5072
IRWIN A . [CONTRIBUTION FROM
THE
PEARL
Yol, T S
IXSTITUTE OF PAPER CHEXISTRY]
Studies on Lignin and Related Products. X1II.l Oxidations of Lignin Model Compounds and their Significance to Lignin Structure2 BY IRWXA. PEARL RECEIVED JULY 9, 19j6 Alkaline cupric oxide and alkaline nitrobenzene oxidation of lignin model compounds such as b-substituted vanillils anti bis-vanillyl compounds with 2-carbon side chains in the LY- and cy -positions yielded the $:me compounds that are obtained from analogous oxidations of coniferous lignosulfonate matcrialc. T h u i , the occurrence of all monomolecular and biinolecular guaiacyl compounds in such coniferous lignosulfonate oxidation mixtures can be nccouiit.ed tor by assuming for n porticin of the lignin complex a-LYlinkages betxeen two C,-C3 structural units. The sl-ntheses of 3,3 ’-diinethoxq-dietie~troland 3,3 ‘dimethoxyhexesterol diacetate are recorded.
A few years ago the possibility of the presence in tion conditions employed, and the state of oxidaa t least a part of the complex lignin structure of a tion of the starting materials precluded aldehyde linkage between the a-carbon atoms of two CC-Ca formation a t the a-carbon positions under oxidizunits was s ~ g g e s t e dand , ~ the hypothesis was sup- ing conditions. Therefore, it is apparent that the ported more recently when it was demonstrated products of oxidation of these model compounds that except for compounds having carbon chains in are very similar t o those obtained from lignosulthe 5-position of the guaiacyl group, the compounds fonate materials under analogous conditions. Alformed in alkaline cupric oxide oxidations of ligno- kaline nitrobenzene oxidation of these compounds sulfonate materials actually could be prepared by gave, in addition to the compounds noted above, similar oxidation of bis-vanillyl compounds con- .5-formylvanillovanillone, and an unidentified cartaining two c6-c units linked through their a- bonyl compound, probably 5-formylvanillil, incarbon atoms.4 Unfortunately, these bis-vanillyl dicating that bis-vanillyl compounds with 5-carbon compounds contained only C6-c units without d- substitution can account for all of the types of carbon chain substitution. The present paper pre- compounds isolated from alkaline cupric oxide and sents the results of a continuing study on the alka- alkaline nitrobenzene oxidations of lignosulfonate line nitrobenzene and alkaline cupric oxide oxida- materials. In order to determine whether the @tions of lignin model compounds with two c6-c and y-carbon atoms of similar guaiacyl comunits linked through their a-carbon atoms and con- pounds with c6-c3 units linked through their ataining 5-carbon side chains and with two c6-c3 carbon atoms would be removed under these oxiunits linked through their a-carbon atoms. The dizing conditions to yield the same reaction prodfirst group of model compounds comprises S-allyl- ucts, three such compounds were oxidized under vanillil, 5,5’-diallylvanillil, 5-propenylvanillil and both alkaline nitrobenzene and alkaline cupric oxide 5,5’-dipropenyl~anillil.~ The second group coni- conditions. Cupric oxide oxidation of 3,3’-dimethoxydiethylprises 3,3‘-dimethoxydiethylstilbestrol,63,3’-dimethoxyhexestrol and 3,3‘-dimethoxydienestrol. stilbestrol yielded a mixture of compounds similar These oxidations are compared with similar oxida- to that obtained from a corresponding lignosulfonate oxidation except t h a t no &substituted guaitions of coniferous lignosulfonate materials. Cupric oxide and oxidation of 5-allylvanillil acyl compounds were obtained. Vanillin, vanillic under conditions reported earlier3 and isolation of acid, acetovanillone, vanillil and vanillovanillone reaction products by means of chromatography were identified in the oxidation products. I t yielded products with and without the 5-carbon should be noted that this representative ‘ 2 6 . 