The Computational and Experimental Investigation of Novel Catalytic Pathways for the Deoxydehydration of trans-Diols in Carbohydrates

Abstract

Currently, extensive research is being carried out on the efficient conversion of biomass- derived feedstock such as cellulose, hemicellulose and lignin into high-value precursors for the chemical industry. However, unlike finite fossil fuel-derived hydrocarbons, bio-based material is heavily functionalised, as it consists of many polar hydroxyl moieties. Deoxydehydration (DODH) reactions provide an efficient method for the conversion of vicinal diols to form high-value olefin products. These in turn are able to undergo a plethora of chemical reactions to be transformed into different functionalities. The deployment of transition metal catalysis, particularly using rhenium, vanadium and molybdenum, has pioneered this field. Most efforts have focused on methyltrioxorhenium- catalysed DODH, which has proven to be most successful. Using this catalyst, it was previously thought that trans-cyclic diols were unable to undergo DODH. In our study, we present a density functional theory investigation into how expanding the ring size of the trans-cyclic diol lowers the free energy barrier of the rate-determining step in this reaction. Contrary to our preliminary computational study, using our reaction conditions experimentally, we found that constrained trans-cyclic diols are able to undergo DODH via an alternative pathway, to form olefin products in surprisingly high yields. To date, considerably less research has been carried out on the Earth-abundant vanadium-catalysed DODH reaction. Prior computational and experimental investigations into the use of vanadium(V)-catalysis, in the form of [n-Bu4N]VO2(dipic), focus on the use of unsustainable solvents such as benzene and chlorobenzene, with stoichiometric reductants to catalyse the DODH reaction of simple aliphatic diols. We present here a computational and experimental study on the vanadium(V)-catalysed DODH reaction of trans-cyclic diols and uncover the light dependence this reaction displays. Furthermore, we present the utility of other vanadium(V) complexes in the catalysis of 1,2-vicinal diols in alcoholic solvents, which also behave as a reductant in the reaction mixture. Here we obtain olefin yields comparable to that of vanadium(V)-catalysed DODH in aromatic solvents with stoichiometric reductants. We highlight side reactions taking place and present potential ways to overcome the byproduct formation through future work.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Neil Burton (Supervisor) & Richard Bryce (Supervisor)