Z-Selective Alkene Isomerization by High-Spin Cobalt(II) Complexes
- 3 January 2014
- journal article
- research article
- Published by American Chemical Society (ACS) in Journal of the American Chemical Society
- Vol. 136 (3), 945-955
- https://doi.org/10.1021/ja408238n
Abstract
The isomerization of simple terminal alkenes to internal isomers with Z-stereochemistry is rare, because the more stable E-isomers are typically formed. We show here that cobalt(II) catalysts supported by bulky β-diketiminate ligands have the appropriate kinetic selectivity to catalyze the isomerization of some simple 1-alkenes specifically to the 2-alkene as the less stable Z-isomer. The catalysis proceeds via an "alkyl" mechanism, with a three-coordinate cobalt(II) alkyl complex as the resting state. β-Hydride elimination and [1,2]-insertion steps are both rapid, as shown by isotopic labeling experiments. A steric model explains the selectivity through a square-planar geometry at cobalt(II) in the transition state for β-hydride elimination. The catalyst works not only with simple alkenes, but also with homoallyl silanes, ketals, and silyl ethers. Isolation of cobalt(I) or cobalt(II) products from reactions with poor substrates suggests that the key catalyst decomposition pathways are bimolecular, and lowering the catalyst concentration often improves the selectivity. In addition to a potentially useful, selective transformation, these studies provide a mechanistic understanding for catalytic alkene isomerization by high-spin cobalt complexes, and demonstrate the effectiveness of steric bulk in controlling the stereoselectivity of alkene formation.Keywords
This publication has 70 references indexed in Scilit:
- Spin Crossover during β-Hydride Elimination in High-Spin Iron(II)– and Cobalt(II)–Alkyl ComplexesOrganometallics, 2013
- Chain-Walking Strategy for Organic Synthesis: Catalytic Cycloisomerization of 1,n-DienesJournal of the American Chemical Society, 2012
- Chelated Ruthenium Catalysts for Z-Selective Olefin MetathesisJournal of the American Chemical Society, 2011
- Stereoselectivity of supported alkene metathesis catalysts: a goal and a tool to characterize active sitesBeilstein Journal of Organic Chemistry, 2011
- Three-Coordinate and Four-Coordinate Cobalt Hydride Complexes That React with DinitrogenJournal of the American Chemical Society, 2009
- Cobalt‐Catalyzed Isomerization of 1‐Alkenes to (E)‐2‐Alkenes with Dimethylphenylsilylmethylmagnesium Chloride and Its Application to the Stereoselective Synthesis of (E)‐AlkenylsilanesChemistry – An Asian Journal, 2009
- Highly Z- and Enantioselective Ring-Opening/Cross-Metathesis Reactions Catalyzed by Stereogenic-at-Mo Adamantylimido ComplexesJournal of the American Chemical Society, 2009
- An active and selective alkane isomerization catalyst: iron- and platinum-promoted tungstated zirconiaChemical Communications, 2001
- Metal-Hydrogen Bond Cleavage Reactions of Transition Metal Hydrides: Hydrogen Atom, Hydride, and Proton Transfer ReactionsComments on Inorganic Chemistry, 1991
- Simplified method of calibrating thermometric nuclear magnetic resonance standardsAnalytical Chemistry, 1975