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Cobalt-Catalyzed Trifluoromethylation−Peroxidation of Unactivated Alkenes with Sodium Trifluoromethanesulfinate and Hydroperoxide Hong-Yu Zhang,† Chao Ge,† Jiquan Zhao,† and Yuecheng Zhang*,†,‡ †

School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, P. R. China National-Local Joint Engineering Laboratory for Energy Conservation of Chemical Process Integration and Resources Utilization, Hebei University of Technology, Tianjin 300130, P. R. China

‡

S Supporting Information *

ABSTRACT: Disclosed herein is an unprecedented cobalt-catalyzed trifluoromethylation−peroxidation of unactivated alkenes. In this process the hydroperoxide acts as a radical initiator as well as a coupling partner. The cheap and readily available sodium trifluoromethanesulfinate serves as the CF3 source in the reaction. Various alkenes are transformed into vicinal trifluoromethylperoxide compounds in moderate to good yields.

T

Scheme 1. Difunctionalization of Alkenes

he trifluoromethyl group is a crucial structural motif in pharmaceuticals, agrochemicals, and functional materials fields due to its excellent ability to promote the metabolic stability and lipophilicity in bioactive molecules.1 Therefore, many efficient methods for trifluoromethylation have been developed.2 CF3-involved difunctionalization of unactivated alkenes, such as carbotrifluoromethylation,3 oxytrifluoromethylation,4 and aminotrifluoromethylation,5 provides a straightforward access to a variety of complicated and valuable trifluoromethylated compounds in a single procedure (Scheme 1a). Despite the significance of the achievements, the scope of CF3-involved difunctionalization is still limited, and the method for introducing a trifluoromethyl group and another different functional group such as a peroxide moiety into alkenes remains challenging. On the other hand, natural products with peroxide groups have raised widespread concerns due to their special chemical and biological properties.6 Moreover, peroxides often serve as key potential intermediates in organic synthesis, because they are readily converted into ketone, alcohol, and epoxy compounds.7 Consequently, several elegant methods of incorporation of peroxide groups into organic molecules via the difunctionalization of the alkenes have been disclosed (Scheme 1b).8 For example, Li’s group has achieved the carbonylation−peroxidation of alkenes,7b and the Klussmann and Loh groups have respectively realized the alkylation− peroxidation of alkenes.9,10 However, to the best of our knowledge, the simultaneous introduction of trifluoromethyl and peroxide moieties into the same molecule has not been achieved. Hence, we describe the first example of trifluor© 2017 American Chemical Society

omethyl-peroxide difunctionalization of unactivated alkenes. The reaction uses low-cost cobalt salt as the catalyst and readily available sodium trifluoromethanesulfinate (Langlois reagent) as the CF3 source. The Langlois reagent was extensively used in the trifluoromethylation reactions, especially in the trifluoromethylation−halogenation of alkenes.11 All allyl, vinyl, chain terminal, and internal alkenes could afford the target products Received: August 16, 2017 Published: September 13, 2017 5260

DOI: 10.1021/acs.orglett.7b02353 Org. Lett. 2017, 19, 5260−5263

Letter

Organic Letters

optimal reaction conditions were obtained, which are 10 mol % of Co(OAc)2·4H2O, 2 equiv of CF3SO2Na, 6.5 equiv of TBHP, 0.6 equiv of K3PO4, and 2 mL of CH3CN for 0.2 mmol of 1a, at a reaction temperature of 75 °C, with an argon atmosphere. With the optimized reaction conditions in hand, the substrate scope and universality of the reaction were explored by using an array of alkenes (Scheme 2). Initial studies were focused on the

in moderate to good yields with good functional group compatibility. We initially chose 4-allylanisole (1a) as the model substrate, sodium trifluoromethanesulfinate (CF3SO2Na) as the CF3 source, and tert-butyl hydroperoxide (TBHP, 70% solution in H2O) as a radical initiator and coupling partner to explore the reaction conditions, inspired by our recently disclosed method for aminotrifluoromethylation of alkenes.12 First, various acetate salts as catalysts were tested. Fortunately, the target molecule (4a) was obtained in 66% yield in the case of Co(OAc)2·4H2O as catalyst, and the other metal acetates including Mn, Cu and Mg were also valid for this transformation (Table 1, entries 1−4). However, the other tested

