Tetrahedrane and Cyclobutadiene: Synthesis and Structure of Highly Strained Hydrocarbons

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Tetrahedrane is a hydrocarbon with highly strained C-C sigma bonds. Because of the extremely strained skeleton, the increase in the energy level of C-C sigma-bond orbitals results in the sigma-pi conjugation. Cyclobutadiene is one of valence isomers of tetrahedrane and a cyclic unsaturated compound with four pi-electrons within a rectangular planar four-membered ring.We have investigated the synthesis, structural analysis and properties of substituted tetrahedranes stabilized by silyl groups. The perfluoroaryl substituted cyclobutadiene derivatives were prepared as air- and moisture-sensitive red solids by the photochemical isomerization of the corresponding tetrahedranes. The first aryl substituted cyclobutadienes were characterized by spectroscopic data as well as by Xray crystallography. We also developed new versatile synthetic routes of substituted tetrahedranes, and reported more than 20 stable derivatives up to now.