Non-oxidative vanadium-catalyzed C-O bond cleavage: application to degradation of lignin model compounds.

Lignocellulosic biomass has recently received great interest as a renewable source of fuel and chemicals. Among the three major components of non-edible lignocellulose (cellulose, hemicellulose, and lignin), extensive efforts have been made to convert cellulose into ethanol and other biofuels. In contrast, research on the conversion of lignin has been limited to its removal from biomass either to enhance the accessibility of chemicals and enzymes to other components of biomass or to prevent the photo-yellowing of paper and pulp. Despite the fact that lignin corresponds to up to 30% of the weight and 40% of the energy content of lignocellulosic biomass, few novel processes aimed at producing high value compounds have been reported. Recently, several reports inspired by the pulp bleaching process have been published regarding the mechanism and product distribution of enzymatic and chemical oxidation reactions. Using dimeric lignin model compounds (e.g. 1) containing a b-O-4 linkage that represents the most common substructure in lignin, aromatic aldehydes were obtained as the main products in low yield. Although these methods show promise for selective conversion of lignin, fundamentally new catalytic processes need to be developed to fully realize lignin s potential as a chemical feedstock. Furthermore, a thorough understanding of the mechanism of these processes is necessary to achieve high selectivity. Aiming to develop a novel method to selectively convert lignin to highly functionalized aromatic compounds, we explored various homogeneous vanadium complexes for the conversion of 1 (Table 1). Most of the vanadium catalysts tested yielded benzylic alcohol oxidation product 4 as the major product in addition to small amounts of C O bond cleavage products 2 and 3 (entries 2–7). In spite of the low yield, the formation of 2 distinguishes this reaction from previous reports: not only is 2 a novel product, but it is also a redox-neutral transformation. Excited by this new reactivity, we explored other vanadium catalysts and found that tridendate Schiff base ligands favor C O bond cleavage over benzylic oxidation (entries 8–11). Higher selectivity for C O bond cleavage was observed when ligands with larger bite angles were employed (entries 8 vs. 9 and 10 vs. 11). The increased reactivity of catalyst 11 compared to 9 (entry 11 vs. 9) may be attributed to its tert-butyl substituents, which enable intermediates from 11 to remain as catalytically active monomeric species instead of forming insoluble aggregates. Thus, through subtle changes in the ligand structure, the reactivity of the vanadium(V)–oxo catalyst was tuned away from simple alcohol oxidation toward the cleavage of the bO-4 carbon–oxygen bond. To understand the role of oxygen in the formally nonoxidative process, the reaction was performed under anaerobic conditions with catalyst 11. After 24 h, the same products were obtained as those under aerobic conditions, albeit with lower conversion, along with a pale purple precipitate (Scheme 1). The collected precipitate exhibited the same analytical properties as independently prepared V complex 12. The EPR spectrum of the dark purple reaction mixture Table 1: Ligand effects on degradation of lignin model compound 1.