Regarding a Persisting Puzzle in Olefin Metathesis with Ru Complexes: Why are Transformations of Alkenes with a Small Substituent Z-Selective?

摘要

An enduring question in olefin metathesis is that reactions carried out with widely accessible Ru dichloro complexes, which typically favor E alkenes, generate Z isomers preferentially when substrates bearing a smaller substituent are used; Z 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 syn-substituted metallacyclobutanes, which is mystifying because catalyst features implemented in the more recently developed and broadly applicable Z-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 Z 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 Z-selective (and enantioselective) olefin metathesis transformations promoted by stereogenic-at-Ru complexes containing a bidentate aryloxide ligand.