Oxidative Coupling of Benzylamines into N-Benzylbenzaldimines with Mn(II)/tert-BuOOH

Abstract

3-Methyllumiflavin promotes conversion of C6H5CH2NH2 to N-benzylbenzaldimine 4 under acid catalyzed thermal conditions. Aerobic oxidative dehydrogenation of C6H5CH2NH2 is catalyzed by molybdenium-vanadium salt to yield 4. Clay-catalyzed reaction of benzylamine is suggested to involve C6H5CH=NH2 that react another C6H5CH2NH2 for the formation of 4. Polypyrrole catalyst is effective in the dehydrogenation of C6H5CH2NH2 with O2 to make 4. The same reaction is also catalyzed by a aniline trimer. Monoamine oxidases catalyze the oxidation of primary amines to give iminium cation that is hydrolyzed to form the aldehydes. Mn(III) ions are known to catalyse various oxidative free radical reactions. We’d like to herein report the oxidative coupling of benzylamines utilizing Mn(II)/tert–BuOOH. Benzylamine was oxidized under various conditions to determine the role of each component for the reaction (Table 1). Entry 2 indicates the importance of the catalyst. The oxidation does not occur in the absence of oxidant, tertbutylhydroperoxide (TBHP) (entry 3). Variously substituted benzylamines undergo coupling reaction to give N-benzylbenzaldimines by the catalysis of Mn=O that is formed from Mn/tert-BuOOH. The reaction takes place within 3-4 h to give the desired product in 93% yield (Table 2). Mn(II) ion may make a complex with TBHP to give Mn=O that may provoke electron transfer from C6H5CH2NH2 forming aminium radical cation, C6H5CH2 H2 1. The intermediate 1 becomes acidic enough to expel benzylic proton to produce C6H5 HH2 2 that is oxidized by Mn(III) with formation of C6H5CH= H2 3. C6H5CHO might result from hydrolysis of 3. However, control experiment under the condition of entry 1 of Table 1 at room temperature shows no trace of benzaldehyde (aldehydic proton: δ = 10 ppm) but indicates instead gradual increase of benzylic CH2 of 4 (chemical shift: δ = 4.84 ppm) with reaction time of 5, 10, 15, 30 min, and 1 h, respectively. This