Quinonemethides as Tentative Structural Elements in Lignin

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o-Quinonemethides as Tentative Structural Elements in Lignin

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Downloaded by TUFTS UNIV on November 28, 2016 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0059.ch006

JOHN M. HARKIN

Forschungsinstitut für die Chemie des Holzes und der Polysaccharide OrganischChemisches Institut der Universität Heidelberg, Germany

During lignification, addition of phenols onto p-quinonemethides leads to labile free or etherified p-hydroxybenzyl aryl ethers. These rearrange slowly in situ or rapidly on treatment with acids or alkalies—e.g. during pulping—to o-hydroxydiphenylmethanes with free or etherified p'hydroxyl groups. These groupings can be readily dehydrogenated either by enzymes (e.g., during lignification) or oxidizing agents (e.g., during pulping or bleaching of pulps) to give stable o-quinonemethides. Structures involving o-quinonemethides may be responsible for part of the carbonyl and part of the free radical contents of lignin andfor some of the color in pulps. Pertinent models have been prepared, and their properties have been examined.

A d l e r and Marton have made a careful study of the carbonyl content of spruce lignin (1, 2, 18, 19). Values for some types of carbonyl groupings known to be present in lignin were determined accurately by various direct methods, but the value given for unconjugated carbonyl (/) was derived by subtracting the sum of these values from the value for the total carbonyl content of lignin. However, the value for the total carbonyl content of lignin is questionable since different amounts are reflected by different methods of assay (13, 19). Hence, the value for the content of unconjugated carbonyl groups in lignin also appears to be somewhat unreliable. Unconjugated carbonyl groups can occur only in the ^-position of the C side chain of the arylpropanoid units in lignin. A simple bio3

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Present address: Division of Wood Chemistry Research, Forest Products Laboratory, Madison, Wis. 53705. 65 Marton; Lignin Structure and Reactions Advances in Chemistry; American Chemical Society: Washington, DC, 1966.


66

LIGNIN STRUCTURE AND REACTIONS

gentic mechanism leading to the formation of β - k e t o n i c groups in lignin is hard to conceive, and including the amount of β - c a r b o n y l postulated by Adler and Marton (/) in the formula scheme for lignin designed by Freu denberg (4, 5, 9) constrains the formula owing to the concurrent reduction in the possibilities of forming β-aryl ether bonds, the most frequent type of interunitary bonding in lignin (6). Therefore we looked for other possible types of carbonyl groupings in lignin which might be more compatible with the current theory about the mode of its biogenesis. Pew et al. (23, 24) have shown that benzyl alcohol derivatives are produced in biphenyl-coupled and diphenyl ether lignin models by the action of peroxidase and hydrogen peroxide, one enzyme known to be involved in lignification. p-Quinonemethides are thought to be intermediates in this reaction. Continued dehydrogenation of the />-hydroxybenzyl alcohols (23, 24) leads to the corresponding ketones (aketones). Similar observations have also been made by the present author with several lignin models—e.g., the biphenyl-coupled dehydro dimers of vanillyl alcohol, apocynol, dihydroconiferyl alcohol, guaiacylpropane-1,3diol, and guaiacylglycerol-/5-coniferyl ether; the production of carbonyl groups was followed by infrared spectroscopy (15). However, even aketones, like ^-ketones, are relatively weak chromophores and can hardly account for the strong coloration of isolated lignins. Other, stronger chromophores seem to be indicated.

