Oxovanadium Complex-Catalyzed Aerobic C-C Bond Cleavage of Biomass-derived Scaffolds

Abstract

The non-sustainable nature of fossil fuels as feedstocks for valuable chemicals, combined with the environmental damage caused by their extraction and combustion, increases the need for the development of a bio-based economy. While industry and public opinion are slowly shifting towards acceptance of this change, efficient technologies for the depolymerization and subsequent separation of lignocellulosic biomass fall short of the ever-increasing demand. In particular, there are currently no efficient, sustainable mass scale methods to convert lignin, the most abundant source of aromatic molecules on Earth. The use of oxovanadium(V) catalyst complexes to aerobically cleave C‒C bonds has been demonstrated previously and remains an attractive option for incorporation into a sustainable bio-based economy. Two new triphenoxyamine oxovanadium(V) catalysts with reduced steric bulk and electron density at the metal center (vs. previously reported complexes) have been synthesized for aerobic oxidative diol C‒C bond cleavage. These complexes were found to cleave less activated and more complex substrates than previous generations, including cyclic diols and polyalcohols. Several insights into the reaction pathways of this class of complex were elucidated through a series of kinetic studies. Experimentally, the rate of C‒C bond cleavage of both pinacol and hydrobenzoin was determined to be unaffected by substitution of the O‒H bonds with deuterium, suggesting that currently proposed mechanisms need to be revised. Multiple catalytic regimes were observed during anaerobic reaction, which were not altered significantly by the brief addition of O2. A series of density functional theory calculations revealed a plausible mechanism for the trialkoxy complex that did not involve a proton transfer in the rate determining step, instead suggesting that ligand-arm dissociation-reassociation play a significant role in the reaction. In a second project, new bisphenoxyamine-N-appended base ligand with less steric hindrance and electron density at the metal center, has been synthesized utilizing similar design principles gained from work with triphenoxyamine catalysts. When reacting with lignin model compound 1,2-diphenyl-2-methoxyethanol, this new complex displays a higher selectivity towards aldehydes and esters (relative to previous bisphenoxyamine-N-appended ligands), leading to a higher rate of C‒C bond cleavage. Investigations into the mechanism of bisphenoxy complexes, as well as the role of the N-appended base in reactivity, were performed using substrate pre-complexed bisphenoxy compounds. Thermolysis at 60 and 100 °C produced almost exclusively oxidative C‒H bond cleavage product benzyl methyl ether, with evidence for overoxidation product benzoic acid observed. Thermolysis of labelled substrate pre-complexed revealed that N-appended base may impede C‒C cleavage of 1,2-diphenyl-2-methoxyethanol by forcing the methyl ether away from the oxovanadium(V) center. Through the use of these multidentate phenoxyamine ligands, advances have been made towards sustainable oxovanadium catalysis in the pursuit of efficient and selective lignocellulosic disassembly for a sustainable bio-based economy.