The Structure and Reactivity of Lignin

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Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 21, 2013 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0059.ch002

The Structure and Reactivity of Lignin ERICH ADLER, KNUT LUNDQUIST, and GERHARD E. MIKSCHE Institutionen för Organisk Kemi, Chalmers Tekniska Högskola, Göteborg, Sweden

Degradation of Björkman spruce lignin by "acidolysis" (4 hours refluxing with 0.2M HCl in dioxane-water) followed by fractionation on dextran and silica gel columns results in isolating several products containing one or two guaiacyl groupings. The monomeric main product (6%) . was ω-hydroxyguaiacylacetone (XII) originating from arylglycerol β-aryl ether systems (II). Isolation of phenylcoumarone (XXII) and the ο,p'-dihydroxystilbene (XXIII) reveals the occurrence in lignin of the trimeric sequence (XXVIII). Furthermore, the ρ,ρ'-dihydroxystilbene (XXV) and the a-methyldesoxybenzoin (XXVI) were isolated and shown to originate from a 1,2-bis -guaiacyl-1,3-propanediol system (XXXIV) either present in lignin or formed on acidolysis from a cyclohexadienone precursor (XXXII). For afifthdimeric product, structure (XXIV) (d,l-3,4-divanillyltetrahydrofuran) is proposed; its origin is still obscure.

For several years, studies regarding the behavior of lignin on "acidolysis," —i.e., heating with 0.2M hydrogen chloride in dioxane-water 9:1— have been carried out in our laboratory. Analytical examination of the changes incurred by this treatment, and more recently, the isolation of various degradation products helped to elucidate some basic structural features of the lignin molecule. Strong analytical support for the presence of the phenylcoumaran system (I) in lignin was obtained a few years ago (S) (Figure 1). Under the conditions of acidolysis, models for system I, namely dihydrodehydrodiconiferyl alcohol (III) (13) and its methyl ether (III, OCH ) were con verted into phenylcoumarone derivatives (VIII and VIII, OCH ). The structure of the phenolic coumarone (VIII) was established by an inde3

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

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Structure and Reactivity


I

I

Phenylcoumaran structure

Arylglycerol-/3-aryl structure

ether

Figure L Structure of phenylcoumaran (I) and arylglycerol β-aryl ether (II) pendent synthesis (29).

Its yield on acidolysis of III was 75%.

The

conversion (III) —> (VIII) may be understood as a sequence of reactions involving ring opening with the formation of the benzylium ion (IV), formation of the cinnamyl alcohol (V) (full line arrows) and the allylium ion (VI), allylic rearrangement with reclosure of the hydrofuran ring to give V I I , and finally, migration of the exocyclic double bond of V I I (Figure 2). As a by-product of the acidolysis reaction, the 0,/>'-dihydroxystilbene (IX) was formed, obviously by the loss of a molecule of formaldehyde from the benzylium ion intermediate (IV) (broken line arrows), a reaction to be regarded as a reverse Prins reaction

(29). HjCr^C^OH

CHjCHjCr^OH HOqjg

rTçT J HC

OCH

ICH3

3

OH

0 HC® 0CH

ICH3

3

r τ [och3i

OH

III

[OCH3] HOÇHg Ϊ

Hi

HJCHJCHJOH

H®|-H 0 2

HÇ—0 -ΗΦ

Figure 2.

Acidolysis of dihydrodehydrodiconiferyl alcohol

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

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LIGNIN STRUCTURE AND REACTIONS


Phenylcoumarone (VIII) has a characteristic ultraviolet and ionizat i o n ^ spectrum, which enabled us to detect dimeric structures of this type in reaction mixtures obtained when B j ô r k m a n spruce lignin was subjected to acidolysis for 20 hours. From the spectrophotometric estimation of the amount of the phenylcoumarone systems formed, we concluded that from a total of 100 phenylpropane units of B j ô r k m a n lignin, about 20 are involved in phenylcoumaran systems (I); in other words, about every 10th phenylpropane unit is linked to one of its neighbors by the cyclic benzyl aryl ether linkage characteristic of I. Acidolysis was also used in our earlier, mainly analytical studies regarding the arylglycerol β-aryl ether system (II). (The usual designation of the C atoms of the propane side chain, as given in (II), is retained. The reversed numbering recently used by K . Freudenberg (16) seems to be appropriate only for the (unsaturated) cinnamyl side chain (cf. (21)). After 48 hours acidolysis, model substance X (R = H ) (J, 22) yielded (6) a mixture of α - h y d r o x y p r o p i o g u a i a c o n e ( X V ) , vanilloyl methyl ketone ( X V I ) , and guaiacylacetone ( X V I I ) in addition to guaiacol (XIII). Products X V - X V I I were detected by paper chromatography after similar acidolysis of B j ô r k m a n lignin, analogous to the results obtained by Hibbert on ethanolysis of wood, which gave the ethyl ether of ketol ( X V ) as well as the ketones ( X V I ) and ( X V I I ) . The nonphenolic model compound X (R = C H ) (4) reacted in a similar way (6), the essential feature of the acidolysis reaction of the model substances thus being the liberation of guaiacol and the formation of C-methyl groups. Analogously, acidolysis of B j ô r k m a n lignin resulted in liberating free phenolic hydroxyl groups as well as forming C-methyl groups. The increase in the amounts of these groups indicated that one-fourth to one-third of the phenylpropane units of spruce lignin has the structure of an arylglycerol, in which the 0-hydroxyl group is etherified with the phenolic hydroxyl group of the adjacent unit, as represented by formula II (Figure 3). 3

