Transition states in pericyclic reaction

The second mechanism, due to the permutational properties of the electronic wave function is referred to as the permutational mechanism. It was introduced in Section I for the H4 system, and above for pericyclic reactions and is closely related to the aromaticity of the reaction. Following Evans principle, an aromatic transition state is defined in analogy with the hybrid of the two Kekule structures of benzene. A cyclic transition state in pericyclic reactions is defined as aromatic or antiaromatic according to whether it is more stable or less stable than the open chain analogue, respectively. In [32], it was assumed that the in-phase combination in Eq. (14) lies always the on the ground state potential. As discussed above, it can be shown that the ground state of aromatic systems is always represented by the in-phase combination of Eq. (14), and antiaromatic ones—by the out-of-phase combination. 

Close inspection of the D-E rules shows that the transition states in pericyclic reactions resemble structures which are topologically... 

The fact that there is at least one antibonding overlap in the cyclic array of orbitals does not reduce aromaticity. Mobius transition states are as aromatic as normal Htickel transition states. In contrast to stable Mobius annulenes, Mobius transition states in pericyclic reactions are quite frequent. They also include c aromatic... 

Evans/Dewar Concept of Aromaticity of Transition States in Pericyclic Reactions... 

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See also in sourсe #XX -- [ ]

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