Molecular Mapping of the Catalytic Cycle of the Cobalt-Catalyzed

ARTICLE pubs.acs.org/Organometallics

Molecular Mapping of the Catalytic Cycle of the Cobalt-Catalyzed Hydromethoxycarbonylation of 1,3-Butadiene in the Presence of Pyridine in Methanol Laszlo T. Mika,† Robert Tuba,† Imre Toth,‡ Stephan Pitter,§ and Istvan T. Horvath*,†,|| †

Institute of Chemistry, E€otv€os University, Pazmany Peter setany 1/A, H-1117 Budapest, Hungary DSM Research, Geleen, The Netherlands § Karlsruhe Institute of Technology Energy Center, Kaiserstrasse 12, 76131 Karlsruhe, Germany Department of Biology and Chemistry, City University of Hong Kong, Kowloon, Hong Kong

)

‡

bS Supporting Information ABSTRACT:

High-pressure in situ IR and NMR investigations of the hydromethoxycarbonylation of 1,3-butadiene (1) to methyl 3-pentenoate (2) in methanol in the presence of Co2(CO)8 and pyridine under carbon monoxide has revealed that the reaction starts by the methanol- and/or pyridine-assisted disproportionation of Co2(CO)8, followed by the establishment of equilibria involving the ionic species [Co(Py)6]2+{[Co(CO)4] }2, [Co(Py)6]2+[MeO] [Co(CO)4] , [PyH]+[Co(CO)4] , and [MeOH2]+[Co(CO)4] . The addition of HCo(CO)4 (3) to pyridine or methanol results in the formation of [PyH]+[Co(CO)4] and [MeOH2]+[Co(CO)4] , respectively, and 3 is not detectable by either IR or NMR. The ionic 1,4-addition of [MeOH2]+[Co(CO)4] to 1 is the only pathway to 2-butenylcobalt tetracarbonyl, CH3CHdCHCH2Co(CO)4 (4), via the protonation of 1 followed by the reaction of the C4 carbocation with the counteranion tetracarbonylcobaltate. In the absence of carbon monoxide, 4 could lose a coordinated carbon monoxide to form (η3-C4H7)Co(CO)3 (7) in a reversible reaction. In the presence of carbon monoxide, 4 is converted to the acylcobalt tetracarbonyl species 5 via CO insertion into the Co carbon bond of 4 followed by the reaction with CO. The pyridine-assisted methanolysis of 5 leads to the formation of the product methyl 3-pentenoate (2) and pyridinium tetracarbonylcobaltate. The key intermediates of the catalytic cycle were isolated and characterized.

’ INTRODUCTION Molecular mapping of the mechanisms of catalytic reactions is crucial to understand catalyst performance and design better catalytic systems. The application of in situ spectroscopy could provide the necessary information to structurally identify key catalytic intermediates and develop the reaction network among substrates, catalyst precursor(s), intermediates, products, and side products.1 3 A better understanding of the mechanism of these reactions could be used to develop greener technologies.4,5 Carbonylation reactions have been used to generate new carbon carbon bond(s) by utilizing carbon monoxide as a readily available and cheap C1 building block.6 While a large variety of carbonylation reactions for the synthesis of aldehydes, r 2011 American Chemical Society

acids, esters, acid halides, and amides have been developed, the analogous carbonylation of 1,3-dienes has rarely been explored.7 The carbonylation of butadiene to vinyl cyclohexene derivatives in the presence of Co2(CO)8 was first reported by Reppe in the early 1940s.8 The cobalt-based catalytic system was applied by DuPont for the synthesis of organic esters using inexpensive raw materials such as isoprene, butadiene, etc.9 Tsuji successfully introduced palladium halides as catalyst precursors for the hydroalkoxycarbonylation of dienes for the synthesis of alkyl-3pentenoates in good yields.10 The hydromethoxycarbonylation Received: July 10, 2011 Published: August 08, 2011 4751

dx.doi.org/10.1021/om200617q | Organometallics 2011, 30, 4751–4764

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Scheme 1. 1,3-Butadiene (1)-Based Synthesis of Adipic Acid and ε-Caprolactam

