Transannular pi-pi Interaction of (4n)pi Systems

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It is our interest to study how two pi-systems mutually interact when they overlap each other in a close proximity. According to the frontier molecular orbital calculations, Greenberg et al. predicted a destabilizing pi-pi interaction in a (4n+2)pi-(4n+2)pi system, a stabilizing interaction in a (4n+2)pi-(4n)pi system, and greater stabilization in a (4n)pi-(4n)pi system than in the (4n+2)pi-(4n)pi system. To verify this interesting prediction experimentally, we considered that [3.3]annulenophanes must be the most suitable models since [3.3]cyclophane permits exact evaluation of pi-pi interaction without interference of the strain effect.As model compounds for the study of (4n+2)(4n+2)interaction (n*2), we synthesized [3.3]cyclophanes containing Vogel's 1,6-methano[10]annulene, which are the first example of [m.n]cyclophanes containing two annulene rings. The synthesis of double-layered annulenophanes containing trans-15,16-dimethyl-dihydropyrene units (14pi) and Paudler's 8b,8c-diazacyclopent[fg]acenaphthylene (12pi) is in progress. The structures and transannular pi-pi interactions of annuleno-phanes were studies based on the spectral properties and theoretical calculations by the MNDO-PM3 method.