The Study On The Metal-free Oxidation Of α-isophorone By Oxygen

Ketoisophorone (KIP) is an important intermediate of vitamin E, which can be obtained by the oxidation of isophorone. Currently, there are two synthetic routes to get KIP. One is the oxidation ofβ-isophorone, which is obtained by the isomerizate ofα-isophorone. The other is the direct oxidation ofα-isophorone to product KIP, eliminating the isomerization step. Due to the effect of conjugate,α-isophorone is more stable thanβ-isophorone from thermodynamic viewpoint. The isomerization ofα-isophorone toβ-isophorone is only about 2%, and a high reaction temperature (> 190℃) is needed, which means the consuming of energy is very large. In addition, the catalytic systems for the oxidation of isophorone usually contain metal, and the catalytic processes need volatile orgainic solvent, resulting in the pollution of the environment. Therefore, this paper studies the direct oxidation ofα-isophorone to KIP by a metal-free N-hydroxyphthalimide (NHPI) as organic catalystusing oxygen. This method offers a new way for the preparation of KIP.The paper firstly investigated the oxidation ofα-isophorone to KIP using NHPI as main catalyst in the presence of organic co-catalyst and organic solvents This process avoids the use of metal completely, and the reaction condition is mild, therefore the energy consumption is very low. In addition, the influence of temperature, solvent, and catalytic amount is optimized. the selectivity of 90% can be achieved under the optimized reacton condition.Secondly, we also investigated the solvent-free oxidation ofα-isophorone by NHPI in the absence of the co-catalyst. It was found that only the catalytic amount of NHPI was used and catalytic oxidation ofα-isophorone was effective. The selectivity of more than 75% can be obtained using 5% NHPI at 60℃for 10 h.Finally, this paper also studied the oxidation mechanism ofα-isophorone in the presence of NHPI. Combining the detected intermediates and products by ESI-MS and GC-MS, we proposed a mechanism for the oxidation ofα-isophorone by NHPI. Furthermore, we certificate the mechanism through the comparative experiment of different reaction substrate. This mechanism, not only explains the reason of the formation of reaction by-products, which is helpful in improving the selectivity, but also provides a new idea to verify the oxidation reaction mechanism of relevant system.