# Cyclopropenium ion

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{{Short description|Chemical compound}}
thumbnail|180 px|The cyclopropenium cation
The '''cyclopropenium ion''' is the cation with the formula {{chem|C|3|H|3|+}}. It has attracted attention as the smallest example of an [aromatic](/source/aromatic) [cation](/source/cation). Its salts have been isolated, and many derivatives have been characterized by [X-ray crystallography](/source/X-ray_crystallography).<ref>{{March6th}}</ref> The cation and some simple derivatives have been identified in the atmosphere of the Saturnian moon [Titan](/source/Titan_(moon)).<ref>
A.Aliab, C.Puzzarinid, "Cyclopropenyl cation – the simplest Huckel's aromatic molecule – and its cyclic methyl derivatives in Titan's upper atmosphere", Planetary and Space Science,
Volume 87, October 2013, Pages 96-105.  
https://doi.org/10.1016/j.pss.2013.07.007</ref>

==Bonding==
With two π electrons, the cyclopropenium cation class obeys [Hückel’s rule](/source/H%C3%BCckel%E2%80%99s_rule)s of aromaticity for {{nowrap|4''n'' + 2}} electrons since, in this case, ''n''&nbsp;=&nbsp;0. Consistent with this prediction, the C<sub>3</sub>H<sub>3</sub> core is planar and the C–C bonds are equivalent. In the case of the cation in [C<sub>3</sub>(SiMe<sub>3</sub>)<sub>3</sub>]<sup>+</sup>{{chem|SbCl|6|−}},<ref>{{cite journal|first1=A. |last1=De Meijere |first2=D. |last2=Faber |first3=M. |last3=Noltemeyer |first4=R. |last4=Boese |first5=T. |last5=Haumann |first6=T. |last6=Muller |first7=M. |last7=Bendikov |first8=E. |last8=Matzner |first9=Y. |last9=Apeloig|journal=J. Org. Chem.|year=1996| volume=61|issue=24 |page=8564|doi=10.1021/jo960478e|title=Tris(trimethylsilyl)cyclopropenylium Cation: The First X-ray Structure Analysis of an α-Silyl-Substituted Carbocation}}</ref> the ring C–C distances range from 1.374(2) to 1.392(2)&nbsp;Å.
[[File:NEZKOS01.png|thumb|Structure of the salt {{nowrap|[C<sub>3</sub>(SiMe<sub>3</sub>)<sub>3</sub>]<sup>+</sup>{{chem|SbCl|6|−}}}}]]

==Syntheses==
Salts of many cyclopropenyl cations have been characterized. Their stability varies according to the steric and inductive effects of the substituents.

Salts of triphenylcyclopropenium were first reported by [Ronald Breslow](/source/Ronald_Breslow) in 1957. The salt was prepared in two steps starting with the reaction of phenyldiazoacetonitrile with [diphenylacetylene](/source/diphenylacetylene) to yield 1,2,3-triphenyl-3-cyclopropene nitrile. Treatment of this with [boron trifluoride](/source/boron_trifluoride) yielded [C<sub>3</sub>Ph<sub>3</sub>]BF<sub>4</sub>.<ref>{{cite journal|last1=Yadav|first1=Arvind|title=Cyclopropenium Ion|journal=Synlett|year=2012|volume=23|issue=16|pages=2428–2429|doi=10.1055/s-0032-1317230|doi-access=free}}</ref><ref name=breslow>{{cite journal |author = [Ronald Breslow](/source/Ronald_Breslow)|title = Synthesis of the ''s''-Triphenylcyclopropenyl Cation |year = 1957 |journal = [J. Am. Chem. Soc.](/source/J._Am._Chem._Soc.) |volume = 79 |issue=19 |pages = 5318 |doi = 10.1021/ja01576a067 |bibcode = 1957JAChS..79Q5318B }}</ref><ref>{{cite journal|journal=Org. Synth.|year=1997|volume=74|page=72|doi=10.15227/orgsyn.074.0072|title=1,2,3-Triphenylcyclopropenium Bromide|first1=Ruo |last1=Xu |first2=Ronald |last2=Breslow}}</ref>

:600px|Cyclopropenium

Per a 1970 paper reporting the parent cation's isolation, "Simple mixing of [3-chlorocyclopropene](/source/3-chlorocyclopropene) with [antimony pentachloride](/source/antimony_pentachloride), [aluminum trichloride](/source/aluminum_trichloride), or [silver fluoroborate](/source/silver_fluoroborate)...[leads] to the salts of cyclopropenyl cation."  The [hexachloroantimonate](/source/hexachloroantimonate) ({{chem|SbCl|6|−}}) salt is indefinitely stable at −20&nbsp;°C.<ref>{{cite journal|title=Cyclopropenyl Cation. Synthesis and Characterization |first1=R. |last1=Breslow |first2=J. T. |last2=Groves |journal=J. Am. Chem. Soc. |year=1970 |volume=92 |issue=4 |pages=984–987 |doi=10.1021/ja00707a040|bibcode=1970JAChS..92..984B }}</ref> 

