Pericyclic reactions electron counting

In a pericyclic reaction, electron counting can be effected in several ways, all equivalent. For example, in the Diels-Alder reaction, one can count the number of conjugated atoms in butadiene and in ethylene, or the number of bonds made (two o and one n bonds) or broken (three n bonds) in the process. In all cases, a total of six intervening electrons are obtained. 

The stereochemistry of any pericyclic reaction can be predicted by counting the total number of electron pairs (bonds) involved in bond reorganization and then applying the mnemonic "The Electrons Circle Around. " That is, thermal (ground-slate) reactions involving an even number of electron pairs occur with either conrotatory or antarafacial stereochemistry. Exactly the opposite rules apply to photochemical (excited-state) reactions. 

An important consequence of the pseudopericyclic mechanism is that the planar (or nearly planar) transition states preclude orbital overlap between the a- and Jt-orbitals. This implies that all pseudopericyclic reactions are allowed. Therefore, Jt-electron count, which dictates whether a pericyclic reaction will be allowed or forbidden, disrotatory, or conrotatory, is inconsequential when it comes to pseudopericyclic reactions. Bimey demonstrated this allowedness for all pseudopericyclic reactions in the study of the electrocyclic reactions 4.11-4.14. 

Huckel was also able to show that if a cyclic conjugated tt-system is irradiated with light so that it goes into the first excited singlet or triplet electronic state, it is especially stable if the number of cyclically conjugated electrons equals [4n]. Hence, photochemically activated pericyclic reactions will proceed suprafacially via a Huckel transition state if the electron count corresponds to [An],... 

Each of these theoretical approaches leads to the same predictions regarding reaction conditions and stereochemistry. For a wide range of reactions, the selection rules can be used empirically, based on a simple method of electron counting, without regard to their theoretical basis. The selection rules for pericyclic reactions relate three features ... 

The stereochemistry of any pericyclic reaction can be predicted by counting the total number of electron pairs (bonds) involved in bond reorganization and then applying the mnemonic The Electrons Circle... 

Therefore, if we derive or remember one rule for a pericyclic reaction, then any time an MO phase change is added the rule will reverse. Two reversals cancel each other. For example, 4n face to face (supra-supra) cycloadditions are not thermally allowed. If we add two electrons, we fill the next highest MO, which has a phase reversal. This means An+2 cycloadditions are thermally favored. Thermal electrocyclic reactions of 4n species go conrotatory, whereas thermal 4n+2 electrocyclic reactions go disrotatory. Thermal sigmatropic reactions of 4n species go supra-inversion or antara-retention. Count arrows to tell whether the pericyclic reaction is 4n or 4n + 2. Phase reversals occur between retention/inversion at the migrating center, between antarafacial/suprafacial migration, with 4n vs. 4n+2 electrons, and between thermal and photochemically excited species. 

Examine the following complex pericyclic reactions, and designate them using the electron count and suprafacial / antarafacial terminology. State if they are allowed or forbidden ba.sed upon the generalized orbital symmetry rule. 

Occasionally, though, you will run across a more exotic pericyclic process, and will want to decide if it is allowed. In a complex case, a reaction that is not a simple electrocyclic ringopening or cycloaddition, often the basic orbital symmetry rules or FMO analyses are not easily applied. In contrast, aromatic transition state theory and the generalized orbital symmetry rule are easy to apply to any reaction. With aromatic transition state theory, we simply draw the cyclic array of orbitals, establish whether we have a Mobius or Hiickel topology, and then count electrons. Also, the generalized orbital symmetry rule is easy to apply. We simply break the reaction into two or more components and analyze the number of electrons and the ability of the components to react in a suprafacial or antarafacial manner. 

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