The Heat of Formation of Cyclobutadiene

摘要

Cyclobutadiene (1) and its derivatives have beckoned to chemists ever since Kekule's deduction of the structure ofbenzene and his attempt to synthesize 1 in 1872.[1] Pioneeringefforts by Willst%tter and Finkelstein were followed by a century of studies which produced many remarkable findings.Room temperature stable derivatives such as tri-tert-butylcyclobutadiene[2] and tetra-tert-butylcyclobutadiene[3] were successfully prepared and spectroscopically characterized, whereas the parent compound was found to be more elusive. It dimerizes in solid matrices at35 K, is a transient reactive intermediate in solution, and has a lifetime of only 2 ms at 0.1 Torr in the gas phase.[1a]Nevertheless, trapping[4] and spectroscopic results have revealed that cyclobutadiene has a ground-state singlet configuration and adopts a rectangular D2h structure which rapidly undergoes automerization. An isolable and room temperature stable complex consisting of 1 in the cavity of a spherical crown ether (that is, a hemicarceplex) as even been prepared,[5] but the thermodynamic stability of cyclobutadiene remains experimentally unknown.