Electroorganic reactions. 38. Mechanism of electrooxidative cleavage of lignin model dimers

Electro-organic reactions. Part 60[1]. The electro-oxidative conversion at laboratory scale of a lignosulfonate into vanillin in an FM01 filter press flow reactor: preparative and mechanistic aspects

“…The changes are fairly well understood to involve quinonemethide intermediates, from studies of lignins and of model compounds [27][28][29][30]. Whilst there is formation of small amounts of low-molecular weight products the main changes are modification of functionality in the polymer.…”

“…The process has been demonstrated in the laboratory, one example being the formation, by chemical oxidation of the corresponding phenol, of a quinonemethide (8) which undergoes [39] base-catalysed or electrochemically initiated polymerisation. Much information on the mechanism of electrooxidation of lignins was obtained by the detailed examination of the oxidation of variously substituted lignin-model compounds (9) at a nickel anode in 2-3 M aqueous NaOH at 150-160°C or, in acetonitrile, by tris-(4-bromophenol)amine radical-cation as a homogeneous single electron oxidant [29]. These experiments allowed identification of functionality that was essential for oxidative cleavage.…”

“…For vanillin formation, this competition is illustrated in Scheme 2; the enol ether (10) has been characterised as an intermediate. It is stable in alkaline solution but decomposes rapidly in neutral or acidic media [28,29].…”

Abrasive Stripping Voltammetric Studies of Lignin and Lignin Model Compounds

“…Lignin, and several model compounds have been shown to undergo homogeneous chemistry, involving oxidation to form relatively stable aromatic radical cation species, usually involving electron transfer, which further decomposes via cleavage of the C a À C b bond in the case of 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (DMP), whilst for 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (MMP), dimerisation of the resonance-stabilised phenoxyl radical is the most likely reaction pathway [8,9], although this suggested mechanism of oxidative cleavage is poorly understood in the literature [10]. Rochefort et al [11] also investigated the mechanism of oxidation of DMP and the cleavage of the a À b carbon bond in aqueous buffer and reported the products as being benzaldehyde and veratraldehyde (methylvanillin), verified in another study by Parpot et al in 1.0 M NaOH [12] and Limosin and co-workers, also in aqueous base [13].…”

Electro-Degradation of Sodium Lignosulfonate

“…Utley and Smith [9] used nickel as the anode and lead as the cathode in their experiments. Pardini and Smith [13] studied the mechanism of electrooxidative cleavage of lignin at nickel anodes in alkaline electrolytes with model dimers and found that polymerization at nickel anodes was predominant during the degradation procedure although aldehydes were formed together with larger amounts of the corresponding carboxylic acids. They also found that the combinations of 4-hydroxyl, a-keto, b-O-aryl, and b-hydroxymethyl functionality were crucial for the oxidation at nickel.…”