Editing Allenes

{{Short description|1=Any organic compound containing a C=C=C group}}
{{for|the given name|Allene (given name)}}
{{For|the organic compound with the common name allene|Propadiene}}
[[File:Propadiene structure.svg|right|thumb|[[Propadiene]], the simplest allene, is also known as allene]]

In [[organic chemistry]], '''allenes''' are [[organic compounds]] in which one [[carbon]] atom has [[double bond]]s with each of its two adjacent carbon atoms ({{chem2|R2C\dC\dCR2}}, where R is [[hydrogen|H]] or some [[organyl group]]).<ref>{{GoldBookRef|title=allenes|file=A00238}}</ref> Allenes are classified as [[diene#Classes|cumulated dienes]]. The parent compound of this class is [[propadiene]] ({{chem2|H2C\dC\dCH2}}), which is itself also called ''allene''. A group of the structure {{chem2|R2C\dC\dCR\s}} is called '''allenyl''', while a substituent attached to an allene is referred to as an '''allenic''' substituent (R is H or some alkyl group). In analogy to [[Allyl group|allylic]] and [[Propargyl group|propargylic]], a substituent attached to a saturated carbon α (i.e., directly adjacent) to an allene is referred to as an '''allenylic''' substituent.  While allenes have two consecutive ('cumulated') double bonds, compounds with three or more cumulated double bonds are called [[cumulene]]s.

== History ==
For many years, allenes were viewed as curiosities but thought to be synthetically useless and difficult to prepare and to work with.<ref name=":0">The Chemistry of the Allenes (vol. 1−3); Landor, S. R., Ed.; cademic Press: London, 1982.</ref><ref name=":2">{{Cite journal|last=Taylor|first=David R.|date=1967-06-01|title=The Chemistry of Allenes|url=https://pubs.acs.org/doi/abs/10.1021/cr60247a004|journal=Chemical Reviews|language=en|volume=67|issue=3|pages=317–359|doi=10.1021/cr60247a004|issn=0009-2665|url-access=subscription}}</ref> Reportedly,<ref name=":1">{{Cite journal|last1=Hoffmann-Röder|first1=Anja|last2=Krause|first2=Norbert|date=2004-02-27|title=Synthesis and Properties of Allenic Natural Products and Pharmaceuticals|url=https://onlinelibrary.wiley.com/doi/10.1002/anie.200300628|journal=Angewandte Chemie International Edition|language=en|volume=43|issue=10|pages=1196–1216|doi=10.1002/anie.200300628|pmid=14991780|issn=1433-7851|url-access=subscription}}</ref> the first synthesis of an allene, [[glutinic acid]], was performed in an attempt to prove the non-existence of this class of compounds.<ref>{{Cite journal|last1=Burton|first1=B. S.|last2=von Pechmann|first2=H.|date=January 1887|title=Ueber die Einwirkung von Chlorphosphor auf Acetondicarbonsäureäther|url=https://onlinelibrary.wiley.com/doi/10.1002/cber.18870200136|journal=Berichte der Deutschen Chemischen Gesellschaft|language=de|volume=20|issue=1|pages=145–149|doi=10.1002/cber.18870200136}}</ref><ref>{{Cite journal|last1=Jones|first1=E. R. H.|last2=Mansfield|first2=G. H.|last3=Whiting|first3=M. C.|date=1954|title=Researches on acetylenic compounds. Part XLVII. The prototropic rearrangements of some acetylenic dicarboxylic acids|url=http://xlink.rsc.org/?DOI=jr9540003208|journal=Journal of the Chemical Society (Resumed)|language=en|pages=3208–3212|doi=10.1039/jr9540003208|issn=0368-1769|url-access=subscription}}</ref> The situation began to change in the 1950s, and more than 300 papers on allenes have been published in 2012 alone.<ref>Data from the [[Web of Science]] database.</ref> These compounds are not just interesting intermediates but synthetically valuable targets themselves; for example, over 150 natural products are known with an allene or cumulene fragment.<ref name=":1" />

==Structure and properties==
===Geometry===
[[File:Allene3D.png|thumb|right|3D view of propadiene (allene)]]
The central carbon atom of allenes forms two [[sigma bond]]s and two [[pi bond]]s. The central carbon atom is [[orbital hybridization#sp hybrids|sp-hybridized]], and the two terminal carbon atoms are [[orbital hybridization#sp2 hybrids|sp<sup>2</sup>-hybridized]]. The bond angle formed by the three carbon atoms is 180°, indicating linear geometry for the central carbon atom. The two terminal carbon atoms are planar, and these planes are twisted 90° from each other. The structure can also be viewed as an "extended tetrahedral" with a similar shape to [[methane]], an analogy that is continued into the stereochemical analysis of certain derivative molecules.

