English

• Over the past few decades, hypervalent iodine chemistry has witnessed remarkable progress [1-15], which was partially sustained by the emergence and application of novel hypervalent iodine reagents [16-25]. Among the vast category of these reagents, heterocyclic iodine(Ⅲ) compounds [26-30], compared to their noncyclic analogues, have demonstrated the merit of higher thermal stability and unique reactivities. Accordingly, a great deal of efforts have been devoted to investigating various heterocyclic iodine(Ⅲ) reagents bearing versatile ligands including azido [31-35], cyano [36-40], trifluoromethyl [41-48] and triflate groups [49-52]. The most famous and best-investigated heterocyclic iodanes are benziodoxoles, which have achieved significant advances in their preparation, structure determination, and synthetic applications [53-68]. In contrast, benziodazoles, an analogous heterocyclic iodane reagent of benziodoxoles, have received relatively less attention [69-74]. To the best of our knowledge, there was a limited number of benziodazole reagents that have been reported till now (Fig. 1a), and only three known transformation patterns mediated by these existing benziodazole reagents 1a-h have been established (Fig. 1b). In the majority of these transformations, benziodazoles were employed as ligand- or atom-transfer reagents for various functionalization reactions including azidation [75-78], trifluoromethylthiolation [79, 80], aroylation [81], fluorination [82], and alkynylation [83] as shown in Fig. 1b, type 1. As an illustration, Zhang's group reported the design and synthesis of hypervalent fluoroiodane(Ⅲ) reagent 1e, which was capable of performing the intramolecular ring expansion/fluorination of unactivated cyclopropanes to afford a diverse array of 4-fully substituted fluoropiperidines [82]. In addition, benziodazoles can also enable the oxidative coupling reactions as solely an oxidant, without incorporating any structural moieties into products (Fig. 1b, type 2). For instance, Wang and coworkers developed a rhenium-catalyzed dehydrogenative olefination of C(sp³)−H bonds mediated by alanine-derived hypervalent iodine(Ⅲ) reagents 1g, which is crucial for oxidizing Re(n) catalyst species from low oxidation states to high ones [84]. Furthermore, benziodazoles may display high electrophilic reactivity and more than one structural moieties can be introduced into the corresponding product (Fig. 1b, type 3). For example, Waser's group demonstrated that both components (alkyne ligand and benziodazole skeleton) of ethynylbenziodazolones (EBZs) 1h can be found in imidate products, which was produced from the oxyalkynylation of diazo compounds mediated by EBZs 1h [85].

  Figure 1

  

  Figure 1.  (a) Established benziodazole-type hypervalent iodine reagents 1a-h. (b) Three transformation patterns mediated by reagents 1a-h and representative examples. (c) Selective 2-iodobenzamido- and triflate-transfer reactions enabled by benziodazole-triflate reagent 1i.

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  However, despite the above advances that have been made in the field of benziodazole-type hypervalent iodine reagents, the development of new-type benziodazole reagents and research on their reactivities are still highly desired. To our knowledge, no former reports have described benziodazole-triflate reagent and investigated its reaction reactivity. In this communication, we reported the first synthesis of benziodazole-type hypervalent triflate-iodine reagent 1i (Scheme 1) and its unique application as both a 2-iodobenzamido-transfer reagent for the synthesis of poly-substituted oxazoles from diverse α-electron withdrawing group substituted carbonyl compounds and a triflate-transfer reagent for the first access to α-triflate β-keto-sulfones via triflation of β-keto-sulfones (Fig. 1c, type 1). Differing from the above benziodazole reagents, this new benziodazole-type reagent 1i can not only facilitate the ligand-transfer reaction (Fig. 1c, type 1) that well corresponds to the first type of the three known transformation patterns, but also enable a new transformation pattern, namely, incorporating the 2-iodobenzamido moiety of the hypervalent iodine reagent into products (Fig. 1c, type 4).

  Scheme 1

  

  Scheme 1.  Preparation of benziodazole-triflate reagent 1i.

