Mechanistic Insights into Water-Catalyzed Formation of Levoglucosenone from Anhydrosugar Intermediates by Means of High-Level Theoretical Procedures

Abstract

Levoglucosenone (LGO) is an important anhydrosugar product of fast pyrolysis of cellulose and biomass. We use the high-level G4(MP2) thermochemical protocol to study the reaction mechanism for the formation of LGO from the 1,4:3,6-dianhydro-α-d-glucopyranose (DGP) pyrolysis intermediate. We find that the DGP-to-LGO conversion proceeds via a multistep reaction mechanism, which involves ring-opening, ring-closing, enol-to-keto tautomerization, hydration, and dehydration reactions. The rate-determining step for the uncatalyzed process is the enol-to-keto tautomerization (ΔG‡298 = 68.6 kcal mol–1). We find that a water molecule can catalyze five of the seven steps in the reaction pathway. In the water-catalyzed process, the barrier for the enol-to-keto tautomerization is reduced by as much as 15.1 kcal mol–1, and the hydration step becomes the rate-determining step with an activation energy of ΔG‡298 = 58.1 kcal mol–1.

Insight into the formation mechanism of levoglucosenone in phosphoric acid-catalyzed fast pyrolysis of cellulose

“…Previous studies have suggested that LG and DGP are key intermediates for LGO formation in H 3 PO 4 -catalyzed cellulose pyrolysis [11][12][13]18] . Based on these studies, the possible pathways responsible for LGO formation are provided in Fig.…”

“…LGO formation from DGP involves breakage of the 3,6-and 1,4acetal bonds, and formation of the 1,6-acetal bond. According to previously proposed pathways [12,13,15] , a possible pathway is illustrated in Fig. 4 (Path 9).…”

“…Alternatively, the 3,6-acetal bond of DGP might break firstly and then 1,6-acetal bond replaced the original 1,4-acetal bond. Based on these assumptions, Wan et al [12] proposed a detailed formation pathway for LGO via DGP conversion. The authors reported that water might catalyze the formation of LGO from DGP based on quantum chemistry calculations.…”

A computational foray into the mechanism and catalysis of the adduct formation reaction of guanine with crotonaldehyde

“…Water is a well-known proton-transfer catalyst [45][46][47][48][49][50][51][52][53][54] and abundant in the biological system. It can be envisaged that participation of an explicit water molecule in all steps of both pathways may facilitate the proton transfers, thereby lowering energy barriers toward the transformations.…”

Seventh Asia-Pacific Conference of Theoretical and Computational Chemistry (APCTCC7)

“…[5] In catalysis research, Karton et al (University of Western Australia) report a detailed mechanistic study on the role of water in the formation of levoglucosenone from anhydrosugar intermediates, an important process in biomass conversion. [6] Wong et al (National University of Singapore) examine the catalytic mechanism of pentanidium-catalyzed asymmetric phase-transfer conjugate addition. [7] The origin of stereoselectivity is unravelled in their DFT study.…”