Letter Cite This: Org. Lett. 2018, 20, 1054−1057
pubs.acs.org/OrgLett
Metal-Free Synthesis of (E)‑Monofluoroenamine from 1‑Sulfonyl1,2,3-triazole and Et2O·BF3 via Stereospecific Fluorination of α‑Diazoimine Ze-Feng Xu, Haican Dai, Lihong Shan, and Chuan-Ying Li* Department of Chemistry, Zhejiang Sci-Tech University, Xiasha West Higher Education District, Hangzhou, 310018, China S Supporting Information *
ABSTRACT: A general, stereospecific, and straightforward method for the rapid preparation of functionalized (E)monofluoroenamines is reported. Rather than transition metals (Rh, Ni, Pd, Cu, Ag, etc.), Et2O·BF3 was employed to promote the formation of α-diazoimine through the Dimroth equilibrium of common 1-sulfonyl-1,2,3-triazole for the first time. An overall migration of fluoride from boron to the diazo-linked carbon of α-diazoimine was achieved. Derivations and late-stage modification of bioactive molecule were conducted. A plausible mechanism was also proposed.
T
conversion of 1,2,3-triazole was rarely studied. To the best of our knowledge, only two cases were reported.10 In 2014, the Abarca group10a disclosed that Selectfluor could participate in the denitrogenative transannulation of pyridotriazoles 4 in MeCN, producing imidazo[1,5-α]pyridines 5 with low efficiency (up to 30% yield). Later in 2016, the Adimurthy group10b improved the yield up to 99% in the same transformation using Et2O·BF3 as the catalyst (Scheme 1B). In continuation of our interest in carbene chemistry,4,8b chloride, bromide, and iodide were introduced as nucleophiles to capture the rhodium carbene giving 1,2-dihydroisoquinolines4c and tetrahydropyridine4b (top of Scheme 1C). Unfortunately, when fluoride was employed as a nucleophile to trap the α-imino rhodium carbene, desulfonylation occurred affording 1H-triazole 7 without any desired fluorination taking place, these side reactions were attributed to the considerable alkalinity11 of the fluoride added in the reaction. It is well-known that fluorine is the most electronegative element, and introduction of fluorine usually brings dramatic changes in the physical and chemical properties of the parent molecule, making it vital in medicine, pesticides, and material sciences.12 Specially, β-fluoroenamine is a very important motif in many drugs and bioactive molecules (Figure 1). For example, 5-fluorouracil I12e,13a,b is an inhibitor of thymidylate synthase employed in treatment of many malignancies such as gastrointestinal cancers and breast cancer. Bearing a similar skeleton, capecitabine II is also employed in the treatment of breast and colorectal cancers.13c 4-Fluoro-2H-isoquinolin-1-one derivatives III have been confirmed to be a tumor necrosis factor.13d The most popular literature protocols for β-fluoroenamine synthesis include aminofluorination of alkynes inter- or intramolecu-
ransformation of transition metal carbene is a powerful strategy for construction of various valuable molecules.1 Following the pioneering work of Fokin, Gevorgyan and coworkers,2 many reports3,4 have demonstrated the great flexibility of 1-sulfonyl-1,2,3-triazole 1 in the synthesis of nitrogencontaining heterocycles and various other compounds.5,6 Because of the excellent efficiency of a rhodium catalyst, αimino rhodium carbene became the most efficient and important intermediate generated by denitrogenation of α-diazoimine 2, which was formed through the Dimroth tautomerization of 1 (top of Scheme 1A). Other transition metals7 (such as Ni,7a,b Pd,7c−e Cu,7f−h Ag7i−k) were also utilized as catalysts in transformations of fused 1,2,3-triazoles (pyridotriazole, for example). Several thermally induced reactions of 1 were also reported8 with ketenimine 3′9 as the key intermediate (bottom of Scheme 1A). Non-transition metal catalyzed or promoted Scheme 1. Background
Received: December 26, 2017 Published: February 5, 2018 © 2018 American Chemical Society
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DOI: 10.1021/acs.orglett.7b04014 Org. Lett. 2018, 20, 1054−1057
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Organic Letters Table 1. Optimization of Reaction Conditionsa
Figure 1. β-Fluoroenamine in drugs and bioactive molecules.
