Phosphine-Based <i>Z</i>‑Selective Ruthenium Olefin Metathesis Catalysts

Whereas a number
of highly Z-selective ruthenium-based
olefin metathesis catalysts bearing N-heterocyclic carbene ligands
have been reported in recent years, Z-selectivity
has so far been difficult to achieve for phosphine-based catalysts.
Guided by predictive density functional theory (DFT) calculations,
we have developed phosphine-based ruthenium olefin metathesis catalysts
giving 70–95% of the Z-isomer product in homocoupling
of terminal alkenes such as allylbenzene, 1-octene, allyl acetate,
and 2-allyloxyethanol. Starting from a moderately selective catalyst,
[P­(Cy)3]­(-S-2,4,6-Ph-C6H2)­ClRu­(CH-o-OiPrC6H4) (4, Cy = cyclohexyl, iPr = isopropyl), obtained by substituting a chloride of the Hoveyda–Grubbs
first-generation catalyst with 2,4,6-triphenylbenzenethiolate, we
moved on to replace Cl and PCy3 by chelating, anionic phosphine
ligands. Such ligands increase selectivity by limiting rotation around
the P–Ru bond and by specifically directing the steric bulk
of the phosphine substituents toward the selectivity-inducing thiolate
ligand. In particular, DFT calculations predicted that o-(dialkylphosphino)­phenolate ligands should improve selectivity
and activity compared to 4. The most promising of these
compounds (8b), based on the o-(di-tert-butylphosphino)­phenolate ligand, directs the
two P-bonded tert-butyl substituents toward the 2,4,6-triphenylbenzenethiolate
and has little steric hindrance trans to the thiolate.
This compound metathesizes terminal olefins such as allylbenzene and
1-octene with Z-selectivities above 80% and allylacetate
above 90%. Although these phosphine-based ruthenium monothiolate catalysts
in general achieve somewhat lower activities and Z-selectivities than their second-generation counterparts, they also
offer examples giving less substrate and product isomerization and
thus higher yields.