The course of the acidolysis reaction of model compound X was then studied in greater detail (/, 2 24 , 2/\ 28). We found that ω-hydroxyguaiacylacetone ( X I I , R = H ) and its methyl ether ( X I I , R = C H ) are intermediates in the acidolysis of guaiacylglycerol /3-(2-methoxyphenyl) ether ( X , R = H ) and its methyl derivative ( X , R = C H ) , respectively. Obviously, their formation is caused by an initial dehydration (step a) and the hydrolysis of the resulting enol aryl ether (step b). Ketol (XII) is further converted by allylic rearrangement of ( X l l a ) and subsequent enol-keto rearrangement (step c) (17, 18> 33) into an equilibrium mixture (28 29) of the isomeric secondary ketols ( X I V and X V ) . Finally, oxidoreduction of the latter ketols (R = H ) provides vanilloyl methyl ketone ( X V I ) and guaiacylacetone ( X V I I ) . It may be pointed out that this oxidoreduction is not given by the nonphenolic ketols ( X I V and X V , R = C H ) (8) which seems to indicate that the quinonemethide derived :

y

3

3

y

3

y


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from the phenolic benzyl alcohol ( X I V , R = H ) is involved in the reaction. Traces of vanillin (XVIII) and vanillic acid ( X I X ) were detected as by products in the acidolysis of the phenolic starting material ( X , R = H ) .

Figure 3. Acidolysis of arylglycerol β-aryl ether model compounds. Conditions: 0.2M HCl in dioxane-water, 9:1\ refluxed for 4 hours.

In the acidolysis of the phenolic β-aryl ether model ( X , R = H ) Reac tion b was much faster than the initial step a\ furthermore, Reaction c was appreciably slower than a. Therefore, the primary ketol (XII) accumu lated in the acidolysis mixture during the first few hours of heating and could be isolated after 4 hours of acidolysis of X (R = H ) in a yield of 53%; only 15% of the starting material had been converted further into the mixture of ( X I V ) and ( X V ) . The total β-aryl ether cleavage was 74%, as indicated by the yield of guaiacol (XIII). (The nonphenolic model com pound X (R = C H ) , reacted at an appreciably lower rate, the corres ponding yields after 4 hours of acidolysis being 34% X I I (R = C H ) , 3% X I V and X V (R = C H ) , and 42% guaiacol.) 3

3

3

On the basis of these model experiments we expected that 4 hours of lignin acidolysis would produce ω - h y d r o x y g u a i a c y l a c e t o n e ( X I I , R = H ) as a reaction product of arylglycerol β-aryl ether structures (II) containing arylglycerol moieties with an uncondensed aryl group. In fact, Lundquist (25) was able to isolate the ketol X I I (R = H ) , which had not been ob tained previously from lignin, in a yield of about 6% of the B j ô r k m a n lignin used, by fractionating the acidolysis mixture on silica gel columns.


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The crude B j ô r k m a n lignin acidolysis mixture contained both polymeric material and a number of more or less low molecular, chromatographically visible products in addition to ketol (XII) and was considered to be a potential source not only of further monomeric but also of dimeric and oligomeric degradation products. The polymeric material was readily removed by filtering the crude mixture of reaction products through a short silica gel column (solvent, dioxane-benzene 1:3). T o separate the products of lower molecular weight present in the filtrate, gel filtration on dextran (Sephadex) columns proved to be very helpful. The use of polar organic solvents or their mixture with water suppressed the undesirable adsorption effects previously encountered when gel filtration was applied to aromatic compounds in aqueous solution, and fractionation according to molecular size (molecular sieving) was accomplished. This is illustrated in Figure 4 which shows the behavior of a model mixture containing three monomeric compounds ( X V ; X V I I I ; X I I , R = H ) , three dimeric compounds ( X , R = H ; X X V I I ; (+)-pinoresinol) and B j ô r k m a n lignin on filtration through Sephadex G 25, dioxane-water 1:1 being used as eluant (31).

Figure 4.

