Bimolecular Coupling as a Vector for Decomposition of Fast-Initiating Olefin Metathesis Catalysts

The correlation between
rapid initiation and rapid decomposition
in olefin metathesis is probed for a series of fast-initiating, phosphine-free
Ru catalysts: the Hoveyda catalyst HII, RuCl2(L)­(CHC6H4-o-OiPr); the Grela catalyst nG (a
derivative of HII with a nitro group para to OiPr); the Piers catalyst PII, [RuCl2(L)­(CHPCy3)]­OTf; the third-generation
Grubbs catalyst GIII, RuCl2(L)­(py)2(CHPh); and dianiline catalyst DA, RuCl2(L)­(o-dianiline)­(CHPh), in all of
which L = H2IMes = N,N′-bis­(mesityl)­imidazolin-2-ylidene.
Prior studies of ethylene metathesis have established that various
Ru metathesis catalysts can decompose by β-elimination of propene
from the metallacyclobutane intermediate RuCl2(H2IMes)­(κ2-C3H6), Ru-2. The present work demonstrates that in metathesis of terminal olefins,
β-elimination yields only ca. 25–40% propenes for HII, nG, PII, or DA, and none for GIII. The discrepancy
is attributed to competing decomposition via bimolecular coupling
of methylidene intermediate RuCl2(H2IMes)­(CH2), Ru-1. Direct evidence for methylidene coupling
is presented, via the controlled decomposition of transiently stabilized
adducts of Ru-1, RuCl2(H2IMes)­Ln(CH2) (Ln = pyn′; n′ = 1, 2, or o-dianiline). These adducts were synthesized by treating in situ-generated
metallacyclobutane Ru-2 with pyridine or o-dianiline, and were isolated by precipitating at low temperature
(−116 or −78 °C, respectively). On warming, both
undergo methylidene coupling, liberating ethylene and forming RuCl2(H2IMes)­Ln. A mechanism is proposed
based on kinetic studies and molecular-level computational analysis.
Bimolecular coupling emerges as an important contributor to the instability
of Ru-1, and a potentially major pathway for decomposition
of fast-initiating, phosphine-free metathesis catalysts.