Trichlorocyclopropenium salts are generated by chloride abstraction from [tetrachlorocyclopropene](/source/tetrachlorocyclopropene):<ref>{{cite journal|last1=Glück|first1=C.|last2=Poingée|first2=V.|last3=Schwager|first3=H.|title=Improved Synthesis of 7,7-Difluorocyclopropabenzene|journal=Synthesis|volume=1987|issue=3|year=1987|pages=260–262|doi=10.1055/s-1987-27908}}</ref> 
:C<sub>3</sub>Cl<sub>4</sub> + AlCl<sub>3</sub> → [C<sub>3</sub>Cl<sub>3</sub>]<sup>+</sup>{{chem|AlCl|4|−}}

[Tetrachlorocyclopropene](/source/Tetrachlorocyclopropene) can be converted to tris(''tert''-butyldimethylsilyl)cyclopropene. Hydride abstraction with [nitrosonium tetrafluoroborate](/source/nitrosonium_tetrafluoroborate) yields the {{chem name|trisilyl-substituted}} cyclopropenium cation.<ref>{{cite journal|last1=Buchholz|first1=Herwig|last2=Surya Prakash|first2=G. K.|last3=Deffieux|first3=Denis|last4=Olah|first4=George|title=Electrochemical preparation of tris(''tert''-butyldimethylsilyl)cyclopropene and its hydride abstraction to tris(''tert''-butyldimethylsilyl)cyclopropenium tetrafluoroborate|journal=Proc. Natl. Acad. Sci.|year=1999|volume=96|issue=18|pages=10003–10005|url=http://www.pnas.org/content/96/18/10003.full.pdf|bibcode=1999PNAS...9610003B|doi=10.1073/pnas.96.18.10003|pmid=10468551|pmc=17831|doi-access=free}}</ref>

:470px|Cyclopropenium synthesis 2

[Amino](/source/Amino)-substituted cyclopropenium salts are particularly stable.<ref>{{cite journal|last1=Bandar |first1=Jeffrey S. |last2=Lambert |first2=Tristan H.|title=Aminocyclopropenium ions: synthesis, properties, and applications|journal=Synthesis |year=2013|volume=45|issue=10|pages=2485–2498|doi=10.1055/s-0033-1338516 }}</ref><ref>{{cite journal|last1=Haley|first1=Michael M.|last2=Gilbertson|first2=Robert D.|last3=Weakley|first3=Timothy J.D.|title=Preparation, X-ray Crystal Structures, and Reactivity of Alkynylcyclopropenylium Salts|journal=Journal of Organic Chemistry|year=2000|volume=65|issue=5|pages=1422–1430|doi=10.1021/jo9915372|pmid=10814104}}</ref> [Calicene](/source/Calicene) is an unusual derivative featuring cyclopropenium linked to a [cyclopentadienide](/source/cyclopentadienide).

:thumb|140px|Calicene features a cyclopropenium ring.

== Reactions ==
===Organic chemistry===
Chloride salts of cyclopropenium esters are intermediates in the use of dichlorocyclopropenes for the conversion of [carboxylic acid](/source/carboxylic_acid)s to [acid chloride](/source/acid_chloride)s:<ref name="lambert">{{cite journal|last1=Hardee|first1=David J.|last2=Kovalchuke|first2=Lyudmila|last3=Lambert|first3=Tristan H.|title=Nucleophilic Acyl Substitution via Aromatic Cation Activation of Carboxylic Acids: Rapid Generation of Acid Chlorides under Mild Conditions|journal=Journal of the American Chemical Society|year=2010|volume=132|issue=14|pages=5002–5003|doi=10.1021/ja101292a|pmid=20297823|bibcode=2010JAChS.132.5002H }}</ref>

:400px|Formation of acid chloride by cyclopropenium derivative

Related cyclopropenium cations are produced in the regeneration of the 1,1-dichlorocyclopropenes from the [cyclopropenone](/source/cyclopropenone)s.

The cyclopropenium chlorides have been applied to peptide bond formation.<ref name=lambert/> For example, in the figure below, reacting a boc-protected amino acid with an unprotected amino acid in the presence of the cyclopropenium ion allows the formation of a [peptide bond](/source/peptide_bond) via acid chloride formation followed by nucleophilic substitution with the unprotected [amino acid](/source/amino_acid).