===Symmetry===
[[File:Allene symmetry.png|none|300px]]
The symmetry and isomerism of allenes has long fascinated organic chemists.<ref>{{March6th}}</ref> For allenes with four identical substituents, there exist two twofold axes of rotation through the central carbon atom, inclined at 45° to the CH<sub>2</sub> planes at either end of the molecule. The molecule can thus be thought of as a two-bladed [[propeller]]. A third twofold axis of rotation passes through the C=C=C bonds, and there is a mirror plane passing through both CH<sub>2</sub> planes. Thus this class of molecules belong to the D<sub>2d</sub> [[point group]]. Because of the symmetry, an unsubstituted allene has no net [[bond dipole moment|dipole moment]], that is, it is a non-polar molecule.
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{{Multiple image
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| image1    = Allene chirality.png
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| footer    = ''R'' and ''S'' configurations are determined by precedences of the groups attached to the axial section of the molecule when viewed along that axis. The front plane is given higher priority over the other and the final assignment is given from priority 2 to 3 (i.e. the relationship between the two planes).
}}
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An allene with two different substituents on each of the two carbon atoms will be [[chirality (chemistry)|chiral]] because there will no longer be any mirror planes. The chirality of these types of allenes was first predicted in 1875 by [[Jacobus Henricus van 't Hoff]], but not proven experimentally until 1935.<ref>{{cite journal |title= Experimental Demonstration of the Allene Asymmetry |first1= Peter |last1= Maitland |first2= W. H. |last2= Mills |journal= Nature |volume= 135 |pages= 994 |year= 1935 |issue= 3424 |doi= 10.1038/135994a0 |bibcode= 1935Natur.135Q.994M |s2cid= 4085600 |doi-access= free }}</ref> Where '''A''' has a greater priority than '''B''' according to the [[Cahn–Ingold–Prelog priority rules]], the configuration of the [[axial chirality]] can be determined by considering the substituents on the front atom followed by the back atom when viewed along the allene axis. For the back atom, only the group of higher priority need be considered.

Chiral allenes have been recently used as building blocks in the construction of organic materials with exceptional chiroptical properties.<ref>{{cite journal|title=Allenes in Molecular Materials|last1=Rivera Fuentes|first1=Pablo|last2=Diederich|first2=François|journal=Angew. Chem. Int. Ed. Engl.|date=2012|volume=51|issue=12|pages=2818–2828|doi=10.1002/anie.201108001|pmid=22308109|doi-access=free}}</ref> There are a few examples of drug molecule having an allene system in their structure.<ref>{{Cite journal|last1=Celmer|first1=Walter D.|last2=Solomons|first2=I. A.|title=The Structure of the Antibiotic Mycomycin|date=1952|url=http://dx.doi.org/10.1021/ja01127a529|journal=Journal of the American Chemical Society|volume=74|issue=7|pages=1870–1871|doi=10.1021/ja01127a529|bibcode=1952JAChS..74.1870C |issn=0002-7863|url-access=subscription}}</ref>  Mycomycin, an antibiotic with tuberculostatic properties,<ref>{{Cite journal|last=Jenkins|first=D.E.|date=1950|title=Mycomycin: a new antibiotic with tuberculostatic properties.|journal=J Lab Clin Med|volume=36|issue=5|pages=841–2|pmid=14784717}}</ref> is a typical example. This drug exhibits enantiomerism due to the presence of a suitably substituted allene system.

Although the semi-localized textbook [[Sigma-pi separation|σ-π separation]] model describes the bonding of allene using a pair of localized orthogonal π orbitals, the full molecular orbital description of the bonding is more subtle.  The symmetry-correct doubly-degenerate HOMOs of allene (adapted to the D<sub>2d</sub> point group) can either be represented by a pair of orthogonal MOs ''or'' as twisted helical linear combinations of these orthogonal MOs.  The symmetry of the system and the degeneracy of these orbitals imply that both descriptions are correct (in the same way that there are infinitely many ways to depict the doubly-degenerate HOMOs and LUMOs of benzene that correspond to different choices of eigenfunctions in a two-dimensional eigenspace).  However, this degeneracy is lifted in substituted allenes, and the helical picture becomes the only symmetry-correct description for the HOMO and HOMO–1 of the C<sub>2</sub>-symmetric {{ill|2,3-Pentadiene|lt=1,3-dimethylallene|de|2,3-Pentadien}}.<ref>{{Cite journal|last1=H. Hendon|first1=Christopher|last2=Tiana|first2=Davide|last3=T. Murray|first3=Alexander|last4=R. Carbery|first4=David|last5=Walsh|first5=Aron|date=2013|title=Helical frontier orbitals of conjugated linear molecules|journal=Chemical Science|language=en|volume=4|issue=11|pages=4278–4284|doi=10.1039/C3SC52061G|doi-access=free|hdl=10044/1/41564|hdl-access=free}}</ref><ref>{{Cite journal|last1=Garner|first1=Marc H.|last2=Hoffmann|first2=Roald|last3=Rettrup|first3=Sten|last4=Solomon|first4=Gemma C.|author-link4=Gemma Solomon|date=2018-06-27|title=Coarctate and Möbius: The Helical Orbitals of Allene and Other Cumulenes|url=|journal=ACS Central Science|volume=4|issue=6|pages=688–700|doi=10.1021/acscentsci.8b00086|issn=2374-7943|pmc=6026781|pmid=29974064}}</ref> This qualitative MO description extends to higher odd-carbon cumulenes (e.g., 1,2,3,4-pentatetraene).
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=== Chemical and spectral properties ===
Allenes differ considerably from other alkenes in terms of their chemical properties.  Compared to isolated and conjugated dienes, they are considerably less stable: comparing the isomeric pentadienes, the allenic 1,2-pentadiene has a heat of formation of 33.6 kcal/mol, compared to 18.1 kcal/mol for (''E'')-1,3-pentadiene and 25.4 kcal/mol for the isolated 1,4-pentadiene.<ref>{{Cite journal|last=Informatics|first=NIST Office of Data and|title=Welcome to the NIST WebBook|url=https://webbook.nist.gov/index.html.en-us.en|access-date=2020-10-17|website=webbook.nist.gov|year=1997|doi=10.18434/T4D303|language=en}}</ref>

