Grubbs’ ruthenium-carbene catalysts, transition

Olefin metathesis is a unique reaction and is only possible by transition metal catalysis. In fact only complexes of Mo, W, Re, and Ru are known to catalyze olefin metathesis. Once it was known that metallocarbenes were the actual catalytic species, a variety of metal carbene complexes were prepared and evaluated as catalysts. Two types of catalysts have emerged as the most useful overall. The molybdenum-based catalysts developed by Schrock and ruthenium-based catalysts developed by Grubbs. 

A significant development for the selective synthesis of alkenes makes use of alkene metathesis. Metathesis, as applied to two alkenes, refers to the transposition of the alkene carbon atoms, such that two new alkenes are formed (2.110). The reaction is catalysed by various transition-metal alkylidene (carbene) complexes, particularly those based on ruthenium or molybdenum. The ruthenium catalyst 84, developed by Grubbs, is the most popular, being more stable and more tolerant of many functional groups (although less reactive) than the Schrock molybdenum catalyst 85. More recently, ruthenium complexes such as 86, which have similar stability and resistance to oxygen and moisture as complex 84, have been found to be highly active metathesis catalysts. 

The theme of this chapter is that transition metals let you do things to organic molecules which are unthinkable without them. Nowhere is this more true than in metathesis reactions, and we finish the chapter with a reminder of the power of the ruthenium catalysts we introduced in Chapter 38. There we discussed the carbene-based mechanism of the reaction, and we showed you some simple examples such as this cyclization of a symmetrical amine to give a five-membered heterocycle using a catalytic amount of the ruthenium complex known as Grubbs 1 catalyst. 

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