Letter pubs.acs.org/OrgLett
Copper-Catalyzed Intermolecular Chloro- and Bromotrifluoromethylation of Alkenes Mingyang Fu, Long Chen, Yongpeng Jiang, Zhong-Xing Jiang,* and Zhigang Yang* Key Laboratory of Combinatorial Biosynthesis and Drug Discovery (Wuhan University), Ministry of Education and Wuhan University School of Pharmaceutical Sciences, Wuhan 430071, China S Supporting Information *
ABSTRACT: A highly practical copper-catalyzed intermolecular halotrifluoromethylation of alkenes has been developed under mild reaction conditions. A variety of Cl/Br-containing trifluoromethyl derivatives were directly synthesized from a wide range of alkenes, including electron-deficient and unactivated alkenes. he incorporation of fluorinated residues into complex molecular architectures has become almost irreplaceable in pharmaceuticals and agricultural chemicals.1 In particular, the CF3 group is considered as a useful structural motif in many biologically active molecules because it can often influence chemical and metabolic stability, lipophilicity, and binding selectivity.2 Therefore, the development of new methods for highly efficient and reliable introduction of a CF3 group into organic molecules has attracted much attention.3 For instance, a number of transition-metal-catalyzed methods for the trifluoromethylation of (hetero) aryl,3d,4 aryl halides,5 alkynes,6 and the corresponding boronic acids7 have been successively reported. In recent years, the trifluoromethylation of alkenes using electrophilic trifluoromethylation reagents, such as Togni’s8 and Umemoto’s9 reagents, has become a research hotspot and many effective methods have been studied intensively.3d,10,11 For example, the groups of Buchwald,11a Wang,11b and Liu11c have independently explored the copper-catalyzed electrophilic allylic trifluoromethylation of unactivated olefins. Furthermore, a variety of alkene difunctionalizations involving trifluoromethylation, such as oxy-, amino-, hydro-, azido-, cyano-, aryl-, and halo-containing trifluoromethylation, have also been developed.12 Although similar vicinal halo trifluoromethylations of alkenes have been reported (Scheme 1a),12a−e,m−r most of these methods required more expensive photoredox catalysts than CuBr2 such as RuCl2(PPh3)2, Ru(bpy)3(PF6)2, [Ir(dF(CF3)ppy) 2 (dtbbpy)]PF 6 , fac-[Ir(ppy) 3 ], Ru(bpy) 3 ]Cl 2 , Ru(phen)3Cl2, and [Cu(dap)2]Cl, base and light, and/or high temperatures. These conditions lead to some limitations on the large scale preparations of vicinal halo trifluoromethyl derivatives. Thus, the development of low cost and more efficient as well as facile large scale preparation approaches for halotrifluoromethylation of alkenes is still desirable and in high demand. Furthermore, halogenated compounds are important building blocks for a variety of useful derivatives, such as amine, hydroxyl, hydrocarbon, and hydrogen. Herein, we report a convenient protocol for the copper-catalyzed chloro- and bromotrifluoromethylation of electron-deficient alkenes and unactivated unactivated alkenes with Togni’s reagent and dihalo sulfoxide
T
© XXXX American Chemical Society
Scheme 1. Previous and Present Works
(SOX2) as the trifluoromethyl and halogen sources, respectively (Scheme 1b). Initially, the chlorotrifluoromethylation of 1-butyl-3-methylene-2-pyrrolidinon 1a with Togni’s reagent 2a was investigated by using chlorotrimethylsilane (TMSCl) as the chlorine source in the presence of 5 mol % CuBr2 in CHCl3 at room temperature for 10 h. We were delighted to find that the desired product 3chloro-1-butyl-3-trifluoroethyl-2-pyrrolidinon 3a was indeed observed with a 50% yield (Table 1, entry 1). Then, different chlorine sources were examined. To our delight, oxalyl chloride