of 1 in the presence of phosphine-modified palladium catalysts resulted in the formation of methyl 3-pentenoate, M3P (2),11 which could be a key intermediate in the production of adipic acid (Scheme 1) and ε-caprolactam.12 There have been significant industrial interests in the hydromethoxycarbonylation of butadiene (1) by DSM,11 DuPont,13 Mitsubishi,14 BASF,15 Rhone/Poluenc,16 and Shell17 using cobalt or palladium catalysts. In the early 1970s BASF began an extensive research program on producing dimethyl adipate, which could be hydrolyzed to adipic acid. The three-stage process is based on 1 using cobalt carbonyls in the presence of pyridine,18 which was even tested on a pilot-plant scale but was not commercialized.19 In the middle of the 1980s, DuPont also began a major program on a butadiene-based technology. In contrast to BASF’s process, it involved the direct dihydrocarboxylation of butadiene to adipic acid. The first step is catalyzed by Pd, Rh, or Ir, resulting in mainly 3-pentenoic acid, followed by a Rhor Ir-catalyzed transformation to adipic acid, 2-methylglutaric acid, and 2-ethylsuccinic acid.20 The advantage of this process was that these byproduct acids could be isomerized with the same catalyst.21 Dicobalt octacarbonyl in the presence of N-containing bases, such as pyridine (Py), collidine, lutidines, etc. can be used as a catalyst for the hydromethoxycarbonylation of 1.22 Matsuda studied the reaction in the presence of pyridine at 100 140 °C and 100 600 bar of CO and obtained M3P (2) in 60% yield. Schaefer published an economic route from olefin to fatty acid esters with this system.23 It has been suggested that the use of N-containing promoters, including both amide-substituted pyridines and some simple amides, can lead to higher yields in the Co-catalyzed hydromethoxycarbonylation of octene-1 to linear esters.24 The exact role of these N-bases remains the subject of active debate. The possible mechanisms of the transition-metal-catalyzed hydromethoxycarbonylation of 1 have been investigated by several groups. One of the earliest proposals25 was based on the catalytic cycle of the hydroesterification of olefins catalyzed by a pyridine-modified cobalt catalyst.26 The catalytic cycle starts by the addition of HCo(CO)4 (3) to a CC double bond, leading to the formation of an alkyl Co(CO)4 species, which can undergo CO insertion to yield an {acyl Co(CO)3} species, followed by its reaction with carbon monoxide to give acyl Co(CO)4. In the case of butadiene, Imyanitov has suggested that

Scheme 2. Proposed Mechanism by Forster and CoWorkers25

[CH3CHdCHCH2COCo(CO)4] could react with pyridine to form [CH3CHdCHCH2COPy]+[Co(CO)4] .27 The last step of the catalytic cycle is the methanolysis of this species to form the products and [PyH]+[Co(CO)4] (Scheme 2). An alternative mechanism by Milstein was based on the formation of the coordinatively unsaturated {MeOCOCo(CO)3} species, which in turn can react with 1 to give the allylic complex (η3-CH2CHCHCH2COOMe)Co(CO)3 (6) (Scheme 3.).28 The reaction of 6 with HCo(CO)4 (3) could result in M3P (2) and the coordinatively unsaturated {Co2(CO)7}. It is important to note that the presence of 6 has not been reported under catalytic conditions. Our preliminary investigations have shown that the cobaltcatalyzed hydromethoxycarbonylation of 1,3-butadiene (1) in the presence of pyridine starts by the disproportionation of Co2(CO)8 to [CoPy6][Co(CO)4]2 followed by the formation of HCo(CO)4 (3). The proposed addition of 3 to 1 leads to the formation of CH3CHdCHCH2Co(CO)4 (4), which, depending on the conditions, can undergo carbon monoxide insertion to yield CH3CHdCHCH2COCo(CO)4 (5) or reversible decarbonylation to form η3-C4H7Co(CO)3 (7). It was also shown that pyridine accelerates the conversion of 7 to M3P (2) and the methanolysis of 5, as depicted in Scheme 4, by increasing the concentration of MeO in solution. 4752

dx.doi.org/10.1021/om200617q |Organometallics 2011, 30, 4751–4764

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Scheme 3. Proposed Mechanism by Milstein and Co-Workers28

Scheme 4. Proposed Catalytic Cycle of the Hydromethoxycarbonylation of 1 by Pyridine-Modified Cobalt Catalysts2

HCo(CO)4 (3) was proposed to be a key species in these mechanisms, though no direct evidence was observed for its presence. The reactivity of 3 is highly dependent on the reaction environment, and its properties in organic solvents and water have been investigated by Reppe,29a Hieber,29b and Sternberg.29c Although its solubility in water is limited (5.6 10 2 mol L 1), the pH of the solution as well as the neutralization curve corresponded to those of a strong acid such as HNO3 and HCl (pKa < 2).29d,e 3 is fairly stable at room temperature in aqueous solution in the absence of oxygen. In contrary, the half-life of a 0.02 M solution of 3 in n-hexane is about 5 h.30a It should be noted that the decomposition of 3 was found to be autocatalytic in n-heptane.30b In the case of aprotic hydrocarbon solvents, HCo(CO)4 (3) could react as a cobalt hydride with a CC double bond according to the Heck and Breslow mechanism,31a which turned out to be also an octacarbonyldicobalt-catalyzed reaction (Scheme 5).31b Ungvary and Marko observed a radical pair containing reaction between 2,3-dimethyl-1,3-butadiene and HCo(CO)4 (3) in saturated hydrocarbon solution, resulting in the analogous compound 2-butenylcobalt tetracarbonyl (4) via exclusive 1,4addition (Scheme 6). In the formation of this product Co2(CO)8 and CO have no effect on the rate of the reaction.32 In the case of 1,3-butadiene (1), the 2,1-addition product could readily be

