Depolymerization of Oxidized Lignin Catalyzed by Formic Acid

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Depolymerization of Oxidized Lignin Catalyzed by Formic Acid Exploits an Unconventional Elimination Mechanism Involving 3c-4e Bonding: A DFT Mechanistic Study Shuanglin Qu, Yanfeng Dang, Chunyu Song, Jiandong Guo, and Zhi-Xiang Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01095 • Publication Date (Web): 21 Sep 2015 Downloaded from http://pubs.acs.org on September 23, 2015

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ACS Catalysis

Depolymerization of Oxidized Lignin Catalyzed by Formic Acid Exploits an Unconventional Elimination Mechanism Involving 3c-4e Bonding: A DFT Mechanistic Study

Shuanglin Qu, Yanfeng Dang, Chunyu Song, Jiandong Guo, Zhi-Xiang Wang*[a],[b] a

School of Chemistry and Chemical Engineering, University of the Chinese Academy

of Sciences, Beijing,100049, China. b

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin,

300072, China.

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Abstract: A DFT study has been performed to gain insight into the formic-acid-catalyzed

depolymerization

of

oxidized

lignin

model

(1ox)

to

monoaromatics, developed by Stahl et al. (Nature 2014, 515, 249-252). The conversion proceeds sequentially via formylation, elimination, and hydrolysis. Intriguingly, the elimination process exploits an unconventional mechanism different from the known ones such as E2 and E1cb. The new mechanism is characterized by passing through an intermediate stabilized by a proton-shared 3c-4e bond (HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟO=Cα)

and

by

shifting

the

3c-4e

bond

to

the

3c-4e

HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟOOCH bond in the joint leaving group that is originally a regular H-bond (HCOO−H⋅⋅⋅OOCH−). According to these characteristics, as well as the important role of the original HCOO−H⋅⋅⋅OOCH− bond, we term the mechanism as E1H-3c4e elimination. The root-cause of the E1H-3c4e elimination is that the poor leaving formate group is less competitive in stabilizing the negative charge resulted from Hβ abstraction by HCOO- base than the nearby carbonyl group (Cα=O) that can utilize the negative charge to form a stabilizing 3c-4e bond with a formic acid molecule. In addition, the study characterizes versatile roles of formic acid in achieving the whole transformation, which accounts for why the HCO2H/NaCO2H medium works so elegantly for 1ox depolymerizaion. Keywords: biomass conversion, lignin depolymerization, elimination mechanism, 3c-4e bond, formic acid, DFT.

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1. Introduction Discovery of strategies to convert biomass to valuable chemicals and fuels is of great importance for sustainable development of mankind.1 Lignocellulose, a typical non-edible biomass, mainly consists of cellulose, hemicelluloses, and lignin.2 Biorefinery of present technologies can efficiently transform (hemi)cellulose into biobased chemicals and fuels, but lignin conversion is much more difficult. Lignin is often treated as a waste or burned for its energy,3 however, lignin, as a heterogeneous aromatic bioploymer, has the potential to offer a renewable source of aromatic building blocks for chemical industry and simply burning lignin wastes the potential.4 It is highly desirable to develop efficient routes for lignin depolymerization, extracting aromatics for further downstream processing.5 Although

lignin

depolymerization is challenging due to its highly complex molecular structures and recalcitrant chemical nature,4,6 methods for lignin deconstruction have been reported.7-10 More recently, Stahl and coworkers have developed an elegant approach to catalytically transform lignin to structurally defined monomeric aromatics.11,12 Exemplified by the experimentally used lignin model (1lig), Scheme 1 recapitulates their strategy. The method breaks the most common structural motif called β−O−4 linkage in lignin through two stages, including aerobic oxidization of Cα alcohol of 1lig to a carbonyl group in 1ox and cleavage of the Cβ−O bond of 1ox with HCO2H/NaCO2H medium giving the monoaromaitcs Ar1 and Ar2. Notably, the second stage is a facile catalytic process, in contrast to the previously reported formic-acid-promoted lignin depolymerizations that used formic acid as a sacrificial hydrogen donor.13 Stahl et al. have proposed a reaction sequence to account for the conversion of 1ox to Ar1 + Ar2, including formylation, elimination, and hydrolysis (Scheme 1).11 Nevertheless, the acting mechanism of formic acid remains unclear, encouraging us to perform a DFT mechanistic study. The energetic and geometric details allow us to "visualize" the transformation for an in-depth understanding. Interestingly, we observed that the remarkable transformation adopts an unconventional elimination mechanism (termed as E1H-3c4e) which is different from the known ones such as E2 3 / 27


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and E1cb.

