Sterically Driven Olefin Metathesis: The Impact of Alkylidene Substitution on Catalyst Activity

The dramatic reactivity difference
between the Grubbs metathesis
catalysts and their resting-state methylidene derivatives was probed
in an integrated crystallographic, solid-state NMR and localized molecular
orbital analysis study. A principal focus was the second-generation
Grubbs system RuCl2(H2IMes)­(PCy3)­(CHR) (GII, R = Ph; GIIm, R = H); supporting studies were carried out with the first-generation
species RuCl2(PCy3)2(CHR)
(GI, GIm). The compiled rate constants for
PCy3 dissociation demonstrate the limited lability of the
methylidene complexes (e.g.,
ca. 275-fold lower for GIIm than GII and
nearly 2000 times lower for the IMes analogue GIIm′). This is important because it impedes catalyst re-entry from the
resting state into the active cycle. The 31P chemical shift
(CS) tensors for the PCy3 ligand exhibited the expected
changes (i.e., those characteristic of an increased Ru–P orbital
interaction) in GIIm relative to GII, as
did GIm vs GI. Greater insight was offered
by the 13C CS tensors. Whereas calculations on truncated
models predict significant differences in 13C CS tensor
values for GII compared with GIIm, the experimental
values are equivalent, implying a compensating effect that weakens
the RuC interaction in the benzylidene complex. Published
X-ray crystallographic parameters for GII and GI reveal that one chloride ligand is displaced below the basal plane
by steric interactions with the benzylidene phenyl group, an effect
absent in GIIm and GIm. During PCy3 loss from the [Ru]CHPh systems, established processes of
alkylidene rotation transform Ph–Cl repulsion into Ph–PCy3 repulsion. Displacing the PCy3 ligand below the
plane does not relieve this conflict, instead incurring steric interactions
with the H2IMes ligand. Enhanced PCy3 lability
in the benzylidene complexes, relative to their methylidene analogues,
is hence proposed to originate in the steric pressure exerted by the
Ph substituent.