Scheme 5. Reaction of H-MLx with 1,3-Butadiene (1) by the Hydride Mechanism

converted to the stable 1-methyl-π-allyl Co(CO)3 complex,33 which could be formed from the 1,2-addition product via previous isomerization only. In the case of protic solvents, the disproportionation of Co2(CO)8 or the dissociation of HCo(CO)4 (3) could result in H+[Co(CO)4] , which in turn could react as an acid via an ionic 1,4-addition mechanism, well established for the addition of HBr to 1,3-butadiene (1) (Scheme 7).34 Beller et al. reported the palladium-catalyzed hydromethoxycarbonylation of 1,3-butadiene (1) to M3P (2) in 70% yield by using Pd(OAc)2, HX, and 1,4-bis(diphenylphosphino)butane, and the intermediacy of a palladium hydride species was proposed (Scheme 8).35 4753


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Scheme 6. Reaction of H-MLx with 1,3-Butadiene (1) by the Radical Mechanism

Scheme 7. Reaction of H-MLx with 1,3-Butadiene (1) by the Ionic Mechanism

Figure 1. Hydromethoxycarbonylation of 1,3-butadiene (1) at different initial cobalt concentrations (98.0 mmol of 1, 98.8 mmol of MeOH, 98.9 mmol of pyridine, 100 bar of CO, 140 °C, and 15 h).

Scheme 8. Proposed Mechanism of the Pd-Catalyzed Methoxycarbonylation of 1 Figure 2. Hydromethoxycarbonylation of 1,3-butadiene (1) at different initial carbon monoxide pressures ([Co2(CO)8] = 0.23 mol/L, 75 mmol of 1, 50 mmol of MeOH, 90 mmol of pyridine, 140 °C, 15 h).

We report here a detailed mechanistic investigation of the pyridine-modified cobalt-catalyzed hydromethoxycarbonylation of 1,3-butadiene (1) in methanol. High-pressure in situ IR and NMR spectroscopic and isotope labeling methods were used to characterize the key intermediates, establish the role of HCo(CO)4 (3), methanol, and pyridine, and develop a molecular map of the catalytic cycle.

’ RESULTS AND DISCUSSION Influence of Different Reaction Parameters. Initial studies of the hydromethoxycarbonylation of 1,3-butadiene (1) to give methyl 3-pentenoate (2) were concerned with the influence on

initial cobalt concentration, initial carbon monoxide pressure, and ratio of pyridine to methanol, respectively. The accelerating effect of different N-containing heterocycles was also investigated. The reaction of 98 mmol of 1,3-butadiene (1) in the presence of 1.6 mmol of Co2(CO)8, 98 mmol of MeOH, and 98 mmol of pyridine (Py) at 100 bar of CO and 140 °C resulted in the formation of both trans-M3P (9.3 mmol) and cis-M3P (3.14 mmol). The effect of the cobalt concentration on the formation of M3P (2) was investigated in the initial concentration range of 0.01 0.2 mmol/mL of Co2(CO)8. Figure 1. shows that the amounts of M3P (trans and cis isomers) are linear with respect to the cobalt concentrations, as expected. The ratio of trans to cis isomer seems to be independent of the cobalt concentration and the ratio of pyridine to methanol as well.40 The influence of the initial carbon monoxide pressure was studied at 0.23 mol/L Co2(CO)8 initial concentration above 60 bar. It was observed that the formation of M3P (2) is linear with respect to the initial CO pressure (Figure 2) and the initial rate of pressure decrease ( dpinit/dt) (Figure 3). Matsuda studied the influence of different picolines, pyridine, and triethylamine on the formation of M3P (2).22 The reinvestigation of the different N-containing heterocycles has revealed that the best conversion could be achieved in the presence of pyridine, and the substituents in the ortho position(s) decrease 4754


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Figure 3. Initial pressure drop during the hydromethoxycarbonylation of 1,3-butadiene (1) at different initial carbon monoxide pressures (0.23 mol/L Co2(CO)8, 75 mmol of 1, 50 mmol of MeOH, 90 mmol of pyridine, 140 °C, 15 h).