Ether Groups in Lignin It was recently demonstrated that spruce lignin contains approxi mately 4% free p-hydroxybenzyl aryl ether I and about 6% etherified />-hydroxybenzyl aryl ether II (10). Compounds containing this type of grouping have been made by polymerizing ^-quinonemethides (//) and have been isolated as intermediates of the in vitro biosynthesis of lignin (8). Such ethers are readily cleaved in the cold by mild acids or alkalies or slowly by water alone. Heating accelerates this hydrolysis. Fast cleavage of the free />-hydroxybenzyl aryl ethers and the slow cleavage of the />-alkoxybenzyl aryl ethers in lignin by the mild alkalinity of sodium borohydride (15) tend to make the values of 0.21-0.24 C O per C unit de termined for spruce milled wood lignin using the N a B H assay method (15,19) too high. 9

4

Generally, the hydrolysis of such ^-hydroxy- or p-alkoxybenzyl aryl ethers is accompanied by condensation of the benzyl carbon atom with unsubstituted positions in a neighboring benzene ring and release of the etherified phenolic hydroxyl group. However, when provisions are made to prevent these condensation reactions—e.g., by conducting the reaction in anhydrous methanol (15) or by removing the compounds liberated be fore they can condense using a mild percolating system (20)—small amounts of low molecular weight hydrolysis products can be isolated from lignin

Marton; Lignin Structure and Reactions Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

6.

HARKIN

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o-quinonemethides


(7). Dissolution of almost 40% of powdered beech wood in this way (20) suggests that the benzyl aryl ether content of beech lignin is much higher than that of spruce lignin. The hydrolysis and condensation of />-hydroxy- or />-alkoxybenzyl aryl ethers lead to derivatives of hydroxydiphenylmethane—e.g., structures of type III. There is evidence that this process occurs to some extent in the tree without the interference of external reagents. This may result from the natural acidity of wood and may represent a sort of aging process, causing greater condensation and strengthening of the lignin. The same type of condensation must take place to a greater extent when lignin comes into contact with acids—e.g., during the isolation of lignins by acidic hydrolysis of the wood polysaccharides—or with alkalies—e.g., during alkaline pulping processes. Any hydroxydiphenylmethane structures formed in this way would be prone to oxidation, either by the phenol dehydrogenases involved in ligni fication while in situ in the wood, by redox processes during pulping, or by oxidizing agents during pulp bleaching. Therefore, we prepared some dihydroxydiphenylmethanes and studied their behavior towards oxidizing agents.

ρ,ρ'-Dihydroxydiphenylmethanes /^'-Dihydroxydiphenylmethanes are readily prepared using a slight adaptation of Pearl's method.

H e observed the formation of diguaiacyl-

methane as a by-product during the oxidation of vanillin to vanillic acid {21). Method. Vanillin was hydrogenated in ethyl acetate using Raney nickel to give vanillyl alcohol (IV, R = H , m.p., 1 1 5 ° C ) , in quantitative yield. The alcohol was then refluxed with silver oxide under nitrogen for 24 hours to give diguaiacylmethane V , R = H , m.p., 1 0 9 ° - 1 1 0 ° C . as the major product. T h e material was purified by chromatography on deac tivated silica gel using chloroform :acetone (9:1 v / v ) as eluant. T h e formaldehyde eliminated during the above reaction is oxidized by the silver oxide to formic acid, and hence condensations of the phenol formaldehyde type are avoided; the formation of polymeric products is thus suppressed. Boiling vanillyl alcohol with excess alkali alone for 1 hour (21) does not afford quantitative yields of diguaiacylmethane but a mixture of several products plus unchanged vanillyl alcohol. The corresponding syringyl derivative was prepared as follows. Vanillin was converted into 5-iodovanillin, m . p . l 8 1 ° - 1 8 2 ° C . , which was then converted with sodium methoxide and copper powder (22) into syringaaldehyde, m . p . l 0 9 ° - 1 1 0 ° C . A trace of free iodine promotes the catalyst in this reaction. The aldehyde was again reduced quantitatively with Raney nickel in ethyl acetate to syringyl alcohol (IV, R = O C H , m . p . l 3 4 ° - 5 ° C . ) which on treatment with alkali and A g 0 as above gave a high yield of bis-3,5-di-0-methylpyrogallylmethane (V, R = O C H , m.p.H2°-3°C). Dehydrogenation of V with laccase in air or peroxidase 3

