Cyclization Strategy To

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An External-Catalyst-Free Trifluoromethylation/Cyclization Strategy To Access Trifluoromethylated-Dihydroisoquinolinones/Indolines with Togni Reagent II Junchao Wang,†,⊥ Kai Sun,†,⊥ Xiaolan Chen,*,†,‡ Tong Chen,§ Yan Liu,† Lingbo Qu,† Yufen Zhao,†,‡ and Bing Yu*,† †

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China The Key Laboratory for Chemical Biology of Fujian Province, Xiamen University, Xiamen 361005, China § High and New Technology Research Center of Henan Academy of Sciences, Zhengzhou 450001, China Org. Lett. Downloaded from pubs.acs.org by WASHINGTON UNIV on 03/06/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: A novel and efficient CF3 radical-involved externalcatalyst-free trifluoromethylation/cyclization methodology to access a group of new trifluoromethylated dihydroisoquinolinones was developed, by reacting different N-allylbenzamides with Togni-II in one pot under mild reaction conditions. Meanwhile, this external-catalyst-free trifluoromethylation/cyclization protocol was also well suitable for being employed to synthesize many valuable trifluoromethylated N-acetylindolines, by reacting N-arylN-allylacetamides with Togni-II. In both reactions, the amide groups of reactants themselves acted as the catalysts to promote the generation of the CF3 radical required for the following radical-cascade trifluoromethylation/cyclization reactions.

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rifluoromethylated compounds have been targeted increasingly in the fields of agrochemicals, organic materials, and, in particular, pharmaceuticals over the past century.1 The trifluoromethyl moiety is found in many notable synthetic drugs, leading to altered physical and physiological behaviors of those compounds with respect to their uptake and metabolism.2 Hence, the development of synthetic methods for incorporation of CF3 groups into biologically and materially valuable compounds is actively pursued in academic research, as it is considerably absent in nature.3 The rapid development of trifluoromethylation reactions over the past decades has largely been driven by the discovery of novel trifluoromethylating reagents.4 Among them are Ruppert−Prakash reagent (TMSCF3) which has been widely used as a nucleophilic trifluoromethylating reagent since reported by Ruppert and coworkers in 1984;5 Langlois reagent (CF3SO2Na), introduced by Langlois and co-workers in 1991 for trifluoromethylation of aromatics;6 trifluoromethanesulfonyl chloride (CF3SO2Cl), which has been widely employed to introduce the CF3 group into aromatic or heteroaromatic systems in photoredox catalysis since introduced by MacMillan et al. in 2011;7 Umemoto reagents (Figure 1), which have been frequently used as electrophilic trifluoromethylating reagents to react with a wide range of nucleophiles including carbanions, silyl enol ethers, enamines, anilines, phenols, phosphines, and thiolates since being developed by Umemoto and co-workers in 1990;8 and Togni reagents (Figure 1) which had been developed by Togni and co-workers9 as convenient electrophilic CF3 sources for selective trifluoromethylation of active methylene compounds, thiols, phenols, and alkylphosphines from 2006 to © XXXX American Chemical Society

Figure 1. Umemoto and Togni reagents.

2008.10 Among all the CF3 sources mentioned above, the laterdeveloped Togni-II is especially attractive and worth mentioning, owing not only to its easy preparation, preservation, and commercially less expensive price (about $62/g) in comparison with Togni-I (about $190/g) but also to its explosive applications in the later on developed CF3 radicalinvolved trifluoromethylation reactions from 2011 to present. In 2011, Buchwald’s group developed a copper-catalyzed allylic trifluoromethylation strategy by reacting Togni-II with terminal olefins using [(MeCN)4Cu]PF6 as the catalyst.11 Interestingly, Wang’s group in the same year reported their independently developed similar trifluoromethylation reaction, with the exception of choosing CuCl rather than [(MeCN)4Cu]PF6 as the catalyst.12 Additionally, Wang’s group went further to prove that Togni-II could react with TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), a well-known radical scavenger, to produce TEMPO−CF3, eventually providing supportive evidence that the CF3 radical was indeed generated in the reaction process. Following this academic Received: February 4, 2019

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DOI: 10.1021/acs.orglett.9b00465 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