Figure 4. Hydromethoxycarbonylation of 1,3-butadiene (1; 0.07 mol) in the presence of different N-containing bases (53 mmol) and 2.3 mmol of Co2(CO)8 under 160 bar of CO at 140 °C after 15 h.

the product formation significantly, though the product formation was even lower (TON = 0.5) in the absence of base (Figure 4). In order to better understand the formation of different isomers, the products of deuteriomethoxycarbonylation were investigated (27 mmol of butadiene, 45.7 mmol of pyridine, 32 mmol of CD3OD, 0.64 mmol of Co2(CO)8, 100 bar of CO, 140 °C). Both trans- and cis-M3P and methyl 2-pentenoate (M2P) were identified in the reaction mixture, as expected. The presence of deuterium could be detected on the fifth carbon of M3P isomers (trans-M3P, CH2D group, 17.3 ppm, JC D = 19.1 Hz; cis-M3P, CH2D group, 12.3 ppm, JC D = 19.1 Hz), on the second carbon atom of M3P isomers (trans-M3P, CHD group, 37.3 ppm, JC D = 19.1 Hz; cis-M3P, CHD group, 32.3 ppm, JC D = 19.1 Hz), and on the fifth carbon atom of M2P ( CH2D group, 11.7 ppm, JC D = 19.1). It is very important to note that only 1,4-addition (linear) products could be detected. In situ NMR investigation of the izomerization of trans-M3P has also established that the formation of trans-M3P, M2P, and H D exchange on the CH2 group of M3P occur in the presence of pyridine and do not involve the cobalt catalyst in methanol. It should be noted that R,β- and R,γ-unsaturated esters and ethers can be isomerized in the presence of bases.36 Mechanistic Investigations. The initial in situ IR studies were focused on the catalytic reaction of 1,3-butadiene (1), carbon monoxide, and methanol in the presence of Co2(CO)8 in pyridine. At the start of the reaction the so-called CoCo salt or

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[CoPy6][Co(CO)4]2 was the only cobalt species in solution at 100 °C under 75 bar of CO (the first spectrum of Figure 5a) and no catalytic conversion of 1 could be detected. Only when the temperature was increased to 140 °C was the transient formation of (η3-C4H7)Co(CO)3 (7) observed, and the appearance of a band at 1739 cm 1 indicated the formation of M3P (2). In order to confirm the presence of 7, we prepared an authentic sample by the reaction of crotyl chloride and NaCo(CO)4.37 Its characteristic IR bands and NMR peaks (Table 1) are markedly different from those of (η3-CH2CHCHCH2COOMe)Co(CO)3 (6), which was proposed as a key intermediate and could be prepared according to Scheme 9. One of the most important facts indicating the presence of 7, instead of 6, is the missing band at 1751 cm 1 due to the methyl ester group in 6. Interestingly, the 1H NMR of 6 shows the presence of peaks for syn and anti isomers, which are absent in the 1H NMR of 7. While the carbonylation of 1 was slowly progressing (Figure 5a), 7 disappeared and [CoPy6][Co(CO)4]2 was again the only detectable cobalt species in solution. The concentration profiles of (η3-C4H7)Co(CO)3 (7) and [CoPy6][Co(CO)4]2 (Figure 5b) suggested that 7 could either be an intermediate or it could be formed by a reversible side reaction of the catalytic cycle. The formation of 7 by the reaction of 1 and 3 is well established in nonpolar environments.33 Calculations for the gasphase addition of the coordinatively unsaturated {HCo(CO)3} to 1 have suggested that the formation of 7 by the irreversible Markovnikov addition of {HCo(CO)3} to 1 is much more favorable than the reversible anti-Markovnikov addition.38 These studies suggest that the classic Heck Breslow mechanism involving {HCo(CO)3} first resulted in 7. In the case of 2,3dimethyl-1,3-butadiene, the formation of the corresponding allylcobalt species takes place through the corresponding alkylcobalt complexes.36 Mirbach proposed that the addition of 1,3-butadiene (1) to an equilibrium mixture of Co2(CO)8 and [Co(MeOH)6][Co(CO)4]2 at 100 °C under 75 bar of CO in methanol leads to the formation of 7,39 which we confirmed by in situ IR measurements (Figure 6). It should be noted that we have already shown that 7 can be converted to M3P (2) and to an equilibrium mixture of Co2(CO)8 and [Co(MeOH)6][Co(CO)4] quantitatively at 140 °C and 75 bar of CO.2 In order to understand the role of HCo(CO)4 (3), we have developed a reliable experimental protocol for the preparation of its solutions in various media. For example, when solid NaCo(CO)4 in n-octane was treated with gaseous dry HCl at 78 °C under nitrogen, the formation of 3 (1991, 2030, 2057, 2117 cm 1) was observed.40 Similar results were obtained when NaCo(CO)4 was treated with gaseous dry HCl in toluene at 78 °C under N2. Alternatively, a solution of 3 in toluened8 can be prepared by bubbling N2 through a flask containing a solution of 3 in n-octane at 30 °C and a connected second flask containing neat toluene-d8 at 78 °C. The characteristic band at 2028 cm 1 indicated the appearance of the hydride species.40 The reaction of HCo(CO)4 (3) with methanol was investigated by bubbling N2 through a flask containing a solution of 3 in n-octane at 30 °C and a connected second flask containing neat MeOH at 78 °C. The in situ IR of the solution in the second flask did not show the characteristic bands of 3 at all, and the only detectable carbonyl band was at 1901 cm 1 (Figure 7), indicating the formation of [MeOH2]+[Co(CO)4] . This result demonstrates that the equilibrium between HCo(CO)4 (3) 4755


