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

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 ≈ iodide < amine ≈ thioether
≈ olefin < isonitrile ≈ 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.