Metallocarbene terms

Hu and coworkers have examined N-H (and O-H and S-H) insertions in the presence of silver salts as well as with copper or rhodium catalysts with styryl diazoacetates (Schemes 8.18 and 8.19, Tables 8.9 and 8.10).47 Two possible products (114/117 and 115/118) were obtained that are derived from either direct insertion or insertion with net transposition (Schemes 8.18 and 8.19). Silver and copper salts tended to favor transposition (Table 8.9, entries 2-5 Table 8.10, entries 2-9), whereas rhodium favored direct insertion (Table 8.9, entry 1 Table 8.10, entry 1). The selectivity differences between the two products were again rationalized in terms of two mechanistic pathways. In the case of rhodium-based catalysts, it was proposed that the reaction occurs via a metallocarbene, whereas with copper and silver catalysts the reaction was interpreted as proceeding by Lewis acid activation. 

On palladium, demethylation (primary-secondary or primary-tertiary C-C bond rupture) is the major hydrocracking reaction. On platinum, the demethylation is still favored, but the other cleavage modes (secondary-secondary and secondary-tertiary C-C bond rupture) become appreciable. On iridium, deethylation predominates, while on nickel, the initial hydrocracking distribution includes a large excess of methane relative to the simple demethylation and deethylation. Finally, on cobalt, extensive cracking to methane accounts for 100% of the overall reaction. These results may be rationalized in terms of metallocarbene chemistry and of the capacities of the different metals to form metallocarbenes. 

The massive synthetic potential of the olefin metathesis reaction has yet to be realized in terms of generality and yields. However, since the metallocarbene addition mechanism has been largely accepted, more useful mechanistic studies have appeared this year. The whole area has been adequately reviewed by the timely publication of the proceedings of the international symposium held at Noordwijkerhout in September 1977. The synthetic utility of this transformation has now been extended by tungsten catalysis to the functionalized olefins of aj-olefinic esters (49), to give reasonable yields (35—40%) of the products as isomeric mixtures (50) in which the trans-isomer predominates. Small amounts of the chlorinated starting materials (51) are also present. The complex tungsten... 

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. 

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