Study of Extremely Long C - C Bond

点击翻译按钮获取中文摘要

Some naphthocyclobutenes substituted with four phenyl groups and with two phenyl and two t Bu groups are prepared. X-ray analysis of these naphthocyclobutenes showed that the cyclobutene C - C bonds are extremely long, 1.72 - 1.73 Å. Especially, the C - C cyclobutene bond of the naphthocyclobutene substituted with two phenyl and two t Bu groups is the longest 1.73 Å. π-Electron locarization in the naphthalene ring is also observed.Reason for the extremely long C - C bond in naphthocyclobutenes would be a steric repulsion among bulky substituents on the four-membered ring. There is one another possible reason, namely, through bond interaction. In order to clarify which reason is more important, naphthocyclobutene derivatives substituted with cis- and trans-diphenyl groups were prepared. X-ray structural analysis of these showed there is no big difference between the bond length of cis- and trans-isomers. These results suggest that the steric repulsion is more important factor.Similarly, anthradicyclobutenes and phenanthradicyclobutenes substituted with phenyl groups were prepared. X-ray structural analysis of these showed that the cyclobutene C - C bonds are also extremely long. Stabilities of these cyclobutane derivatives would be due to the following reasons : ring opening of the cyclobutene should produce sterically crowded biradical or resonance energy lost quinodimethane.