[5,5]-SIGMATROPIC REARRANGEMENT - DFT PREDICTION OF A DIRADICAL MECHANISM FOR A WOODWARD-HOFFMANN ALLOWED THERMAL PERICYCLIC REACTION - ENGLISH, ENGLISH

The mechanisms of the degenerate [5,5] sigmatropic rearrangements of 5 ,5a,10,10a-tetrahydroheptalene and (Z,Z)-1,3,7,9-decatetraene were exp lored with restricted and unrestricted Becke3LYP/6-31G hybrid HF-DFT calculations. The restricted DFT wavefunctions for the synchronous, co ncerted transition structures for these formally Woodward-Hoffmann all owed 10-electron rearrangements are unstable with respect to unrestric ted wavefunctions. A stepwise diradical mechanism is predicted for bot h reactions. The 9 kcal/mol preference for the [5s,5s] mechanism over the [5a,5a] pathway in the rearrangement of decatetraene results prima rily from the geometric distortions that the system must adopt in the [5a,5a] transition structure. The geometric and electronic characteris tics of the [5,5] and [3,3] (Cope) rearrangements are compared.