One‐Pot Catalytic Conversion of Lignin‐Derivable Guaiacols and Syringols to Cyclohexylamines

Abstract

Cyclic primary amines are elementary building blocks to many fine chemicals, pharmaceuticals, and polymers. Here, a powerful one‐pot Raney Ni‐based catalytic strategy was developed to transform guaiacol into cyclohexylamine using NH3 (7 bar) and H2 (10 bar) in up to 94 % yield. The methodology was extendable to the conversion of a wider range of guaiacols and syringols into their corresponding cyclohexylamines. Notably, a crude bio‐oil originating from the reductive catalytic fractionation of birch lignocellulose was transformed into a product mixture rich in 4‐propylcyclohexylamine, constituting an interesting case of catalytic funneling. The isolated yield of the desired 4‐propylcyclohexylamine reached as high as 7 wt % (on lignin basis). Preliminary mechanistic studies pointed at the consecutive occurrence of three key catalytic transformations, namely, demethoxylation, hydrogenation, and amination.

Aniline Derivatives from Lignin under Mild Conditions Enabled by Electrochemistry

“…After establishing optimal conditions for the spiroannulation of 1G , we turned our attention to the electrochemical oxidation of several additional lignin‐derived phenols of interest (Table 2). The oxidation of 4‐propanolsyringol ( 1S ), another promising building block for the synthesis of nitrogen‐containing chemicals, [10–11,38‐39] was next tested to evaluate the effect of the double phenol ortho ‐substitution by electron‐donor ‐OMe groups. The double substitution at C‐2 and C‐6 positions influences the electronic properties of the aromatic ring, imparts steric hindrance and consequently the keto‐enol tautomerization step [40] .…”

“…Throughout the last years, our group has successfully developed reductive catalytic fractionation (RCF) approaches mediated by copper porous metal oxides (Cu 20 PMO) towards the highly selective production of 4‐propanolguaiacol 1G (>90 %) from pine lignocellulose finding a wide array of promising applications for this bio‐based platform chemical [23a,27] . Furthermore, a notable high yield of crude aromatic bio‐oil and selectivity towards formation of 4‐propylguaiacol ( 2G ) has been reported by Sels and co‐workers when RCF was mediated by Ru/C [44] .…”

“…More specifically for the electrochemical oxidation of lignin‐derived phenols, we envisage that the inherent functionalities might play an active role in the formation of more reactive cyclohexadienone intermediates. For instance, lignin‐derived 4‐propanolguaiacol ( 1G ) and 4‐propanolsyringol ( 1S ), recurrent platform chemicals obtained by reductive catalytic fractionation (RCF), [27] undergo spiroannulation reactions towards the formation of spirodienones, [28,29] previously demonstrated by chemical oxidation. Additionally, the anodic oxidation of 2‐alkoxyphenols such as 4‐propylguaiacol ( 2G ), [30,31] 4‐ethylguaiacol ( 3G ) and respective syringol units ( 2‐3S ), might also lead to the generation of ortho / para quinone monoketals [32] in the presence of MeOH as both solvent and nucleophile (See Figure 1B).…”

Sustainable lignin modifications and processing methods: green chemistry as the way forward

“…This approach has already been successfully demonstrated for the conversion of lignin model compounds (or lignin-derived platform chemicals) such as guaiacol, syringol and phenol in cyclohexylamines or anilines. [167][168][169][170][171][172] The latter molecules are important precursors for the synthesis of nitrogen-containing alkyl and aryl chemicals. Primary amines can be applied avoiding the use of ammonia.…”

Deriving high value products from depolymerized lignin oil, aided by (bio)catalytic funneling strategies

“…13). 160 Another example of catalytic funneling by means of amination concerned the amination of a softwood (pine) derived RCF lignin oil. In contrast to the above, this ran over a hydrogen borrowing methodology and gave highly pure 1G amine (as its HCl salt) in 4.6 wt% yield on a lignin basis.…”