Pericyclic transition structure geometries

Fig. 5.35 Geometry predicted by CASSCF ab initio calculations of the two possible transition structure geometries for the Diels-Alder reaction between ethene and butadiene. (Figure adapted from Houk KN, J Gonzalez and Y Li 1995. Pericyclic Reaction Transition States Passions and Punctilios 1935-1995. Accounts of Chemical Research 28 81-90.)...

The name Houk has become synonymous with calculations on the transition states of pericyclic reactions. For two decades, as increasingly sophisticated types of electronic structure calculations became feasible for such reactions, Ken s group used these methods to investigate the geometries and energies of the transition structures. Ken s calculations showed that, in the absence of unsymmetrical substitution, bond making and bond breaking occur synchronously in pericyclic reactions. 

The orbital phase theory includes the importance of orbital symmetry in chanical reactions pointed out by Fukui [11] in 1964 and estabhshed by Woodward and Holiimann [12,13] in 1965 as the stereoselection rule of the pericyclic reactions via cyclic transition states, and the 4n + 2n electron rule for the aromaticity by Hueckel. The pericyclic reactions and the cyclic conjugated molecules have a conunon feature or cychc geometries at the transition states and at the equihbrium structures, respectively. 

Pericyclic reactions are routinely classified as"allowed"or"forbidden"with a particular structure for the transition state. In practice, this classification means that one geometry for the reaction has a low energy transition state (allowed) or that a different geometry has a very high energy transition state (forbidden). To determine whether a reaction is allowed or forbidden, a handful of approaches exist. We will examine one approach frontier molecular orbital theory. 

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