Cyclization Strategy

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Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Visible-Light Induced Radical Perfluoroalkylation/Cyclization Strategy To Access 2‑Perfluoroalkylbenzothiazoles/ Benzoselenazoles by EDA Complex Yan Liu,†,‡ Xiao-Lan Chen,*,†,§ Kai Sun,† Xiao-Yun Li,† Fan-Lin Zeng,† Xiao-Ceng Liu,† Ling-Bo Qu,† Yu-Fen Zhao,†,§ and Bing Yu*,† †

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China College of Biological and Pharmaceutical Engineering, Xinyang Agriculture & Forestry University, Xinyang 464000, China § The Key Laboratory for Chemical Biology of Fujian Province, Xiamen University, Xiamen 361005, China

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‡

S Supporting Information *

ABSTRACT: A novel and practical fluoroalkyl radical-initiated cascade reaction was developed to access diverse 2-fluoroalkylbenzothiazoles by reacting various fluoroalkyl radical sources, including perfluoroalkyl iodide (ICnF2n+1, n = 3−8, 10), ICF(CF3)2, ICF2COOEt, ICF2CF2Cl, or ICF2CF2Br, tetramethylethane-1,2-diamine (TMEDA), and 2-isocyanoaryl thioethers in tetrahydrofuran under nitrogen atmosphere and blue-light irradiation conditions. Furthermore, this one-pot protocol could well be expanded to access various 2-fluoroalkylbenzoselenazoles starting from (2-isocyanophenyl)(methyl)selane, perfluoroalkyl iodides (ICnF2n+1, n = 3−8) or ICF2COOEt and TMEDA.

F

luorine-containing organic compounds have been targeted increasingly in the fields of pharmaceuticals, agrochemicals, and materials science over the past century.1 The fluorinated alkyl moieties can be found in many drugs, as the incorporation of fluorinated alkyl groups into medicinal agents could dramatically enhance their physicochemical and biological properties such as lipophilicity, bioavailability and metabolic stability, hence leading to significant clinical results of them.2 In comparison with the introduction of the trifluoromethyl group into biologically and pharmaceutically valuable molecules, the introduction of perfluoroalkyl (CnF2n+1, n ≥ 2) moieties as potential pharmacophores has remained considerably less explored. However, the development of new synthetic methodologies for incorporation of perfluoroalkyl groups (Rf) into different functionalities would open up additional opportunities for chemists to synthetically access much more biologically and materially potential compounds. Benzothiazole is a privileged structural moiety observed in numerous drugs, natural products, fluorescence probes, biological dyes, and agrochemicals.3 Numerous benzothiazoles attached with a trifluoromethyl substituent at the 2-position have been synthesized over the past half-century. Figure 1 shows six benzothiazoles attached with a trifluoromethyl group at the 2position, all of which exhibit remarkable pharmacological properties, such as anti-inflammatory, anti-Alzheimer, antibacterial, antitumor, and anticardiovascular disorder properties.4 Five typical synthetic methodologies previously published concerning the synthesis of 2-trifluoromethyl substituted benzothiazoles are shown in Scheme 1a−e, including condensation of 2-aminothiophenols with trifluoroacetic acid, which is a traditional method toward 2-trifluoromethyl substituted benzothiazoles, as © XXXX American Chemical Society

Figure 1. Representative biologically active 2-trifluoromethylbenzothiazoles.

exampled by Scheme 1a;5 transition-metal-catalyzed annulation of trifluoromethylimidoyl chlorides with hydrosulfide hydrate (Scheme 1b);6 intramolecular oxidative cyclization of trifluoromethylated thiobenzanilides (Scheme 1c);7 Sandmeyer trifluoromethylation as exampled by Scheme 1d;8 and photocatalytic CF3 radical-initiated cascade cyclization by reaction of 2-isocyanoaryl thioethers with Umemoto reagent (Scheme 1e).9 Notably, even though the synthetic methods toward 2trifluoromethyl substituted benzothiazoles have extensively been explored, while in sharp contrast, the synthetic method toward 2-perfluoroalkyl substituted benzothiazoles has considerably lagged behind. Regrettably, so far, only one 2perfluoroalkyl-substituted benzothiazole, 2-perfluorobutylbenzothiazole, has been synthesized by Hirano, Uchiyama, and coReceived: April 6, 2019

