Metallocarbenes Schrock-type

Scheme 2.5 Schematic presentation of M-carbene bonding in Fischer-Tropsch and Schrock types of metallocarbenes.

We should emphasize that Fischer-Tropsch and Schrock type of carbenes are extreme representations of metal carbenoids [17-21]. Furthermore, representation of the metal-carbene bonds in terms of singlet and triplet carbenes is just a convenient way to describe these bonds. For example, both types of metallo-carbenes can be explained on the basis of a singlet carbene fragment very strong it-back donation would lead to the Schrock-type of metallocarbene, while no back donation or a weak n-back donation corresponds to the Fischer-Tropsch type of metallocarbene. Of course, it is also possible to understand both types of metallo-carbenes solely on the basis of the triplet carbene fragment. Once again, the use of either both models or only one of them to understand the nature of the M-carbene bond is just a matter of preference. 

Thus, the relatively weak Schrock-type oilandrelatively strong Fischer-Tropsch type of metallocarbenes are more attractive for the C-H, C=C, and C=C bond alkylation by diazocarbenes. The aforementioned delicate balance can be achieved by utilizing combination of... 

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. 

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