Selective, Nickel-Catalyzed Hydrogenolysis of Aryl Ethers

Abstract

Selective hydrogenolysis of the aromatic carbon-oxygen (C-O) bonds in aryl ethers is an unsolved synthetic problem important for the generation of fuels and chemical feedstocks from biomass and for the liquefaction of coal. Currently, the hydrogenolysis of aromatic C-O bonds requires heterogeneous catalysts that operate at high temperature and pressure and lead to a mixture of products from competing hydrogenolysis of aliphatic C-O bonds and hydrogenation of the arene. Here, we report hydrogenolyses of aromatic C-O bonds in alkyl aryl and diaryl ethers that form exclusively arenes and alcohols. This process is catalyzed by a soluble nickel carbene complex under just 1 bar of hydrogen at temperatures of 80 to 120°C; the relative reactivity of ether substrates scale as Ar-OAr>>Ar-OMe>ArCH(2)-OMe (Ar, Aryl; Me, Methyl). Hydrogenolysis of lignin model compounds highlights the potential of this approach for the conversion of refractory aryl ether biopolymers to hydrocarbons.

Lignin depolymerization (LDP) in alcohol over nickel-based catalysts via a fragmentation–hydrogenolysis process

“…31 Other catalysts, such as CuCr oxide, 32 Co-Mo-S/Al 2 O 3 , 33 activated carbon-, alumina-or silica-supported Ru [34][35][36][37] or Pt, 38,39 have also been reported in hydrogenation of lignin or model compounds to monomeric phenols. Nickel-based catalysts have shown excellent chemoselectivity for aromatic products or high activity for C-O bond cleavage, as reported by Hartwig 40 and Rinaldi. 41,42 We recently reported a strategy of catalytic conversion of lignosulfonate into 4-ethylguaiacol and 4-propylguaiacol over heterogeneous nickel catalysts.…”

Depolymerization Pathways for Branching Lignin Spirodienone Units Revealed with ab Initio Steered Molecular Dynamics

“…Thus, depolymerization of unprocessed lignin remains of significant interest 8,16 and has been pursued experimentally with polyoxometalate catalysts 17 , base-catalyzed hydrolysis [18][19] , acidolysis [20][21] , ionic liquid treatment 22 , reduction with hydrosilanes 23 , photochemical degradation 24 , and ball milling 25 . In addition to general depolymerization, efforts have also included selective strategies via homogeneous catalysts [26][27][28][29][30] and enzymes 31 . A route to selective depolymerization that both prevents repolymerization and generates a specific set of monomers 32 has only recently had success with the aid of metal catalysts [33][34] capable of producing up to 50% yield.…”

Reductive Cleavage of C—O Bond in Model Compounds of Lignin

“…[31] Typically, a 100 mL round bottom flask was loaded with copper(I) iodide (1 mmol), pyridine-2-carboxylic acid (2 mmol), aryl iodide (10 mmol), phenol (12 mmol), potassium phosphate (20 mmol), DMSO (30 mL) and a magnetic stir bar. The reaction flask was sealed with a septum in an argon atmosphere.…”

“…[3][4][5] The 4-O-5 linkage (e.g., diaryl and aryl alkyl ethers) is a representative C-O linkage in lignin, and its catalytic cleavage is considered as an efficient way to depolymerize lignin for production of aromatics. [3,4,[6][7][8][9][10][11] Various metal catalysts such as Ni, [12][13][14][15] Ru, [16,17] Cu, [18][19][20][21] Pd/Zn, [22] V [23][24][25][26] and Fe, [27] have been reported for catalysing the cleavage of aryl C-O in lignin model compounds including diaryl ethers and aryl alkyl ethers via oxidation or hydrogenlysis. However, harsh reaction conditions including high temperature and complicated reaction systems are generally required, and the target product yields are relatively low.…”