Nonconjugated Diradicals

Abstract

Few reactions are as conceptually simple as the stretching of a carbon-carbon bond in a carbocyclic ring to afford a bond-broken species, a diradical (1-11). The list of thermal and photochemical reactions that appear to involve diradical structures as intermediates is long, much longer than the handful documented to proceed by concerted processes (12). To use the diradical hypothesis as a predictive tool, it is necessary to define a set of properties for diradicals. However, because most nonconjugated diradicals are short-lived, few direct data on their behavior exist. To date most studies on the properties of diradicals have been confined to indirect methods. In this chapter, rather than present a compendium of all thermal and photochemical reactions thought to involve nonconjugated diradical pathways, we review the literature selectively to find general features of diradical behavior. Specific questions we address are: (a) What are the relative rates of the reaction of diradicals? (b) Is there evidence for diradicals as common intermediates? (c) What is the influence on diradical behavior of variables such as method of generation, substitution, temperature, and spin state? (d) What are the magnitude and consequences of interactions between the radical centers? (e) What are the lifetimes of diradicals? The most-studied nonconjugated diradicals are the 1,3- and 1,4-diradicals. It may seem logical to start with the smaller 1,3-diradical where the radical centers are "insulated" by only one methylene unit. However, the trimelthylene story is complex, perhaps because the radical centers may interact significantly. In contrast, tetramethlyene seems to be more well behaved with regard to our chemical intuition of a "classical" diradical. For this reason, we review the 1,4-diradical first as a model of conventional diradical behavior and then proceed to the more complicated 1,3-system.

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