Bicyclobutane
Bicyclobutane is an organic compound with the formula C4H6. It is a bicyclic molecule consisting of two cis-fused cyclopropane rings, and is a colorless and easily condensed gas. Bicyclobutane is noted for being one of the most strained compounds that is isolatable on a large scale
Manufacture and properties
Bicyclobutane is highly strained — its strain energy is estimated at 63.9 kcal mol−1. It is a nonplanar molecule, with a dihedral angle between the two cyclopropane rings of 123°.The first reported bicyclobutane was the ethyl carboxylate derivative, C4H5CO2Et, which was prepared by dehydrohalogenation the corresponding bromocyclobutanecarboxylate ester with sodium hydride. The parent hydrocarbon was prepared from 1-bromo-3-chlorocyclobutane by conversion of the bromocyclobutanecarboxylate ester, followed by intramolecular Wurtz coupling using molten sodium. The intermediate 1-bromo-3-chlorocyclobutane can also be prepared via a modified Hunsdiecker reaction from 3-chlorocyclobutanecarboxylic acid using mercuric oxide and bromine:
Stereochemical evidence indicates that bicyclobutane undergoes thermolysis to form 1,3-butadiene with an activation energy of 41 kcal mol−1 via a concerted pericyclic mechanism.
Derivatives
Bicyclobutanes are explored in medicinal chemistry as covalent reactive groups.In the simplest case, double cyclopropanation of acetylene with a copper catalyst gives a 1:1 mixture of cyclopropenes and symmetric bicyclobutanes. Other symmetric bicyclobutanes form from functionalization of benzvalene.
A synthetic approach to more substituted bicyclobutane derivatives involves ring closure of a suitably substituted 2-bromo-1-cyclopropane with magnesium in THF, or methyllithium in diethyl ether.
Substituted bicyclobutanes can also be prepared from the reaction of iodo-bicyclopentanes with amines, thiols, and sulfinate salts: