Editing Allenes (section)

== Use and occurrence ==

Allene itself is the most commonly used member of this family; it exists in equilibrium with [[propyne]] as a component of MAPP gas.<ref>{{cite book |doi=10.1002/14356007.m22_m01 |chapter=Propyne |title=Ullmann's Encyclopedia of Industrial Chemistry |year=2008 |last1=Buckl |first1=Klaus |last2=Meiswinkel |first2=Andreas |isbn=978-3527306732 }}</ref>

===Research===
The reactivity of substituted allenes has been well explored.<ref name=":72">{{Cite journal|last=Pasto|first=Daniel J.|date=January 1984|title=Recent developments in allene chemistry|url=https://linkinghub.elsevier.com/retrieve/pii/S004040200191289X|journal=Tetrahedron|language=en|volume=40|issue=15|pages=2805–2827|doi=10.1016/S0040-4020(01)91289-X|url-access=subscription}}</ref><ref name=":82">{{Cite journal|last=Ma|first=Shengming|date=2009-10-20|title=Electrophilic Addition and Cyclization Reactions of Allenes|url=https://pubs.acs.org/doi/10.1021/ar900153r|journal=Accounts of Chemical Research|language=en|volume=42|issue=10|pages=1679–1688|doi=10.1021/ar900153r|pmid=19603781|issn=0001-4842|url-access=subscription}}</ref><ref>{{Cite journal|last1=Yu|first1=Shichao|last2=Ma|first2=Shengming|date=2012-03-26|title=Allenes in Catalytic Asymmetric Synthesis and Natural Product Syntheses|url=https://onlinelibrary.wiley.com/doi/10.1002/anie.201101460|journal=Angewandte Chemie International Edition|language=en|volume=51|issue=13|pages=3074–3112|doi=10.1002/anie.201101460|pmid=22271630|url-access=subscription}}</ref><ref name=":9">{{Cite journal|last=Ma|first=Shengming|date=2005-07-01|title=Some Typical Advances in the Synthetic Applications of Allenes|url=https://pubs.acs.org/doi/10.1021/cr020024j|journal=Chemical Reviews|language=en|volume=105|issue=7|pages=2829–2872|doi=10.1021/cr020024j|pmid=16011326|issn=0009-2665|url-access=subscription}}</ref> 

