Regioselectivity of Enzymatic and Photochemical Single Electron Transfer Promoted Carbon−Carbon Bond Fragmentation Reactions of Tetrameric Lignin Model Compounds

Abstract

New types of tetrameric lignin model compounds, which contain the common β-O-4 and β-1 structural subunits found in natural lignins, have been prepared and carbon-carbon bond fragmentation reactions of their cation radicals, formed by photochemical (9,10-dicyanoanthracene) and enzymatic (lignin peroxidase) SET-promoted methods, have been explored. The results show that cation radical intermediates generated from the tetrameric model compounds undergo highly regioselective C-C bond cleavage in their β-1 subunits. The outcomes of these processes suggest that, independent of positive charge and odd-electron distributions, cation radicals of lignins formed by SET to excited states of sensitizers or heme-iron centers in enzymes degrade selectively through bond cleavage reactions in β-1 vs β-O-4 moieties. In addition, the findings made in the enzymatic studies demonstrate that the sterically large tetrameric lignin model compounds undergo lignin peroxidase-catalyzed cleavage via a mechanism involving preliminary formation of an enzyme-substrate complex.

An Introduction to Model Compounds of Lignin Linking Motifs; Synthesis and Selection Considerations for Reactivity Studies

“…Others have taken the approacho fs ynthesizing as pecific linkingm otif or as eries of linking motifs and then combining them in discrete non-dimerization reactions to generate the desired oligomeric product. Each linking motif is combined with af unctional group or masked functional group, which can be used in subsequent steps to build-up the desired oligomer.T his approach is somewhat more versatile as it does not necessarily result in symmetrical modelc ompounds.T his approach has been used to synthesize trimers [58,[150][151][152][153][154] , and tetramers [151] (3 linking motifs (b-O-4) (b-O-4) (b-O-4)).A n example of this approach reported by Lahive et al [56] is shown in Scheme18i nwhich a( b-O-4) (b-5) model compound of general structure 58 is synthesized.T he approach essentially follows the b-5 Method1aa pproach of dimerizing 21 to generate the b-5 Type C-1 as outlined in Scheme 5. This was followed by methylation or tert-butyldimethylsilyle ther (TBS) phenol protection and an oxidative cleavage step to install an aldehyde group.T his allowed the application of the Nakasubo methodo fb-O-4 synthesis as outlined in Scheme 1b,f ollowed by reduction and, if necessary,T BS removal to give access to the desired (b-O-4) (b-5) model compound.…”

“…This approach is somewhat more versatile as it does not necessarily result in symmetrical model compounds. This approach has been used to synthesize trimers (2 linking motifs (β‐O‐4) (β ‐ 1), (β ‐ 5) (β‐O‐4) and multiple (β‐O‐4) (β‐O‐4)), and tetramers (3 linking motifs (β‐O‐4) (β‐O‐4) (β‐O‐4)). An example of this approach reported by Lahive et al .…”

Proton‐Coupled Electron Transfer in Photoredox Catalytic Reactions

“…Fragmentation of C–C bonds is also induced by photochemical electron transfer , , . When PCET is involved, such reactions can be carried out under mild conditions and complex polyfunctionalized compounds can be transformed.…”

Ruthenium‐Catalyzed CC Bond Cleavage in Lignin Model Substrates

“…[4,5] Hence,t he efficient catalytic cleavage of this specific linkage could pave the way for selective lignin depolymerization and utilization.Owing to their tunable reactivity and the accessibility of the polymer for molecular catalysts,h omogeneous catalysts hold great potential for the selective fragmentation of lignin. [4b] Several reductive, [6] oxidative, [7,8] and redox-neutral methods [9] for the cleavage of the b-O-4 linkage in lignin model compounds have been reported recently (Scheme 1). Thec leavage of CÀCb onds is so far limited to oxidative procedures and radical pathways.…”