Scheme 2. Substrate Scopea

Table 1. Selected Reaction Conditions Optimizationa

entry

catalyst (10 mol %)

base (0.6 equiv)

t (°C)

solvent

yield (%)b

1 2 3 4 5 6 7 8 9 10c 11c 12c 13c 14c 15 16e 17e 18e 19e,f

Mn(OAc)2·4H2O Cu(OAc)2·H2O Mg(OAc)2·3H2O Co(OAc)2·4H2O Co(acac)2 CoCl2·6H2O Co(OAc)2·4H2O Co(OAc)2·4H2O Co(OAc)2·4H2O Co(OAc)2·4H2O Co(OAc)2·4H2O Co(OAc)2·4H2O Co(OAc)2·4H2O Co(OAc)2·4H2O Co(OAc)2·4H2O Co(OAc)2·4H2O Co(OAc)2·4H2O Co(OAc)2·4H2O Co(OAc)2·4H2O

− − − − − − − − − CsCO3 K2CO3 K3PO4 KOAc DABCOd K3PO4 K3PO4 K3PO4 K3PO4 K3PO4

65 65 65 65 65 65 55 75 85 75 75 75 75 75 75 75 75 75 75

CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN DMF EtOAc CH3CN

61 56 54 66 trace trace 63 70 58 60 63 72 64 59 75 79 36 68 35

a Unless specifically noted otherwise, reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), 3a (0.9 mmol), catalyst (0.02 mmol), base (0.12 mmol), solvent (2 mL), stirred at 75 °C under an argon atmosphere. bYield of isolated product. cBase (0.16 mmol). dDABCO = triethylenediamine. eTBHP (1.3 mmol). fUnder an air atmosphere. a

Reagents and conditions: 1 (0.2 mmol), CF3SO2Na (0.4 mmol), tertbutyl hydroperoxide (70% solution in H2O) (1.3 mmol), Co(OAc)2· 4H2O (0.02 mmol), K3PO4 (0.12 mmol), CH3CN (2 mL), 75 °C, under an argon atmosphere. Yields given are for isolated products. b Without potassium phosphate. cThe diastereomeric ratio determined by crude 19F NMR. dCumyl hydroperoxide (contains ca. 20% aromatic hydrocarbon) (1.3 mmol).

cobalt salts were invalid (Table 1, entries 5−6). Next, the reaction temperature was optimized and a yield of 70% of 4a was obtained at 75 °C (Table 1, entries 7−9). Then, various bases (0.8 equiv) were tested and the yield of 4a increased to 72% with K3PO4 as base (Table 1, entries 10−14). Encouraged by this result, we further screened the loading of K3PO4 and found that 0.6 equiv of K3PO4 was beneficial for the reaction and the yield of 4a increased to 75% (Table 1, entry 15). Here, it is inferred that the possible role of K3PO4 is to act as a buffer. Finally, the ratio of tert-butyl hydroperoxide to 4-allylanisole was optimized and a yield as high as 79% of the product (4a) was obtained in a molar ratio of 6.5:1 of tert-butyl hydroperoxide to 4-allylanisole (Table 1, entry 16). Also, different solvents were screened and the results showed that CH3CN was the best among all the tested solvents including DMF and EtOAc (Table 1, entries 17 and 18). The reaction was also conducted under an air atmosphere, but the yield of 4a decreased dramatically (Table 1, entry 19). Consequently, the

allyl alkenes, and the corresponding products were obtained in good yields (4a−4o). Next, a series of substituted styrenes were also examined. Pleasingly, various target trifluoromethylperoxide compounds were obtained in moderate yields (4p− 4y). In addition, long chain alkenes were also used as substrates and the corresponding products were received in moderate to good yields (4z−4dd). Unfortunately, a low yield was obtained in the case of benzyl acrylate, a representative of acrylate derivatives, as the substrate (4ee). Moreover, reactions could also proceed well in the cases of indene and 1,2-dihydronaphthalene as substrates, being the representatives of internal 5261