4 3 side chain. Much of the original 5-allylvanillil dimer yielded acetovanillone as one of the products was recovered as its isomer, 5-propenylvanillil, of oxidation, and it is possible t o account for this under the conditions of the experiment. Other end-product of oxidation without assuming an aldol compounds isolated and identified were vanillil, configuration for the side chain in the C6-Ca struc5-propenylvanillovanillonej 5-allylvanillil, %pro- ture of lignin with subsequent reverse aldol under penylvanillic acid, 5-carboxymethylvanillil, 5-car- alkaline conditions to yield acetovanillone and forboxyvanillil, 5-carboxyvanillic acid and vanillic m a l d e h y ~ l e . ~It is entirely possible, however, that acid. Almost identical results were obtained with an aldol configuration is formed as an intermediate 5-propenylvanillil, and similar results were ob- in the oxidation of the instant C6-C3 dimer when its tained with 5,5’-diallylvanillil and 5,5’-dipropenyl- a - a carbon linkage is broken. Nitrobenzene oxidation of 3,3‘-dimethoxydivanillil except that in the case of the bis-substituted compounds, the yields of unsubstituted guaiacyl ethylstilbestrol gave essentially the same compounds as the chief products of oxidation. Paper chrocompounds were much less. Thus, no aldehydes were formed from the un- matography indicated even the same minor prodsaturated side chains under the cupric oxide oxida- ucts. Similar results were obtained when 3,3’diinethoxydienestrol and 3,3’-dimethoxyhexestrol ( I ) For paper XI1 of this series, see THISJ O U R N A L , 78, 4133 (1966,). ( 2 ) Presented before t h e Diviaion of Cellulose Chemistry at its were oxidized with nitrobenzene although in the Lignin Symposium a t the 130th hleeting of t h e Ameiican Chemical SI. lstter case all yields were a little lower. l’anderciety, Atlantic City. Xew Jersev, September 10 to 21, 19,50, linde and co-workers8recently reported the alkaline (3) I. A. Pearl and E . E. Dickey, THISJ O U R N A L , 74, 014 (1952). (4) I . A. Pearl and D. I,, Beyer, i b i d . , 7 6 , 2221 (19.54). ( 5 ) I. A. Pearl, i b i d . , ‘77, 2828 (19X). ( G ) I . A . Pearl, i b i d , 78, 4133 (1036).
( 7 ) A . v . Wacek and K . Kmtzl, J . P o l y m e r Sci , 3, ,539 (1918). (8) I-stalsmelting of the nitrobenzene, the mixture analyzed its described a t 246-247' and having the following maxima in its ultraviolet absorption spectrulu: ,A 235 m p , e 2642(!; A,,,,,, above. From tlie bisulfite-soluble fraction, ill addition to the other products noted, there was recovered a light yellow 323 inp, E 18500. Analyses arid properties correspond with crystalline solid melting a t 178-1i'Yo, with Xi 0.10 aiicl 5-carboxymethylvanillil. 10 : 3 : 3 butanol-pyridine-\1.ater Ri 0.85, and not depressing Anal. Calcd. for C18H16Ob: C , 60.00; H, 4.48. Pound: a mixed melting point with 5-forn~ylvanillova11illo1ie.~ C, 60.01: H , 4.64. The formation of this compound in the present reaction Fraction C contained two compounds having Ki's 0.11 indicates that the product isolated originally from the (5-carboxymethylvanillil) and 0.06. T h e fraction was oxidation of lignosulfonate material must have the assigned covered with acetone and filtered. The acetone filtrate structure. was cliroinatographed on paper, and the K f0.06 band was The other, allyl- and propenyl\wlillil compounds were eluted with 505;; acetone and concentrated under reduced oxidized by the micro method of Stone and Blundell,l2 and pressure to give colorless crystals of 5-carboxyvanillil the reaction mixtures analyzed by means of paper chrmiiatoghydrate melting a t 261-262" and not depressing a mixed raphy. All products were compared with identical mamelting point with 5-carboxyvanillil prepared by oxidation terials isolated in the macro-studies described above. of 5-propenylvanillil diacetate5 with potassium permaiigaCupric Oxide Oxidation of 3,3'-Dimethoxydiethylstilnate in boiling acetone and hydrolysis of the resulting 5- bestrol.