Gelfiltrationof model mixture. Conditions: Sephadex G25 dioxane-water', 1:1.


y

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If a reaction mixture obtained after 4 hours acidolysis of B j ô r k m a n lignin (spruce), after neutralization and removal of the polymeric material, were filtered through a Sephadex G 25 column, the elution curve (Figure 5) exhibited three peaks (26). The effluent fractions corresponding to peaks A and Β were subjected to further fractionation by chromatography on silica gel columns. We found that fraction A contained only monomeric products while fraction Β was a mixture of dimeric products (Figures 6 and 7).

Effluent volume Figure 5.

Elution curve of reaction mixture

In fraction A , which contained 12% of the lignin used the main con stituent was the expected ω - h y d r o x y g u a i a c y l a c e t o n e (XII), which again was isolated in a yield of 6% of the lignin. It was accompanied by minor amounts of its two rearrangement products ( X I V , X V ) (25, 26) and their two oxidoreduction products ( X V I , X V I I ) as well as small quantities of vanillin (XVIII) and vanillic acid ( X I X ) . These "monomeric" lignin degradation products are identical with those obtained on acidolysis of the β-aryl ether model ( X , R = H ) as reported above, and their rela tive amounts, especially the relative abundance of the primary ketol (XII), also agree with the results of the model experiment. In our opinion, these results constitute decisive proof that the arylgly cerol 0-aryl ether structure (II) occurs in lignin. Recently, small amounts of guaiacylglycerol 0-coniferyl ether have been isolated by Nimz (35) after percolating pre-extracted sprucewood for 8 days with 2% aqueous acetic acid at 1 0 0 ° C . This particular β-aryl


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FRACTION A (12 % )

Figure 6.

Effluent fraction A FRACTION Β (5 7.)

Figure 7.

Effluent fraction Β

ether must be present in lignin as an end group, bound to the rest of the molecule by an easily hydrolyzable linkage, presumably a benzyl aryl ether linkage. The arylglycerol β-aryl ether linkages may remain largely un affected by the hydrolytic extraction method of Nimz whereas cleavage of such linkages will occur throughout the whole lignin molecule in the con siderably more drastic acidolysis procedure (0.2M H Q ) used in our work. Therefore, isolating 6% ω - h y d r o x y g u a i a c y l a c e t o n e (XII) from B j ô r k man lignin also provides quantitative information. If this yield is compared with the yield of X I I from the model compounds X , considering the fact that only the noncondensed arylglycerol units of lignin can give rise to free ketol (XII), it tends to indicate that about 25% of the phenylpropane units of lignin are arylglycerol units connected to an adjacent unit by a β-aryl ether link. This figure, which would include both noncon-


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densed and condensed arylglycerol units, agrees with our previous estimate (6) which was based on purely analytical data. However, it may well be considered as a lower limit, since C — C condensation between benzyl al coholic groups and phenolic nuclei may occur during the acidolysis reaction, which would result in a decreased yield of ketol (XII). Although in the 4 hours of acidolysis of the model X such condensation seems to be negli gible (cf. the high yield of guaiacol, X I I I ) , it may be favored in the acidoly sis of lignin by closer vicinity of the reacting groups. A small amount of the previously unknown ^ - h y d r o x y p h e n y l - ω hydroxyacetone ( X X ) , obviously derived from />-hydroxyphenylglycerol β-aryl ether elements, was also isolated from fraction A , and its structure was established by synthesis {26). Finally, coniferaldehyde ( X X I ) was found {26) in fraction A in a quan tity of about 0.5% of the lignin. The major portion of X X I probably originates from coniferaldehyde end groups linked to the lignin molecule by hydrolyzable ether bonds—i.e., a- and β-aryl ether bonds. In agree ment with this view, we found that borohydride-reduced B j ô r k m a n lignin, when subjected to acidolysis, gave only traces of coniferaldehyde. The fraction of the Sephadex eluate, which corresponded to peak Β (Figure 5), contained 5% of the weight of the B j ô r k m a n lignin used in the acidolysis experiment. We expected it to contain dimeric acidolysis products. So far, five pure compounds ( X X I I - X X V I ) have been obtained from fraction B . Structure ( X X I V ) (3,4-divanillyltetrahydrofuran) for one of these products, although still awaiting final confirmation, is proposed on the basis of ultraviolet, infrared, and mass spectra of the substance and of further analytical data. The ( — )-enantiomer of compound ( X X I V ) (m.p., 117°C.) has been prepared by Freudenberg and Knof {14) from ^-pinoresinol, and on the basis of chromatographic evidence it was suggested as being present in the extractives of sprucewood. Since the acidolysis product has m.p. 1 3 5 ° C , it is apparently not identical with the substance described by Freudenberg and Knof; the possibility that our product might be the racemate of X X I V was then examined by synthesizing the latter from

The Structure and Reactivity of Lignin

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