Research Article pubs.acs.org/acscatalysis
Abnormal Mesoionic Carbene Silver Complex: Synthesis, Reactivity, and Mechanistic Insight on Oxidative Fluorination Qilun Liu,†,¶ Zheliang Yuan,†,¶ Hao-yang Wang,*,† Yang Li,‡ Yichen Wu,† Tao Xu,† Xuebing Leng,† Pinhong Chen,† Yin-long Guo,† Zhenyang Lin,*,‡ and Guosheng Liu*,† †
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China ‡ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China S Supporting Information *
ABSTRACT: A silver-catalyzed intramolecular amination of alkynl-imine substrates has been extensively studied to build various isoquinoline derivatives efficiently. However, most of these transformations are limited to hydroamination, and the related oxidative reaction is quite rare. Importantly, the mechanistic details are still unknown, which retarded further progress in the field. In this work, a novel abnormal mesoionic carbene silver complex (MIC)nAg(I) was isolated and fully characterized as the key intermediate. Further investigation on the oxidative transformation of the silver complex reveals that successful oxidative halogenation could be achieved with NXS (X = Cl, Br, and I), as well as F+ reagent. Surprisingly, the fluorination reaction occurred in the presence of both strong (SelectFluor) and weak (NFSI) fluorinating reagents, although the F-Py-type reagent, whose oxidative potential lies between, is ineffective. Further mechanistic studies disclosed that (1) from kinetic data, the (MIC)Ag(I) complex was proved to be the reactive intermediate in the fluorination reaction, and pyridyl-oxazoline (Pyox) ligand could significantly improve this transformation; (2) from DFT calculation results, two different mechanistic pathways were suggested to be involved, a metathesis process in the case of NFSI promoted by the chelation of sulfonyl group toward the silver center and a redox process in the case of SelectFluor due to its strong oxidative potential. KEYWORDS: alkynl-imine, abnormal mesoionic carbene, silver complex, oxidative, fluorination
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lines and its derivatives (Scheme 1, top),2 and a few example of oxidative transformation were explored recently.3 For the abovementioned catalytic processes, a common heteroaryl-Ag complex was proposed as the key intermediate; however, up to now, such an intermediate has never been isolated and characterized, which rendered the poor understanding of its reactivity with different oxidants and overall redox catalytic system (Scheme 1, bottom). During the last decades, a range of precious metal (e.g., Pd, Rh, etc.)-catalyzed oxidative processes have been reported, and the related mechanisms have been extensively studied.4 The emphasis on better mechanistic understanding is essential for
INTRODUCTION Silver-catalyzed reactions have emerged as important synthetic methods for a variety of organic transformations.1 Due to their Lewis acidity, silver complexes have been demonstrated as efficient catalysts to activate the carbon−carbon triple bond toward the nucleophilic attack. Over the years, a large number of silver-catalyzed nucleophilic cyclization of alkynes have been developed to construct heterocycles.2 Among these reactions, it is generally believed that the (hetero)aryl- or vinyl-Ag complex is involved as a key intermediate, and these sp2 C−Ag bonds are prone to undergo protonolysis. Thus, the related transformations mainly focused on broadening the nucleophile scope to achieve hydrooxygenation, hydroamination, and other reactions as reported in the last several decades. For instance, Ag(I)-catalyzed amination of the alkynl-imine substrate 1 has been extensively studied for the efficient synthesis of isoquino© XXXX American Chemical Society
Received: August 25, 2015 Revised: September 30, 2015
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mixture contaminated by AgNO3, possibly due to the fast precipitation. Meanwhile, it was found that addition of Pyox could slow down the cyclization reaction slightly (see Supporting Information). We assumed that this observation might be helpful to get pure silver complexes. Delightfully, two pure AgI complexes 4a and 5a were obtained from the reaction mixture of 1a and AgNO3 in the presence of 20 mol % and 100 mol % of Pyox, respectively. Complexes 4a and 5a were stable at or below 0 °C in solid state or in polar solvents (DMF or DMA), but gradually decomposed to give protonolysis product at room temperature. Their structures were confirmed as abnormal mesoionic carbene silver complexes by NMR and elemental analysis (Scheme 2).8