2

3


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LIGNIN STRUCTURE AND REACTIONS

and hydrogen peroxide or with neutral inorganic oxidizing agents—e.g., M n 0 , P b 0 , heavy metal salts, and air, or potassium persulfate—led to a yellow-colored solution containing some of the />-quinonemethide (VI). 2

2

Results. This solution exhibits a high molar extinction with X at 400 ιτιμ; the carbonyl absorption in the infrared appears at 6μ. The crude quinonemethide (VI) can be prepared in aqueous solution or in al m a x

most any organic solvent. When its solution is made alkaline with sodium carbonate or when V is oxidized in weakly alkaline solution—e.g., with potassium ferricyanide or simply with air, a permanent violet color is formed with X at 345 and 575-580 ιτιμ. The violet color is caused by the formation of the highly conjugated phenoxide ion V I I . If caustic alkali is used, the violet color is only transient, owing to nucleophilic addi tion of hydroxyl ion onto the ^-quinonemethide to form the corresponding benzhydrol derivative VIII. The /^-quinonemethide V I exhibits the nor mal reactions of />-quinonemethides—e.g. decoloration by electrolytes such as mineral or organic acids, phenols or methanol owing to addition reac tions. Already during its preparation (e.g., by shaking solutions of V in dioxan with manganese dioxide), some further dehydrogenation of the quinonemethide V I occurs, leading to the free semiquinone radical (IX), which can be detected in the solution of the crude oxidized product by E P R spectroscopy. The mixture obtained on dehydrogenation of V thus con tains some unchanged V , some of the />-quinonemethide V I , and some of the free radical I X ; it is therefore impossible at present to give definite values for the molar extinctions of the quinonemethides or quantitative data on the free radical concentrations.


m a x

Other Lignin Models More lignin-like models were made following the method of Gierer et al. (14). The yields were improved by applying the dilution principle. Dilute solutions of vanillyl alcohol ( X , R = R ' = H ) or syringyl alcohol ( X , R = O C H , R ' = H ) were added slowly dropwise to refluxing 66% aqueous ethanol containing creosol (XI) with 1% H Q as catalyst. In this way intermolecular condensation of the alcohols is circumvented. The products consist of mixtures of about 80% 6-vanillylcreosol ( X I I , R = R ' = H , m . p . l l 2 - 5 ° C . ) plus a little 5-vanillylcreosol (VIII, R = R ' = H , m . p . l l 4 ° - 5 C . ) or 80%) 6-syringylcreosol (XII, R = O C H , R ' = H , m . p . l 2 r - 2 ° C . ) plus about \0% 5-syringylcreosol ( X I I , R = O C H , R ' = H ) . The syringylcreosol described by Gierer et al. (14) is actually the 6-isomer X I I and not the 5-isomer X I I I . This agrees with the find ings of Sarkanen for electrophilic substitution of creosol (J, 25). With acidic catalysts, a benzyl carbonium ion derived from the vanillyl or syringyl alcohol makes an electrophilic attack on the creosol nucleus. Under alkaline conditions the condensation leads to 5-vanillylcreosol 3

0

0

3

3


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HARKIN

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o-Quinonemethides

C I

c

C I

I

c

I

c

c

I

C

I OCH

I

I

HC— OCH,

OCH,

O—CH

3

y \

I

Ο

I

HC

I

I

I

I I

c

Ο—CH

c

c

I I O—CH HC

c

OH

Ο


OCH, —ο OCH,

OCH, OH

—Ο III

II OCH,

CH~—OH

2

I OH

R

+ 2 Ag + H C O O H + H 0 2

Ag 0 2

R

I

O C H 3

OH

R

/ x / \ I OCH3 OH

IV R

R

R

R OH

0==CH-

Quinonemethides as Tentative Structural Elements in Lignin

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