Ruthenium Phosphinimine Complex as a Fast-Initiating Olefin Metathesis Catalyst with Competing Catalytic Cycles

Abstract

A ruthenium-based olefin metathesis (OM) catalyst bearing a monodentate triphenylphosphinimine ligand, Ru1, was synthesized, characterized, and its activity for the homocoupling of terminal alkenes was investigated. Utilizing 1-hexene as a model substrate, the empirical rate law for Ru1 was found to be first-order in alkene and complex (indicating that both species were involved in the rate-limiting step), with a rate constant of 0.697 ± 0.050 M–1 s–1. Moreover, the experimentally determined activation parameters ΔS ⧧ and ΔH ⧧ (−48.7 ± 5.1 eu and 3.19 ± 0.15 kcal/mol, respectively) were consistent with an associative or associative interchange ligand substitution reaction. When considering the ΔG ⧧ (298 K) value of 17.7 kcal/mol, Ru1 ranked among the fastest initiating ruthenium-based OM catalysts reported in the literature. Density functional theory (DFT) calculations were also performed to explore potential catalytic mechanisms. Two pathways were considered: a traditional mechanism where the phosphinimine ligand de-coordinated and an alternative mechanism where the phosphine donor de-coordinated. Although the energy differences between the two pathways were typically fairly small (1.4–3.5 kcal/mol), the alternative pathway with phosphine de-coordination was energetically more favorable. It is anticipated, however, that both cycles are working in tandem during the catalytic reaction. In addition to kinetic studies, the stability of Ru1 was explored using 1-hexene as a model substrate. The phosphinimine catalyst was found to be mildly oxygen-sensitive and moisture-tolerant. Furthermore, Ru1 was determined to be prone to bimolecular decomposition, through the crystallographic characterization of a key degradation product. There was also strong evidence for NH exchange between the tricyclohexylphosphine and triphenylphosphinimine moieties. Lastly, the substrate scope of Ru1 in regard to α-olefins was explored. Catalytic efficiency dropped with more electron-deficient alkenes, as well as with increasing steric bulk on the substrate, which was consistent with the proposed catalytic mechanism.

Competitive Isomerization and Catalyst Decomposition During Ring-Closing Metathesis

“…These ammonium salts are readily prepared via a double alkenylation of dibutylamine with the desired bromoalkene, followed by salt metathesis with NaPF 6 to give the desired charge-tagged complexes (Figure 1). All derivatives were characterized by 1 H and 13 C NMR, ESI-MS, and in two cases by X-ray analysis (see Supporting Information). We used GII [33,34] in all the following experiments in this study.…”

“…Dichloromethane (Supelco, HPLC grade, stabilizer-free) was dried over calcium hydride and distilled and degassed before use. NMR spectra were recorded on a Bruker Avance III 300 Hz NMR spectrometer at standard conditions and referenced to the residual proton or 13 C atom of the solvent. NMR chemical shifts are reported in ppm relative to tetramethylsilane at 0 ppm.…”

“…1 H NMR (300 MHz, CDCl 3 ) δ 5.74 (ddt, J = 16.9, 10.2, 6.7 Hz, 2H), 4.99 -4.81 (m, 4H), 3.14 -3.00 (m, 8H), 2.04 -1.90 (m, 4H), 1.59 -1.46 (m, 8H), 1.40 -1.18 (m, 28H), 0.92 (t, J = 7.2 Hz, 6H). 13…”

“…[10][11][12] The activity of the Grubbs-type catalysts is exemplified by the dissociation of a phosphine or pyridine ligand from the initial 16 ecomplex to form the catalytically active 14 especies, to which alkene coordination and subsequent reaction with the metal alkylidine occurs. The other L-type ligand has also been varied beyond the phosphines and carbenes of GI and GII, and examples containing phosphinimine [13] and phosphite [14] ligands are known. It is widely accepted that the mechanism proceeds via a [2+2] cycloaddition in which an unsaturated Ru-alkylidine complex binds one of the C=C bonds of a bis-alkene to form a four-membered ring.…”

Competitive isomerization and catalyst decomposition during ring-closing metathesis

“…[11][12][13] The activity of the Grubbs-type catalysts is initiated by the dissociation of a phosphine or pyridine ligand from the initial 16 e − complex to form the catalytically active 14 e − species, to which alkene coordination and subsequent reaction with the metal alkylidene occurs. The other L-type ligand has also been varied beyond the phosphines and carbenes of GI and GII, and examples containing phosphinimine 14 and phosphite 15,16 ligands are known. It is widely accepted that the mechanism proceeds via a [2 + 2] cycloaddition in which an unsaturated Ru-alkylidene complex binds one of the CC bonds of a bis-alkene to form a four-membered ring.…”