larly.14,15 However, atom economy and high cost were issues in the methodology encountered which posed limits on its application. From a practical point of view, more diverse strategies concerning safety and cost of readily available substrates and reagents are still highly desirable. The formation of compound 7 inspired us to explore some kind of “prolonged release” fluorine source to avoid the unpleasant side reactions. On the other hand, we envisioned that an appropriate Lewis acid may coordinate to 2′ which resulted in the formation of diazonium ion 2″ (Scheme 2A).1h If
entry
x
additive (equiv)
temp (°C)
yield (%)b
1 2 3 4d 5d 6d 7 8e
1.0 1.0 1.5 1.5 1.5 1.5 1.5 1.5
− − − − HBF4 (1.0) TBAF·3H2O (0.2) TBAF·3H2O (0.2) TBAF·3H2O (0.2)
rt reflux reflux reflux reflux reflux reflux reflux
44 54 59 68 20 80 (71c) 78 − (70c)
a
General reaction conditions: 1a (44.6 mg, 0.2 mmol), Et2O·BF3, additive, 1,2-DCE (2.0−2.5 mL), N2 atmosphere. bDetermined by 1H NMR with 1,3,5-trimethoxylbenzene as the internal standard. cIsolated yield. d4 Å molecular sieve was added. eThe reaction was carried out in 1.0 mmol scale. Ms = methylsulfonyl, TBAF = tetrabutylammonium fluoride, DCE = 1,2-dichloroethane.
Scheme 2. Initial Hypothesis and Findings
reserved (70% isolated yield, entry 8). When TBAX (X = Cl, Br, I) was also used as an additive, only 8a was isolated in 51−63% yields without other halogenated products formed. The reaction scope of the transformation proved to be quite general (Scheme 3). The influence of the sulfonyl group was Scheme 3. Reaction Scope
a nucleophilic part was set up in the Lewis acid, then a rapid shift of Nu from the boron to carbon would occur with the expulsion of nitrogen. The following hydrolysis would lead to enamine 8. As a result, if the nucleophilic part is a fluorine source, then the desired useful β-fluoroenamine could be obtained with no free fluoride participation in the reaction. We tried several fluorinated Lewis acids, and gratifyingly, when 1-(methylsulfonyl)-4-phenyl1H-1,2,3-triazole 1a was stirred with Et2O·BF3 (1.0 equiv) in 1,2dichloroethane at rt, (E)-β-fluoroenamine 8a was generated stereospecifically in 44% yield. The configuration of the CC was determined by the coupling constant of 3JF−H (25.4 Hz), which was a typical value for a cis coupling constant in fluoroalkenes16 (Scheme 2B). Using 1a as a template substrate, a series of reactions were carried out in order to establish the optimal conditions (Table 1). Elevating the temperature from rt to reflux promoted the yield of 8a to 54% in the presence of 1.0 equiv of Et2O·BF3 in DCE (entries 1−2). When the dosage of Et2O·BF3 increased to 1.5 equiv, the yield of 8a was improved slightly (entry 3). Interestingly, 4 Å molecular sieves had a positive effect on the yield of 8a (68%, entry 4). The addition of HBF4, which is the source of both the fluoride and proton, resulted in a decreased yield (20%, entry 5). According to Sander and co-workers, migration of F from the boron to carbene carbon was reversible.17 Hence, Lewis bases were utilized for the purpose of coordinating to the newly formed −BF2 group to interrupt fluorine migrating back to the boron, and fortunately, 0.2 equiv of TBAF·3H2O could increase the yield of the desired product to 80% (71% isolated yield, entry 6). To our surprise, in the presence of TBAF·3H2O, 4 Å molecular sieves were not necessary (78%, entry 7). Furthermore, when the reaction was carried out on a larger scale (1.0 mmol), the yield of 8a was also
evaluated first, and both alkyl sulfonyl substituted 8a, 8b and aryl sulfonyl substituted 8c−e were produced in moderate to good yields (48−70%). Triazoles bearing weaker electron-donating alkyls in the benzene ring afforded the corresponding products in moderate yields (8f−h, 54−62%). Ortho-substituted 8i and 8j were generated smoothly in 59% and 55% yields. Notably, in these two experiments, no intramolecular product (bromine and cyano group as nucleophile) was isolated, which indicated that the intermolecular fluorination predominates. Halogens are valuable substituents in organic synthesis, and several halogenated aryl fluoroenamines were produced conveniently (8k−q, 66−85% yields). Substrates incorporating oxo functionalities were also tested. While 8r bearing an ester group was obtained in acceptable yield, 8s was isolated in only 28% yield, which may be because of the coordination of carbonyl with BF3. Since ortho-substituted compounds are of great synthetic 1055
DOI: 10.1021/acs.orglett.7b04014 Org. Lett. 2018, 20, 1054−1057
