Combination of Lewis Basic Selenium Catalysis and Redox Selenium

Letter pubs.acs.org/OrgLett

Combination of Lewis Basic Selenium Catalysis and Redox Selenium Chemistry: Synthesis of Trifluoromethylthiolated Tertiary Alcohols with Alkenes Zechen Zhu, Jie Luo, and Xiaodan Zhao* Institute of Organic Chemistry & MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, P. R. China S Supporting Information *

ABSTRACT: A new and efficient method for diaryl selenide catalyzed vicinal CF3S hydroxylation of 1,1-multisubstitued alkenes has been developed. Various trifluoromethylthiolated tertiary alcohols could be readily synthesized under mild conditions. This method is also effective for the intramolecular cyclization of alkenes tethered by carboxylic acid, hydroxy, sulfamide, or ester groups and is associated with the introduction of a CF3S group. Mechanistic studies have revealed that the pathway involves a redox cycle between Se(II) and Se(IV) and Lewis basic selenium catalysis.

O

ization of alkenes is a great strategy to quickly construct these molecules along with incorporation of a functional group. In 2015, our group reported that trifluoromethylthioamination of alkenes could proceed smoothly to afford CF3S products under Lewis basic selenium catalysis with the aid of acid.8a In continuation of our interest in the synthesis of different CF3S compounds, our new goal was to construct CF3S alcohols to supplement the library of CF3S compounds available through the oxytrifluoromethylthiolation of alkenes. Because the hydroxytrifluoromethylthiolation of 1,1-dialkyl and 1,1-aryl,alkyl-substituted alkenes is challenging and has not been developed to date,11i,j we initiated HO-trifluoromethylthiolation by using (3-methylbut-3-en-1-yl)benzene (1a) as the model substrate. When 1a was treated with known electrophilic N-CF3S-saccharin (2) under conditions suitable to our previous reactions on the CF3S amination of alkenes in the presence of H2O,8a the desired product was observed in trace amounts, even in different conventional solvents. We realized that the former conditions were acidic and disfavored the formation of product. The main reasons might lie in the two issues: (i) H2O is a weak nucleophile and acid can further weaken the nucleophilicity of H2O, and (ii) competitive elimination promoted by acid could occur to form allyl products instead of difunctionalization products particularly when an alkyl group is connected to the double bond of the substrate.11a,13 Considering the above difficulties, we turned our attention to conducting the reaction without the addition of extra acid (Table 1). Not surprisingly, when 1a reacted with CF3S reagent 2 in the presence of H2O using electron-rich diaryl selenide as the catalyst in various common solvents such as MeCN, dichloromethane, THF, and toluene, the desired product 3a

rganoselenium catalysis has received considerable attention from chemists because of its merits in organic synthesis: mild reaction conditions, good functional group tolerance, and excellent selectivities.1 Furthermore, some transformations which are difficult to achieve by other methods could be accomplished by selenium catalysis.2 As an important part of this field, systems that merge a catalytic amount of an organoselenium compound and oxidants, i.e., N-halosuccinimide,3 peroxides,4 and fluoropyridinium salts,2d have been frequently utilized for the synthesis of various valuable compounds by delivering oxygen and halogen into substrates. In these redox catalytic cycles, the selenium valence changes between divalence and tetravalence. Using organoselenium compounds as Lewis base catalysts is another important part of selenium catalysis.5−8 The relevant studies are most focused on the functionalization of alkenes. In such catalysis, organoselenium compounds activate electrophilic reagents such as halogenating and sulfenylating ones to form selenium-captured ionic complexes, which trigger the following alkene functionalization. Despite great achievements in this direction, developing new reaction systems to solve challenging synthesis is still highly desirable. Herein, we report our discovery that the combination of Lewis basic selenium catalysis and redox selenium chemistry enables difunctionalization of multisubstituted alkenes to efficiently produce trifluoromethylthiolated tertiary alcohols under oxidative conditions. The efficient synthesis of CF3S compounds has attracted much attention in recent years9 and is quite important for the discovery of pharmaceuticals and agrochemicals because a fluorinated moiety can adjust the chemical, physical, and biological properties of parent molecules.10 Until now, many approaches have been developed to construct diverse CF3S molecules.8,11,12 Among the developed methods, difunctional© 2017 American Chemical Society

