High Efficient Catalytic System For Selective Oxidation Of Alcohols Based On The Activation Of Dioxygen

The oxidation of alcohols to their corresponding aldehydes and ketones plays important role both in fundamental research and industrial production. The use of molecular oxygen as the terminal oxidant is of economic and environmental benefits in the oxidation of alcohols. Currently,2,2,6,6-tetramethylpiperidyl-l-oxyl (TEMPO) has been widely used in the aerobic oxidation of alcohols due to its high efficacy and selectivity. However, TEMPO alone could not directly catalyze the oxidation of alcohols, and its work relies on the assistance of various cocatalysts which play the important role in activating dioxygen. The reported cocatalysts for dioxygen activation included generally two sorts:nitric oxide and the transition metallic complexes. However, the corrosion of the often existed HBr for the former and the less effectiveness of the latter, especially Cu complexes, were considered to be tough problems. Therefore, efforts on developing new TEMPO-based catalytic systems with inexpensive component and high catalytic activity are still necessary for the aerobic oxidation of alcohols.In this paper, TEMPO was combined with Co(NO3)2 and dimethylglyoxime (DH2) for the selective aerobic oxidation of alcohols under milde conditions. The performance of this catalytic system was investigated in detail. Major progresses of this dissertation are as follows:1. The catalytic activities of Co(NO3)2 was compared with other transition metal salts under the same conditions using the aerobic oxidation of benzyl alcohol as the model reaction. Consequently, Co(NO3)2 had been proved to be an effective metal center, and displayed relatively high catalytic activity in comparation with other metal salts.2. The optimization of the Co(N03)2/DH2/TEMPO catalyzed reaction conditions was studied in detail. It was found that benzyl alcohol could be smoothly and rapidly oxidized to benzyl aldehyde with 1:4:1 mole ratio of Co(NO3)2, DH2 and TEMPO at 70℃under 0.4 MPa O2 in dichloromethane, and a complete conversion and 100% selectivity of benzyl aldehyde were obtained using 1mol% Co(NO3)2 within 3 h. 3. Under the optimized conditions, various alcohols including benzylic, allylic, aliphatic, and cycloaliphatic alcohols could be quantitatively oxidized to the corresponding aldehydes or ketones. After GC showed the reaction completed, the product was simply isolated with 99% purity by silica-gel column chromatography to yield the corresponding carbonyl compound.4. A possible mechanism was proposed for Co(NO3)2/DH2/TEMPO catalyzed oxidation:cobaloxime and NOx were firstly generated in situ during the oxidation by the reaction of Co(NO3)2 and DH2, and both of them played roles for the activation of dioxygen, and then participated in two concerted but different catalytic routes respectively which were conducted via oxoammonium cation mechanism.In summary, a novel three-component catalytic system Co(N03)2/DH2/TEMPO for aerobic oxidation of alcohols under mild conditions to their corresponding carbonyl compounds was developed. This new catalytic system displayed great performance in the catalytic aerobic oxidation of various alcohols, and showed good application prospect.