(166f) First-Principles Investigation of the Metal Catalyzed Epimerization of Sugars

Epimerization of carbohydrates is widely used for the production of rare sugars and has found immense importance in biology, medicine and food industries. Inorganic metal catalysts have been identified as potential catalysts for sugars epimerization, to replace existing enzymatic processes. In the present work, we investigated the homogeneous and heterogeneous metal catalyzed epimerization of glucose to mannose using density functional theory. Sn-beta zeolite, a heterogeneous catalyst, in combination with borate salts catalyzes the epimerization of glucose. Borate salts form a complex with glucose at C₁ and C₂carbons, preventing the isomerization and thus, indirectly promoting epimerization. The Lewis acidic metal centre (Sn-OH) catalyzes the glucose ring opening, followed by enolization, which is catalyzed by the Brønsted acidic silanol (Si-OH) group of Sn-beta zeolite. The reaction further proceeds through the formation of distorted three membered transition state. The intramolecular 1,2 carbon shift was found to be the rate determining step, with an activation barrier of 26.3 kcal/mol. This study was further extended to analyze activities of homogeneous metal catalysts such as Molybdenum (M), Tungsten (W) and Vandium (V), in their oxoanion complex forms, to catalyze the rate limiting 1,2 carbon shift associated with the epimerization reaction. The experimentally observed difference in the activities of these homogeneous metal complexes, in catalyzing 1,2 carbon shift in glucose, at different pH was explained. Formation of polynuclear metal oxoanions at higher pH reduces the structural flexibility of the metal complex. Our studies suggest that, upon binding with glucose, their ability to restructure the carbon backbone in glucose decreases, resulting in lower activity towards 1, 2 carbon shift at higher pH. Bonding interaction analysis was carried out to quantify the flexibility/rigidity of the metal oxoanion complexes. Comparison of catalytic activities of Mo, W and V (as dimetalate) showed that vanadate is more active than the popular molybdate complex and W is the least active of them all. The trend in the catalytic activities of these metals is explained on the basis of the participation of higher orbitals in the complexation of metal oxoanions with glucose. Additionally, the flexibility/rigidity of the metal complex is also shown to be a descriptor of its catalytic activity for the 1,2 carbon shift reaction. The present investigation can be extended to identify potential inorganic homogeneous and heterogeneous catalysts to catalyze the 1,2 carbon shift associated with the epimerization of sugars.

References:

1. W.R. Gunther, Y. Wang, Y. Ji, V.K. Michaelis, S.T. Hunt, R.G. Griffin, Y. Román-Leshkov, Nat. Commun. 3 (2012) 1109.
2. R. Bermejo-Deval, R.S. Assary, E. Nikolla, M. Moliner, Y. Román-Leshkov, S.J. Hwang, A. Palsdottir, D. Silverman, R.F. Lobo, L.A. Curtiss, M.E. Davis, Proc. Natl. Acad. Sci. USA 109 (2012) 9727-9732.
3. B.K. Chethana, S.H. Mushrif, J. Catal. 323 (2015) 158-164.
4. V. Bilik, Chem. Zvesti 26 (1972) 183-186.
5. V. Bilik, L. Petrus, J. Zemek, Chem. Zvesti 32 (1978) 242 -251.