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Figure 5. (a) In situ IR spectra of the reaction of 11.3 mmol of Co2(CO)8 with 28.2 mmol of 1 and 39.5 mmol of MeOH in 55 mL of pyridine under 75 bar of CO at 100 °C and then at 140 °C. (b) Absorbance profiles of the IR bands: (4) at 1885 cm 1, [CoPy6][Co(CO)4]2; (0) at 1984 cm 1 and (]) at 2055 cm 1, (η3-C4H7)Co(CO)3 (7); () at 1739 cm 1, M3P (2).

Table 1. Characterization of Allylcobalt Tricarbonyl Compounds compd (η3-C4H7)Co(CO)3 (7)

IR ν(CO) in n-pentane, cm

1

13

C NMR in d4-MeOH, ppm

2057 (s)

19.2 (CH3), 45.8 (CH2), 71.8 (CH), 84.0 (CH), 171.7 (CdO)

1988 (vs) (η3-CH2CHCHCH2COOMe)Co(CO)3 (6)

2067 (vs)

syn isomer: 36.8 (CH2), 48.0 (CH2), 51.2 (CH3), 66.6 (CH), 83.3 (CH),

1999 (vs)

anti isomer: 36.8 (CH2), 48.0 (CH2), 51.0 (CH3), 67.2 (CH), 82.2 (CH),

171.7 (CdO), 202.9 (CtO) 171.7 (CdO), 202.9 (CtO) 1751 (w)

and [MeOH2]+[Co(CO)4] in methanol is shifted to the latter: e.g., the amount of 3 is below the detection limit, not to mention the coordinatively unsaturated {HCo(CO)3}. Similarly, the in situ 1H NMR of HCo(CO)4 (3) in toluene-d8, prepared by the addition of gaseous and dry HCl to Na[Co(CO)4], shows the presence of a hydride signal at 11.55 ppm, which disappeared after the addition of pyridine (Figure 8). Monitoring the reaction by in situ IR confirmed the presence of

HCo(CO)4 (3), its disappearance after the addition of pyridine, and the appearance of a very strong and only band at 1882 cm 1, indicating the formation of [Co(CO)4] (Figure 9). Similarly, the reaction of 3 with MeOH in toluene-d8 resulted in complete conversion to [Co(CO)4] . In the well-known Heck and Breslow mechanism, the dissociation of CO from HCo(CO)4 (3) leads to the formation of {HCo(CO)3}, which then can react with the carbon carbon 4756


Organometallics double bond.31a When 3 was transferred to a solution of hexene-1 in MeOH, the formation of [Co(CO)4] and a small amount of Co2(CO)8 was detected at 30 °C (Figure 10). No significant Scheme 9. Synthesis of (η3-CH2CHCHCH2COOMe)Co(CO)3 (6)

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changes were observed by the further addition of hexene-1. When butadiene (1) was added, the formation of 4 and additional Co2(CO)8 was detected with a concomitant decrease of the intensity of the band at 1902 cm 1 for [Co(CO)4] . When the reaction mixture was warmed to room temperature, the formation of (η3-C4H7)Co(CO)3 (7) and [Co(CO)4] can be detected. These results show that HCo(CO)4 (3) in MeOH readily forms [MeOH2]+[Co(CO)4] , which cannot react with hexene-1 but will react with butadiene (1) to form the 2-butenylcobalt tetracarbonyl intermediate (4) at low temperatures. The stability of 4 is limited, and it can be converted to 7 at higher temperatures. On the basis of these observations the Markovnikov addition involving HCo(CO)4 (3) and/or {HCo(CO)3} to 1 in methanol seems to be not operational. The formation of the alkenylcobalt tetracarbonyl intermediate (4) and its conversion to (η3-C4H7)Co(CO)3 (7) was confirmed in the absence of hexene-1 (Figures 11 and 12).

Figure 6. In situ IR spectra of the reaction of 26 mmol of 1 and 11.4 mmol of Co2(CO)8 in 1246 mmol of MeOH at 70 bar of CO and 100 °C.

Figure 7. In situ IR spectra observed during the transfer of 11.1 mmol of gaseous HCo(CO)4 into neat MeOH at 4757

78 °C by N2.


Organometallics The reaction of 3 with 1 in methanol under 10 bar of carbon monoxide at low temperature resulted in the corresponding acyl

Figure 8. In situ NMR (a) of HCo(CO)4 (3) in toluene-d8 and (b) after the addition of pyridine at room temperature under N2.

