Preparation and kinetic stabilization of highly strained paracyclophanes

In this Account we describe the preparation, structures, and properties of the most highly strained paracyclophanes known to date, i.e. [4]- and [1.1]paracyclophanes and their derivatives. The preparation of these cyclophanes has been accomplished through the photochemical valence isomerization of corresponding 1,4-bridged Dewar benzene derivatives. These strained paracyclophanes are prone to polymerize and are not stable enough to isolate: [4]paracyclophane is rapidly consumed even below - 100 degreesC in fluid solution, though it remains intact indefinitely in a frozen organic glass at 77 K, while [1.1]paracyclophane is stable only below -20 degreesC in solution. By introducing substituents which sterically hinder access to the bridgehead carbon atoms by other reagents, however, one can remarkably stabilize these species kinetically so as to allow the measurement of H-1 NMR spectrum of the former and the isolation of the latter as crystals. The H-1 NMR spectrum of the [4]paracyclophane strongly support, in conjunction with theoretical calculations, the sustenance of considerable aromaticity despite the extreme bending of its benzene ring. [4]Paracyclophane is so strained that it is thermodynamically less stable than the corresponding Dewar isomer and the thermal conversion of the benzene form to the Dewar form is observed for the first time. A fully unsaturated derivative of [4]paracyclophane is also generated and confirmed to prefer the structure of 1,2,3,4-tetradehydro[4]paracyclophane rather than that of pi-bond isomer, bicyclo[4.2.2]deca-1,3,5,7,9-pentaene. The stabilized [1.1]paracyclophane is found to undergo efficient photochemical transformation into a benzene p,p'-dimer structure, from which the former is thermally regenerated upon mild heating. Structural characteristic features of the [1.1]paracyclophane and the benzene pp'-dimer are revealed by X-ray crystallography. Limits for experimentally accessible strained paracyclophane are also discussed.