Support for a Dioxyallyl Cation in the Mechanism Leading to

Article pubs.acs.org/joc

Support for a Dioxyallyl Cation in the Mechanism Leading to (−)-Levoglucosenone Ben W. Greatrex,*,† Jan Meisner,‡ Stephen A. Glover,† and Warwick Raverty§ †

School of Science and Technology, University of New England, Armidale, New South Wales 2351, Australia Institute for Theoretical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany § Circa Group, Bio-21 Institute, Melbourne, Victoria 3052, Australia ‡

S Supporting Information *

ABSTRACT: Levoglucosenone (LGO) is the major product formed when cellulose is pyrolyzed in the presence of acid at temperatures between 170 and 350 °C. The current intense interest in biomass conversion has led to a number of reports on its preparation; however, there is still uncertainty on the mechanism leading to LGO. We propose a new mechanism which involves a C2−C1 hydride shift followed by intramolecular trapping of a dioxyallyl cation. The reaction has been modeled using DFT calculations from the known LGO precursors levoglucosan and 1,4:3,6-dianhydro-α-D-glucopyranose to a common intermediate with calculated barriers of 10.6 and 13.5 kcal·mol−1, respectively. A discussion of the literature on the formation of LGO from late pathway intermediates is also provided.

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INTRODUCTION The chiral synthon (−)-levoglucosenone 1 (LGO) is obtained as a major product when cellulose is pyrolyzed under reduced pressure in the presence of acid. The potential for this carbohydrate derived synthon has long been recognized, and recent successes in scaling the synthesis of LGO have enabled new applications.1,2 Reactions to generate LGO are performed at temperatures up to 400 °C usually with sulfuric acid or phosphoric acid catalysts either at low concentration3 or with the removal of char in a continuous process.4,5 In contrast, the acid-free pyrolysis at much higher temperatures generates levoglucosan 2 (LG) as the major product, and very little LGO is observed in the pyrolysis oils.6−8 Suitable substrates for the formation of LGO include purified cellulose, waste paper, and a variety of lignocellulosic materials such as sugar cane bagasse, wheat straw, and even sawdust.9−11 A large number of publications have discussed the preparation and use of LGO, yet the mechanism leading to its formation is still an area of contention.3,7,12−17 The confusion arises probably because a variety of carbohydrate structures can lead to LGO, although with vastly different yields.18 Cellulose currently gives the highest yields, and the transformation of this biopolymer is the most industrially relevant.3,18 A mechanism to explain the selectivity of the acidcatalyzed pyrolysis was first proposed by Broido et al.12 in 1973, and extensive discussion has been reported recently by Huber.3 Recently, we showed that α-ketoketals such as 6 can be formed from inosose mesylates such as 4, and the reaction proceeded through a neutral dioxyallyl 5a or the protonated dioxyallyl cation 5b (Scheme 1).19,20 While the elimination and subsequent reaction of the mesylate 4 used base, α-ketoacetals © 2017 American Chemical Society

Scheme 1. Levoglucosenone (1), Levoglucosan (2), and the Trapping of Dioxyallyl Systems with Alcoholsa

a Red color indicates the common α-ketoacetal/α-ketoketal functional group.

9 can also be formed under acidic conditions via allylic oxocarbenium ions such as 8 when α,α′-dihydroxyketones 7 are heated in the presence of alcohols in a process first reported by Mattox (Scheme 1).21,22 The similarities of the ketalization reaction outcomes to the structure of LGO suggested to us that the reaction yielding LGO involved an intramolecular variant of the Mattox process with both oxygens of the ketal endocyclic. Received: August 20, 2017 Published: October 24, 2017 12294

DOI: 10.1021/acs.joc.7b02109 J. Org. Chem. 2017, 82, 12294−12299

Article

The Journal of Organic Chemistry

Scheme 2. Previous Mechanisms and the Proposed Mechanism Showing the Formation of the Dioxyallyl Cation 28 (Red) from Cellulose via Levoglucosan (2) or 1,4:3,6-Dianhydro-α-D-glucopyranose 12