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DOI: 10.1021/acs.orglett.9b01175 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 1. Previous and Current Synthetic Methods

Scheme 2. Two Types of EDA Complexes Acting as VisibleLight Absorbances

donor−acceptor (EDA) complex is actually formed in situ by two structurally small reactant molecules, one acting as electron donor (D) and the other acting as electron acceptor (A). Upon light irradiation, an excited EDA complex ([EDA]*) is formed. After that, the reactive [EDA]* complex cleaves to form a radical pair ([D+•/A−•]). The later generated radical cation D+• and radical anion A−• can subsequently undergo different pathways such as additions, eliminations, rearrangements, etc.14 It can be seen from Scheme 2 that the overall process of the formation of D+• and A−• involves a single electron transfer (SET) process from D to A. In fact, Scheme 2a represents only one type of EDA complex reported previously. Scheme 2b shows another type of EDA complex. In this case, BL functions as the electron acceptor (A) of EDA complex. BL is finally transferred into a neutral radical (B•) along with an anion (L−).15 Nowadays, development of synthetic methodologies enabled by light-irradiation of EDA complexes is being enthusiastically pursued in organic chemistry. An obvious advantage of those methodologies is that they did not need any external added photocatalysts, in particular those expensive and potentially toxic metal complexes, exhibiting an attractive environmentally friendly feature. During the past decade, an explosive development of CF3 radical initiated cascade cyclization reactions, aiming at concise construction of structurally diverse CF3-substituted heterocycles and carbocycles, has been witnessed.16 Those cascade cyclization reactions were able to incorporate the trifluoromethyl group into the cyclic ring construction within one reaction step, exhibiting high atom-/step-economy and operational simplicity. As part of our continuous interest in the development of convenient radical-involved reactions,17 we herein disclose an efficient fluoroalkyl radical-initiated fluoroalkylation/cyclization methodology to access diverse 2-fluoroalkylbenzothiazoles, by reacting commercially available fluoroalkyl radical sources, including perfluoroalkyl iodide (ICnF2n+1, n = 3−8, 10), ICF(CF3)2, ICF2COOEt, ICF2CF2Cl or ICF2CF2Br, tetramethylethane-1,2-diamine (TMEDA), and structurally simple 2isocyanoaryl thioethers in tetrahydrofuran (THF) under nitrogen atmosphere and blue-light irradiation conditions. Meanwhile, this protocol was well-suited to be expanded to access structurally novel and biologically potential 2-fluoroalkylbenzoselenazoles, starting from (2-isocyanophenyl)(methyl)selane, perfluoroalkyl iodides (ICnF2n+1, n = 3−8) or ICF2COOEt and TMEDA, as illustrated in Scheme 1g. In both reactions, the perfluoroalkyl radicals required for the following radical-cascade perfluoroalkylation/cyclization reactions were generated from EDA complexes formed in situ from fluoroalkyl iodides (electron acceptors) and TMEDA (electron donor). We initiated our study by establishing optimal experimental conditions using the model reaction of (2-isocyanophenyl)(methyl)sulfane (1a) with C4F9I (2d) in the presence of various