Scheme 1-I. As shown in Scheme 1-I and -II, the single electron donors (SEDs) in fact acted as the catalysts in those CF3-radical-initiated cascade trifluoromethylation/cyclization reactions. They include the catalysts we just mentioned in Scheme 1a−d. Over the past decade, the rapid development of Togni-II-involved trifluoromethylation reactions has always been accompanied by the discovery of reliable SEDs. In comparison with the widely used transition-metal SEDs, the later developed organic base catalysts, such as DIPEA,17 dppBz,18 and DBN17,19 reported by Liu’s group in 2015 and 2016, exhibited a special advantage due to the capacity of tolerating different kinds of functional groups that otherwise would form a strong coordination with the transition-metal catalysts to impede the reaction efficiency.17 It is especially worth noticing that the previous Togni-II-involved radical trifluoromethylation reactions usually required the external SEDs, those metal- or organic-based catalysts mentioned in Scheme 1-I. Herein, we would like to present a novel externalcatalyst-free trifluoromethylation/cyclization strategy. As illustrated in Scheme 2-I, reactant A is an aromatic amide attached

turning point, a prominent development in CF3 radical initiated cascade cyclization reactions for the concise construction of diverse trifluoromethyl moiety-containing heterocycles and carbocycles by using Togni-II as a reliable CF3 radical source has been witnessed.13 Those cascade reactions were able to incorporate the CF3 group simultaneously into a cyclic ring construction in one synthetic step under mild reaction conditions, showing remarkable operational simplicity and atom/step economy. Among numerous such examples are copper-catalyzed trifluoromethylation/ cyclization to access trifluoromethylated carbocycles and Nheterocycles, a milestone work developed by Sodeoka’s group in 2013 (Scheme 1a);14 Bu4NI-catalyzed preparation of Scheme 1. External Single-Electron-Donor (SED)-Catalyzed Cascade Trifluoromethylation/Cyclization with Togni-II

Scheme 2. An External-Catalyst-Free Catalyzed Cascade Trifluoromethylation/Cyclization with Togni-II

with an unconjugated double bond. We have a good reason to believe that it is the internal amide group in A that acts as the SED of Togni-II to initiate the formation of CF3 radical along with the 2-iodobenzoate anion (G), reasonably attributing to the easy formation of the resonance stabilized A•+. Afterward, the CF3 radical is added to the unactivated double bond of another A, giving a radical intermediate B. Then, via a SET process from radical B to A•+, cation intermediate C is formed, accompanied by regeneration of A. Alternatively, through another SET process from radical B to Togni-II, cation C can also possibly be formed along with the formation of the CF3 radical and anion G. The final trifluoromethylated heterocycle D is obtained after deprotonation of C. It is well-known that heterocycles are essential structural skeletons existing in abundant pharmaceutically and biologically valuable molecules.20 Dihydroisoquinolinone-containing heterocycles are prevalent in natural products, agro-

trifluoromethylated phenanthridines, another attractive work reported later in the same year by Studer’s group (Scheme 1b);15 fac-Ir(ppy)3-catalyzed synthesis of CF3-substituted dihydroquinolines and 1-azaspiro[4.5]decanes reported by Xia’s group in 2015 (Scheme 1c);16 and DIPEA/DBNcatalyzed chemoselective carbotrifluoromethylation and oxytrifluoromethylation by tuning the catalysts and solvent system, reported by Liu’s group in 2016 (Scheme 1d).17 It is especially interesting to observe from the numerous published Togni-II-involved radical trifluoromethylation reactions that the formation of the free CF3 radical was all achieved via a single electron transfer (SET) from an external single electron donor (SED) to Togni-II, as illustrated in B

DOI: 10.1021/acs.orglett.9b00465 Org. Lett. XXXX, XXX, XXX−XXX

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tron-withdrawing (−Cl, −Br, and −I) and electron-donating (−Me) substituents at the 4-position of benzene rings, reacted with Togni-II smoothly under the optimized reaction conditions, giving the resulting 4-substituted products in good to excellent yields (3a−e). No obvious electronic effects were observed in those just mentioned cases (3a−e). Meanwhile, N-methyl-N-(2-methylallyl)benzamides bearing −Me and −OMe at the 2-position and −Cl and −Br at the 3-position of benzene rings reacted with Togni-II smoothly, rendering products 3f−i in moderate to good yields. Finally, benzamides attached with different N-alkyl or -allyl substituents were employed to react with Togni-II, giving all the corresponding products (3j−n) in satisfactory yields. The indolic moiety is a core structure of many drugs and exists in many naturally occurring alkaloids.23 In the following research, we attempted to extend this external-catalyst-free Togni-II-involved trifluoromethylation/cyclization protocol to the synthesis of a number of biologically valuable trifluoromethylated N-acetylindolines, by reacting various N-aryl-Nallylacetamides with Togni-II under the optimized conditions (Scheme 4). As it can be seen from cases 5a−j, a group of N-

