Mechanism of Vanadium-Catalyzed Selective CâO ... - ACS Publications

Subscriber access provided by UCL Library Services

Article

Mechanism of Vanadium-Catalyzed Selective C -O and C-C Cleavage of Lignin Model Compound Yuan-Ye Jiang, Long Yan, Hai-Zhu Yu, Qi Zhang, and Yao Fu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00239 • Publication Date (Web): 24 May 2016 Downloaded from http://pubs.acs.org on May 25, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Mechanism of Vanadium-Catalyzed Selective C−O and C−C Cleavage of Lignin Model Compound Yuan-Ye Jiang,a Long Yan,a Hai-Zhu Yu,b Qi Zhang,a and Yao Fua,* a

Collaborative Innovation Center of Chemistry for Energy Materials, CAS Key Laboratory of

Urban Pollutant Conversion, Department of Chemistry, University of Science and Technology of China, Hefei 230026 b

Department of Chemistry and Centre for Atomic Engineering of Advanced Materials, Anhui

University, Hefei, 230601

ABSTRACT: Efficient depolymerization methods are critical to the sustainable production of fuels and chemicals from biomass. Ligand-controlled selective C(sp3)−O and Ar−C(sp3) cleavages of β-O-4 lignin model compounds were realized with the vanadium catalysts under redox-neutral condition or air atmosphere. To clarify the mechanism and the origin of selectivity, a joint theoretical and experimental study was performed herein. First, with the aid of density functional theory (DFT) calculations, an updated mechanism involving VV, VIV and VIII complexes was discovered for the C(sp3)−O cleavage process catalyzed by the Schiff base vanadium complexes with an overall free energy barrier of 34.9 kcal/mol. Meanwhile, a detailed catalytic cycle involving novel stepwise O−O/Ar−C(sp3) cleavage was clarified for the

1 Environment ACS Paragon Plus

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ar−C(sp3) cleavage process catalyzed by the bis(8-oxyquinolate) coordinated vanadium complexes, owing an overall free energy barrier of 28.8 kcal/mol. Further analysis based on the energetic span model revealed that the switchable selectivity results from the different T1(ground triplet state)-HOMOs-separation/charge dispersion effects of ligands and the different formal oxidation states of TOF-determining transition state (TDTS) in the C(sp3)−O and Ar−C(sp3) cleavage processes. Finally, control experiments of base and oxygen pressure were conducted to validate the conclusions from DFT studies regarding the role of bases and the TDTS step in the Ar−C(sp3) cleavage process.

KEYWORDS: lignin model compounds, vanadium, C−O bond cleavage, C−C bond cleavage, aerobic oxidation, density functional theory, spin crossover, two-state reactivity

1. Introduction With the depletion of fossil fuels, biomass is regardforwrad. ed as a promising alternative for the production of fuels and chemicals nowadays.1 As a unique large-volume renewable biomass mainly constituted by methoxy-substituted phenyl and phenolic subunits, lignin is receiving increasing attention for the production of aromatic chemicals.2 Efficient depolymerization of lignin is critical to the success of chemical conversion of lignin. Under such circumstances, C−O and C−C cleavages of lignin model compounds (alkyl aryl ethers, β-O-4 lignin models, syringyl alcohol etc.) have grown rapidly in recent years with the development of various metal catalysts311

, photocatalysts12 as well as metal-free methods.13 For example, Hartwig et al. reported a Ni-

catalyzed C(benzyl)–O and Ar−O cleavage reactions using hydrogen as a reductant.3a Alternatively, Baker and Loh et al. reported a Cu-catalyzed C(sp3)−C(sp3) cleavage reaction4a,b


Page 2 of 37

Page 3 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

employing oxygen as the oxidant. Meanwhile, Bozel, Canevali and co-workers reported the oxidative Ar−C(sp3) cleavage of phenolic lignin model compounds with cobalt catalysis.5 Very recently, Stahl et al. reported the C(sp3)−O cleavage of lignin to aromatics by combining metalfree oxidation with formic-acid-induced formylation.13 On the other hand, bond cleavage reactions under redox-neutral conditions were also explored. For instance, Bergman and Ellman et al. disclosed the Ru-catalyzed C(sp3)–O cleavage reaction.6a Klankermayer and co-workers recently made a leap forward by achieving the Ru-catalyzed C(sp3)−C(sp3) cleavage of the lignin model compounds.6b In spite of these encouraging successes, efforts are still necessary in the aspects of the cost of catalysts, the handling safety (avoiding flammable and explosive reactants like pure O2 or high-pressure H2) and the bond cleavage selectivity. Regarding these issues, the vanadium-catalyzed C−O and C−C cleavages of the lignin model compound have shown outstanding performance.14 In 2010, Toste and co-workers reported the C(sp3)−O cleavage of non-phenolic β-O-4 lignin model compounds, which generates enones and phenols by using Schiff base vanadium complex S1 as the catalyst (Scheme 1a).14a This reaction is formally redox-neutral. Nonetheless, the reaction can be accelerated under air due to the prevention of catalysts’ precipitation. Later on, Hanson, Silks and co-workers discovered the Ar−C(sp3) cleavage of the phenolic β-O-4 lignin model compound under air atmosphere using the bis(8-oxyquinolate) vanadium complex Q1 as the catalyst (Scheme 1b).14b Interestingly, under identical conditions, the C(sp3)−O cleavage of the same phenolic substrate is preferred if the catalyst is changed to S1 (similar to the aforementioned results reported by Toste et al). Hence, the studies by Toste, Hanson and Silks et al. provide a cost-efficient and relatively safe tool for ligand-controlled selective C(sp3)−O and Ar−C(sp3) cleavage of β-O-4 lignin model compounds.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1. Vanadium-catalyzed C−O or C−C cleavage of (a) non-phenolic β-O-4 lignin model compound and (b) phenolic β-O-4 lignin model compound Mechanistic studies have been performed on the aforementioned reactions. For the C(sp3)−O cleavage process, Toste et al. proposed a one-electron catalytic cycle involving VIV and VV complexes (Path A, Scheme 2a).14a As shown in Scheme 2a, the VV catalyst S1 first undergoes a ligand exchange with the benzylic hydroxyl group of the lignin model compound to generate the intermediate A. Then an intramolecular hydrogen transfer forms the ketyl-radicalcoordinated VIV complex B, followed by C−O cleavage to generate the aryloxy radical and C. Thereafter, hydroxyl group elimination on C gives D and the enone product. Finally, catalyst S1 is regenerated through subsequent oxidation of intermediate D by the aryloxy radical. Different from Toste’s proposal, Hanson et al. suggested that vanadium-catalyzed aerobic oxidation of alcohols in the presence of bases might give the triplet VIII complex F.15 According to their


