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{{short description|Chemical reaction which forms a cyclic molecule}}
[[File:Non-ionic Cycloadditions.png|thumb|261x261px|Non-ionic Cycloadditions]]
In [[organic chemistry]], a '''cycloaddition''' is a [[chemical reaction]] in which "two or more [[Unsaturated hydrocarbon|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 [[Multiplicity (chemistry)#Molecules|bond multiplicity]]". The resulting reaction is a [[cyclization]] reaction. Many but not all cycloadditions are [[Concerted reaction|concerted]] and thus [[pericyclic]].<ref name="goldbook">{{Citation|title=cycloaddition|url=https://goldbook.iupac.org/html/C/C01496.html|work=IUPAC Compendium of Chemical Terminology|year=2009|publisher=IUPAC|doi=10.1351/goldbook.C01496|isbn=978-0-9678550-9-7|access-date=2018-10-13|doi-access=free|url-access=subscription}}</ref> Nonconcerted cycloadditions are not pericyclic.<ref>{{Citation|title=pericyclic reaction|url=https://goldbook.iupac.org/html/P/P04491.html|work=IUPAC Compendium of Chemical Terminology|year=2009|publisher=IUPAC|doi=10.1351/goldbook.P04491|isbn=978-0-9678550-9-7|access-date=2018-10-13|doi-access=free|url-access=subscription}}</ref> 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]]: {{nowrap|(''i'' + ''j'' + …)}} where the variables are the numbers of linear atoms in each reactant. The product is a cycle of size {{nowrap|(''i'' + ''j'' + …)}}. In this system, the standard [[Diels-Alder reaction]] is 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 />
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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 (''syn''/''syn'' stereochemistry) in most cases. Very few examples of [[antarafacial]]-antarafacial (''anti''/''anti'' stereochemistry) reactions have also been reported. There are a few examples of thermal cycloadditions which have 4''n'' π electrons (for example the [2 + 2]-cycloaddition). These proceed in a suprafacial-antarafacial sense (''syn''/''anti'' stereochemistry), such as the cycloaddition reactions of [[ketene]] and [[Allenes|allene]] derivatives, in which the [[orthogonal]] set of [[p orbital]]s allows the reaction to proceed via a crossed [[transition state]], although the analysis of these reactions as [<sub>π</sub>2<sub>s</sub> + <sub>π</sub>2<sub>a</sub>] is controversial. Strained alkenes like ''trans''-cycloheptene derivatives have also been reported to react in an antarafacial manner in [2 + 2]-cycloaddition reactions.
[[William von Eggers Doering|Doering]] (in a personal communication to [[Robert Burns Woodward|Woodward]])
<ref>{{Citation |last1=Izzotti|first1=Anthony|
last2=Gleason|first2=James|
date=2022-06-07|
title=Do Antarafacial Cycloadditions Occur? Cycloaddition of Heptafulvalene with Tetracyanoethylene|
url=https://chemistry-europe.onlinelibrary.wiley.com/doi/epdf/10.1002/chem.202201418|
journal=Chemistry: A European Journal|volume=28 |issue=49 |pages=e202201418 |
doi=10.1002/chem.202201418|pmid=35671245 |url-access=subscription}}
</ref>
Erden and Kaufmann had previously found that the cycloaddition of heptafulvalene and N-phenyltriazolinedione also gave both suprafacial-antarafacial and suprafacial-suprafacial products. <ref>{{Cite journal|last1=Erden|first1=Ihsan|last2=KauFmann|first2=Dieter|date=1981-01-01|title=Cycloadditionsreaktionen des heptafulvalens|url=https://dx.doi.org/10.1016/0040-4039%2881%2980058-5|journal=Tetrahedron Letters|language=de|volume=22|issue=3|pages=215–218|doi=10.1016/0040-4039(81)80058-5|issn=0040-4039|url-access=subscription}}</ref>
[[File:14plus2.png|center|frameless|400x400px]]
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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|bibcode=2000JAChS.122.7817M }}</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:
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===Diels-Alder reactions===
The [[Diels-Alder reaction]] is perhaps the most important and commonly taught cycloaddition reaction. Formally it is a [4+2] cycloaddition reaction and exists in a huge range of forms, including the [[inverse electron-demand Diels–Alder reaction]], [[
:[[File:Diels-Alder (1,3-butadiene + ethylene) red.svg|300px|
Reactions involving heteroatoms are known
===Huisgen cycloadditions===
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===Nitrone-olefin cycloaddition===
The [[nitrone-olefin 3+2 cycloaddition|Nitrone-olefin cycloaddition]] is a (3+2)cycloaddition.
:[[File:NitrGen.
===Cheletropic reactions===
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==Other==
Other cycloaddition reactions exist: [[(4+3) cycloaddition
==Formal cycloadditions==
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===Iron-catalyzed 2+2 olefin cycloaddition===
Iron<nowiki>[</nowiki>[[Diiminopyridine|pyridine(diimine)]]] catalysts contain a redox active ligand in which the central iron atom can coordinate with two simple, unfunctionalized olefin double bonds. The catalyst can be written as a resonance between a structure containing unpaired electrons with the central iron atom in the II oxidation state, and one in which the iron is in the 0 oxidation state. This gives it the flexibility to engage in binding the double bonds as they undergo a cyclization reaction, generating a cyclobutane structure via C-C reductive elimination; alternatively a cyclobutene structure may be produced by beta-hydrogen elimination. Efficiency of the reaction varies substantially depending on the alkenes used, but rational ligand design may permit expansion of the range of reactions that can be catalyzed.<ref>{{cite journal|author1=Jordan M. Hoyt|author2=Valeria A. Schmidt|author3=Aaron M. Tondreau|author4=Paul J. Chirik|author-link4=Paul Chirik|date=2015-08-28|title=Iron-catalyzed intermolecular [2+2] cycloadditions of unactivated alkenes|journal=Science|volume=349|issue=6251|pages=960–963|bibcode=2015Sci...349..960H|doi=10.1126/science.aac7440|pmid=26315433|s2cid=206640239 }}</ref><ref>{{cite journal|author1=Myles W. Smith|author2=Phil S. Baran|author-link2=Phil S. Baran|date=2015-08-28|title=As simple as [2+2]|journal=Science|volume=349|issue=6251|pages=925–926|bibcode=2015Sci...349..925S|doi=10.1126/science.aac9883|pmid=26315420 |s2cid=42226757 }}</ref>
==References==
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