Mechanistic Studies of the Oxidative Dehydrogenation of Methanol

Jun 9, 2009 - David M. Pearson and Robert M. Waymouth*. Department of Chemistry, Stanford University, Stanford, California 94305-5080. Organometallics...
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Organometallics 2009, 28, 3896–3900 DOI: 10.1021/om900217s

Mechanistic Studies of the Oxidative Dehydrogenation of Methanol Using a Cationic Palladium Complex David M. Pearson and Robert M. Waymouth* Department of Chemistry, Stanford University, Stanford, California 94305-5080 Received March 20, 2009

The catalytic oxidation of methanol in the presence of the cationic [neocuproinePd(OAc)]2[OTf]2 (OTf = trifluoromethylsulfonate), 1, yields methyl formate. With benzoquinone as a terminal oxidant 4-hydroxyphenyl formate is also formed competitively. Mechanistic and labeling studies implicate formaldehyde and methyl hemiformal as key intermediates. Beta-hydrogen elimination from Pd-OCH3 or Pd-OCH2OCH3 intermediates is proposed as a key step in the generation of formaldehyde and methyl formate, respectively.

Introduction The catalytic oxidation of alcohols serves not only as a practical process to the synthetic chemist but also as a key transformation in the methanol fuel cell.1-3 The catalytic electro-oxidation of methanol on hetereogeneous electrodes in fuel cells typically proceeds through carbon monoxide (CO) intermediates; the strong affinity of hetereogeneous electrode catalysts for CO leads to poisoning and consequently high overpotentials.1,4-6 The oxidation of methanol to CO has also been observed for homogeneous group 10 complexes.7,8 As part of our efforts to develop low-temperature alcohol oxidation catalysts,9 we sought catalyst systems that could effect methanol oxidation by mechanisms that might avoid CO as an intermediate. Herein we report investigations of methanol oxidation by the cationic palladium complex [(neocuproine)Pd(OAc)]2[OTf]2 (1)9 and demonstrate that this complex oxidizes methanol to methyl formate. Mechanistic studies provide evidence for a two-step mechanism involving formaldehyde and formal intermediates. Significantly, CO is not an intermediate in this process.

*Corresponding author. E-mail: [email protected]. (1) Carrette, L.; Friedrich, K. A.; Stimming, U. ChemPhysChem 2000, 1 (4), 162–193. (2) Hogarth, M. P.; Ralph, T. R. Plat. Met. Rev. 2002, 46 (4), 146– 164. (3) Olah, G. A. Angew. Chem., Int. Ed. 2005, 44 (18), 2636–2639. (4) Beden, B.; Hahn, F.; Juanto, S.; Lamy, C.; Leger, J. M. J. Electroanal. Chem. 1987, 225 (1-2), 215–225. (5) Jusys, Z.; Kaiser, J.; Behm, R. J. Electrochim. Acta 2002, 47 (2223), 3693–3706. (6) Burstein, G. T.; Barnett, C. J.; Kucernak, A. R.; Williams, K. R. Catal. Today 1997, 38 (4), 425–437. (7) Periana, R. A.; Mironov, O.; Taube, D.; Bhalla, G.; Jones, C. J. Science 2003, 301 (5634), 814–818. (8) Portnoy, M.; Milstein, D. Organometallics 1994, 13 (2), 600–609. (9) Conley, N. R.; Labios, L. A.; Pearson, D. M.; Mccrory, C.; Chidsey, C. E. D.; Waymouth, R. M. Organometallics 2007, 26 (23), 5447–5453. (10) Starchevsky, M. K.; Hladiy, S. L.; Pazdersky, Y. A.; Vargaftik, M. N.; Moiseev, I. I. J. Mol. Catal. A: Chem. 1999, 146 (1-2), 229–236. pubs.acs.org/Organometallics

The selective oxidation of methanol to methyl formate has been previously observed for selected homogeneous10-13 and heterogeneous catalysts;14-18 limited mechanistic studies suggest that these can proceed through a hemiformal intermediate.11,19 Methyl formate is an important chemical feedstock, which can readily provide a variety of C1 to C3 compounds of significant value.20-22 Industrially, methyl formate is produced by the carbonylation of methanol in the presence of sodium methoxide,23 but is also catalyzed by metal carbonyls24 or homogeneous Pt phosphine complexes.25 Methyl formate can also be prepared by the catalytic reduction of CO2 in methanol by Pd(diphos)2 and H2.26 The dehydrogenation of methanol to methyl formate with concomitant generation of H2 has been reported with heterogeneous catalysts (typically Cu based) at high temperatures.14-18

