Grubbs-type, Ru-based systems

Second-generation catalysts supported through the NHC ligand, reported by 

Blechert [24] followed this initial report with a second, where the first example of a supported Grubbs-Hoveyda catalyst (5) was described. Catalyst 12 displayed better recyclability in RCM, although again, no leaching data was presented. The authors additionally noted that the high activity observed in RCM did not translate to the more challenging cross metathesis (CM) reactions of electron-deficient alkenes, where the catalyst displayed reduced activity. 

Numerous other examples of supported catalyst systems have since been described in the literature from Shi [29], Grubbs [30], Thieuleux and Coperet [31, 32], and Pleixats and Wong Chi Man [33, 34]. All of these examples describe some form of a silica-supported analog, either from grafting onto commercial silica, or through the preparation of a hybrid organic-inorganic material prepared by sol-gel chemistry, with the functionality incorporated into the silica matrix. 

The authors speculated that the observed difference in activity was the result of the increased stability available to 24 due to its flexible tether and ability to interact with the silica surface via stabilizing Ru-O interactions similar to those present in a Grubbs-Hoveyda-type catalyst. This provided a means of stabilizing the reactive, 14-electron intermediate from 24 that was not available to the analogous, active species from 25, which in turn led to an increased catalyst lifetime for hybrid catalyst 24. 

In a differing approach to the development of insoluble, supported catalytic materials, the use of soluble polymer supports offers a possible method to facilitate the separation, subsequent recovery, and reuse of catalyst complexes. It is important to note that this often requires relatively large quantities of a non-solvent for the precipitation and recovery of the catalytic material. This is an obvious limitation, as the excess waste generates an environmental concern however, other techniques can be employed to facilitate the recovery. Examples of this include liquid/liquid phase separations or the selective precipitation of the product or catalyst from the reaction media through the use of different stimuli, such as temperature or pH. 

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