Page 4 of 37

Page 5 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

proposal, a competitive pathway leading to E and F from A via the base-promoted benzylic deprotonation, spin crossover and protonation of the oxo-ligand is plausible in the presence of pyridine (Path B, Scheme 2a). From F, Path B and Path A might converge either in the isomerization step between F and B or the C(sp3)−O cleavage on F to form C. In addition to above proposals, we speculate that oxidation of VIV complex C by phenoxy radical may also occur before the hydroxyl elimination. Regarding the Ar−C(sp3) cleavage process, hydrogen atom abstraction from the lignin model compound by the vanadium catalyst to form a superoxovanadium intermediate was pointed out by Hanson and co-workers (Scheme 2b).14b It is similar to the mechanism of Co-catalyzed Ar−C(sp3) cleavage reactions.5,16 Although this proposal seems plausible, the details remain obscure. More importantly, although effort has been put into mechanistic studies on vanadium catalysis,17 the mechanism of V-catalyzed C−O/C−C cleavage of β-O-4 lignin model compound, and especially the origin of the ligand-controlled selectivity, is still unclear to date.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 2. Mechanistic proposals for vanadium-catalyzed (a) C−O cleavage of β-O-4 lignin model compound and (b) C−C cleavage of phenolic β-O-4 lignin model compound Given the versatility of vanadium catalysts and our long-term interest in chemical conversion of biomass,18 here a combined theoretical and experimental study was performed on the mechanism of the vanadium-catalyzed selective C(sp3)−O and Ar−C(sp3) bond cleavages of the phenolic β-O-4 lignin model compound. An updated mechanism involving VIII, VIV and VV complexes was located for the C(sp3)−O bond cleavage process and a completed catalytic cycle involving novel stepwise O−O/Ar−C(sp3) cleavage mechanism was clarified for the Ar−C(sp3) bond cleavage process. These calculation results are consistent with the previous experimental observations as well as the additional control experiments in our study. Based on the above results, the different T1-HOMOs-separation/charge dispersion effects of ligands and the different


Page 6 of 37

Page 7 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

formal oxidation states of vanadium in the TDTSs are proposed to be responsible for the switchable selectivity. 2. Computational Methods DFT calculations were performed by employing G09 program19 in solution phase (solvent = pyridine) with SMD solvent model.20 The M06 method21 was adopted as our method in conjunction with an ultrafine integration grid.22 6-31G(d) was used for all main group elements while LANL2DZ and the associated basis set23 with the polarization function (ζ(f) = 1.751)24 were used for vanadium. Geometry optimizations were performed without symmetry restraints unless mentioned otherwise. Frequency analysis based on the optimized structures was performed to verify the stationary point as an intermediate or transition state, and also to obtain the thermodynamic corrections. Intrinsic reactant coordinate (IRC) analysis25 was performed to check whether transition states connect correct stationary points or not at the same lever of theory. The intermediates and transition states bearing unpaired electrons (in doublet, triplet or quartet state) were investigated with unrestricted M06 method. The spin contamination of the unrestricted calculations is less than 5%. Minimum energy crossing points (MECP) between the energy profiles of different spin states were located by using the program developed by Harvey et al.26 Note that MECPs are not stationary points in the full dimensions of either of potential energy surfaces, the thermodynamic corrections of MECPs based on standard frequency analysis is meaningless. We added the electron gap between MECPs and the corresponding precursors (to evaluate the difficulties of spin crossover process only) to the relative Gibbs free energies of the precursors, to get the formal Gibbs free energies of MECPs.27 1.9 kcal/mol is added to every species to account for the change in standard states from gas phase to aqueous solution.28 NBO version 3.1 implemented in Gaussian 09 was used for natural bond orbital (NBO) analysis.29


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Solution-phase Gibbs free energies referencing to 1 mol/L and 298.15 K were used for discussion unless mentioned otherwise. 3. Results and discussion A 4:1 mixture of the diastereomers of the lignin model compound was employed as the substrate in the reported experiments,14a,b and the major erythro-isomer 1e was chosen as the model substrate for our computational study (Scheme 3). To reduce computational cost, the tertiary butyl groups of Schiff base ligand in S1 are replaced with hydrogen atoms in calculations. Thus, a simplified vanadium complex S1′′ was used as our model catalyst. Note that S1′′ show closely similar selectivity as S1 in Toste’s C(sp3)−O cleavage experiments.14a To distinguish the spin states of vanadium complexes in the following discussions, the symbols -s, -d, -t and -q were used to denote the singlet, doublet, triplet and quartet states respectively. 3.1 Mechanism for C−O Cleavage by Schiff Base Vanadium Catalysts The singlet state of S1′′ is remarkable more stable than the triplet state by 42.3 kcal/mol, thus we initiated our theoretical study from the singlet complex S1′′-s. It first undergoes ligand exchange with the benzylic hydroxyl group of 1e via a four-membered transition state (TSS1-s, Scheme 3), and forms the methanol-coordinated intermediate S2-s. The energy barrier of this step is 17.7 kcal/mol. S2-s releases methanol to generate the more stable intermediate S3-s, positioning the benzylic C−H bond closer to the oxo ligand. The benzylic C−H bond starts to break at this stage. Both the intramolecular hydrogen transfer via TSS2-s (Path A, Scheme 2a; Figure 1) and the pyridine-promoted deprotonation via TSS2-py-s (Path B, Scheme 2a) were investigated, and the former is kinetically favored by 6.1 kcal/mol. Meanwhile, the energies of S1′′-t, S2-t and S3-t are


Page 8 of 37

Page 9 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

significantly higher than those of the singlet species, which implies that the triplet state pathway is energetically disfavored and therefore was excluded for this stage.

Scheme 3. Calculated free energy profile of C−O cleavage of phenolic β-O-4 lignin model compound by vanadium catalyst S1′′ (in kcal/mol) The transition state TSS2-s leads to the formation of S4-s, which could directly undergo C(sp3)−O cleavage via TSS3-s. Though the energy barrier of this elementary step is only 15.7 kcal/mol, the overall energy barrier is as high as 38.6 kcal/mol (refers to S1′′-s), indicating that this pathway is less possible. By contrast, the triplet state pathway becomes favorable at this stage. It starts with the spin crossover from S4-s to form the triplet intermediate S4-t via MECP1, with an energy decrease of 19.6 kcal/mol. MECP1 is less stable than S4-s by only 0.9 kcal/mol, suggesting that this spin crossover is feasible. Our results are also supported by Hanson and Cundari’s work that the triplet state of VIII complexes is relatively more stable than the ones


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 37

in singlet state.15 Furthermore, the triplet ground state of some other VIII complexes have also been confirmed with electron paramagnetic resonance.30 From S4-t, the triplet C(sp3)−O cleavage occurs via TSS3-t with an overall energy barrier of 30.7 kcal/mol. Thus, the triplet pathway is kinetically favored over the singlet state pathway (TSS3-t vs TSS3-s) by 7.9 kcal/mol. As shown in Scheme 3, the highly exergonic transformation of S4-s to S4-t provides a critical driving force for the triplet state C(sp3)−O cleavage pathway, presumably caused by the formation of ketone from the unstable ketyl radical. As shown in Figure 1, the V-O-Cbenzyl angle is 167.8o in S4-s but shortens to 137.3o in S4-t. Meanwhile, the V−Obenzylic bond length is 1.872 Å in S4-s, much shorter than that (2.139 Å) in S4-t. Furthermore, the spin density distribution on the vanadium center is 2.02 in S4-t and 1.10 in TSS3-t. These results imply that the oxidation state of the vanadium formally changes from +3 to +4 in the C(sp3)−O cleavage step, rather than the persistent +4 in Path A (Scheme 2a).


