Direct and selective hydrogenolysis of arenols and aryl methyl ethers

Abstract

For valorization of biomass, the conversion of lignin to deoxygenated bulk aromatic compounds is an emerging subject of interest. Because aromatic rings are susceptible to metal-catalysed hydrogenation, the selective hydrogenolysis of carbon-oxygen bonds still remains a great challenge. Herein we report direct and selective hydrogenolysis of sp 2 C-OH bonds in substituted phenols and naphthols catalysed by hydroxycyclopentadienyl iridium complexes. The corresponding arenes were obtained in up to 99% yields, indicating the possible production of arenes from lignin-derived bio-oils. Furthermore, the same catalysts were applied to the unprecedented selective hydrogenolysis of the sp 3 C-O bonds in aryl methyl ethers. Thus, the hydrodeoxygenation of vanillylacetone, a lignin model compound, afforded alkylbenzenes as the major products via triple deoxygenation.

Selective production of arenes via direct lignin upgrading over a niobium-based catalyst

“…This suggests that the lignin was first depolymerized to phenolic monomers via multiple steps of C–O hydrogenolysis to cleave C–O–C ether bonds, and the resultant phenolic monomers were sequentially converted to arenes and cycloalkanes by further hydrogenolysis over Ru/Nb 2 O 5 . Although depolymerization of model compounds of lignin over homogeneous or heterogeneous catalysts is widely reported to generate phenols by the cleavage of aromatic C–O bonds in aryl ethers23242526, further hydrogenolysis of the C aromatic –OH bond in phenols to produce fully deoxygenated hydrocarbons is problematic and hardly achieved. Interestingly, the Ru/Nb 2 O 5 catalyst integrated the depolymerization and hydrogenolysis steps in lignin conversion to selectively produce arenes.…”

“…Preserving the aromatic rings while selectively cleaving the C–O bonds in phenols is a highly challenging task because the C–O bond in aryl ethers is strong, especially for that in phenol (414 kJ mol −1 ). There is therefore a well-known competition between the hydrogenolysis of C–O bonds and the hydrogenation of the aromatic rings during the lignin hydrodeoxygenation23242526. Unfortunately, the latter is thermodynamically favoured and, as a result, selective production of arenes from lignin upgrading is rarely reported27.…”

Chemo‐ and Regioselective Hydrogenolysis of Diaryl Ether C−O Bonds by a Robust Heterogeneous Ni/C Catalyst: Applications to the Cleavage of Complex Lignin‐Related Fragments

“…Theb enzyl-protected biaryl ether 10 was cleaved effectively in the presence of 7.5 mol %Ni/C and as toichiometric amount of the base KHMDS [17] to furnish eight aromatic monomers described by structures 11-14 in 85 %c ombined yield (Scheme 1). Theb enzyl-protected biaryl ether 10 was cleaved effectively in the presence of 7.5 mol %Ni/C and as toichiometric amount of the base KHMDS [17] to furnish eight aromatic monomers described by structures 11-14 in 85 %c ombined yield (Scheme 1).…”

Concerted Nucleophilic Aromatic Substitution Reactions

“…Theproposal of ac oncerted mechanism was backed up by computational analysis,w here as ingle transition state was observed for the conversion of 140!141 with formation of ap artial negative charge,c onsistent with ac S N Ar process (DG* = 14.7 kcal mol À1 ,F igure 2). Chibasd emethoxylation studies feature deprotonated amines as nucleophiles.D emethoxylation by ah ydroxycyclopentadienyl iridium hydride nucleophile has been proposed as ac oncerted nucleophilic aromatic substitution by Kusumoto and Nozaki, [59] although no mechanistic evidence has yet been revealed to support this.…”