Propellane

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.

[1.1.1] propellane

[1.1.1] Propellane was first synthesized in 1982 [ " [1.1.1] Propellane" Kenneth B. Wiberg and Frederick H. Walker J. Am. Chem. Soc.; 1982; 104(19) pp 5239 - 5240; DOI|10.1021/ja00383a046.] converting the 1,3-di-carboxylic acid of [bicyclopentane|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 [Organic Syntheses, Coll. Vol. 10, p.658 (2004); Vol. 75, p.98 (1998) [http://www.orgsyn.org/orgsyn/prep.asp?prep=v75p0098 Online article] .] 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. [" [n] Staffanes: a molecular-size "Tinkertoy" construction set for nanotechnology. Preparation of end-functionalized telomers and a polymer of [1.1.1] propellane" Piotr Kaszynski and Josef Michl J. Am. Chem. Soc.; 1988; 110(15) pp 5225 - 5226; DOI|10.1021/ja00223a070] 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). [ " [2.2.2] Propellane system" Philip E. Eaton and George H. Temme J. Am. Chem. Soc.; 1973; 95(22) pp 7508 - 7510; DOI|10.1021/ja00803a052] The organic synthesis ("scheme 3") [Reaction sequence: photochemical [2+2] cycloaddition of ethylene on cyclohexene system 1 to bicyclic 2 followed by elimination reaction with potassium t-butoxide of acetic acid to cyclobutene 3 followed by another cycloaddition with ethylene to 4. This compound is converted to the diazo ketone 5 by deprotonation (acetic acid, sodium methoxide) and reaction with tosyl azide which then undergoes Wolff rearrangement to ketene 6. Ozonolysis forms ketone 7 and another sequence of diazotation and rearrangement forms the [2.2.2] propellane with a dimethylamide substituent after reaction of the final ketene with dimethylamine] features two Wolff rearrangement reactions.

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This propellane is also in-stable: 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 [3.3.1.1^3,7.0^1,3] decane is a [1.3.3] propellane of the adamantane family. It can be prepared by oxidation of 1,3-dihalo-adamantanes ["Tetracyclo [3.3.1.13,7.01,3] decane. Highly reactive 1,3-dehydro derivative of adamantane" Richard E. Pincock and Edward J. Torupka J. Am. Chem. Soc.; 1969; 91(16) pp 4593 - 4593; DOI|10.1021/ja01044a072] 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: ["Formation of Alternating Copolymers via Spontaneous Copolymerization of 1,3-Dehydroadamantane with Electron-Deficient Vinyl Monomers" Shin-ichi Matsuoka, Naoto Ogiwara, and Takashi Ishizone J. Am. Chem. Soc.; 2006; 128(27) pp 8708 - 8709; (Communication) DOI|10.1021/ja062157i]

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References