cyclopropene

The molecule has a triangular structure. The reduced length of the double bond compared to a single bond causes the angle opposite the double bond to narrow to about 51° from the 60° angle found in cyclopropane.{{cite journal |title= Structure of 3-cyanocyclopropene by microwave spectroscopy and ab initio molecular orbital calculations. Evidence for substituent-ring double bond interactions |author= Staley, S. W. |author2= Norden, T. D. |author3= Su, C.-F. |author4= Rall, M. |author5= Harmony, M. D. |journal= J. Am. Chem. Soc. |year= 1987 |volume= 109 |issue= 10 |pages= 2880–2884 |doi= 10.1021/ja00244a004 }} As with cyclopropane, the carbon–carbon bonding in the ring has increased p character: the alkene carbon atoms use sp^2.68 hybridization for the ring.{{cite journal |journal= Tetrahedron |volume= 38 |issue= 5 |year= 1982 |pages= 645–655 |doi= 10.1016/0040-4020(82)80206-8 |title= The geometry of small rings: Molecular geometry of cyclopropene and its derivatives |author= Allen, F. H. }}

The first confirmed synthesis of cyclopropene, carried out by Dem'yanov and Doyarenko, involved the thermal decomposition of trimethylcyclopropylammonium hydroxide over platinized clay at approximately 300 °C. {{Cite book|last1=Hart|first1=Harold|url=https://books.google.com/books?id=qhwSBQAAQBAJ|title=Advances in Alicyclic Chemistry|last2=Karabatsos|first2=G. J.|publisher=Academic Press Inc.|year=1966|volume=1|location=New York and London|page=55|isbn=9781483224206|language=en}} This reaction produces mainly trimethylamine and dimethylcyclopropyl amine, together with about 5% of cyclopropene. Later Schlatter improved the pyrolytic reaction conditions using platinized asbestos as a catalyst at 320–330 °C and obtained cyclopropene in 45% yield.{{Cite journal|last=Schlatter|first=Maurice J.|date=1941-06-01|title=The Preparation of Cyclopropene|url=https://doi.org/10.1021/ja01851a068|journal=Journal of the American Chemical Society|volume=63|issue=6|pages=1733–1737|doi=10.1021/ja01851a068|issn=0002-7863|url-access=subscription}}

Cyclopropene can also be obtained in about 1% yield by thermolysis of the adduct of cycloheptatriene and dimethyl acetylenedicarboxylate.{{Cite book|url=https://books.google.com/books?id=GVeGAwAAQBAJ&dq=alder+jacobs+cyclopropene+1528&pg=PA2713|title=Houben-Weyl. Methods of Organic Chemistry – Cyclopropanes, Authors Index, Compound Index|publisher=George Thieme Verlag|year=1997|isbn=978-3-13-101644-7|editor-last=de Meijere|editor-first=Armin|volume=E 17d|location=Stuttgart, New York|pages=2712–2713|language=en}}

Allyl chloride undergoes dehydrohalogenation upon treatment with the base sodium amide at 80 °C to produce cyclopropene in about 10% yield.{{cite journal |author= Closs, G.L. |author2= Krantz, K.D. |title= A Simple Synthesis of Cyclopropene |journal= Journal of Organic Chemistry |year= 1966 |volume= 31 |issue= 2 |pages= 638 |doi= 10.1021/jo01340a534 }}

:{{chem2 | CH2\dCHCH2Cl + NaNH2 -> C3H4 + NaCl + NH3 }}

The major byproduct of the reaction is allylamine. Adding allyl chloride to sodium bis(trimethylsilyl)amide in boiling toluene over a period of 45–60 minutes produces the targeted compound in about 40% yield with an improvement in purity:{{OrgSynth |author= Binger, P. |author2= Wedermann, P. |author3= Brinker, U. H. |year= 2000 |title= Cyclopropene: A New Simple Synthesis and Its Diels-Alder reaction with Cyclopentadiene |volume= 77 |pages= 254 |collvol= 10 |collvolpages= 231 |prep=v77p0254}}

:{{chem2 | CH2\dCHCH2Cl + NaN(TMS)2 -> C3H4 + NaCl + NH(TMS)2 }}

1-Methylcyclopropene is synthesized similarly but at room temperature from methallylchloride using phenyllithium as the base:{{cite journal |author= Clarke, T. C. |author2= Duncan, C. D. |author3= Magid, R. M. |title= An Efficient and Convenient Synthesis of 1-Methylcyclopropene |journal= J. Org. Chem. |year= 1971 |volume= 36 |issue= 9 |pages= 1320–1321 |doi= 10.1021/jo00808a041 }}

:{{chem2 | CH2\dC(CH3)CH2Cl + LiC6H5 ->}} {{chem2|CH3\sC3H3|link=1-Methylcyclopropene}} + LiCl + {{chem2|C6H6}}

Treatment of nitrocyclopropanes with sodium methoxide eliminates the nitrite, giving the respective cyclopropene derivative. The synthesis of purely aliphatic cyclopropenes was first illustrated by the copper-catalyzed additions of carbenes to alkynes. In the presence of a copper catalyst, ethyl diazoacetate reacts with acetylenes to give cyclopropenes. 1,2-Dimethylcyclopropene-3-carboxylate arises via this method from 2-butyne. Copper, as copper sulfate and copper dust, are among the more popular forms of copper used to promote such reactions. Rhodium acetate has also been used.

Studies on cyclopropene mainly focus on the consequences of its high ring strain. At 425 °C, cyclopropene isomerizes to methylacetylene (propyne).

:{{chem2 | C3H4 -> H3CC\tCH }}

Attempted fractional distillation of cyclopropene at –36 °C (its predicted boiling point) results in polymerization. The mechanism is assumed to be a free-radical chain reaction, and the product, based on NMR spectra, is thought to be polycyclopropane.

Cyclopropene undergoes the Diels–Alder reaction with cyclopentadiene to give endo-tricyclo[3.2.1.0^2,4]oct-6-ene. This reaction is commonly used to check for the presence of cyclopropene, following its synthesis.

:Image:CyclopropeneDielsAlder.PNG{{clear-left}}

• Malvalic acid is a toxic cyclopropene fatty acid that occurs in cottonseed oil.
• 1-Methylcyclopropene (1-MCP) is used to slow the ripening in fruits.{{ cite journal |author= Beaudry, R. |author2= Watkins, C. |title= Use of 1-MCP on Apples |journal= Perishable Handling Quarterly |year= 2001 |issue= 108 |pages= 12|publisher= University of California }}{{cite journal |journal= Postharvest Biology and Technology |volume= 32 |issue= 2 |date=May 2004 |pages= 193–204 |doi= 10.1016/j.postharvbio.2003.11.009 |title=Inhibition of ethylene action by 1-methylcyclopropene extends postharvest life of "Bartlett" pears |author= Trinchero, G. D. |author2= Sozzi, G. O. |author3= Covatta, F. |author4= Fraschina, A. A. }}
• Borirenes, phosphirenes, and silirenes are boron-, phosphorus-, and silicon-substituted cyclopropenes, with the formula {{chem2|RBC2R'2, RPC2R'2}}, and {{chem2|R2SiC2R'2}}.
• Cyclopropene fatty acids a class of naturally occurring cyclopropenes.

{{Reflist}}

• {{Commonscatinline|Cyclopropene}}

{{cycloalkenes}}

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Category:Cycloalkenes

Category:Gases

Category:Annulenes