Cyclopropanation Reactions

CYCLOPROPANATION REACTIONS

Formation of two single bonds from a carbon atom is also well known for building up carbon skeletons. In this process, a three-membered ring is formed by reaction of a difunctional carbon atom with an olefin. Because of the strain of three-membered rings, their synthesis is not trivial and a small number of reactions which effectively append three-membered rings to molecules are important and widely used.

The Simmons–Smith cyclopropanation utilizes methylene diiodide and a zinc–copper couple to produce a carbenoid intermediate. This intermediate reacts with olefins to give cyclopropanes. The geometry of the double bond is preserved in the cyclopropane.

This addition is sensitive to steric biases in the olefin, and the methylene group will enter from the least hindered side of the molecule. Alcohol substituents in the olefin will facilitate the reaction and guide the methylene group syn to the alcohol.

The base-promoted α eliminations of chloroform or bromoform provide a simple method for the production of dihalocarbenes. These add readily to olefinic double bonds to give 1,1-dihalocyclopropanes. The halogens can be removed (one or both) by reduction; the most common method is to use tri-n-butyltin hydride.

Another common method for forming cyclopropanes is to react α-diazoketones or esters with olefins under the influence of copper or, better yet, rhodium or ruthenium catalysis. Again a metal carbenoid intermediate is produced which reacts with the olefin.

The importance of this strategy is that functionalized cyclopropanes are pro-duced which can be further manipulated. The process can also be carried out intramolecularly with high efficiency.