Highly Efficient Ruthenium Catalysts for the Formation of Tetrasubstituted Olefins via Ring-Closing Metathesis

Abstract

A series of ruthenium-based metathesis catalysts with N-heterocyclic carbene (NHC) ligands have been prepared in which the N-aryl groups have been changed from mesityl to mono-ortho-substituted phenyl (e.g., tolyl). These new catalysts offer an exceptional increase in activity for the formation of tetrasubstituted olefins via ring-closing metathesis (RCM), while maintaining high levels of activity in ring-closing metathesis (RCM) reactions that generate di- and trisubstituted olefins.

A Second-Generation Synthesis of the Cyanthiwigin Natural Product Core

“…In a nitrogen-filled glovebox, Pd(OAc) 2 (1.4 mg, 6.3 µmol) was weighed into a 20-mL scintillation vial and dissolved in solvent (10 mL). In a separate 1-dram vial, (S)-CF 3 -tBu-PHOX (15b) (3.7 mg, 6.3 µmol) was dissolved in solvent (1 mL).…”

“…Commercial reagents (Sigma Aldrich or Alfa Aesar) were used as received with the exception of palladium(II) acetate (Sigma Aldrich) which was stored in a nitrogen-filled govebox. The modified GrubbsHoveyda II catalyst (S3) 2 was donated by Materia Inc. and used without further purification. The (S)-t-BuPHOX ligand 15a, 3 the (S)-CF 3 -t-BuPHOX ligand 15b, 4 and bis(3,5-dimethoxydibenzylideneacetone)palladium 5 were prepared according to known procedures.…”

NHC Backbone Configuration in Ruthenium-Catalyzed Olefin Metathesis

“…Surprisingly, two isomeric compounds, corresponding to different conformations of the N-tolyl substituents (10a-synGII and 10b-synGII, Figure 8), were obtained. Since complex 7-synGII as well as GIItol exist as a mixture of rotational isomers both in the solid state and in solution [18,26,48], the obtainment of separate, stable rotational isomers was strictly related to the steric pressure exerted by the bulky phenyl groups on the backbone. Phosphine-containing complexes 10a-synGII and 10b-synGII were easily converted in the corresponding phosphine-free catalysts 10a-synHGII (95% yield) and 10b-synHGII (84% yield) by treatment with 2-isopropoxystyrene.…”

“…Modifications of the NHC ligand include the nature of the ring backbone (saturated or unsaturated), substitution at the nitrogen atoms and at the carbon atoms of the backbone, ring-size variation, introduction of heteroatoms in the skeleton, and introduction of chirality [11][12][13][14][15]. Among these, modifications of the steric and electronic properties of substituents on the backbone and/or the nitrogen atoms have had a significant impact on catalyst activity, stability and selectivity in several metathesis applications [16][17][18][19][20][21][22][23][24][25][26][27][28][29].…”

Synthesis and Reactivity of Olefin Metathesis Catalysts Bearing Cyclic (Alkyl)(Amino)Carbenes

“…[1,2] Many metathesis catalysts based on the [L 2 X 2 Ru=CHR] scaffold have been synthesized in an effort to increase catalyst stability, activity, and substrate scope. [3][4][5][6][7][8][9][10] A significant gain in these areas was achieved after exchanging a single PCy 3 ligand of 1 with H 2 IMes (H 2 IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene),an N-heterocyclic carbene (NHC), to produce catalyst 2 ( Figure 1). [5] These results are attributed to the increased σ-donor ability of H 2 IMes over PCy 3 , which increases the affinity for π-acidic olefins relative to σ-donating phosphines.…”