Unexpected Latency of Z-Stereoretentive Ruthenium Olefin Metathesis Catalysts Bearing Unsymmetrical N-heterocyclic Carbene or Cyclic(alkyl)(amino)carbene Ligands

A set of ruthenium complexes bearing a CAAC or uNHC ligand and a dithiocatechol fragment have been obtained and characterized spectroscopically. The activity and Z-selectivity of the newly obtained catalysts were studied in selected model CM, self-CM, and RCM olefin metathesis reactions. Intriguingly, and in contrast to structurally related NHC-bearing catalysts Ru4a and Ru4b, the CAAC and uNHC analogues showed no or only very little activity in olefin metathesis. Interestingly, despite being not productive in metathesis reactions conducted in solution, Ru8 enabled the synthesis of a model 16-membered macrocyclic lactone of valuable musk smell with excellent chemoselectivity (no C-C double-bond migration was observed) at a concentration 40 times higher than that typically used by organic chemists in similar macrocyclizations (200 mM instead of 5 mM) with excellent Z-selectivity. Unfortunately, also here the conversion was low.