The C–H bonds of allenes are considerably weaker and more acidic compared to typical vinylic C–H bonds: the bond dissociation energy is 87.7 kcal/mol (compared to 111 kcal/mol in ethylene), while the [[Proton affinity|gas-phase acidity]] is 381 kcal/mol (compared to 409 kcal/mol for ethylene<ref>{{Cite book|last=Alabugin|first=Igor V.|url=http://doi.wiley.com/10.1002/9781118906378|title=Stereoelectronic Effects: A Bridge Between Structure and Reactivity|date=2016-09-19|publisher=John Wiley & Sons, Ltd|isbn=978-1-118-90637-8|location=Chichester, UK|language=en|doi=10.1002/9781118906378}}</ref>), making it slightly more acidic than the propargylic C–H bond of propyne (382 kcal/mol).  

The <sup>13</sup>C NMR spectrum of allenes is characterized by the signal of the sp-hybridized carbon atom, resonating at a characteristic 200-220 ppm. In contrast, the sp<sup>2</sup>-hybridized carbon atoms resonate around 80 ppm in a region typical for alkyne and nitrile carbon atoms, while the protons of a CH<sub>2</sub> group of a terminal allene resonate at around 4.5 ppm — somewhat upfield of a typical vinylic proton.<ref>{{Cite book |last1=Pretsch |first1=Ernö |last2=Bühlmann |first2=Philippe |last3=Badertscher |first3=M. |url=https://www.worldcat.org/oclc/405547697 |title=Structure determination of organic compounds : tables of spectral data |date=2009 |publisher=Springer |isbn=978-3-540-93810-1 |edition=Fourth, Revised and Enlarged |location=Berlin |oclc=405547697}}</ref> 

Allenes possess a rich cycloaddition chemistry, including both [4+2] and [2+2] modes of addition,<ref>{{Cite journal|last1=Alcaide|first1=Benito|last2=Almendros|first2=Pedro|last3=Aragoncillo|first3=Cristina|date=2010-01-28|title=Exploiting [2+2] cycloaddition chemistry: achievements with allenes|url=https://pubs.rsc.org/en/content/articlelanding/2010/cs/b913749a|journal=Chemical Society Reviews|language=en|volume=39|issue=2|pages=783–816|doi=10.1039/B913749A|pmid=20111793|issn=1460-4744|hdl=10261/29537|hdl-access=free}}</ref><ref>{{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> as well as undergoing formal cycloaddition processes catalyzed by transition metals.<ref>{{Cite journal|last1=Alcaide|first1=Benito|last2=Almendros|first2=Pedro|date=August 2004|title=The Allenic Pauson−Khand Reaction in Synthesis|url=http://doi.wiley.com/10.1002/ejoc.200400023|journal=European Journal of Organic Chemistry|language=en|volume=2004|issue=16|pages=3377–3383|doi=10.1002/ejoc.200400023|issn=1434-193X|url-access=subscription}}</ref><ref>{{Cite journal|last1=Mascareñas|first1=José L.|last2=Varela|first2=Iván|last3=López|first3=Fernando|date=2019-02-19|title=Allenes and Derivatives in Gold(I)- and Platinum(II)-Catalyzed Formal Cycloadditions|url= |journal=Accounts of Chemical Research|language=en|volume=52|issue=2|pages=465–479|doi=10.1021/acs.accounts.8b00567|issn=0001-4842|pmc=6497370|pmid=30640446}}</ref>  Allenes also serve as substrates for transition metal catalyzed hydrofunctionalization reactions.<ref>{{Cite journal|last1=Zi|first1=Weiwei|last2=Toste|first2=F. Dean|date=2016-08-08|title=Recent advances in enantioselective gold catalysis|url=https://pubs.rsc.org/en/content/articlelanding/2016/cs/c5cs00929d|journal=Chemical Society Reviews|language=en|volume=45|issue=16|pages=4567–4589|doi=10.1039/C5CS00929D|pmid=26890605|issn=1460-4744|url-access=subscription}}</ref><ref>{{Cite journal|last1=Lee|first1=Mitchell|last2=Nguyen|first2=Mary|last3=Brandt|first3=Chance|last4=Kaminsky|first4=Werner|last5=Lalic|first5=Gojko|date=2017-12-04|title=Catalytic Hydroalkylation of Allenes|journal=Angewandte Chemie International Edition|language=en|volume=56|issue=49|pages=15703–15707|doi=10.1002/anie.201709144|pmid=29052303|doi-access=free}}</ref><ref>{{Cite journal|last1=Kim|first1=Seung Wook|last2=Meyer|first2=Cole C.|last3=Mai|first3=Binh Khanh|last4=Liu|first4=Peng|last5=Krische|first5=Michael J.|date=2019-10-04|title=Inversion of Enantioselectivity in Allene Gas versus Allyl Acetate Reductive Aldehyde Allylation Guided by Metal-Centered Stereogenicity: An Experimental and Computational Study|url= |journal=ACS Catalysis|volume=9|issue=10|pages=9158–9163|doi=10.1021/acscatal.9b03695|pmc=6921087|pmid=31857913}}</ref>

