Cleavage reactions, lignins

Substitution Reactions on Side Chains. Because the benzyl carbon is the most reactive site on the propanoid side chain, many substitution reactions occur at this position. Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide stmeture. In a reversal of the ether cleavage reactions described, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers by acid-catalyzed etherifications or transetherifications with alcohol or phenol. The conversion of a benzyl alcohol or ether to a sulfonic acid group is among the most important side chain modification reactions because it is essential to the solubilization of lignin in the sulfite pulping process (17). 

As for (i-0-4 ethereal bond cleavage, reaction of the primary cation-radical with solvent water under the same conditions of bio-oxidation was shown to form an arylglycerol and the corresponding phenoxy radical (Kirk et al. 1986, Fabbri et al. 2005) (Scheme 8.22). Since the p-0-4 ethereal bond is the most abundant type of interunit linkage in the lignin polymer, this ethereal bond cleavage represents an important depolymerization reaction. 

Side-Chain Cleavage Reactions of 0-1 and fl-O-4 Lignin Model Compounds Catalyzed by Lignin Peroxidase. 

In order to eluddate the mechanism of the further transformations of the primary lignin decomposition products including the cleavage reactions, we synthesized some of the important primary products labelled with Cu and introduced these into the cultures of fungi or enzymes. Thus, it could be shown, for example, that the breakdown of the side chain of ferulic acid occurs at the double bond vanillic acid is found. During polymerization in the presence of phenoloxidases, in the case of carboxyl-labelled ferulic acid, about 60% of the activity is split off as Cli02. The polymers labelled in the 2 and 3 position in the side chain or in the methoxyl group contain the whole applied activity. 

Under alkaline conditions isolating lignin degradation products which are essentially of a phenyl.ethyl rather than a phenylpropyl nature is structurally important and requires a lignin structure by which the 7-carbon may be removed as a result of a 0-7 carbon-carbon cleavage reaction, either by direct alkaline hydrolysis or alkali-catalyzed hydrogenolysis. 

It is not surprising to us that the current CIDEP study was not able to pinpoint the role of the excited singlet reaction. Nevertheless, the results of the solid state experiments at 77 K discussed earlier support the recent CIDNP product analysis of similar lignin model reactions which strongly indicate a primary direct 0-cleavage reaction from the excited singlet state (9,11-12). 

Yan JF, Pla F, Kondo R, Dolk M, McCarthy JL (1984) Lignin 21. Depolymerization by bond cleavage reactions and degelation. Macromolecules 17 2137-2142... 

Adhesive Feedstocks from Lignin Mechanistic Studies on the Oxidative-Cleavage Reaction of Some Lignin Model Compounds... 

A proposed scheme for increasing the reactivity of lignin and thus enhancing its usefulness as an alternative adhesive feedstock is presented in Figure 1. The first step is to perform an oxidative-cleavage reaction to form phenolic benzaldehyde compounds. 

The details of the reaction conditions used in this study have been described elsewhere (Dershem, S. M., et al., Holzforschung Fisher, T. H., et al., J. Org. CAem., in press). To test the importance of a p-hydroxyl substituent, the kinetics of oxidation of three benzyl alcohols p-hydroxybenzyl alcohol, (1), m-hydroxybenzyl alcohol, (2), and 4-hydroxy-3-methoxybenzyl alcohol, (3), were examined under alkaline nitrobenzene oxidation conditions. Some l-(4-hydroxyphenyl)-2-(4 -substituted phenyl)ethanols, (4), were synthesized as / -l lignin model compounds and subjected to alkaline nitrobenzene oxidation at 120 °C to study substituent effects. For controls, some of these compounds were reacted with or without nitrobenzene, alkaline catalyst, or water. In an effort to determine the effects of substituents on the oxidative-cleavage reaction of 4-hydroxystilbenes (5), a series of competitive rate experiments using both nitrobenzene and copper(II) as the oxidants in 2N NaOH was performed (Dershem, S. M., et al., Holzforschung, in press). 

The reaction depicted in Figure 3 can also be applied to the copper(II) oxidative-cleavage of lignin. In this case, copper(II) is reduced to copper(I) as the lignin is oxidized. Interestingly, copper(I) can be reoxidized to copper(II) by air (23). This autoxidation of copper(I) could explain why air was necessary for the copper(II) oxidation of stilbenes whereas, when nitrobenzene was used, identical experimental results were obtained under either an air or nitrogen atmosphere (Table II). 

Alkaline hydrolysis of lignin increases the number of reactive benzylic hydroxyl groups and may also be important in further depolymerizing the lignin once the oxidative-cleavage reaction has occurred. The formation of a p-electron-withdrawing -CHO substituent on aryl lignin units should increase the rate of hydrolysis of the ether bonds (26). Hydrolysis also forms p-phenylate ions, which then protect the benzaldehyde from further reaction via the Cannizzaro reaction, as mentioned earlier. 

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