Evidence that naphthocyclobutene, phenanthrodicyclobutene, and anthrodicyclobutene derivatives are not contaminated by their cyclobutene ring-opened isomers

It has not been directly proven that the title cyclobutene derivatives which have an extremely long Csp3Csp3 bond length of the cyclobutene ring, are not contaminated by their cyclobutene ring-opened isomers. In order to clarify the uncertainties, NO addition experiments were carried out. When the cyclobutene ring is cleaved easily, NO may react with the cyclobutene ring. Although NO did not react with naphthocyclobutene and anthrodicyclobutene derivatives, NO reacted with anthrodicyclobutene derivatives at their central benzene nuclei instead of their cyclobutene rings. These results clearly show that all the cyclobutene derivatives do not exist in an equilibrium with any cyclobutene ring-opened isomer.