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==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
[[William von Eggers Doering|Doering]] (in a personal communication to [[Robert Burns Woodward|Woodward]]) discovered that [[Fulvalene (compound class)|heptafulvalene]] and tetracyanoethylene can react in a suprafacial-antarafacial [14 + 2]-cycloaddition. This result was later confirmed and extended by Erden and Kaufmann, who reported the suprafacial-antarafacial cycloaddition of heptafulvalene with ''N''-phenyltriazolinedione.<ref>{{Cite journal|last=Erden|first=Ihsan|last2=KauFmann|first2=Dieter|date=1981-01-01|title=Cycloadditionsreaktionen des heptafulvalens|url=https://www.sciencedirect.com/science/article/pii/0040403981800585|journal=Tetrahedron Letters|language=de|volume=22|issue=3|pages=215–218|doi=10.1016/0040-4039(81)80058-5|issn=0040-4039}}</ref>
[[File:14plus2.png|center|frameless|400x400px]]
==Photochemical cycloadditions and their stereochemistry==
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The [[nitrone-olefin 3+2 cycloaddition|Nitrone-olefin cycloaddition]] is a (3+2)cycloaddition.
:[[File:NitrGen.png|Nitrone olefin cycloaddition]]
===Iron-catalyzed 2+2 olefin cycloaddition===▼
Iron[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|title=Iron-catalyzed intermolecular [2+2] cycloadditions of unactivated alkenes|author1=Jordan M. Hoyt |author2=Valeria A. Schmidt |author3=Aaron M. Tondreau |author4=Paul J. Chirik |journal=Science|date=2015-08-28|volume=349|issue=6251|pages=960–963|doi=10.1126/science.aac7440 |pmid=26315433|bibcode=2015Sci...349..960H}}</ref><ref>{{cite journal|title=As simple as [2+2]|author1=Myles W. Smith |author2=Phil S. Baran |journal=Science|date=2015-08-28|volume=349|issue=6251|pages=925–926|doi=10.1126/science.aac9883|bibcode=2015Sci...349..925S}}</ref>▼
===Cheletropic reactions===
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[[Image:3+3-cycloaddition.svg|center|600px|Intermolecular Formal [3+3] Cycloaddition Reaction]]
▲===Iron-catalyzed 2+2 olefin cycloaddition===
▲Iron[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
==References==
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