Regarding a Persisting Puzzle in Olefin Metathesis with Ru Complexes: Why are Transformations of Alkenes with a Small Substituent <i>Z</i>‑Selective?

An enduring question in olefin metathesis is that reactions carried out with widely accessible Ru dichloro complexes, which typically favor <i>E</i> alkenes, generate <i>Z</i> isomers preferentially when substrates bearing a smaller substituent are used; <i>Z</i> enol ethers, alkenyl sulfides, 1,3-enynes, alkenyl halides, or alkenyl cyanides can be prepared reliably with reasonable efficiency and selectivity. Transformations thus proceed via the more hindered <i>syn</i>-substituted metallacyclobutanes, which is mystifying because catalyst features implemented in the more recently developed and broadly applicable <i>Z</i>-selective catalysts are absent in the Ru dichloro systems. Herein, we describe experimental and computational investigations that offer a plausible rationale for these puzzling selectivity trends. The following will be demonstrated. (1) Kinetic <i>Z</i> selectivity depends on the relative barrier for olefin association/dissociation versus metallacyclobutane formation/cleavage. There can be appreciable stereochemical control when metallacyclobutane formation/breakage is turnover-limiting. (2) Stereoelectronicnot purely stericeffects are central: achieving the p-orbital overlap needed for alkene formation while minimizing steric repulsion between the incipient olefin substituent and a catalyst’s anionic ligand during the cycloreversion step is crucial. We show that similar stereoelectronic factors are probably operative in the more recently introduced <i>Z</i>-selective (and enantioselective) olefin metathesis transformations promoted by stereogenic-at-Ru complexes containing a bidentate aryloxide ligand