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Cite This: J. Am. Chem. Soc. 2018, 140, 15194−15199
Cobalt-Catalyzed Trifluoromethoxylation of Epoxides Jie Liu,§,† Yongliang Wei,§,† and Pingping Tang*,†,‡ †
State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China ‡ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, China
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S Supporting Information *
ABSTRACT: A catalytic ring-opening reaction of epoxides by nucleophilic trifluoromethoxylation of trifluoromethyl arylsulfonate has been developed based on the use of a cobalt catalyst. This reaction provides an efficient, simple route for directly construction of a wide range of vicinal trifluoromethoxyhydrins under mild conditions. In addition, this method can convert terminal epoxides into target products with good chemo- and regioselectivity.
N
ucleophilic ring-opening of epoxides represents an important method to obtain, in single step, bifunctional organic molecules.1−3 Accordingly, various nucleophiles have been reported in the past few years, such as amines,4 thiols,5 hydroxyl,6−10 phenol,11,12 carboxylic acids,13−15 azide,16,17 cyanide,18−20 halogen21−32 and so on. In comparison, the trifluoromethoxy group (OCF3) is rarely used as the nucleophile in the ring-opening reaction of epoxides, even though OCF3-containing compounds are of great interest in new drug and agrochemical design due to its unique properties.33−59 It would be the poor nucleophilicity and unstability of trifluoromethoxy anion limited its application in this area. To the best of our knowledge, there has been only one report about the construction of vicinal trifluoromethoxyhydrins through the ring-opening of epoxides by now.60 In that case, the toxic fluorophosgene with high pressure (4 MP) was used as the source of the trifluoromethoxy anion (−OCF3) for nucleophilic attacking, which apparently increased the risk and difficulty of operation. Therefore, the nucleophilic ringopening of epoxides by trifluoromethoxy anion is extremely challenging. Herein, we present a first example of cobaltcatalyzed ring-opening reaction of epoxides with trifluoromethyl arylsulfonate (TFMS) under mild conditions (Figure 1). And a bimetallic cooperative catalytic process was determined by NMR and dynamic analysis as well. Recently, our group reported a series of applications of trifluoromethyl arylsulfonate (TFMS) as the trifluoromethoxylation reagent for the introduction of trifuoromethoxyl group into the target molecules.61−65 And its trifluoromethoxy anion − OCF3 could be generated in situ by AgF, CsF (Figure 1a). We wondered whether it could be possible to achieve the nucleophilic ring-opening of epoxides by trifluoromethoxy anion from TFMS. However, we realized that two issues have to be considered: (a) fluoride salts is not suitable to activate the TFMS to generate the −OCF3 since the nucleophilic ringopening of epoxides by fluoride anion could happen;27−32 and © 2018 American Chemical Society
Figure 1. Cobalt-catalyzed trifluoromethoxylation of epoxides
(b) how to stabilize the trifluoromethoxy anion. To address the first issue, we hypothesis that in the presence of catalytic SalenCoX (Figure 1c), axial anions X of SalenCoX could be used to react with TFMS to generate −OCF3 first, and ringopening of epoxide (1) with −OCF3 was achieved to generate the intermediate A, which react with fluorophosgene generated in situ from −OCF3 to form intermediate B, and followed workup to give the desired vicinal trifluoromethoxyhydrins (3). The fluoride decomposed from −OCF3 was further used to generate −OCF3 with TFMS during the reaction (Figure 1b). Received: September 23, 2018 Published: October 29, 2018 15194
DOI: 10.1021/jacs.8b10298 J. Am. Chem. Soc. 2018, 140, 15194−15199
Communication
Journal of the American Chemical Society
TFMS first. As shown in Table 2, Five-, six-, seven-membered ring substrates (1a to 1d, 1g to 1l) were successfully converted
Our investigation started using 2,3-epoxy-1,2,3,4-tetrahydronaphthalene (1a) as the model substrate to optimize the reaction conditions. Different axial anions X of SalenCoX were evaluated as shown in Table 1. Compared with other Lewis
Table 2. Substrate Scope for Ring-Opening Reaction of Meso-Epoxidesa
Table 1. Screening of Reaction Conditionsa
Entry
X
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15b 16c 17c
OTs Cl BF4 SbF6 SCN OTf NO3 OAc PhCO2 CF3CO2 DNP DNP DNP DNP DNP − −
n
Bu4N+DNP− (mol %)
Yield (%)d
− − − − − − − − − − − 10 20 40 10 − 10
17 46 0 0 8 21 47 27 70 63 73 88 72 62 97 0 0
a
a
General conditions: 1a (1.0 equiv), 2 (2.0 equiv), cobalt (10 mol %), MeCN, 25 °C, N2. b3.0 equiv of 2 was used. cNo cobalt catalyst was used. dYields were determined by 19F NMR with benzotrifluoride as a standard. DNP = 2,4-dinitrophenoxy.