Much like acetylenes, electron-poor allenes are unstable.  Tetrachloroallene polymerizes quantitatively to perchloro(1,2-dimethylenecyclobutane) at −50&nbsp;°C.<ref>{{cite journal|doi=10.1002/ange.19630750113|department=Zuschriften [Letters]|lang=de|title=Perchlor-propadien-(1.2), ein hochreaktives Allen|trans-title=Perchloro-1,2,-propadiene, a highly reactive allene|first1=A.|last1=Roedig|first2=G.|last2=M&auml;rkl|first3=B.|last3=Heinrich|page=88|journal=Angewandte Chemie|volume=75|year=1963|issue=1|bibcode=1963AngCh..75...88R }}</ref>

Cyclic allenes with fewer than 10 ring atoms are [[ring strain|strained]].  Those with fewer than 8 atoms generally only form unstable [[aryne]]-like intermediates.<ref>{{cite journal|journal=Pure Appl. Chem.|volume=78|issue=2|page=451|year=2006|doi=10.1351/pac200678020451|publisher=[[IUPAC]]|title=Synthesis and reactivity of new strained cyclic allene and alkyne precursors|first1=Diego|last1=Peña|first2=Beatriz|last2=Iglesias|first3=Iago|last3=Quintana|first4=Dolores|last4=Pérez|first5=Enrique|last5=Guitián|first6=Luis|last6=Castedo}}</ref><ref name=BicycleThesis>{{cite thesis|url=https://etd.lib.metu.edu.tr/upload/3/12610317/index.pdf|title=Bicyclic strained allenes|institution=[[Middle East Technical University]]|first=Benan|last=Kilba&scedil;|type=PhD|date=Jan 2009}}</ref>{{rp|pp=6&ndash;7}}  The latter are sometimes stabilized by [[non-Kekule molecule|diradical]] or [[ylidic]] resonance structures.<ref name=BicycleThesis/>{{rp|pp=14&ndash;15}}<ref>{{cite journal |last1=Kelleghan |first1=Andrew&nbsp;V. |last2=Tena&nbsp;Meza |first2=Arismel |last3=Garg |first3=Neil&nbsp;K. |date=2023-11-09 |title=Generation and reactivity of unsymmetrical strained heterocyclic allenes |journal=Nat. Synth. |volume=3 |issue=3 |pages=329–336 |doi=10.1038/s44160-023-00432-1 |pmc=11031199 |pmid=38645473}}</ref>

==Synthesis==
Although allenes often require specialized syntheses, the parent allene, [[propadiene]] is produced industrially on a large scale as an equilibrium mixture with [[methylacetylene|propyne]]:
:<chem>H2C=C=CH2 <=> H3C-C#CH</chem>
This mixture, known as [[MAPP gas]], is commercially available.  At 298 K, the Δ''G°'' of this reaction is –1.9 kcal/mol, corresponding to ''K''<sub>eq</sub> = 24.7.<ref>{{Cite journal|last1=Robinson|first1=Marin S.|last2=Polak|first2=Mark L.|last3=Bierbaum|first3=Veronica M.|last4=DePuy|first4=Charles H.|last5=Lineberger|first5=W. C.|date=1995-06-01|title=Experimental Studies of Allene, Methylacetylene, and the Propargyl Radical: Bond Dissociation Energies, Gas-Phase Acidities, and Ion-Molecule Chemistry|url=https://doi.org/10.1021/ja00130a017|journal=Journal of the American Chemical Society|volume=117|issue=25|pages=6766–6778|doi=10.1021/ja00130a017|bibcode=1995JAChS.117.6766R |issn=0002-7863|url-access=subscription}}</ref>

