A detailed examination of boronic acid–diol complexation

Carbohydrates as ligands: coordination equilibria and structure of the metal complexes

The dehydration of glucose and other hexose carbohydrates to 5- (hydroxymethyl)furfural (HMF) was investigated in imidazolium-based ionic liquids with boric acid as a promoter. A yield of up to 42 % from glucose and as much as 66 % from sucrose was ob- tained. The yield of HMF decreased as the concentration of boric acid exceed- ed one equivalent, most likely as a con- sequence of stronger fructose-borate chelate complexes being formed. Com- putational modeling with DFT calcula- tions confirmed that the formation of 1:1 glucose-borate complexes facilitat- ed the conversion pathway from glu- cose to fructose. Deuterium-labeling studies elucidated that the isomeriza- tion proceeded via an ene-diol mecha- nism, which is different to that of the enzyme-catalyzed isomerization of glu- cose to fructose. The introduced non- metal system containing boric acid pro- vides a new direction in the search for catalyst systems allowing efficient HMF formation from biorenewable sources.

M. Hlaibi

Papers

Stability constants of borate complexes of oligosaccharides