Reaction conditions: 1 (1.0 equiv), TFMS (2, 3.0 equiv), catalyst I (0.1 equiv), nBu4N+DNP− (0.1 equiv), MeCN, N2, 25 °C. Yields of isolated products are given. b0.15 equiv of catalyst I and n Bu4N+DNP− were used. c0.2 equiv of catalyst I and nBu4N+DNP− were used. d4 equiv of TFMS (2) was used. e5 equiv of TFMS (2) was used. fNo nBu4N+DNP− was used. gYields were determined by integration of the 19F NMR spectrum using benzotrifluoride as an internal standard.
acids (see Supporting Information for more details), 17% desired product 3a was found through workup of intermediate B when 10% mol SalenCoOTs was applied as the catalyst. Encouraged by this result, the electron-withdrawing axial anion X such as PhCO2−, CF3CO2− and DNP (Table 1, entries 9 to 11) exhibited the best catalytic activity, and DNP gave the highest yield 73% (Table 1, entry 11). In contrast, anions with poor nucleophilicity, such as BF4− and SbF6−, extremely suppressed the product formation. And TFMS was left without any decomposition in those cases. The role of axial anion DNP is not only easy to dissociate from central metal for the activation of epoxide but also can react with TFMS to generate − OCF3. In 2010, Langlois and co-workers reported nBu4N+ could stabilize the − OCF 3 . 66 Based on this result, n Bu4N+DNP− was applied in this reaction. The highest yield 88% was achieved when 10% mol nBu4N+DNP− was used (Table 1, entry 12), while further increasing the amount of ammonium salt had a negative effect on the reaction (Table 1, entries 12 to 14). The yield was further up to 97% when 3.0 equiv of TFMS was used (Table 1, entry 15). The control reactions in the absence of cobalt catalyst showed none of the trifluoromethoxylation product (Table 1, entries 16 and 17). After extensive optimization of reaction conditions, 97% product 3a could be obtained as the highest yield when the reaction of 1a was conducted with 10% mol catalyst I, 10% mol nBu4N+DNP−, 3 equiv TFMS (2) in MeCN under an N2 atmosphere at room temperature. With the optimized conditions in hand, we then investigated the scope of the ring-opening reaction of meso-epoxides with
into the desired vicinal trifluoromethoxyhydrins with yield range from 45% to 95%. Acyclic substrates 1e and 1f were also smoothly transformed into the desired products (3e, 3f) with excellent yields. Moreover, the transformation was compatible with the presence of some functionalities (ester, ether and amide) in the substrates. It is worth mentioning that less than 5% fluorination byproduct through ring opening of epoxides by fluoride was observed. Besides meso-epoxides, rac-epoxides were also explored, and the results are listed in Table 3. In general, these terminal epoxides were easier to be converted into the vicinal trifluoromethoxy-hydrins with yield range from 62% to 93% (3m to 3r). Ether, nitrile and halogen were all tolerated. Notably, the catalytic system displayed excellent regioselectivities in the ring-opening of substrates 1m to 1p. There only secondary alcohols 3m to 3p were formed. The high regioselectivities were found with substrates 1q and 1r, and less than 2% primary alcohol byproduct was found in the reaction. Inspired by these results, we are aware that this catalytic system could retain the stereochemistry at the methine carbon during the ring-opening process. Therefore, chiral pure substrate (S)-1m was used and gave the product 3m in 88% yield with >99% ee (Supporting Information). This indicated that catalyst I predominately induced the ring opening at the methylene carbon of (S)-1m and the stereochemistry at the methine 15195
DOI: 10.1021/jacs.8b10298 J. Am. Chem. Soc. 2018, 140, 15194−15199
Communication
Journal of the American Chemical Society
TFMS in MeCN was monitored by 19F NMR first to indicate the existence of SalenCoOCF3 (around −18.8 ppm, Figure 2).61,70,71 Moreover, kinetic study about catalyst II revealed a
Table 3. Substrate Scope for Ring-Opening Reaction of RacEpoxides with TFMSa
Figure 2. 19F NMR spectra of a mixture of catalyst II and TFMS in CH3CN.
