Ruthenium Olefin Metathesis Catalysts Featuring N-Heterocyclic Carbene Ligands Tagged with Isonicotinic and 4-(Dimethylamino)benzoic Acid Rests: Evaluation of a Modular Synthetic Strategy

Abstract

A modular and flexible strategy towards the synthesis of N-heterocyclic carbene (NHC) ligands bearing Brønsted base tags has been proposed and then adopted in the preparation of two tagged NHC ligands bearing rests of isonicotinic and 4-(dimethylamino)benzoic acids. Such tagged NHC ligands represent an attractive starting point for the synthesis of olefin metathesis ruthenium catalysts tagged in non-dissociating ligands. The influence of the Brønsted basic tags on the activity of such obtained olefin metathesis catalysts has been studied.

Ruthenium Complex Bearing a Hydroxy Group Functionalised N‐Heterocyclic Carbene Ligand – A Universal Platform for Synthesis of Tagged and Immobilised Catalysts for Olefin Metathesis

“…Recently, we developed the practical high-yielding synthesis of racemic salt 1. [23] We realised that it can be utilised in the synthesis of Hoveyda-Grubbs catalyst NHC ligand (Scheme 1). To do so, the free carbene was generated in situ by treatment of 1 with KHMDS and reacted with Hov I to provide "protected" complex 2 in a good yield of 76 %.…”

“…Of interest, the same strategy applied not to the Hoveydatype precursor 3 a, but to the related indenylidene complex 3 b failed completely in providing the corresponding acylated complexes (Scheme 2b). [23] Such differences of behaviour between 3 a and 3 b are assumed to rely on the greater stability of phosphine-free complex 3 a under basic conditions required for acylation reaction. Indeed, the decomposition of complex 3 b was systematically observed during all attempted reactions with acyl chloride and the excess of triethylamine in DCM, and even when the corresponding indenylidene complexes were formed, such derivatives degraded quickly during the workup involving column chromatography on silica gel.…”

“…Indeed, the decomposition of complex 3 b was systematically observed during all attempted reactions with acyl chloride and the excess of triethylamine in DCM, and even when the corresponding indenylidene complexes were formed, such derivatives degraded quickly during the workup involving column chromatography on silica gel. [23] The new catalysts (4-9) [27] were fully characterised by 1 H and 13 C NMR spectroscopy as well as HRMS and/or elemental analysis and IR spectroscopy. The signals of the benzylidene protons in the NMR spectra ranged from 16.36 to 16.45 ppm, which is typical of Hoveyda-type complexes, and the IR spectra of all these complexes exhibited an intense band at about 1760 cm À 1 , confirming the presence of ester units.…”

“…Interestingly, the severely reduced activity of catalyst 7 can be explained by a self-poisoning process of the Ru centre via coordination of the basic Lewis nitrogen atom of the pyridine unit an effect that has been demonstrated previously. [23] Such coordination of the pyridine with the Ru centre appears to disrupt the catalytic cycle and accelerate the decomposition of the catalytic species. To circumvent this limitation of complex 7, the addition of a Brønsted acid was considered to quaternise the basic pyridine nitrogen atom.…”

“…In our previous work, we added 1.1 equivalents of HCl in dioxane to a DCM solution of catalyst 7 and to our delight, this led to a dramatic improvement in the overall conversion of 10 up to 89 % after 60 minutes. [23] In the current study we decided to investigate the behaviour of complexes 6 and 7 (catalysts with Brønsted base moieties) in the RCM reaction of 10 in the presence of another Brønsted acid, para-toluenesulfonic acid monohydrate (pTsOH, 1.1 equiv. ), which can be considered as a model of a solid support functionalised with sulfonic acid groups that we planned to use in the next part of the study.…”