Rearrangement of perfluorocyclobutyne†

Abstract

The energies of cyclobutyne, perfluorocyclobutyne and related compounds have been calculated at levels from MP2/6-31G* through QCISD(T)/6-311+G*, and also at B3LYP and B3PW91/6-311+G*. The relative energies found with the larger basis sets are quite consistent. Cyclobutyne is calculated to be a transition state at most levels of theory, leading to cyclopropylidenecarbene. However, the latter does not have C_2v symmetry, but rather undergoes a tilt of the carbene carbon toward a ring carbon leading to a distorted C_s structure. Perfluorocyclobutyne is calculated to have a significant activation energy for rearrangement at the MP2 level, but higher levels of theory suggest that it will rearrange spontaneously to perfluorocyclopropylidenecarbene, again with a distorted structure. The calculated heats of hydrogenation of the cyclobutynes are about 70 kcal mol^–1 greater than for acetylene. The energy of a process by which perfluorocyclobutyne or perfluorocyclopropylidinecarbene might be formed is also calculated.