Simmons–Smith reaction

The Simmons - Smith reaction is a name reaction in organic chemistry, which was named after its developers Howard Ensign Simmons, Jr., and RD Smith. This chemical reaction is used for the synthesis of derivatives of cyclopropane. Is an addition reaction, with a carbene is added across a double bond without free carbene is present in the reaction mixture.

Overview reaction

An alkene reacts with the reagents diiodomethane and zinc to a cyclopropane and zinc iodide.

Mechanism

First of diiodomethane and excess activated zinc, which is available by use of the zinc - copper couple, a Zinkcarben ( carbenoid ) generated. This organozinc compound is similar to the Grignard reagent. Depending on the solvent, the substrate has a different structure, which corresponds to the Schlenk equilibrium in the Grignard reaction.

In the next reaction step, reacted with a carbenoid alkene. The methyl group of the carbenoid adds this in a concerted reaction to the double bond of the alkene. In a transient state, electrons are moved, after which cyclopropanecarboxylic and zinc iodide formed.

The Simmons-Smith reaction occurs stereospecifically. This means that to be obtained with the use of cis- alkenes cis cyclopropane derivatives, and the use of the trans - alkene the corresponding trans - derivatives. In place of the zinc - copper couple and diethylzinc can be used as the zinc source.

Contains the alkene used at a chiral center is a functional group that can form a coordinate bond with zinc ions, as may be reported induces a stereo information that leads to the preferential formation of one stereoisomer.

By sterically bulky radicals of attack can be prevented from this page and the cyclopropanation is favored by the other side instead.

Asymmetric Simmons-Smith reaction

By using chiral ligands, the Simmons-Smith reaction can also be carried out enantioselectively. This was achieved in 1992 using chiral disulfonamide ligands.

Use

The Simmons-Smith reaction is used for example for the preparation of spiro-compounds. The figure shows the synthesis of Rotan.

A further application is the indirect α - methylation of ketones.