The two [[Pi bond|π-bonds]] are located at the 90° angle to each other, and thus require a reagent to approach from somewhat different directions. With an appropriate substitution pattern, allenes exhibit axial chirality as predicted by Van 't Hoff as early as 1875.<ref>Van ’t Hoff, J. H. ''La Chimie dans l’Espace''; P.M. Bazendijk, 1875; p. 43.</ref><ref name=":9" /> Protonation of allenes gives cations '''11''' that undergo further transformations.<ref>{{Cite book |last=Fleming |first=Ian |url=https://www.worldcat.org/oclc/607520014 |title=Molecular orbitals and organic chemical reactions |date=2010 |publisher=Wiley |isbn=978-0-470-68949-3 |edition=Reference |location=Hoboken, N.J. |pages=526 |oclc=607520014}}</ref> Reactions with soft electrophiles (e.g. Br<sup>+</sup>) deliver positively charged [[Onium ion|onium ions]] '''13'''.<ref name=":102">{{Cite book|last=Brandsma|first=L.|url=https://www.worldcat.org/oclc/162570992|title=Synthesis of acetylenes, allenes and cumulenes : methods and techniques|date=2004|publisher=Elsevier|others=H. D. Verkruijsse|isbn=978-0-12-125751-4|edition=1st|location=Amsterdam|oclc=162570992}}</ref> Transition-metal-catalysed reactions proceed via allylic intermediates '''15''' and have attracted significant interest in recent years.<ref>{{Cite journal|last=Ma|first=Shengming|date=2006-01-01|title=Transition-metal-catalyzed reactions of allenes|journal=Pure and Applied Chemistry|volume=78|issue=2|pages=197–208|doi=10.1351/pac200678020197|s2cid=97529095|issn=1365-3075|doi-access=free}}</ref><ref>{{Cite journal|last1=Bates|first1=Roderick W.|last2=Satcharoen|first2=Vachiraporn|date=2002-03-06|title=Nucleophilic transition metal based cyclization of allenes|url=http://xlink.rsc.org/?DOI=b103904k|journal=Chemical Society Reviews|volume=31|issue=1|pages=12–21|doi=10.1039/b103904k|pmid=12108979|url-access=subscription}}</ref> Numerous cycloadditions are also known, including [4+2]-, (2+1)-, and [2+2]-variants, which deliver, e.g., '''12''', '''14''', and '''16''', respectively.<ref name=":72" /><ref>{{Cite journal|last1=Cherney|first1=Emily C.|last2=Green|first2=Jason C.|last3=Baran|first3=Phil S.|date=2013-08-19|title=Synthesis of ent -Kaurane and Beyerane Diterpenoids by Controlled Fragmentations of Overbred Intermediates|journal=Angewandte Chemie International Edition|language=en|volume=52|issue=34|pages=9019–9022|doi=10.1002/anie.201304609|pmc=3814173|pmid=23861294}}</ref><ref>{{Cite journal|last=Wiesner|first=K.|date=August 1975|title=On the stereochemistry of photoaddition between α,β-unsaturated ketones and olefins|url=https://linkinghub.elsevier.com/retrieve/pii/0040402075850824|journal=Tetrahedron|language=en|volume=31|issue=15|pages=1655–1658|doi=10.1016/0040-4020(75)85082-4|url-access=subscription}}</ref><ref>{{Cite journal|last1=Rahman|first1=W.|last2=Kuivila|first2=Henry G.|date=March 1966|title=Synthesis of Some Alkylidenecyclopropanes from Allenes 1|url=https://pubs.acs.org/doi/abs/10.1021/jo01341a029|journal=The Journal of Organic Chemistry|language=en|volume=31|issue=3|pages=772–776|doi=10.1021/jo01341a029|issn=0022-3263|url-access=subscription}}</ref>

[[File:Overview_allene_reactivity_Zhurakovskyi.svg]]

===Occurrence===
[[File:Fucoxanthin.svg|thumb|422 px|[[Fucoxanthin]], the most abundant of all carotinoids, is the light-absorbing [[pigment]] in the [[chloroplast]]s of [[brown algae]], giving them a brown or olive-green color.]]
Numerous natural products contain the allene functional group. Noteworthy are the pigments [[fucoxanthin]] and [[peridinin]]. Little is known about the biosynthesis, although it is conjectured that they are often generated from alkyne precursors.<ref>{{cite book |title= Modern Allene Chemistry |chapter= 18. Allenic Natural Products and Pharmaceuticals |first1= Norbert |last1= Krause |first2= Anja |last2= Hoffmann-Röder |editor1-first= Norbert |editor1-last= Krause |editor2-first= A. Stephen K. |editor2-last= Hashmi |year= 2004|pages= 997–1040 |doi= 10.1002/9783527619573.ch18 |isbn= 9783527619573 }}</ref>

Allenes serve as ligands in [[organometallic chemistry]]. A typical complex is Pt([[hapticity|η<sup>2</sup>]]-allene)(PPh<sub>3</sub>)<sub>2</sub>. Ni(0) reagents catalyze the cyclooligomerization of allene.<ref>Otsuka, Sei; Nakamura, Akira "Acetylene and allene complexes: their implication in homogeneous catalysis" Advances in Organometallic Chemistry 1976, volume 14, pp. 245-83. {{doi|10.1016/S0065-3055(08)60654-1}}.</ref> Using a suitable catalyst (e.g. [[Wilkinson's catalyst]]), it is possible to reduce just one of the double bonds of an allene.<ref>{{cite journal |last1 = Bhagwat|first1=M. M.|last2= Devaprabhakara|first2=D. |journal = [[Tetrahedron Letters]] |pages = 1391–1392 |year = 1972 |doi = 10.1016/S0040-4039(01)84636-0 |title = Selective hydrogenation of allenes with chlorotris-(triphenylphosphine) rhodium catalyst |volume = 13 |issue = 15}}</ref>