Origins of initiation rate differences in ruthenium olefin metathesis catalysts containing chelating benzylidenes

J Am Chem Soc. 2015 May 6;137(17):5782-92. doi: 10.1021/jacs.5b01144. Epub 2015 Apr 21.

Abstract

A series of second-generation ruthenium olefin metathesis catalysts was investigated using a combination of reaction kinetics, X-ray crystallography, NMR spectroscopy, and DFT calculations in order to determine the relationship between the structure of the chelating o-alkoxybenzylidene and the observed initiation rate. Included in this series were previously reported catalysts containing a variety of benzylidene modifications as well as four new catalysts containing cyclopropoxy, neopentyloxy, 1-adamantyloxy, and 2-adamantyloxy groups. The initiation rates of this series of catalysts were determined using a UV/vis assay. All four new catalysts were observed to be faster-initiating than the corresponding isopropoxy control, and the 2-adamantyloxy catalyst was found to be among the fastest-initiating Hoveyda-type catalysts reported to date. Analysis of the X-ray crystal structures and computed energy-minimized structures of these catalysts revealed no correlation between the Ru-O bond length and Ru-O bond strength. On the other hand, the initiation rate was found to correlate strongly with the computed Ru-O bond strength. This latter finding enables both the rationalization and prediction of catalyst initiation through the calculation of a single thermodynamic parameter in which no assumptions about the mechanism of the initiation step are made.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Alkenes / chemistry*
  • Benzylidene Compounds / chemistry*
  • Catalysis
  • Chelating Agents / chemistry*
  • Crystallography, X-Ray
  • Magnetic Resonance Spectroscopy
  • Models, Molecular
  • Molecular Structure
  • Organometallic Compounds / chemical synthesis
  • Organometallic Compounds / chemistry*
  • Quantum Theory
  • Ruthenium / chemistry*
  • Thermodynamics

Substances

  • Alkenes
  • Benzylidene Compounds
  • Chelating Agents
  • Organometallic Compounds
  • Ruthenium