(11) Lastovyak, Y. V.; Gladii, S. L.; Pasichnyk, P. I.; Starchevskii, M. K.; Pazderskii, Y. A.; Vargaftik, M. N.; Moiseev, I. I. Kinet. Catal. 1994, 35 (4), 512–515. (12) Lloyd, W. G. J. Org. Chem. 1967, 32 (9), 2816–2818. (13) Nishimura, T.; Kakiuchi, N.; Onoue, T.; Ohe, K.; Uemura, S. J. Chem. Soc., Perkin Trans. 1 2000, 1915-1918. (14) Cant, N. W.; Tonner, S. P.; Trimm, D. L.; Wainwright, M. S. J. Catal. 1985, 91 (2), 197–207. (15) Iwasa, N.; Yamamoto, O.; Akazawa, T.; Ohyama, S.; Takezawa, N. J. Chem. Soc., Chem. Commun. 1991, 1322–1323. (16) Liu, Z.-T.; Lu, D.-S.; Guo, Z.-Y. Appl. Catal., A 1994, 118 (2), 163–171. (17) Forzatti, P.; Tronconi, E.; Busca, G.; Tittarelli, P. Catal. Today 1987, 1 (1-2), 209–218. (18) Miyazaki, E.; Yasumori, I. Bull. Chem. Soc. Jpn. 1967, 40 (9), 2012–2017. (19) Pocker, Y.; Davis, B. C. J. Chem. Soc., Chem. Commun. 1974, 803–804. (20) Ikarashi, T. Chem. Econ. Eng. Rev. 1980, 12 (8), 31–34. (21) Lee, J. S.; Kim, J. C.; Kim, Y. G. Appl. Catal. 1990, 57 (1), 1–30. (22) Roeper, M. Erdoel Kohle, Erdgas, Petrochem. 1984, 37 (11), 506– 510. (23) Aguilo, A.; Horlenko, T. Hydrocarbon Process. 1980, 59 (11), 120. (24) Darensbourg, D. J.; Gray, R. L.; Ovalles, C.; Pala, M. J. Mol. Catal. 1985, 29 (2), 285–290. (25) Head, R. A.; Tabb, M. I. J. Mol. Catal. 1984, 26 (1), 149–158. (26) Inoue, Y.; Sasaki, Y.; Hashimoto, H. J. Chem. Soc., Chem. Commun. 1975, No. 17, 718–719.

Published on Web 06/09/2009

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Table 1. Catalytic Oxidation of 2-Heptanol and Methanola entry alcohol

oxidant (M)

catalyst time, (mol % Pd) h

conv yield selectivity (%) (%) (%)e

MeOH air 1 (5) 2 13 11b 85 1 (5) 24 21 19b 90 MeOH O2 2-hep- BQ (0.25) 1 (5) 1 94 94c tanol 4 MeOH BQ (0.25) 1 (5) 24 82 64b,d 78 5 MeOH BQ (0.25) 2 (5) 24 3 33 6 MeOH BQ (0.25) 3 (5) 24 12 4b a Reaction conditions: alcohol (0.2 M) in CD3CN at 50 °C. b Yield of methyl formate. c Yield of 2-heptanone. d 4-Hydroxyphenyl formate is formed in 6% yield. e Selectivity for methyl formate is defined as SelMF = 2[MF]/([MeOH]i - [MeOH]), where [MF] and [MeOH] are the observed concentrations of methyl formate and methanol, respectively, and [MeOH]i is the initial concentration of methanol. 1 2 3

Figure 1. Ligand oxidation of cationic palladium complex 1 during the aerobic oxidation of alcohols.

Homogeneous Pd complexes have been shown to be competent alcohol oxidation catalysts.9,12,13,27-32 We recently reported the synthesis of the cationic palladium complex [(neocuproine)Pd(OAc)]2[OTf]2 (1) and its use toward aerobic oxidation of 2-heptanol.9 This catalyst showed high initial rates for alcohol oxidation at room temperature, which we attributed to the presence of an internal acetate base and a noncoordinating, anionic triflate, providing an open coordination site. However, the aerobic oxidation of 2heptanol was accompanied by the oxidation of the proximal methyl group of the ligand; the resultant tridentate complex 2 was inactive (Figure 1).

