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.

Scheme 7.18 Mechanism of cross-metathesis using a chelated ruthenium catalyst [75].

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... 

Figure 7.20 3D structures of transition states leading to Z- and f-2-butene products in the homodimerization of propene catalyzed by an adamantyl-chelated ruthenium catalyst. 

In a subsequent report, Wang et al. [77] performed a DFT investigation on the homodimerization of 3-phenyl-l-propene, an experimentally used substrate, with a similar chelated ruthenium catalyst. These computations also agreed with the previous report in that the side-bound mechanism is favored. The same authors later reported a computational study on the same reaction, only using a chelated nitrato catalyst in place of the carboxylate-bound catalysts used in earlier calculations [78]. This study also found the reaction to occur via a side-bound mechanism. 

Put together, these computational studies and our computational results indicate that the chelated ruthenium catalysts shown in Scheme 7.16 all undergo side-bound olefin metathesis. The Z-selectivity is attributed to the ligand-substrate steric repulsions present in these side-bound transition states. 

Wang et al. [54, 79] also investigated the mechanism of initiation of the chelated ruthenium catalyst to form the active ruthenium alkylidene complex. 

In 2012, Grubbs and Houk et al. [76] investigated the decomposition pathways of the chelated ruthenium catalysts using both X-ray crystallography and DFT calculations. Several decomposition products for the C-chelated ruthenium catalysts have been observed under different conditions (Scheme 7.21) (a) when catalyst 22 was exposed to an excess of CO gas at -78 °C, an alkyl ruthenium complex (23)... 

Endo K, Grubbs RH. Chelated Ruthenium Catalysts for Z-Selective Olefin Metathesis. J dm Chem Soc. 2011 133(22) 8525-8527. 

---