Pentacoordinate Ruthenium(II) Catecholthiolate and Mercaptophenolate Catalysts for Olefin Metathesis: Anionic Ligand Exchange and Ease of Initiation

摘要

The investigations disclosed offer insight regarding several key features of Ru-based catecholthiolate olefin metathesis catalysts. Factors influencing the facility with which the two anionic ligands undergo exchange and those that affect the rates of catalyst release are elucidated by examination of more than a dozen new complexes. These studies shed light on how different chelating groups can influence Ru-S bond strength and, as a result, the facility of catecholthiolate rotation. The trans influence series ether < ester approximate to iodide < amine approximate to thioether approximate to olefin < isonitrile approximate to phosphite has been established through X-ray structural analysis and shown to correlate well with the barrier for catecholthiolate rotation (trans effect) determined by variable-temperature NMR experiments and computational studies (DFT). It is found that, apart from electronic factors, chelate geometry has a more notable effect on the rate of catalyst release (five- vs six-membered chelate ring and mono vs bidentate ligand). Polytopal processes involving pentacoordinate Ru(II) carbene complexes are shown to be distinct from previously reported fluxional events that involve tetracoordinate species and which are capable of causing diminished polymer syndiotacticity. Ru mercaptophenolate complexes have been synthesized and isolated as a single diastereomer (O-C trans to the NHC). This latter set of species promotes representative olefin metathesis reactions readily and gives Z selectivity levels that are higher than those when the corresponding catecholate systems are used, but less so in comparison to catecholthiolate complexes. A rationale for variations in stereoselectivity is presented.