The Influence of Various N-Heterocyclic Carbene Ligands on Activity of Nitro-Activated Olefin Metathesis Catalysts

A set of nitro-activated ruthenium-based Hoveyda-Grubbs type olefin metathesis catalysts bearing sterically modified N-hetero-cyclic carbene (NHC) ligands have been obtained, characterised and studied in a set of model metathesis reactions. It was found that catalysts bearing standard SIMes and SIPr ligands (4a and 4b) gave the best results in metathesis of substrates with more accessible C–C double bonds. At the same time, catalysts bearing engineered naphthyl-substituted NHC ligands (4d–e) exhibited high activity towards formation of tetrasubstituted C–C double bonds, the reaction which was traditionally Achilles’ heel of the nitro-activated Hoveyda–Grubbs catalyst.


General Remarks
All air and moisture sensitive reactions were carried out under argon atmosphere in pre-dried glassware using Schlenk techniques. The catalyst preparation was carried out under argon in pre-dried glassware using Schlenk techniques. The anhydrous solvents were dried by distillation over the following drying agents and were transferred under argon: THF (over sodium benzophenone ketyl and distilled under argon), toluene (Na), n-pentane, n-hexane, n-pentane, CH2Cl2 (CaH2). Flash column chromatography was performed using Merck silica gel 60 (230-400 mesh). Analytical thin-layer chromatography (TLC) of all reactions was performed on silica gel 60 F254 TLC plates. Visualization was performed with standard potassium permanganate stains or UV light. NMR spectra were recorded in CD2Cl2; using

Synthesis of 4b
Similar to the preparation of 4a, 5 (150 mg, 0.68 mmol) was added to solution of complex 2b (690 mg, 0.68 mmol) in toluene

Synthesis of 4d
Similar to the preparation of 4c, 5

Model metathesis reactions General comparative RCM (NMR measurements)
Solutions of dienes 6 and 8 (0.08 mmol) in degassed CD2Cl2 0.75 mL was placed in NMR tube. Solutions of catalysts 1 mol% was freshly prepared and added to NMR tube in 0.05 mL of CD2Cl2. Progress of the reactions was monitored by means of NMR spectroscopy and their products were unambiguously identified by comparing with published NMR spectra available in literature.
Single-crystal X-ray measurements of the scattered intensities of reflections for CuClPCy3 The experimental details for CuClPCy3 are summarized in Table 1S.
The refinement was based on F 2 for all reflections except those with negative intensities.
Weighted R factors wR and all goodness-of-fit S values were based on F 2 , whereas conventional R factors were based on the amplitudes, with F set to zero for negative F 2 . The  Tables Vol.C. 8 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in the idealized positions.    Hydrogen atoms omitted for clarity. Figure 4S. Overlay of molecules from the 4a structure (black) with the 4b (magenta), 4c structure (blue), 4d structure (green), 4f structure (grey), 4e' structure (red) and 4e'' (yellow) and 4g (grey).
The CuClCy3 molecules ( Figure 5S) crystallize in the triclinic P-1 space group with half of the molecule in the asymmetric unit. The crystal structure of the complex was previously determined. 9 Each copper(I) atom is in trigonal coordination to a tricyclohexylphosphine ligand and two bridging chloride ligands. The bond distance between Cu-Cl1, Cu-Cl1 i and Cu-P1 is 2.3239(7) Å, 2.2931(5) Å and 2.1853(6), respectively. The Cu1 i -Cl1 i -Cu1 angle between is 83.57(5) making the central ring a rhomboid. The i superscript stands for the symmetry operations 1-x, -y, -z. Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms omitted for clarity.