Coordinating functional groups ruthenium carbene

Oxygen-functional groups (e.g., ester, amide, ketone, and ether) are beneficial for the metathesis in some studies, while they occasionally have a negative influence on the reaction. It is suggested that both effects are caused by a coordination of these polar groups onto ruthenium of the generated carbene from Ru-catalyst and olefin of substrate. Therefore, when the chelate structure becomes too stable, Lewis acid such as Ti(OTr)4 is frequently employed as an additive (e.g., conversion of 3 into 4 in Scheme 24.3). In the case of more active [Ru]-II as a catalyst, Ti(0 Pr)4 is not necessarily required for the RCM of 5 ... 

Very recently, several five and six-coordinate ruthenium monocarbene complexes have also been structurally characterized by the groups of Che [141] and Miyamoto [142-144], X-ray crystal structure determinations revealed Ru = C distances of 1.806(3)-1.876(3) A, comparable to those in the previously reported analogues (Table 3). In one case, (TPFPP)Ru[C (p-C6H40Me)2], the complex is a one-dimensional coordination polymer [141]. In this particular complex, the carbene groups function as bridges through the carbene carbon atom and one of the two methoxy oxygen atoms. Many of these complexes were also characterized by H and C NMR [141]. 

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