Coarctate and Pseudocoarctate Reactions: Stereochemical Rules.

Coarctate reactions are a distinct class of concerted reactions. They differ from other concerted reactions, such as S(N)2, E2, or pericyclic reactions, in such a way that atoms are involved in the transition state at which two bonds are made and two bonds are broken simultaneously. Similar to pericyclic reactions, an electron count can be used to predict reactivity and stereochemistry. The "coarctate rules" are derived from a topological variant of the aromaticity of the transition state principle. Pseudocoarctate transition states exhibit disconnections in the topology of orbital overlap and do not necessarily follow the coarctate rules.