Ruthenium Complexes Bearing Two N-Heterocyclic Carbene Ligands in Low Catalyst Loading Olefin Metathesis Reactions

Abstract

The synthesis of ruthenium indenylidene complexes containing mixed N-heterocyclic carbene ligands featuring one sterically small NHC [IMeMe (1,3,4,5-tetramethylimidazol-2-ylidene), I i PrMe (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), and ICy (1,3-dicyclohexylimidazol-2-ylidene)] and one larger congener SIMes (1,3-bis-(2,4,6 trimethylphenyl)imidazolin-2-ylidene) is described. Characterization by X-ray diffraction allowed for calculation of percent buried volume of the small NHCs in these complexes and for speculation on the relationship NHC/catalytic activity. In addition, these complexes displayed an enhanced stability but also a high ability to promote ring-closing metathesis reactions at catalyst loadings as low as 0.05 mol %.

Bis(Cyclic Alkyl Amino Carbene) Ruthenium Complexes: A Versatile, Highly Efficient Tool for Olefin Metathesis

“…These complexes show a distorted square pyramidal geometry with the carbon atom of the indenylidene ligand in the apical position (Figure ). A similar geometry of the coordination sphere can be found in CAAC‐benzylidenes (for example, 12 a ) and (NHC)(NHC′)indenylidene complexes ( 5 ) . The CAACs exhibit stronger σ‐donation abilities than the NHCs.…”

“…Ligation of two identical [bis(NHC)Ru] or different [(NHC)(NHC′)Ru] NHCs to the ruthenium center has been significantly less explored. Some of those complexes revealed interesting features such as good efficiency in ring‐opening metathesis polymerization (ROMP) ( 4 ), effectiveness in the formation of tetrasubstituted double bonds ( 5 ), or mechanochemical activation ( 6 ) . In general, the complexes containing two identical NHCs exhibit low activity, as exemplified by catalyst 6 , whereas the ligation of two different NHCs is somewhat tedious.…”

Olefin Metathesis Featuring Ruthenium Indenylidene Complexes with a Sterically Demanding NHC Ligand

“…15 While several catalysts can efficiently convert di‐ and trisubstituted dienes into the corresponding RCM product in short reaction times using classical catalyst loadings (1–5 mol %), at very low loadings (and to the best of our knowledge) the catalyst loading limits are 25 ppm15c and 250 ppm,15b respectively, for the formation of di‐ and trisubstituted olefins such as 9 b and 8 b . Additionally, for challenging substrates such as 9 b , loadings of at least 2000 ppm are usually necessary to achieve near quantitative yields 3d. 15b, c…”

The catalytic performance of Ru–NHC alkylidene complexes: PCy3 versus pyridine as the dissociating ligand

“…An alternative approach to phosphine-free Ru-metathesis catalysts uses pyridines as placeholders. Originally, complex G was synthesized as a precursor for mixed NHC-phosphine complexes other than D [ 25 – 28 ] or, very recently, for the synthesis of Ru-alkylidenes with two different NHC ligands [ 29 ]. Comparatively little information is available concerning the catalytic activity of these pyridine-NHC complexes.…”