linear correlation between rate constants (k) vs [catalyst II]2, reflecting a second-order dependence on catalyst. And the same conclusion was also obtained by analyzing the data with the variable time normalization analysis graphical methods developed by Burés (see more detail in the Supporting Information).72,73 These observations provide solid evidence that a bimetallic cooperation exists in the ring-opening reaction of epoxides with TFMS. Jacobsen further proposed two limiting geometries for the enantioseletivity-determining transition state: “head to head” and “head to tail”. And only the “head to tail” geometry could induce the asymmetric ring opening of epoxides, which was confirmed by a dimeric salen chromium complex (similar to catalyst III in Figure 3) who
a
Reaction conditions: 1 (1.0 equiv), TFMS (2, 4.0 equiv), catalyst I (0.1 equiv), nBu4N+DNP− (0.1 equiv), MeCN, N2, 25 °C. Yields of isolated products are given. b3 equiv of TFMS (2) was used. c5 equiv of TFMS (2) was used. d0.15 equiv of catalyst I and nBu4N+DNP− were used. r.r. ratio was determined by integration of the 1H NMR or 19 F NMR spectrum.
carbon was kept. Unfortunately, the lower regioselectivity was observed when styrene epoxide 1s was used due to conflicting steric and electronic biases to nucleophilic attack.16,69 The conversion of asymmetric inner epoxy-alkane 1t shown the steric factor had a big influence on the ring-opening position. The less hindered carbon of epoxy group was more favored attack of −OCF3 to form the product 3t as the main product. Except regioselectivity, catalyst I also shown its excellent chemoselectivity in the ring-opening of substrate 1u with two different epoxy groups, and the ring-opening only occurred at the terminal epoxy group due to more steric hindered structure of left epoxy group. Enantioselective ring opening of epoxides by trifluoromethoxylation anion was also investigated. Initially, the reaction was conducted with the chiral pure catalyst II. Unfortunately, the desymmetrization of meso-epoxide (1a) only gave the desired product (3a) in 95% yield with 6% ee, and the kinetic resolution of rac-epoxide (1m) afforded the desired product (3m) in 80% yield with 13% ee. In order to explain this phenomenon, we performed some preliminary studies which help us gain more insight into the reaction mechanism. According to Jacobsen’s reports, the metallic salen complex behave as a Lewis acid to activate epoxide and also as a nucleophile delivery agent through a bimetallic cooperation, which is the key point to realize the high enantioselective ringopening of epoxides.67−69 Hence, a mixture of catalyst II and
Figure 3. Ring-opening reaction of substrate 1a with TFMS catalyzed by catalyst III.
can simulate the “head to tail” geometry through its intramolecular bimetallic pathway.74,75 To our surprise, when chiral catalyst III was applied in the ring opening of 1a with TFMS, except trifluoromethoxylated product 3a (47%, 19F NMR yield), there was also fluorinated product 3aa (40%, 19F NMR yield). And in contrast with trifluoromethoxylated product 3a still with low level of stereocontrol (ee = 12%), fluorinated product 3aa gave a remarkable result (87% ee) (Figure 3). This reveals that fluorinated product 3aa was produced through the intramolecular bimetallic pathway (“head to tail”) just like the previous report,31,74,75 but trifluoromethoxylated product 3a was not. Hence, obviously one cobalt center of catalyst II could hardly induce the nucleophile −OCF3 to approach to an epoxide activated by 15196
DOI: 10.1021/jacs.8b10298 J. Am. Chem. Soc. 2018, 140, 15194−15199
Communication
Journal of the American Chemical Society another cobalt center through the “head to tail” geometry. The barrier may be caused by the steric repulsion between CF3 and the activated epoxide or salen ligand. And this may explain the loss of stereo control of catalyst II in the ring opening of epoxides with TFMS even if the reaction proceeds with a bimetallic cooperative catalysis. In conclusion, we have reported the first ring-opening reaction of epoxides with trifluoromethyl arylsulfonate (TFMS) to construct vicinal trifluoromethoxyhydrins under mild conditions. This method enables the transformation of various epoxides into the corresponding trifluoromethoxylation products and a range of functional groups are compatible. Especially terminal epoxides can be converted into the products with good chemo- and regioselectivity. Mechanism studies reveal the reaction may be achieved through a bimetallic cooperation. Further efforts will focus on designing new chiral catalyst to achieve the vicinal trifluoromethoxyhydrins with high enantioselectivity.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b10298. Experimental procedures and characterization of all new compounds including 1H, 13C and 19F NMR spectra (PDF) Data for C11H11F3O2 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Pingping Tang: 0000-0002-8296-5695 Author Contributions §
J.L. and Y.W. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the State Key Laboratory of Elemento-organic Chemistry for generous start-up financial support. This work was supported by the National Key Research and Development Program of China (2016YFA0602900), NFSC (21522205, 21672110) and the Fundamental Research Funds for the Central Universities.
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DOI: 10.1021/jacs.8b10298 J. Am. Chem. Soc. 2018, 140, 15194−15199
Communication
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