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  Oxazole ring, a privileged five-membered heterocyclic motif, is commonly found in a variety of biologically active natural products and pharmaceuticals [86-90], including the anti-diabetic agent AD-506 [91], the nonsteroidal anti-inflammatory drug oxaprozin [92], the anti-mycobacterial natural product texaline [93], and the anti-pancreatic cancer agent PC-046 [94]. The construction of oxazole skeleton has been continuously studied for decades, leading to numerous significant achievements. Traditional methods for the synthesis of oxazole compounds include the 2-acyl aminoketone Robinson-Gabriel dehydration [95-98] and Cornforth rearrangement [99]. In recent years, more diverse synthetic methods have been reported for the assemblage of oxazole framework [100-107]. As metal or Lewis acid catalysts remain virtually indispensable for these reactions, it should be highly desirable to develop novel additive-free synthetic method for the synthesis of oxazoles. In this research, we demonstrated an alternative straightforward, catalyst-free approach to accessing oxazole derivatives through a benziodazole-triflate-mediated oxidative cyclization of diverse α-electron withdrawing group substituted carbonyl compounds.

  The benziodazole-type hypervalent-iodine reagent 1i could be readily prepared in 85% yield as a white, microcrystalline solid, from the ligand exchange reaction between hypervalent chloro‑iodine(Ⅲ) compound 2 and silver triflate (AgOTf) in chloroform under N₂ atmosphere condition (Scheme 1). It is noteworthy that reagent 1i can also be accessed on a large-scale of ten grams without significant diminishment of the yield. Furthermore, this new reagent 1i is bench stable and can be stored in a dry environment at room temperature for months without decomposition.

  Benziodazole-triflate reagent 1i was characterized by ¹H, ¹³C, and ¹⁹F NMR spectroscopy. The ¹³C NMR chemical shift of the C−I ipso-carbon (117.4 ppm) had a positive incremental deshielded shift of about 24 ppm from that of iodoarene, which is consistent with the typical characteristics of the reported benziodazole compounds [75-85, 108-112]. Thermogravimetry-differential thermal analysis (TG-DTS) showed that reagent 1i decomposed at 154 ℃ to form a brown tar (see Supporting information for details). This reagent exhibited good solubility in common organic solvents including dimethyl sulfoxide, acetonitrile, tetrahydrofuran, chloroform, toluene, etc. A single crystal of 1i (CCDC: 2172493) was grown in the chloroform at room temperature, and X-ray crystallography showed that 1i has an approximate T-shaped structure (O10−I−N9 bond angle 164.22°) and a I(III)-OTf bond (2.391 Å) [113], indicating the larger bond interactions beyond the ordinary van der Waals attraction.

  With reagent 1i in hand, we initially envisaged that it can be used as a potential triflate-transfer reagent for introduction of the triflate moiety by reacting with an appropriate nucleophile. To our surprise, when reagent 1i (0.65 mmol, 1.5 equiv.) was treated with the commercially available ethyl benzoylacetate (3a, 0.5 mmol, 1.0 equiv.) in acetonitrile at 80 ℃, an exclusive oxidative cyclization reaction occurred to give ethyl 2-(2-iodophenyl)−4-phenyloxazole-5-carboxylate 4a as crystalline solids in 56% yield, rather than the expected triflated product (Table 1, entry 1). The structure of compound 4a was undoubtedly established by X-ray crystal crystallography (Table 2, 4a, CCDC: 2207130). This outcome clearly indicated that in this transformation, the benziodazole-type hypervalent-iodine reagent 1i was used as a 2-iodobenzamido-transfer reagent, rather than the predicted triflate-transfer reagent.

  Table 1

  

  Table 1.  Optimization of the reaction conditions.^a

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  Motivated by this appealing finding, we then continued our investigation by optimizing the reaction conditions, using ethyl benzoylacetate 3a as model substrate (Table 1). Solvents screening indicated that aprotic solvents were generally superior to protic solvents, with toluene proved to be the best one in promoting the formation of the poly-substituted oxazole 4a (entries 1–7). Subsequent temperature testing revealed that either increasing or decreasing temperatures from 80 ℃ was not beneficial for this transformation (entries 8 and 9). To our delight, increasing the loading of reagent 1i from 1.5 equiv. to 1.75 equiv. can further improve the yield of product 4a to 81% (entry 10). Extra attempts to improve the reaction outcome by employing additives including AgOTf, Cu(OTf)₂, and BF₃·Et₂O turned out to be unfruitful (entries 11–13).