Scheme 1. Ag(I)-Catalyzed Amination of Alkynes
further exploration of new oxidative reactions. In this vein, our group has recently described a new reaction mode for silvercatalyzed oxidative fluorination with very mild electrophilic fluorinating reagent, N-fluoro-bisbenzene-sulfonamide (NFSI).5 In order to make this oxidative transformation more synthetically useful, we sought to isolate the proposed organosilver complex and gain more insights on its reactivity toward different oxidants, as well as the mechanistic pathway. Herein, we report our latest discovery of the silver-catalyzed oxidative amination of alkynes with alkynl-imine substrates. An abnormal mesoionic carbene (MIC) silver complex was successfully isolated and characterized. Further investigation on the oxidative reactivities of silver complexes toward different oxidants has been conducted. Finally, due to our interests on the C−F bond formation from the metal center,6 the detailed mechanism on the fluorination of the MIC−silver complex with different electrophilic fluorinating reagents has been studied. The results of these studies suggested that distinctive mechanistic pathways may apply: a σ-metathesis process is involved in the case of NFSI and a redox pathway involving Ag(III) in the case of SelectFluor.
Scheme 2. Synthesis of (MIC)Ag Complexes 4a and 5a
X-ray diffraction study confirmed that 5a is a bismesoionic carbene silver complex [(F-MIC)2AgNO3] (Figure 1). In 5a,
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RESULTS AND DISCUSSION In our previous report on the silver-catalyzed aminofluorination of alkynes for the straightforward synthesis of fluorinated heteroarenes, we found that pyridyl-oxazoline (Pyox) ligand played an important role for the highly selective fluorination.5b,c In addition, the choice of F+ source is also crucial: NFSI and SelectFluor exhibited efficient reactivities to provide 2, but F-Py type reagent was ineffective (eq 1). This trend of reactivity Figure 1. Structures of 5a (left) and 5b (right) in the solid state. Ellipsoids are set at 50% probability. Hydrogen atoms and NO3− are omitted for clarity. Selected bond lengths [Å] and angles [°]: Ag1−C1 2.108(5), Ag1−C17 2.107(4), F1−C5 1.356(6), F2−C21 1.349(5), N1−C8 1.325(7), N1−C9 1.386(6), N1−C14 1.510(6), C1−C2 1.422(7), C1−C9 1.400(6), C9−C10 1.554(7); C1−Ag1−C17 175.47(18), C9−C1−C2 116.9(4), C9−C1−Ag1 122.0(4), C2− C1−Ag1 120.8(3). For 5b, see Supporting Information.
the Ag1−C1 bond (2.108(5)Å) is considerably shorter than that of a previously reported mesityl-Ag complex (2.22(3)Å)9 and slightly longer than that of the (IMes)2AgOTf complex (2.067(4)Å).10 The 13C NMR of C1 (177.3) is closer to that of (IMes)2AgOTf (183.6 ppm).10 The orientation of the two 6membered MIC rings in 5a are slightly twisted relative to one another. Analogous complex 5b was also isolated and characterized (see Figure 1 and Supporting Information). Very interestingly, mixing 5a and 5b resulted in immediate formation of the heteroleptic complex 5c with an intensive signal of [5c-NO3]+
observed is inconsistent with the oxidation potential of the F+ reagents.7 These interesting observations prompted us to gain more insight into this chemistry. Synthesis and Characterization of (MIC)nAg(I) Complexes. In order to address the C−F bond formation in eq 1, we initially attempted to isolate the heteroaryl-Ag intermediate. When substrate 1a was treated with one equivalent of AgNO3 in acetonitrile, a white solid was immediately precipitated from solvent. However, elemental analysis indicated that it was a 6733
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Table 1. Reaction of (MIC)nAg(I) Complexesa
at m/z 579,11 suggesting a fast ligand redistribution between 5a and 5b (Figure 2A,B, and eq 2).12 However, no reaction
Figure 2. ESI-MS studies of MIC silver complexes: (A) 5a in DMA; (B) 5a and 5b in DMA; (C) 4a in DMA; (D) addition of extraneous AgNO3 (1 equiv) to the solution of 5a in DMA.
