Reaction of (Z)-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 conditions. The structure of 4 was confirmed by x-ray crystallographic analysis. Semi-empirical PM3 calculations and the measurement of the He I photoelectron spectrum of 2 indicated that the ionization potential of 2 is considerably lower than that of paracyclophane (1), which gave a [4 + 2] cycloadduct (3) with TCNE. The unusual pericyclic selectivity and regioselectivity are discussed on the basis of PM3 calculations. The regioselectivity is ascribed to the higher π bond order of the bridgehead aromatic bond of 2, which is on the same side of the bridge double bond, than that of the bond on the opposite side of the bridge double bond.
|Number of pages||6|
|Journal||Journal of Physical Organic Chemistry|
|State||Published - 1 Jan 1996|