(717g) Mechanistic Study of Diaryl Ether Bond Cleavage during Lignin Hydrogenolysis Using Advanced Lignin Model Compounds

Lignin is a renewable, CO₂-neutral, and widely available aromatic resource for replacement of fossil-derived products. Reductive catalytic fractionation (RCF) of lignin is one of the most promising lignin upgrading methods for lignin valorization. Catalytic cleavage of β-ether (alkyl–O–aryl) has been widely studied in the past. However, the cleavage of another ether linkage, the 4–O–5 (diaryl ether) has not been brought to the forefront. With a higher bond dissociation energy (BDE) compared to aliphatic ethers, the diaryl ether bond is more challenging to cleave. Hydrogenolytic cleavage of strong aryl C­–O bonds over heterogeneous metal catalysts requires high temperatures and H₂pressures. Hartwig et al. did a series of elegant studies on the mechanism of diaryl ether cleavage under a homogeneous nickel catalyst with the aid of NaOtBu base at low hydrogen pressure. Lercher et al. proposed a mechanism of palladium-catalyzed hydrolytic cleavage of diaryl ether bonds. Li et al. reported a palladium-catalyzed cross-coupling of diaryl ethers with pyrrolidine. These recently reported mechanisms contradict the conventional acid-catalyst ether bond cleavage mechanism.

As a polymer biosynthesized via the general phenylpropanoid pathway, three monolignols, p-coumaryl alcohol (H), coniferyl alcohol (G), and sinapyl alcohol (S), are involved in lignification. The extra methoxyl, phenol, and sidechain on the aromatic ring dramatically affect the electron distribution and the chemical properties of the substrate. Thus, using proper lignin model compounds is crucial for lignin depolymerization studies; minor changes in the structure of lignin model compounds may have a huge effect on the reaction mechanism. Diaryl ether linkages in lignin, the presence of which was recently confirmed by using appropriate lignin model compounds, are more complicated than simple diphenyl ethers. Thus, it is necessary to examine the reaction mechanism of diaryl ethers in lignin using authentic lignin model compounds under catalytical hydrogenolysis condition.

In this work, a set of lignin model compounds from dimers to tetramers has been subjected to the hydrogenolysis reaction. A set of new meta-substituted aromatic products were first identified by NMR and high-resolution MS. The mechanism of the formation of the rearrangement products and sidechain-truncation products are discussed.