Introduction
Cyclopropanation is a chemical process that results in the formation of cyclopropane rings, denoted as (CH2)3. The high ring strain inherent in cyclopropanes makes their synthesis challenging, typically necessitating the use of highly reactive entities such as carbenes, ylids, and carbanions.
The Simmons–Smith reaction is a notable example of cyclopropanation, which employs iodomethylzinc iodide as the active carbenoid. This compound is usually produced by reacting diiodomethane with a zinc-copper couple. Cost-effective modifications have been developed that use more affordable reagents like dibromomethane or diazomethane along with zinc iodide. The reactivity of the system can be enhanced by replacing the zinc-copper couple with diethylzinc.
While free carbenes can be used for cyclopropanation reactions, their use is limited due to the difficulty in producing them in a stable form and the tendency for carbene dimerization. However, dihalocarbenes such as dichlorocarbene and difluorocarbene are relatively stable and can react to form geminal dihalo-cyclopropanes.
Reaction
The Simmons-Smith cyclopropanation reaction:
Cyclopropanation using free carbenes: