Iridium-catalysed primary alcohol oxidation and hydrogen shuttling for the depolymerisation of lignin

Abstract

A new Ir catalysed approach for the selective cleavage of the Cα–Cβ bond in lignin β-O-4 units, allowing access to novel and tuneable monomeric product mixtures.

Selective Primary Alcohol Oxidation of Lignin Streams from Butanol‐Pretreated Agricultural Waste Biomass

“…Previous reports on β‐ O ‐4 γ‐oxidation have focused on the generation of aldehydes (lignin γ‐oxALD ) in model systems and en route to lignin depolymerization . To our knowledge, only three reports have described the preparation of model β‐ O ‐4 γ‐carboxylic acids and organosolv lignin γ‐oxCA .…”

Selective Depolymerisation of γ‐Oxidised Lignin via NHC Catalysed Redox Esterification

“…and the NHC catalysed redox‐esterification of α‐phenoxyaldehydes by Smith et al., we proposed a new two‐step depolymerisation route for butanosolv lignins (Figure c). We believed that the incorporation of BuOH into the β‐aryl ethers would be key to achieving this selective transformation as it not only simplifies the selectivity challenges in alcohol oxidation, but also prevents the usually observed retro‐aldol Cα‐Cβ fragmentation pathway following conversion of the primary alcohol to the aldehyde …”

“…We believed that the incorporation of BuOH into the β-aryl ethers would be key to achieving this selective transformation as it not only simplifies the selectivity challenges in alcohol oxidation, but also prevents the usually observed retro-aldol Cα-Cβ fragmentation pathway following conversion of the primary alcohol to the aldehyde. [12,20] We started our investigations with the synthesis of a series of oxidised dimeric model compounds (1 a-d) (Scheme S1) which represent the different types of units that would be found in oxidised soft or hardwood butanosolv lignins (lignin γÀ°x ). The NHC catalysed redox-esterification step was then investigated using 1 a-d and a previously reported triazolium based pre-catalyst 2.…”

“…Thus, a beech butanosolv lignin was prepared according to previous reports [18] and the oxidation of the primary alcohol groups to aldehydes in Scheme 1. Previously reported two-step oxidative approaches to lignin depolymerisation via a) secondary [6][7][8][9][10][11] or b) primary alcohol [12][13][14] oxidation of native type β-aryl ether linkages lignin compared to this work c) employing selective oxidation and NHC catalysis for the depolymerisation of butanosolv lignin βaryl ether linkages. the β-aryl ether linkages was attempted using the biphasic TEMPO/NCS catalytic system developed by Einhorn et al [22] In contrast to our previous reports on the direct TEMPO-catalysed oxidation of this alcohol to carboxylic acids, [23] attempts to perform this reaction on the butanosolv lignin failed to provide the expected aldehyde product.…”

Using Fractionation and Diffusion Ordered Spectroscopy to Study Lignin Molecular Weight.

“…[1,2] However, the valorization of lignin remains a challenge due to its complex structure. [3][4][5][6][7][8][9] Its inherent heterogeneity is a result of at least three contributing factors: i) Lignin is made up of variable amounts of 3 possible C9 monomers (H, G, and S, Figure 1A); ii) different types of linkages are formed by coupling of the C9 monomers, the most abundant of which is the β-O-4 linkage ( Figure 1B); iii) Lignin has considerably higher variation in chain lengths compared to standard polymers leading to a high polydispersity (2.0-9.0 depending on the source and pretreatment method used, Figure 1C). [10] It is becoming evident that for any lignin application, discrete regions of the bulk molecular weight distribution (MWD) will be most suitable.…”