The first allene to be synthesized was [[penta-2,3-dienedioic acid]], which was prepared by Burton and Pechmann in 1887. However, the structure was only correctly identified in 1954.<ref>{{Cite journal|last1=Jones|first1=E. R. H.|last2=Mansfield|first2=G. H.|last3=Whiting|first3=M. C.|date=1954-01-01|title=Researches on acetylenic compounds. Part XLVII. The prototropic rearrangements of some acetylenic dicarboxylic acids|url=https://pubs.rsc.org/en/content/articlelanding/1954/jr/jr9540003208|journal=Journal of the Chemical Society (Resumed)|language=en|pages=3208–3212|doi=10.1039/JR9540003208|issn=0368-1769|url-access=subscription}}</ref>

Laboratory methods for the formation of allenes include:
*from geminal dihalocyclopropanes and organolithium compounds (or metallic sodium or magnesium) in the [[Skattebøl rearrangement]] ([[Doering–LaFlamme allene synthesis]]) via rearrangement of cyclopropylidene carbenes/carbenoids
*from reaction of certain terminal [[alkyne]]s with [[formaldehyde]], [[copper(I) bromide]], and added base ([[Crabbé–Ma allene synthesis]])<ref>{{OrgSynth|title=One-Step Homologation of Acetylenes to Allenes: 4-Hydroxynona-1,2-diene [1,2-Nonadien-4-ol]|last1=Crabbé|first1=Pierre|last2=Nassim|first2=Bahman|last3=Robert-Lopes|first3=Maria-Teresa|collvol=7|collvolpages=276|volume=63|page=203|date=1985|prep=CV7P0276|doi=10.15227/orgsyn.063.0203}}</ref><ref>{{OrgSynth|title=Buta-2,3-dien-1-ol|first1=Hongwen|last1=Luo|first2=Dengke|last2=Ma|first3=Shengming|last3=Ma|year=2017|vol=94|page=153–166|prep=v94p0153|doi=10.15227/orgsyn.094.0153}}</ref>
*from propargylic halides by S<sub>N</sub>2′ displacement by an organocuprate<ref>{{Cite journal|last1=Yoshikai|first1=Naohiko|last2=Nakamura|first2=Eiichi|date=2012-04-11|title=Mechanisms of Nucleophilic Organocopper(I) Reactions|url=https://pubs.acs.org/doi/10.1021/cr200241f|journal=Chemical Reviews|language=en|volume=112|issue=4|pages=2339–2372|doi=10.1021/cr200241f|pmid=22111574|issn=0009-2665|url-access=subscription}}</ref>
*from [[dehydrohalogenation]] of certain [[halide|dihalides]]<ref>{{OrgSynth|title=Allene|last1=Cripps|first1=H. N.|last2=Kiefer|first2=E. F.|collvol=5|collvolpages=22|volume=42|page=12|date=1962|prep=CV5P0022|doi=10.15227/orgsyn.042.0012}}</ref>
*from reaction of a triphenylphosphinyl ester with an acid halide, a [[Wittig reaction]] accompanied by dehydrohalogenation<ref>{{cite journal|last1=Lang|first1=Robert W.|last2=Hansen|first2=Hans-Jürgen|title=Eine einfache Allencarbonsäureester-Synthese mittels der Wittig-Reaktion|trans-title=A simple synthesis of allene carboxylic acid esters by means of the Wittig reaction|journal=[[Helv. Chim. Acta]]|volume=63|issue=2|pages=438–455|date=1980|doi=10.1002/hlca.19800630215}}</ref><ref>{{OrgSynth|title=α-Allenic Esters from α-Phosphoranylidene Esters and Acid Chlorides: Ethyl 2,3-Pentadienoate [2,3-Pentadienoic acid, ethyl ester]|last1=Lang|first1=Robert W.|last2=Hansen|first2=Hans-Jürgen|collvol=7|collvolpages=232|volume=62|page=202|date=1984|prep=CV7P0232|doi=10.15227/orgsyn.062.0202}}</ref>
*from propargylic alcohols via the [[Myers allene synthesis]] protocol—a [[stereospecific]] process
*from metalation of allene or substituted allenes with BuLi and reaction with electrophiles (RX, R<sub>3</sub>SiX, D<sub>2</sub>O, etc.)<ref>{{Cite journal|last1=Michelot|first1=Didier|last2=Clinet|first2=Jean-Claude|last3=Linstrumelle|first3=Gérard|date=1982-01-01|title=Allenyllithium Reagents (VI)1. A Highly Regioselective Metalation of Allenic Hydrocarbons2. A Route to Mono, DI, TRI or Tetrasubstituted Allenes|url=https://doi.org/10.1080/00397918208061912|journal=Synthetic Communications|volume=12|issue=10|pages=739–747|doi=10.1080/00397918208061912|issn=0039-7911|url-access=subscription}}</ref>
The chemistry of allenes has been reviewed in a number of books<ref name=":0" /><ref>{{Cite book|url=https://www.worldcat.org/oclc/501315951|title=The chemistry of ketenes, allenes and related compounds. Part 1|series=PATAI'S Chemistry of Functional Groups|date=1980|publisher=Wiley|editor-last=Patai|editor-first=Saul|isbn=978-0-470-77160-0|location=Chichester|oclc=501315951}}</ref><ref>{{Cite book|url=https://www.worldcat.org/oclc/520990503|title=The chemistry of ketenes, allenes and related compounds. Part 2|series=PATAI'S Chemistry of Functional Groups|date=1980|publisher=Wiley|editor-last=Patai|editor-first=Saul|isbn=978-0-470-77161-7|location=Chichester|oclc=520990503}}</ref><ref name=":10">{{Cite book|last1=Brandsma|first1=L.