Bimolecular Coupling in Olefin Metathesis: Correlating Structure and Decomposition for Leading and Emerging Ruthenium−Carbene Catalysts

Abstract

Bimolecular catalyst decomposition is a fundamental, long-standing challenge in olefin metathesis. Emerging ruthenium–cyclic(alkyl)(amino)carbene (CAAC) catalysts, which enable breakthrough advances in productivity and general robustness, are now known to be extraordinarily susceptible to this pathway. The details of the process, however, have hitherto been obscure. The present study provides the first detailed mechanistic insights into the steric and electronic factors that govern bimolecular decomposition. Described is a combined experimental and theoretical study that probes decomposition of the key active species, RuCl 2 (L)(py)(=CH 2 ) 1 (in which L is the N-heterocyclic carbene (NHC) H 2 IMes, or a CAAC ligand: the latter vary in the NAr group (NMes, N-2,6-Et 2 C 6 H 3 , or N-2-Me,6- i PrC 6 H 3 ) and the substituents on the quaternary site flanking the carbene carbon (i.e., CMe 2 or CMePh)). The transiently stabilized pyridine adducts 1 were isolated by cryogenic synthesis of the metallacyclobutanes, addition of pyridine, and precipitation. All are shown to decompose via second-order kinetics at −10 °C. The most vulnerable CAAC species, however, decompose more than 1000-fold faster than the H 2 IMes analogue. Computational studies reveal that the key factor underlying accelerated decomposition of the CAAC derivatives is their stronger trans influence, which weakens the Ru−py bond and increases the transient concentration of the 14-electron methylidene species, RuCl 2 (L)(=CH 2 ) 2 . Fast catalyst initiation, a major design goal in olefin metathesis, thus has the negative consequence of accelerating decomposition. Inhibiting bimolecular decomposition offers major opportunities to transform catalyst productivity and utility, and to realize the outstanding promise of olefin metathesis.

Catalytic Decomposition of Long‐Chain Olefins to Propylene via Isomerization‐Metathesis Using Latent Bicyclic (Alkyl)(Amino)Carbene‐Ruthenium Olefin Metathesis Catalysts

“…This can be explained by the intrinsic decomposition of the metathesis catalysts via β-elimination, leading to RuÀ H species inducing double-bond isomerization. [24][25][26] The positive effect of increasing RuH loading on propylene yield showed that the "bottleneck" of propylene formation is certainly the rate of isomerization. The purity of the ethylene gas used strongly influenced the ISOMET activity of the catalysts.…”

“…[22,23] These intermediates, when heated in the presence of ethylene excess, may take part in β-elimination reactions and lead to fast intrinsic catalyst decomposition. [24][25][26] The recent development of new carbene ligands has resulted in catalysts with higher stability, functional group tolerance, and improved catalytic activity (Scheme 2). [27][28][29][30][31][32] For example, replacing the N-heterocyclic carbene (NHC) ligand of the second generation Hoveyda-Grubbs catalyst (HG2) by CAAC-5 carbenes significantly improves the efficiency of olefin metathesis (Scheme 3, BG).…”

Routes to High-Performing Ruthenium–Iodide Catalysts for Olefin Metathesis: Ligand Lability Is Key to Efficient Halide Exchange

“…While chlororuthenium catalysts (Chart ) dominate these applications, iodide analogues offer important advantages. − Long-overlooked because of their slower metathesis reactions, − iodide catalysts such as nG-I 2 have recently been shown to offer improved productivities in the synthesis of macrocycles via ring-closing metathesis (mRCM, , a metathesis manifold of keen interest for the production of antiviral therapeutics), and increased selectivity for metathesis of terminal versus internal olefins . Their tolerance for ethylene , (the coproduct in metathesis of terminal olefins) is also striking: it is due in part to relatively slow bimolecular decomposition. , Indeed, their ethylene-tolerance is second only to that of cyclic (alkyl)­(amino) carbene (CAAC) derivatives, examples of which appear in Chart . , Heightened stability toward water adds further potential, most prominently for opportunities in chemical biology.…”

“…The new catalysts hold intriguing potential in mRCM in view of evidence that bulky ligands accelerate cyclization of conformationally flexible dienes (e.g., musk precursor 2 ; Table ). Also important is the slower bimolecular decomposition of RuI 2 (L)­(CH 2 ) . Catalyst lifetime is critical for such mRCM reactions because cyclization typically proceeds via a concentration-dependent ring–chain equilibrium, in which oligomerization is kinetically preferred and the oligomers liberate the desired products via backbiting .…”

“…5 Their tolerance for ethylene 11,12 (the coproduct in metathesis of terminal olefins) is also striking: it is due in part to relatively slow bimolecular decomposition. 6,12 Indeed, their ethylenetolerance is second only to that of cyclic (alkyl)(amino) carbene (CAAC) derivatives, examples of which appear in Chart 1. 13,14 Heightened stability toward water 6 adds further potential, most prominently for opportunities in chemical biology.…”