Development of ChiralC₂-SymmetricN-Heterocyclic Carbene Rh(I) Catalysts through Control of Their Steric Properties

Abstract

Chiral square-planar Rh(I) complexes based on new C2-symmetric NHC ligands have been synthesized selectively in a few steps as single diastereoisomers. These chiral pre-catalysts were applied to the asymmetric transfer hydrogenation of 1-phenylpropanone and to the 1,2-addition of arylboronic acids to aldehydes. We demonstrated a proper functionalization of the aromatic rings connected to the nitrogen atoms of the NHC ligand improved significantly the asymmetric induction of the chiral Rh(I) NHC catalysts. Bulky substituents allowed a better control of the steric features of the catalyst quadrants because they behaved as conformational and chirality relays of the NHC chiral backbone.

Arylation of aldehydes catalyzed by fluorinated NHC–Rh(i) complexes

“…19,20 This pioneering work opened the door to other relevant examples involving a wide variety of catalysts. [21][22][23][24][25][26][27][28][29][30][31][32][33][34] So, the use of Rh-NHC complexes for the arylation of carbonyls was described by Fu ¨rstner and co-workers. 35 They described an assisted catalytic system based on RhCl 3 Á3H 2 O and the in situ formation of NHC ligands.…”

Ru-Catalyzed Asymmetric Addition of Arylboronic Acids to Aliphatic Aldehydes via P-Chiral Monophosphorous Ligands

“…Classic methods for constructing such chiral molecules include asymmetric hydrogenation of ketones [11][12][13], asymmetric hydrogen transfer reduction of ketones or unsaturated ketones [14][15][16][17][18][19][20][21], along with the asymmetric addition of organometallic reagents to aldehydes [22][23][24]. As an alternative, strategies via the asymmetric homologation reaction [25][26][27][28], tandem Michael-MPV reaction and subsequent reductive desulfurization [29], as well as the asymmetric addition of aldehydes with arylboronic acids were also established [30,31]. It is worth noting that the tandem α-alkylation/asymmetric reactions of ketones with alcohols [32] and the asymmetric Guerbet reaction of alcohols [33,34] are green and high-efficient strategies.…”

Steric Tuning of Sulfinamide/Sulfoxides as Chiral Ligands withC1, Pseudo-meso, and Pseudo-C2Symmetries: Application in Rhodium(I)-Mediated Arylation

“…Transition metal enantioselective catalysis remains an active area of development in synthetic methodology. − Despite the great progress made in this field, one of the aspects that has been and remains a cardinal challenge in this area is the design and development of new effective ligands . The steric, electronic, and overall symmetry of the ligands are important elements to be considered in their design and preparation.…”