Looking for opportunities to increase the stability of stereoretentive ruthenium catalysts, we obtained a series of complexes with a modified ether group in the benzylidene ligand. The best of these proved to be the catalyst Ru21 with a benzyl substituent on a chelating oxygen atom. Although this complex was still sensitive to the influence of atmospheric oxygen, its use typically led to expected products with yields higher than for the benchmark Ru5 and Ru7 while maintaining excellent selectivity. DFT calculations reveal that the rate-limiting step of the oxygen-driven decomposition is the oxygen attack on the Ru-C bond. The resulting transition state resembles the ruthenacyclobutane intermediate of the metathesis catalytic cycle and leads to Ru-C bond cleavage and the final, very stable Ru(IV)-oxo product. The rate of decomposition depends mainly on the NHC structure and the bulker the NHC, the slower the decomposition rate.