Cyclopropane

Cyclopropane is a cycloalkane molecule with the molecular formula C₃H₆, consisting of three carbon atoms linked to each other to form a ring, with each carbon atom bearing two hydrogen atoms. The bonds between the carbon atoms are a great deal weaker than in a typical carbon-carbon bond. This is the result of the 60° angle between the carbon atoms, which is far less than the normal angle of 109.5°. For bonds between atoms with sp³ hybridised orbitals. This angle strain has to be subtracted from the normal C-C bond energy, making the resultant compound more reactive than acyclic alkanes and other cycloalkanes such as cyclohexane and cyclopentane. This is the banana bond description of cycloalkanes.

There is also torsional strain because the hydrogen atoms are held in the eclipsed conformation.

However, cyclopropanes are more stable than a simple angle strain analysis would suggest. Cyclopropane can also be modeled as a three-center-bonded orbital combination of methylene carbenes. This results in the Walsh orbital description of cyclopropane, where the C-C bonds have mostly pi character. This is also why cyclopropanes often have reactivity similar to alkenes. This is also why carbenes can easily add into alkenes to produce cyclopropanes. Cyclopropanes taken to the extreme are tetrahedranes and propellanes.

Cyclopropane is an anaesthetic when inhaled, but has been superseded by other agents in modern anaesthetic practice. This is due to its extreme reactivity under normal conditions: when the gas is mixed with oxygen there is a significant risk of explosion.

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Safety

Because of the strain in the carbon-carbon bonds of cyclopropane, the molecule has an enormous amount of potential energy. In pure form, it will break down to form linear hydrocarbons, including "normal", non-cyclic propene. This decomposition is potentially explosive, especially if the cyclopropane is liquified, pressurized, or contained within tanks. Explosions of cyclopropane and oxygen are even more powerful, because the energy released by the formation of normal propane is compounded by the energy released via the oxidation of the carbon and hydrogen present. At room temperature, sufficient volumes of liquified cyclopropane will self-detonate. To guard against this, the liquid is shipped in cylinders filled with tungsten wool, which prevents high-speed collisions between molecules and vastly improves stability. Pipes to carry cyclopropane must likewise be of small diameter, or else filled with unreactive metal or glass wool, to prevent explosions. Even if these precautions are followed, cyclopropane is dangerous to handle and manufacture, and is no longer used for anaesthesia.

Cyclopropanes

Cyclopropanes are a class of organic compounds sharing the common cyclopropane ring, in which one or more hydrogens may be substituted. These compounds are found in biomolecules; for instance, the pyrethrum insecticides (found in certain Chrysanthemum species) contain a cyclopropane ring.

Organic synthesis

Cyclopropanes can be prepared in the laboratory by organic synthesis in various ways and many methods are simply called cyclopropanation:

• addition of zinc to 1,3-dichloropropane in the Freund reaction (1882)^[2]
• addition to an alkene of a zinc carbenoid in the Simmons-Smith reaction (1958) for example to cinnamyl alcohol.^[3] In one adaptation^[4] an amide is reacted with two equivalents of dichloromethane aided by titanium tetrachloride and magnesium:

a possible reaction mechanism for this cyclopropanation was proposed^[5]:

• addition to an alkene of a carbene such as dibromocarbene in the synthesis of propellane^[6] or methyl diazoacetate^[7]
• nucleophilic displacement of a leaving group by a carbon nucleophile in a 1,3 relationship, for example the synthesis of cyclopropylacetylene from 5-chloro-1-pentyne.^[8] Another example can be found in the Bingel reaction. An asymmetric reaction creating three stereocenters is demonstrated in a reaction of cyclohexenone with bromonitromethane assisted by trans-2,5-dimethylpiperazine as a base and a pyrrolidine based tetrazole organocatalyst^[9][10]:

• an intramolecular Wurtz coupling for example in the synthesis of bicyclo[1.1.0]butane^[11]
• Rearrangement reaction of certain cyclobutane compounds for instance the conversion of 1,2-cyclobutanediol to cyclopropanecarboxaldehyde^[12]
• photochemical rearrangement reaction of 1,4-dienes to vinylcyclopropanes in the di-pi-methane rearrangement^[13]

Organic reactions

Although cyclopropanes are formally cycloalkanes, they are very reactive due to considerable strain energy and due to double bond character.

• Cyclopropyl groups participate in cycloaddition reaction such as the formal [5+2]cycloaddition shown below:

This asymmetric synthesis is catalyzed by a rhodium BINAP system with 96% enantiomeric excess^[14].

• Cyclopropyl groups also engage in many rearrangement reactions. An extreme example is found in the compound bullvalene. A cyclopropane ring is an intermediate in the Favorskii rearrangement. Certain methylenecyclopropanes are found to convert to cyclobutenes^[15]:

This reaction is catalyzed by platinum(II) chloride in a carbon monoxide environment. The proposed reaction mechanism is supported by deuterium labeling^[16].

In another version of the same reaction^[17] the catalyst is PdBr₂ is prepared in situ from palladium(II) acetate and copper(II) bromide and the solvent is toluene.

References