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species 5 (Figure 13). When the solution was warmed, 5 was converted to 7, indicating its reversible formation from 4, as expected. The reaction of 3 with 1 in benzene-d6, as an apolar media, leads to the formation of 4, not 7, which suggests the prevailing path is either the 1,4-addition of [MeOH2]+[Co(CO)4] to 1 (Scheme 10) or the radical reaction of 3 and 1 to the corresponding alkenyl species 4. The reaction of [MeOH2]+[Co(CO)4] with 1 could lead to the formation of branched alkylcobalt tetracarbonyl species, which could easily insert carbon monoxide to form terminally unsaturated product by 1,2-addition. Only the formation of the linear ester was detected in both catalytic and mechanistic experiments, and the branched methyl 2-methylbut-3-enoate could not be observed. In order to rule out the role of a radical pathway (Scheme 6), we have also investigated the hydromethoxycarbonylation of 1 in the presence of an equimolar amount of p-hydroquinone, a well-known radical trap, with respect to the cobalt catalyst. In situ IR showed the transient formation of (η3-C4H7)Co(CO)3 (7) and the formation of M3P in both the absence (Figure 6) and presence of p-hydroquinone.40 These results suggest that the radical mechanism can be excluded.

Figure 9. In situ IR spectrum of HCo(CO)4 (3) in toluene-d8 and spectra after addition of pyridine at room temperature under N2.

Figure 10. In situ IR spectra of the addition of 3 (6 mmol) to a solution of hexene-1 (a, 8 mmol) in MeOH (6 mL) followed by the addition of hexene-1 (b, 4 mmol) and then 1 (72 mmol) at 10 °C under N2 (# denotes Co2(CO)8). 4758


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Figure 11. IR spectra of the reaction of 14.2 mmol of 3 with 105.5 mmol of 1 in MeOH (30 mL) under N2. The first spectrum shows 3 in MeOH at 78 °C (* denotes Co2(CO)8 at 2040 and 2074 cm 1).

Figure 12. IR spectra of the formation of 7 from 4. The reaction started at 0 °C and finished at 25 °C (* denotes Co2(CO)8 at 2040 and 2074 cm 1).

Figure 13. In situ IR spectra of the reaction of 3 (20.2 mmol) with 1 (86 mmol) in MeOH (55 mL) under 10 bar of CO: formation of 5 at 68 °C and 7 above 0 °C. The first spectrum shows HCo(CO)4 in MeOH at 68 °C (* denotes Co2(CO)8 2040 and 2074 cm 1). 4759


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Scheme 10. Possible Pathways in the Reaction of DCo(CO)4 and 1,3-Butadiene (1)

Figure 14. 1H NMR spectrum of the reaction mixture of 1 and Co2(CO)8 in MeOH-d4 at 70 bar of CO and 100 °C after 12 h.

In order to obtain additional insights into the mechanism of the reaction, we have investigated the reaction of 1 and Co2(CO)8 in MeOH-d4. Surprisingly, the reaction leads to the formation of the monodeuterated (η3-C4H6D)Co(CO)3 exclusively. The peak for the methyl group of the allyl ligand at 21 ppm (t, JC D = 20.2 Hz) shows the presence of one deuterium atom (Figures 14 and 15). These results indicate that only one deuterium atom was incorporated directly from CD3OD, which was both the solvent and the reactant in the reaction. Furthermore, no isomerization occurred of the coordinated allyl ligand during and after its formation, suggesting a very selective and specific path (Scheme 11). It should be noted that [PyH]+[Co(CO)4] does not react with butadiene (1).

Next, we tried to prepare and characterize the alkyl and acyl intermediates and investigated their transformations. One of the possible ways to prepare these compounds is based on the reaction of NaCo(CO)4 and crotyl bromide.2 The formation 7 from 4 and 5 above 0 °C indicated that both alkyl and acyl species, depending on the reaction conditions, can undergo reversible decarbonylation (Figures 12 and 13). Similar intermediates were observed for 2,3-dimethyl-1,3butadiene under the same conditions.40 When carbon monoxide was bubbled into the solution of the corresponding alkyl compounds (2092 cm 1), CO insertion proceeded at low temperatures (acyl species: 2001, 2022, 2043, 2105 cm 1) and the allyl type complex (1982, 2051 cm 1) was formed above 0 °C. 4760


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Figure 15.

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13

C NMR spectrum of (η3-C4H6D)Co(CO)3 in MeOH-d4 (# denotes 1, and * denotes 7).

Scheme 11. Formation of (η3-C4H6D)Co(CO)3 by the Ionic 1,4-Addition of [MeOD2]+[Co(CO)4] to 1

formation of the product methyl 3-pentenoate (2) and pyridinium tetracarbonylcobaltate.

