Water was observed to catalyze Korcek cyclization, keto-enol tautomerization, and β-elimination reactions, but had little effect on hydrogen migration rearrangement and retro-aldol reaction, and sometimes even increased the activation energy (see Fig. for a comparison of water-catalyzed and non-catalyzed transition state geometries and activation energies). provided by several low-barrier routes to keto-enol tautomerization of D-Glucose (Fig. ). Tautomerization of D-Glucose to hexene-1,2,3,4,5,6-hexanol (HEH) involves the direct transfer of a hydrogen atom from an α carbon (index 2 in Fig. ) to the carbonyl oxygen through the breaking a σ and π bond. This uncatalyzed breaking of two bonds and forming two bonds (i.e., a b2f2