Ethylene-Triggered Formation of Ruthenium Alkylidene from Decomposed Catalyst

Ethylene
is known to readily decompose ruthenium-based olefin metathesis
catalysts, such as Grubbs second-generation catalyst (GII), by forming the unsubstituted ruthenacyclobutane (Ru-2) that may undergo a 1,2-H shift and liberate propene. The resulting
alkylidene loss has been assumed to be irreversible. Yet, by reacting
(SIMes)­(η6-p-cymene)­RuCl2 (1), the p-cymene-stabilized alkylidene-free
fragment resulting from loss of propene from Ru-2, with
ethylene, we show that the methylidene analogue of GII (GIIm) and other Ru alkylidenes are formed, along with
catalytic amounts of propene and butenes, and can be stabilized by
tricyclohexylphosphine (PCy3) at 50 °C in C6D6. An almost 20-fold increase in activity for ring-closing
metathesis of diethyl diallylmalonate (DEDAM) on pretreatment of 1 with ethylene suggests that the reversibility of the alkylidene
loss may be used to develop longer-lived metathesis catalysts and
processes. Mechanistic density functional theory (DFT) calculations
suggest that the connection between 1 and GIIm involves oxidative coupling of two ethylene molecules to form a
key metallacyclopentane intermediate (M49). A 1,2-H shift
in M49 gives the methyl-substituted ruthenacyclobutane M303, which, on cycloreversion, liberates propene and GIIm. Alternatively, successive H-shifts starting in M49 may give 1-butene (fast reaction) and 2-butene (slower)
with a lower barrier than that of Ru alkylidene. The lower predicted
barrier is consistent with butene, especially 1-butene, being the
dominating product at the start of the experiments, in particular
at lower temperature.