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

Abstract

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, RuCl(L)(═CHCH- o-O Pr); the Grela catalyst nG (a derivative of HII with a nitro group para to OPr); the Piers catalyst PII, [RuCl(L)(═CHPCy)]OTf; the third-generation Grubbs catalyst GIII, RuCl(L)(py)(═CHPh); and dianiline catalyst DA, RuCl(L)( o-dianiline)(═CHPh), in all of which L = HIMes = 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 RuCl(HIMes)(κ-CH), 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 RuCl(HIMes)(═CH), Ru-1. Direct evidence for methylidene coupling is presented, via the controlled decomposition of transiently stabilized adducts of Ru-1, RuCl(HIMes)L(═CH) (L = py ; 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 RuCl(HIMes)L. 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.

Synthesis of Well‐Defined Poly(norbornene) Containing Carbon Nanodots by Controlled ROMP

“…A complete shift in the SEC trace to the high‐molecular‐weight region was observed while maintaining a narrow dispersity of 1.11, indicating the successful preparation of the block copolymer. A small high molecular shoulder might arise from the bimolecular coupling . Notably, the block copolymer containing an HBC unit was achieved for the first time.…”

Acrylate Esters by Ethenolysis of Maleate Esters with Ru Metathesis Catalysts: an HTE and a Technoeconomic Study

“…Currently, ethenolysis of oleates serves as a benchmark reaction for the search of improved metathesis catalysts, often including screening of large catalyst libraries . Achieving high productivity in ethenolysis can be hampered by deactivation of intermediate Ru methylidene species, [Ru=CH 2 ], by a bimolecular coupling or methylidene abstraction, both processes being facilitated by the small size and the high reactivity of the methylidene ligand . In addition, a β ‐hydrogen elimination pathway contributes significantly to decomposition of unsubstituted ruthenacyclobutane intermediates .…”

Alkene Metathesis for Transformations of Renewables

“…[7,8] In particular, olefin metathesis is one of the modern catalysed reaction which as impacted the world of homogeneous organometallic catalysis over the last 25 years. [9][10][11] Continuous improvements of catalysts performances [12] and stability with better knowledge of activation and deactivation pathways [13][14][15] have rendered this process compatible with the transformation of bio-sourced compounds of variable purity. [16] In this chapter, we will review the main recent achievements and progress obtained in the valorisation of terpenes and lipids thanks to olefin metathesis.…”