Some Comments on Forbidden and Allowed Reactions

When Woodward and Hoffmann developed the conservation of orbital symmetry, they introduced the terms allowed and forbidden to describe reactions such as the [4+2] and [2+2] cycloadditions, respectively. This terminology caught on, and has become fairly standard in the field. With the benefit of a historical perspective, though, we can now see that these terms are too definitive. 

Similarly, a reaction that is allowed is simply one that does not have such an electronic barrier. This does not automatically mean, however, that the reaction will be favorable. Steric interactions or other factors could make the reaction quite slow. All we know is that no additional barrier due to electronic factors contributes to the overall activation energy. 

The two cycloaddition reactions we have discussed so far illustrate some of these points. The [2+2] cycloaddition is forbidden. However, olefins can dimerize to make cyclobutanes. It is just that the reaction is not concerted, but rather involves a biradical intermediate. The [4+2] cycloaddition is allowed, but in fact the concerted cycloaddition of ethylene and butadiene requires high temperature and pressure. It does occur by a concerted allowed path, but the activation barrier is high. 

With this perspective the terms forbidden and allowed seem overly sweeping. Something like disfavored and not disfavored would be less dramatic (and less grammatical) but closer to the reality of the science. However, forbidden and allowed are firmly entrenched in the pericyclic literature, and we will use them here. We hope this discussion, however, will provide you with the proper perspective and will discourage any tendency to interpret the terminology too literally. 

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