Non-aromatic Claisen-type rearrangements

In the non-aromatic Claisen-type rearrangements, a sigma bond breaks, a sigma bond is formed, and two t-bonds migrate concertedly. The basic reaction is illustrated below 

These reactions are 6-center and, unless sterically inhibited, proceed through cyclohexane-type chair transition states. Incorporation of oxygen at one or more of the possible positions (excluding peroxides) develops all of the possible reaction subclasses of interest to this review, viz. 

Gas phase kinetic data have been reported for only two of these reaction subclasses, allyl esters (2), and vinyl allyl ethers (4). Subclasses (5), (6) and (1) have very fast reverse processes (see activation energy predictions), but could be observed in terms of loss of optical activity at the 4-position. It would be interesting to look for these reactions. 

It is clear that in the activation process of the Claisen rearrangements, three internal rotations of the ground states will be transformed to torsions or out of plane vibrations in the transition state. These are the internal rotations around the 2,3 3,4 and 4,5 bonds. The method of O Neal and Benson appears to give a reason- 

ESTIMATED ACTIVATION ENERGIES FOR COPE AND CLAISEN-TYPE REACTIONS 

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Three main reaction types are considered below, bond fissions, reverse ene reactions, and non-aromatic Claisen-type rearrangements. 

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