Herrmann-Type, BisNHCs

As was discussed with precatalyst 36, NHCs can be employed as the dissociating ligand. The NHC is reputed to be a strong a-donor ligand and, therefore, a poor leaving group. In precatalyst 2, the Cyj P exclusively dissociates relative to the 

H2lMes ligand. In a bis(NHC) complex, one NHC must be rendered as a better leaving group to allow dissociation to form a 14-electron, reactive intermediate. 

The initiation of the Grubbs precatalysts is affected by many factors, but the structure of the precatalyst itself plays the paramount role in determining reactivity. In the evaluation of new catalysts, the initiation process describes the temporal relationship between a precatalyst and its active form. The development of a perfect catalyst is a difficult proposition because contradictory attributes must be simultaneously optimized, and particular applications place certain demands upon the catalyst. In some cases, precatalysts are completely dormant, and are only initiated under the appropriate conditions (e.g.. Section 9.7). Ultimately, initiation rate is related to the required catalyst loading for an application. If initiation is slow, either due to the structure of the precatalyst or the reaction conditions, then higher loadings of precatalyst may be needed. 

The Grubbs pyridine solvates have proven valuable in materials applications and as synthetic intermediates. These solvates are not particularly shelf-stable, and they easily lose one pyridine in solution. However, they are well-suited toward applications involving electron-poor alkenes (e.g., acrylonitrile), where their rapid initiation and lack of phosphine to intercept the intermediate carbenes are particularly kinetically advantageous. 

The Piers catalyst has played a leading role in the identification of ruthenacy-clobutane intermediates. This precatalyst is distinctive from the others because it is a stable, 14-electron complex, and can be considered to be pre-initiated since no ligand dissociation is needed to enter the first turn of the catalytic cycle. 

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Scheme 9.4 Protonation of a latent precatalyst Herrmann-Type, BisNHCs [45].

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