Page 11 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 1. Optimized structures of selected intermediate and transition states. Bond length is shown in angstrom (black) and bond angle is shown in degree (blue). After the C(sp3)-O dissociation step, the intermediate S5-t containing a phenoxyl radical and a VIV center is formed. The spin density of vanadium in S5-t is 1.04. From S5-t, the extrusion of phenoxyl radical followed by the elimination of hydroxyl group on the VIV complex via TSS4-d could occur (Path A, Scheme 2a). However, this pathway is ruled out due to the extremely high overall energy barrier (40.4 kcal/mol). We speculate that the presence of the unstable phenoxyl radical results in the disfavored hydroxyl group elimination on VIV complex. Alternatively, the reaction pathway may divert to a pre-oxidation of the VIV complex by the phenoxyl radical, which generates 2-methoxyphenol and a VV complex. Interestingly, this oxidation step could be realized via another spin crossover, i.e. from S5-t to S5-s via MECP2. In this process, the hydrogen atom spontaneously moves from the vanadium complex to the phenoxyl radical without going through a formal transition state, as indicated by our calculation results. This step is remarkably exergonic by 20.0 kcal/mol. MECP2 is more stable than S5-t by 12.3 kcal/mol, implying that the oxidation via the spin crossover is feasible. The release of the phenol product from S5-s generates the VV complex S6-s, from which the hydroxyl group elimination occurs via TSS4-s to generate the vanadium(V) hydroxide complex S7-s and the enone product. Compared to TSS4-d, the hydroxyl group elimination transition state TSS4-s is kinetically more favorable by 17.1 kcal/mol. Similarly, the triplet state pathway at this stage (S6t and S7-t) could be excluded due to its high energy demand. Therefore, the formation of the enone is favorably achieved by the oxidation of the VIV intermediate (with the phenoxy radical) and the following hydroxyl group elimination. The ligand exchange of S7-s with 1e might form S3-s via TSS5-s (Scheme 4), and S3-s can start the next C(sp3)−O cleavage process as shown in Scheme 3. On the other hand, ligand


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 37

exchange between S7-s and methanol (formed in the transformation from S2-s to S3-s) via TSS6s could regenerate the original catalyst S1′′-s to start the next catalytic cycle. Since the transformation between S7-s and S1′′-s is more feasible than the following C(sp3)−O cleavage process, there is a fast equilibrium between S7-s and S1′′-s and the relevant overall energy barriers of the C(sp3)−O cleavages are identical (refers to the more stable S1′′-s). In this case, the C(sp3)−O cleavages catalyzed by S7-s and S1′′-s are comparative under catalytic conditions.

Scheme 4. Calculated free energy profiles of competitive C(sp3)−O cleavage catalyzed by S1′′ and S7 (in kcal/mol) 3.2 Mechanism of C−C Cleavage by Bis(8-oxyquinolate) Vanadium Catalysts After examining the mechanism of the C−O cleavage catalyzed by the Schiff base vanadium catalysts, we then turned to the Ar−C(sp3) cleavage of 1e catalyzed by bis(8-oxyquinolate) vanadium catalyst Q1. Referring to the recent studies of hydrogen transfer of oxovanadium complexes,17a we first examined the hydrogen transfer between the phenolic hydroxyl group of 1e and the oxo ligand of Q1 (Scheme 5). Further calculations revealed that the ground state of Q1 is also singlet state. As illustrated in Scheme 5, Q1-s combines with 1e to form Q2-s by noncovalent interaction in the first instance. From Q2-s, hydrogen transfer in singlet state could occur but no transition state was located due to the increasing energy with the Ooxo−Hphenol


Page 13 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

distance shortening (see Figure S1 for the energy surface scan). On the other hand, the triplet hydrogen transfer pathway starts with the spin crossover via MECP3 to form Q1-t. Then Q2-t is formed via the interaction between Q1-t and 1e. Afterwards, Q2-t undergoes the hydrogen transfer process via TSQ1-t, resulting in the phenoxyl-radical-containing vanadium hydroxide complex Q3-t. TSQ1-t is an early transition state with a long Ooxo−Hphenol bond length of 1.437 Å. Because of the high energy demand for the spin crossover from Q1-s to Q1-t (energy barrier is 47.1 kcal/mol), this triplet hydrogen transfer is excluded. It is noteworthy that the triplet hydrogen transfer after TSQ1-t is highly exergonic while the singlet hydrogen transfer from Q2-s is endergonic. Thus, a crossing point possibly exists to connect these two pathways and it is located as MECP4 (see Figure S2 for the details). MECP4 is structurally similar to Q3-t with a short Ooxo−Hphenol bond length of 1.061 Å. Meanwhile, the energy of MECP4 is higher than that of Q2-s by 30.0 kcal/mol, showing that this hydrogen transfer via the spin crossover process is more feasible than the corresponding pure singlet process or the spin crossover process via MECP3. Q3-t releases the phenoxyl radical and generates doublet Q4-d, from which proton transfer could occur to form the iPrOH-coordinated intermediate Q5-d. For this step, the pathways assisted by two hydroxyl groups (via TSQ2-d), one hydroxyl group (via TSQ2-iso1-d) and no hydroxyl group (via TSQ2-iso2-d) were all considered. By comparison, the proton shuttle mechanism via TSQ2-d is the most favorable with a total energy barrier of 31.0 kcal/mol. Note that the quartet state proton transfer process was considered but excluded due to the high energies of the related intermediates (Q4-q and Q5-q). In addition, proton transfer from the benzylic hydroxyl group of 1e to the isopropoxide in Q1 was examined, but this pathway is endergonic by 48.9 kcal/mol and we also excluded this pathway.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 37

Scheme 5. Calculated free energy profile of the generation of phenoxyl radical from lignin model compound by Q1 (in kcal/mol) From Q5-d, the Ar−C(sp3) cleavage stage starts. We found that this stage prefers low spin states (singlet or doublet), and the results of related triplet and quartet states were put into supporting information without further discussion herein. First, the ligand exchange of Q5-d with oxygen could form η2-oxygen complex Q6-d or η1-oxygen complex Q7-d. Q6-d is more stable than Q7-d, and this result is consistent with the crystal structures of vanadium-dioxygen complexes.31 The spin density is distributed almost equally on the two oxygen atoms of O2 in Q6-d with the numbers of 0.55 and 0.57, respectively. By contrast, the spin density mainly spreads on the uncoordinated oxygen atom of O2 in Q7-d with the number of 0.71 (the spin density on the coordinated oxygen atom of O2 is 0.37). From Q7-d, the uncoordinated oxygen atom couples with the phenoxyl radical to form the superoxovanadium intermediate Q8-s with an energy decrease of 17.9 kcal/mol. For the following Ar−C(sp3) cleavage on Q8-s, a direct C−C cleavage mechanism was first examined. Relaxed energy surface scan indicates a continuous energetic increase with the Ar−C(sp3) bond lengthening. For example, the electronic


Page 15 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

energy increases by 34.2 kcal/mol (refers to that of Q8-s) when the Ar−C(sp3) bond length reaches 2.656 Å (Figure S3), suggesting this pathway is less possible. The difficulty of this mechanism probably lies in the simultaneous generation of two unstable radical species after the direct Ar−C(sp3) cleavage. Therefore, we next considered the concerted hydrogen transfer/Ar−C(sp3) cleavage mechanism via TSQ3-iso-s (Figure 2). This mechanism generates 2,6-dimethoxybenzoquinone 2, an aldehyde and a vanadium hydroxide complex without the formation of any radical species. Indeed, this step is highly exergonic by about 70 kcal/mol whereas kinetically disfavored with a high overall energy barrier of 37.2 kcal/mol. In addition, the concerted hydrogen transfer/Ar−C(sp3) cleavage pathway in the absence of vanadium was also excluded due to the high overall energy barrier of 37.8 kcal/mol (Scheme S1). On this occasion, we noticed a partial formation of V−O bond (1.818 Å, Figure 2), and a pre-cleavage of O−O bond in the superoxovanadium complex Q8-s. A normal O−O bond length is calculated to be 1.206 Å in free dioxygen. However, it is enlarged to 1.405 Å in Q8-s. This phenomenon hints at a possible mechanism in which the O−O cleavage on Q8-s occurs first to generate oxovanadium(V) radical Q9-d and an oxygen radical Int1 (Figure 2). Then the oxygen radical undergoes Ar−C(sp3) cleavage to generate 2,6-dimethoxybenzoquinone 2 and the carbon radical Int2. The radical coupling of Int2 and Q9-d forms a C−O bond and generates Q10-s, from which intramolecular proton transfer occurs to give the β-hydroxyaldehyde 3 and the vanadium(V) hydroxide complex Q11-s. The calculation results verifies that this stepwise O−O/Ar−C(sp3) cleavage is indeed operative with a total energy barrier of 28.8 kcal/mol (refers to Q2-s in Scheme 5). In this mechanism, the O−O cleavage (via TSQ3-s) is the rate-determining step while the Ar−C(sp3) cleavage (via TSQ4-d) and the subsequent intramolecular proton transfer (via TSQ5-s) are much more feasible. The radical coupling of Int2 and Q9-d is highly


