Abstract
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.
Highlights
First transition metal complexes bearing N-heterocyclic carbene (NHC) ligands were studied independently by Wanzlick [1] and Öfele [2] in the late 1960s, these intriguing species remained unexplored for many years
All complexes were synthesised via the stoichiometric metathesis-ligand exchange reaction according to a procedure initially disclosed by Hoveyda [71] and illustrated on Scheme 1
The synthesis of complexes 4c–e was carried out using appropriate Grubbs second generation complexes 1c–e as the source of ruthenium [40,66,68,73]
Summary
First transition metal complexes bearing N-heterocyclic carbene (NHC) ligands were studied independently by Wanzlick [1] and Öfele [2] in the late 1960s, these intriguing species remained unexplored for many years. Catalyst 4a bearing a well-known SIMes ligand (Figure 1, NHC structures: a) was chosen as the benchmark, while the less known SIPr (Figure 1, 4b) [53] and the new complexes with Me2 IMes [40] and with two naphthalene based ligands (Figure 1, NHC structures: c–e) developed by Dorta, were studied in detail [66,67,68,69] These five complexes were characterised structurally and tested in model olefin metathesis reactions [70] to check how steric properties of the different NHC ligands influence structural and catalytic properties of the resulting.
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