Oxidative Deamination of sec-Alkyl Primary Amines with 3,5-Di-tert-butyl-1,2-benzoquinone: A Second Look

Article abstract of DOI:10.1021/jo00261a002

Corey's oxidative deamination with 3,5-di-tert-butyl-1,2-benzoquinone (1) was studied with four sec-alkyl primary amines: cyclohexylamine (2a), cycloheptylamine (2b), 2-butanamine (2c), and 3-pentanamine (2d).The condensation products of 1 with 2a-d were identified as the respective Schiff bases (4a-d) of 2-amino-4,6-ditert-butylphenol (5) resulting from rapid, spontaneous prototropic rearrangement of the intermediate quinone imines (3a-d).MNDO calculations on model systems confirm that the rearrangement is thermodynamically highly favored (ΔΔH_f=16.5 kcal/mol).Acidic hydrolisis of 4a-d gave the corresponding ketones (6a-d) and aminophenol 5 in high yields.Aminophenol 5 was identified as the source (via oxidative coupling) of the intensely blue 2,4,6,8-tetra-tert-butyl-1-phenoxazinone (12) accompanying all aerated reactions.Attempted regeneration of 1 via air oxidation/hydrolysis of 5 instead gave 12 in high yields; electrochemical or dichromate oxidation of 5 in strongly acidic media, however, gave 1 in 64 and 56percent yield, respectively.Rapid hydrolysis of the corresponding quinone iminium ion to 1 was confirmed by cyclic voltammetry of 5 in acidic media, which displayed two cathodic waves (E_Pc = 0.409 and 0.307 V, respectively), the more negative corresponding to 1.