Scheme 1. Schematic illustration of lignin depolymerization method developed by Stahl et al.11,12

2. Computational Details In this study, we used Gaussian 09 program14 to perform standard DFT computations. Using actual compounds, all structures were optimized and characterized to be energy minima or transition states (TSs) at the B3LYP15/6-31G(d,p) level. The energies were then improved by M06-2X16/6-311++G(d,p) single-point calculations with solvation effects of formic acid accounted by SMD17 (solvation model density) solvent model which is based on the quantum mechanical charge density of a solute molecule interacting with a continuum description of the solvent. M06-2X has been widely used and shown to achieve high accuracy for organic systems.18,19 The refined single-point energies were then corrected to enthalpies and free energies at 298.15 K and 1 atm, using the gas phase B3LYP/6-31G(d,p) harmonic frequencies. The combined use of two DFT functionals has been applied to reasonably account for various catalytic reactions.20 To ascertain the reliability of the used computational method, we recalculated the energetics of the elimination step at other DFT levels (e.g. ω-B97XD,21 TPSSTPSS,22 BP8623 and M06-2X), which draw the 4 / 27


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same conclusions except for some numerical differences (vide infra). It should be emphasized that the above thermal corrections based on the ideal gas phase model inevitably overestimate entropy contributions to free energies for reactions in solvent, in particular for reactions involving multi-component change, because of ignoring the suppressing effect of solvent on the rotational and transitional freedoms of substrates. The entropy overestimation by ideal gas phase model was also demonstrated

by

experimental

studies.24,25

Because

no

standard

quantum

mechanics-based approach is available to accurately calculate entropy in solution, we adopted the approximate approach proposed by Martin et al.26 According to their approach, a correction of 4.3 kcal/mol applies to per component change for a reaction at 298.15 K and 1 atm (i.e., a reaction from m- to n-components has an additional correction of (n−m) × 4.3 kcal/mol). Previously, we applied the correction protocol for mechanistic studies of various catalytic reactions and found such corrected free energies were more reasonable than enthalpies and uncorrected free energies,27 although the protocol is by no means accurate. We also noted that other correction factors (e.g. ca 1.9 or 5.4 kcal/mol) were adopted in literatures.28 Wiberg bond indices (WBIs) were calculated at the M06-2X/6-311++G(d,p) level according to the natural orbital (NBO) method.29 We discuss the mechanism in terms of the corrected free

energies and give the enthalpies for references in the brackets in the relevant figures. Total energies and Cartesian coordinates of all optimized structures are given in the

Supporting Information (SI).

3. Results and Discussion 3.1 Depolymerization mechanism of 1ox Experimentally, Stahl et al.11 have demonstrated a three-stage sequence for the depolymerization of 1ox to Ar1 and Ar2, including formylation, elimination, and hydrolysis (Scheme 1). In the following, we discuss the mechanism of the transformation in terms of the three stages. 5 / 27


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Formylation stage: Figure 1 illustrates our computed pathway for the stage. The optimized structures of the stationary points in Figure 1 are given in Figure S1 in SI. The stage starts with the electrophilic attack of formic acid at the hydroxyl group of 1ox, forming a gem-diol intermediate 3, followed by dehydration of 3 to generate the formylation product 5. The attack involves two formic acid molecules, as depicted by TS1, one acting as an electrophile and the other serving as a hydrogen transfer shuttle (H-shuttle) to mediate the hydrogen transfer from the hydroxyl group of 1ox to the carbonyl group of the attacking formic acid molecule. The barrier for the electrophilic attack is 16.5 kcal/mol (TS1 relative to the ternary hydrogen bond (H-bond) complex 2). The dehydration of 3 spans TS2 with a barrier of 16.4 kcal/mol, leading to 5. The dehydration also involves a formic acid H-shuttle. The formic acid H-shuttles in TS1 and TS2 are indispensable, because the barriers (TS1' and TS2', the counterparts of TS1 and TS2) without H-shuttles are too high (>42.0 kcal/mol) to be accessible. The role of H-shuttle in facilitating hydrogen transfer was reported previously by others30 and us.27a,27d,27e Water available in the system can also be a H-shuttle to mediate hydrogen transfer, but it is much less effective than formic acid (see TS1w and TS2w in Figure 1), because formic acid is a stronger proton donor than water. In addition, compared to formic acid, water is minor in the present system. Thus, water is not considered as a H-shuttle hereafter. Overall, the formylation stage overcomes a barrier of 18.4 kcal/mol (TS2 relative to 2) and releases 3.7 kcal/mol of energy. The exergonic stage with low barriers is in agreement with the experimental observation that the formylation could proceed fast at room temperature.11 The stage consumes a formic acid molecule, but can be recovered in the next elimination stage (vide infra). The reaction was carried out in the HCO2H/NaCO2H medium, thus formic acid can form hydrogen bond (H-bond) with the substrate. We verified that the H-bond between a formic acid and the carbonyl group (Cα=O) of 1ox has no essential effect on the barriers of the stage (see Figure S2).