Combined with our interest in LGO as a chiral synthon23−25 and the considerable discourse around the mechanism, we examined pathways leading to LGO in more detail using published experimental data and DFT computation. We now propose that the mechanism is likely to include a common stabilized dioxyallyl intermediate formed after a C2−C1 hydride shift, which can be accessed from known intermediates on the pathway from cellulose to LGO. Some of the many mechanisms proposed for the formation of LGO are shown in Scheme 2. Broido et al. proposed that a C3 cation 10 formed from LG under acid catalyzed conditions underwent a 1,2-hydride shift, leading to ketone 11 with subsequent β-elimination affording LGO.12 The appeal of this mechanism derives from the retention of the C1−C6 anhydro linkage that is found in the starting material and the product. Modeling by Sarotti established that the Broido mechanism is unlikely, as some isolevoglucosenone 3 should also be formed as hydride can also migrate from the 4-position.26 Kawamoto et al. studied the pyrolysis of cellulose and LG in the sulfolaneH3PO4 system and concluded that although LG undoubtedly leads to LGO, not all cellulose can transition through LG to LGO, and an additional pathway must be in operation.27

Shafizadeh et al. demonstrated that 1,4:3,6-dianhydro-α-Dglucopyranose (12) was a potential intermediate, as this compound leads to LGO upon pyrolysis in good yield and can be observed as a component in pyrolysis mixtures (Scheme 2).14 This transformation requires several C−O bonds to break and a structural reorganization to give a 1,6-anhydro linkage. Shafizadeh et al. also reported the formation of 3,6anhydroglucose when the char formed from acid-treated partially pyrolyzed cellulose was hydrolyzed with water, which supported the finding that 12 and 3,6-anhydrocellulose are formed during acid-catalyzed pyrolysis. A mechanism for LGO formation from 12 involving the syn-elimination of the 3,6bridged ether in 13 was proposed by the authors. This mechanism explains the lack of isolevoglucosenone in pyrolysis mixtures but is not consistent with the behavior of 3,6anhydroglucosides under acidic conditions.14 The 3,6-anhydro linkage is retained when 12 is treated under both thermal and aqueous acidic conditions, and cleavage of the 1,4-anhydrobridge gives the anomeric oxocarbenium ion which undergoes further reaction.14 A number of other studies support the possibility of 3,6anhydro linkages in acid-catalyzed cellulose pyrolysis in the 12295


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Figure 1. Calculated relative Gibbs free energies (kcal·mol−1) at 300 K for the transformation 12 (+ H3O+) → 1 (+ H2O + H3O+) and 2 (+ H3O+) → 1 (+ 2H2O + H3O+) in THF.

for the process was a tautomerization, which is a facile process in solution, and delocalized structure 17 is an energy maxima, whereas the model we propose (vide infra) has electron delocalized minima. The model by Yu et al. also made no attempt to explain the observation of lower temperature dehydration of cellulose under acidic conditions. Recently, the formation of 3,6-anhydrosugars, including LG from glucose, has been modeled at 500 °C by Lu et al.; however, pathways to LGO were not explored.29

sequence leading to LGO. Dobele et al. have shown that, under conditions of slow pyrolysis, in the presence of 1.5−5.4% phosphoric acid, the glucose monomers within the cellulose undergo dehydration without significant chain cleavage at temperatures up to 250 °C.28 No significant quantities of LGO were formed until temperatures greater than 280 °C were reached, with the highest yields of LGO being observed in the temperature range 330−380 °C. On the basis of these observations, the authors concluded that the phosphoric acidcatalyzed pyrolysis of cellulose involves dehydration of the cellulose, while the polymer chains remain largely intact. The acid-catalyzed depolymerization of these chains with release of LGO occurred only as a secondary process at temperatures above 280 °C. Much lower temperatures are required when the pyrolysis reaction is carried out in polar aprotic solvents such as THF. Huber et al. reported the highest yields of LGO to date from 1% w/w cellulose in THF at 170−190 °C under high pressure, indicating considerable stabilization of the rate limiting step by the solvent.3 In a recent gas-phase DFT study, Yu et al. identified the most favorable pathway leading to LGO starting with neutral glucose as a model and proposed a concerted elimination from C2 leading to enediol 16.15 The proposal by Yu et al. used neutral glucose and ignored the requirement of acid for LGO selectivity in this reaction. Furthermore, the calculated rate limiting step

