Transition Structures for the Aromatic Claisen Rearrangements by the Molecular Orbital Method

Ab initio calculations were performed for eight Claisen rearrangements, eqs 1−8. Transition-state (TS) structures of [3,3] sigmatropic rearrangements of reactions 1−4 are similar, but their activation energies (E a's) are different, E a(1) < E a(2) and E a(3) < E a(4). From the intermediate of reaction 3, a hydrogen is moved intermolecularly to form the product, o-allyl phenol. The lower reactivities of reactions 2 and 4 relative to reactions 1 and 3 are ascribed to large endothermicities in the sigmatropic rearrangements, respectively. Chair-type transition states are more favorable than boat-type transition states in reactions 1−4. The allyl group is released from the in-plane C−X (X = O or N) σ bond and is captured by the π-type lone-pair electrons. The sulfur- and phosphorus-containing rearrangements, reactions 5−8, are computed to have smaller activation energies but are to be less exothermic than those of oxgyen- and nitrogen-containing rearrangements.