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Figure 1. Free energy profile for 1ox undergoing formylation stage.

Elimination stage: Scheme 2 illustrates our explorations for the mechanism of elimination stage with the free energy profiles shown in Figure 2 and the key optimized structures displayed in Figure 3. To begin with, the synperiplanar 5 (the formylation product of 1ox) converts to the elimination-preferable antiperiplanar 6, though the process costs 1.5 kcal/mol. A straightforward mechanism (i.e. E2 mechanism) was proposed for the elimination.11 To characterize the proposed E2 elimination pathway, starting from 6, we first examined the pathway according to the classical E2 mechanism, only using a base (i.e. HCOO-) to extract the Hβ of 6. Geometric optimizations aiming at a concerted TS for simultaneous hydrogen abstraction and dissociation of the formate group converged to TS3a. Compared to the bond lengths [R(Cβ−Hβ)=1.096 Ǻ and R(Cγ−O)=1.443 Ǻ] in 6, the two bonds in TS3a are stretched to 1.578 and 1.492 Ǻ (see Figure 3), respectively, showing somewhat features of E2 elimination. However, IRC (intrinsic reaction 7 / 27


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coordinate) calculation starting from TS3a led to an intermediate 7a (Figure S3 in SI). Although the Cβ−Hβ bond in 7a is further stretched to 1.869 Ǻ, the formate group does not leave at all with a bond length of R(Cγ−O)=1.508 Ǻ. Thus, TS3a does not describe a concerted E2 elimination. Actually, 7a is a carbanion similar to the proton-removed intermediate in a typical E1cb elimination, with the formal negative charge located on Cβ. Attempts to locate a TS to dissociate the formate group of 7a resulted in TS4a. The elongated Cγ−O bond (1.522 Ǻ, compared to 1.508 Ǻ in 7a) in TS4a shows the leaving tendency of the formate group, but IRC calculation (Figure S4 in SI) led TS4a to another intermediate 8a with an even shortened Cγ−O bond (1.482 Ǻ, compared to the 1.508 Ǻ in 7a). From 7a to 8a, the newly formed HCO2H moiety attached to Cβ through a weak O−H⋅⋅⋅Cβ hydrogen bond [(R(O−H) = 1.033 Ǻ and R(H⋅⋅⋅Cβ) = 1.869 Ǻ] swings to the carbonyl group (Cα=O) and the formal negative charge on Cβ shifts to the carbonyl O atom, forming a proton-shared 3c-4e (three-center, four-electron) bond. The 3c-4e bonding nature can be formally described as HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟO=Cα and verified by the short distance of the middle proton from

the neighbor oxygen atoms

[R(OƟ⋅⋅⋅H⊕)=1.079 and R(H⊕⋅⋅⋅ƟO=Cα)=1.390 Ǻ] and Wiberg bond indices (WBIs) of the two bonds (0.489 and 0.224, respectively). Note that the values are obviously different from those [R(O⋅⋅⋅H)=1.758 and R(H⋅⋅⋅O)=0.988 Ǻ and WBIs of the two bonds (0.038 and 0.653, respectively)] of the regular HCOOH⋅⋅⋅O=Cα H-bonds, e.g. in 6c (see Figure 3). Supporting the charge shift to form 3c-4e bond, the Cα−Cβ single (1.436 Ǻ) and Cα=O double (1.251 Ǻ) bonds in 7a turn to formal double (1.360 Ǻ) and single (1.342 Ǻ) bonds in 8a, respectively. The conversion of 7a to 8a is somewhat similar to the keto-enol tautomerization, however, unlike the convention that an enol tautomer is less stable than a keto tautomer, the "enol" form 8a is 13.7 kcal/mol more stable than the "keto" form 7a, which can be attributed to the stabilization of the 3c-4e bonding in 8a. TS4a was obtained by optimization in terms of electronic energy, but TS4a is even slightly lower than 7a in terms of free energy (by 0.5 kcal/mol). Therefore, Hβ-abstraction and charge shift would take place concertedly, leading to 8a directly. Indeed, when geometric optimizations were carried 8 / 27


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out in solution at the M06-2X(SMD)/6-31G(d,p) level, 7a and TS4a could not be located (vide infra). The same holds true for the process of 7b/7c→TS4b/TS4c. It is difficult to locate a TS (labeled as TS5a) for the departure of the formate group from 8a, which could be due to the formation of the 3c-4e bond that stabilizes the negative charge. To verify the reasoning, we intended to remov the HCO2H moiety from 7a to invalid the 3c-4e bond. The resultant 8a' can proceed to dissociate the formate group via TS5a'. The pathway (6→TS3a→7a→8a'→ TS5a'→10) corresponds to a typical E1cb mechanism, however, it is less favorable than the green or black pathway discussed below.