’ EXPERIMENTAL SECTION

The final step of the reaction is the pyridine-assisted methanolysis of 5, which was already reported in detail in our preliminary communication.2 In summary, the pyridine-modified cobalt-catalyzed hydromethoxycarbonylation of 1,3-butadiene (1) to methyl 3-pentenoate (2) in the presence of pyridine in methanol under carbon monoxide pressure involves the activation of methanol by [CoPy6][Co(CO)4]2 and the formation of [MeOH2]+[Co(CO)4] (Scheme 12). The addition of gaseous HCo(CO)4 (3) to pyridine or methanol results in the formation of [PyH]+[Co(CO)4] and [MeOH2]+[Co(CO)4] , respectively, and 3 is not detectable at all by either IR or NMR. Surprisingly, the 1,4-addition of [MeOH2]+[Co(CO)4] to 1 is the exclusive pathway to 2-butenylcobalt tetracarbonyl (4) via the protonation of 1 followed by the reaction of the C4 carbocation with the counteranion tetracarbonylcobaltate. In the absence of carbon monoxide, 4 loses a coordinated carbon monoxide to form (η3-C4H7)Co(CO)3 (7) in a reversible reaction. In the presence of carbon monoxide, 4 is converted to the acylcobalt tetracarbonyl species 5 via CO insertion into the CC bond of the 2-butenyl ligand of 4 followed by reaction with CO. The pyridine-assisted methanolysis of 5 leads to the

Materials. The solvents were purchased from Aldrich Chemical Co., dried in the usual way, and stored under nitrogen. Co2(CO)8 was obtained from Strem Chemical Co. and purified via recrystallization from previously dried and deoxygenated n-hexane. 1,3-Butadiene was obtained from Fluka and transferred to a 30 mL stainless steel bomb before use. The deuterated solvent was purchased from Eurisotop Inc. The organometallic compounds were prepared under an inert atmosphere using standard Schlenk techniques. General Techniques. All high-pressure NMR experiments were performed in a 10 mm sapphire tube, equipped with a titanium head,41 using a Bruker Avance 250 MHz spectrometer. Infrared spectra were recorded with a ReactIR 1000 Reaction Analysis System attached to an atmospheric or high-pressure silicon (SiComp) probe head. In situ highpressure IR experiments were carried out in a custom-manufactured 300 mL stainless steel reactor equipped with a high-pressure IR probe head and high-pressure gas system, as shown in Figure 16. The catalytic carbonylations of 1,3-butadiene (1) were performed in an automated (Honeywell HC-900) Parr-4500 PASCAR (parallel screening of catalytic reactions) system containing six 100 mL highpressure stainless steel reactors (pmax 350 bar, Tmax 350 °C). The reactors were operated in parallel, each one equipped with inlet and exhaust valves, a safety rupture disk, and a pressure transducer in addition to an internal K-type thermocouple. In addition to a Parr PID-A462EE temperature controller, electrically heated jackets and internal cooling fingers served to control the desired temperature with an accuracy of (0.5 °C. All six vessels were stirred with a magnetic stirrer system. The temperature, pressure, and stirring speed were recorded continuously on a PC and served for the online monitoring and evaluation of the reaction progress. The catalyst solution including methanol and pyridine was prepared under an argon atmosphere using standard Schlenk techniques and transferred into the previously argon flushed reactor with a syringe through a rubber septum equipped ball valve. 1,3-Butadiene (1) was loaded into a high-pressure cylinder and transferred into the reactor with 10 bar overpressure of carbon 4761


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Scheme 12. Catalytic Cycle of the Hydromethoxycarbonylation of Butadiene (1) by Pyridine-Modified Cobalt Catalysts in Methanol

Figure 16. Schematic drawing of the in situ IR measurement apparatus. monoxide. The reaction was stopped after 14 h, and after the reactor was cooled to room temperature, the gases were exhausted. For the analysis 0.2 mL of the reaction mixture was dissolved in 1 mL of methylene chloride followed by the addition of 25 μL of cyclohexane as internal standard. GC analyses were performed on an Agilent 6890N instrument using a HP5 capillary column with cyclohexane as the internal standard. The catalytic deuteromethoxycarbonylation was performed in a 25 mL high-pressure Hasteloy-C Parr reactor. A 218.7 mg portion

