Z has become an ever more important letter for olefin metathesis with the advent of a ruthenium catalyst that is highly selective for preparing Z (cis) alkenes (J. Am. Chem. Soc., DOI: 10.1021/ja202818v). In the beginning, nearly all ring-opening and cross-metathesis catalysts predominantly yielded E (trans) alkenes. That began to change when MIT’s Richard R. Schrock and Boston College’s Amir H. Hoveyda led a team that developed highly Z-selective molybdenum and tungsten monoaryloxide-pyrrolide catalysts (C&EN, Sept. 13, 2010, page 33). Caltech’s Robert H. Grubbs and coworkers subsequently reported a ruthenium version featuring an N-heterocyclic carbene (NHC) ligand, but its Z-selectivity wasn’t quite on par with the Schrock-Hoveyda catalysts (C&EN, Nov. 15, 2010, page 37). Grubbs and Koji Endo are now reporting a revised version in which an adamantyl substituent on the NHC ligand is additionally coupled to the ruthenium center (shown above, R = tert-butyl). The resulting chelated structure improves the catalyst’s selectivity so that it’s comparable with molybdenum and tungsten catalysts. All of the Z-selective catalysts are welcome, Grubbs and Endo note, because they are each distinctly versatile for preparing complex natural products and stereoregular polymers.