Scheme 2. Possible pathways explored for elimination stage.

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Figure 2. Free energy profiles corresponding to the pathways shown in Scheme 2.

Figure 3. B3LYP/6-31G(d,p) optimized geometries of key stationary points labeled in Scheme 2, along with key bond lengths in angstroms (key structures of the black pathway are given in Figure S5 in SI). Trivial atoms are omitted for clarity. Values in parentheses are Wiberg bond indices. Values in brackets are imaginary frequencies of TSs. 10 / 27


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To promote the departure of the formate group from 8a, as proposed by Stahl et al. in their proposed E2 elimination pathway,11 we added a formic acid molecule to 8a to form a H-bond with the formate group, resulting in 8b. The TS (TS5b) for the elimination thus could be located. Interestingly, as the joint HCOOH^OOCH species departs gradually, the initial H-bond (HCOOa−H⋅⋅⋅ObOCH) evolves to a 3c-4e bond represented by HCOOaƟ⋅⋅⋅H⊕⋅⋅⋅ƟObOCH. The evolution is magnified by the elongated R(Oa−H)/the 1.215/1.215(9)]

shortened and

the

R(H⋅⋅⋅Ob)

[0.994/1.715Ǻ(8b)→1.023/1.553(TS5b)→

decreased

WBI(Oa−H)/the

[0.635/0.056(8b)→0.579/0.118(TS5b)→0.359/0.359(9)].

increased Compared

WBI(H⋅⋅⋅Ob) to

the

dissociation of the formate group from 8a, the departure of the group from 8b is accelerated by the formation of the 3c-4e bonded leaving species (9), because the formation of the 3c-4e bond stabilizes the negative charge accumulated on the formate group as it departs. Thus the H-bond in 8b is not just a simple H-bond but plays an important role in promoting the departure of the formate group. From another point of view, the dissociation of 9 from 8b gradually shifts the HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟO=Cα 3c-4e bond in 8b to the HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟOOCH 3c-4e bond in 9 which originally is a H-bond in 8b. We analyzed that the assumed TS5a (if it exists) must be higher than TS5b, because the Cγ−O bond length (1.482 Ǻ) in 8a is obviously shorter than the 1.520 Ǻ in 8b. Note that, alternative to the formation of 8b from addition of HCO2H to 8a, 8b can also be obtained via the pathway 6 + HCOO- + HCO2H→TS3b→7b → TS4b→8b. Figure 4 examines the NBO charge evolution along with the elimination process: 6 + HCOO- + HCO2H → TS3b → 8b→TS5b→10a + 9. As the joint leaving group (HCOOH^OOCH) departs, the leaving group gradually gains negative charge (-0.354 → -0.416 → -0.418 → -0.711 → -1.0 e in blue), while the group including HCOOand Hβ losses negative charge (-0.759 → -0.224 → -0.162 → -0.068 → 0.072e in red). The Cα=O group in 8b bears much greater negative charge (-0.507e) than the -0.072e in 10a, reflecting the significant difference of the HCOO⋅⋅⋅H⋅⋅⋅O=Cα bonding in 8b (a 3c-4e bond) and in 10a (a H-bond). In addition, the charge variations from (-0.669e) on HCOO⋅⋅⋅H⋅⋅⋅O=Cα and -0.418e (QL) in 8b to the corresponding values (0.0e in 10a 11 / 27


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and -1.0e of 9) are consistent with the shift of the HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟO=Cα 3c-4e bond in 8b to the HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟOOCH 3c-4e bond in 9.

