Cyclopropenium ion

The cyclopropenium ion is the cation with the formula C
3H+
3. It 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.[1] The cation and some simple derivatives have been identified in the atmosphere of the Saturnian moon Titan.[2]

The cyclopropenium cation

Bonding

With two π electrons, the cyclopropenium cation class obeys Hückel’s rules of aromaticity for 4n + 2 electrons since, in this case, n is zero. 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]+SbCl−
6,[3] the ring C–C distances range from 1.374(2) to 1.392(2) Å.

Structure of the salt [C3(SiMe3)3]+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-2-cyclopropene nitrile. Treatment of this with boron trifluoride yielded [C3Ph3]BF4.[4][5][6]

The parent cation, [C3H3]+, was reported as its hexachloroantimonate (SbCl−
6) salt in 1970.[7] It is indefinitely stable at −20 °C.

Trichlorocyclopropenium salts are generated by chloride abstraction from tetrachlorocyclopropene:[8]

C3Cl4 + AlCl3 → [C3Cl3]+AlCl−
4

Tetrachlorocyclopropene can be converted to tris(tert-butyldimethylsilyl)cyclopropene. Hydride abstraction with nitrosonium tetrafluoroborate yields the trisilyl-substituted cyclopropenium cation.[9]

Amino-substituted cyclopropenium salts are particularly stable.[10][11] Calicene is an unusual derivative featuring cyclopropenium linked to a cyclopentadienide.

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:[12]

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.[12] 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.

This method of mildly generating acid chlorides can also be useful for linking alpha-anomeric sugars.[13] 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.

Additionally, some synthetic routes make use of cyclopropenium ring openings yielding an allylcarbene cation. The linear degradation product yields both a nucleophilic and electrophilic carbon centers.[14]

Organometallic compounds
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.[15]

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.[16]

See also

References
  1. Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 978-0-471-72091-1
  2. 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
  3. De Meijere, A.; Faber, D.; Noltemeyer, M.; Boese, R.; Haumann, T.; Muller, T.; Bendikov, M.; Matzner, E.; Apeloig, Y. (1996). "Tris(trimethylsilyl)cyclopropenylium Cation: The First X-ray Structure Analysis of an α-Silyl-Substituted Carbocation". J. Org. Chem. 61 (24): 8564. doi:10.1021/jo960478e.
  4. Yadav, Arvind (2012). "Cyclopropenium Ion". Synlett. 23 (16): 2428–2429. doi:10.1055/s-0032-1317230.
  5. Ronald Breslow (1957). "Synthesis of the s-Triphenylcyclopropenyl Cation". J. Am. Chem. Soc. 79 (19): 5318. doi:10.1021/ja01576a067.
  6. Xu, Ruo; Breslow, Ronald (1997). "1,2,3-Triphenylcyclopropenium Bromide". Org. Synth. 74: 72. doi:10.15227/orgsyn.074.0072.
  7. Breslow, R.; Groves, J. T. (1970). "Cyclopropenyl Cation. Synthesis and Characterization". J. Am. Chem. Soc. 92 (4): 984–987. doi:10.1021/ja00707a040.
  8. Glück, C.; Poingée, V.; Schwager, H. (1987). "Improved Synthesis of 7,7-Difluorocyclopropabenzene". Synthesis. 1987 (3): 260–262. doi:10.1055/s-1987-27908.
  9. Buchholz, Herwig; Surya Prakash, G. K.; Deffieux, Denis; Olah, George (1999). "Electrochemical preparation of tris(tert-butyldimethylsilyl)cyclopropene and its hydride abstraction to tris(tert-butyldimethylsilyl)cyclopropenium tetrafluoroborate" (PDF). Proc. Natl. Acad. Sci. 96 (18): 10003–10005. Bibcode:1999PNAS...9610003B. doi:10.1073/pnas.96.18.10003. PMC 17831. PMID 10468551.
  10. Bandar, Jeffrey S.; Lambert, Tristan H. (2013). "Aminocyclopropenium ions: synthesis, properties, and applications". Synthesis. 45 (10): 2485–2498.
  11. Haley, Michael M.; Gilbertson, Robert D.; Weakley, Timothy J.D. (2000). "Preparation, X-ray Crystal Structures, and Reactivity of Alkynylcyclopropenylium Salts". Journal of Organic Chemistry. 65 (5): 1422–1430. doi:10.1021/jo9915372.
  12. Hardee, David J.; Kovalchuke, Lyudmila; Lambert, Tristan H. (2010). "Nucleophilic Acyl Substitution via Aromatic Cation Activation of Carboxylic Acids: Rapid Generation of Acid Chlorides under Mild Conditions". Journal of the American Chemical Society. 132 (14): 5002–5003. doi:10.1021/ja101292a. PMID 20297823.
  13. Nogueira, J. M.; Nguyến, S. H.; Bennett, C. S. (2011). "Cyclopropenium Cation Promoted Dehydrative Glycosylations Using 2-Deoxy- and 2,6-Dideoxy-Sugar Donors". Journal of the American Chemical Society. 13 (11): 2184–2187. doi:10.1021/ol200726v. PMID 21548642.
  14. Yoshida, Zen'ichi; Yoneda, Shigeo; Hirai, Hideo (1981). "A Novel Synthesis of Pyrroles by the Reactions of Tris(alkylthio)cyclopropenium Salt with Amines". Heterocycles. 15 (2): 865. doi:10.3987/S-1981-02-0865.
  15. Chiang, T.; Kerber, R. C.; Kimball, S. D.; Lauher, J. W. (1979). "(η3-Triphenylcyclopropenyl) Tricarbonylcobalt". Inorganic Chemistry. 18 (6): 1687–1691. doi:10.1021/ic50196a058.
  16. Jiang, Yivan; Freyer, Jessica; Cotanda, Pepa; Brucks, Spencer; Killops, Kato; Bandar, Jeffrey; Torsitano, Christopher; Balsara, Nitash; Lambert, Tristan; Campos, Luis (2015). "The evolution of cyclopropenium ions into functional polyelectrolytes" (PDF). Nature Communications. 6 (1): 1–7. Bibcode:2015NatCo...6E5950J. doi:10.1038/ncomms6950.
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