Unexpected oxidation of β-isophorone with molecular oxygen promoted by TEMPO

A novel and efficient protocol for the oxidation of beta-isophorone (beta-IP) using molecular oxygen without any additives catalyzed by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) has been established. The generally accepted catalytic mechanism of alcohols by the oxoammonium cation (TEMPO+) derived from TEMPO indeed fails to explain our experimental observations, because a favorable radical-based process is confirmed by electron spin resonance measurements. Our results show that a plateau of the time-dependence curve is observed in the oxidation of beta-IP with TEMPO at low temperature, which is quite different from that of N-hydroxyphthalimide (NHPI). The proposed mechanism of this catalytic process is also compared with that of NHPI. The theoretically characterized reaction pathways show that unlike the phthalimide N-oxyl radical, TEMPO promotes the oxidation via its interaction with the active intermediate hydroperoxide (ROOH) rather than its initial interaction with beta-IP, and byproduct water also assists the alpha-H atom transfer from ROOH to TEMPO. In addition to the intensive oxidation of alcohols catalyzed by TEMPO, the present study widens its specific applications in the active C-H bonds of hydrocarbons, and also provides new insights into its promoted metal-free oxidation.