Cycloaddition: Difference between revisions

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A '''cycloaddition''' is a [[pericyclic]] [[chemical reaction]], in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity."<ref name=goldbook>{{cite web|last=International Union of Pure and Applied Chemistry (IUPAC)|title=Cycloaddition|url=http://goldbook.iupac.org/C01496.html|accessdate=26 February 2014}}</ref> The resulting reaction is a [[cyclization]] reaction. Many but not all cycloadditions are [[Concerted reaction|concerted]]. As a class of [[addition reaction]], cycloadditions permit carbon–carbon bond formation without the use of a [[nucleophile]] or [[electrophile]].

Cycloadditions can be described using two systems of notation. An older, but still common, notation is based on the size of linear arrangements of atoms in the reactants. It uses [[parentheses]]: (''i'' + ''j'' + …) where the variables are the numbers of linear atoms in each reactant. The product is a cycle of size (''i'' + ''j'' + …). In this system, the standard [[Diels-Alder reaction]] a (4 + 2)cycloaddition, the [[1,3-dipolar cycloaddition]] is a (3 + 2)cycloaddition and [[cyclopropanation]] of a carbene with an alkene a (2 + 1)cycloaddition.<ref name=goldbook />

A more recent, IUPAC-preferred notation uses [[square brackets]] to indicate the number of ''electrons'', rather than carbon atoms, involved in the formation of the product. In the [''i'' + ''j'' + …] notation, the standard Diels-Alder reaction is a [4 + 2]cycloaddition, the 1,3-dipolar cycloaddition is [4 + 2].<ref name=goldbook />

==Thermal cycloadditions and their stereochemistry==

Thermal cycloadditions are those cycloadditions where the reactants are in the ground electronic state. They usually have (4''n'' + 2) π electrons participating in the starting material, for some integer n. These reactions 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 4''n'' π electrons (for example the [2 + 2] cycloaddition);. theseThese proceed in a suprafacial-antarafacial sense, such as the dimerisation of [[ketene]], in which the [[orthogonal]] set of [[p orbital]]s allows the reaction to proceed via a crossed [[transition state]].

==Photochemical cycloadditions and their stereochemistry==

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[[Image:Bpe-resorcinol-cycloaddition.png|right|thumb|Cycloaddition of ''trans''-1,2-bis(4-pyridyl)ethene]]

[[Supramolecular chemistry|Supramolecular effects]] can influence these cycloadditions. The cycloaddition of ''trans''-1,2-bis(4-pyridyl)ethene is directed by [[resorcinol]] in the [[solid-state chemistry|solid-state]] in 100% [[chemical yield|yield]].<ref>{{cite journal |author1=L. R. MacGillivray |author2=J. L. Reid |author3=J. A. Ripmeester | title = Supramolecular Control of Reactivity in the Solid State Using Linear Molecular Templates | year = 2000 | journal = [[J. Am. Chem. Soc.]] | volume = 122 | issue = 32 | pages = 7817–7818 | doi=10.1021/ja001239i}}</ref>

Some cycloadditions instead of π bonds operate through strained [[cyclopropane]] rings;, as these have significant π character. For example, an analog for the Diels-Alder reaction is the [[quadricyclane]]-[[DMAD]] reaction:

[[Image:Qcane.png|500px|center]]

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==