Ru‐Based Olefin Metathesis Catalysts Bearing pH‐Responsive N‐Heterocyclic Carbene (NHC) Ligands: Activity Control via Degree of Protonation

Abstract

= 1,3-bis-(2Ј,6Ј-dimethyl-4Ј-dimethylaminophenyl)-4,5-dihydroimidazol-2-ylidene] were used for the ring-opening metathesis polymerization (ROMP) of exo-7-oxanorbornene derivative 7 in the presence of various amounts of acid. Upon gradual protonation of the NMe 2 groups of the H 2 Tap ligand, the metathesis activity of both catalysts were gradually reduced due Over the last decade, olefin metathesis has emerged as a powerful technique in organic [1] and polymer synthesis.[2] In particular Ru-based, single-site catalysts such as Grubbs' first-and second-generation catalysts 1 and 2, and Hoveyda-Grubbs catalyst 3 have gained scientific and commercial importance due to their high tolerance towards moisture and functional groups, [3][4][5] as these catalysts have significantly expanded the scope of metathesis substrates. In terms of overall activity and thermal stability, catalysts 2 and 3 bearing an N-heterocyclic carbene (NHC) ligand, most commonly the H 2 IMes [bis(2,4,6-trimethylphenyl)dihydroimidazol-2-ylidene] ligand, are superior. [4,5] This is due to their slower rates of initiation which is overcompensated by the fact that these catalysts promote extremely fast metathesis propagation. [6,7] On the flip side however, the high propagation rates accomplished with these catalysts makes them unsuitable for controlled ring-opening metathesis polymerization (ROMP) in contrast to catalyst 1. In fact, only small fractions of catalysts 2 and 3 initiate in a standard ROMP reaction. [7] Apart from reaction temperature and structural parameters, only a few controls have been recognized to influence the activity of Ru-based olefin metathesis catalysts. The addition of Brønsted [8][9][10][11][12][13][14][15] or Lewis [15][16][17] to electronic changes of the N-heterocyclic carbene (NHC) ligand donor capability. The investigation of the ROMP polymer 8, DFT calculations and measurements of the initiation kinetics prove that the reduced activity is solely due to reduced rates of propagation.

Improved Molecular Weight Control in Ring‐Opening Metathesis Polymerization (ROMP) Reactions with Ru‐Based Olefin Metathesis Catalysts Using N Donors and Acid: A Kinetic and Mechanistic Investigation

“…Only one example has been reported for which acid addition has caused deceleration of the metathesis reaction 23. It was demonstrated that propagation rates could be controlled by means of an acid addition resulting in an improved molecular weight control through a gradual propagation deceleration 24…”

Preferential Encapsulation and Stability of La3N Cluster in 80 Atom Cages: Experimental Synthesis and Computational Investigation of La3N@C79N

“…We have used this approach in prior investigations of various late transition metal complexes 15–17. To examine the performance of this method, geometry optimization was carried out for Sc 3 N@I h -C 80 .…”

Grubbs–Hoveyda type catalysts bearing a dicationic N-heterocyclic carbene for biphasic olefin metathesis reactions in ionic liquids

“…We were attracted by NHC ligands containing a diamino function at the aromatic ring as realized in 1 [ 20 – 22 ] since such ligands can be permanently quaternized to the corresponding dicationic species via double alkylation. Additionally, they remain structurally closely related to mesitylene-based NHC ligands.…”