Chiral phosphoric acid catalyzed asymmetric transfer hydrogenation of quinolines in a sustainable solvent

The use of diethyl carbonate as a sustainable solvent for organocatalytic asymmetric transfer hydrogenations of 2-substituted quinolines using highly efficient chiral phosphoric acid catalysts with Hantzsch esters as a hydrogen source is reported for the first time. The asymmetric transfer hydrogenation reaction in diethyl carbonate provides enantiomerically pure 1,2,3,4-tetrahydroquinolines with high yields and excellent enantioselectivities (up to 99% ee). These results clearly confirm that this green and sustainable solvent is an excellent replacement for organic solvents, which are harmful to the environment, and transition metal based catalysts are not required. The effects of different chiral phosphoric acids, solvents, catalyst loading, temperature effect, and reaction time on the conversion and enantioselectivity of desired product are discussed.

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(S)-2-Methyl-1,2,3,4-tetrahydroquinolineC10H13NEe = 87%[α]D22 = −75.1 (c 1.0, CHCl3)Source of chirality: (R)-3,3′-Bis(2,4,6-triisopropylphenyl)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphateAbsolute configuration: (S)

(S)-7-Chloro-2-methyl-1,2,3,4-tetrahydroquinolineC10H12ClNEe = 98%[α]D20 = −76.4 (c 0.5, CHCl3)Source of chirality: (R)-3,3′-Bis(2,4,6-triisopropylphenyl)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphateAbsolute configuration: (S)

(S)-2-Phenyl-1,2,3,4-tetrahydroquinolineC15H15NEe = 99%[α]D25 = −39.3 (c 1.0, CHCl3)Source of chirality: (R)-3,3′-Bis(2,4,6-triisopropylphenyl)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphateAbsolute configuration: (S)

(S)-7-Chloro-2-phenyl-1,2,3,4-tetrahydroquinolineC15H14ClNEe = 97%[α]D20 = −54.1 (c 0.9, EtOAc)Source of chirality: (R)-3,3′-Bis(2,4,6-triisopropylphenyl)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphateAbsolute configuration: (S)

(S)-2-(4-Chlorophenyl)-1,2,3,4-tetrahydroquinolineC15H14ClNEe = 98%[α]D20 = −40.2 (c 1.2, CHCl3)Source of chirality: (R)-3,3′-Bis(2,4,6-triisopropylphenyl)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphateAbsolute configuration: (S)

(S)-2-(p-Tolyl)-1,2,3,4-tetrahydroquinolineC16H17NEe = 96%[α]D20 = −23.4 (c 1.0, CHCl3)Source of chirality: (R)-3,3′-Bis(2,4,6-triisopropylphenyl)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphateAbsolute configuration: (S)

(S)-2-(4-Methoxyphenyl)-1,2,3,4-tetrahydroquinolineC16H17NOEe = 98%[α]D25 = −27.1 (c 1.0, CHCl3)Source of chirality: (R)-3,3′-Bis(2,4,6-triisopropylphenyl)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphateAbsolute configuration: (S)