Influence of Substituents in a Six-Membered Chelate Ring of HG-Type Complexes Containing an N→Ru Bond on Their Stability and Catalytic Activity

An efficient approach to the synthesis of olefin metathesis HG-type catalysts containing an N→Ru bond in a six-membered chelate ring was proposed. For the most part, these ruthenium chelates can be prepared easily and in high yields based on the interaction between 2-vinylbenzylamines and Ind II (the common precursor for Ru-complex synthesis). It was demonstrated that the increase of the steric volume of substituents attached to the nitrogen atom and in the α-position of the benzylidene fragment leads to a dramatic decrease in the stability of the target ruthenium complexes. The bulkiest (i)Pr substituent bonded to the nitrogen atom or to the α-position does not allow the closing of the chelate cycle. N,N-Diethyl-1-(2-vinylphenyl)propan-1-amine is a limiting case; its interaction with Ind II makes it possible to isolate the corresponding ruthenium chelate in a low yield (5%). Catalytic activity of the synthesized complexes was tested in RCM reactions and compared with α-unsubstituted catalysts obtained previously. The structural peculiarities of the final ruthenium complexes were thoroughly investigated using XRD and NMR analysis, which allowed making a reliable correlation between the structure of the complexes and their catalytic properties.