:650px|Peptide catalysis by cyclopropenium ions

This method of mildly generating acid chlorides can also be useful for linking alpha-[anomeric](/source/anomeric) [sugar](/source/monosaccharide)s.<ref>{{cite journal|last1=Nogueira|first1=J. M.|last2=Nguyến|first2=S. H.|last3=Bennett|first3=C. S.|title=Cyclopropenium Cation Promoted Dehydrative Glycosylations Using 2-Deoxy- and 2,6-Dideoxy-Sugar Donors|journal=Journal of the American Chemical Society|volume=13|year=2011|issue=11|pages=2184–2187|doi=10.1021/ol200726v|pmid=21548642}}</ref> After using the cyclopropenium ion to form the chloride at the [anomeric carbon](/source/anomeric_carbon), the compound is iodinated with [tetrabutylammonium iodide](/source/tetrabutylammonium_iodide). This iodine can thereafter be substituted by any [ROH](/source/hydroxyl) group to quickly undergo alpha-selective linkage of sugars.

:450px|Sugar linkage

Additionally, some synthetic routes make use of cyclopropenium ring openings yielding an {{chem name|[allylcarbene cation](/source/allylcarbene_cation)}}. The linear degradation product yields both a nucleophilic and electrophilic carbon centers.<ref>{{cite journal|last1=Yoshida|first1=Zen'ichi|last2=Yoneda|first2=Shigeo|last3=Hirai|first3=Hideo|title=A Novel Synthesis of Pyrroles by the Reactions of Tris(alkylthio)cyclopropenium Salt with Amines|journal=Heterocycles|year=1981|volume=15|issue=2|pages=865|doi=10.3987/S-1981-02-0865|doi-access=free}}</ref>

:400px|A proposed mechanism of the ring opening of a cyclopropenium ion to form an allylcarbene cation

===Organometallic compounds===
thumb|right|Structure of Ph<sub>3</sub>C<sub>3</sub>Co(CO)<sub>3</sub> viewed down the C<sub>3</sub> symmetry axis.
Many complexes are known with cyclopropenium ligands.  Examples include [M(C<sub>3</sub>Ph<sub>3</sub>)(PPh<sub>3</sub>)<sub>2</sub>]<sup>+</sup> (M = Ni, Pd, Pt) and Co(C<sub>3</sub>Ph<sub>3</sub>)(CO)<sub>3</sub>.  Such compounds are prepared by reaction of cyclopropenium salts with low valent [metal complex](/source/metal_complex)es.<ref>{{cite journal|last1=Chiang |first1=T. |last2=Kerber |first2=R. C. |last3=Kimball |first3=S. D. |last4=Lauher |first4=J. W.|title=(η<sup>3</sup>-Triphenylcyclopropenyl) Tricarbonylcobalt|journal=Inorganic Chemistry|year=1979|volume=18|issue=6 |pages=1687–1691|doi= 10.1021/ic50196a058}}</ref>

== As polyelectrolytes==
Because many substituted derivatives are known, cyclopropenium salts have attracted attention as possible [polyelectrolytes](/source/polyelectrolytes), relevant to technologies such as [desalination](/source/desalination) and [fuel cell](/source/fuel_cell)s. The tris(dialkylamino)cyclopropenium salts have been particularly evaluated because of their high stability.<ref>{{cite journal|last1=Jiang|first1=Yivan|last2=Freyer|first2=Jessica|last3=Cotanda|first3=Pepa|last4=Brucks|first4=Spencer|last5=Killops|first5=Kato|last6=Bandar|first6=Jeffrey|last7=Torsitano|first7=Christopher|last8=Balsara|first8=Nitash|last9=Lambert|first9=Tristan|last10=Campos|first10=Luis|title=The evolution of cyclopropenium ions into functional polyelectrolytes|journal=Nature Communications|volume=6|year=2015|issue=1|pages=1–7|doi=10.1038/ncomms6950|pmid=25575214|pmc=4354017|bibcode=2015NatCo...6E5950J|url=https://cloudfront.escholarship.org/dist/prd/content/qt2h53h39q/qt2h53h39q.pdf?t=p9xau4|doi-access=free}}</ref>

==See also==
* [Phosphirenium ion](/source/Phosphirenium_ion)
{{clear}}
== References ==
{{reflist}}

Category:Non-benzenoid aromatic carbocycles
Category:Cations
Category:Cyclopropenes

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Adapted from the Wikipedia article [Cyclopropenium ion](https://en.wikipedia.org/wiki/Cyclopropenium_ion) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Cyclopropenium_ion?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.