--I mixture of 13 g . of 3,3'-dirnetIios~diethylcarboxyvanillil diacetate. The 10 :3 :3 butanol-pyridinestilbestrol diacetate,6 40 g. of sodium hydroxide, 825 g. of water Rfis 0.49 and the ultraviolet absorption spectrum has water and 100 ml. of ethanol was boiled under reflux one the following maxima: A,, 230 nip, E 26720; A s ~ l o u ~ ~ a rhour and then distilled under reduced pressure until all 292 mp, E 14770; ,,,A 330 m p , E 18710. ethanol was removed, maintaining the volume constant with Anal. Calcd. for C'7H160g: C, 56.04; H, 4.43. Found: water. The mixture was transferred to a stainless steel stirring autoclave and heated a t 170' for 3 hours and processed C, 56.10; H , 4.44. Fraction D was cliroinatographed on paper and found to as described above for 5-allylvanillil. Tlic reaction mixturc contain only 5-carboxpvanillil melting at 261-262" and was anal>-zedin the same mariner employed for similar oxidations of vanillil and its reduction products.* The main prodvanillil which was crystallized from acetic acid t o yield ucts of oxidation were vanillil, vanillic acid and acctovatiilcrystals nielting at 230-231' which did not depress a mixed lone, with smaller amounts of vanillin and vanillovanillonc. melting point with authentic vanillil. Fraction E was the largest fraction, comprising almost -111 products were identified by isolation and coinparisoil half of the bicarbonate-soluble material. This fraction with authentic samples. -1 number of other compounds were indicated by paper chromatography, but these have contained only one product, vanillic acid melting at 208not been isolated and identified as yet. 210" and not depressing a mixed melting point with authentic Other Oxidations of Compounds Related to 3,3'-Divanillic acid. Analysis of Bisulfite-soluble Fraction.-This relatively methoxydiethylstilbestro1.--.7,3' - Dirnethoxydiethylstilbestrol diacetate, 3,8'-di1nethoxydienestrol and 3,3'-dismall fraction was worked up in the same manner as the inethoxyhexestrol diacetate were oxidized with alkaline bicarbonate-soluble fraction to yield vanillic acid, vanillil, 5-propenylvanillil, 5-carboxymethylvanillil, 5-carbosyvan- nitrobenzene by the Stone and Blundell procedure, a i d the oxidation mixtures analyzed by means of paper chromatogillil and 5-carboxyvanillic acid. The total recovery of identified products from this raphy and comparison with the results of the large scale experiment is given in Table I . Because of the high re- cupric oxide oxidation of 3,3'-ditnethoxydietliylstilbestrol. covery of 5-propenylvanillil under these conditions, the XI1 oxidations gave the same products by chroniatogrnphy, experiment was repeated a t 180" for 5 hours and, in this but yields of individual compounds varied in the several experiments. The yields of all products appeared to be a case, the yields of all products, especially vanillil and little less in the experiment with 3,3'-dimethoxyhexestrol vanillic acid, increased a t the expense of 5-propenylvanillil. Other Oxidations of Allyl and Propenylvanillil Com- diacetate. p0unds.-~5,5'-Diallylvanillil, 5-propenylvanillil and 5,5'Acknowledgment.-The author wishes to thatik dipropenylvanillil were oxidized with alkaline cupric oxide Mr. Harold Willemsen for the analyses and ultraunder the same conditions, and the reaction products were treated in the same manner to give substantially the same violet absorption spectra reported in this paper. results. Iiidividual yields varied somewhat, and yields of __ unsubstituted oxidation products were lower. I n addition, tlie bis-allyl and bis-propenyl substituted compounds ga1-e ( 1 3 ) J . E . Stone arid hi. J. Ijlundell, d i s d l . ('hcui.,23, iil [1!131)
