A periodic-DFT study of retro-aldol fragmentation of fuctose on MoO3

Abstract

We use Density-Functional Theory calculations to investigate the mechanism of the retro-aldol fragmentation of fructose to glyceraldehyde (GLA) and dihydroxyacetone (DHA) on MoO3. We find that the reaction follows the classic mechanism of CC bond activation by proton abstraction from the βOH group of the sugar and stabilization of the transition state and resulting enolate intermediate by charge delocalization through coordination of the sugar’s carbonyl group to a Lewis acidic Mo atom. The deprotonation of fructose and the CC bond scission take place in a concerted fashion. On MoO3, fructose can also undergo epimerization to d-hamamelose through a 1,2-carbon shift along a pathway that is as probable kinetically as the one that leads to retro-aldol fragmentation but is somewhat more favourable thermodynamically. We see that hamamelose formation does not have an adverse effect on the overall yield of alkyl lactates, because hamamelose can itself undergo retro-aldol fragmentation to GLA, which can further be converted to alkyl lactates through a cascade of thermodynamically favorable reactions involving isomerization via 1,2-hydrogen shift, dehydration and ketalization, all catalyzed by the Lewis acidic zeolite Sn-MFI.