Critical analysis of published work shows that there is no evidence that any Diels-Alder reaction takes place in a synchronous manner.On the contrary, reactions of unsymmetrical dienes or dienophiles have been shown to take place via very unsymmetrical transition states. There is no evidence that the same is not true for symmetrical reactants. Indeed, all the available evidence can be interpreted in terms of a very unsymmetrical transition state, close to a biradical or zwitterion. This conclusion has now been confirmed by detailed kinetic studies of the (reverse) Diels-Alder reactions of maleic anhydride with furan, 2-methylfuran, and 2,5-dimethylfuran.

The potential energy surfaces for the fragmentation of the radical anions of p-nitrochlorobenzene and p- and m-chloroacetophenones were explored using first principle methods. The behavior of these compounds, stabilized by pi acceptors, is compared to that shown by the unsubstituted halobenzenes (PhX, X = F, Cl, Br, I). The presence of pi and sigma radical anions was inspected as well as the intramolecular electron transfer (intra-ET) from the pi to the sigma surface, responsible for the dissociation of these intermediates. The profiles obtained with the B3LYP functional in the gas phase and in the presence of a polar solvent are in agreement with the spectroscopic evidence and with the experimentally observed reactivity of the compounds under study. The stability of the radical anion of p-nitrochlorobenzene and the adiabatic and endothermic nature of its dissociation are explained. The order of the rate constants for dissociation m-chloroacetophenone < p-chloroacetophenone is interpreted on the basis of the differences in the adiabatic character of the intra-ET of both isomers which is ascribed to the nodal properties of their SOMOs. In the halobenzene family, the electronic factors responsible for the intra-ET are analyzed. The stabilization of the sigma surface exerted by the different halogens and its effect on the rate constants for dissociation are explained.

The S RN 1 (Unimolecular Radical Nucleophilic Substitution) is a process through which nucleophilic substitution is achieved on aromatic and aliphatic compounds that do not react or react slowly through polar nucleophilic mechanisms. This transformation takes place through electron transfer steps with radicals and radical anions as intermediates. The S RN 1 mechanism was proposed for the first time in 1966 for the substitution of alkyl halides with electron‐withdrawing groups and in 1970 for the substitution of unactivated aryl halides. Since then, the scope of the process has increased considerably, and it constitutes an important synthetic method for achieving the substitution of unactivated aromatic and heteroaromatic substrates, vinyl halides, perfluoroalkyl iodides, and activated and non‐activated alkyl compounds. Nucleophiles that can be used include anions derived from carbon or heteroatoms, which react to form a new carbon‐carbon or carbon‐heteroatom bond. Several reviews have been published in relation to activated and non‐activated aliphatic S RN 1 reactions, to aromatic photoinitiated reactions, to S RN 1 reactions at aromatic centers, to reactions performed with electrochemical techniques, and to the synthetic applications of the process. This chapter attempts to cover aromatic S RN 1 reactions including all the relevant literature up to the end of 1996.

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