  With the optimized conditions established (Table 1, entry 10), we then investigated the generality and scope of this newly established transformation by testing various α-electron withdrawing group- substituted carbonyl compounds (Scheme 2). First, various aryl β-keto esters with electron-rich or electron-poor benzene rings were evaluated, and it was found that they were all successfully converted to corresponding poly-substituted oxazole products 4b-4f in moderate to good yields. In addition, the incorporation of halogens into the ortho-, meta- or para-positions of the aromatic ring in substrates exhibited good compatibility to this approach, with the corresponding products 4g-4n obtained in satisfactory yields. Interestingly, the method was equally applicable to substrates bearing heteroaromatic R substituents such as furyl- or thienyl-groups, conveniently affording the desired products 4o and 4p, albeit with deteriorated efficiency. Furthermore, the α-aromatic ring in substrates is not indispensable and can be replaced by a methyl or a cyclohexyl group, with the corresponding alkyl-substituted oxazole products 4q and 4r achieved in acceptable yields. The substrate scope for this exclusive oxidative cyclization methodology could be further extended to alternative types of carbonyl compounds bearing different α-electron withdrawing groups including cyano, amido, and acyl groups, realizing the access to more abundant substituted oxazoles 4s-4z in acceptable yields. The structure of amide-substituted oxazole product 4w (CCDC: 2301016) was further corroborated by X-ray crystal analysis. It is worth noting that when the alkyl-substituted carbonyl substrate, i.e., 1-phenyl-2-propanone was subjected to the standard conditions, the reaction did not afford the expected oxazole or triflated product.

  Scheme 2

  

  Scheme 2.  Synthesis of poly-substituted oxazoles via reagent 1i-mediated oxidative cyclization. Reaction conditions: substrate 3 (0.5 mmol, 1.0 equiv.), reagent 1i (0.875 mmol, 1.75 equiv.) in toluene (10 mL) at 80℃ for 12 h.

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  Intriguingly, when tosylacetophenone (Scheme 3, 5b), a carbonyl compound substituted with an α-electron-withdrawing sulfone group was treated with reagent 1i under the same optimized conditions, a triflation reaction occurred smoothly to provide α-triflate substituted product 6b in 64% yields, with the isolation of the oxazole product in only trace amount. This exceptional result suggested that reagent 1i can not only serve as a 2-iodobenzamido-transfer reagent but also as a triflate-transfer reagent, depending on the categories of the substrates being employed.

  Scheme 3

  

  Scheme 3.  Synthesis of α-triflated β-keto-sulfones via reagent 1i-mediated triflation. Reaction conditions: substrate 5 (0.5 mmol, 1.0 equiv.), reagent 1i (0.875 mmol, 1.75 equiv.) in toluene (10 mL) at 80℃ for 12 h.

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  Motivated by this finding, we then came to investigate the scope and utility of this direct triflation method (Scheme 3). The results manifested that diverse β-keto-sulfones 5 bearing either electron-donating or electron-withdrawing group on the benzene ring were viable substrates for the method, furnishing the corresponding α-triflate-substituted products 6a-6o in moderate to good yield. The structure of 6b (CCDC: 2256361) was unambiguously determined by X-ray crystal analysis. Remarkably, this protocol could also be extended to the substrate bearing a thiophene ring, a representative motif of the heteroaryl fragments, delivering the corresponding thienyl-containing product 6p in acceptable yield. Notably, substrates substituted with a strong electron-withdrawing CF₃ group on either of the two aromatic rings were also converted to the corresponding triflated products 6d and 6m, albeit in relatively lower yields.

  To gain insight into the mechanisms of the exclusive oxidative cyclization and the triflation reaction mediated by reagent 1i, radical scavenger TEMPO was introduced into the above two reaction systems under standard conditions, respectively (Schemes 4a and b). For the reaction between reagent 1i and ethyl benzoylacetate 3a, the formation of desired oxazole product 4a was fully inhibited with the employment of 2 equiv. of TEMPO (Scheme 4a). Instead, the cross coupling adduct 7a was separated in 89% yield and the other adduct 8 was detected by high resolution mass spectroscopy (HRMS). Similar outcomes were observed for the reaction between reagent 1i and tosylacetophenone 5b, with product 6b formed in trace amount and the cross-coupling products 9a and 8 detected by HRMS (Scheme 4b). However, further control experiments indicate that the reaction of substrates 3a and 5b with TEMPO in the absence of benziodazole-triflate reagent 1i also afforded the corresponding adduct 7a and 9a, respectively (Schemes 4c and d). Thus, the results from the above radical scavenger experiments cannot provide convincing evidence that the reaction undergoes radical pathways. Furthermore, BHT was introduced as an alternative radical scavenger to the corresponding reaction of substrates 3a and 5b under standard conditions. It was found that products 4a and 6b could be achieved in 54% and 58% yield, respectively (Schemes 4e and f). Additionally, a radical clock experiment was also designed and conducted. When (2-propenyl)-3-oxobutanoate 3aa was subjected to the standard conditions, it was oxazole product 4aa rather than any radical cyclized product that was obtained (Scheme 4g). These outcomes might indicate that both reactions did not undergo radical pathway and ionic mechanism is more applicable for the two transformations.

