Tricyclo[2.1.0.01,3]pentane

Article abstract of DOI:10.1021/ja00076a025

The reaction of 1-bromo-2-(chloromethyl)bicyclo[1.1.0]butane with methyllithium was examined. At -78 °C, 1-lithio-2-(chloromethyl)bicyclobutane is formed, and at -50 °C, it reacts to form cyclopentadiene. Evidence is presented that tricyclo[2.1.0.0^1,3]pentane, an isomer of [1.1.1]propellane, is formed as an intermediate. It could be trapped with phenylthiol to give 2-vinylcyclopropyl phenyl sulfide, and a ¹³CNMR signal attributed to the tricyclopentane also was observed. The course of its conversion to cyclopentadiene, which must involve inversion of configuration at one carbon center, was explored theoretically. Initially, one of the central C-C bonds lengthens along with rotation of the attached hydrogen, leading to a transition state. This is followed by further rotation leading to inversion of configuration at the C-H bond and cleavage of the other central C-C bond, forming cyclopentenyl carbene. The latter would be expected to undergo a rapid hydrogen migration giving cyclopentadiene. The calculated activation energy (～23 kcal/mol) was in reasonable agreement with that estimated from the experimental data (～16 kcal/mol). The heat of formation of tricyclopentane was estimated to be 132 kcal/mol, leading to a strain energy of 143 kcal/mol. This makes it one of the most highly strained of known hydrocarbons on a per carbon basis.