occurred between 5a and Pyox. We speculated that the bond strength between the abnormal mesoionic carbene (MIC) and Ag(I) is much stronger than the bond between Pyox and Ag(I).
a
Reaction condition: the reaction was conducted in 0.0125 mmol scale, 19F NMR yield with CF3-DMA (N,N-dimethyltrifluoroacetamide) as internal standard. bThe yield in parentheses was obtained in the absence of ligand Pyox. TMPy = 2,4,6-trimethylpyridine.
4a, addition of Pyox significantly accelerated the reaction rate and suppressed the protonolysis product (see Figure 3A and For the complex (MIC)AgI 4a, attempts to grow single crystals were unsuccessful. Beside elemental analysis and NMR, the identity of its structure was further studied by electrospray mass spectrometry: the observation of signal [5a-NO3]+ at m/z 597 indicated that the (F-MIC)2AgI complex 5a was formed in the gas phase (Figure 2C and eq 3). This behavior is consistent with that of an N-heterocylic carbene silver complex reported earlier.13 Furthermore, treatment of 5a with extraneous AgNO3 could immediately give 4a (Figure 2D). To the best of our knowledge, this is the only report on well-characterized mesoionic carbene silver complexes. Reactivity of (MIC)nAg(I) Complexes. With the silver complexes in hand, further studies were conducted to evaluate their reacitivities toward various oxidants in the presence of ligand Pyox. As shown in Table 1, when complexes 4a and 5a were initially treated with different electrophilic fluorinating reagents, the reactions of SelectFluor and NFSI afforded the corresponding fluorination product 2a in excellent yields (entries 1−2). In contrast, the reaction of F-Py and F-TMPy (N-fluoro-2,4,6-trimethylpyridine) failed to obtain 2a. Instead, the silver complexes underwent protonolysis process to give isoquinonium as the sole product (entries 3−4).14 Furthermore, treatment of silver complexes 4a and 5a with NXS (X = Cl, Br, I) also generated the related oxidation products 3a−3c in good yields (entries 5−7).15 Mechanism Studies on the Fluorination of (MIC)nAg(I) Complexes. The results discussed above indicated that electrophilic fluorinating reagents having high (SelectFluor) and low (NFSI) oxidative potential are both beneficial for the fluorination of the silver complexes, but F-Py reagents whose oxidative potential lies between are ineffective. In order to address these observations, further mechanistic insights were surveyed. With NFSI as Oxidant. The initial investigation was focused on the fluorination of the silver complexes 4a and 5a using NFSI as the fluorinating reagent. For the fluorination of
Figure 3. Effect of Pyox and AgNO3 on fluorinations of 4a and 5a: left (A), [4a] = 0.025 M, [NFSI] = 0.038 M, [Pyox] = 0.025 M; right (B), [5a] = 0.013 M, [NFSI] = 0.038 M, [Pyox] = 0.025 M, [AgNO3] = 0.0025 M.
Supporting Information). A similar ligand effect was also observed in the case of 5a but with an obvious induction period (blue and dark lines in Figure 3B). Very interestingly, when extraneous AgNO3 (20 mol %) was added, the induction period disappeared (red and green lines in Figure 3B). We reasoned that the induction period might be the result of the slow fluorination of the silver complex 5a.16 In the presence of AgNO3, 5a was instantly transformed into the more reactive species 4a (Figure 2D), and its fast fluorination resulted in the disappearance of induction period. On the basis of this analysis, we attributed the drastically enhanced reaction rate and disappearance of induction period (red line in Figure 3B) to the formation of Pyox·4a, which might act as the truly active species for the fluorination. Furthermore, kinetic studies revealed that the reaction rate exhibited the first-order dependence on the concentrations of Pyox (Figure 4A) and NFSI (Figure 4B). These results implied that both Pyox·4a and 6734
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Figure 4. Kinetic data for the fluorination of 5a with NFSI in the presence of AgNO3 (20 mol %): left (A), dependence on the concentration of Pyox; right (B), dependence on the concentration of NFSI.
