The Stereochemistry of the Cope Rearrangement: Qualitative Theory (OCAMS) and Computation (AM1)

Contrary to common preconceptions, orbital correlation diagrams can be useful for deducing the mechanism and stereochemistry of degenerate sigmatropic rearrangements. The correlation lines have to be drawn between the reactant and the postulated transition state, rather than between the homomeric reactant and product. In the case of the Cope rearrangment, this is done most economically with Orbital Correspondence Analysis in Maximum Symmetry (OCAMS), by drawing correspondence lines between molecular orbitals of the reactant and those of a formal superposition (STS) of alternative transition structures (TS) in a symmetry point group that includes those of both as subgroups. The alternative boat (C 2v ) and chair (C 2h ) transition structures postulated for the degenerate Cope rearrangement of 1,5‐hexadiene are formally combined to an STS of D 2h symmetry. OCAMS then selects its C 2h component, indicating that the pathway via the chair‐like transition state is preferred, in agreement with experimental and computational results for both the Cope rearrangement and the isoelectronic Claisen rearrangement. Rearrangement through the less favored boat pathway is shown to arise from a changed electronic configuration in the transition state. Computational evidence is presented showing that the unexpectedly facile rearrangement of polycyclic molecules like semibullvalene and barbaralane, which are constrained to react via the boat pathway, is primarily due to steric and stereoelectronic factors that are explicable in molecular orbital terms, rather than to the incursion of a biradicaloid transition state.