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 37

exergonic by 65.3 kcal/mol and no transition state exists therein. Different from the concerted hydrogen transfer/Ar−C(sp3) cleavage mechanism via TSQ3-iso-s, the O−O bond is almost broken in TSQ3-s (the bond length is 1.900 Å) whereas the Ar−C(sp3) bond length is nearly unchanged (1.537 Å, Figure 2).

Figure 2. Calculated free energy profiles of Ar−C(sp3) cleavage on bis(8-oxyquinolate) vanadium complex (in kcal/mol) and optimized structures of selected intermediates and transition states (bond length is shown in angstrom). Similar to S7, the vanadium(V) hydroxide complex Q11 isomerizes to Q1 via ligand exchange. From Q11 and Q1, both the Ar−C(sp3) cleavage of 1e and the dehydration of βhydroxyaldehyde 3 to generate the final product are possible (Scheme 6a). The simplified energy profiles of the four pathways are shown in Scheme 6b while the detailed results were put into supporting information (Scheme S2-S5). The Ar−C(sp3) cleavage catalyzed by Q11-s proceeds


Page 17 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

via the spin-crossover-involved hydrogen transfer (MECP6) (blue line, Scheme 6b). The regeneration of Q1-s from Q11-s is realized via the ligand exchange transition state TSQ6-s, and the following Ar−C(sp3) cleavage is the same as that presented in Scheme 5 and Figure 2 (red line, Scheme 6b). For the dehydration process, a mechanism including ligand exchange with the β-hydroxyaldehyde (via TSQ7-s and TSQ13-s) and the base-assisted deprotonation (TSQ8-s) was elucidated (black line and green line respectively, Scheme 6b). By comparison, Q11-catalyzed dehydration has the lowest energy barrier among the four pathways while the Ar−C(sp3) cleavage catalyzed by Q1 is the secondary feasible pathway. Because the two pathways have close energy barriers, they might be competitive in the actual reactions. Furthermore, the Ar−C(sp3) cleavage catalyzed by Q11 is kinetically less favored than that by Q1, indicating that Q1 plays a more important role than Q11 in the C−C cleavage of lignin model compound.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 6. Competitive transformations from Q11 and Q1 (a) four possible pathways (b) calculated free energy profile in kcal/mol 3.3 Origin of ligand-controlled bond cleavage To explore the origin of ligand-controlled selectivity, we further investigated the Ar−C(sp3) cleavage catalyzed by the Schiff base vanadium complexes S1′′ and S7, as well as the C(sp3)−O cleavage catalyzed by bis(8-oxyquinolate) vanadium complex Q1 (Scheme S6-S8). It is found

Scheme 7. Calculated free energy profile of key steps in the C−OAr and Ar−C(sp3) cleavage of lignin model compound 1e by catalysts (a) S1′′ and (b) Q1


Page 18 of 37

Page 19 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

that the Ar−C(sp3) cleavage catalyzed by S1′′ or S7 proceeds via the same mechanism as shown in Scheme 5 and Figure 2. The C(sp3)−O cleavage mechanism with Q1 is very similar to those with S1′′ or S7 except for the benzylic C(sp3)−H cleavage stage. In the case of Q1, benzylic C(sp3)−H cleavage proceeds via a pyridine-assisted deprotonation, a spin crossover and a protonation step (Path B, Scheme 2a). It is also noted that S1′′ and S7 can react with the primary hydroxyl group and the phenolic hydroxyl group of 1e by fast ligand exchange. The corresponding ligand exchange generates the stable intermediate S17-s (Scheme 7) but the subsequent transformations were kinetically disfavored compared with the pathway starting from the benzylic hydroxyl group (Scheme S9 and S10). Ligand exchanges of Q1 with the primary hydroxyl group and the phenolic hydroxyl group of 1e were also considered but they are kinetically disfavored over the reactions between Q1 and benzylic hydroxyl group (Scheme S11). Based on all these results, the energetic span model developed by Kozuch and Shaik is employed to clarify the bond cleavage selectivity.32,33 Since S7 has similar selectivity with S1′′, only the key free energy profiles of S1′′ and Q1 are discussed in the manuscript (Scheme 7). The details of S7 were put into Supporting Information as Scheme S9. According to energetic span model, the turnover frequency (TOF) of a catalytic reaction is related to the calculated energy profile by Eqs. 1 and 2, []

/ ∏ [] ! − !"% & '(( ') ' = "# !"# − !"% + ∆!, & '(( ') -

TOF =

Eq. 1 Eq. 2

where KB is the Boltzmann constant, T is the temperature, h is the Planck constant, R is the gas constant, [R] is the concentration of the consumed reactants and the [P] is the concentration of


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

the generated products in the transformations from the TOF-determining intermediate (TDI) to TOF-determining transition state (TDTS). The term δE is the energetic span and Gibbs free energies were used to calculate δE.32 According to the catalytic process shown in Scheme 7a, S17-s is the common TDI for both the C(sp3)−O and the Ar−C(sp3) cleavage processes. The TDTS of the C(sp3)−O cleavage process catalyzed by S1′′, i.e. TDTSS(C-O), is the intramolecular hydrogen transfer transition state TSS2-s. Therefore, the corresponding energetic span δES(C-O) is 33.5 - (-1.4) = 34.9 kcal/mol according to equation 2. Similarly, the TDTS of the Ar−C(sp3) cleavage catalyzed by S1′′, which is labelled as TDTSS(C-C), is the spin-crossover-involved proton transfer MECP7 with an energetic span of 37.3 kcal/mol. According to equation 1, the TOFs of both the corresponding C(sp3)−O and Ar−C(sp3) cleavage processes are given in Eqs. 3 and 4.

TOF./012 =

TOF./002 =

3456/7382 9:

3456/7372 9:

Eq. 3

[MeOH]

Eq. 4

Furthermore, the ratio of TOFS(C-O) and TOFS(C-C) can be calculated through Eq. 5. >1?@/A3B2 >1?@/A3A2

=

456/7372 3 456/7382 9:

C [DEFG]

Eq. 5

Because MeOH generates from S1′′, [MeOH] is estimated to be the initial [S1′′], i.e. 8.5*10-3 mol/L. Therefore, the ratio of TOFS(C-O) and TOFS(C-C) is 3.6*103, meaning that the C(sp3)−O cleavage is significantly kinetically favored. Similarly, the ratio of TOFS(C-O) and TOFS(C-C) is estimated to be 3.0*105 for S7 (see Scheme S12 for details). Thus the calculated results are qualitatively consistent with the observed selectivity for Schiff base vanadium catalysts.