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RESULTS AND DISCUSSION A plausible mechanism for the formation of LGO from cellulose proceeding through 12 and LG can be postulated based on the pyrolytic reaction conditions, the Mattox rearrangement, and our previous dioxyallyl trapping experiments with alcohols and is shown in Scheme 2. We propose that two pathways to LGO involve either formation of a 3,6anhydro linkage (Pathway A) or through LG (Pathway B), and both involve the formation of a stabilized cationic dioxyallyl species 28. The first step in Pathway A is the formation of a 3,6-anhydro linkage in the cellulose, which could occur via two processes: an acid-catalyzed SN2 substitution at C6 with the nucleophilic C3 alcohol, or the acid-catalyzed loss of the hydroxyl group at C3, giving rise to a secondary cation which is trapped by the C612296


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The computed Gibbs free energies of hydronium or water were then summed with the energy of the structure at each point on the reaction coordinate. For the calculation of the Gibbs free energy, we included zero-point energies and calculated thermal effects of the differences in harmonically approximated vibrations. The computational model provided clear routes to the product 1 for both Pathways A and B. Starting with 12, the ring-opening of 22 by breaking the C4−C1 anhydro linkage, which afforded 23 via TS1 (+3.65 kcal·mol−1) was calculated to be exergonic by 2.4 kcal·mol−1. This change reflected the ring-strain present in the starting material and the relative stability of the oxocarbenium ion 23. A transition state TS2 was found for the hydride shift from C2 to C1, which converted 23 to the protonated ketone 24 with a calculated barrier of 13.5 kcal· mol−1 relative to that of 12. This saddle point was the highest point of Gibbs free energy and corresponded to the rate-determining step for the transformation from 12 to 1. The barrier is comparable to that found by Assary et al. for the hydride transfer for the formation of hydroxymethylfurfural from fructose (9.6 kcal·mol−1).32 The solution phase bimolecular tautomerization and proton transfer from 24 giving 25 promoted by strong acid catalysts is expected to be facile in solution, and barriers for the proton-transfer reactions have not been calculated. The ring-opening of 25 to give dioxyallyl 28 has a small barrier of 4.4 kcal·mol−1 and is significantly exergonic by −12.6 kcal· mol−1. Indeed, dioxyallyl cation 28 was calculated to be the most stable stationary point along the coordinate 12 to 1. The low barrier for this exergonic step is in line with the Hammond postulate. TS3 is early and resembling reactant, which is reflected in the similarities between the conformation of 25 and TS3; 25 required little distortion from the equilibrium geometry to reach TS3. Trapping of the dioxyallyl cation 28 by the C6 alcohol was calculated to the endergonic by 4.3 kcal· mol−1. Proton transfer from the oxonium product 29 to give oxonium 30 resulted in little change in energy, and the elimination of water and a proton from 30 gave rise to final product 1. Starting with 12, the entire process was marginally exergonic with ΔG = −0.27 kcal·mol−1 at 300 K. Pathway B shows the reaction starting with LG (2), which can also proceed to a common intermediate in dioxyallyl cation 28. The calculation performed in THF also gave a lower barrier (10.56 kcal· mol−1) for the rate limiting Wagner−Meerwein reaction from 26 to 27. The product of the rearrangement, intermediate 27, was significantly more stable than the starting material, as it was free to adopt a 4C1 conformation with groups equatorial. Proton transfer from the carbonyl in 27 to the neighboring hydroxyl to give 33 was slightly exergonic by 1.54 kcal·mol−1. The oxonium species 33 formed a hydrogen bond between the protonated C3 alcohol and the C6oxygen, which stabilized the molecule in a 1C4 conformation. The minima was 3 kcal·mol−1 more stable than any local minima found starting the geometry optimization using a 4C1 conformation. Transformation of species 33 into 28 required a tautomerization at C1 and loss of the water molecule, which formed an intramolecular bond with the C6−O during the transformation. The two pathways also differ in the relative conformation and stability of the starting materials used in the calculations. Whereas 12 exists in a boat conformation for the pyranose ring, LG has a 1C4 conformation; therefore, the relative barriers for the two pathways are not directly comparable. The transition states TS1−7 and stationary point 28 found in Pathways A and B are shown in Figure 2. The C1 oxocarbenium ion character found in TS1−4, TS6−8, and 28 can be seen, as these structures possess a relatively short C1−O5 bond while the stationary point 28 best highlights the electronic distribution in these cyclic dioxallyl cationic species. The ring in dioxyallyl cation 28 is flattened to give maximal orbital overlap which results in a near coplanarity of the O5−C1−C2−C3 atoms, which subtend a dihedral angle of only 11°. The C1−C2 bond in 28 is relatively long compared to the C2−C3 bond, which indicates a dominant canonical structure with a single bond between C1−C2 and an enol-like C2−C3 bond. The differences between C1 and C3 are also evident in the LUMO for 28 (Figure 2), which shows a larger contribution from the C1 2pz compared to that