Results and Discussion Attempts to oxidize methanol with 1 under the aerobic conditions previously used for 2-heptanol9 were unsuccessful. Very slow rates were observed at room temperature; at higher temperatures (50 °C), the catalytic oxidation of methanol in CD3CN in the presence of air or oxygen as the terminal oxidant selectively generates methyl formate, albeit in low yields (Table 1). Under these conditions (1 atm of air, 50 °C), palladium black rapidly precipitates from solution. A slight improvement was observed if 1 atm of O2 was used in place of air; in both cases low turnover numbers (TONs) were observed (entries 1 and 2). These results suggest that under these conditions, air and oxygen are not kinetically competent oxidants for the reduced palladium species. The low conversions observed under our conditions using O2 at 50 °C prompted us to investigate benzoquinone33-37 as a terminal oxidant. The oxidation of 2-heptanol at room temperature using 1 and benzoquinone exhibited slower rates than those previously observed with air, but resulted in a longer-lived catalyst. At 50 °C, catalytic oxidation of 2-heptanol with 1 in the presence of benzoquinone yielded 2-heptanone in 94% yield after 1 h (Table 1, entry 3). Catalytic oxidation of methanol with 1 in the presence of benzoquinone proceeded to 82% conversion after 24 h at 50 °C, significantly slower than that observed for 2-heptanol (27) Stahl, S. Angew. Chem., Int. Ed. 2004, 43 (26), 3400–3420. (28) Stoltz, B. M. Chem. Lett. 2004, 33 (4), 362–367. (29) Sigman, M. S.; Jensen, D. R. Acc. Chem. Res. 2006, 39, 221–229. (30) Schultz, M. J.; Sigman, M. S. Tetrahedron 2006, 62, 8227–8241. (31) Ten Brink, G.-J.; Arends, I. W. C. E.; Sheldon, R. A. Adv. Synth. Catal. 2002, 344 (3-4), 355–369. (32) Arends, I.; Tenbrink, G.; Sheldon, R. J. Mol. Catal. A: Chem. 2006, 251 (1-2), 246–254. (33) Amatore, C.; Cammoun, C.; Jutand, A. Synlett 2007, No. 14, 2173–2178. (34) Grennberg, H.; Gogoll, A.; Baeckvall, J. Organometallics 1993, 12 (5), 1790–1793. (35) Baeckvall, J.; Bystroem, S.; Nordberg, R. J. Org. Chem. 1984, 49 (24), 4619–4631. (36) Baeckvall, J.; Hopkins, R.; Grennberg, H.; Mader, M.; Awasthi, A. J. Am. Chem. Soc. 1990, 112 (13), 5160–5166. (37) Popp, B. V.; Thorman, J. L.; Stahl, S. S. J. Mol. Catal. A: Chem. 2006, 251 (1-2), 2–7.

Figure 2. Pathways for the formation of methyl formate from methanol.

oxidation (Table 1, entries 3 and 4). The lower rate of methanol oxidation relative to that of other alcohols has been observed previously in aerobic oxidations using palladium.38 The major product was methyl formate (78% selectivity); low yields of 4-hydroxyphenyl formate were also identified by NMR. In accord with our previous work on the aerobic oxidation of 2-heptanol,9 (neocuproine)Pd(OAc)2, 2, was ineffective as a catalyst under these conditions (entry 5). The dicationic complex [(neocuproine)Pd (CH3CN)2][OTf]2, 3, was slightly more effective for methanol oxidation than 2, but showed significantly lower conversions and selectivities for the formation of methyl formate than 1 (entries 4 and 6). The lower rates for 2-heptanol oxidation and lower selectivity for methyl formate using benzoquinone as a terminal oxidant suggest that this oxidant and/or the reduced hydroquinone may not be entirely innocent. To investigate the role of benzoquinone, the concentration of benzoquinone was varied from 0.1 to 0.4 M (0.5 to 2 equiv, respectively) in the oxidation of methanol with 1 (5 mol % Pd) at 50 °C. In all cases the reaction profile is characterized by an initial sharp loss in selectivity for methyl formate. The selectivity for methyl formate increases gradually with increasing conversion. This behavior becomes more pronounced as the starting concentrations of benzoquinone are increased. These observations strongly suggest the buildup of an unobservable intermediate that is trapped reversibly by benzoquinone or hydroquinone. Acid-catalyzed transesterification of crude reaction mixtures with benzyl alcohol did not result in an increase in the total amount of formate (total yield of benzyl formate = initial (38) Noronha, G.; Henry, P. M. J. Mol. Catal. A: Chem. 1997, 120 (1-3), 75–87.

