Effect of Intramolecular Paterno—Buchi Reaction on the Thermodynamics and Kinetics of Nearly Degenerate[3,3]-Sigmatropic Shift in Fluxional Polycycles

摘要

In reactions with weak dienophiles, cyclooctatetraene (COT) often yields 2:1 adducts possessing thefluxional bicyclo[5.1.0]octadiene moiety. They undergo fast, nearly degenerate Cope rearrangement withan activation barrier similar to that of the parent dihydrobullvalene. Irradiation to excite the carbonylmoiety induces an intramolecular Paterno–Buchi cyclization yielding endo-oxetanes and significantlychanging the Cope-averaged NMR spectra. In this paper we examine the effect of skeletal distortioncaused by intramolecular [2 + 2]-photoaddition on thermodynamics and the activation barrier of the[3,3]-sigmatropic tautomerism. Our finding is that such a distortion lifts the energetic degeneracy of thetwo valence tautomers, while not affecting the activation barrier.