|last2=Verkruijsse|first2=H.D.|url=https://www.worldcat.org/oclc/162570992|title=Synthesis of acetylenes, allenes and cumulenes : methods and techniques|date=2004|publisher=Elsevier|isbn=978-0-12-125751-4|edition=1st|location=Amsterdam|oclc=162570992}}</ref> and journal articles.<ref name=":2" /><ref name=":7">{{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>{{Cite journal|last1=Zimmer|first1=Reinhold|last2=Dinesh|first2=Chimmanamada U.|last3=Nandanan|first3=Erathodiyil|last4=Khan|first4=Faiz Ahmed|date=2000-08-01|title=Palladium-Catalyzed Reactions of Allenes|url=https://pubs.acs.org/doi/10.1021/cr9902796|journal=Chemical Reviews|language=en|volume=100|issue=8|pages=3067–3126|doi=10.1021/cr9902796|pmid=11749314|issn=0009-2665|url-access=subscription}}</ref><ref name=":8">{{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=Alcaide|first1=Benito|last2=Almendros|first2=Pedro|last3=Aragoncillo|first3=Cristina|date=2010|title=Exploiting [2+2] cycloaddition chemistry: achievements with allenes|url=http://xlink.rsc.org/?DOI=B913749A|journal=Chem. Soc. Rev.|language=en|volume=39|issue=2|pages=783–816|doi=10.1039/B913749A|pmid=20111793|hdl=10261/29537 |issn=0306-0012|hdl-access=free}}</ref><ref>{{Cite journal|last=Pinho e Melo|first=Teresa M. V. D.|date=July 2011|title=Allenes as building blocks in heterocyclic chemistry|url=http://link.springer.com/10.1007/s00706-011-0505-7|journal=Monatshefte für Chemie - Chemical Monthly|language=en|volume=142|issue=7|pages=681–697|doi=10.1007/s00706-011-0505-7|s2cid=189843060|issn=0026-9247|url-access=subscription}}</ref><ref>{{Cite journal|last1=López|first1=Fernando|last2=Mascareñas|first2=José Luis|date=2011-01-10|title=Allenes as Three-Carbon Units in Catalytic Cycloadditions: New Opportunities with Transition-Metal Catalysts|url=https://onlinelibrary.wiley.com/doi/10.1002/chem.201002366|journal=Chemistry – A European Journal|language=en|volume=17|issue=2|pages=418–428|doi=10.1002/chem.201002366|pmid=21207554|issn=0947-6539|url-access=subscription}}</ref><ref>{{Cite journal|last1=Aubert|first1=Corinne|author2-link=Louis Fensterbank|last2=Fensterbank|first2=Louis|last3=Garcia|first3=Pierre|last4=Malacria|first4=Max|last5=Simonneau|first5=Antoine|date=2011-03-09|title=Transition Metal Catalyzed Cycloisomerizations of 1, n -Allenynes and -Allenenes|url=https://pubs.acs.org/doi/10.1021/cr100376w|journal=Chemical Reviews|language=en|volume=111|issue=3|pages=1954–1993|doi=10.1021/cr100376w|pmid=21391568|issn=0009-2665|url-access=subscription}}</ref><ref>{{Cite journal|last1=Krause|first1=Norbert|last2=Winter|first2=Christian|date=2011-03-09|title=Gold-Catalyzed Nucleophilic Cyclization of Functionalized Allenes: A Powerful Access to Carbo- and Heterocycles|url=https://pubs.acs.org/doi/10.1021/cr1004088|journal=Chemical Reviews|language=en|volume=111|issue=3|pages=1994–2009|doi=10.1021/cr1004088|pmid=21314182|issn=0009-2665|url-access=subscription}}</ref> Some key approaches towards allenes are outlined in the following scheme:<ref name=":3">{{Cite journal|last=Sydnes|first=Leiv K.|date=2003-04-01|title=Allenes from Cyclopropanes and Their Use in Organic SynthesisRecent Developments|url=https://pubs.acs.org/doi/10.1021/cr010025w|journal=Chemical Reviews|language=en|volume=103|issue=4|pages=1133–1150|doi=10.1021/cr010025w|pmid=12683779|issn=0009-2665|url-access=subscription}}</ref><ref name=":4">{{Cite journal|last1=Brummond|first1=Kay|author-link=Kay Brummond|last2=DeForrest|first2=Jolie|date=March 2007|title=Synthesizing Allenes Today (1982-2006)|url=http://www.thieme-connect.de/DOI/DOI?10.1055/s-2007-965963|journal=Synthesis|language=en|volume=2007|issue=6|pages=795–818|doi=10.1055/s-2007-965963|issn=0039-7881|url-access=subscription}}</ref><ref name=":5">{{Cite journal|last1=Yu|first1=Shichao|last2=Ma|first2=Shengming|date=2011|title=How easy are the syntheses of allenes?|url=http://xlink.rsc.org/?DOI=C0CC05640E|journal=Chemical Communications|language=en|volume=47|issue=19|pages=5384–5418|doi=10.1039/C0CC05640E|pmid=21409186|issn=1359-7345|url-access=subscription}}</ref><ref name=":6">{{Cite journal|last1=Tejedor|first1=David|last2=Méndez-Abt|first2=Gabriela|last3=Cotos|first3=Leandro|last4=García-Tellado|first4=Fernando|date=2013|title=Propargyl Claisen rearrangement: allene synthesis and beyond|url=http://xlink.rsc.org/?DOI=C2CS35311C|journal=Chem. Soc. Rev.|language=en|volume=42|issue=2|pages=458–471|doi=10.1039/C2CS35311C|pmid=23034723|hdl=10261/132538 |issn=0306-0012|hdl-access=free}}</ref>

