The Woodward-Hoffmann rules and molecular orbitals

The characteristic feature of all pericyclic reactions is the concertedness of all the bond making and bond breaking, and hence the absence of any intermediates. Naturally, organic chemists have worked hard, and devised many ingenious experiments, to prove that this is true, concentrating especially on the Diels-Alder reaction. The following is an oversimplified description of some of the most telling experiments. 

The rates of Diels-Alder reactions are little affected by the polarity of the solvent. If a zwitterionic intermediate were involved, the intermediate would be more polar than either of the starting materials, and polar solvents would solvate it more thoroughly. Typically, a large change of solvent dipole moment, from 2.3 to 39, causes an increase in rate by a factor of only 10. In contrast, stepwise ionic cycloadditions take place with increases in rate of several orders of magnitude in polar solvents. This single piece of evidence rules out stepwise ionic pathways for most Diels-Alder reactions, and the only stepwise mechanism left is that involving a diradical. 

Deuterium substitution on the four carbon atoms changing from trigonal to tetrahedral as the reaction proceeds, gives rise to inverse secondary kinetic isotope effects, small but measurable, both for the diene and the dienophile 3.1. If both bonds are forming at the same time, the isotope effect when both ends are deuterated is geometrically related to the isotope effects at each end. If the bonds are being formed one at a time, the isotope effects are arithmetically related. It is a close call, but the experimental results, both for cycloadditions and for cycloreversions, suggest that they are concerted. 

Another way of testing how one end of the dienophile affects the other end is to load up the dienophile with up to four electron-withdrawing groups and 

We now turn to the ideas, based on MO theory, which have been advanced to explain the patterns of reactivity that all pericyclic reactions show. 

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