Salicylaldimine Ruthenium Alkylidene Complexes: Metathesis Catalysts Tuned for Protic Solvents

Abstract

Tuning the electronic and steric environment of olefin metathesis catalysts with specialized ligands can adapt them to broader applications, including metathesis in aqueous solvents. Bidentate salicylaldimine ligands are known to stabilize ruthenium alkylidene complexes, as well as allow ring-closing metathesis in protic media. Here, we report the synthesis and characterization of exceptionally robust olefin metathesis catalysts bearing both bidentate salicylaldimine and N-heterocyclic carbene ligands, including a trimethylammoniumfunctionalized complex adapted for polar solvents. NMR spectroscopy and X-ray crystallographic analysis confirm the structures of the complexes. Although the N-heterocyclic carbenesalicylaldimine ligand combination limits the activity of these catalysts in nonpolar solvents, this pairing enables efficient ring-closing metathesis of both dienes and enynes in methanol and methanol-water mixtures under air.

A Highly Active Aqueous Olefin Metathesis Catalyst Bearing a Quaternary Ammonium Group

“…[15] After our preliminary report, other catalysts bearing polar quaternary ammonium groups in the benzylidene fragment were reported by Mauduit, Grela and co-workers (13 ac), [16] Raines (14) [17] and very recently by Grubbs (15 a,b; Figure 3). [18] As a continuation of our work in the field of organometallic catalysis in water, [3] we extended our preliminary study on the quaternary ammonium iodide 12.…”

New Indenylidene‐Schiff Base‐Ruthenium Complexes for Cross‐Metathesis and Ring‐Closing Metathesis

“…[30] Furthermore, some salicylaldiminato-type ruthenium complexes have been water-adapted (catalysts for aqueous metathesis) and are excellent for biological applications and green chemistry. [31] Taking into account the highly desirable attributes of this type of ligands, some new indenylidene Schiff base Ru-based complexes have been conveniently designed and prepared (3a-f) exhibiting a good tolerance towards organic functionalities, air and moisture. The new indenylidene complexes have been successfully applied in olefin metathesis reactions such as cross metathesis (CM) and ring-closing metathesis (RCM).…”

Aqueous Olefin Metathesis

“…[58] Catalyst 77 not only initiates metathesis reactions in dichloromethane faster than the parent Hoveyda-Grubbs complex 4 b, but also promotes various metathesis reactions (RCM, CM, and enyne metathesis) in aqueous mixtures [58] and in neat water. [59] Other catalysts containing polar quaternary ammonium groups in the benzylidene fragment were later reported by Raines and co-workers (complex 78), [60] Mauduit, Grela, and co-workers (complexes 79 and 80), [61] and Jordan and Grubbs (complexes 81 and 82; Scheme 22). [62] Comparison of the results of model RCM and CM reactions leads to the conclusion that, although the application profiles of these catalysts are in general quite similar, there are some interesting differences (Tables 3 and 4).…”