Stereochemistry of electrocyclic reactions

A striking illustration of the relationship between orbital symmetry considerations and the outcome of photochemical reactions can be found in the stereochemistry of electrocyclic reactions. In Chapter 11, the distinction between the conrotatory and the disrotatory mode of reaction as a function of the number of electrons in the system was... 

Thermal and photochemical electrocyclic reactions always take place with opposite stereochemistry because the symmetries of the frontier orbitals are always different. Table 30.1 gives some simple rules that make it possible to predict the stereochemistry of electrocyclic reactions. 

The observed stereochemistry of electrocyclic reactions is codified in the following table, where the total number of electrons (N) involved in the major bonding changes is expressed as a multiple (4n, or not a multiple (4n + 2), of four. 

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. 

Treatment for the Rate of Bimolecular, Gas Phase Reactions , if The symmetry rules allowing some reactions and forbidding others were first proposed by Robert B. Woodward and Roald Hoffmann in two letters to the editor Stereochemistry of Electrocyclic Reactions and Selection Rules for Sigmatropic Reactions , Journal of American Chemical Society, 87 (1965) 395, 2511 as well as by Kenichi Fukui and Hiroshi Fujimoto in an article published... 

Woodward R, Hoffmann R. Stereochemistry of electrocyclic reactions. J Am Chem Soc. 1965 87 395-397. 

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