Cyclopropenium ion

{{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 cation. Its salts have been isolated, and many derivatives have been characterized by X-ray crystallography.<ref>{{March6th}}</ref> The cation and some simple derivatives have been identified in the atmosphere of the Saturnian moon Titan.<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 rules of aromaticity for {{nowrap|4''n'' + 2}} electrons since, in this case, ''n''&nbsp;=&nbsp;0. Consistent with this prediction, the C₃H₃ core is planar and the C–C bonds are equivalent. In the case of the cation in [C₃(SiMe₃)₃]⁺{{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|[C₃(SiMe₃)₃]⁺{{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 in 1957. The salt was prepared in two steps starting with the reaction of phenyldiazoacetonitrile with diphenylacetylene to yield 1,2,3-triphenyl-3-cyclopropene nitrile. Treatment of this with boron trifluoride yielded [C₃Ph₃]BF₄.<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|title = Synthesis of the ''s''-Triphenylcyclopropenyl Cation |year = 1957 |journal = 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 with antimony pentachloride, aluminum trichloride, or silver fluoroborate...[leads] to the salts of cyclopropenyl cation." The 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:<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> :C₃Cl₄ + AlCl₃ → [C₃Cl₃]⁺{{chem|AlCl|4|−}}

Tetrachlorocyclopropene can be converted to tris(''tert''-butyldimethylsilyl)cyclopropene. Hydride abstraction with 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-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 is an unusual derivative featuring cyclopropenium linked to a 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 acids to acid chlorides:<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 cyclopropenones.

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 via acid chloride formation followed by nucleophilic substitution with the unprotected amino acid.

:650px|Peptide catalysis by cyclopropenium ions

This method of mildly generating acid chlorides can also be useful for linking alpha-anomeric sugars.<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, the compound is iodinated with tetrabutylammonium iodide. This iodine can thereafter be substituted by any ROH 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}}. 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 Ph₃C₃Co(CO)₃ viewed down the C₃ symmetry axis. Many complexes are known with cyclopropenium ligands. Examples include [M(C₃Ph₃)(PPh₃)₂]⁺ (M = Ni, Pd, Pt) and Co(C₃Ph₃)(CO)₃. Such compounds are prepared by reaction of cyclopropenium salts with low valent metal complexes.<ref>{{cite journal|last1=Chiang |first1=T. |last2=Kerber |first2=R. C. |last3=Kimball |first3=S. D. |last4=Lauher |first4=J. W.|title=(η³-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, relevant to technologies such as desalination and fuel cells. 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 {{clear}} == References == {{reflist}}

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