Origins of Carbene-Metal Complexes

For the purpose of examining potential routes to the formation of the initial carbene-metal entity, it is convenient to classify catalysts into three major, albeit rather arbitrary, categories. The first group comprises catalyst systems utilizing a relatively stable and well-characterized car-bene which is attached to the metal. The second class consists of systems that are activated by organometallic cocatalysts which presumably form cr-bonded R—M transients, wherein M is the transition metal. The third category includes catalyst combinations that do not involve a prior car-bene or an organometallic component. 

Catalysts Possessing Stable Carbene-Metal Ligands 

Following Casey s experiment (5) depicted in Eq. (4), Katz and coworkers (26, 27) demonstrated that the stable W(0) carbene complex (CO)5W=CPh2 may also be employed as a metathesis catalyst. 

In comparison with other catalysts, (diphenylcarbene)tungstenpenta-carbonyl is surprisingly sluggish (26). This observation is significant as it relates to diverse views regarding the carbene-to metallocyclobutane interconversion. Whereas Casey emphasizes a need to accommodate the incoming olefin within the coordination sphere of the metal prior to rearrangement to a metallocycle [Eq. (5)], Katz (28) has described the process essentially as a dipolar attack of a polarized carbene-metal (R2C+—M-) on the olefin [Eq. (6)]. The latter does not specify a need for w-complexation of the olefin as a precondition to metathesis. 

Casey s scheme calls for an empty coordination site on the metal in addition to the complexed carbene. Recently, Casey demonstrated that carbonyls of (CO)5W=CPh2 are vulnerable to exchange processes under relatively mild reaction conditions (29)  

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