Lignin‐Derived Thioacidolysis Dimers: Reevaluation, New Products, Authentication, and Quantification

Abstract

Lignin structural studies play an essential role both in understanding the development of plant cell walls and for valorizing lignocellulosics as renewable biomaterials. Dimeric products released by selectively cleaving β–aryl ether linkages between lignin units reflect the distribution of recalcitrant lignin units, but have been neither absolutely defined nor quantitatively determined. Here, 12 guaiacyl‐type thioacidolysis dimers were identified and quantified using newly synthesized standards. One product previously attributed to deriving from β–1‐coupled units was established as resulting from β–5 units, correcting an analytical quandary. Another longstanding dilemma, that no β–β dimers were recognized in thioacidolysis products from gymnosperms, was resolved with the discovery of two such authenticated compounds. Individual GC response factors for each standard compound allowed rigorous quantification of dimeric products released from softwood lignins, affording insight into the various interunit‐linkage distributions in lignins and thereby guiding the valorization of lignocellulosics.

An Introduction to Model Compounds of Lignin Linking Motifs; Synthesis and Selection Considerations for Reactivity Studies

“…The dimerization of 4-propyl guaiacol provides the 4-O-5 dimer as the minor component (8 %) in the product mixture whereas the 5-5-linked primary product 42 is obtained in a5 6% yield, as discussed in the previouss ection. [107] As imilar enzymatic dimerizationr eactionh as been reportedw ith ap henolic G-G b-O-4 Type C, giving only 2.9 %o f the 4-O-5 coupled dimer. [23]…”

“…Enzymatic dimerization (b-5 Method1b) using HRP hasp roved quite easily scalable as it is carriedo ut in aqueous conditions and has been used frequently to carry out this conversion with yields in the range of 30-50 %. [103][104][105]107] From ap ractical perspective, b-5 Method1bh as significant advantages over b-5 Method 1a; that is, less use of organic solvents, no necessity to go through extensive drying and degassing procedures, the generation of less waste, and ease of scale-up lead to ap reference for the use of this method. Access to b-5 models with S-S substitution patterns is not possible due to the lack of af ree 5-position on the aromatic ring.…”

“…[3] Model compounds that have been used to study the 4-O-5 motif range from the widely used simple and commercially available diphenyl ether [140][141][142] to substituted diphenyl ethers synthesized via Cucatalyzed arylation of phenols (49)w ith aryl halides( 48), [140,143] to products of radicald imerization reactions. [107,144,145] Examples of the models that have been used are shown in Figure 8. Linking motif models of the type 4-O-5 Type Ac ontain ether-linked aryl groups;h owever,t hese lack the aromatic substitution patterns seen in lignin.L inking motif modelso ft he type 4-O-5 Type Bw ith lignin-like substitution patterns on the aromatic rings offer ac loser match to the native 4-O-5 linkage.…”

Mechanistic Study of Diaryl Ether Bond Cleavage during Palladium‐Catalyzed Lignin Hydrogenolysis

“…[33] Access to the diaryl ether core of the 4-O-5' model 2 was achieved through a biomimetic oxidative coupling reaction of propylguaiacol promoted by horseradish peroxidase and H 2 O 2 . [34] This reaction and synthetic steps leading to the other model compounds are elaborated in the Supporting Information (Scheme S1).…”

On the Meaning and Origins of Lignin Recalcitrance: A Critical Analysis of the Catalytic Upgrading of Lignins Obtained from Mechanocatalytic Biorefining and Organosolv Pulping

“…In this assay, BF 3 etherate acts as a catalyst for the stepwise and selective thioethylation (with EtSH) of any alcohol or ether group . β‐ O ‐4 linkages are selectively cleaved by thioacidolysis, rendering characteristic monoaromatic species 2‐methoxy‐4‐(1,2,3‐ tris (thioethyl)propyl)phenol, 1 , and 2,6‐dimethoxy‐4‐(1,2,3‐ tris (thioethyl)propyl)phenol, 2 , as primary products, derived from G‐ and S‐units, respectively (Scheme S1, SI). Moreover, various other minor products are formed from the thioacidolysis procedure (a detailed description is provided in our recent publication).…”