Propellane

1,3-dehydroadamantanes are [1.3.3]propellanes of the adamantane family.

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[1.1.1]propellane

[1.1.1]Propellane was first synthesized in 1982^[1] converting the 1,3-di-carboxylic acid of bicyclo[l.l.l]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 (!)

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^[2] 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.^[3] 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.

[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).^[4] The organic synthesis (scheme 3)^[5] features two Wolff rearrangement reactions.

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[3.3.1.13,7.01,3]decane is a [1.3.3]propellane of the adamantane family. It can be prepared by oxidation of 1,3-dihalo-adamantanes^[6] 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.

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:^[7]

References

1. ^ [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.
2. ^ Organic Syntheses, Coll. Vol. 10, p.658 (2004); Vol. 75, p.98 (1998) Online article.
3. ^ [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
4. ^ [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
5. ^ 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
6. ^ 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
7. ^ 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