Quantum-chemical study of the hydrogenation and hydrogenolysis of xylose

Selective terminal C–C scission of C5-carbohydrates

“…As described earlier, this mechanism will not have a preference for the terminal positions. 18,19 Most importantly, a retro-aldol mechanism fails to explain why starting from xylose, threitol is the first product to appear ( Table 2, entries 11 and 12) and starting from arabinose, erythritol is the initial product ( Table 2, entries 18 and 19). The intermediate tetrulose products have a carbonyl group at the C2 position, which after hydrogenation should give both erythritol and threitol independent of the starting aldose (see RA mechanism in Scheme 2).…”

“…The formation C2 and C3 products is not only a matter of statistics: quantum chemical calculations clearly show a preference for other products than C4. 18,19 Thus, conditions favouring retro-aldol reactions are applied when ethylene glycol and propylene glycol are the desired products. The formation of these products is non-selective and occurs at high temperatures (200-240 °C), H 2 -pressures (60-100 bar H 2 ) and alkaline conditions.…”