(0.64 mmol) of Co2(CO)8 was dissolved in a mixture of 45.7 mmol of pyridine and 32 mmol of CD3OD in a Schlenk flask under nitrogen. The catalyst solution was transferred into the previously nitrogen flushed reactor via a rubber septum. 1,3-Butadiene (1) was loaded into a highpressure cylinder and transferred into the reactor with 50 bar overpressure of carbon monoxide followed by pressurization up to 100 bar of CO. The reaction mixture was heated to 140 °C and magnetically stirred for 14 h. The reaction mixture was cooled to room temperature and transferred to a Schlenk flask. All volatile compounds were vacuum-transferred to another Schlenk flask, which was cooled to 198 °C with liquid nitrogen. The products were analyzed by NMR spectroscopy and GC-MS. Preparation of NaCo(CO)4. A 29.7 g portion (0.74 mol) of pulverized NaOH was added to a solution of 11.29 g (33.03 mmol) of dicobalt octacarbonyl in 160 mL of tetrahydrofuran at room temperature under nitrogen. The reaction took place within 1/2 h, and a purple precipitate was formed. The reaction mixture was filtered under N2 at 78 °C. During the evaporation of the solution under vacuum, the temperature was slowly increased to room temperature and the residual suspension was dried at 85 90 °C under vacuum. A white powder of sodium cobalt tetracarbonyl was formed; yield 34.3 mmol, 77.9%. HCo(CO)4 was obtained by the reaction of sodium cobalt tetracarbonyl and dry HCl. A 0.26 mol portion of dry hydrochloric acid was slowly bubbled through a mixture of 20.2 mmol of NaCo(CO)4 in 35 mL of n-octane at 78 °C. The reaction was monitored by in situ IR over about 2 h. Characteristic IR bands (ν(CO) n-octane, cm 1): 1993 (w), 2028 (vs), 2063 (vs), 2117 (w).

Preparation of HCo(CO)4 Solution in Toluene-d8 and Methanol. A solution of 3 in the corresponding solvent can be prepared by bubbling N2 through a flask containing a solution of 3 in n-octane at 30 °C and a connected second flask containing neat toluened8 or MeOH at 78 °C. The transfer was monitored by in situ IR. 4762


Organometallics Preparation of (η3-C4H7)Co(CO)3. A 21.6 mmol portion of crotyl chloride was added to a solution of 23.9 mmol of NaCo(CO)4 in 22 mL of tetrahydrofuran. During the reaction a white precipitate was formed, which was filtered under N2 at 0 °C. This solution was concentrated under vacuum at the same temperature. The product was characterized by IR and NMR spectroscopy (Table 1). Preparation of (η3-C4H6D)Co(CO)3. The synthesis was carried out under a N2 atmosphere as follows: 0.16 g (2.96 mmol) of 1,3butadiene was introduced into the high-pressure NMR tube at 80 °C. In the meantime 0.289 g (0.84 mmol) of Co2(CO)8 was dissolved in 1.5 mL of CD3OD in a Schlenk flask at 0 °C and then added to 1,3-butadiene at 78 °C and pressurized with 50 bar of CO. Over the next 6 h the NMR tube was heated at 100 °C while the color of the solution changed from brown to orange. NMR measurements of this solution were performed at room temperature. In addition to the signals of 1,3-butadiene, η3-C4H6DCo(CO)3 could be detected. 13C NMR (δ, CD3OD, ppm): 19.2 (CH3), 45.8 (CH2), 71.8 (CH), 84.0 (CH). Preparation of (η3-CH2CHCHCH2COOCH3)Co(CO)3. The synthesis was carried out according to Scheme 9.28a To a solution of 254 mg of NaCo(CO)4 (1.31 mmol) in 30 mL of diethyl ether under N2 at 40 °C was added 0.13 mL (1.41 mmol) of methyl chlorooxoacetate. After this addition a yellow solution with a light green precipitate was formed. After 1/2 h the temperature was increased to 25 °C in order to complete the decarbonylation. The reaction mixture was stored at room temperature for 3 h, and next 28 g (0.52 mol) of gaseous 1,3-butadiene was slowly bubbled through the mixture for 4 h. During this time a light orange solution was formed, which was filtered under nitrogen, and diethyl ether was evaporated under vacuum. The residual orange oil was washed with 10 mL of dried n-pentane and filtered. After filtration the solvent was removed and a clear orange oil was obtained as the product. The product was characterized by IR and NMR spectroscopy (see Table 1). The 13C NMR is shown in the Supporting Information.

’ ASSOCIATED CONTENT

bS Supporting Information. Figures giving a plot of the effect of the pyridine/methanol ratio on the formation of M3P and additional IR and NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author

*Address correspondence to the City University of Hong Kong. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was partially supported by DSM Research, Geleen, The Netherlands and the Hungarian National Scientific Research Fund (Grant No. OTKA-T032850). The donation of the ReactIR 1000 instrument by Applied Systems Inc, a MettlerToledo Company, is greatly appreciated. The authors thank the German Ministry of Research and Education (BMBF) and Karlsruhe Institue of Technology (former Forshungszentrum Karlsruhe) for financial support of the PASCAR system. The European Union and the European Social Fund have provided financial support to the project under the grant agreement no. MOP 4.2.1./B-09/KMR-2010-0003. We thank Prof. Ferenc TA Ungvary (University of Pannonia) for helpful discussions. ’ REFERENCES (1) (a) Csihony, S.; Mika, L. T.; Vlad, G.; Barta, K.; Mehnert, C. P.; Horvath, I. T. Collect. Czech. Chem. Commun. 2007, 72, 1094.

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Molecular Mapping of the Catalytic Cycle of the Cobalt-Catalyzed

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