Back Cover: Protection Group Effects During α,γ‐Diol Lignin Stabilization Promote High‐Selectivity Monomer Production (Angew. Chem. Int. Ed. 5/2018)

Abstract

Selective hydrogenolysis … …ofisolated lignin is achieved using tuned protection groups during mild acid-catalyzed lignin extraction and an appropriate hydrogenolysis catalyst. In their Communication on page 1356 ff., J. S. Luterbacher et al. report near-theoretical yields of monomers and, with wild-type hardwood, recovery of as few as two major products.Asingle major product is produced using high-syringyl transgenic hardwood as the source of lignin.

Chemicals from Lignin: A Review of Catalytic Conversion Involving Hydrogen

“…In this strategy, formaldehyde hinders the formation of new C−C bonds by forming a stable six‐membered 1,3‐dioxane structure with 1,3‐diols on lignin side‐chains, which blocks the formation of benzylic cations, thus preventing repolymerization. Subsequently, the same group further developed other diol protection reagents, such as acetaldehyde and propionaldehyde . The subsequent hydrogenolysis catalyzed by Pd/C generated lignin monomers at near‐theoretical yields based on Klason lignin, and with high selectivity to a single 4‐ n ‐propanolsyringol product (80 %) in the case of the transgenic hybrid poplar.…”

An uncondensed lignin depolymerized in the solid state and isolated from lignocellulosic biomass: a mechanistic study

“…[21][22][23] After the cleavage of the β-O-4-aryl ether bond, the resulting C2-aldehyde (a phenylacetaldehyde fragment) and Hibbert's ketone (HK) end-groups are vulnerable to aldol condensation under acidic conditions, forming other inert C-C linkages. 10,24 Previous studies have explored different means of avoiding or reducing the condensation reactions by introducing trapping/capturing reagents or pretreatment by, for example, trapping the benzyl carbocation using aromatic compounds such as phenol, 22,23 protecting the α-carbon from condensation by forming a 1,3-dioxane structure with the αand γ-hydroxyls of lignin using formaldehyde 12 or other aldehydes, 25 or in situ capturing of the unstable postcleavage carbonyls, e.g., the C2-aldehyde and HK, by producing acetals using diols such as ethylene glycol. 10,11,15 Intrinsically, the success of lignin depolymerization to produce uncondensed lignin relies on the competition between ether cleavage and condensation.…”