Synthesis of Planar Chiral [2.2]Paracyclophanyl Imidazo[1,5-a]pyridinium Salts for the Rhodium-Catalyzed Asymmetric Arylation

Abstract

Several novel flexibility-restricted imidazo[1,5-a]pyridinium triflates (abbreviated as imidazolium salts) were synthesized from (4S _p,13R _p)-(−)-4-amino-13-bromo[2.2]paracyclophane and pyridylaldehyde. These imidazolium salts can be used as nitrogen-containing heterocyclic carbene precursors in asymmetric catalysis and here they are applied in the Rh-catalyzed asymmetric 1,2-addition of arylboronic acids to aldehydes. After optimizing the catalytic situations and testing a series of substrates, moderate enantioselectivity and good yield were obtained.

GRAPHICAL ABSTRACT

Keywords:

• Aldehyde
• asymmetric arylation
• N-heterocyclic carbene
• [2.2]paracyclophane
• planar chirality

ACKNOWLEDGMENTS

Financial support from the National Natural Science Foundation of China (Grant Nos. 20441004 and 20671059) and the Department of Science and Technology of Shandong Province is gratefully acknowledged

Notes

^a Reaction conditions: [Rh(OAc)₂]₂ (3 mol%), 5d (3 mol%), KOBu-t (1 eq), arylboronic acids (2 eq), N₂, 80 °C, 2 h.

^b Isolated yield.

^c Determined by chiral HPLC (CHIRALPAK IA Columns) analysis.

^a Reaction conditions: [Rh(OAc)₂]₂ (3 mol%), ligand 5 (3 mol%), KOBu-t (1 eq), arylboronic acids (2 eq), N₂, MeOH/DME (5:1), reflux, 2 h.

^b Isolated yield.

^c Determined by chiral HPLC (CHIRALPAK IA Columns) analysis.

^a Reaction conditions: [Rh(OAc)₂]₂ (3 mol%), ligand 5d (3 mol%), KOBu-t (1 eq), arylboronic acids (2 eq), N₂, MeOH/DME (5:1), reflux, 2 h.

^b Isolated yield.

^c Determined by chiral HPLC (CHIRALPAK IA Columns) analysis.