  Scheme 4

  

  Scheme 4.  Control experiments.

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  To gain an insight into the mechanism of above reactions, we performed DFT calculations on the reaction of the new hypervalent iodine reagent 1i and 3a, as well as the reaction of 1i and 5a, and the corresponding calculated Gibbs free energy profiles are depicted in Fig. 2, Fig. 3, respectively.

  Figure 2

  

  Figure 2.  DFT-computed potential energy profile of the reaction of 3a and 1i. The results of Hirshfeld charges analysis are shown (standard state: 25 ℃, 1 mol/L).

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  Figure 3

  

  Figure 3.  DFT-computed potential energy profile of the reaction of 5a and 1i (standard state: 25 ℃, 1 mol/L).

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  As to the reaction of 1i and 3a, the calculation results show that the reaction is triggered by the coordination of IM1 with the electrophilic iodine(Ⅲ) atom of 1i, where IM1 is formed by the tautomerization of 3a. The ligand exchange step needs to overcome an energy barrier of 21.3 kcal/mol, and IM2 is generated via ligand exchange with the OTf anion still remaining around the iodine(Ⅲ) center. Subsequently, an intramolecular proton shift occurs to give IM3, which leads to a decrease in energy of 9.2 kcal/mol. The OTf anion attacks the activated carbon atom bonded to I^III to form IM4, where the formed imide and α-oxytriflated β-ketone ester are connected through the hydrogen bonding interaction. The Hirshfeld charge analysis of IM4 shows that the more positive charge (0.16) is concentrated on the carbonyl carbon atom (C1) rather than the α-carbon atom (C2, 0.05), which indicates that the carbonyl carbon would be more favorable for the nucleophilic attack of the imide nitrogen. The nitrogen atom attacks the carbonyl carbon to form IM5 via TS3, which needs to overcome an energy barrier of 18.7 kcal/mol. After that, the oxygen on the benzoyl group attacks the α-carbon atom connecting OTf group to give IM6 via TS4 with an activation energy of 28.0 kcal/mol relative to IM4; this process is the rate-limiting step in the reaction. Then, an addition-elimination step occurs to remove the acetyl group and generate the by-product TfOAc. Finally, the product 4a is obtained by a dehydration process. The Gibbs free energy profile of the overall reaction shows that the formation of 4a is highly exergonic by 27.3 kcal/mol.

  For another reaction (Fig. 3), it starts with the coordination of IM1 to the electrophilic iodine(Ⅲ) atom of 1i, with an activation energy of 27.5 kcal/mol, to give IM2; this process is the rate-limiting step in the reaction. Subsequently, the OTf anion attacks the activated carbon atom bonded to I^III to form IM4 with an energy barrier of 23.5 kcal/mol.

  To understand the origin of the different chemoselectivity of two reactions, we calculated the energy barrier of the intermolecular cyclization process for IM4. The nitrogen atom on the imide attacks the carbonyl group and this step would lead to the formation of IM5. Then, the oxygen atom on the benzoyl group of the imide attacks the carbon atom bonded to OTf group through TS4, which has to overcome an activation energy barrier of 35.7 kcal/mol relative to IM4_s. It is a too high barrier to climb under the used reaction conditions. We think that the high energy barrier of TS4 is caused by the steric hindrance of the sulfonyl group adjacent to the α-carbon linking the OTf group. In TS4, the O3-O4 bond length is 2.63 Å, which is shorter than the O1-O2 bond length (2.85 Å) in TS4. It illustrates that compared to TS4, the nucleophilic attack of oxygen atom at the carbon atom bonded to OTf in TS4 requires overcoming greater steric hindrance.

