Ruthenium-Based Complexes Bearing Saturated Chiral N-Heterocyclic Carbene Ligands: Dynamic Behavior and Catalysis

Abstract

The synthesis of Ru-based catalysts, presenting saturated chiral C 2 symmetric (3, 5) and C 1 symmetric (4) N-heterocyclic carbene (NHC) ligands bearing N-(S)-phenylethyl groups, was carried out. Variable-temperature NMR studies were conducted to investigate the interconversion of atropisomers in solution. The complex behaviors were rationalized evaluating the rotation barrier of alkylidene and NHC groups around the C−Ru bonds, by DFT calculations. Comparison between NMR data and DFT calculations suggested that interconversion between different atropisomers, which occurs at room temperature, is due to the free rotation of the benzylidene group around the RuC bond. The activity and stereoselectivity of 3−5 were investigated in ring-closing metathesis (RCM), asymmetric ring-closing metathesis (ARCM), cross-metathesis (CM), and ring-opening metathesis polymerization (ROMP). 4 showed the highest activity in all reactions and gave a significantly low E:Z ratio in the CM reaction. Modest enantioselectivity in the ARCM of an achiral triene was observed in the presence of C 2 symmetric catalyst 5.

NHC Backbone Configuration in Ruthenium-Catalyzed Olefin Metathesis

“…However, most ruthenium complexes bearing only alkyl side chains were unstable and did not provide better catalytic results compared with the parent catalysts GII and HGII [30][31][32][33]. In 2008, the first examples of monophosphine Ru complexes bearing monodentate saturated NHC ligands which combine benzyl chiral groups on the nitrogens with methyl backbone substituents were reported in the literature (1-synGII and 1-antiGII, Figure 2) [21]. Both complexes, presenting S-phenylethyl N-substituents and syn (1-synGII) or R,R-anti (1-antiGII) methyl groups on the backbone, were prepared in around 40% yield by deprotonation of the corresponding imidazolinium salts obtained after condensation of the corresponding chiral diamines.…”

“…Catalyst 1-antiGII [21], that bears a C2 symmetric NHC ligand with four stereogenic centers, since, in addition to the R,R methyl substituted backbone, they have S-phenylethyl N-substituents, gave modest enantioselectivity (33% ee) in the asymmetric ring-closure of 18, probably due to the minor role of the chiral phenylethyl groups in transferring the asymmetric information from the backbone to the substrate.…”

“…[Ru] CH Cl , RT, 1h Besides the above described catalysts with phenyl substituents on the backbone, also ruthenium complexes bearing symmetrical NHC ligands with anti methyl groups on the backbone were explored in the ARCM of model substrate 18 (Scheme 4). Catalyst 1-antiGII [21], that bears a C 2 symmetric NHC ligand with four stereogenic centers, since, in addition to the R,R methyl substituted backbone, they have S-phenylethyl N-substituents, gave modest enantioselectivity (33% ee) in the asymmetric ring-closure of 18, probably due to the minor role of the chiral phenylethyl groups in transferring the asymmetric information from the backbone to the substrate.…”

“…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].…”

Asymmetric catalysts for stereocontrolled olefin metathesis reactions

“…18 Pioneering attempts to install chirality near the metal by asymmetric N-substituents did not provide promising results (see also ref. 19), therefore the NHC's backbone was targeted for the installation of stereoinformation. Since the stereocenters at these positions are remote from the metal, aromatic N-substituents were used to transfer the chirality to the olefin's coordination sphere by a well defined twist around the N-aryl bond.…”