Cyclopropene

Cyclopropene is the simplest cycloalkene. It has a triangular structure and chemical formula 34. While the simplest, it is one of the more unstable cycloalkenes due to the high ring strain. Because of this ring strain cyclopropene is both difficult to prepare and interesting to study.

Early syntheses
The first confirmed synthesis of cyclopropene, carried out by Dem'yanov and Doyarenko, involved the thermal decomposition of trimethylcyclopropylammonium hydroxide over platinized clay at 320-330 °C under a CO2 atmosphere. This reaction produces mainly trimethylamine and dimethylcylcopropyl amine, together with about 5% of impure cyclopropene. Cyclopropene can also be obtained in about 1% yield by thermolysis of the adduct of cycloheptatriene and dimethyl acetylenedicarboxylate.

Modern syntheses from allyl chlorides
Allyl chloride undergoes dehydrohalogenation upon treatment with the base sodium amide at 80 °C to produce cyclopropene in ~10% yield.
 * CH2=CHCH2Cl +  NaNH2  →  C3H4 (cyclopropene)  +  NaCl  +  NH3

The major byproduct of the reaction is allyl amine. Adding allyl chloride to sodium bis(trimethylsilyl)amide in boiling toluene over a period of 45-60 min. produces the targeted compound in about 40% yield with an improvement in purity:
 * CH2=CHCH2Cl + NaN(TMS)2 → C3H4 (cyclopropene) +  NaCl  +  NH(TMS)2

1-Methylcyclopropene is synthesized similarly but at room temperature from methallylchloride using phenyllithium as the base:
 * CH2=C(CH3)CH2Cl +  LiC6H5 → CH3C3H3 (1-methylcylopropene)  +  LiCl  +  C6H6

Derivative syntheses
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 following reaction. Using copper sulfate as the catalyst, the addition of carbenes derived from ethyl diazoacetate to acetylene yielded the appropriate cyclopropene. By this route, 1,2-dimethylcyclopropene was formed by addition of methylene to 2-butyne, and 1,2-dimethylcyclopropene-3-carboxylate was made by adding carbomethoxycarbene to 2-butyne. Copper has proved to be useful as a catalyst in a variety of cyclopropene syntheses. Copper sulfate and copper dust are among the more popular forms of copper used.

Chemical reactions of cyclopropene and its derivatives
Studies on cyclopropene mainly focus on the consequences of its high ring strain, and the release thereof. At 425 °C, cyclopropene isomerizes to methylacetylene.
 * C3H4 → H3CCCH

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[3.2.1.02,4]oct-6-ene. This reaction is commonly used to check for the presence of cyclopropene, following its synthesis.

Applications of cyclopropenes
1-Methylcyclopropene (1-MCP) is used to inhibit fruits' and flowers' sensitivity to ethylene, particularly apples, thus slowing the process of ripening in fruits. It works by inhibiting the perception of ethylene, which results in the reduction of respiration, aroma production, and softening, all signs of ripening and is used in the floral industry to inhibit floral senescence [see refs 6, 7]. While simple exposure to 1-MCP is enough to begin the inhibiting process, optimal conditions found for apples call for air concentration of about 0.25 to 1 ppm, temperatures slightly above room temperature, and an exposure time of 12-16 h. SmartFresh is a product based on this cyclopropene and in use by the produce industry.