alcohol. The synthesis of 3,6-anhydroglucosides is usually achieved synthetically by the conversion of the C6 alcohol into a good leaving group and cyclization under basic conditions. Furthermore, the stereochemistry at C3 is retained in the reaction; the C6 alcohol cannot be involved in the water displacement at C3 as the σ* orbital is not accessible in the 1C4 conformation. The next step in the reaction pathway is the formation of the oxocarbenium ion 23, which begins the depolymerization. Both exocyclic and endocyclic cleavage mechanisms operate during carbohydrate hydrolysis; however, the pathway leading to LGO requires an endocyclic ring-opening even if operating on a glucose moiety still on cellulose. As found by Shafizadeh et al., cation 23 can be trapped by the C4 oxygen, leading to 12, which is volatile and can be detected in distillates. Trapping of the C1 oxocarbenium ion in 23 could occur with a hydroxyl group on a terminal glucose unzipping the polymer or with a glycosidic ether internal to the polymer breaking the cellulose chain.30 Typical conditions used to generate LGO (270−350 °C, 1−30 mbar) do not promote the accumulation of water in the reaction mixture, and so the oxocarbenium ions that are produced are unlikely to be trapped by liberated water, leading to carbohydrate monomers in solution. The protonation of 12 and ring-opening gives oxocarbenium ion 23, which we propose undergoes a 1,2-hydride shift, resulting in the formation of protonated ketone 24. A precedent for this can be found in the results reported by Raines et al. using deuterium labeling.31 They showed that a 1,2-hydride shift from C2 to C1 is responsible for the thermal rearrangement of xylose to xylulose, demonstrating that this mechanism operates during the pyrolysis of sugars. A C2 to C1 hydride shift may also be responsible for the conversion of fructose to hydroxymethylfurfural (HMF) in aqueous solution, a reaction which is also promoted by heat and acid.32 The enolization at C1 to give 25 and proton exchange with C6−O followed by elimination will result in a stabilized dioxyallyl cation 28, which can be trapped by the liberated C6 alcohol. A similar delocalized intermediate was also proposed by Assary and Curtiss et al. in their DFT study of the conversion of glucose and fructose to HMF.32 Deprotonation of C6−O and protonation of C4−O in 29 and a final β-elimination then results in the formation of LGO. Pathway B shows the formation of LGO proceeding through LG and occurs when the formation and trapping of the oxocarbenium ion by the C6 alcohol precedes the formation of the 3,6-anhydro bridge. The reaction again involves a C2−C1 hydride shift and generates the same dioxyallyl intermediate 28. The conformation required to generate the oxyallyl cation is similar in both pathways as an axial C3 alcohol is required in 27 to give overlap from the developing carbocation at C3 and the enol system.

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COMPUTATIONAL CHEMISTRY

To investigate the proposed mechanism, we computationally modeled the sequence 12 to 1 and the sequence LG through to 1 using DFT with the results shown in Figure 1. For all calculations, we applied the M06-2X functional33 with the def2-TZVP basis set34 implemented in Turbomole v. 7.0.1.35 For geometry optimizations, we used the DLFind library36 interfaced via Chemshell.37 Solvation effects were included implicitly via the conductor like screening model (COSMO) with a value of ε = 7.58 for THF. The choice of THF was due to the relatively low temperatures required for transformation in this solvent.3 Transition states were confirmed as having a single imaginary frequency. For protonations and deprotonations, a water molecule or hydronium ion were used as proton acceptor or donor, respectively. 12297


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The Journal of Organic Chemistry Scheme 4. Formation of LGO from Ketone 35 from Gallagher et al.38

which lithium perchlorate gave the best results. The authors proposed that a delocalized dioxyallyl cation 37 was trapped by the C6 alcohol, giving rise to LGO, which “again illustrates the thermodynamic sink that LGO represents”.38 Thus, although the generality of the pathway was not recognized in their work, the Gallagher group synthesized an almost perfect mechanistic probe in structure 36, which clearly demonstrates the feasibility of Pathway A starting at cation 25.