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reaction14 to generate the corresponding α-trifluoromethyl ketone 5x in the presence of 1 equiv of DABCO (Scheme 5, eq 4).

alkenes, and afford the expected products in moderate yields but with low dr values (4ff−4gg). Encouraged by these results on simple alkenes, we applied this reaction to natural quinine as a challenging substrate. Surprisingly, although quinine bears both quinoline and hydroxyl groups, the reaction can still proceed smoothly and the desired product was received in 40% yield under the standard reaction conditions (4hh). In addition, cumyl hydroperoxide as another radical initiator and coupling partner was tested, and the corresponding vicinal trifluoromethyl-peroxide product was obtained under the standard reaction conditions in 48% yield. After analysis of the substrate scope, additional experiments were carried out to gain insight into the reaction mechanism. To ascertain whether this reaction proceeded via a radical pathway or not, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a radical inhibitor was added in the reaction; only a trace amount of the desired product was detected (Scheme 3, eq 1).

Scheme 5. Transformations of Peroxide Group

In conclusion, we have developed an original cobalt-catalyzed trifluoromethyl-peroxide difunctionalization of alkenes. Both the catalyst and CF3 source are cheap and readily available. A variety of allyl, vinyl, chain terminal, and internal alkenes with different functional groups are well tolerated in this transformation. Moderate to good yields of the expected products were obtained. This transformation provides a convenient and economic method for the construction of vicinal trifluoromethyl-peroxide derivatives, which is useful in the organic synthesis.

Scheme 3. Additional Experiments

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ASSOCIATED CONTENT

S Supporting Information *

Besides, no TEMPO−CF3 adduct was detected, but a TEMPOcaptured allyl product 7a was obtained. However, when 2.0 equiv of BHT (2,6-di-tert-butyl-p-cresol), another well-known radical inhibitor, was added in the reaction, the formation of the difunctionalized product was suppressed; instead, a BHTcaptured intermediate 5a was obtained in 48% yield (Scheme 3, eq 2). These results indicated that a radical addition mechanism might be involved in the reaction. According to these findings and previous reports,7b,8e,9,10,13 a plausible mechanism is proposed and shown in Scheme 4.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02353. Experimental details and characterization data for all new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Scheme 4. Plausible Reaction Mechanism

Hong-Yu Zhang: 0000-0002-1186-6614 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21476057), the Natural Science Foundation of Hebei Province (CN) (Grant No. B2016202393, B2015202284), and the Program for the Top Young Innovative Talents of Hebei Province (CN) (Grant No. BJ2017010).

Initially, the Co(III) tert-butyloxy complex (1A) and trifluoromethyl radical (1B) are respectively generated from Co(OAc)2 and CF3SO2Na in the presence of TBHP. Upon formation of intermediate 1B, it undergoes addition to the C C double bond of the substrate to afford the alkyl radical (1C). Meanwhile, 1A reacts with TBHP to give the Co(III) intermediate tert-butylperoxy complex (1D). Eventually crosscoupling between 1D and 1C provides the target product (4a) along with a Co(II)-catalyst. In order to prove the usefulness of this reaction in organic synthesis, the transformations of the peroxide moiety on the substrates were studied. When 4a was hydrogenated over palladium−carbon in methanol, the peroxide moiety was transformed to a hydroxyl group, and a vicinal hydroxyltrifluoromethyl compound (6a) was obtained (Scheme 5, eq 3). In addition, 4x can undergo a Kornblum−DeLaMare

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