Studies on the condensation of lignins in alkaline media. Part II. The formation of stilbene and arylcoumaran structures through neighbouring group participation reactions

Abstract

JOSEF GIERER and INGEGERD PETTERSSON. Can. J. Chem. 55, 593 (1977). The alkaline condensation of p-hydroxyarylglycerol-a-aryl ether units in lignin with other phenolic structures and the subsequent reactions of the condensation products have been elucidated using appropriate model compounds. It is shown that the primary condensation of simple phenols (2,6-xylenol or resorcinol) into the a-position of phenolic compounds of the f3-aryl ether type is followed by an aryl participation (A,-3) reaction resulting in the cleavage of the a-aryl ether bond and formation of the corresponding stilbene derivative. When resorcinol is used as reaction partner, the condensation is also followed by the cleavage of the P-aryl ether linkage through another type of neighbouring group participation reaction involving the phenolate anion ortho to the site of condensation and giving rise to the corresponding arylcoumaran derivative.The consequences of these findings for the course of alkaline delignification processes are briefly discussed. Chem. 55, 593 (1977). On a ClucidC, utilisant des composes modiles approprits, le mecanisme de la condensation alcaline d'unites d'ether p-hydroxyarylglycCro1 a-aryle de la lignine avec d'autres structures phCnoliques et les rCactions subsCquentes des produits de condensation. On a montr6 que la condensation primaire de phenols simples (xylenol-2,6 ou rCsorcino1) en position a des composes phCnoliques du type ether a-aryle est suivie par une reaction de participation du groupe aryle (A1-3) qui conduit au bris du lien ether P-aryle et a la formation du derive stilbene correspondant. Quand on utilise le resorcinol comme partenaire de la rCaction, la condensation est aussi suivie par une coupure du lien ether a-aryle par un autre type de reaction de participation de groupe voisin impliquant I'anion phenolate ortho par rapport au site de condensation et donnant lieu au dCrivC arylcoumarane correspondant.On discute des consCquences de ces resultats pour les processus de delignification alcaline. [Traduit par le journal]Numerous studies on the alkaline condensation of lignins have shown that the benzylic position in phenolic units constitutes the main site of reaction in this complex process (1-6). In most of these studies, model compounds of the phydroxybenzyl alcohol type were reacted with simple phenols and the resulting diarylmethanes were isolated. The condensation reaction was formulated as a nucleophilic addition of an ortho-or para-cyclohexadienone carbanion to the appropriate para-quinonemethide, followed by proton or side-chain elimination with rearomatization (7). Results from studies using model compounds of the y-hydroxyaryl coumaran (8) and P-arylglycerol (9) types indicated that this conjugate addition reaction is not prevented by the presence of an aryi or an aroxy group in the adjacent P-position (cf. also condensations in acidic media (10)). Thus, carbanions formed from phenolic lignin structures compete, in fact, with the other nucleophiles present in the pulping liquor (SH-, OH-ions) ...

Characterization of four different lignins as a first step toward the identification of suitable end‐use applications

“…The sulfonation of lignin under alkaline conditions and a high temperature was reported in the past. 44,45 Also, it should be highlighted that the sulfonate and carboxylate groups and total charge density of LS2 were higher than those of other samples. These results are in harmony with observed peaks of carboxylate and sulfonate groups in FTIR analysis.…”

Conversion of technical lignins to functional materials with retained polymeric properties

“…Specifically, under acidic conditions, phenols were introduced at the aposition of lignin [94][95][96][97][98][99][100]. Phenolation can also occur under alkaline conditions, and the reaction mechanism is similar to that of condensation that occurs during kraft cooking [101,102]. Kobayashi et al [103][104][105] subjected kraft lignin to phenolation and observed a 2-2.5 times higher reactivity than that of unmodified kraft lignin.…”

Toward Thermoplastic Lignin Polymers; Part II: Thermal & Polymer Characteristics of Kraft Lignin & Derivatives

“…Thermally induced events possibly occurring within softwood kraft lignin operating via radical mechanisms at low temperatures. (Gierer 1980;Gierer and Pettersson 1977;Kawamoto et al 2007a;Kawamoto et al 2007b;Kawamoto et al 2008a;Kawamoto et al 2008b;Nakamura et al 2007;Ohashi et al 2011) At relatively low temperatures, radicals stabilized and trapped by steric factors within the solid structure of lignin (Sazanov and Gribanov 2010) may be activated (Scheme 2A). Phenoxy radicals can be generated via an H abstraction process when interacting with the thermally activated radicals (•R).…”

“…Since β-O-4 linkages have been reported to survive the kraft pulping process (Froass et al 1996;Jiang and Argyropoulos 1999;Jiang and Argyropoulos 1994;Liitiä et al 2003), the reported formation of formaldehyde at relatively low temperatures (Fenner and Lephardt 1981) can be explained by γ elimination of methylol (CH 2 -OH) groups within residual β-O-4 linkages in kraft lignin (Scheme 3A) (Gierer 1980;Gierer and Pettersson 1977) via quinone methide intermediates, which are readily formed from the phenoxy radicals derived from the phenolic OH groups. In addition to the elimination of the γ methylol (CH 2 -OH) group of the β-O-4 linkage creating a stable enol ether (Scheme 3A) (Gierer 1980;Gierer and Pettersson 1977;Kawamoto and Saka 2007), homolytic C β -O bond scission is a competing reaction pathway, responsible for the formation of two new phenoxy radicals (Scheme 3B) (Nakamura et al 2007).…”

“…In addition to the elimination of the γ methylol (CH 2 -OH) group of the β-O-4 linkage creating a stable enol ether (Scheme 3A) (Gierer 1980;Gierer and Pettersson 1977;Kawamoto and Saka 2007), homolytic C β -O bond scission is a competing reaction pathway, responsible for the formation of two new phenoxy radicals (Scheme 3B) (Nakamura et al 2007). Derived from phenoxy radicals, the quinone methide intermediate can also undergo a re-aromatization process through a series of resonancestabilized radical mesomers (Schemes 3A and 3B) (Kawamoto et al 2007a,b).…”