Comparative studies on oxidation of lignin model compounds with hydrogen peroxide using Mn(IV)-Me3TACN and Mn(IV)-Me4DTNE as catalyst

Comparative studies are conducted on the kinetics and reaction mechanism for the oxidation of lignin model compounds, 1-(3,4-dimethoxyphenyl)ethanol (1), 1-(3,4-dimethoxyphenyl)-1-propene (2) and E-1,2-diphenylethene (3) with hydrogen peroxide at reaction temperatures below 80degreesC using [LMn(IV)(mu-O)(3)Mn(IV)L](PF6)(2) (C-1) and [L'Mn(IV)(mu-O)(3)Mn(IV)]-(ClO4)(2) (C-2) as catalyst. The disappearance rate of 1 and 2 in the first phase of C-1-catalyzed oxidation increases up to the temperature range of 50-60 degreesC then decreases with increasing reaction temperature. The cause for the slow down of the disappearance rate is not known. Based on the kinetic data and reaction products identified, C-1 is found to be more effective as catalyst in the oxidation of 1 and 2 than C-2, but less effective in the oxidation of 3. In the C-1- and C-2-catalyzed oxidation, 1 is oxidized to the corresponding a-carbonyl derivative 4, while 2 readily undergoes epoxidation of the conjugated double bond to produce the corresponding epoxides 5 and 6. The anti-Markovnikov nucleophilic addition of hydroxide and hydroperoxide anions on 5 and 6 then produced alpha,beta-diol 7 and benzaldehyde derivative 8, which are non-catalytic reactions and rate determining steps. The compound 3 also undergoes epoxidation of conjugated aliphatic double bond producing the corresponding epoxide 9. However, none of the corresponding a,p-diol and benzaldehyde derivative is detected in the reaction mixture. In addition, 2 is more susceptible to expoxidation than 3. On the basis of the kinetics and reaction mechanism of the reactions, the catalytic cycles of the C-1- and C-2-catalyzed oxidation of 1-3 are postulated. (C) 2003 Elsevier B.V. All rights reserved.