Probing the Lewis Acid Catalyzed Acyclic Pathway of Carbohydrate Conversion in Methanol by In Situ NMR

Abstract

Future bioindustries will rely on the formation of diverse chemicals at high yield through various reaction pathways. These pathways include reactions to a series of alpha‐hydroxy esters and acids that can be formed from the conversion of C3–C6 carbohydrates by Lewis acidic catalysts in alcohols and water. Definitive kinetic and mechanistic insights to support the development of carbohydrate conversion processes are arguably less developed than for analogous biocatalytic processes. Here, we visualize acyclic pathways of carbohydrate dehydration, using the acyclic C4 carbohydrate erythrulose as a probe molecule for the conversion by homogeneous SnCl4 in methanol. In situ studies allow the detection of previously postulated intermediates, identify the branch point to competing products and provide energetic and mechanistic insight by kinetic analysis. Reversibility of reactions, stereoselectivity and differential propensity for deuterium incorporation in competing pathways can be tracked by the formation of compounds with asymmetric mass distribution.

Mechanism and malleability of glucose dehydration to HMF: entry points and water-induced diversions

“…In a typical catalytic experiment, 50 mg monosaccharide and 2-4.5 mg CrCl 3 •6H 2 O (depending on whether 1 H or 13…”

“…Kinetic in situ experiments were acquired as series of 1 H or 13 C NMR spectra in a pseudo-2D fashion using a 600 MHz Bruker Avance III NMR spectrometer equipped with a SmartProbe, using d 6 -DMSO as the lock substance in all circumstances. Free induction decays were sampled as 16384 complex data points using a spectral width of 16 ppm during an acquisition time of 1.7 seconds in the case of 1 H spectra, while 64k complex data points (spectral width of 240 ppm) were sampled for 1.8 seconds (aq) in the case of 13 C spectra. Inter-scan recycle delays of 2 seconds were used for the series of 1 H spectra.…”

“…Inter-scan recycle delays of 2 seconds were used for the series of 1 H spectra. Recycle delays of 3 seconds were used for 13 C spectra. Automatic shimming by Bruker Topspin Topspin 3.5 pl7 software during the experiment was employed.…”

“…[3][4][5][6][7][8] The most efficient approaches to convert glucose to HMF entail the use of suitable Lewis acidic catalysts to convert glucose to fructose prior to dehydration. 2,[8][9][10][11][12][13] In particular, chromium chloride (as CrCl 2 or CrCl 3 ) has been established as particularly efficient in converting glucose to HMF. 11,14 The stoichiometric dehydration of cyclic fructose to HMF is a Brønsted acid catalysed reaction.…”

“…3,18 Catalysts of interest for the conversion of carbohydrates to value-added chemicals include paramagnetic Lewis acids. At millimolar concentrations, paramagnetic catalysts constitute paramagnetic cosolutes that accelerate the T 1 relaxation of analyte 13 C nuclear spins. 19,20 Such conditions permit sensitivity improved, quantitative in situ NMR spectroscopy with higher time resolution than experiments in the absence of paramagnetic catalysts could have.…”