Page 21 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

As to the C(sp3)−O cleavage process catalyzed by Q1, the TDTSQ(C-O) is the C(sp3)−O cleavage transition state TSQ17-t rather than the benzylic C−H cleavage transition state TSQ15-s. The reasons can be attributed to the lower energy of TSQ15-s and the fact that the benzylic C−H cleavage is promoted with assistance of the high concentration of pyridine (pyridine is used as the solvent in experiments). Similarly, the O−O cleavage transition state TSQ3-s is the TDTS for the Q1-catalyzed Ar−C(sp3) cleavage (TDTSQ(C-C)) rather than TSQ2-d or MECP4 due to the low concentration of oxygen and the generated phenoxyl radical at this stage (see supporting information for details of determining TDTSQ(C-C)). On the other hand, Q2-s is the most stable intermediate in the transformations connecting TSQ17-t and TSQ3-s so that it represents the common TDI for these two competitive processes. Accordingly, the TOFs of the C(sp3)−O cleavage and the Ar−C(sp3) cleavage catalyzed by Q1 are obtained through Eqs. 6 and 7, respectively.

TOFH/012 =

TOFH/002 =

345I/7382

9:

Eq. 6

[FL ]

Eq. 7

[JK,FG]

345I/7372 9:

C

[JK,FG]

where δEQ(C-O) = 32.7 - (-1.1) = 33.8 kcal/mol and δEQ(C-C) = 27.7 - (-1.1) = 28.8 kcal/mol. Thus, the ratio of TOFQ(C-O) and TOFQ(C-C) can be worked out via Eq. 8, >1?

>1?M/A3B2 =

M/A3A2

45I/7372 3 45I/7382 9:

C [FL ]

Eq. 8

Based on the solubility of oxygen in pyridine,34 [O2] is estimated as 1.2*10-3 mol/L. Thus, the ratio of TOFQ(C-O) and TOFQ(C-C) can be worked out via Eq. 8 as 0.7 for Q1. The ratio qualitatively supports the experimental observation that Ar−C(sp3) cleavage is preferred over C(sp3)−O cleavage by using the bis(8-oxyquinolate) vanadium catalyst. 21 Environment ACS Paragon Plus

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 37

According to the above results, the ligand-controlled switchable selectivity is realized by two factors. First, changing catalyst S1′′ to Q1 significantly accelerates the hydrogen transfer between the lignin model compound and the oxo ligand of vanadium catalysts. In this process, vanadium catalysts exhibit two-state reactivity35 wherein the spin state of catalysts changes from singlet to triplet in the kinetically favored pathway. The singlet-triplet energy gap of Q1 (29.7 kcal/mol) is remarkably lower than that of S1′′ (42.3 kcal/mol), suggesting that Q1 is more competent to the spin state change. It is known that a larger spatial separation of the unpairedelectrons-occupied orbitals can lead to a smaller singlet-triplet energy gap because the repulsion between the unpaired electrons is smaller therein.36 Consistent with this rule, we found that the HOMO of Q1-t primarily spreads on one 8-oxyquinolate ligand while the HOMO-1 of the Q1-t primarily spreads around the vanadium and on the other 8-oxyquinalinate ligand. By contrast, the HOMO and HOMO-1 of S1′′-t have more overlap not only around the vanadium but also on the methoxide ligand and the Schiff base ligand (Figure S5). In this case, the bis(8-oxyquinolate) ligands increase the spatial separation of the unpaired-electrons-occupied orbitals of the ground triplet state (T1) to lower the singlet-triplet energy gap, thus the spin-crossover-involved hydrogen transfer is facilitated. Second, changing catalyst S1′′ to Q1 enlarged the energy difference between the O−O bond cleavage transition state and the TDTSs of C(sp3)−O cleavage processes. The energy difference is 3.2 kcal/mol (TSS8-s vs TSS2-s) when S1′′ is used as the catalyst while enlarged to 5.0 kcal/mol (TSQ3-s vs TSQ17-t) with Q1 as the catalyst. The NBO charges support that bis(8-oxyquinalinate) ligand has a stronger charge dispersion effect on vanadium center compared to the Schiff base ligand. For example, the NBO charges of vanadium are 0.802 and 0.934 in TSS8-s and TSS3-t respectively. However, they are much smaller in TSQ3-t (0.679) and TSQ17-t (0.799),


Page 23 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

respectively. In addition, we noticed that the formal oxidation states of vanadium in the O−O bond cleavage transition state (TSQ3-s and TSS8-s) is +5 while those involved in the TDTSs of Ar−C(sp3) cleavage process are lower, i.e. from +5 to +4 in TSS2-s and from +3 to +4 in TSQ17-t. Taking these two factors into account, we proposed that the ligands with a stronger charge dispersion effect (such as the bis(8-oxyquinolate)) can stabilize those TDTSs bearing higher oxidation state vanadium center (i.e, the vanadium involved in the O−O cleavage transition state) with a larger degree, and thus benefit the corresponding Ar−C(sp3) cleavage process. With this proposal, it is also understandable that Q1 promotes a deprotonation mechanism (via TSQ15-s) for the benzylic C−H bond cleavage while S1′′ prefers the intramolecular hydrogen transfer (via TSS2-s). This is because that the anionic vanadium species generated in the deprotonation mechanism could be better stabilized by the bis(8-oxyquinolate) ligand than by the Schiff base ligand. 3.4 Experimental mechanistic study It was previously found that benzylic deuteration of the β-O-4 lignin model compound slows the conversion in the S1-catalyzed C(sp3)−O cleavage reactions. Meanwhile, the yields of the C(sp3)−O cleavage products were almost the same in either pyridine or less basic acetonitrile.14b These observations could be well rationalized by our proposed mechanism, in which the intramolecular hydrogen transfer between the benzylic C−H bond and the oxo ligand is the ratedetermining step while the pyridine-assisted deprotonation pathway is much disfavored (Scheme 3). To further examine the validity of our calculated results, the following experiments were conducted. First, bases (pyridine as a solution or NEt3 as an additive) were usually used in Q1-


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

catalyzed Ar−C(sp3) cleavage of phenolic β-O-4 lignin model compound.14b However, our calculation results indicates that the base is not essential for the Ar−C(sp3) cleavage (Scheme 5 and Figure 2) but accelerates the dehydration of β-hydroxyaldehyde 3 to generate acrolein derivative (TSQ8-s Vs TSQ8-HT-s in Scheme S3). Consistent with this conclusion, 2,6dimethoxybenzoquinone 2, β-hydroxyaldehyde 3 and ketone 5 are still obtained as the major products associated with a low yield of acrolein derivative 4 when Q1-catalyzed oxidation of 1 is conducted in ethyl acetate (Scheme 8). When a catalytic amount of NEt3 is added, the yield of 3 decreases while the yield of 4 increases, supporting the acceleration effect of bases on the dehydration step. The yields of oxidation product 5 are almost the same under the two conditions. This result is consistent with the calculation result that intramolecular hydrogen transfer pathway and base-assisted deprotonation pathway have close free energy barriers in the generation of 5-coordinated intermediate Q32-t (33.3 Vs 32.0 kcal/mol, Scheme S8). Thereafter, Q32-t can release 5 and react with another VV complex to generate two VIV complexes (or a bimetallic complex). The following aerobic oxidation of the VIV complexes with O2 can regenerate VV catalysts to finish the catalytic cycle.14a,17b


