Article pubs.acs.org/Organometallics
Revealing the Ligand Effect on Copper(I) Disproportionation via Operando IR Spectra Ben Cheng,† Hong Yi,† Chuan He,† Chao Liu,† and Aiwen Lei*,†,‡ †
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei, People’s Republic of China National Research Center for Carbohydrate Synthesis, Jiangxi Normal University, Nanchang 330022, Jiangxi, People’s Republic of China
‡
S Supporting Information *
ABSTRACT: The ligand effect on CuI disproportionation is investigated through operando IR. It is found that the CuI complexes formed from the reaction of CuI with β-diketone enolate, β-keto ester enolate, and β-keto amide enolate could disproportionate to CuII and Cu0 species, respectively. The distinct disproportionation rates of CuI complexes chelated by different βdicarbonyl ligands provide a useful insight into the nature of copper catalytic reactions, which will be helpful for designing ligands and understanding the mechanism of Cu-catalyzed coupling reactions.
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INTRODUCTION Since the 1860s, Cu-mediated C−C and C−X (X = O or N) coupling reactions, such as the Glaser−Hay reaction,1 Ullmann reaction,2 Cadiot−Chodkiewicz reaction,3 and Castro−Stephens reaction,4 were found to have wide applications in both academic and industrial areas. In the past decades, the Ullmanntype coupling reactions5 have been well developed, while the mechanism is still unclear. In these Cu-driven coupling reactions, various oxidation states of copper including Cu0, CuI, and CuII might be involved. When associated with ligands, the processes employing CuI are usually faster than those with CuII, suggesting that the active catalyst may be a Cu(I) species rather than a Cu(II) species.6 Recently, some groups have reported that CuI species might be the active catalyst in Cucatalyzed C−X (X = O or N) coupling reactions. Hartwig and co-workers demonstrated that CuI amidate complexes7 and aryloxide complexes8 were respectively the active catalyst in Cucatalyzed C−N and C−O bond coupling reactions. In fact, the oxidation states of Cu species could change easily in such C−X coupling reactions. Jutand and co-workers have proven that aryl halides could oxidize a Cu0 precursor to the active CuI complex in Ullmann-type reactions by using electrochemical techniques.9 Then, by using NMR and UV−vis spectroscopy, they found that alcohols or amines could reduce the CuII precursor to CuI, which could undergo further β-hydride elimination, giving a Cu0 intermediate. Meanwhile, the Cu0 intermediate could also undergo comproportionation with CuII to afford CuI species.10 Although some progress has been achieved, until now most of the mechanistic investigations were focused on the nitrogen ligands.7−11 The widely used oxygen ligands such as 1,3-diketone have received less attention.11a,f,g,12 Generally, the choice of other parameters (ligand, base, solvent) is more crucial than the copper source (CuI in most cases) in many Cu-catalyzed cross-coupling reactions.5c © XXXX American Chemical Society
Especially, the ligand effect on the Cu catalyst usually play a critical role in the reaction. In 2006, Buchwald and coworkers13 reported a highly selective Ullmann-type C−N bond forming reaction at room temperature. The reaction rate was highly accelerated by utilizing β-diketone as the ligand. Since then, Cu-catalyzed C−N,14 C−O,14c,d,15 and C−S14d coupling reactions involving β-diketonate complexes have become a hot topic in organic synthesis. However, compared to the booming methodology work development, only a few reports investigated the mechanism of a Cu catalyst with a β-diketone ligand. Recently, our group16 reported the rapid disproportionation of a CuI complex to Cu0 and CuII species. By using XANES (in situ X-ray absorption near-edge structure)/EXAFS (extended X-ray absorption fine structure) and operando IR spectroscopy (IR characterization of a “working” condition17), we found a CuI complex formed immediately upon treatment of CuI and a β-diketone nucleophile, which was labile and disproportionated to nano-Cu0 and Cu(acac)2 rapidly. Further computational calculations and experimental results indicated that this CuI β-diketonate complex was the active catalyst in the Cu-catalyzed arylation reaction. In this work, we would like to communicate our direct observation of the formation of Cu complexes between CuI and a range of β-dicarbonyl compounds by using the operando IR technique. It was found that these CuI complexes could disproportionate to CuII and Cu0 species, respectively, with distinct rates.
