Graphite-supported gold nanoparticles as efficient catalyst for aerobic oxidation of benzylic amines to imines and N-substituted 1,2,3,4-tetrahydroisoquinolines to amides: synthetic applications and mechanistic study.
Selective oxidation of amines using oxygen as terminal oxidant is an important area in green chemistry. In this work, we describe the use of graphite-supported gold nanoparticles (AuNPs/C) to catalyze aerobic oxidation of cyclic and acyclic benzylic amines to the corresponding imines with moderate-to-excellent substrate conversions (43-100%) and product yields (66-99%) (19 examples). Oxidation of N-substituted 1,2,3,4-tetrahydroisoquinolines in the presence of aqueous NaHCO3 solution gave the corresponding amides in good yields (83-93%) with high selectivity (up to amide/enamide=93:4) (6 examples). The same protocol can be applied to the synthesis of benzimidazoles from the reaction of o-phenylenediamines with benzaldehydes under aerobic conditions (8 examples). By simple centrifugation, AuNPs/C can be recovered and reused for ten consecutive runs for the oxidation of dibenzylamine to N-benzylidene(phenyl)methanamine without significant loss of catalytic activity and selectivity. This protocol "AuNPs/C+O2" can be scaled to the gram scale, and 8.9 g (84 % isolated yield) of 3,4-dihydroisoquinoline can be obtained from the oxidation of 10 g 1,2,3,4-tetrahydroisoquinoline in a one-pot reaction. Based on the results of kinetic studies, radical traps experiment, and Hammett plot, a mechanism involving the hydrogen-transfer reaction from amine to metal and oxidation of M-H is proposed.
Department of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China.
Man-Ho
MH
So
Yungen
Y
Liu
Chi-Ming
CM
Ho
Chi-Ming
CM
Che
D000577:Amides / Q000138:chemical synthesis*; D000588:Amines; D002384:Catalysis; D006046:Gold; D006108:Graphite / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry; D007097:Imines / Q000138:chemical synthesis*; D007700:Kinetics; D053758:Nanoparticles / Q000737:chemistry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D013379:Substrate Specificity; D044005:Tetrahydroisoquinolines / Q000737:chemistry
D000577:Amides; D000588:Amines; D007097:Imines; D044005:Tetrahydroisoquinolines; C014843:1,2,3,4-tetrahydroisoquinoline; D006046:Gold; D006108:Graphite; D006859:Hydrogen; D010100:Oxygen