Page 25 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Scheme 8. Oxidation of phenolic lignin model compound with the catalyst Q1 Second, we found increasing oxygen pressure to 1 atm accelerates the Q1-catalyzed Ar−C(sp3) cleavage of 1 in the initial 4 hours. However, unknown side reactions consuming the products are also accelerated (solid precipitates are formed), and the yields nearly keep steady while the conversion of 1 keeps increasing with the reaction proceeding. On the other hand, lowering the oxygen pressure qualitatively decreases reaction rates. For example, the conversion of 1 and the yield of 2 are 52.0% and 18.0% respectively after 12 hours under the oxygen pressure of 0.02 atom. Under an air atmosphere, the corresponding conversion and yield reach 100% and 40.5% respectively after 12 hours (for more details see Table S4). These phenomena support our conclusion that O−O cleavage is the TDTS of the Q1-catalyzed Ar−C(sp3) cleavage process. (Scheme 7). 3.5 Overall mechanisms for C−C and C−O cleavage


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

Based on the above results and discussions, we proposed a VV-VIV-VIII-VIV-VV catalytic cycle for the C(sp3)−O cleavage of the β-O-4 lignin model compound catalyzed by the Schiff base vanadium complexes (Scheme 9). It starts with the ligand exchange of the VV catalyst with the benzylic hydroxyl group of the lignin model compound. Then the intramolecular hydrogen transfer occurs to generate the VIV intermediate, which quickly isomerizes to the VIII intermediate via a spin crossover. Thereafter, the C(sp3)−O cleavage occurs and generates the VIV intermediate, from which the oxidation of vanadium catalyst occurs via another spin crossover to form the VV intermediate. Finally, the hydroxyl group elimination and the ligand exchange regenerates the active VV catalyst to finish the catalytic cycle.

Scheme 9. Overall mechanistic proposals for C(sp3)−OAr and Ar−C(sp3) bond cleavage of lignin model compound by dif-ferent vanadium catalysts For the Ar−C(sp3) cleavage of the phenolic β-O-4 lignin model compounds catalyzed by bis(8-oxyquinolate) vanadium complexes, we proposed a VV-VIV-VV catalytic cycle. It starts with a spin-crossover-involved intermolecular hydrogen transfer between the benzylic hydroxyl


Page 27 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

group of lignin model compound and the oxo ligand of vanadium(V) catalysts, which generates a VIV intermediate and a phenoxyl radical. Thereafter, the VIV intermediate undergoes proton transfer (via a proton shuttle mechanism) and generates an alcohol-ligated VIV intermediate. Thereafter, the VIV intermediate releases the alcohol and combines with oxygen and the phenoxyl radical to form a superoxovanadium intermediate. From the superoxovanadium intermediate, a stepwise O−O/Ar−(sp3) cleavage occurs to break the Ar−(sp3) bond first, followed by the radical coupling to generate a gem-diolate vanadium intermediate. Finally, the intramolecular proton transfer and the ligand exchange occur to regenerate the active VV catalyst and the β-hydroxyaldehyde. Dehydration of the β-hydroxyaldehyde could also be completed by the hydroxide VV intermediate with the aid of bases as discussed in Scheme 6. According to these two mechanisms, both the different formal oxidation state of vanadium in the TDTS and the different T1-HOMOs-separation/spin dispersion effect of ligands determine the selectivity of bond cleavage. The better T1-HOMOs-separation effect of bis(8-oxyquinolate) ligands decreases the singlet-triplet energy gap, thus promoting the spin-crossover-invovled hydrogen transfer between lignin model compound and the oxo ligand of vanadium catalysts. On the other hand, the formal oxidation state of vanadium in the TDTS of the Ar−C(sp3) cleavage is +5 while that of the C(sp3)−O cleavage is lower. The bis(8-oxyquinolate) ligand has a stronger charge dispersion effect compared to the single Schiff base ligand. Thus, the former provides a more significant stabilization effect on the TDTS of the Ar−C(sp3) cleavage. Due to the two factors, the preferred bond cleavage selectivity changes from C(sp3)−O cleavage for Schiff base vanadium catalysts to Ar−C(sp3) cleavage for bis(8-oxyquinolate) vanadium catalysts. 4. Conclusion


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 37

In conclusion, a joint theoretical and experimental study was performed to investigate the mechanism of vanadium-catalyzed selective C(sp3)−O and Ar−C(sp3) cleavage of the phenolic βO-4 lignin model compound. An updated VV-VIV-VIII-VIV-VV catalytic cycle was located for the C(sp3)−O cleavage process. This mechanism corroborates the previous mechanistic proposals on the elementary steps. Nonetheless, the sequence of elementary steps and the oxidation state of the vanadium center are different from the previous proposal. What is more important, a completed catalytic cycle was elucidated for the Ar−C(sp3) cleavage process and a novel stepwise O−O/Ar−C(sp3) cleavage mechanism is located for the key Ar−C(sp3) cleavage stage. In both systems, the two-state reactivity of vanadium catalysts facilitates the C(sp3)−O cleavage, the oxidation of vanadium by phenoxyl radical, and the intermolecular hydrogen transfer of the phenolic hydroxyl group with the oxo ligand. These spin crossover phenomena highlight the unique transformation patterns of vanadium in redox reactions. Meanwhile, the different T1HOMOs-separation/charge dispersion effects of ligands and the different formal oxidation states of vanadium in TDTSs are the origin of the ligand-controlled switchable selectivity. Experimental studies further validate our theoretical results that bases are not essential for the Ar−C(sp3) cleavage but facilitate the dehydration step, and the rate-determining step of the Ar−C(sp3) cleavage process is the O−O bond cleavage. We hope that the present manuscript provides deeper insights into vanadium chemistry and inspires further development of efficient catalytic conversions for renewable carbon sources.

ASSOCIATED CONTENT


Page 29 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Supporting Information. The free energy profiles omitted in the manuscript, potential energy scan, selectivity analysis for S7 and determination of the TDTS in Q1-catalyzed C-C cleavage process, graphics of the HOMO and HOMO-1 of Q1-t and S1′′-t, calculated free energies and Cartesian coordinates of all intermediates, transition states and MECPs. Experimental procedure and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank the NSFC (21325208, 21172209, 21361140372), the 973 Program (2012CB215306), FRFCU (WK2060190025, WK2060190040, FRF-TP-14-015A2), CAS (KJCX2-EW-J02), PCSIRT and the supercomputer center of the University of Science and Technology of China. We thanks Dr. Ju-Long Jiang for good advice. REFERENCES


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 37

(1) For selected recent reviews: (a) Tuck, C. O.; Pérez, E.; Horváth, I. T.; Sheldon, R. A.; Poliakoff, M. Science 2012, 337, 695−699. (b) Morais, A. R. C.; da Costa Lopes, A. M.; BogelŁukasik, R. Chem. Rev. 2015, 115, 3−27. (c) Besson, M.; Gallezot, P.; Pinel, C. Chem. Rev. 2014, 114, 1827−1829. (d) Straathof, A. J. J. Chem. Rev. 2014, 114, 1871−1908. (e) MäkiArvela, P.; Simakova, I. L.; Salmi, T.; Murzin, D. Y. Chem. Rev. 2014, 114, 1909−1971. (f) Lanzafame, P.; Centi, G.; Perathoner, S. Chem. Soc. Rev. 2014, 43, 7562−7580. (2) (a) Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski, T. J.; Tuskan, G. A.; Wyman, C. E. Science 2014, 344, 709. (b) Xu, C.; Arancon, R. A. D.; Labidid, J.; Luque, R. Chem. Soc. Rev. 2014, 43, 7485−7500. (3) For selected Ni-catalyzed relevant reactions: (a) Sergeev, A. G.; Hartwig, J. F. Science 2011, 332, 439−443. (b) Tobisu, M.; Chatani, N. Acc. Chem. Res. 2015, 48, 1717−1726. (c) Tobisu, M.; Chatani, N. ChemCatChem 2011, 3, 1410−1411. (d) Tobisu, M.; Morioka, T.; Ohtsuki, A.; Chatani, N. Chem. Sci. 2015, 6, 3410−3414. (e) He, J.; Zhao, C.; Lercher, J. A. J. Am. Chem. Soc. 2012, 134, 20768−20775. (4) For selected Cu-catalyzed relevant reactions: (a) Sedai, B.; Baker, R. T. Adv. Synth. Catal. 2014, 356, 3563−3574. (b) Zhang, J.; Liu, Y.; Chiba, S.; Loh, T. -P. Chem. Commun. 2013, 49, 11439−11441. (c) Strassberger, Z.; Alberts, A. H.; Louwerse, M. J.; Tanase, S.; Rothenberg, G. Green Chem. 2013, 15, 768−774. (d) Nanayakkara, S.; Patti, A. F.; Saito, K. Green Chem. 2014, 16, 1897−1903. (5) For selected Co-catalyzed relevant reactions: (a) Cedeno, D.; Bozell, J. J. Tetrahedron Lett. 2012, 53, 2380−2383. (b) Elder, T.; Bozell, J. J.; Cedenob, D. Phys. Chem. Chem. Phys. 2013, 15, 7328−7337. (c) Njiojob, C. N.; Rhinehart, J. L.; Bozell, J. J.; Long, B. K. J. Org. Chem.


