THERMAL [2-[6]PARACYCLOPH-3-ENE WITH TETRACYANOETHYLENE(2] CYCLOADDITION OF (Z))

Reaction of (Z)-[6]paracycloph-3-ene (2) with tetracyanoethylene (TCNE ) gave a [2+2] cycloadduct (4), which represents the first example of thermal [2+2] cycloaddition of a benzene derivative under mild conditi ons, The structure of 4 was confirmed by x-ray crystallographic analys is, Semi-empirical PM3 calculations and the measurement of the He I ph otoelectron spectrum of 2 indicated that the ionization potential of 2 is considerably lower than that of [6]paracyclophane (I), which gave a [4+2] cycloadduct (3) with TCNE, The unusual pericyclic selectivity and regioselectivity are discussed on the basis of PM3 calculations, T he regioselectivity is ascribed to the higher pi bond order of the bri dgehead aromatic bond of 2, which is on the same side of the bridge do uble bond, than that of the bond on the opposite side of the bridge do uble bond.