chemicals, and pharmaceuticals, and they are well-known for their fascinating pharmacological activities, such as antitumor, antiallergic, anti-inflammatory, and their widespread application in cancer diagnosis.21 As part of our continuing interest in radical reactions,22 herein, we disclose a novel and efficient external-catalyst-free Togni-II-involved radical trifluoromethylation/cyclization methodology to access various trifluoromethylated dihydroisoquinolinones, by reacting different Nallylbenzamides with Togni-II in one pot under mild reaction conditions. Meanwhile, this protocol was also well suitable to synthesize various trifluoromethylated N-acetylindolines, by reacting N-aryl-N-allylacetamides with Togni-II (Scheme 2a). In both reactions, the amide groups of reactants themselves acted as the catalysts to initiate the formation of the CF3 radical required for the following radical-cascade trifluoromethylation/cyclization reactions. We started the meaningful research from establishing optimal experimental conditions using the model reactions of N-methyl-N-(2-methylallyl)benzamide 1a with Togni-II in one pot for 12 h, as summarized in Table S1. After extensive experimentation, the optimized reaction conditions were established as follows: 1a (0.5 mmol), Togni-II (1.5 equiv), and NaOAc (2 equiv) were mixed in DMF (3 mL) at 80 °C for 12 h in one pot. We concluded from our screening results that the yields of product 3a could always be significantly improved in the absence of CuI, compared to those under the reaction conditions with the presence of CuI, basically ruling out the necessity of adding any external previously reported catalysts, those metal- or organic-based catalysts. With the optimized conditions in hand, we then explored the substrate scope of this external-SED-free protocol for the synthesis of various trifluoromethylated dihydroisoquinolinones, by examining different substituted N-allylbenzamides (1). The results are summarized in Scheme 3. As can be seen from cases 3a−e, a group of starting N-methyl-N-(2methylally)benzamides, mainly including those bearing elec-

Scheme 4. Substrate Scope for the Synthesis of Substituted Indolinesa

Scheme 3. Substrate Scope for the Synthesis of Substituted 3,4-Dihydroisoquinolin-1(2H)-onesa a

Reaction conditions: 4 (0.5 mmol), Togni-II (1.5 equiv), NaOAc (2 equiv), DMF (3 mL), 80 °C, 12 h. Isolated yields were given.

(2-methylallyl)-N-phenylacetamides, mainly including those bearing electron-withdrawing (−F, −Cl, −Br, −I, and −CF3) and electron-donating (−Me, −OMe, −iPr, and −tBu) substituents at the 4-position of benzene rings, reacted smoothly with Togni-II, giving the resulting 4-substituted products 5a−j in good to excellent yields. No obvious electronic effects were observed in those cases (5a−j). To our delight, the fragile methylthio group in starting N-(2methylallyl)-N-(2-methylthiophenyl)acetamide survived under the reaction conditions, rendering the product 5k in a moderate yield. Moreover, N-allyl-N-phenylacetamide was also applicable for this reaction, affording product 5l in a good yield. With respect to the synthetic method toward trifluoromethylated N-acetylindolines, Liang’s group just reported a cascade synthetic method to access some trifluoromethylated N-acetylindolines but using CF3SO2Na rather than Togni-II as the trifluoromethylating reagent; however, a strong oxidant (1.5 equiv of K2S2O8) was required by this reaction for oxidizing CF3SO2Na to release SO2 and the

a

Reaction conditions: 1 (0.5 mmol), Togni-II (1.5 equiv), NaOAc (2 equiv), DMF (3 mL), 80 °C, 12 h. Isolated yields were given. C

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Organic Letters CF3 radical.24 In contrast, without addition of any additional oxidant, our newly developed external-catalyst-free trifluoromethylation/cyclization methodology allowed more fragile Naryl-N-allylacetamides to be employed as our reactants, such as the one attached with the methylthio group, showing high functional tolerance. Additionally, all our related syntheses toward trifluoromethylated N-acetylindolines were achieved within half the reaction time of what Liang’s method required, showing a much higher reaction efficiency. To provide supportive evidence for our proposed externalcatalyst-free CF3-radical-involved trifluoromethylation/cyclization mechanism, two control experiments were conducted initially (Scheme 5). TEMPO and BHT (2,6-di-tert-butyl-4-

