The Cope Rearrangement Transition Structure Is Not Diradicaloid, but Is It Aromatic?

摘要

Any problem that has generated nearly a hall century of controversy is not solved easily. What is the "precise mechanism" of the parent, degenerate Cope rearrangement of 1,5-hexadiene (1) (Scheme 1)? Does the favored pathway with chairlike transitionstructures follow a stepwise mechanism via a C_(2h)-symmetrical intermediate 3 (3a) and two equivalent C_2-sym-metrical transition structures. 2 and 2', or is this a synchronous, fully concerted process in which the C_(2h)-symmetrica! structure 3a describes the only transition structure? What is the mechanism of the alternative route via boatlike transition structures, which are roughly 11 kcalmol~(-1) higher in energy? Do the transition states or intermediates have "diyl" character, in other words "diradicaloid" (singlet diradical) character, or are they better described as "delocalized" or "aromatic"?