Cyclopropenes

Cyclopropenes are hydrocarbons in which a three-membered ring contains a carbon–carbon double bond. The parent compound cyclopropene has the formula C3H4 and is the simplest member of the cyclopropene family. The double bond is part of a very small ring, which imposes substantial angle strain and leads to high reactivity compared with larger cycloalkenes.

Synthesis of cyclopropenes typically involves transforming alkynes or vinyl precursors. A common route is cyclopropanation of alkynes with carbenes or carbenoids (for example, dichlorocarbene) to give vinylcyclopropane derivatives, which can be converted to cyclopropenes by dehalogenation or dehydrogenation. Other methods include dehalogenation of 1,1-dihalocyclopropanes or rearrangements of cyclopropadienes. These routes take advantage of the inherent strain and reactivity of small rings.

Reactivity: The combination of ring strain and the endocyclic double bond makes cyclopropenes highly reactive toward additions and cycloadditions. They readily participate in transformations such as cycloadditions with dienes or dienophiles, ring-opening reactions to give allyl-type products, and various catalytic processes to construct more complex frameworks. Substitution on the ring can modulate stability and reactivity.

Applications: Cyclopropenes are valuable building blocks in organic synthesis and materials chemistry. Substituted cyclopropenes are used to access larger molecules and in some contexts as reactive partners in bioorthogonal ligations or strain-promoted reactions. Their chemistry illustrates how extreme ring strain affects the behavior of small-ring alkenes.