Metathesis reactions have had a tremendous impact in organic synthesis, enabling a variety of applications in polymer chemistry, drug discovery and chemical biology.
Although challenging, the possibility to perform aqueous metatheses has become an attractive alternative, not only because water is a more sustainable medium, but also to exploit biocompatible conditions.
Such cycloaddition reactions between two alkenes to give cyclobutanes is symmetry forbidden and occurs only photochemically.
However, the presence of d-orbitals on the metal alkylidene fragment breaks this symmetry and the reaction is quite facile.
This metallacycle then breaks up in the opposite fashion to afford a new alkylidene and new olefin.
If this process is repeated enough, eventually an equilibrium mixture of olefins will be obtained.
Polymerization of terminal acetylenes is complicated by the potential for the R group to insert alpha or beta with respect to the metal.
It is extremely challenging to always get a beta insertion and generate a polymer with reproducible properties.
Two important features of these catalysts are that they are 100% active and have been fully characterized by NMR and X-ray crystallography.
The success of these catalysts stems from their coordinative and electronic unsaturation (making them electrophilic) and their bulky ligands (prevents bimolecular decomposition).