workers, via their developed diethylzinc-mediated crosscoupling reaction of 2-iodobenzothiazole with perfluorobutyl iodide in the presence of CuI catalyst with argon protection, as illustrated in Scheme 1f, in which relatively expensive 2iodobenzothiazole as well as unstable diethylzinc were employed.10 It is well-known that diethylzinc is an organometallic compound that reacts violently with water, ignites upon contact with air, and needs to be handled in an inert atmosphere. Thus, exploring an economically and feasibly synthetic methodology to access more structurally diverse 2-perfluoroalkylbenzothiazoles remains urgent and necessary. One of the major targets in modern synthetic chemistry is to develop green synthetic methods that take into account environmental impacts in the selection of reactants and reaction conditions. The use of visible-light to promote chemical processes is increasingly growing in importance as it represents an environmentally friendly alternative to the existing traditional synthetic methods.11 However, as matter of fact, most organic molecules do not absorb radiations in visible region. Therefore, an external photocatalyst (e.g., ruthenium and iridium polypyridyl complexes or organic dyes) is usually required to absorb light in the ultraviolet−visible spectral range,12 while, over the past decade, many electron donor−acceptor (EDA) complexes have been found to be capable of acting as visible light absorbances.13 As illustrated in Scheme 2a, the electron B

DOI: 10.1021/acs.orglett.9b01175 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

A gram-scale experiment toward perfluorobutylbenzothiazole (3ad) was then carried out under irradiation of blue LEDs at room temperature for 1 h via our specially designed reactor (for more details, see Figure S1). We were delighted to find that 3ad was obtained in 70% isolated yield. Selenium is a chemical element with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium. Selenium is an important ingredient existing in many multivitamins and dietary supplements. It is also a component of the antioxidant enzymes (glutathione peroxidase and thioredoxin reductase) and three deiodinase enzymes.18 Over the past few years, a number of synthetic protocols have been developed for accessing structurally new organoselenium compounds, some of which are potentially valuable drug candidates.19 Much regrettably, so far, no synthetic methodologies toward fluoroalkyl groupcontaining organoselenium heterocycles have been found reported in the literature. Much to our delight, this newly developed protocol could well be extended to reach a number of 2-fluoroalkylbenzoselenazoles (5) by reacting perfluoroalkyl iodides (ICnF2n+1, n = 3−8) or ICF2COOEt (2), TMEDA, and (2-isocyanophenyl)(methyl)selane (4a), under irradiation of 25 W blue LEDs with N2 protection at 30 °C for 12 h, as illustrated in Scheme 4. The corresponding products 5ac−5al were gained in

bases under irradiation of 25 W LEDs with N2 protection at 30 °C, as summarized in Table S1. After an extensive experimentation, the optimized reaction conditions were established as follows: 1a (0.2 mmol), 2d (2.0 equiv), and TMEDA (2.0 equiv) were mixed in THF (2 mL) under irradiation of 25 W blue LEDs with N2 protection at 30 °C for 1 h. Having the optimal conditions in hand, we next examined the reaction scope by reacting a wide range of fluoroalkyl iodides 2, TMEDA, and various (2-isocyanophenyl)(alkyl)sulfanes 1, as summarized in Scheme 3. We appropriately expanded some Scheme 3. Radical-Initiated Cascade Reaction toward 2Perfluoroalkylbenzothiazolesa

Scheme 4. Radical-Initiated Cascade Reaction Toward 2Perfluoroalkylbenzoselenazolesa

a

Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), and TMEDA (0.4 mmol) in THF (2 mL) under irradiation of 25 W LEDs with N2 protection at 30 °C for 1 h. R = Me unless noted otherwise. Isolated yields were given. bReaction time was 3 h. cReaction time was 6 h. d Reaction time was 12 h. eR = Et. fR = Ph. gR = Et.