and thionyl chloride could afford the desired product in 96% and 95% yields, respectively (Table 1, entries 2 and 3). By employing phosphorus trichloride, a 78% yield could be afforded (Table 1, entry 4). A significant drop in the reactivity was found when CuCl was used as a chlorine source (20%, Table 1, entry 5). Considering thionyl chloride is a cheap and commonly used reagent, SOCl2 was chosen as the best chlorine source for the following investigation. Among the solvents tested (Table 1, entries 6−11), dioxane slightly decreased the yield to 88%. Other solvents, such as DCM, DCE (1, 2-dichloroethane), MeOH, DMSO, and DMF, were found to be less effective for this Received: October 23, 2015
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DOI: 10.1021/acs.orglett.5b03080 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Optimization of Reaction Conditionsa
Scheme 2. Scope of Chlorotrifluoromethylationa,b
entry
CF3+
chlorine source
solvent
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15c 16d
2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2b 2c 2d 2a 2a
TMSCl (COCl)2 SOCl2 PCl3 CuCl SOCl2 SOCl2 SOCl2 SOCl2 SOCl2 SOCl2 SOCl2 SOCl2 SOCl2 SOCl2 SOCl2
CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 dioxane DCM DCE MeOH DMSO DMF CHCl3 CHCl3 CHCl3 CHCl3 CHCl3
50 96 95 78 20 88 73 60 66 71 49 37 NR NR NR 95
a
Reaction conditions: 1a (0.2 mmol), CF3+ (0.3 mmol, 1.5 equiv), CuBr2 (5 mol %) and [Cl] (0.2 mmol, 1.0 equiv in solvent (1.0 mL) at rt for 10 h. bYields was determined by 19F NMR spectroscopy using PhCF3 as an internal standard. cWithout CuBr2. dReaction on a 1.0 mmol scale with a catalyst loading of 1 mol %.
reaction. For comparison, other electrophilic CF3+ reagents were also studied. Using Togni’s reagent 2b led to much lower yield, probably due to its lower reactivity (37% yield, Table 1, entry 12). When Umemoto’s reagents 2c and 2d were employed, no reaction occurred (Table 1, entries 13−14). Moreover, the controlled experiment indicated the copper catalyst was essential in catalyzing the reaction (Table 1, entry 15). The reaction was carried out on a 1.0 mmol scale, and reducing the catalyst loading to 1 mol % led to no loss of reactivity (Table 1, entry 16). Under the optimal reaction conditions (Table 1, entry 16), we investigated the scope of the copper-catalyzed three-component chlorotrifluoromethylation of alkenes. First, different alkyl and aryl substituted groups on the nitrogen atom of 2-pyrrolidinon (1a−e) were compatible with the reaction conditions affording the corresponding products with excellent yields (3a−e, Scheme 2). Meanwhile, a single crystal of the N-phenyl substituted chlorotrifluoromethyl 2-pyrrolidinon 3c was obtained, and the product structure was also confirmed. Six-membered α,βunsaturated cyclic amides were then examined, which led to the corresponding adducts 3f−g in good yields. Furthermore, internal α,β-unsaturated 2-pyrrolidinon was also a suitable substrate for the reaction and afforded the desired product 3h in good yield and diastereoselectivity (anti/syn = 10:1). Second, linear unsaturated amides were tested under the same reaction conditions. Only a 45% yield was obtained when N,Ndiethylmethacrylamide was used as a substrate. When oxalyl chloride was employed as the chlorine source and dioxane was used instead of CHCl3, a high yield was observed (Scheme 2, 3i,
a
Reaction conditions: 1 (2.0 mmol), 2a (3.0 mmol, 1.5 equiv), CuBr2 (1 mol %) and SOCl2 (2.0 mmol, 1.0 equiv in CHCl3 (10.5 mL) at rt for 10 h. bYield of isolated product. cOxalyl chloride (2.0 equiv) was used as the chlorine source, and dioxane was used as solvent. dThe results from trans- and cis-alkenes were obtained in brackets and parentheses, respectively.