  In order to demonstrate the synthetic utility of the obtained oxazole products and α-triflate β-keto-sulfones, 4a and 6a were chosen as representative compounds to perform some further derivatization reactions (Schemes 5a and b). By reacting with sodium hydroxide in methanol, the ester-substituted oxazole 4a could undergo alcoholysis give to the corresponding carboxylic acid 10a in 94% yield [114]. In addition, as an organic halide, iodine-containing 4a is also a suitable precursor for further transition metal-catalyzed cross-coupling reactions. Specifically, treating 4a with methylboronic acid and potassium carbonate in the presence of bis(triphenylphosphine)palladium(Ⅱ) chloride smoothly initiated a Suzuki-Miyaura cross-coupling reaction, providing product 11a in 80% yields [115]. Moreover, Pd/C mediated hydrodefunctionalization of 4a in hydrogen atmosphere with the catalysis of DABCO could reductively remove the iodine atom on the benzene ring in the substrate, affording deiodinated product 12a in 75% yields [116]. This final transformation clearly indicated that the iodine atom in all products could be readily removed by hydrogenative reduction, thus addressing the concern that the iodine atom in the products might be a redundant substituent. Furthermore, when 6a was treated with potassium phtalimide, resulting in the substitution of the triflate group with a phthalimide group, this transformation could achieve the formation of 2-(2-oxo-2-phenyl-1-(phenylsulfonyl)ethyl)isoindoline-1, 3‑dione 13 in 48% yield.

  Scheme 5

  

  Scheme 5.  (a) Derivatization of the obtained poly-substituted oxazole 4a. (b) Derivatization of the obtained α-triflate β-keto-sulfone compound 6a.

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  In conclusion, we realized the preparation of a novel benziodazole-triflate reagent 1i and had its typical λ³-iodane T-shaped structure confirmed by X-ray crystallography. The feature of this new reagent was that it can be used as both a 2-iodobenzamido- and a triflate-transfer reagent, reacting with diverse α-electron withdrawing group substituted carbonyl compounds to produce a series of poly-substituted oxazoles or α-triflate β-keto-sulfones. The ionic mechanisms with selectivity were proposed and testified by both control experiments and DFT calculations. This work not only supplements the type of hypervalent iodine reagents, but also represents an exclusive transformation pattern mediated by benziodazole-type hypervalent iodine reagents. Further studies on the application of this reagent 1i as well as the mechanism on its divergent reactivities are still in progress in our labs.

  Declaration of competing interest

  We herein declare that all authors (Yadong Li, Feng-Huan Du, Junjie Li, Jun Xu, Zhifang Yang, Shanshan Li, Chi Zhang and Yunfei Du) have seen and approved the submission of this manuscript and there is no interest conflicts between/among all authors.

  CRediT authorship contribution statement

  Yadong Li: Writing – original draft, Methodology, Formal analysis, Data curation. Feng-Huan Du: Writing – original draft, Software, Methodology, Formal analysis, Data curation. Junjie Li: Writing – original draft, Software, Methodology, Formal analysis, Data curation. Jun Xu: Software, Formal analysis, Data curation. Zhifang Yang: Formal analysis, Data curation. Shanshan Li: Formal analysis, Data curation. Chi Zhang: Writing – review & editing, Supervision, Resources, Project administration, Methodology, Funding acquisition. Yunfei Du: Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Conceptualization.

  Acknowledgments

  We acknowledge the National Natural Science Foundation of China (Nos. 22071175, 22071116) and Tianjin Graduate Research and Innovation Project (No. 2021YJSB196) and for financial supports.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110338.

  
  
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• 

  Figure 1  (a) Established benziodazole-type hypervalent iodine reagents 1a-h. (b) Three transformation patterns mediated by reagents 1a-h and representative examples. (c) Selective 2-iodobenzamido- and triflate-transfer reactions enabled by benziodazole-triflate reagent 1i.

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  Scheme 1  Preparation of benziodazole-triflate reagent 1i.

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  Scheme 2  Synthesis of poly-substituted oxazoles via reagent 1i-mediated oxidative cyclization. Reaction conditions: substrate 3 (0.5 mmol, 1.0 equiv.), reagent 1i (0.875 mmol, 1.75 equiv.) in toluene (10 mL) at 80℃ for 12 h.

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  Scheme 3  Synthesis of α-triflated β-keto-sulfones via reagent 1i-mediated triflation. Reaction conditions: substrate 5 (0.5 mmol, 1.0 equiv.), reagent 1i (0.875 mmol, 1.75 equiv.) in toluene (10 mL) at 80℃ for 12 h.

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  Scheme 4  Control experiments.

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  Figure 2  DFT-computed potential energy profile of the reaction of 3a and 1i. The results of Hirshfeld charges analysis are shown (standard state: 25 ℃, 1 mol/L).

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  Figure 3  DFT-computed potential energy profile of the reaction of 5a and 1i (standard state: 25 ℃, 1 mol/L).

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  Scheme 5  (a) Derivatization of the obtained poly-substituted oxazole 4a. (b) Derivatization of the obtained α-triflate β-keto-sulfone compound 6a.

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  Table 1.  Optimization of the reaction conditions.^a

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