Figure 4. Charge evolution in the elimination process (6 + HCOO- + HCO2H→ TS3b→ 8b→TS5b→10a +9)

The transformation was carried out in the HCO2H/NaCO2H medium, thus we next considered the elimination more realistically. We added two formic acid molecules to 6 to form H-bonds with its formate and carbonyl (Cα=O) groups, respectively, giving 6c. The elimination pathway of 6c by HCOO- base is illustrated by the black pathway in Scheme 2 and Figure 2. As compared, the elimination pathway of 6c is mechanistically similar to that of 6 (the green pathway). The elimination also involves the formation of a 3c-4e bonding intermediate (i.e. 8c) and a shift of the 3c-4e bond in 8c to that in 9 (more geometric and NBO results of 8c, TS3c and TS5c are given in Figure S5). The relative barriers of TS3c to 6c and TS5c to 8c (19.0 and 10.9 12 / 27


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kcal/mol, respectively) are close to those (19.3 and 11.0 kcal/mol, respectively) of TS3b to 6 and TS5b to 8b. Therefore, the initial H-bond formed with the carbonyl group (Cα=O) has no essential influence on the elimination, in contrast to the important role of the H-bond initially formed with the leaving formate group (see the blue pathway and the green pathway). The higher TS3c than TS5c is in accord with the D-labeling experiment, which showed that the Cβ−Hβ bond cleavage is the rate-determining step11. On the contrary, TS3a is lower than TS5a' in the typical E1cb mechanism (see above).

Figure 5. Comparing the bond 3c-4e bonding of 9 with that of a well-studied 3c-4e bonding species FƟ⋅⋅⋅H⊕⋅⋅⋅ƟF. Bond lengths are in Angstrom and values in parentheses are WBIs. (a) Comparisons of geometric structures and WBIs. (b) Comparisons of molecular orbitals. We assigned 8a, 8b, 8c, and 9 to be 3c-4e bonding intermediates. To verify the 3c-4e bonding nature, using 9 as a representative, Figure 5 compares the bonding of 9 with that of a well-recognized 3c-4e bonding species FƟ⋅⋅⋅H⊕⋅⋅⋅ƟF (FHF) 31. The WBI (0.359) of the two bonds in 9 is close to that (0.356) of FHF (Figure 5a). In addition, both 9 and FHF have similar canonical molecular orbitals involved in the 3c-4e bonding31,32 (Figure 5b). In both cases, the WBI sums (ca 0.7) of the two bonds are less than the ideal value (i.e. 2) for a 4e bond, which is due to that two of the four 13 / 27


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electrons occupy a nonbonding orbital (HOMO). Similar molecular orbitals can also be identified in 8a, 8b, and 8c (see Figure S6). A typical E2 elimination is a concerted process in which H-abstraction and departure of leaving group takes place simultaneously. A typical E1cb elimination is a stepwise process involving two TSs (one for H-abstraction and one for departure of a leaving group) and an intermediate between the two TSs. The intermediate is the conjugate base (abbreviated as "cb" in the E1cb terminology) of the substrate. In the present elimination, the conjugate base intermediates (e.g. 7a, 7b, and 7c) and the TSs (TS4a/TS4b/TS4c) could be located in terms of gas phase electronic energy, but these TSs disappears in terms of free energy, thus in reality, the conjugate base intermediates would not exist even as transient species (true local minima) like that involved in the conventional E1cb elimination. The 3c-4e bonding intermediates (8a/8b/8c) are characteristic species for the elimination mechanism. Moreover, the H-bond prior formed with the formate group also plays an important role in promoting the departure of the leaving group. To symbolize these characteristics of the present stepwise elimination, we term the elimination mechanism as E1H-3c4e. Both E1cb and E1H-3c4e pathways include two steps, H-abstraction via TS3a (E1cb) and TS3b/TS3c (E1H-3c4e) and departure of the leaving group via TS5a'(E1cb) and TS5b/TS5c(E1H-3c4e). Because the intermediates (8a', 8a, 8b, and 8c) generated from H-abstraction can interconvert each other via association or dissociation of HCO2H molecule(s), as illustrated in Scheme 2, the preference of E1H-3c4e or E1cb mechanism only depends on the barrier heights of TS5b/TS5c relative to TS5a'. Because TS5a' is monomolecular and TS5b/TS5c are tri-/ tetra-molecular, respectively, TS5a' suffers less entropy penalty than TS5b/TS5c (see Figure 2). Therefore, high temperature would favor E1cb and disfavor E1H-3c4e mechanism. The energies in Figure 2 were estimated at 298.15 K, which show that E1cb mechanism is less favorable than E1H-3c4 mechanism, TS5a' being 5.4/9.1 kcal/mol higher than via TS5b/TS5c, respectively. If using the experimental temperate (110.0°) to correct entropy contributions, the energies of TS5a', TS5b, and TS5c are 22.6, 21.0, and 19.5 kcal/mol, respectively. Again, E1cb mechanism is still 14 / 27


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less favorable than E1H-3c4e mechanism via TS5b/TS5c. However, it can be anticipated that, when temperature is further raised to high enough, E1cb mechanism would suppress E1H-3c4e mechanism. To corroborate the E1H-3c4e elimination mechanism, we recalculated the elimination stage (the green pathway in Scheme 2 and Figure 2) at other five levels of DFT calculations. Figure 6 compares the energetic results. Understandably, the energies predicted by different DFT levels vary to some extent of degree, but the pathways are quite similar. In particular, the key TSs (TS3b and TS5b) and the 3c-4e bonding intermediate 8b, which characterize the E1H-3c4e mechanism, could be located

at

all

the

DFT

levels.

