Ethylene-Triggered Formation of Ruthenium Alkylidene from Decomposed Catalyst

Wietse Smit,Marco Foscato,Vidar R Jensen,Giovanni Occhipinti

doi:10.1021/acscatal.0c02206

Abstract

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.

Highlights

Ethylene reduces the stability of any Ru alkylidene formed, this substrate facilitates alkylidene observation and analysis of the reaction mixture by not isomerizing via double-bond migration and by being invisible in self-metathesis
Propene is likely to originate from ethenolysis of 2-butene (Scheme 2) and from decomposition of Ru-2 via Scheme 1 and is a known indicator of ruthenium alkylidene
We have shown that ruthenium methylidenes GIIm and GIm form on reaction of PCy3 and ethylene, a known ruthenium alkylidene metathesis catalyst poison, with 1

Summary

■ INTRODUCTION

Olefin metathesis has evolved to become one of the broadest applicable catalytic technologies for the assembly of carbon−carbon bonds,[1] popular, functional-group-tolerant, and easy-to-handle ruthenium-based catalysts such as Grubbs second-generation catalyst (GII, Scheme 1)[2] suffer from a vulnerability that limits their further industrial uptake in valorization of renewable feedstocks and production of natural products and pharmaceuticals:[3−6] the low stability of the key catalytic intermediates, which typically results in catalyst decomposition after only a few thousand turnovers.[7−11] rapid decomposition is observed with ethylene as a substrate,[12−14] which so far has precluded the use of this, the smallest and most atom-economic coupling partner in metathesis “cracking” of unsaturated plant oils.[4,15−17]. Barriers for the key hydride transfer reactions of a Ru-bound s-cis-butadiene complex were found to be lower than those of the corresponding s-trans complex (see the Supporting Information), but the s-cis-butadiene complex could so far not be connected to the rest of the reaction network with sufficiently low barriers to be competitive Apart from this lack of connection in the cis-formation pathway, the calculations reflect the experiments and offer mechanistic explanations for all of the key observations: the fact that 1-butene is the first and major product at low temperatures and the somewhat slower formation of 2-butene, the promoting effect of temperature on the alkylidene formation, and the intimate connection between alkylidene and propene formation. Once alkylidene has been formed, propene is produced catalytically via ethenolysis of 2-butene

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES

A General Decomposition Pathway for Phosphine-Stabilized Metathesis Catalysts