Received: August 3, 2017 Published: September 6, 2017 4940

DOI: 10.1021/acs.orglett.7b02406 Org. Lett. 2017, 19, 4940−4943

Letter

Organic Letters Table 1. Condition Optimizationa

Scheme 1. Substrate Scope of Oxytrifluoromethylthiolation of Alkenesa

entry

cat.

solvent

atmosphere

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15c

C1 C1 C1 C1 C1 C2 C3 C4 C5 C6 C7 none C1 C1 C1

MeCN CH2Cl2 THF toluene MeNO2 MeNO2 MeNO2 MeNO2 MeNO2 MeNO2 MeNO2 MeNO2 MeNO2 MeNO2 MeNO2

under air under air under air under air under air under air under air under air under air under air under air under air under N2 O2 balloon O2 balloon

0 0 0 0 74 28 50 47 58 0 0 0 17 87 (83) 52

a

Conditions: 1a (0.05 mmol), 2 (1.3 equiv), H2O (5 equiv), catalyst (10 mol %), solvent (1 mL), room temperature, 24 h. bRefers to NMR yield using CH2Br2 as the internal standard. Isolated yield on 0.2 mmol scale is in the parentheses. cNo additional H2O.

a Conditions: alkene (0.2 mmol), 2 (1.3 equiv), H2O (5 equiv), C1 (10 mol %), MeNO2 (4 mL), O2 balloon, room temperature, 24 h. b From 2-phenyl-2-butene (Z/E = 2.7:1). cFrom (Z)-oct-4-ene.

was not formed (entries 1−4). To our delight, the use of MeNO2 as the solvent allowed the formation of product 3a in good yield (entry 5). Encouraged by this outcome, further optimization was carried out using MeNO2 as the solvent. Several other selenium catalysts were screened (entries 6−11), but the yield of 3a did not improve. It was found that less and more electron-rich selenide catalysts led to lower yields than catalyst C1 or even no product. The results indicate that the efficiency of trifluoromethylthiolation depends on the catalyst, with the appropriate electron density on the selenium atom being essential. Product was not observed when the reaction was performed in the absence of selenide (entry 12). Interestingly, when the reaction was carried out under an N2 atmosphere, the product was obtained in only 17% yield (entry 13). In contrast, the yield improved to 87% under an O2 atmosphere (entry 14). These results show that O2 plays an important role in the reaction. Furthermore, when no extra water was added to the reaction, the product was still produced in 52% yield (entry 15). The required trace water for the reaction might come from solvent. With the optimal conditions in hand, the substrate scope was evaluated (Scheme 1). 1,1-Aryl,methyl-substituted alkenes underwent difunctionalization to generate the corresponding tertiary alcohols in 38−98% isolated yields (3b−l). An electronwithdrawing group on the phenyl ring did not affect the reaction much, but electron-rich aryl groups on substrates lowered the yields of products (3k, 38%; 3l, 57%). A variety of styrene derivatives with different substituents such as phenyl,

ethyl, propyl, isopropyl, and benzyl at the α position worked efficiently to give the corresponding products in good yields (3m−q, 73−87%). This method is effective for other 1,1dialkyl-substituted alkenes as well (3r, 52%). Other types of alkenes such as tri-, tetra-, mono-, and 1,2-disubstituted ones were tested under similar conditions. They afforded the desired products in moderate to good yields (3s−v, 45−81%). It is noteworthy that a sterically hindered tetrasubstituted alkene still underwent difunctionalization to form the desired product in moderate yield. Similar conditions proved suitable for the intramolecular cyclization of nucleophile-tethered alkenes (Table 2). All of the alkenes with carboxylic acid, hydroxy, sulfamide, and ester groups worked efficiently to give the corresponding products in good to excellent yields. The selectivity was excellent for the reactions. For example, when the olefinic carboxylic acid 4b was utilized as the substrate, only the 5-exo-trig cyclization product 5b was formed in good yield (entry 2). In addition, by this method, trifluoromethylthiolated tetrahydrofuran derivative 5d and pyrrolidine derivative 5e could be prepared in high yields (entries 4 and 5). Interestingly, olefinic ester 4f underwent the cyclization to afford product 5a, which could be formed from the reaction of acid 4a under similar conditions. It is noted that hydroxylated products were not observed in these transformations. The results indicate that this new method has good generality. 4941