Recently, Ritter and co-workers proposed a multinuclear silver complex to address fluorination of aryl-metal reagent, in which a high-valent bimetallic (MIC)nAgII·(AgII) complex was involved as the key intermediate, and its reductive elimination could lead to the C−F bond formation.17 In this scenario, multinuclear silver complexes (F-MIC)nAgI·(LAgI) (L = Pyox) might also be generated in current reactions and act as active species to form C−F bond. If so, (1) the interaction of complex 5a and extraneous AgNO3 should occur to give multinuclear silver complexes; (2) the related fluorination reaction should be accelerated by increasing the concentration of AgNO3. Indeed, the 1H NMR signals of 5a shifted significantly with increasing the amount of AgNO3 (see Figure 5A), and the similar phenomena was observed in the case of complex 4a (see Figure 5C). For the fluorination reaction, addition of 20−30 mol % AgNO3 did accelerate the fluorination of complex 5a and eliminated the induction period.18 However, the reaction rate was significantly decreased when the amount of AgNO3 was further increased (above 30 mol %, Figure 5B). Furthermore, only a reduced-rate effect on addition of extraneous AgNO3 was observed in the case of 4a (Figure 5D). These observations suggested that a multinuclear silver complex is unlikely to be the active species in the fluorination step, and therefore, the reaction should involve a different mechanism from Ritter’s bimetallic pathway. We attribute the accelerated-up-rate of 5a by small amount of AgNO3 to the generation of more reactive complex L·4a (L = Pyox or DMA) and the reduced rate by excess AgNO3 to the formation of less reactive multisilver complex with unclear structure (see Scheme 3). For the fluorination of complex 4a, as shown in Scheme 3, there are two scenarios to address the C−F bond formation: (1) complex L·4a (L = Pyox or DMA) can be oxidized by NFSI to form a high-valent AgIII complex, int·I, followed by reductive elimination to form C−F bond (path a); (2) complex L·4a might react directly with NFSI to provide C−F bond via a metathesis process (path b). In order to clarify the fluorination mechanism, DFT calculations were conducted.19 The MIC Ag(I) carbene complex A, a model complex for DMA·4a in our calculations, is considered as the active species. The fluorination of complex A with NFSI can occur to give the intermediate C, in which the product 2a is coordinated to the Ag center, through two possible pathways, paths a (black) and b (blue) in Figure 6. From the energy profiles, we can clearly see that path b via a metathesis process (with an energy barrier of 8.7 kcal/mol) is much more favorable than path a via a
Figure 5. Top: Efect of extraneous AgNO3 on the complex 5a: left (A), reaction of complex 5a with AgNO3 (1−20 equiv); right (B), fluorination of complex 5a with NFSI in the presence of AgNO3 (0.1− 2 equiv) and Pyox (25 mol %). Bottom: Effect of extraneous AgNO3 on the complex 4a: left (C), complex 4a with AgNO3; right (D), complex 4a with NFSI in the presence of AgNO3 and Pyox (25 mol %).