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RESULTS AND DISCUSSION Bases Were Different. In Cu-catalyzed Ullmann-type coupling reaction, whether a ligand was involved or not, a
Received: October 19, 2014
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strong base is not indispensable, and a weak base such as K2CO3, K3PO4, and Cs2CO3 is effective. Because of the weak basicity of K3PO4, it would take about 10 min to form potassium enolate by the reaction of acetylacetone and a 1.5 stoichiometric amount of K3PO4.16 In addition, the residual K3PO4, insoluble in DMSO, may effect the observation of a Cu0 precipitate. What is more, when K3PO4 was added to the DMSO solvent of N,N-diethylacetoacetamide 1c, we could see that the peak (1638 cm−1) of 1c was no longer consumed (Figure 1a), suggesting that K3PO4 is not basic enough to
Figure 2. (a) Overall three-dimensional Fourier transform IR (3DFTIR) profile of the stoichiometric reaction between 2a and CuI. (b) Kinetic profiles of the stoichiometric reaction between 2a and CuI. (c) ConcIRT spectra of 2a, 3a, and 4a. (d) ConcIRT spectra of 4a and Cu(acac)2.
Figure 1. (a) Overall three-dimensional Fourier transform IR (3DFTIR) profile of the reaction between 1c and K3PO4. (b) Overall 3DFTIR profile of the reaction between 1c and KOtBu.
peak of 2a at 1616 cm−1 was consumed promptly, and a new peak (1594 cm−1) that belongs to 3a (tetrahedral-structured ate complex containing two acetylacetonate molecules)16 appeared immediately. 3a is unstable in our current conditions and will decay directly after its formation; a new peak belonging to 4a appeared at 1582 cm−1. The whole decay process was complete in 2 min. In the stoichiometric reaction, the ratio of enolate and CuI was 1:1, and the IR detection of CuI and CuII complexes was very obvious. As shown in eq 1 (Scheme 1), only half of the CuI reacted with enolate 2a in the beginning. However, along with the disproportionation of the CuI complexes 3a (Scheme 1, eq 2), 2a would be released constantly, and the released enolates 2a would react with the remaining CuI. Therefore, overall (Scheme 1, eq 3), a 1:1 ratio will result in half the amount of CuII and another half-amount of Cu0 together with 1 equiv of KI. Data analysis with the iC IR software revealed the IR spectra in the 1480−1660 cm−1 region of the three species, 2a (green
transfer 1c to its potassium enolate. To prepare the Cu complex with β-dicarbonyl compounds, a strong base was employed. As shown in Figure 1b, the strong base KOtBu can react with 1c to form potassium enolate (2c) in 2−3 min at room temperature. Therefore, a stoichiometric amount of KOtBu was utilized to react with β-dicarbonyl compounds to form the corresponding potassium enolate. Stepwise Stoichiometric Reaction between KOtBu, CuI, and Acetylacetone 1a. To elucidate the reaction between CuI and β-diketone enolate and the following disproportionation, we used operando IR to monitor the stoichiometric reaction of potassium enolate 2a and CuI. 2a formed in situ by reaction between a stoichiometric amount of acetylacetone (1a) and KOtBu in dry DMSO, and the process (2−3 min) was monitored by operando IR. As shown in Figure 2a, after the peak of 2a at 1616 cm−1 became steady, a stoichiometric amount of CuI was added to the mixture. The B
dx.doi.org/10.1021/om501053k | Organometallics XXXX, XXX, XXX−XXX
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2−3 min, which was monitored by operando IR. As shown in Figure 4a, after the peak of 2b at 1656 cm−1 became steady, a
Scheme 1. Equation of the Reaction of 2a and CuI
line), 3a (blue line), and 4a (red line), which are shown in Figure 2c. The peaks of 2a are at 1511 and 1616 cm−1, the peaks of 3a are at 1511 cm−1 (ν(CC) stretching modes) and 1594 cm−1 (ν(CO) stretching modes), and the peaks of 4a are at 1523 cm−1 (ν(CC) stretching modes) and 1582 cm−1 (ν(CO) stretching modes); all the peaks are the same as what we reported in the stepwise stoichiometric reaction between acetylacetone, K3PO4, and CuI.16 We also monitored an authentic sample of Cu(acac)2 by operando IR, and the ConcIRT spectrum is shown in Figure 2d (black line). By comparing the ConcIRT spectrum with 4a, we believe that 4a must be bis(acetylacetone)Cu(II). To get further insights into this mechanism, we used electron paramagnetic resonance (EPR) to study this process. As shown in Figure 3, a strong
Figure 4. (a) Overall three-dimensional Fourier transform IR (3DFTIR) profile of the stoichiometric reaction between 2b and CuI. (b) Kinetic profiles of the stoichiometric reaction between 2b and CuI. (c) ConcIRT spectra of 2b, 3b, and 4b. (d) ConcIRT spectra of 4b and the product of the reaction between 2b and CuBr2.
