in7.[6] The product was not isolated but kept in solution at −196 °C.
[1.1.1]Propellane spontaneously reacts withacetic acid to yield amethylidenecyclobutaneester (4 above).
[1.1.1]Propellane undergoes apolymerization reaction where the central C–C bond is split and connected to adjacentmonomer units, resulting instaffanes.[7]
Aradical polymerization initiated bymethyl formate andbenzoyl peroxide results in a distribution ofoligomers. Ananionic addition polymerization withn-butyllithium results in a fully polymerized product.X-ray diffraction of the polymer shows that the connecting C–C bonds havebond lengths of only 1.48 Å, significantly shorter than the normal 1.54 Å.
The compound1,3-dehydroadamantane, which can be viewed as a bridged [1.3.3]propellane, also polymerizes in a similar way.
