Stereochemistry orbital symmetry rules, electrocyclic

We have now considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbitals, orbital correlation diagrams, and transition-state aromaticity. All arrive at the same conclusions about stereochemistry of electrocyclic reactions. Reactions involving 4n + 2 electrons will be disrotatory and involve a Huckel-type transition state, whereas those involving 4n electrons will be conrotatory and the orbital array will be of the Mobius type. These general principles serve to explain and correlate many specific experimental observations made both before and after the orbital symmetry rules were formulated. We will discuss a few representative examples in the following paragraphs. 

One attraction of electrocyclic ring closures for use in synthesis is related to fact that they proceed by means of clearly defined mechanisms. In a single step, one can form a ring and control relative or even absolute stereochemistry at multiple sites in a molecule leading to a rapid increase in complexity. Relative stereochemistry is controlled by orbital symmetry rules (Table 19.1), making the prediction of stereochemistry straightforward. The 7t-electron count always refers to the ring-open compound. 

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. 

Woodward-Hoffman rules The stereochemistry of electrocyclic processes, based on the symmetry of molecular ir orbitals. 

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