Figure 3. EPR spectra of the product of the reaction between 2a and CuI.
EPR signal could be observed after CuI was added to the DMSO solution of 2a. This result disclosed that CuII species could be formed in this transformation. Besides this, in the reaction of 2a and CuI we also clearly observed the formation of a precipitate, suggesting that Cu metal was formed in the reaction. So we deduce that as soon as its formation, 3a disproportionated to Cu(0) and bis(ethyl acetoacetato)Cu(II) in nearly 2 min. Stepwise Stoichiometric Reaction between KOtBu, CuI, and Ethyl Acetoacetato 1b. To elucidate the reaction between CuI and β-keto ester enolate and the following disproportionation, we used operando IR to monitor the stoichiometric reaction of potassium enolate 2b and CuI. 2b was formed in situ by the reaction between a stoichiometric amount of ethyl acetoacetato (1b) and KOtBu in dry DMSO in
stoichiometric amount of CuI was added to the mixture. The peak of 2b at 1656 cm−1 disappeared promptly, and a new peak (1631 cm−1) that belongs to 3b appeared immediately. 3b is unstable in our current conditions and will decay directly after its formation; a new peak belonging to 4b appeared at 1605 cm−1. The whole decay process was complete in 25 min. Data analysis revealed the IR spectra in the 1450−1700 cm−1 region of the three species, 2b (green line), 3b (blue line), and 4b (red line), which are shown in Figure 4c. The peaks of 2b are at 1474, 1519, and 1656 cm−1, the peaks of 3b are at 1503 and 1631 cm−1, and the peaks of 4b are at 1517 and 1605 cm−1. We also monitored the stoichiometric reaction of potassium enolate 2b and CuBr2 by operando IR. After half a C
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stoichiometric amount of CuBr2 was added to the DMSO solution of 2b, two new peaks (Figure 4d, black line), which we believed to belong to bis(ethyl acetoacetato)Cu(II), appeared and quickly accumulated to their maximum value. By comparing the ConcIRT spectrum with 4b in Figure 4d, we believe that 4b must be bis(ethyl acetoacetato)Cu(II). What is more, as shown in Figure 5, a strong EPR signal could be
Figure 5. EPR spectrum of the product of the reaction between 2b and CuI.
observed after the reaction of 2b and CuI. This result also disclosed that CuII species could be formed in this transformation. Besides this, in the reaction of 2b and CuI we also clearly observed the formation of a precipitate, suggesting that Cu metal was formed in the reaction. All these experiments indicated that 3b could disproportionate to Cu0 and bis(ethyl acetoacetato)Cu(II) in about 25 min. Stepwise Stoichiometric Reaction between KOtBu, CuI, and N,N-Diethylacetoacetamide 1c. To elucidate the reaction process between Cu(I) and β-keto amide enolate, we used operando IR to monitor the stoichiometric reaction of potassium enolate 2c and CuI. 2c was formed in situ by reaction between a stoichiometric amount of N,N-diethylacetoacetamide (1c) and KOtBu in dry DMSO. As shown in Figure 6a, after the peak of 2c at 1598 cm−1 became steady, a stoichiometric amount of CuI was added to the mixture. The peak of 2c at 1598 cm−1 was consumed promptly, and a new peak (1579 cm−1) that belongs to 3c appeared immediately. 3c is unstable in our current conditions and will decay directly after its formation; a new peak belonging to 4c appeared at 1589 and 1564 cm−1. The whole decay process was complete in 10 min. Data analysis revealed the IR peaks in the 1480−1630 cm−1 region were the three species 2c (green line), 3c (blue line), and 4c (red line), which are shown in Figure 6c. The peaks of 2c are at 1481, 1508, and 1598 cm−1, the peaks of 3c are at 1507 and 1579 cm−1, and the peaks of 4c are at 1500, 1523, 1564, and 1589 cm−1. We also monitored the stoichiometric reaction of potassium enolate 2c and CuBr2 by operando IR. After half a stoichiometric amount of CuBr2 was added to the DMSO solution of 2c, four new peaks (Figure 6d, black line), which we believe to belong to bis(N,N-diethylacetoacetamide)Cu(II), appeared and quickly accumulated to their maximum value. By comparing the ConcIRT spectrum with that of 4c in Figure 7d, we believed 4c must be bis(N,Ndiethylacetoacetamide)Cu(II). As shown in Figure 7, a strong EPR signal could be observed for the reaction of 2c and CuI. This result also disclosed that CuII species could be formed in this transformation. Besides this, in the reaction of 2c and CuI we also clearly observed the formation of a precipitate, suggesting that Cu0 metal was formed in the reaction. So we deduce that 3c disproportionated to Cu0 and bis(N,Ndiethylacetoacetamide)Cu(II) in nearly 10 min.
