N‐Heterocyclic Carbene and Phosphine Ruthenium Indenylidene Precatalysts: A Comparative Study in Olefin Metathesis

Abstract

Kinetic studies on ring-closing metathesis of unhindered and hindered substrates using phosphine and N-heterocyclic carbene (NHC)-containing ruthenium-indenylidene complexes (first and second generation precatalysts, respectively) have been carried out. These studies reveal an appealing difference, between the phosphine and NHC-containing catalysts, associated with a distinctive rate-determining step in the reaction mechanism. These catalysts have been compared with the benzylidene generation catalysts and their respective representative substrates. Finally, the reaction scope of the two most interesting precatalysts, complexes that contain tricyclohexylphosphine and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (SIMes), has been investigated for the ring-closing and enyne metathesis for a large range of olefins. Owing to their high thermal stability, the SIMes-based indenylidene complexes were more efficient than their benzylidene analogues in the ring-closing metathesis of tetrasubstituted dienes. Importantly, none of the indenylidene precatalysts were found to be the most efficient for all of the substrates, indeed, a complementary complex-to-substrate activity relationship was observed.

Homobimetallic Ruthenium Vinylidene, Allenylidene, and Indenylidene Complexes: Synthesis, Characterization, and Catalytic Studies

“…This substrate was prepared by alkylation of 1,1-diphenylpropynol with nbromopropane in the presence of sodium hydride according to standard procedures. [30][31][32] At room temperature in dichloromethane, its reactivity toward ruthenium-ethylene complex 7a closely matched the one observed with 1,1-diphenylpropynol and resulted in the formation of vinylidene complex 8b in almost quantitative yield (Scheme 3). Spectroscopic evidence for an Ru=C=CHR unit included a sharp, medium-intensity IR band at 1669 cm À1 (n C=C ) and a doublet at 4.00 ppm with a coupling constant 4 J P,H = 3.8 Hz for the vinylidene proton.…”

“…In accordance with earlier observations, [18,20,37] it was nevertheless slightly less efficient than its monometallic parent 15, which afforded a full conversion of diethyl diallylmalonate into the corresponding RCM product within a few minutes at room temperature. [32] We next investigated the self-metathesis of styrene in the presence of homobimetallic ruthenium complexes 3a, 8a, b, 9, and 11. Experiments were carried out by adding 0.002 molar equivalent of catalyst to a stock solution of the olefin (1 M in toluene) stirred in an oil bath at 85 8C under an inert atmosphere [Eq.…”

Evaluation of an olefin metathesis pre-catalyst with a bulky and electron-rich N-heterocyclic carbene

“…Therefore, kinetic experiments were carried out, in which the RCM of 3 to form 4 (catalysed by each complex) was monitored by 1 (Figure 4 (b)). …”

“…Catalyst syntheses were performed in a MBraun glovebox containing dry Ar and less than 1 ppm oxygen. 1 …”

“…1,2 To allow the fine-tuning of the electronic 3 and steric 4 properties affecting the catalytic properties of the metal centres to which they are co-ordinated, numerous NHC ligands have been reported in the literature. 5 In particular, NHCs have played key roles in gold 6 and palladium 7 catalysis, and have proved to be a breakthrough in ruthenium-catalysed olefin metathesis.…”