Cycloaddition: Difference between revisions

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Revision as of 08:52, 13 October 2018

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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." The resulting reaction is a [[cyclization]] reaction. Many but not all cycloadditions are [[Concerted reaction|concerted]].<ref name="goldbook">{{cite web|url=http://goldbook.iupac.org/C01496.html|title=Cycloaddition|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]] 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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[[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==

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===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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==Other==

Other cycloaddition reactions exist: [[4+3 cycloaddition|[4+3] cycloadditions]], [[6+4 cycloaddition|[6+4] cycloadditions]], [[Woodward-Hoffmann rules|[2 + 2] photocycloadditions]], and [[4+4 photocycloaddition| [4+4] photocycloadditions]]

==Formal cycloadditions==