Transition state aromaticity Mobius topologies

Transition state aromaticity (Huckel and Mobius topologies)... 

We have considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbital, orbital correlation diagrams, and transition state aromaticity. All arrive at the same conclusions about the stereochemistry of electrocyclic reactions. Reactions involving 4n + 2 electrons are disrotatory and involve a HUckel-type transition structure, whereas those involving 4n electrons are conrotatory and the orbital array are of the Mobius type. These general principles serve to explain and correlate many specific experimental observations. The chart that follows summarizes the relationship between transition stmcture topology, the number of electrons, and the feasibility of the reaction. 

In the butadiene example, the conrotatory transition state has a Mobius topology, and will be aromatic four electrons are present). Thus, the conrotatory process is allowed. The disrotatory transition state is a Hiickel system, and will require two or six electrons for aromaticity. Since in butadiene only four electrons are present, the disrotation pathway will be forbidden. 

Figure 15.17 B shows the aromatic transition state analysis of these reactions. We draw a picture of an opening pathway with the minimum number of phase changes and examine the number of nodes. The four-electron butadiene-cyclobutene system should follow the Mobius/conrotatory path, and the six-electron hexatriene-cyclohexadiene system should follow the Hiickel/disrotatory path. As such, aromatic transition state theory provides a simple analysis of electrocyclic reactions. The disrotatory motion is always of Hiickel topology, and the conrotatory motion is always of Mobius topology.

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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