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Figure 3. Isotopic differences as a consequence of reaction pathway in the oxidation of CD3OH.

Figure 4. 1H NMR of the oxidation of a 13CH3OH and CD3OH mixture using 1 (2.5 mol %) and benzoquinone after 100 min at 50 °C. The ratio of H4:HD3:DH3 is 38:38:12.

yield of methyl + 4-hydroxyphenyl formates). We suggest that the lower selectivity in the presence of excess benzoquinone may be a consequence of condensation reactions between benzoquinone and formaldehyde (vide infra, see Supporting Information). Several mechanisms for the oxidative dehydrogenation of methanol to methyl formate have been proposed (Figure 2).10-19 These mechanisms implicate formaldehyde as a key intermediate. Formaldehyde can react further, yielding formic acid (pathway A), carbon monoxide (pathway B), methyl formate (pathway C), or methyl hemiformal (pathway D). We performed several mechanistic and labeling studies with 1 in an effort to distinguish among these reaction pathways. In situ 13C NMR monitoring of the oxidation of 13CH3OH with 1 in the presence of benzoquinone provided no evidence for the buildup of 13CO or labeled formic acid (H13CO2H) under standard conditions. Furthermore, when formic acid and 13CH3OH were subjected to the reaction conditions, only doubly labeled methyl formate (H13CO13 2 CH3) was observed. These studies suggest that oxidation of methanol to formic acid followed by esterification (pathway A) is not significant under the reaction conditions employed (see Supporting Information). The oxidation of methanol under an atmosphere of CO with 2.5 mol % 1 (5 mol % Pd) at room temperature led

Figure 5. Mechanisms of the (a) Tishchenko and (b) Cannizzaro disproportionation reactions.

to the formation of a Pd carbonyl adduct after 5 min and complete suppression of methanol oxidation at 50 °C (see Supporting Information). At higher catalyst loadings (5 mol % 1, 10 mol % Pd), the formation of methyl formate was observed, but the rate was roughly half that of oxidations done in the absence of CO. Oxidation of 13CH3OH under 1 atm of CO resulted in no detectable incorporation of CO into the methyl formate, nor was 13CO observed during the reaction in the absence of CO. These results reveal that exogenous CO is not incorporated into

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Figure 6. Proposed mechanism for the oxidation of methanol to methyl formate by 1 and benzoquinone.

methyl formate and imply that carbon monoxide is not an intermediate (pathway B) in the catalytic oxidation of methanol by 1. To further probe the feasibility of pathway B, we investigated the oxidation of CD3OH under standard reaction conditions (see Supporting Information). Substitution of deuterium into the methyl position resulted in noticeable decrease in rate. After 48 h a 51% yield of methyl formate was obtained. Comparison of the deuterium resonances at the formyl and methyl positions gave a ratio of 1.1:3.0, suggesting quantitative conservation of deuterium in the formyl position. The generation of deutero-formate products disfavors a carbonylation mechanism (pathway B), as insertion of CO into the palladium-methoxide bond and subsequent protonation by acetic acid39 or other proton sources would yield protio-formates (Figure 3). Consequently, these labeling experiments are most consistent with pathways C and D (Figure 2). To address the possibility of a Tishchenko or Cannizzaro type disproprotionation mechanisms (pathway C), the oxidation of a 1:1 mixture of 13CH3OH and CD3OH was carried out and monitored by 1H and 13C NMR. After 100 min the reaction had reached 30% conversion. Three discrete methyl formate isotopomers, H*CO2*CH3, H*CO2CD3, and DCO2*CH3, were observed (Figure 4, DCO2CD3 could not be observed under these conditions) in a ratio of 38:38:12 at 30% conversion (100 min) and 37:33:14 at 67% conversion (24 h, 50 °C). The absence of any isotope exchange into the methyl signals (CH2DOY or CD2HOY, Y=C(O)H(D) or H) argues against a Tishchenko or Cannizzaro type disproportionation, as either disproportion mechanism (Figure 5) should lead to scrambling of isotopes into the methyl groups. Similarly, oxidation of a mixture of CH3OH and CD3OH and analysis by GC/MS yielded d0-, d1-, d3-, and d4-methyl formates; the absence of significant amounts of d2-methyl formates