[[File:Overview_common_allene_syntheses_Zhurakovskyi.svg]]

One of the older methods is the Skattebøl rearrangement<ref name=":3" /><ref>{{Cite journal|last1=Skattebøl|first1=Lars|last2=Nilsson|first2=Martin|last3=Lindberg|first3=Bengt|last4=McKay|first4=James|last5=Munch-Petersen|first5=Jon|date=1963|title=The Synthesis of Allenes from 1,1-Dihalocyclopropane Derivatives and Alkyllithium.|journal=Acta Chemica Scandinavica|language=en|volume=17|pages=1683–1693|doi=10.3891/acta.chem.scand.17-1683|issn=0904-213X|doi-access=free}}</ref><ref>{{Cite journal|last1=Moore|first1=William R.|last2=Ward|first2=Harold R.|date=December 1962|title=The Formation of Allenes from gem-Dihalocyclopropanes by Reaction with Alkyllithium Reagents 1,2|url=https://pubs.acs.org/doi/abs/10.1021/jo01059a013|journal=The Journal of Organic Chemistry|language=en|volume=27|issue=12|pages=4179–4181|doi=10.1021/jo01059a013|issn=0022-3263|url-access=subscription}}</ref> (also called the Doering–Moore–Skattebøl or Doering–LaFlamme<ref>{{Cite journal|last=Fedoryński|first=Michał|date=2003-04-01|title=Syntheses of gem -Dihalocyclopropanes and Their Use in Organic Synthesis|url=https://pubs.acs.org/doi/10.1021/cr0100087|journal=Chemical Reviews|language=en|volume=103|issue=4|pages=1099–1132|doi=10.1021/cr0100087|pmid=12683778|issn=0009-2665|url-access=subscription}}</ref><ref>{{Cite book|last1=Kurti|first1=Laszlo|last2=Czako|first2=Barbara|url=https://www.worldcat.org/oclc/850164343|title=Strategic Applications of Named Reactions in Organic Synthesis : Background and Detailed Mechanisms.|date=2005|publisher=Elsevier Science|isbn=978-0-08-057541-4|location=Burlington|pages=758|oclc=850164343}}</ref> rearrangement), in which a gem-dihalocyclopropane '''3''' is treated with an organolithium compound (or dissolving metal) and the presumed intermediate rearranges into an allene either directly or via carbene-like species. Notably, even strained allenes can be generated by this procedure.<ref>{{Cite journal|last1=Shi|first1=Min|last2=Shao|first2=Li-Xiong|last3=Lu|first3=Jian-Mei|last4=Wei|first4=Yin|last5=Mizuno|first5=Kazuhiko|last6=Maeda|first6=Hajime|date=2010-10-13|title=Chemistry of Vinylidenecyclopropanes|url=https://pubs.acs.org/doi/10.1021/cr900381k|journal=Chemical Reviews|language=en|volume=110|issue=10|pages=5883–5913|doi=10.1021/cr900381k|pmid=20518460|issn=0009-2665|url-access=subscription}}</ref> Modifications involving leaving groups of different nature are also known.<ref name=":3" /> Arguably, the most convenient modern method of allene synthesis is by [[sigmatropic rearrangement]] of [[Propargyl group|propargylic substrates]].<ref name=":4" /><ref name=":5" /><ref name=":6" /> Johnson–Claisen<ref name=":6" /> and Ireland–Claisen<ref>{{Cite journal|last1=Ireland|first1=Robert E.|last2=Mueller|first2=Richard H.|last3=Willard|first3=Alvin K.|date=May 1976|title=The ester enolate Claisen rearrangement. Stereochemical control through stereoselective enolate formation|url=https://pubs.acs.org/doi/abs/10.1021/ja00426a033|journal=Journal of the American Chemical Society|language=en|volume=98|issue=10|pages=2868–2877|doi=10.1021/ja00426a033|bibcode=1976JAChS..98.2868I |issn=0002-7863|url-access=subscription}}</ref> rearrangements of ketene acetals '''4''' have been used a number of times to prepare allenic esters and acids. Reactions of vinyl ethers '''5''' (the Saucy–Marbet rearrangement) give allene aldehydes,<ref>{{Cite journal|last1=Kurtz|first1=Kimberly C.M.|last2=Frederick|first2=Michael O.|last3=Lambeth|first3=Robert H.|last4=Mulder|first4=Jason A.|last5=Tracey|first5=Michael R.|last6=Hsung|first6=Richard P.|date=April 2006|title=Synthesis of chiral allenes from ynamides through a highly stereoselective Saucy–Marbet rearrangement|url=https://linkinghub.elsevier.com/retrieve/pii/S0040402006001268|journal=Tetrahedron|language=en|volume=62|issue=16|pages=3928–3938|doi=10.1016/j.tet.2005.11.087|url-access=subscription}}</ref> while propargylic sulfenates '''6''' give allene sulfoxides.<ref>{{Cite journal|last1=Mukai|first1=Chisato|last2=Kobayashi|first2=Minoru|last3=Kubota|first3=Shoko|last4=Takahashi|first4=Yukie|last5=Kitagaki|first5=Shinji|date=March 2004|title=Construction of Azacycles Based on Endo-Mode Cyclization of Allenes|url=https://pubs.acs.org/doi/10.1021/jo035729f|journal=The Journal of Organic Chemistry|language=en|volume=69|issue=6|pages=2128–2136|doi=10.1021/jo035729f|pmid=15058962|issn=0022-3263|url-access=subscription}}</ref><ref>{{Cite journal|last1=Mukai|first1=Chisato|last2=Ohta|first2=Masaru|last3=Yamashita|first3=Haruhisa|last4=Kitagaki|first4=Shinji|date=October 2004|title=Base-Catalyzed Endo-Mode Cyclization of Allenes: Easy Preparation of Five- to Nine-Membered Oxacycles|url=https://pubs.acs.org/doi/10.1021/jo0488614|journal=The Journal of Organic Chemistry|language=en|volume=69|issue=20|pages=6867–6873|doi=10.1021/jo0488614|pmid=15387613|issn=0022-3263|url-access=subscription}}</ref> Allenes can also be prepared by [[nucleophilic substitution]] in '''9''' and '''10''' (nucleophile Nu<sup>−</sup> can be a hydride anion), [[1,2-elimination]] from '''8''', proton transfer in '''7''', and other, less general, methods.<ref name=":4" /><ref name=":5" />