4) that acted as SEDs to promote the decomposition of TogniII to the CF3 radical needed. Thus, extra control experiments to support the reliability of the hypothesis were pursued and carried out as illustrated in Scheme 4c. We mixed N,Ndimethylbenzamide, 1,1-diphenylethylene, and Togni-II in CH3CN and kept the solution refluxing for 12 h at 80 °C. Afterward, trifluoromethyl substituted 1,1-diphenylethylene 8 was successfully separated and identified (Scheme 4c-i). In contrast, no product 8 could be gained after keeping 1,1diphenylethylene and Togni-II refluxing in CH3CN for 12 h at 80 °C (Scheme 5c-ii), strongly suggesting N,N-dimethylbenzamide was capable of acting as the SED of Togni-II to produce the CF3 radical required (Scheme 5c-iv). Furthermore, in order to determine whether or not DMF could act as the SED of Togni-II under our reaction conditions, we conducted a control experiment using solvent DMF instead of CH3CN (Scheme 5c-iii). We mixed 1,1-diphenylethylene and Togni-II in DMF and kept the solution refluxing for 12 h at 80 °C. However, the 19F NMR of the final reaction mixture indicates that no product 8 was produced in this case (details see the Supporting Information), suggesting that solvent DMF could not act as the SED of Togni-II under these reaction conditions. Thus, a decisive conclusion can be reached that it is indeed the internal aromatic amide groups in the reactants 1 and 4 that acted as SEDs to initiate the decomposition of Togni-II to the CF3 radical needed for the following trifluoromethylation/cyclization reactions. This conclusion was further evidenced by our especially designed experiments with the aid of EPR technology (for details see the Supporting Information). In conclusion, we developed a novel and efficient externalcatalyst-free trifluoromethylation/cyclization methodology to access a group of trifluoromethylated dihydroisoquinolinones, by reacting different N-allylbenzamides with Togni-II in one pot under mild reaction conditions. Meanwhile, this externalcatalyst-free trifluoromethylation/cyclization protocol was also well suitable for being employed to synthesize many biologically valuable trifluoromethylated N-acetylindolines, by reacting N-aryl-N-allylacetamides with Togni-II in one pot under mild reaction conditions, exhibiting good functional group tolerance and reaction efficiency. The discovery that the internal amide groups in N-allylbenzamides and N-aryl-Nallylacetamides can promote the decomposition of Togni-II and both the reactants thus can act as their own catalysts toward the syntheses of two biologically important heterocycles, trifluoromethylated dihydroisoquinolinones and Nacetylindolines, opens the possibility to access much more structurally diverse trifluoromethylated heterocycles by means of this newly developed efficient and environmentally sustainable strategy.

Scheme 5. Control Experiments

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methylphenol), two commonly used radical scavengers, were subjected to the reaction under standard conditions, respectively. As a result, the reactions toward 3a were totally suppressed, suggesting that the cascade reaction may involve radical processes (Scheme 5a and b). It is worth mentioning that, after the control reaction was performed with BHT under optimized conditions (Scheme 5b), product 6 was successfully isolated and identified, indicating the CF3 radical was produced from Togni-II and trapped by BHT (for details see the Supporting Information).25 However, we were not satisfied with only carrying out those two control experiments, because we still lacked reliable evidence to support our hypothesis that it is the internal aromatic amide groups in the reactants (1 and

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00465. Experimental details and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. D

DOI: 10.1021/acs.orglett.9b00465 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters ORCID

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Kai Sun: 0000-0003-2135-0838 Xiaolan Chen: 0000-0002-3061-8456 Yufen Zhao: 0000-0002-8513-1354 Bing Yu: 0000-0002-2423-1212 Author Contributions ⊥

J.W. and K.S. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Centre of Advanced Analysis & Computational Science (Zhengzhou University) and financial support from National Natural Science Foundation of China (21501010), the 2017 Science and Technology Innovation Team in Henan Province (22120001).

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DOI: 10.1021/acs.orglett.9b00465 Org. Lett. XXXX, XXX, XXX−XXX