Because

the

geometries

at

the

M06-2X(SMD)/6-311++G(d,p)//M06-2X(SMD)/6-31G(d,p) level were optimized in solution, this level should be superior to other levels. At this level, the 3c-4e bonding intermediate 8b is more pronounced/stable. We have reasoned that 7a/7b/7c and TS4a/TS4b/TS4c would not exist in reality. Consistently, 7b and TS4b could not be located at the M06-2X(SMD)/6-31G(d,p) level, thus the Hβ-abstraction directly leads to the 3c-4e bonding intermediate 8. In short, the results in Figure 6 affirms the E1H-3c4e elimination mechanism.

Figure 6. Energetic comparisons of the elimination stage at other DFT levels.

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Hydrolysis stage: Figure 7 describes the course for 10 (the product of the elimination stage) to undergo hydrolysis with the corresponding optimized structures given in Figure S7 and S8 in SI. This stage proceeds via hydration of the Cβ=Cγ double bond of 10, forming a hemiacetal intermediate 15, followed by C−O bond cleavage, furnishing the monoaromatic products Ar1 and Ar2. This stage invokes a proton which was simulated by H3O+ in our calculations. First, H3O

＋

approaches 10,

forming a H-bond complex 11. Then, the terminal Cγ of the double bond is protonated by passing a barrier of 18.9 kcal/mol (TS6 relative to 11), resulting in a carbocation 12 with the formal positive charge on Cβ. In 12, the H2O moiety bridges the carbonyl O atom and one of the H atoms of Cγ through H-bond interaction. The intermediate 12 is not stable, as depicted by TS7, the O atom of water moiety can readily bond to the carbocation Cβ via nucleophilic attack, meanwhile, an O−H bond is broken, transferring the H atom to the carbonyl group, which leads 12 to 13. The transition state (TS7) for the nucleophilic attack could be located in terms of electronic energy in the gas phase, but it disappears after corrected by solvation and thermal effects. Thus the nucleophilic attack could be downhill straightforwardly. Subsequently, the formate anion grabs the proton on the carbonyl group of 13, leading to a more stable H-bond complex 14. Releasing the formic acid moiety from 14 gives the hemiacetal 15, which then undergoes 1,3-H transfer to break the C−O bond, leading to Ar1 and Ar2. The direct 1,3-H transfer via TS8' requires to surmount a very high barrier (40.6 kcal/mol, TS8' relative to 15). Again, a formic acid H-shuttle can significantly facilitate the H-transfer, as described by TS8, with a much lower barrier (16.1 kcal/mol, TS8 relative to 16). This stage uses the two parts (H+ and HCOO-) of formic acid, which is recovered by proton abstraction from 13 to 14. In the discussions above, we do not consider the H-bond between formic acid and the Cα=O group, but our computations show that such a H-bond has very little effects on the mechanism of the hydrolysis stage (see Figure S9).

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Figure 7. Free energy profile for 10 undergoing hydrolysis.

The combination of the three stages gives eq 1, which is exergonic by 35.1 kcal/mol. Eq 1 involves an equilibrium (eq 2) for formic acid dissociation. Subtracting the energy cost for formic acid dissociation, the net depolymerization of 1ox to Ar1 + Ar2 is exergonic by 8.8 kcal/mol (eq 3), which is the overall thermodynamic driving force for the catalytic depolymerization, without consuming formic acid and formic anion.

The formylation stage uses formic acid molecule as a substrate and H-shuttles. The elimination stage utilizes the conjugate base (HCOO-) of formic acid to deprotonate Hβ and the formic acid molecule to facilitate the departure of the formate 17 / 27


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leaving group. The hydrolysis stage requires acidic character of formic acid (represented by H3O+) and its conjugate base. These roles of formic acid rationalize why the HCO2H/NaCO2H medium worked so well for the conversion. The addition of NaCO2H further increases the concentration of formate anion, thus benefiting the reaction.