Figure 6. (a) Overall three-dimensional Fourier transform IR (3DFTIR) profile of the stoichiometric reaction between 2c and CuI. (b) Kinetic profiles of the stoichiometric reaction between 2c and CuI. (c) ConcIRT spectra of 2c, 3c, and 4c. (d) ConcIRT spectra of 4c and the product of the reaction between 2c and CuBr2.
Figure 7. EPR spectra of the product of the reaction between 2c and CuI.
Comparing the Dispropotion Rate of 3a, 3b, and 3c. Last, we compared the disproportionation rate of CuI species associated with three kinds of β-dicarbonyl compounds in D
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Figure 8. In the three reactions above, we set the time when CuI species accumulate to their maximum value as the beginning of
Article
ASSOCIATED CONTENT
S Supporting Information *
General procedure of the stepwise stoichiometric reaction involving CuI and CuII and the EPR experiment are available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the 973 Program (2011CB80860, 2012CB725302), the National Natural Science Foundation of China (21390400, 21025206, 21272180, and 21302148), the Research Fund for the Doctoral Program of Higher Education of China (20120141130002), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1030). The Program of Introducing Talents of Discipline to Universities of China (111 Program) is also appreciated.
Figure 8. Kinetic profiles of CuI species in the stoichiometric reaction between three potassium enolates and CuI.
the disproportionation and fit the first ConcIRT of three CuI complexes at the same point. We can see that CuI 3c, associated with N,N-diethylacetoacetamide enolate, disproportionated faster than CuI 3b, associated with ethyl acetoacetato enolate, and slower than CuI 3a, associated with acetylacetone enolate.
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CONCLUSIONS In conclusion, we have used operando IR to directly observe three complexes generated from CuI and different β-dicarbonyl compounds. The three CuI species all could disproportionate to corresponding CuII species and a Cu0 precipitate, but the disproportionation rates were different: CuI associated with an anionic acetylacetone ligand disproportionated the fastest of the three CuI species, CuI associated with an anionic N,Ndiethylacetoacetamide ligand disproportionated more slowly, and CuI associated with an anionic ethyl acetoacetato ligand disproportionated the slowest.
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REFERENCES
EXPERIMENTAL SECTION
General Comments. All of the chemicals except dimethyl sulfoxide (DMSO) were commercially available without further purification. DMSO was distilled from calcium hydride and then dried for 24 h. For the ReactIR stepwise stoichiometric reaction experiments, the reaction spectra were recorded using an IC 15 from Mettler-Toledo AutoChem. Data manipulation was carried out using iC IR software, version 4.2. EPR spectra were recorded on a Bruker A200 spectrometer. General Procedure of the Stepwise Stoichiometric Reaction between KOtBu, CuI, and β-Dicarbonyl Compounds. The IR probe was inserted through an adapter into the middle neck of a threenecked reaction vessel that contained a magnetic stirring bar, another neck was capped by a rubber plug for injections, and the last neck was connected to a nitrogen line. Following evacuation under vacuum and flushing with nitrogen three times, 4 mL of DMSO solution, diatomaceous earth (150 mg), and acetylacetone 1a (100 mg, 1.0 mmol) were added to the vessel. Then KOtBu (112 mg, 1.0 mmol) was added to the mixture. Two or more minutes later, CuI (190 mg, 1.0 mmol) was added to the mixture. The mixture was allowed to stir at 25 °C, and the whole process was monitored by operando IR though the probe. The stepwise stoichiometric reaction involving acetoacetic ester 1b (130 mg, 1.0 mmol) and N,N-diethylacetoacetamide 1c (157 mg, 1.0 mmol) followed the same procedure, except for the addition of diatomaceous earth. E
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