Synthesis of novel planar chiral Ag and Rh N-heterocyclic carbene complexes derived from [2.2]paracyclophane and their application in ultrasound assisted asymmetric addition reactions of organoboronic acids to aldehydes

Three novel planar chiral N-heterocyclic carbene silver and rhodium complexes based on [2.2]paracyclophane have been prepared. These could be used as catalysts/precatalysts for the asymmetric 1,2-addition of organoboronic acids to aldehydes. We optimized the reaction conditions and have applied ultrasonic irradiation in the asymmetric arylation for the first time. Under ultrasound irradiation, the combination of planar chiral NHC–Ag complex 5 and RhCl3 can achieve higher catalytic activities in the asymmetric addition of organoboronic acids to aldehydes.

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Chloro[N,N′-bis[(Rp)-(+)-4-[2.2]paracyclophanyl]imidazol-2-ylidene]silverC35H33AgClN2[α]D20=+73.3 (c 0.21, CH2Cl2)Source of chirality: N,N′-bis[(Rp)-(−)-4-[2.2]paracyclophanyl] imidazolium triflateAbsolute configuration: (Rp)

Chloro(η2,η2-1,5-cyclo-octadiene)-[N,N′-bis[(Rp)-(+)-4-[2.2]paracyclophanyl]imidazole-2-ylidene]rhodiumC43H44ClN2Rh[α]D20=+47 (c 0.2, CH2Cl2)Source of chirality: N,N′-bis[(Rp)-(−)-4-[2.2]paracyclophanyl] imidazolium triflateAbsolute configuration: (Rp)

Bromo[N,N′-bis[(Rp)-(+)-12-methoxy-4-[2.2]paracyclophanyl] imidazol-2-ylidene]silverC37H36AgBrN2O2[α]D20=+125.2 (c 0.27, CH2Cl2)Source of chirality: N,N′-bis[(Rp)-(−)-12-methoxy-4-[2.2]paracyclophanyl]imidazolium triflateAbsolute configuration: (Rp)