Alkene Metathesis Mechanism: Scission and Reformation of C=C Bonds

Abstract

Alkene or olefin metathesis reaction, catalyzed by organometallic complexes, involves the redistribution of groups at each end of the C=C double bond to produce new alkene molecules. The reaction mechanism involves two stages, that are intermediate formation and catalytic cycle. Each stage consists of two key steps namely ring-forming [2 + 2] cycloaddition and ring-opening fragmentation.

The first stage involves the initiation of the reaction by [2 + 2] cycloaddition of the starting alkene to the metallocarbene complex. There are two possibilities of cycloaddition reaction. Each results in the formation of an unstable four-membered metallacycle complex.

 

The fragmentation of each complex gives a metal alkylidene intermediate each with the corresponding parts of the starting alkene. These intermediates serve as the catalyst for the second stage of the mechanism.

               

In the catalytic cycle, each metal alkylidene intermediate reacts parallely with the starting alkene via ring-forming cycloaddition and rin-opening fragmentation. The reactions generate a new recombined alkene, and an ethylene molecule with simultaneous regeneration of metal alkylidene for the next catalytic cycle.

Protocol

Alkene metathesis occurs in two stages: Intermediate formation and catalytic cycle. Each stage comprises two steps: ring-forming [2 + 2] cycloaddition and ring-opening fragmentation.

In the first stage, the reaction is initiated by the [2 + 2] addition of the starting alkene to the metallocarbene complex.

There are two possible cycloadditions, each resulting in an unstable four-membered metallacycle complex.

The complexes undergo fragmentation to form corresponding metal alkylidene intermediates, each containing the resp.......