(m/z = 62, e2%)40 provides further evidence against the disproportionation of formaldehyde as a mechanism for generation of methyl formate (pathway C, Figure 2). These results are most consistent with a mechanism in which methanol is first oxidized to formaldehyde, followed by the formation of a methyl hemiformal (pathway D), which is subsequently oxidized to methyl formate (Figure 6).41,42 Support for the intermediacy of methyl hemiformal was obtained by monitoring the oxidation of 13CH3OH at early reaction times; by 13C NMR, two doublets at 54.8 and 90.7 ppm (2JC-C = 2.5 Hz) could be observed (Figure S13), which we attribute to the methyl and formal carbons of hemiformal. The key steps in the oxidation of methanol are proposed to involve the beta-hydrogen elimination from either Pd-OCH3 or Pd-OCH2OCH3 intermediates (Figure 6). The monocationic [(neocuproine)Pd(OAc)]+ likely facilitates binding of methanol (or methyl hemiformal); deprotonation of the bound methanol by the PdOAc followed by beta-hydrogen elimination generates formaldehyde.43 Condensation of formaldehyde with methanol affords the hemiformal, which upon binding to Pd and beta-hydride elimination yields methyl formate. A similar mechanism could account for the formation of 4-hydroxyphenyl formate in the later stages of the reaction as the concentration of hydroquinone increases. Protiated methanol is oxidized more rapidly than deuterated methanol: analysis of the product distribution for the oxidation of mixtures of CH3OH(13CH3OH)/CD3OH yields an integrated isotope effect of approximately HC(O) OCX3/DC(O)OCX3 (X = H or D) = 3 for the formation of the methyl formate. According to the mechanism of Figure 6, the formation of methyl formate involves two sequential beta-hydrogen elimination steps. Comparison of the initial rates of CD3OH and CH3OH consumption yielded a kinetic isotope effect (KIE) of kH/kD =1.4,44 similar to that

(39) Acetic acid would be generated by methanolysis of the Pd acetate bond by HOCD3. (40) Products of 62 m/z remained less than 2% of the observable products over the course of the reaction. Given the reported isotopic purity of CD3OH (99.5% D) and the natural abundance of 13C in these sources, we estimate 62 m/z should account for 1% of the total observable isotopes and conclude that the level observed is within our experimental error.

(41) Schultz, M. J.; Hamilton, S. S.; Jensen, D. R.; Sigman, M. S. J. Org. Chem. 2005, 70, 3343–3352. (42) Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. J. Org. Chem. 1999, 64, 6750–6755. (43) It is not clear if formaldehyde remains bound to Pd or if it is released into solution. (44) Reactions were run separately, and the isotope effect was estimated from the initial rates of methanol consumption.

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previously reported for other Pd catalysts.31,38,45,46 We also observed an isotope effect of kOH/kOD =1.3 by comparison of the initial rates of oxidation of CH3OH and CH3OD, implicating deprotonation as a key step in the oxidation of methanol by 1.44 In summary, we report the selective oxidation of methanol to methyl formate by the cationic palladium complex [(neocuproine)Pd(OAc)]2[OTf]2 (1) in the presence of benzoquinone as a terminal oxidant. Mechanistic studies implicate the beta-hydride elimination of palladium methoxide to generate formaldehyde followed by formation of methyl hemiformal and beta-hydrogen elimination to generate (45) Mueller, J. A.; Jensen, D. R.; Sigman, M. S. J. Am. Chem. Soc. 2002, 124 (28), 8202–8203. (46) Steinhoff, B. A.; Stahl, S. S. Org. Lett. 2002, 4 (23), 4179–4181.

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methyl formate. Labeling studies indicate that carbon monoxide is not an intermediate in these oxidation reactions, illustrating that under appropriate conditions, methanol can be oxidized by mechanisms that do not involve CO as an intermediate.

Acknowledgment. We thank C. E. D. Chidsey and S. R. Lynch for helpful discussions. This work was supported by the Global Climate and Energy Project (33454) at Stanford University. D.M.P. gratefully acknowledges Stanford University for a graduate fellowship. Supporting Information Available: Text and figures providing experimental details, 1H, 13C, and 2H NMR spectra, and mass spectra. This material is available free of charge via the Internet at http://pubs.acs.org.