Page 31 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

2015, 80, 1771−1780. (d) Biannic, B.; Bozell, J. J. Org. Lett. 2013, 15, 2730−2733. (e) Biannic, B.; Bozell, J. J.; Elderb, T. Green Chem. 2014, 16, 3635−3642. (f) Zhu, C.; Ding, W.; Shen, T.; Tang, C.; Sun, C.; Xu, S.; Chen, Y.; Wu, J.; Ying, H. ChemSusChem 2015, 8, 1768−1778. (6) For selected Ru-catalyzed relevant reactions: (a) Nichols, J. M.; Bishop, L. M.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2010, 132, 12554−12555. (b) vom Stein, T.; den Hartog, T.; Buendia, J.; Stoychev, S.; Mottweiler, J.; Bolm, C.; Klankermayer, J.; Leitner, W. Angew. Chem. Int. Ed. 2015, 54, 5859−5863. (c) Weickmann, D.; Plietker, B. ChemCatChem 2013, 5, 2170−2173. (d) vom Stein, T.; Weigand, T.; Merkens, C.; Klankermayer, J.; Leitner, W. ChemCatChem 2013, 5, 439−441. (e) Wu, A.; Patrick, B. O.; Chung, E.; James, B. R. Dalton Trans. 2012, 41, 11093−11106. (g) Huang, Y. -B.; Yan, L.; Chen, M. -Y.; Guo, Q. -X.; Fu, Y. Green Chem. 2015, 17, 3010−3017. (7) For selected Ir-catalyzed relevant reactions: (a) Haibach, M. C.; Lease, N.; Goldman, A. S. Angew. Chem. Int. Ed. 2014, 53, 10160−10163. (b) Kusumoto, S.; Nozaki, K. Nature Commun. 2015, 6, 6296. (8) For selected Fe-catalyzed relevant reactions: (a) Ren, Y.; Yan, M.; Wang, J.; Zhang, Z. C.; Yao, K. Angew. Chem. Int. Ed. 2013, 52, 12674−12678. (b) Napoly, F.; Jean-Gérard, L.; GouxHenry, C.; Draye, M.; Andrioletti, B. Eur. J. Org. Chem. 2014, 781−787. (c) Fan, X.; Cui, X. M.; Guan, Y. -H.; Fu, L.-A.; Lv, H.; Guo, K.; Zhu, H. -B. Eur. J. Org. Chem. 2014, 498−501. (9) For selected Rh-catalyzed relevant reactions: (a) Yan, N.; Yuan, Y.; Dykeman, R.; Kou, Y.; Dyson, P. J. Angew. Chem. Int. Ed. 2010, 49, 5549−5553. (b) Yoon, J. S.; Choi, J. -W.; Suh, D. J.; Lee, K.; Lee, H.; Ha, J. -M. ChemCatChem 2015, 7, 2669−2674. (c) Chatterjee, M.; Chatterjee, A.; Ishizaka, T.; Kawanami, H. Catal. Sci. Technol. 2015, 5, 1532−1539.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

(10) For selected Re-catalyzed relevant reactions: (a) Harms, R. G.; Markovits, I. I. E.; Drees, M.; Herrmann, W. A.; Cokoja, M.; Kühn, F. E. ChemSusChem 2014, 7, 429−434. (b) Leiva, K.; Martinez, N.; Sepulveda, C.; García, R.; Jiménez, C. A.; Laurenti, D.; Vrinat, M.; Geantetb, C.; Fierro, J. L. G.; Ghampson, I. T.; Escalon, N. Appl. Catal., A: Gen. 2015, 490, 71−79. (11) For selected Pd-catalyzed relevant reactions: (a) Chen, Z.; Zeng, H.; Gong, H.; Wang, H.; Li, C. -J. Chem. Sci. 2015, 6, 4174−4178. (b) Galkin, M. V.; Samec, J. S. M. ChemSusChem 2014, 7, 2154−2158. (c) Jan, O.; Marchand, R.; Anjos, L. C. A.; Seuﬁtelli, G. V. S.; Nikolla, E.; Resende, F. L. P. Energy Fuels 2015, 29, 1793−1800. (12) (a) Nguyen, J. D.; Matsuura, B. S.; Stephenson, C. R. J. J. Am. Chem. Soc. 2014, 136, 1218−1221. (b) Mitchell, L. J.; Moody, C. J. J. Org. Chem. 2014, 79, 11091−11100. (13) (a) Rahimi, A.; Ulbrich, A.; Coon, J. J.; Stahl, S. S. Nature, 2014, 515, 249−252. (b) Rahimi, A.; Azarpira, A.; Kim, H.; Ralph, J.; Stahl, S. S. J. Am. Chem. Soc. 2013, 135, 6415−6148. (14) (a) Son, S.; Toste, F. D. Angew. Chem. Int. Ed. 2010, 49, 3791−3794. (b) Hanson, S. K.; Wu, R.; Silks, L. A. Angew. Chem. Int. Ed. 2012, 51, 3410−3413. (c) Hanson, S. K.; Baker, R. T.; Gordon, J. C.; Scott, B. L.; Thorn, D. L. Inorg. Chem. 2010, 49, 5611−5618. (d) Sedai, B.; Díaz-Urrutia, C.; Baker, R. T.; Wu, R.; Silks, L. A.; Hanson, S. K. ACS Catal. 2011, 1, 794−804. (e) Zhang, G.; Scott, B. L.; Wu, R.; Silks, L. A.; Hanson, S. K. Inorg. Chem. 2012, 51, 7354−7361. (f) Chan, J. M. W.; Bauer, S.; Sorek, H.; Sreekumar, S.; Wang, K.; Toste, F. D. ACS Catal. 2013, 3, 1369−1377. (g) Sedai, B.; Díaz-Urrutia, C.; Baker, R. T.; Wu, R.; Silks, L. A.; Hanson, S. K. ACS Catal. 2013, 3, 3111−3122. (h) Mottweiler, J.; Puche, M.; Räuber, C.; Schmidt, T.; Concepciûn, P.; Corma, A.; Bolm, C. ChemSusChem 2015, 8, 2106−2113.