reaction time based on TLC tracing results as specified in Scheme 3. As it can be seen, seven perfluoroalkyl iodides (ICnF2n+1, n = 3− 8, 10) and four other fluoroalkyl iodides, including ICF(CF3)2, ICF2COOEt, ICF2CF2Cl, and ICF2CF2Br, reacted smoothly with (2-isocyanophenyl)(methyl)sulfane (1a) itself, giving the resulting 2-fluoroalkylbenzothiazoles (3ac−3an) in good to excellent yields. Meanwhile, a group of (2-isocyanophenyl)(methyl)sulfanes bearing different substituents on the benzene ring reacted well with C4F9I, affording the corresponding fluoroalkylbenzothiazoles (3bd−3jd) in moderate to excellent yields. Electronic effects were examined via cases of 3bd−3gd. The results showed that (2-isocyanophenyl)(methyl)sulfanes bearing electron donating groups (Me, OMe, and OEt) at 5position of benzene ring did not make obvious differences in yield (3bd, 87%; 3cd, 93%; 3cd, 88%), compared with that of 3ad (92%) resulted from using an unsubstituted reactant (1a); however, in contrast, (2-isocyanophenyl)(methyl)sulfanes bearing electron-withdrawing halo groups (F, CI, and Br) at 5position of benzene ring brought about relatively low yields of the corresponding products (3ed−3gd). It is worth noting that 2isocyanophenylsulfanes attached with ethylthio, phenylthiol, and benzylthiol groups (SEt, SPh, and SBn) (1) could react with C4F9I as well, delivering the same product (3ad) in 78%, 42%, and 86% yields, respectively. Among the target products obtained, the structure of 3ae was further confirmed by X-ray crystallography.

a Reaction conditions: 4a (0.2 mmol), 2 (0.4 mmol), and TMEDA (0.4 mmol) in THF (2 mL) under irradiation of 25 W blue LEDs with N2 protection at 30 °C for 12 h unless noted otherwise. Isolated yields were given. bMeCN as solvent. cReaction time was 3 h.

mediate to good yields. We notice that by far no radical-initiated cascade reaction concerning starting from 4a to the related 2substituted benzoselenazoles has been reported. Comparably, only several 2-substituted benzoselenazoles, including 2-NH2, 2R (alkyl/phenyl), 2-OR, or 2-SR substituted benzoselenazoles have been synthesized via commonly used condensation and cyclization methods.20 Thus, this newly developed perfuoroalkyl radical initiated cascade reaction provides, for the first time, a feasible way to access biologically potential 2-perfluoroalkylbenzoselenazoles, starting from structurally simple (2isocyanophenyl)(methyl)selane and easily available perfluoroalkyl iodides. Among the target 2-perfluoroalkylated benzoselenazoles synthesized, the structure of 5ad was further confirmed by X-ray crystallography. Perfluoroalkyl iodides (Rf-I) reportedly can form EDA complexes with a number of Lewis bases, by shifting the lone pair of the Lewis base to the σ* antibonding orbital of the C−I bond of the Rf-I. Visible-light irradiation on the EDA complex can further trigger a single electron transfer, allowing an easy access to the desired perfluoroalkyl radical under mild reaction conC

DOI: 10.1021/acs.orglett.9b01175 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters ditions.21 In order to prove that the EDA complexes were indeed formed in situ by Rf-I with TMEDA in our reaction system, a UV−vis absorbance experiment, an 19F NMR titration experiment, and a Job’s plot analysis were performed by using C4F9I as electron acceptor and TMEDA as electron donor. We observed formation of the 1:1 complex between C4F9I and TMEDA with a binding constant of 1.45 M−1 in CDCl3 (see the Supporting Information for details). In order to provide convincing evidence to support the conclusions that the perfluoroalkyl radicals required for our radical-cascade perfluoroalkylation/cyclization reactions were indeed generated from EDA complexes formed in advance by perfluoroalkyl iodides and TMEDA, four deliberately designed experiments were carried out with the aid of EPR technology, as illustrated in Figure 2. After we kept a solution of C4F9I and PBN