Scheme 3. Possible Mechanism for the Fluorination of (MIC)nAgI Complex by NFSI
process (with an energy barrier of 18.6 kcal/mol) involving oxidative addition followed by reductive elimination. When ligand Pyox was used instead of DMA, indeed, a lower barrier of 7.7 kcal/mol was calculated for the metathesis step of Pyox· 4a, but a higher barrier of 9.1 kcal/mol was calculated for the biscarbene silver complex 5a. If we assume the Arrhenius equation for the rate constant, the calculated barrier differences imply that Pyox would accelerate the reaction by 5 times, and for 5a, the reaction rate would be 50% of the rate observed for the fluorination of complex DMA·4a. These computational results are in good agreement with the experimental results: the initial rate of reaction of Pyox·4a is 6 times that of DMA·4a, and the initial rate of reaction of DMA·4a is 2.6 times that of 6735
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transition state, likely because SelectFluor is sterically hindered and the lack of a chelating donor group toward Ag(I) complex. However, we found a process involving an oxidative addition of SelectFluor to (MIC)AgI 4a with very low barrier of 7.6 kcal/ mol to produce a Ag(III) intermediate (B). The calculation results are consistent with the fact that the presence of the strong oxidant SelectFluor promotes the oxidative addition followed by reductive elimination. The results discussed above revealed that the fluorination of silver complex 4a undergoes different pathways to construct heteroaryl C−F bond, which is dependent on the choice of different electrophilic fluorination reagents: a redox fluorination is involved in the case of strong oxidative reagent SelectFluor, but a metathesis process proceeded in the case of NFSI. For the case of F-Py, it is possible that its oxidative potential is not high enough to achieve fast oxidation of Ag(I) to Ag(III), resulting in the side protonolysis process; on the other hand, due to the lack of a chelating donor group in F-Py, the metathesis process does not occur.
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CONCLUSION In conclusion, we have isolated and characterized the mesoionic carbene Ag complexes from silver-mediated amination of alkyne, and their reactivities were investigated. The investigation on the fluorination of these silver complexes with NFSI reveals that the bis(MIC) silver complex 5a does exist in the reaction, but it is not the active species for the fluorination. Instead, the mono-MIC silver complex 4a exhibits good reactivity, and ligand Pyox could significantly enhance the reaction rate by ligation to form Pyox·4a, which might be the bona f ide reactive intermediate. DFT calculations suggest that σ-metathesis process is a favorable pathway over redox process for the C−F formation due to the chelation effect of NFSI with Ag(I). Alternatively, the redox fluorination process is involved in the case of SelectFluor due to its high oxidative potential. It is expected that the mechanism of C−F bond formation from MIC-Ag complexes could offer new perspective for novel organic transformation. Further exploration on the related chemistry is in progress in our laboratory.
Figure 6. Top part: free energy profiles calculated for the fluorination of (MIC)AgI 4a based on the mechanism proposed in Scheme 3 (dark line: path a, blue line: path b). The relative free energies are given in kcal/mol. Bottom part: the metathesis barriers calculated for the fluorination of Pyox·4a and 5a on the basis of the model complexes A′ and A″, respectively.
5a.20 We speculate that coordination of the ligand Pyox weakens the Ag(I)-C bond in Pyox·4a, facilitating the metathesis process. For 5a, it is the steric effect that increases the metathesis barrier. With SelectFluor as Oxidant. The reaction with SelectFluor was also monitored by using 19F NMR. We found that the fluorination of silver complexes 4a is much faster, even in the very low temperature (−50 °C), which does not allow us to measure the reaction rate.21 In order to get more information, the further DFT calculations based on the pathways in Scheme 3 were also carried out.20 Figure 7 showed the energy profile calculated for the fluorination of (MIC)AgI 4a with SelectFluor. We failed to locate the σ-metathesis
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b01885. Experimental details, characterization data, and tables giving Cartesian coordinates and electronic energies for all of the calculated structures (PDF) Crystallographic information (CIF) X-ray diffraction data (PDF) Crystallographic information (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions ¶
These authors (Q.L. and Z.Y.) contributed equally.
Figure 7. Free energy profile calculated for the fluorination of (MIC)AgI 4a with SelectFluor.