Page 33 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(15) Wigington, B. N.; Drummond, M. L.; Cundari, T. R.; Thorn, D. L.; Hanson, S. K.; Scott, S. L. Chem. Eur. J. 2012, 18, 14981−14988. (16) Zombeck, A.; Drago, R. S.; Corden, B. B.; Gaul, J. H. J. Am. Chem. Soc. 1981, 103, 7580−7585. (17) Selected mechanistic studies on V-catalyzed reactions: (a) Waidmann, C. R.; Zhou, X.; Tsai, E. A.; Kaminsky, W.; Hrovat, D. A.; Borden, W. T.; Mayer, J. M. J. Am. Chem. Soc. 2009, 131, 4729−4743. (b) Hanson, S. K.; Baker, R. T.; Gordon, J. C.; Scott, B. L.; Sutton, A. D.; Thorn, D. L. J. Am. Chem. Soc. 2009, 131, 428−429. (c) Kirillova, M. V.; Kuznetsov, M. L.; Kozlov, Y. N.; Shul’pina, L. S.; Kitaygorodskiy, A.; Pombeiro, A. J. L.; Shul’pin, G. B. ACS Catal. 2011, 1, 1511−1520. (d) Efremenko, I.; Neumann, R. J. Am. Chem. Soc. 2012, 134, 20669−20680. (e) Cheng, M. -J.; Goddard, W. A., III. J. Am. Chem. Soc. 2013, 135, 4600−4603. (f) Dietl, N.; Wende, T.; Chen, K.; Jiang, L.; Schlangen, M.; Zhang, X.; Asmis, K. R.; Schwarz, H. J. Am. Chem. Soc. 2013, 135, 3711−3721. (g) Cheng, M. -J.; Nielsen, R. J.; Goddard, W. A., III. Chem. Commun. 2014, 50, 10994−10996. (h) Zhang, X.; Wang, Z.; Qiao, N.; Qu, S.; Hao, Z. ACS Catal. 2014, 4, 1500−1510. (i) Gupta, R.; Hou, G.; Renirie, R.; Wever, R.; Polenova, T. J. Am. Chem. Soc. 2015, 137, 5618−5628. (18) (a) Deng, L.; Li, J.; Lai, D. -M.; Fu, Y.; Guo, Q. -X. Angew. Chem. Int. Ed. 2009, 48, 6529−6532. (b) Lai, D. -M.; Deng, L.; Guo, Q. -X.; Fu, Y. Energy Environ. Sci. 2011, 4, 3552−3557. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.;


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 37

Nakai, H.; Vreven, T.; Montgomery, J. A. Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.; Gaussian 09, Revision D.01; Gaussian, Inc., Wallingford CT, 2013. (20) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378−6396. (21) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241. (22) (a) Gräfenstein, J.; Izotov, D.; Cremer, D. J. Chem. Phys. 2007, 127, 214103. (b) Johnson, E. R.; Becke, A. D.; Sherrill, C. D.; DiLabio, G. A. J. Chem. Phys. 2009, 131, 034111. (c) Wheeler, S. E.; Houk, K. N. J. Chem. Theory Comput. 2010, 6, 395−404. (23) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310. (24) Ehlers, A. W.; Böhme, M.; Dapprich, S.; Gobbi, A.; Höllwarth, A.; Jonas, V.; Köhler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111−114. (25) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161−4163. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363−368. (26) Harvey, J. N.; Aschi, M.; Schwarz, H.; Koch, W. Theor. Chem. Acc. 1998, 99, 95−99. (27) For selected examples of using electronic gaps to estimate the feasibility of spin crossover see: (a) Dhuri, S. N.; Cho, K. -B.; Lee, Y. -M.; Shin, S. Y.; Kim, J. H.; Mandal, D.; Shaik, S.; Nam, W. J. Am. Chem. Soc. 2015, 137, 8623−8632. (b) Popp, B. V.; Stahl, S. S. J. Am. Chem.


Page 35 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Soc. 2007, 129, 4410−4422. Note that we also tested performing frequency analysis on MECPs to obtain the approximate free energies of MECPs. For a relevant example see: Kalman, S. E.; Petit, A.; Gunnoe, T. B.; Ess, D. H.; Cundari, T. R.; Sabat, M. Organometallics 2013, 32, 1797−1806. We found that the free energy profiles obtained from the two methods show similar trends (Table S2). (28) (a) Li, H.; Hall, M. B. J. Am. Chem. Soc. 2015, 137, 12330−12342. (b) Plata, R. E.; Singleton, D. A. J. Am. Chem. Soc. 2015, 137, 3811−3826. (29) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO, version 3.1, 1996. (30) For the VIII complexes with triplet ground state: (a) Krzysteka, J.; Ozarowskia, A.; Telserb, J.; Crans, D. C. Coord. Chem. Rev. 2015, 301-302, 123−133. (b) Ye, S.; Neese, F.; Ozarowski, A.; Smirnov, D.; Krzystek, J.; Telser, J.; Liao, J. -H.; Hung, C. -H.; Chu, W. -C.; Tsai, Y. -F.; Wang, R. -C.; Chen, K. -Y.; Hsu, H. -F. Inorg. Chem. 2010, 49, 977−988. (c) Hessen, B.; Lemmen, T. H.; Luttikhedde, H. J. G.; Teuben, J. H.; Petersen, J. L.; Huffman, J. C.; Jagner, S.; Caulton, K. G. Organometallics 1987, 6, 2354−2362. (31) For the crystal structures of vanadium-dioxygen complexes: (a) Nica, S.; Pohlmann, A.; Plass, W. Eur. J. Inorg. Chem. 2005, 2032−2036. (b) Si, T. K.; Chakraborty, S.; Mukherjee, A. K.; Drew, M. G.B.; Bhattacharyya, R. Polyhedron 2008, 27, 2233−2242. (c) Süss-Fink, G.; Stanislas, S.; Shul’pin, G. B.; Nizova, G. V.; Stoeckli-Evans, H.; Neels, A.; Bobillier, C.; Claude, S. J. Chem. Soc., Dalton Trans. 1999, 3169−3175. (32) (a) Kozuch, S.; Shaik, S. Acc. Chem. Res. 2011, 44, 101−110. (b) Kozuch, S.; Shaik, S. J. Phys. Chem. A 2008, 112, 6032−6041. (33) For the examples of application of energetic span model: (a) Dudnik, A. S.; Weidner, V. L.; Motta, A.; Delferro, M.; Marks, T. J. Nature Chem. 2014, 6, 1100−1107. (b) Gellrich, U.;


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 37

Meißner, A.; Steﬀani, A.; Kahny, M.; Drexler, H. -J.; Heller, D.; Plattner, D. A.; Breit, B. J. Am. Chem. Soc. 2014, 136, 1097−1104. (c) Chauvier, C.; Tlili, A.; Gomes, C. D. N.; Thuéry, P.; Cantat, T. Chem. Sci. 2015, 6, 2938−2942. (34) Battino, R.; Rettich, T. R.; Tominaga, T. J. Phys. Chem. Ref. Data 1983, 12, 163−178. (35) (a) Schröder, D.; Shaik, S.; Schwarz, H. Acc. Chem. Res. 2000, 33, 139−145. (b) Shaik, S.; Danovich, D.; Fiedler, A.; Schröder, D.; Schwarz, H. Helv. Chim. Acta 1995, 78, 1393−1407. (36) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T. Coord. Chem. Rev. 2011, 255, 2622−2652.


Page 37 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

For Table of Contents Only


Mechanism of Vanadium-Catalyzed Selective CâO ... - ACS Publications

Recommend Documents

Mechanism of Vanadium-Catalyzed Selective CâO ... - ACS Publications