3.2 Further discussions Importance of pre-oxidation: Stahl et al. have shown that the pre-oxidation of Cα alcohol in 1lig to carbonyl group in 1ox is crucial for the depolymerization. To understand the importance of the pre-oxidation and to corroborate our E1H-3c4e elimination mechanism, we used the unoxidized lignin model 1lig to compute the formylation and elimination stages by focusing on key TSs and intermediates, as shown in Figure 8 (the optimized structures of key stationary points are given in Figure S10 in SI). Compared to the formylation of 1ox (Figure 1), the pre-oxidation does not facilitate the formylation significantly in terms of both kinetics and thermodynamics. This is reasonable. Because the Cγ-position (where the formylation takes place) is two carbon atoms away from the functional group on Cα, being a carbonyl or hydroxyl group on Cα does not affect the formylation stage much. Indeed, the replacement of the C=O group in 1ox with CH2 group (hereafter we denoted the resulting molecule as 1CH2) also does not influence the formylation stage much. The barriers corresponding to TS1OH and TS2OH are 13.9 and 16.6 kcal/mol, respectively (see Figure S11 in SI).

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Figure 8. Pathway for unoxidized 1lig undergoning formylation and elimination. However, the pre-oxidation influences the elimination stage significantly in terms of both mechanism and energetics. 5OH undergoes elimination by following a conventional E2 elimination mechanism with a concerted TS (i.e. TS3OH in Figure 8). The lengths of the two breaking bonds in TS3OH, R(Cβ−H)=1.573Ǻ and R(Cγ−O)=1.840Ǻ, characterize the concerted nature of the E2 mechanism. The elimination barrier measured from 5OH is 38.8 kcal/mol, which is 18.0 kcal/mol higher than its counterpart (20.8 kcal/mol, TS3b relative to 5 in Figure 2). The much lower elimination barrier of 5 than 5OH demonstrates the importance of the pre-oxidation of lignin. The advantages of pre-oxidation can be understood as follows. First, the stronger electron-withdrawing carbonyl group in 6 than the hydroxyl group in 6OH makes the Hβ in 6 more acidic than that in 6OH, as reflected by the larger positive NBO charge on Hβ in 6 (+0.242e) than in 6OH (+0.225e). Second, in the case of 1ox, when the protonic Hβ is gradually pulled off from Cβ of 6, the p orbital of the sp2 Cα can form a π-bond with Cβ, resulting in a Cα=Cβ double bond. Thus, the gradually accumulated negative charge on Cβ can transfer to the carbonyl O-atom via the π-conjugation (see HOMO of TS3b in Figure 9), which can be further stabilized via forming a 3c-4e bond with a formic acid molecule, finally giving 8b (Scheme 2). 19 / 27


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However, this cannot happen to 6OH in the case of 1lig because of the sp3 Cα. Third, in the case of 1ox, when the formate group leaves gradually, the p orbital of the sp2 Cα can form a π-conjugation with the developing Cβ=Cγ double bond, which stabilizes the transition state (TS5b). Moreover, the π-conjugation helps the shift of the COOƟ⋅⋅⋅H⊕⋅⋅⋅ƟO=Cα 3c-4e bond in 8b to the HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟOOCH 3c-4e bond in 9. In contrast, the sp3 Cα in 6OH prevents forming such a π-conjugation in TS3OH. The differences are shown by comparing the HOMOs of TS5b and TS3OH in Figure 9. The finding of E2 elimination pathway of 1lig system corroborates our proposed E1H-3c4e mechanism involved in the elimination stage of 1ox. Note that the elimination of 1CH2 system also follows E2 mechanism (Figure S11 in SI).

Figure 9. Comparing the HOMOs of TS3b and TS5b to that of TS3OH.

Origins of the E1H-3c4e elimination. Formate group is a relatively poor leaving group. To characterize the E1H-3c4e elimination further, we took into account of both good leaving groups (e.g. X = Cl and Br) and poor leaving groups (e.g. X=F, OH, OMe, and NMe2). Starting from 6_X given by replacing the formate group in 6 with X groups, we computed their elimination pathways. Because a H-bond on the carbonyl group (Cα=O) does not affect the elimination mechanism essentially (vide supra), we only computed the pathway by following the green one in Scheme 2 20 / 27


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except for two cases for verification (vide infra).

