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In the classification of
Selection rules, resembling the Woodward-Hoffmann rules, have been proposed to explain patterns in reaction
In the classification of organic reaction
Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical rea ...
s by transition state
In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked ...
topology
In mathematics, topology (from the Greek words , and ) is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing ho ...
, a coarctate reaction (from L. ''coarctare'' "to constrict") is a third, comparatively uncommon topology
In mathematics, topology (from the Greek words , and ) is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing ho ...
, after linear topology and pericyclic
In organic chemistry, a pericyclic reaction is the type of organic reaction wherein the transition state of the molecule has a cyclic geometry, the reaction progresses in a concerted fashion, and the bond orbitals involved in the reaction overlap ...
topology (itself subdivided into Hückel and Möbius topologies).
Transition state topologies
Reactions of linear topology are the most common, and consist of all transformations whose transition states are acyclic, including addition, elimination, substitution, and (some types of) fragmentation reactions. In contrast, in pericyclic reactions, the atoms under chemical change form a closed cycle, and include reactions like the Diels-Alder reaction andCope rearrangement
The Cope rearrangement is an extensively studied organic reaction involving the ,3sigmatropic rearrangement of 1,5-dienes. It was developed by Arthur C. Cope and Elizabeth Hardy. For example, 3-methyl-hexa-1,5-diene heated to 300 °C yield ...
, among many others.
In contrast to these types of reactions, a coarctate reaction is characterized by a doubly cyclic transition state, in which at least one atom undergoes the simultaneous making and breaking of two bonds. Thus, the topology of the transition state of a coarctate reaction is a constricted cycle that meets with itself (resembling a figure eight) while the topology of pericyclic and linear reactions are a circle (or Möbius strip) and line segment, respectively. The concept was first proposed by Herges.
Examples
The most well-known example of a coarctate transition state is that of the epoxidation of an olefin bydimethyldioxirane
Dimethyldioxirane (DMDO), also referred to as Murray's reagent in reference to Robert W. Murray, is a dioxirane derived from acetone and can be considered as a monomer of acetone peroxide. It is a powerful yet selective oxidizing agent which fin ...
. In this transition state, the oxygen atom transferred to the olefin forms a cycle with the acetone leaving group and a cycle with the olefin undergoing epoxidation. Another well-studied reaction is the fragmentation of spirocyclic ozonides into formaldehyde, CO2, and an olefin.
Selection rules, resembling the Woodward-Hoffmann rules, have been proposed to explain patterns in reaction activation energy
In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...
related to transition state topology or orbital symmetry.
References
{{Reflist Organic reactions