Olefin metathesis catalysts, modifications on the NHC ligand

Olefin metathesis is one of the most important synthetic tools, both in organic and olefin chemistry. Olefin metathesis allows to shorten production time of organic compounds and it is characterized by a smaller amount of by-products and waste in comparison with the classical ways of the synthesis of these compounds. Simultaneously, the conditions at which metathesis is performed are environmentally friendly. This method allows obtaining very valuable compounds, such as: anti-inflammatory agents, fungicides and many others. This causes that olefin metathesis has an important place in the organic synthesis and develops constantly. The first catalysts had undefined structure, therefore, it was difficult to predict the mechanism of their action. Big development metathesis came with the synthesis of catalysts with a clearly defined structure. This article explains what olefin metathesis is and shows applications of olefin metathesis. This work presents catalysts, which possess transition metal in the structure, such as: ruthenium, molybdenum or tungsten. It also shows the comparison of catalysts possess ruthenium (Grubbs catalyst first and second generation and Hoveyda-Grubbs catalyst) taking into account their structure and properties. This work describe carbenes, which improve thermal stability of catalysts compare with complexes containing phosphine. Researchers receive new compounds through modifications on the NHC ligand. In this work are presented modifications of the NHC ligand complexes of ruthenium described in the literature and their influence on such catalytic properties as: activity, selectivity, and efficiency. Modifications concern: heteroatoms, bulky groups and groups, which change electron density. Despite many studies and receiving large amounts of catalysts for this reaction, still a problem is the obtaining of an universal catalyst for all types transitions, that would have a high activity, high efficiency and which could be easily recovered after the reaction