Cyclopropanation

Cyclopropanation refers to any chemical process which generates cyclopropane rings. It is an important process in modern chemistry as many useful compounds bear this motif; for example pyrethroids and a number of quinolone based antibiotics (Ciprofloxacin, Sparfloxacin, etc.). However the high ring strain present in cyclopropanes makes them challenging to produce and generally requires the use of highly reactive species, such as carbenes, ylids and carbanions. Many of the reactions proceed in a cheletropic manner.

Several methods exist for converting alkenes to cyclopropane rings using carbene type reagents. As carbene's themselves are highly reactive it is common for them to be used in a stabilised form, referred to as a carbenoid.

In the Simmons–Smith reaction the reactive carbenoid is iodomethylzinc iodide, which is typically formed by a reaction between diiodomethane and a zinc-copper couple. This approach avoids the limitations and safety concerns associated with diazo compounds, which are commonly used in other carbenoid process. However, a major drawback is the high cost of diiodomethane. Modifications involving cheaper alternatives have been developed, such as dibromomethane or diazomethane and zinc iodide. The reactivity of the system can also be increased by exchanging the zinc‑copper couple for diethylzinc, however as this reagent is pyrophoric it must be handled carefully.

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