Chemoselective Metal-Free Aerobic Alcohol Oxidation in Lignin

Abstract

An efficient organocatalytic method for chemoselective aerobic oxidation of secondary benzylic alcohols within lignin model compounds has been identified. Extension to selective oxidation in natural lignins has also been demonstrated. The optimal catalyst system consists of 4-acetamido-TEMPO (5 mol %; TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) in combination with HNO3 and HCl (10 mol % each). Preliminary studies highlight the prospect of combining this method with a subsequent oxidation step to achieve C-C bond cleavage.

Oxidation of Alcohols and Activated Alkanes with Lewis Acid-Activated TEMPO

“…9 We also explored the reaction of complexes 1 and 2 with 1,2-diphenyl-2-methoxyethanol (8), a common -1 lignin model compound. 10,12,21 Oxidation of 8 with complex 1 results in complete consumption of the starting material and formation of 2-methoxy-1,2-diphenylethanone (9) in 75% yield (Table 1, Entry 3). No other oxidation products were observed in the reaction mixture, according to 1 H NMR spectroscopy.…”

“…Herein, we report the reactivity of 1 and 2 towards a variety of alcohols and activated alkanes, including xanthene. 10,[16][17][18][19][20][21][22][23][24] The latter substrate is significant, because its reactivity confirms that these oxidations can proceed via a concerted proton coupled electron transfer (CPET) step, as was previously surmised. 14 To test the role of the Lewis acid in activating the TEMPO moiety, we also explored the reactivity of TEMPO with FeBr 3 in Et 2 O.…”

Heterogeneously catalyzed lignin depolymerization

“…A chemoselective method for the aerobic oxidation of secondary benzylic alcohols, in the presence of unprotected primary alcohols (abundant in lignin), has also been developed by combining 4-acetamido-TEMPO (as a radical oxidant) together with HNO 3 and HCl [83,84]. This approach has been applied to wood pulp delignification, in which oxidation of the C a alcohol to a ketone favours the breakdown of b-O-4 linkages.…”

Review on Catalytic Cleavage of C–C Inter-unit Linkages in Lignin Model Compounds: Towards Lignin Depolymerisation

“…This oxidation methodology has been extended to the depolymerisation of real lignin (Aspen lignin). Through detailed analysis of the product mixture, they propose C-C inter-unit cleavage [67].…”