Figure 10. Mechanisms of elimination of HX (X=Br and Cl). Optimized geometries of TS3_Cl and TS3_Br are given in Figure S13 in SI. For good leaving groups (X= Br and Cl), the eliminations obey the classical E2 mechanism (see Figure 10), each of which features a concerted TS with low barrier (16.9 and 18.5 kcal/mol, respectively) and is exergonic by 13.3 and 10.9 kcal/mol, respectively. No proton-shared 3c-4e intermediate similar to 8b could be located. We reaffirmed that a prior formed H-bond between formic acid and the Cα=O group does not alter the E2 mechanism ( Figure S13). For poor leaving groups (X=F, OH, OMe, and NR2), the eliminations follow the E1H-3c4e mechanism, as illustrated by energy profiles (Figure 11) similar to that of 6 elimination (the green path in Figure 2). The characteristic 3c-4e intermediates (8_X, X= F, OH, OMe, and NR2) for the eliminations could be located (see Figure S14 for detailed geometric and NBO results). Comparing the kinetics and thermodynamics of the five elimination pathways, the eliminations with X=OH, OMe, and NMe2 are obviously more difficult than the elimination of formate group, because these groups are poorer leaving groups (than formate) and the prior HCO2H⋅⋅⋅X H-bond cannot be transformed to a HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟX 3c-4e bond, forming 3c-4e bonded leaving species (Figure S15). Nevertheless, the elimination for X=F kinetically comparable to and 21 / 27


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thermodynamically 6.7 kcal/mol more favorable than the formate elimination, because HCO2H⋅⋅⋅F H-bond can be converted to FƟ⋅⋅⋅H⊕⋅⋅⋅ƟOOCH 3c-4e bond, forming a more stable leaving species (Figure S15). The comparisons reaffirm the importance of the prior H-bond on the leaving group that can be transformed to a HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟX 3c-4e bond. Among the five 3c-4e bonded intermediates (8_X), 8_NMe2 is the lowest along the elimination pathway of 6_NMe2 (blue), which implies that the intermediate could be detected experimentally, thus providing a possibility for experimental verification of the E1H-3c4e elimination mechanism.

Figure 11. Comparisons of elimination mechanisms for poor leaving groups whose leaving ability are close to that of formate group. Optimized geometries of 8_X, TS5_X and 9_X along with Wiberg bond indices are given in Figure S14 and 15 in SI.

On the basis of the discussions above, we conclude that whether the elimination adopts E2 or E1H-3c4e mechanism depends on the leaving ability of the leaving group. When a base pulls off the Hβ atom, the Cβ atom would gradually accumulate negative charge. If X (e.g. Cl and Br) is a good leaving group which is favorable for stabilizing the negative charge resulted from the Hβ-abstraction, the negative charge 22 / 27


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would prefer transferring to the leaving group, resulting in a H-bonded (HCO2H⋅⋅⋅X-) leaving species, thus following the typical E2 mechanism. If X is a poor leaving group that dislikes/avoids the negative charge, the negative charge would prefer migrating to the nearby carbonyl group (Cα=O), forming a 3c-4e bonding intermediate via interacting with a HCO2H molecule, thus gearing the elimination away from the classical E2 mechanism to the new E1H-3c4e mechanism.

4. Conclusions We have performed DFT computations to understand formic-acid-catalyzed depolymerization of oxidized lignin model to monoaromatics. The conversion proceeds via formylation, elimination, and hydrolysis. The study characterizes various roles of formic acid in promoting the transformation, First, as a whole, it acts as a substrate to convert the hydroxyl group of 1ox to a better leaving formate group, and as a shuttle to mediate various hydrogen transfer events, and as a promoter to facilitate the departure of the poor leaving formate group. Second, it uses its parts, formate anion for Hβ abstraction and proton for protonation of Cβ=Cγ double bond. These roles of fromic acid account for why the HCO2H/NaCO2H medium works so elegantly for 1ox depolymerizaion. The study unravels an unconventional elimination mechanism (i.e. E1H-3c4e) which is different from the known ones such as E2 and E1cb mechanisms. The E1H-3c4e mechanism is characterized by passing through an intermediate stabilized by a proton-shared 3c-4e bond (HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟO=Cα) and by shifting the 3c-4e HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟO=Cα bond to the 3c-4e (HCOOƟ⋅⋅⋅H⊕⋅⋅⋅ƟOOCH) bond in the joint leaving group that is originally a regular H-bond (HCOO−H⋅⋅⋅OOC(H)−). The root-cause of the E1H-3c4e elimination is that the poor leaving group (formate) is less competitive in stabilizing the negative charge resulted from Hβ abstraction by HCOOthan the nearby carbonyl group (Cα=O) that can utilize the negative charge to form a stabilizing 3c-4e bond with a formic acid molecule. The elimination process in the 1lig case complies E2 mechanism but with much higher barrier, indicating the importance 23 / 27


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of pre-oxidation and supporting our E1H-3c4e mechanism for the elimination in the 1ox case.

Supporting Information Available Computational details, additional computational results, total energies and Cartesian coordinates of all optimized structures. This information is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Authors Email: [email protected] (Z.X.W.)

Acknowledgement: We acknowledge the support from the National Science Foundation of China (Grant No. 21173263 and 21373216) and the National Basic Research Program of China (973 Program, 2015CB856500).

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