85% vs 45%). Then, substrates bearing Et, nBu, Bn, or H on the α-position of the amide were also suitable substrates for the reaction and afforded the corresponding products 3j−m with good yields (70−88%). When phenyl substitution on the αposition was tested under the reaction conditions, an eliminated product 3n was obtained, probably because the bulky phenyl group hinders binding of Cl to the amide. Next, under the similar reaction conditions with (COCl)2 as the chlorine source, unactivated alkenes such as 1-dodecylene, 3-(benzyloxy)propene, and phenylbutene underwent the chlorotrifluoromethylation smoothly, delivering the corresponding products 3o−q in 75−85% yields. To examine the effect of substrate conformation, two pairs of geometric isomers of alkenes were synthesized and subjected to the chlorotrifluoromethylation reaction under the optimal reaction conditions. It was found that, regardless of the alkene geometry of the substrate (trans or cis), B
DOI: 10.1021/acs.orglett.5b03080 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
In order to gain more insight into the reaction mechanism, several control experiments were conducted under the standard conditions (Scheme 5). When 1.5 equiv of radical scavenger
the corresponding adducts 3r and 3s were obtained in similar yields and the same product diastereomeric ratio (3r, anti/syn = 5:1; 3s was only diastereomer obtained). A 45% yield was obtained when tetrasubstituted alkene 1-methyl-3-isopropylideneindolin-2-one 1t was used as a substrate. Inspired by the chlorotrifluoromethylation of alkenes, we turned our attention to bromotrifluoromethylation of alkenes. Subsequently, the substrate scope of the bromotrifluoromethylation of alkenes was examined in the presence of 1 mol % CuBr2 and thionyl bromide (Scheme 3). A series of α,β-
Scheme 5. Control Experiments
Scheme 3. Scope of Bromotrifluoromethylationa,b TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) was added into the reaction system, the desired transformation was almost shut down and a small amount of the TEMPO−CF3 adduct was detected. To our surprise, no trifluoromethyl-substituted compound was observed except for Togni’s reagent 2a when 1a was treated in the absence of chlorine source SOCl2. It indicated that the chlorine source plays a critical role in the threecomponent reaction. According to the experimental results and related publications,11,12 we proposed a probable mechanism (Scheme 6). First, Scheme 6. Plausible Reaction Mechanism
a
Reaction conditions: 1 (2.0 mmol), 2a (3.0 mmol, 1.5 equiv), CuBr2 (1 mol %), and SOBr2 (2.0 mmol, 1.0 equiv in 10.5 mL CHCl3) at rt for 10 h. bYield of isolated product. cPBr3 (2.0 equiv) as the bromine source and dioxane as the solvent.
unsaturated cyclic amide substrates, as well as linear unsaturated amides with different alkyl substituted groups on the α-position of the amide, and and unactivated alkyene substrate could be smoothly converted to Br-containing trifluoromethyl products 4a−k with good yields (52−91%, Scheme 3). It is worth noting that thionyl bromide could not work well on linear unsaturated amides and unactivated alkene. Instead, good results were obtained by using phosphorus tribromide as the bromine source and dioxane as the solvent under the current catalytic system. To probe the potential scalability of this method, we performed Cu(II)-catalyzed bromotrifluoromethylation of N,N-diethylacrylamide on a gram scale with a 1 mol % catalyst loading. Pleasantly, as shown in Scheme 4, the reaction proceeded smoothly and the desired product 4j yield was increased from 52% to 71%.
Togni’s reagent 2a is activated by CuBr2, generating radical ·CF3 by a single-electron-transfer (SET) process, with the release of Cu3+ species. The stabilized radical ·CF3 was added to an electron-deficient alkene, and the radical intermediate A was formed. Following the second SET process, carbocation intermediate B is afforded, with regeneration of the active Cu2+ catalyst to restart the catalytic cycle. Finally, chlorotrifluoromethylation product 3a is obtained after the cationic intermediate B is nucleophilically attacked by Cl− from thionyl chloride. In summary, we have developed a copper-catalyzed intermolecular chloro- and bromotrifluoromethylation of alkenes under mild reaction conditions. The reactions were catalyzed by a copper(II) catalyst to give the Cl-/Br-containing trifluoromethyl derivatives in high yields (up to 97%). In particular, the procedure is capable of tolerating a relatively wide range of substrates for the bromotrifluoromethylation, and good results (52−91% yield) can also be obtained. Furthermore, an excellent yield can also be obtained on a gram scale, which showed the potential value of the catalyst system. Further studies on catalytic
Scheme 4. Scale-Up of Cu(ll)-Catalyed Bromotrifluoromethylation
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DOI: 10.1021/acs.orglett.5b03080 Org. Lett. XXXX, XXX, XXX−XXX
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asymmetric intermolecular halofunctionalization reactions of alkenes and elucidating the catalytic mechanism are in progress.
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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/acs.orglett.5b03080. Crystallographic data for 3c (CIF) Experimental procedures, compounds characterizations, copies of 1H/19F/13C NMR, MS/HRMS spectra (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 21402144, 21372181, and 21572168), the Fundamental Research Funds for the Central Universities (No. 2042014kf0030) and the Innovation Seed Fund of Wuhan University School of Medicine.
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REFERENCES
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