Rearrangement pathways, potential energy

Scheme 16.5). Thus, the mechanism of the thermal decomposition of 7 has not been fully clarified, yet although the aryne contraction pathway has been established for some benzannellated derivatives of 4. A number of theoretical studies has been devoted to rearrangements on the CeKU potential energy surface. At... 

These trajectories do provide a means for reconciling all of the data into a cohesive fi amework. The [2-1-2] and [2-1-4] prodncts can be formed from passage over the same TS. This explains why both products are observed even at short reaction times and precludes the necessity of the two-step ([4-1-2] followed by the Claisen rearrangement) mechanism. It obviates the requirement for climbing the very large barriers associated with the direct [2-1-2] pathways that were problematic when just examining the potential energy surfaces. Trajectory analysis allows for an interpretation of the KIEs. TST adequately accounts for the KIEs for Reaction 8.7. 

The rearrangement assisted by HF has the same electronic profile as the uncatalyzed pathway. That is, the symmetrical structure corresponds to a minimum on the vibrationless potential energy surface that disappears upon inclusion of zero-point energy. With either NH4+ or H3O+ as the acid, the symmetrical species is a transition structure on the vibrationless surface. 

Product formation requires loss of the plane of symmetry that exists in the reactant. Therefore, to establish the existence of an energy barrier to product formation, it is not valid to simply find an energy maximum along a reaction pathway that preserves this symmetry plane. For a energy maximum to be a true transition structure rather than a mountain top on the potential-energy surface for rearrangement of 14 to 15, the force constant for rotation around the C-N bond at the Cj energy maximum also must be positive. 

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