LAC (hydroxylactone (1R,5S)-1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one) is one of the most interesting products of the pyrolysis of cellulose and represents a useful chiral building block in organic synthesis. A computational investigation at the DFT level on the mechanism of formation of LAC shows that this species can be obtained following two reaction paths, path A and path B, starting from a well-known pyrolysis product (ascopyrone P). A series of internal rearrangements involving in all cases a proton transfer leads directly to LAC (path B). An alternative path (path A) can be also followed. From this path, via a "gate" connecting the two reaction channels, it is possible to reach path B and form LAC. In both cases, the rate-determining step of the process is the initial keto-enol isomerization. We found that water, which is present in the reaction mixture, "catalyzes" the reaction by assisting the proton transfers present in all the steps of the process. In particular, water lowers the barrier of the rate-determining step that becomes 40.9 kcal mol-1 (79.4 kcal mol-1 in the absence of water). The corresponding computed rate constant is 4.3×10 s-1 at 500 °C, a value which is consistent with the presence of LAC in the absence of metal catalysts. The results of this study on the non-catalyzed process underpin the important role played by water in the formation of pyrolysis products of cellulose where proton transfer is a key mechanistic step.
Keywords: cellulose; density functional calculations; organic synthesis; reaction mechanism; water catalysis.
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