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Propellane
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Propellanes are a class of tricyclic organic compounds sharing a common carbon-carbon covalent bond. They are characterized by the presence of carbon with an inverted tetrahedral geometry, large steric strain and high reactivity and are for these reasons much studied in organic chemistry. The simplest propellanes are [1.1.1]propellane (C5H6) and [2.2.2]propellane (C8H12).
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Encyclopedia
Propellanes are a class of tricyclic organic compounds sharing a common carbon-carbon covalent bond. They are characterized by the presence of carbon with an inverted tetrahedral geometry, large steric strain and high reactivity and are for these reasons much studied in organic chemistry. The simplest propellanes are [1.1.1]propellane (C5H6) and [2.2.2]propellane (C8H12). They derive their name from the obvious resemblance to a propeller.
1,3-dehydroadamantanes are [1.3.3]propellanes of the adamantane family.
Bonding
The central bond in [1.1.1]propellane connects two inverted bridgehead atoms with a bond length of only 160 pm. The bond strength is disputed, varying from 59–65 kcal/mol to no strength at all. The energy of the biradical state is calculated to be 80 kcal/mol higher. Recently the type of bonding in this molecule is explained in terms of charge-shift bonding
[1.1.1]propellane
[1.1.1]Propellane was first synthesized in 1982 converting the 1,3-di-carboxylic acid of bicyclo[1.1.1]pentane 1.1 in a Hunsdiecker reaction to the corresponding dibromide 1.2 followed by a coupling reaction with n-butyllithium in scheme 1. The product is isolated by column chromatography at -30°C (!)
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Another synthesis starts with dibromocarbene addition to the alkene bond of 3-chloro-2-(chloromethyl)propene 1.6 followed by deprotonation by methyllithium and nucleophilic displacements in 1.7 not isolated but kept in solution at -196 °C.
The instability of this propellane is evident from its thermal isomerization to 3-methylenecylobutene 1.5 at 114 °C with a chemical half-life of 5 minutes and its spontaneous reaction with acetic acid also to a cyclobutane (1.4).
The strain energy is estimated to be 102 kcal/mol (427 kJ/mol).
Polymerization
[1.1.1]propellane is a monomer in polymerization reactions to so-called [n]staffanes. A radical polymerization initiated by methyl formate and benzoyl peroxide results in a distribution of oligomers (scheme 2) but an anionic addition polymerization with n-butyllithium results in a truly polymerized system. X-ray diffraction of the polymer shows that the connecting C-C bonds have bond lengths of only 148 pm.
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[2.2.2]propellane
The synthesis of the next homologue, [2.2.2]propellane by the group of Philip Eaton of cubane fame, precedes that of [1.1.1]propellane (1973). The organic synthesis (scheme 3) features two Wolff rearrangement reactions.
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This propellane is also unstable: the chemical half-life to isomerization to the monocyclic amide 3.11 at room temperature in solution is 28 minutes. The strain energy is estimated to be 93 kcal/mol (390 kJ/mol).
1,3-dehydroadamantanes
1,3-dehydroadamantane or tetracyclo[3.3.1.13,7.01,3]decane is a [1.3.3]propellane of the adamantane family. It can be prepared by oxidation of 1,3-dihalo-adamantanes and are just as unstable as the other small propellanes. On standing in solution the compound reacts with oxygen from air (half-life 6 hours) to a peroxide which converts to a di-hydroxide by reaction with lithium aluminium hydride.
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Polymerization
Being a propellane, dehydropropellanes can be polymerized as well. In scheme 4 it is reacted with acrylonitrile in a radical polymerization initiated with lithium metal in tetrahydrofuran. The resulting copolymer is alternating with glass transition temperature of 217 °C:
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