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CONCLUSIONS

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ASSOCIATED CONTENT

We proposed a new mechanism for the formation of LGO from cellulose involving the known precursors to LGO, LG, and 1,4:3,6-dianhydro-α-D-glucopyranose (12), which consists of a series of oxonium and oxocarbenium ions. The key step for the reaction process is an energetically accessible Wagner− Meerwein rearrangement starting with the anomeric oxocarbenium ion with the hydride moving from C2 to C1. The calculated barrier for this transformation is relatively low, which is consistent with the observation that 12 and LG are minor components in the acid catalyzed pyrolysis of cellulose and are consumed before they distill under the reaction conditions. The product of the rearrangement can enolize, and this leads to a highly stabilized dioxyallyl cation which can be trapped by the C6-OH, giving the bicyclic system present in LGO. Importantly, both reaction pathways proceed through the oxocarbenium ion at C1, which is formed when cellulose and anhydroglucosides such as LG or 12 are heated in acid. The DFT study elucidated the key steps, relative energies, and transition states for this process, which may assist when designing conditions that favor the production of LGO.

Figure 2. Transition states TS1−8 and 28 with key bond distances found on the pathway from cellulose to LGO and the LUMO for 28. of C3, which means that nucleophilic additions are expected at C1 and not at C3, as is found experimentally. To ascertain whether the reaction was proceeding through a neutral dioxyallyl 34 or a dioxyallyl cation 28, we compared the energies of the two species in Scheme 3. The deprotonation reaction is significantly endergonic with ΔG = 42.2 kcal·mol−1, indicating that a neutral dioxyallyl is unlikely to be important in this reaction sequence.

Scheme 3. Comparison of Dioxyallyl Cation 28 and Neutral Dioxyallyl 34

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b02109. Cartesian coordinates and energies of all transition states and stationary points (PDF)

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The reaction pathways shown in Figure 1 demonstrate that the rate limiting step in the reaction is the hydride transfer from C2 to C1; therefore, the formation of LGO from one of the intermediates following the transition state should be facile. Although not part of formal mechanistic studies regarding the formation of LGO from cellulose, the work of Gallagher and co-workers is particularly enlightening in this regard.38 While studying the reactions of electrophiles with enol ether 36, formed from 3-O-benzyl-1-deoxy3,6-anhydroglucopyran-2-ulose (35), the authors observed the formation of LGO in moderate yield (Scheme 4). The reaction proceeded at subambient temperatures with a variety of Lewis acids, of

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ben W. Greatrex: 0000-0002-0356-4966 Jan Meisner: 0000-0002-1301-2612 Notes

The authors declare no competing financial interest. 12298


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(33) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215. (34) Weigend, F.; Häser, M.; Patzelt, H.; Ahlrichs, R. Chem. Phys. Lett. 1998, 294, 143. (35) Turbomole V7.0 2015. A development of University of Karlsruhe and Forschungzenstrum karlsruhe GmbH, 1989−2007, http://www. turbomole.com, accessed February 3, 2016. (36) Kästner, J.; Carr, J. M.; Keal, T. W.; Thiel, W.; Wander, A.; Sherwood, P. J. Phys. Chem. A 2009, 113, 11856. (37) Metz, S.; Kästner, J.; Sokol, A. A.; Keal, T. W.; Sherwood, P. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2014, 4, 101. (38) Griffin, A.; Newcombe, N. J.; Gallagher, T. In Levoglucosenone and Levoglucosans, Chemistry and Applications: Atl Pr Scientific Pub: Washington, DC, 1994; Vol. 1, p 23.

ACKNOWLEDGMENTS The authors thank Prof. Richard Furneaux for helpful discussions on his previous work. J.M. was financially supported by the GermanResearch Foundation (DFG) within the Cluster of Excellence in Simulation Technology (EXC 310/2) at the University of Stuttgart.

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Support for a Dioxyallyl Cation in the Mechanism Leading to

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