Scheme 5. Proposed Mechanism

I−, is generated from the EDA complex. After that, Rf• adds to the terminal carbon of the isocyano group of 1a, rendering imidoyl radical A. A then undergoes an intramolecular cyclization, forming sulfonium radical B. Starting from B, two reaction pathways (A and B) are possible. Via pathway A, B initially is oxidized by 6 into sulfonium cation C via an intermolecular SET process. A nucleophilic attack of I− on the methyl carbon in cation C leads to the formation of target product 3, along with MeI, which subsequently receives a nucleophilic attack by TMEDA, giving quaternary ammonium salt 7. The ion peak in HRMS at m/z 131.1538 is well consistent with the formation of quaternary ammonium 9. Alternatively, by pathway B, radical B directly reacts with radical cation 6 via a concerted protoncoupled electron transfer (PCET) process, to give the target product 3 and iminium ion 8 along with the release of CH4. The ion peak in HRMS at m/z 115.1230 is well consistent with the formation of iminium 8. The further quantum yield measurement indicated that 0.2 equiv of product was formed for every photon absorbed, ruling out the possibility of chain propagation process, which is well consistent with our proposed mechanism shown in Scheme 5 (for details see the Supporting Information). In conclusion, we report a novel and practical fluoroalkyl radical-initiated cascade reaction to access diverse 2-fluoroalkylbenzothiazoles by reacting commercially available fluoroalkyl radical sources including perfluoroalkyl iodide (ICnF2n+1, n = 3− 8, 10), ICF(CF3)2, ICF2COOEt, ICF2CF2Cl or ICF2CF2Br, TMEDA, and structurally simple 2-isocyanoaryl thioethers in THF under nitrogen atmosphere and blue-light irradiation conditions. Meanwhile, this one-pot protocol could be wellexpanded to access a number of novel and biologically potential 2-fluoroalkylbenzoselenazoles starting from (2isocyanophenyl)(methyl)selane, perfluoroalkyl iodides (ICnF2n+1, n = 3−8) or ICF2COOEt and TMEDA. The Rf radicals required for the radical-cascade perfluoroalkylation/ cyclization reactions were generated from EDA complexes formed in situ from Rf-I and TMEDA. An eminent advantage of this photochemical strategy is that it did not need any external added photocatalysts, in particular those expensive and potentially toxic metal complexes frequently required by many previous photochemical reactions, exhibiting a remarkably benign and environmentally friendly feature.

Figure 2. EPR spectra in the presence of PBN.

(tert-butyl-α-phenylnitrone), a spin trap, in MeCN under irradiation of blue LEDs for 0.5 h, no product PBN-C4F9 was produced, reminding that C4F9I could not cleave itself to yield perfluorobutyl radical under the given conditions (Figure 2-I). Meanwhile, after a solution of C4F9I and PBN with added THF or 1a was irradiated by blue LEDs for 0.5 h, no adduct PBN-C4F9 was detected in both cases, indicating that neither THF nor 1a could act as the electron donor of C4F9I to yield C4F9 radical (Figure 2-II, III). Finally, we kept a solution of TMEDA, C4F9I, and PBN in MeCN under irradiation of blue LEDs for 0.5 h, and in this case, a spectrum signal resulted from the spin adduct C4F9PBN appeared as a triplet of singlets, strongly evidencing that TMEDA could act as the electron donor of C4F9I to produce C4 F9 radical required for the following radical-cascade perfluoroalkylation/cyclization reactions (Figure 2-IV). Additionally, it was found that the model reaction could be totally suppressed by radical scavengers, TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)-oxidanyl), BHT (2,6-di-tert-butyl-4-methylphenol), and CuCl, evidencing that the reaction might proceed via a radical process (for details see Supporting Information). Subsequently, we employed high-resolution mass spectrometry (HRMS) to analyze the reaction solution of the model reaction as illustrated in Figure S7. Two peaks appearing at m/z 115.1230 and 131.1538, corresponding to ions [C6H15N2]+ and [C7H19N2]+, respectively, were observed. The mechanism is proposed based on the experimental outcomes and previous reports (Scheme 5). Initially perfluoroalkyl radical (Rf•), along with radical cation TMEDA+• (6) and

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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.9b01175. D

DOI: 10.1021/acs.orglett.9b01175 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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Experimental details and characterization data (PDF) Accession Codes

CCDC 1905455−1905456 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

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

Xiao-Lan Chen: 0000-0002-3061-8456 Kai Sun: 0000-0003-2135-0838 Yu-Fen Zhao: 0000-0002-8513-1354 Bing Yu: 0000-0002-2423-1212 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), 2017 Science and Technology Innovation Team in Henan Province (22120001).

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DOI: 10.1021/acs.orglett.9b01175 Org. Lett. XXXX, XXX, XXX−XXX