TEMPO Catalyzed Oxidation Of C-H Bonds And The Investigations Of Related Mechanism

Selective oxo-functionalizations of hydrocarbons have been concern of scientists, and the oxidation is one of the most important reactions to achieve this aim. Conventionally, harsh conditions are often required which are harmful for the environments. With the development of industrial technology, green chemistry becomes more and more attractive and developing environmental benign technologies for the oxidation are urgent needed. In the past decades, scientists have paid much attention on non-metal reaction system, and the metal-free catalysts have been popular recommended. Organic free radicals are one of these species, which have been widely used as catalysts in the oxidation. For example, the "non-persistent" N-oxyl radicals, such as phthalimide-N-oxyl (PINO), which are usually generated from the corresponding hydroxylamine, are often used in the oxidation of hydrocarbons due to their high reactivity. However, the instability of themselves limits their applications. On the contrary, the "persistent" N-oxyl radicals, as TEMPO, are stable towards water and air, which can be directly purchased from commerce. As far as we know, utilizing the "persistent" radical TEMPO in the oxidation of hydrocarbon is barely reported. Thus, we carried out a series of combined experimental and theoretical investigations on the oxidation of hydrocarbons catalyzed by TEMPO and its analogues.Firstly, a novel and efficient protocol for the oxidation of β-isophorone (β-IP) using molecular oxygen without any additives catalyzed by TEMPO has been established, and 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 P-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 PINO, TEMPO promotes the oxidation via its interaction with the active intermediate hydroperoxide (ROOH) rather than its initial interaction with β-IP, and byproduct water also assists the a-H atom transfer from ROOH to TEMPO.Secondly, screening other suitable substrates for this reaction is of great importance in widening the application of TEMPO as a catalyst. Herein, various substrates have been investigated by the density functional theory calculations to gain a further understanding. It is found that the activation energy for hydrogen transformation is not only correlated with the dissociation energy of C-H bond, but also with the structure of the substrate. Substituent effects of the double functional groups, especially the electron withdrawing (conjugated) groups, lower the hydrogen transformation barriers of allylic C-H bonds apparently. Besides, conjugation effects play important role in modification of the substrate as the larger scale of the conjugation favors the hydrogen transformation. Furthermore, the asymmetric chemical surroundings of the allylic C-H bonds also make this process go smoothly.Thirdly, the reactivity of TEMPO can be notably enhanced via its coordination to a series of Lewis acids (LAs). This coordination largely alters the electronic features of TEMPO, especially the spin density located on N/O atoms, thereby influencing its reactivity. Aerobic oxidation of a-isophorone catalyzed by TEMPO has achieved comparable performance to that catalyzed by PINO with the assistance of LAs, but the latter efficiency is depressed by LAs. Remarkably changed structural and thermochemical properties of nitroxyl radicals induced by LAs cause the diversely altered reactivity. Persistent and nonconjugated nitroxyl radicals with alkyl groups around N-0 parts like TEMPO can be activated, whereas non-persistent ones, with N-0 parts conjugated with carbonyl groups like PINO are deactivated by LAs. TEMPO analogues combined with strong but less steric-hindrance LAs exhibit the highest catalytic activity.In summary, the oxidation of P-IP with molecular oxygen is promoted by TEMPO without any additives and its proposed mechanism is studied by the combined ESR and therotical techniques. As TEMPO is barely used as a catalyst in the oxidation of hydrocabons, both the features of the substrates and improving the activity of TEMPO have been overall investigated by experimental and therotical methods.