DOI: 10.1021/acs.orglett.7b02406 Org. Lett. 2017, 19, 4940−4943

Letter

Organic Letters Table 2. CF3S-Lactonization, -Etherification, and -Amination of Nucleophile-Tethered Alkenesa

Scheme 3. Mechanistic Exploration

selenoxide 6 was formed in 2% NMR yield. This oxidation occurred only in MeNO2. Other solvents such as dichloromethane and acetonitrile were not effective at all. Selenoxide 6 could be completely reduced to selenide C1 along with the formation of TfOH and saccharin in less than 10 min when it was treated with CF3S reagent (Scheme 3b). TfOH was detected by 19F NMR. In our previous study,8 acid was essential to assist the selenide catalyst in activating the CF3S reagent for difunctionalization of the alkenes. The formation of trace TfOH could be the reason that the selenide/O2/MeNO2 system is efficient for oxytrifluoromethylthiolation of alkenes. When selenoxide 6 served as the catalyst precursor, product 3a was still formed under N2 atmosphere in moderate yield, which further proved the reduction of selenoxide 6 in the reaction (Scheme 3c). To elucidate the role of acid in these reactions, they were carried out under an N2 atmosphere in the presence of acid. When 1 mol % of TfOH was added to the reaction, product 3a could be generated in 70% yield in MeNO2 (Scheme 3d). Under similar acidic conditions, the replacement of MeNO2 with dichloromethane as the solvent led to no product 3a at all and some allylic byproducts. Higher loading of acid resulted in a lower yield. These results reflect that appropriate loading of acid is important for the reaction. On the basis of these mechanistic studies, a plausible reaction pathway is proposed as shown in Scheme 4. Selenide C1 is first oxidized under O2/MeNO2 conditions to produce the diaryl selenoxide 6, after which the selenoxide is reduced to C1 by

a

Conditions: alkene (0.1 mmol), 2 (1.3 equiv), C1 (10 mol %), MeNO2 (2 mL), O2 balloon, room temperature, 24 h. bIsolated yield.

To address why this transformation needs oxygen and where the hydroxy group comes from in the products, labeling experiments were conducted with alkene 1l as the substrate (Scheme 2). When the reaction of 1l with 2 was performed Scheme 2. Labeling Experiments

Scheme 4. Proposed Reaction Mechanism under an 18O2 atmosphere, in the presence of unlabeled H2O, the alcohol product 3l was obtained without 18O labeling. In contrast, when the reaction was run with unlabeled O2 in the presence of H218O, the product was formed in 90% isotopic purity with 18O labeling. These results clearly indicate that the hydroxyl group in the products comes from water, not from oxygen. The outcome rules out the hydroxy group originating from O2 via a radical pathway.14 To further elucidate the role of O2 and the reaction mechanism, more control experiments were carried out (Scheme 3). It was found that electron-rich diaryl selenide C1 could be oxidized under O2/MeNO2 conditions (Scheme 3a). The oxidation process was very slow. In 24 h, diaryl 4942

DOI: 10.1021/acs.orglett.7b02406 Org. Lett. 2017, 19, 4940−4943

Letter

Organic Letters

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CF3S reagent 2 along with the formation of TfOH. Selenide C1 and the acid coactivate the CF3S reagent to form intermediate I. Formation of the episulfonium ion II and attack by H2O ultimately leads to product 3 and regenerates catalyst C1 and TfOH. In summary, we have developed an efficient approach of selenide-catalyzed vicinal CF3S hydroxylation of alkenes under unique O2/MeNO2 conditions. The desired products were obtained in moderate to excellent yields. The developed system merged redox selenium chemistry into Lewis basic selenium catalysis to enable the challenging CF3S hydroxylation of multisubstituted alkenes. Mechanistic studies have proven that the redox cycle between Se(II) and Se(IV) is crucial for the whole transformation.

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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/acs.orglett.7b02406. Experimental details, characterization data, and NMR spectra of new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaodan Zhao: 0000-0002-2135-5121 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Sun Yat-Sen University, the “One Thousand Youth Talents” Program of China, and the Natural Science Foundation of Guangdong Province (Grant No. 2014A030312018) for financial support.

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DOI: 10.1021/acs.orglett.7b02406 Org. Lett. 2017, 19, 4940−4943