Unfortunately, the commercially available Z-selective complex Ru1 failed to produce even traces of macrocycle 2, which can be attributed to its instability under reactive distillation conditions (Scheme 1A). Interestingly, the stereoretentive complex Ru2 produced a larger amount of the desired Z-isomer as compared to previously studied Z-selective molybdenum-based catalysts. Namely, the mRCM reaction of 1 conducted in PAO-6 at 110 °C with 10 mol% of Ru2 led to 2 in 52% of yield as a 58:42 Z/E mixture (Scheme 1A). However, it was reported that similar macrocyclic musk has been previously prepared using Ru2 under standard high-dilution conditions in a solution (3 mM in THF, 40 °C, 1 h) offering much higher Z-selectivity level (up to 95:5 Z/E). Despite the non-perfect selectivity exhibited by Ru2 (58:42 Z/E) in the high-concentration mRCM reaction we have seen this result as the most promising, and therefore decided to focus on the Zstereoretentive class of Ru olefin metathesis catalysts. Apparently, rather harsh conditions of high-temperature reactive distillation require significantly more stable catalysts to allow high stereocontrol in the metathesis step. The mechanism of stereoretentive catalysts degradation in the presence of terminal olefins via 1,2-sulphide shift was studied in detail. According to Hoveyda et al. the reactive alkylidene-ruthenium species formed due to the Chauvin mechanism are attacked by the sulphide anion of the dichlorocatecholthiolate ligand placed opposite to the NHC ligand (trans effect). This 1,2-sulphide shift leads to the generation of catalytically inactive Sruthenium complex (Scheme 1C). Logically, we reasoned that by replacing the strong nucleophilic thiocatecholate dianion T (such as in Ru2) by a weaker nucleophile stabilised by resonance may perhaps render such designed disulphide ligand less prone to trigger catalysts decomposition via 1,2-sulphide shift mechanism. At the same time, we were afraid that the higher stability can come at the cost of lover catalyst activity and selectivity, as it was reported before. pg. 3 of 19 Based on basic textbook knowledge and supported by Density Functional Theory (DFT) calculations, we selected a 2,3-dithioquinaxoline as a precursor for such resonance stabilised ligand (Q), leading to a likely thermally more stable complex Ru3 (Scheme 1B). In fact, nucleophilicity of the sulphide atoms is seen reduced when looking at the corresponding natural charges of each ligand (from -0.44 e -in T to -0.39 e -in Q). A decrease on the Mulliken charges on sulphur (anionic species) is observed in the following order: dithiocatecholate (-0.633258), dichlorodithiocatecholate T (-0.549249) and dithioquinoxaline Q (-0.504840). This can be associated to the increased resonance of the new system, already demonstrated in the NBO analysis (see SI for further details). As a result the delocalization of the charge onto the additional aromatic ring of dithioquinoxaline, absolute hardness of the ligand (η), that is the half of the HOMO-LUMO gap, increases by 0.035 eV for Ru3 with respect to Ru2, particularly for the further stabilization of the HOMO orbital (0.078 eV) compared to the one of the LUMO (0.043; Scheme 1D).