Amine‐Mediated Bond Cleavage in Oxidized Lignin Models

Abstract

Introducing amines/ammonia into lignin cracking will allow novel bond cleavage pathways. Herein, a method of amines/ammonia‐mediated bond cleavage in oxidized lignin β‐O‐4 models was studied using a copper catalyst at room temperature, demonstrating the effect of the amine source on the selectivity of products. For primary and secondary aliphatic amines, lignin ketone models underwent oxidative Cα−Cβ bond cleavage and Cα−N bond formation to generate aromatic amides. For ammonia, the competition between oxygen and ammonia determined the selectivity between Cα−N and Cβ−N bond formation, generating amides and α‐keto amides, respectively. For tertiary amines, the lignin models underwent oxidative Cα−Cβ bond cleavage to benzoic acids. Control experiments indicated that amines act as nucleophiles attacking at the Cα or Cβ position of the oxidized β‐O‐4 linkage to be cleaved. This study represents a novel example that the breakage of oxidized lignin model can be regulated by amines with a copper catalyst.

Sustainable Production of Benzylamines from Lignin

“…Obviously, the present pathways bring advantages over reported b-O-4 conversion routes that need additional C a ÀOH oxidation step in the presence of oxidants and obtained different amination products. [14] Inspired by the above results, the production of benzylamines from the conversion of realistic lignin was investigated (Scheme 4). Direct amination of lignin feedstock using current condition is hard to realize, mainly because dehydrogenation of native lignin to form keto-functional lignin obeying the mechanism of lignin model compound in Scheme 3 over Pd/C did not occur, instead the competitive hydrogenolysis reaction provided small amount of phenol derivatives.…”

“…For example, lignin monomers (phenol, guaiacol and other monophenols) were converted to N-substituted anilines using ammonia, organic amine, hydrazine hydrate, and methyl glycinate as amine resource (Scheme 1, routes 1,2). [13] Amination of the lignin dimers was also explored, affording corresponding a-ketone amide [14] and isoxazole derivatives [15] (Scheme 1, routes 3,4) as the major products. However, the hydroxyl group is more difficult to be transformed than the carbonyl group, in this process, the oxidation of the b-O-4 model compounds to their a-ketone derivatives is an indispensable step in order to guarantee the amination reaction.…”

“…However, the hydroxyl group is more difficult to be transformed than the carbonyl group, in this process, the oxidation of the b-O-4 model compounds to their a-ketone derivatives is an indispensable step in order to guarantee the amination reaction. [14,15] Besides the above investigations, another important progress has been made in the conversion of diphenyl ether (4-O-5 linkage in lignin) over Pd(OH) 2 /C catalysts. [16] Formal cross-coupling of diaryl ethers with organic amines was achieved via dual C(Ar)ÀO bond cleavage, leading to amine derivatives as the targeted products.…”

NHC–palladium-catalyzed ionic liquid-accelerated regioselective oxyarylation of alkynes with diaryl ethers

“…Initially, the oxidative addition of Pd(0) with diaryl ether gives Ar–Pd–Oar intermediate I . 20 Then, the ionic liquids via hydrogen bonding activate the –Oar motifs of the Pd–Oar species to form palladium complexes II . 21 Simultaneously, the ligand exchange of palladium complexes II affords the Ar–Pd species III through the loss of phenol derivatives.…”

Catalytic N-methyl amidation of carboxylic acids under cooperative conditions

“…8 However, the synthesis of N -methyl amides compounds relies heavily on non-catalytic approaches. 5,9 Catalytic approaches were also investigated by Hisaeda, 10 Kundu, 11 Li, 12 Guo, 13 Yu, 14 Maruoka, 15 Wang, 16 Chen, 17 Lamaty 18 and their co-workers starting from nitriles, primiary amides, aldoximes, aldehydes, lignin, carbamoylsilane and alcohols. Until recently, Thakur, 19 Marce, 20 Sadeghzadeh 21 and their co-workers developed elegant N -methyl amidation approach starting from carboxylic acids under nano-MgO, diatomite Earth@IL/ZrCl 4 and Mg(NO 3 ) 2 ·6H 2 O catalysis respectively, while limitations like poor substrate scope or sophisticated tailored catalyst still persist.…”