Origins of Initiation Rate Differences in Ruthenium Olefin Metathesis Catalysts Containing Chelating Benzylidenes

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.