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Cycloaddition
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A cycloaddition is a pericyclic chemical reaction, in which two p bonds are lost and two s bonds are gained. The resulting reaction is a cyclization reaction.
Cycloadditions are usually described by the backbone size of the participants. This would make the Diels-Alder reaction a [4 + 2]cycloaddition, and the 1,3-dipolar cycloaddition a [3 + 2]cycloaddition. This type of reaction is non-polar addition reaction.
mal cycloadditions usually have 4n + 2 p electrons participating in the starting material.
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Encyclopedia
A cycloaddition is a pericyclic chemical reaction, in which two p bonds are lost and two s bonds are gained. The resulting reaction is a cyclization reaction.
Cycloadditions are usually described by the backbone size of the participants. This would make the Diels-Alder reaction a [4 + 2]cycloaddition, and the 1,3-dipolar cycloaddition a [3 + 2]cycloaddition. This type of reaction is non-polar addition reaction.
Reaction mechanism
Thermal cycloadditions usually have 4n + 2 p electrons participating in the starting material. These occur, for reasons of orbital symmetry, in a suprafacial-suprafacial or antarafacial-antarafacial manner (rare). There are a few examples of thermal cycloadditions which have 4n p electrons (for example the [2 + 2] 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.
Cycloadditions in which 4n p electrons participate can also occur as a result of photochemical activation. Here, one component has an electron promoted from the HOMO (p bonding) to the LUMO (p* antibonding). Orbital symmetry is then such that the reaction can proceed in a suprafacial-suprafacial manner. An example is the DeMayo reaction. Another example is shown below, the photochemical dimerization of cinnamic acid.
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Note that not all photochemical (2+2) cyclizations are cycloadditions; some are known to operate by radical mechanisms.
Some cycloadditions instead of p bonds operate through strained cyclopropane rings; as these have significant p character. For example, an analog for the Diels-Alder reaction is the quadricyclane-DMAD reaction:
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 [i+j+...] takes electrons into account and not atoms. In this notation the DA reaction and the dipolar reaction both become a [4+2]cycloaddition. The reaction between norbornadiene and an activated alkyne is a [2+2+2]cycloaddition.
Types of cycloaddition
Two major cycloaddition reactions are :
Formal cycloadditions
Cycloadditions often have metal-catalyzed and stepwise radical analogs, however these are not strictly speaking pericyclic reactions. When in a cycloaddition charged or radical intermediates are involved or when the cycloaddition result is obtained in a series of reaction steps they are sometimes called formal cycloadditions to make the distinction with true pericyclic cycloadditions.
One example of a formal [3+3]cycloaddition between a cyclic enone and an enamine catalyzed by n-butyllithium is a Stork enamine / 1,2-addition cascade reaction:
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