Non-chelated ruthenium catalysts

As described in the previous sections, computational studies from various groups have indicated that the reaction of non-chelated ruthenium catalysts involves a 14e Ru-alkylidene complex as the active catalyst (Scheme 7.17). The olefin approaches the catalyst from the bottom (i.e., trans to the NHC ligand). The side-bound pathway, in which the olefin approaches cis to the NHC ligand, is disfavored. 

Scheme 7.17 Bottom- and side-bound pathways of olefin metathesis using the second-generation, non-chelated ruthenium catalysts.

Electronic preferences of this type are not present with the non-chelated ruthenium catalysts. As illustrated in Figure 7.19c, the NHC ligand without chelation preferentially adopts a conformation that positions the NHC re orbital perpendicular to the ruthenium-aikyiidene bond. Thus, in the bottom-bound transition... 

The thermal stabihty of complex 57a was found to be improved relative to the corresponding first-generation ruthenium-indenylidene complex 6 and Hoveyda catalysts 56 [70]. At 110 °C, complex 57a showed only 50% decomposition after 6 d, whereas 50% decomposition ofthe Hoveyda-I complex 56 was reached within 2 d. The non-chelated ruthenium-indenylidene 6 survived only a few hours at 80 C (Scheme 14.29) [70]. 

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