== 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>

==Delta convention==
Many rings or ring systems are known by semisystematic names that assume a maximum number of noncumulative bonds. To unambiguously specify derivatives that include cumulated bonds (and hence fewer hydrogen atoms than would be expected from the skeleton), a lowercase delta may be used with a subscript indicating the number of cumulated double bonds from that atom, e.g. 8δ<sup>2</sup>-benzocyclononene. This may be combined with the λ-convention for specifying nonstandard valency states, e.g. 2λ<sup>4</sup>δ<sup>2</sup>,5λ<sup>4</sup>δ<sup>2</sup>-thieno[3,4-c]thiophene.<ref>{{cite journal|url=http://www.chem.qmul.ac.uk/iupac/hetero/De.html|title=Nomenclature for cyclic organic compounds with contiguous formal double bonds (the δ-convention)|journal=Pure Appl. Chem.|volume=60|page=1395-1401|year=1988|doi=10.1351/pac198860091395|s2cid=97759274|doi-access=free}}</ref>

==See also==
*Compounds with three or more adjacent carbon–carbon double bonds are called [[cumulene]]s.

==References==
{{CC-notice|cc=by2.5|url=https://ora.ox.ac.uk/objects/uuid:d7e4bbea-4009-4e7e-838c-46b4bb81bc13|author= Oleksandr Zhurakovskyi}}

{{Reflist}}

==Further reading==
* Brummond, Kay M. (editor). ''[http://www.beilstein-journals.org/bjoc/browse/singleSeries.htm?sn=14 Allene chemistry]'' (special thematic issue). ''[[Beilstein Journal of Organic Chemistry]]'' '''7''': 394–943.

==External links==
*[https://www.organic-chemistry.org/synthesis/C1C/chains/allenes.shtm Synthesis of allenes]

{{Functional group|state=expanded}}
{{Authority control}}

[[Category:Cumulated dienes| ]]
[[Category:Jacobus Henricus van 't Hoff]]