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Organic Letters flexibility, in addition to 8i, 8j, and 8o, more ortho-substituted products were produced. For instance, carbon−carbon triple bond loaded 8t was obtained in good yield (76%). A Michael acceptor acrylate group was also compatible in the procedure (8u, 65% yield); however, a vinyl group with a phenyl substituent did not work well in this reaction (8v, 37% yield). In cases of naphthyl substituted products, 1-naphthyl fluoroenamine 8w was generated in an acceptable yield (49%), whereas 2-naphthyl fluoroenamine 8x, in an unsatisfactory 31% yield. Alkoxy was not tolerated, and for strong electron-withdrawing groups (−NO2, −CF3, and −CN) on the aryl ring, only a trace amount of the desired products was detected. Triazoles generated from alkyl alkynes were decomposed after treatment under standard conditions (see Supporting Information (SI)). Notably, all reactions were quenched after triazoles 1 were consumed completely, so for cases with low yields, no substrates were recovered. Several derivations were then performed to illustrate the potential of this protocol in fluorinated compounds synthesis. For example, 8e could be reduced to 2-fluoro-2-phenylethan-1amine 9 by 2.0 equiv of NaBH3CN in good yield (60%, eq 1). 3-
Scheme 4. Proposed Reaction Mechanism
fluoride from boron to carbon accomplished the formal insertion of carbene to the B−F bond, and intermediate 17a was generated. Fluoride in the solution could coordinate to the boron of 17a giving 18a, which may interfere in the equilibrium between 16a and 17a. A 1,3-shift of the BF3 group from carbon to nitrogen led to the formation of 15a, which gave birth to the desired 8a after hydrolization. Unfortunately, no signals related to proposed intermediates were obtained by in situ MS. Additionally, trace HF would be formed from H2O and BF3, and protonation of the diazo group followed by substitution of diazonium with fluoride and tautomerization would also give 8a (path c). We used protonic acid to catalyze the reaction with TBAF as the fluoride source; no 8a was obtained (see SI), which indicated that the participation of Et2O·BF3 in the activation of triazole is necessary. Although the origin of the stereospecificity was unclear at the present stage, it was speculated that the dipolar repulsion between the C−F bond and the C−N or CN bond may play vital roles. In conclusion, an unprecedented user-friendly protocol for the synthesis of valuable (E)-monofluoroenamine from 1-sulfonyl1,2,3-triazole was achieved. Et2O·BF3 acted as both an electrophile and a fluorine resource in this mild transformation. For the first time, the denitrogenative transformation of easily available 1-sulfonyl-1,2,3-triazole was mediated by Et2O·BF3, and no transition metal was necessary in the stereospecific synthesis of densely functionalized products. This method is synthetically appealing in the preparation of fluorine-containing compounds since several derivations of the products and the late-stage modification of estrone were conducted efficiently. Further studies on the mechanism of the unique stereospecific reaction and its application in the construction of more complicated bioactive molecules containing the β-fluoroenamine scaffold are underway.
Fluoroindol 10 could be obtained in 47% yield by a copper catalyzed intramolecular C−N bond coupling reaction from 8o (eq 2). 4-Fluorodihydroisoquinolines 11 and 12 could be generated smoothly from 8i and 8u by a simple intramolecular SN2 reaction and intramolecular Michael addition, respectively (eqs 3 and 4). Moreover, 1y, derived from estrone, could give rise to the desired β-fluoroenamine product 8y in good yield after treatment with Et2O·BF3 under standard conditions, demonstrating the capacity of the protocol in late-stage modification of the bioactive molecule (eq 5). According to literature17−19 and our understanding, a tentative mechanism was proposed (Scheme 4). With the addition of TBAF, fluoride in TBAF could coordinate to BF3 to form BF4−, which could decompose back to BF3 and F−. In path a, an addition−elimination reaction was involved as follows: when Et2O·BF3 was introduced, the imino nitrogen atom coordinated to BF3 first, giving complex 13a. The following fluoride addition to the carbon−carbon double bond in 13a produced 14a, which underwent an elimination of N2 delivering 15a, and finally, 8a was generated after protonation. Alternatively, an insertion− migration process was proposed in path b: the diazo-linked carbon, rather than the imino nitrogen, coordinated to BF3 accompanied by release of N2 giving 16a. 1,2-Migration of
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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.7b04014. Experimental procedures, characterization data, and NMR spectra for new compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chuan-Ying Li: 0000-0001-9400-6830 Notes
The authors declare no competing financial interest. 1056
DOI: 10.1021/acs.orglett.7b04014 Org. Lett. 2018, 20, 1054−1057
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Organic Letters
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ACKNOWLEDGMENTS We are grateful for the support of this work by the National Natural Science Foundation of China (21372204), the Program for Innovative Research Team of Zhejiang Sci-Tech University (13060052-Y), and Zhejiang Sci-Tech University 521 project.
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