Cycloaddition

A Cycloaddition is a pericyclic reaction in which the net result is loss of two pi bonds and gain of two sigma bonds.

Related Topics:
Pericyclic - Pi bond - Sigma bond

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Two major cycloaddition reactions are

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• Diels-Alder reaction
• 1,3-dipolar cycloaddition

Cycloadditions are usually described by the backbone size of the participants. This would make the Diels-Alder a cycloaddition, and the 1,3-dipolar cycloaddition a cycloaddition. This type of reaction is non-polar addition reaction.

Related Topics:
[4 + 2]cycloaddition - [3 + 2]cycloaddition - Addition reaction

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There are related cyclizations such as alkyne trimerization and the Ene reaction.

Related Topics:
Alkyne trimerization - Ene reaction

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Thermal cycloadditions usually have 4n + 2 pi electrons participating in the starting material. These occur, for reasons of orbital symmetry, in a suprafacial-suprafacial or antarafacial-antarfacial manner (rare). There are a few examples of thermal cycloadditions which have 4n pi electrons; (for example the cycloaddition), these proceed in a suprafacial-antarafacial sense, such as the dimerisation of ketene, in which the orthogonal set of p orbitals allows the reaction to proceed via a crossed transition state.

Related Topics:
Orbital symmetry - Suprafacial - Antarafacial - Ketene - Orthogonal - P orbital - Transition state

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Cycloadditions in which 4n pi electrons participate can also occur as a result of photochemical activation. Here, one component has an electron promoted from the HOMO (pi bonding) to the LUMO (pi antibonding). Orbital symmetry is then such that the reaction can proceed in a suprafacial-suprafacial manner.

Related Topics:
Photochemical - HOMO - LUMO

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Cycloadditions often have metal-catalyzed and stepwise radical analogs, however these are not strictly speaking pericyclic reactions.

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Some cycloadditions instead of pi bonds operate through strained cyclopropane rings; as these have significant pi character. For example, an analog for the diels-alder reaction is the quadricyclane-DMAD reaction:

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In the (i+j+...) cycloaddition notation i and j refer to the number of atoms involved in the cycloaddition. In this notation a Diels-Alder reaction is a (4+2)cycloaddition and a 1,3-dipolar addition such as the first step in ozonolysis is a (3+2)cycloaddition. The IUPAC preferred notation however, with takes electrons into account and not atoms. In this notation the DA reaction and the dipolar reaction both become a cycloaddition. The reaction between norbornadiene and an activated alkyne is a cycloaddition.

Related Topics:
Ozonolysis - IUPAC - Norbornadiene - Alkyne

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