Notes
The authors declare no competing financial interest. 6736
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Canta, M.; Solà, M.; Costas, M.; Ribas, X. J. Am. Chem. Soc. 2011, 133, 19386−19392. (k) Yao, B.; Wang, Z.-L.; Zhang, H.; Wang, D.-X.; Zhao, L.; Wang, M. X. J. Org. Chem. 2012, 77, 3336−3340. (7) For the order of oxidative potential of F+ reagent, see ref 3b. (8) For some reviews on the abnormal mesoionic (MIC) carbene complexes, see: (a) Schuster, O.; Yang, L.; Raubenheimer, H. G.; Albrecht, M. Chem. Rev. 2009, 109, 3445−3478. (b) Kruger, A.; Albrecht, M. Aust. J. Chem. 2011, 64, 1113−1117. (c) Crabtree, R. H. Coord. Chem. Rev. 2013, 257, 755−766. For the recent elegant synthesis of MIC-Au complex via cyclization of alkynes, see: (d) Ung, G.; Soleilhavoup, M.; Bertrand, G. Angew. Chem., Int. Ed. 2013, 52, 758−761. For the isolation of the other MIC−Ag complex, see: (e) Heath, R.; Müller-Bunz, H.; Albrecht, M. Chem. Commun. 2015, 51, 8699−8701. (9) (a) Meyer, E. M.; Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. Organometallics 1989, 8, 1067−1079. (b) Furuya, T.; Ritter, T. Org. Lett. 2009, 11, 2860−2863. (10) For reviews on the NHC silver complexes, see: (a) Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105, 3978−4008. For the (IMes)2AgOTf, see: (b) Arduengo, A. J., III; Dias, H. V. R.; Calabrese, J. C.; Davidson, F. Organometallics 1993, 12, 3405−3409. (11) The m/z values of the Ag complex cations throughout this paper were calculated on the basis of the isotope of 107Ag. (12) The mixture of 4a and 5a only gave one set of 1H NMR signals, which significantly shifted from that of the pure 4a or 5a at range from room temperature to −50 °C. This observation implied that a faster equilibrium exists between 4a and 5a in the solution. Thus, it is difficult to monitor the catalytic reaction in situ with 1H NMR as an efficient tool to provide accurate information of silver intermediate. (13) The reactivity of mono-NHC silver complexes in gas phase were summarized in ref 10, and for the individual case, see: (a) Tulloch, A. A. D.; Danopoulos, A. A.; Winston, S.; Kleinhenz, S.; Eastham, G. J. Chem. Soc. Dalton. Trans. 2000, 4499−4506. (b) Garrison, J. C.; Simons, R. S.; Talley, J. M.; Wesdemiotis, C. A.; Tessier, C. A.; Youngs, W. J. Organometallics 2001, 20, 1276−1278. (14) Compared to (MIC)nAg(I) complexes 4a and 5a, the fluorination of (IMes)2Ag(OTf) with these electrophilic fluorinating reagents failed to generate the C−F bond. (15) In all these reactions, except the NO3 anion, some extra anions were existed in the mixture, which derived from oxidants. The final products might contain different anions. Thus, the anion of products did not present in Table 1. (16) The sluggish fluorination of complex 5a could release AgNO3, which can react with 5a to generate active species 4a immediately, resulting in fast enhancement of the reaction rate after the induction period. (17) For the pioneering oxidative fluorination with silver catalyst, see: (a) Tang, P.; Furuya, T.; Ritter, T. J. Am. Chem. Soc. 2010, 132, 12150−12154. (b) Furuya, T.; Strom, A. E.; Ritter, T. J. Am. Chem. Soc. 2009, 131, 1662−1663. (c) Furuya, T.; Ritter, T. Org. Lett. 2009, 11, 2860−2863. (18) When AgNO3 (0.1 mol %) was added, the reaction rate did increase, but the induction period was still observed (see Figure 5B). (19) The density functional theory calculations were performed at the M06(CPCM)//B3LYP level. (20) For details, see the Supporting Information. (21) When the reaction was monitored by 19F-NMR at −50 °C, around 40% yield was obtained in the first scanning, which is hard to give accurate initial rate.
ACKNOWLEDGMENTS We are grateful for financial support from the National Basic Research Program of China (973-2011CB808700), the National Nature Science Foundation of China (Nos. 21225210, 21202185, 21421091, and 21172250), the Science and Technology Commission of the Shanghai Municipality (11JC1415000, 12ZR1453400), and the CAS/SAFEA International Partnership Program for Creative Research Teams. YL and ZL thank the Research Grants Council of Hong Kong (HKUST 603313). GL thanks Prof. Liang Zhao @ Tsinghua University for helpful discussion.
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