Transformation of lignin model compounds toN-substituted aromaticsviaBeckmann rearrangement

Abstract

Here we present the highly effective cleavage of C–C bonds in lignin model compounds for the production of N-substituted aromatics in up to 96% total yield, including benzonitriles and amides, via oxime formation followed by Beckmann rearrangement (BR).

Catalytic [3+3] Annulation of β‐Ketoethers and Cyclopropenones via C(sp3)—O/C—C Bond Cleavage under Transition‐Metal Free Conditions

“…[ 5 ] These strategies usually require harsh conditions or transition‐ metal‐based catalysis. Only some transition‐metal catalysis free routes for the C—O bond cleavage in keto aryl ethers have been developed in recent years, [ 6‐9 ] through ionic liquid inducing oxidation process or classic Baeyer‐Villiger oxidation reaction with stoichiometric oxidant reagent (Scheme 1a). [ 8 ]…”

Redox-neutral photocatalytic strategy for selective C–C bond cleavage of lignin and lignin models via PCET process

“…Wang and co-workers [24] reported a photocatalytic oxidative strategy for C-C bond cleavage along with C b -O bond cleavage in lignin b-O-4 linkages using a mesoporous graphitic carbon nitride catalyst (mpg-C 3 N 4 ), furnishing 4-methoxybenzaldehyde, 4-methoxybenzoic acid and 2methoxyphenol in 36%, 37% and 22% yield, respectively (Scheme 1c). From the above overview, there are still some challenges to be solved in the selective cleavage of C-C bonds of lignin and lignin model compounds, such as harsh reaction conditions [18,19], inefficient C-C bond cleavage, low product yield and selectivity as well as being accompanied with the cleavage of C-O bonds or loss of c-CH 2 OH sometimes [24,[26][27][28][29]. Therefore, it is a great impetus for researchers to develop new strategy to solve these problems.…”

“…Oxidation of a-OH into ketone structure with a lower dissociation energy of C-O bond has been demonstrated to be an effective strategy for lignin depolymerization [30][31][32][33][34]. Recently, we successfully applied the classic organic named reaction to the selective cleavage of C-C bonds in oxidized b-O-4 lignin models and produced various useful platform chemicals in high to excellent yields, such as multifunctional esters through Baeyer-Villiger oxidation [28] and N-substituted aromatics via Beckmann rearrangement [29]. However, instead of pretreating a-OH or c-OH at the expense of extra energy cost and multi-steps, it would be more desirable to develop a depolymerization strategy with better atom economy for selective C-C bond cleavage by taking full advantage of these a-OH groups of lignin rather than pretreating or protecting them.…”

Photocatalytic Cleavage of Aryl Ether in Modified Lignin to Non-phenolic Aromatics

“…Aerobic oxidation, − retro-aldol reaction, decarbonylation, and hydrogenation methods have been performed to cleave the aliphatic C–C bond and afford the aromatic acids, aldehydes, and so on. Using sodium azide, oxidation, and rearrangement reactions could induce the cleavage of the C aryl –C α bond. Despite their versatility, most of these transformations did not involve the cleavage of the aryl ether bonds (C 4′ –O); thus, the depolymerized products were limited to phenolic monomers.…”