# Electrophilic Amide Activation under Mild Conditions Using 2,2,2-Trifluoroethyl Iodonium Salt: Access to Esters and Benzoxazoles

Tiancheng Zhong, Hugo Amistadi-Revol, Julie Tremblais, Sébastien Prévost

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Tiancheng Zhong, Hugo Amistadi-Revol, Julie Tremblais, Sébastien Prévost. Electrophilic Amide Activation under Mild Conditions Using 2,2,2-Trifluoroethyl Iodonium Salt: Access to Esters and Benzoxazoles. Organic Letters, 2025, 27 (32), pp.8858-8862. 10.1021/acs.orglett.5c02352. hal-05346628

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# Electrophilic Amide Activation under Mild Conditions Using 2,2,2-Trifluoroethyl Iodonium Salt: Access to Esters and Benzoxazoles

Tiancheng Zhong, Hugo Amistadi-Revol, Julie Tremblais and Sébastien Prévost*

Laboratoire de Synthèse Organique (LSO – UMR 7652), CNRS, Ecole Polytechnique, ENSTA, Institut Polytechnique de Paris, 828 boulevard des Maréchaux, 91120 Palaiseau (France)

*Supporting Information Placeholder*

<figure><img src="image_2.png" /></figure>

**ABSTRACT:** The amide functional group is a key chemical moiety widely found in nature, yet it is generally considered poorly reactive. We report an electrophilic activation strategy using 2,2,2-trifluoroethyl(mesityl)iodonium triflate under very mild conditions. This approach enables the conversion of amides into esters, as well as heteroaromatic derivatives.

Amides are among the most common chemical moieties found in Nature, notably in amino acids.¹ Compared to other carbonyl groups, amides are generally more stable due to the delocalization of the nitrogen lone pair with the carbonyl bond. This resonance stabilization makes them highly resistant to a broad range of reaction conditions, particularly less reactive toward nucleophiles compare to other carbonyl moieties.² Their inherent stability also contributes to the effectiveness of amide as directing groups in C–H activation reactions, which typically require harsh conditions.³ Because of this high stability, classical hydrolysis of amides generally requires strongly acidic or basic conditions, which may be incompatible with other functional groups present in the molecule.⁴ The lower reactivity of amides compared to other carbonyl derivatives can also been seen as an advantage when chemoselectivity is desired. This feature has driven significant interest in amide activation strategies.⁵ Regarding the direct transformation of amides into esters, recent advances have been made (Scheme 1.a). Building on the pioneering work of Garg’s group on nickel-catalyzed amide activation, transition-metal catalysis approaches have been developed, particularly using nickel or cobalt catalysts.⁶,⁷ Alternatively, the π-electron-donating ability of the amide nitrogen has enabled successful electrophilic activation. In this context, various metallic Lewis acids have been shown to efficiently activate amide moieties, allowing their transformation into esters.⁸ Additionally, a few examples of metal-free electrophilic activation have been reported.⁹ Recently, metal-free activation of amides has also been explored through the use of nucleophiles.¹⁰ However, such conditions typically require highly reactive substrates, such as twisted amides.¹¹ Therefore, there is a strong

interest in developing methods that enable the activation of simple amides under mild conditions.

In this context, we recently reported a C8–H trifluoroethylation reaction of 1-carbonylnaphthalenes.¹² Interestingly, during this study, when specific tertiary amides were used as directing group, we occasionally observed, at room temperature, the formation of 2,2,2-trifluoroethyl 1-naphthoate C instead of expected C8-fluoroalkylated naphthalene B (Scheme 1.b).

Given the known challenges associated with the transformation of amides, we considered this unexpected result a promising lead. Moreover, fluoroalkylated iodonium salts are known as powerful electrophiles,¹³ which suggested the potential for developing a mild methodology to convert amides into fluorinated esters or other valuable functional groups via electrophilic activation (Scheme 1.c).

Herein, we report a new method for the electrophilic activation of amides using 2,2,2-trifluoroethyl(mesityl)iodonium triflate at room temperature, enabling their transformation into either esters or benzoxazoles under mild and metal-free conditions.
Scheme 1. Esterification of Amides

<figure><img src="image_3.png" /></figure>

Transition-metal-catalyzed C–N activation

<figure><img src="image_4.png" /></figure>

Electrophilic activation

<figure><img src="image_5.png" /></figure>

Nucleophilic activation

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

At the outset of our investigation, we assessed the necessity of both the Pd(II) catalyst and trifluoroacetic acid in the esterification reaction. Pleasingly, we found that the transformation could proceed at room temperature using only 2,2,2-trifluoroethyl(mesityl)iodonium triflate **10c** (Table S1). A full optimization study was conducted using (2-iodophenyl)(pyrrolidin-1-yl)methanone **1a** as the model substrate (see the Supporting information). The optimal conditions were identified as using 2 equivalents of iodonium salt **10c** in dichloromethane at room temperature for 20 hours. Following the reaction, an aqueous acidic treatment was performed to fully hydrolyze the imidate triflate intermediate and deliver the expected fluoroester.¹⁴

With the optimized conditions in hand, we next explored the scope of the reaction with various amides (Scheme 2). Pyrrolidine amide **1a** proved to be the most efficient substrate, affording desired 2,2,2-trifluoroethyl 2-iodobenzoate **11a** in 86% NMR yield. Other tertiary amides, including morpholine amide **2a**, Weinreb amide **3a** and dimethyl amide **4a**, were also compatible with the reaction conditions. Secondary amides could also be converted to corresponding ester **11a**, although benzyl- and phenylamides gave only moderate yields. Unfortunately, 8-aminoquinoline amide **8a**, commonly used as a directing group in C–H activation, remained unreactive. Finally, benzamide **9a** was successfully converted to ester in a satisfying yield of 53%.

Scheme 2. Evaluation of the Amide Group<sup>a,b</sup>

<figure><img src="image_9.png" /></figure>

<sup>a</sup>General reaction conditions: **1a–9a** (0.1 mmol, 1 equiv), **10c** (2 equiv) in CH₂Cl₂ (0.5 mL) at 20 °C for 20 h then THF (2 mL) and HClaq (3M, 2 mL) added, stirred at 20 °C for 2 h. <sup>b</sup>NMR yield of product **11a** determined using 1,3,5-trimethoxybenzene as the internal standard.

We then evaluated the scope of the reaction across a broad range of pyrrolidine amides (Scheme 3). First, *ortho*-substituted aromatic amides were tested under the optimized reaction conditions. 2,2,2-Trifluoroethyl 2-iodobenzoate **11a** was isolated in 83% yield, consistent with the previously reported NMR yield. The reaction demonstrated good compatibility with various functional groups, including halogens, alkyl groups, methoxy, ketone and nitro groups, affording corresponding esters in moderate to excellent yields (43-99%). Notably, in some case **(11d** and **11i)**, the isolated yields were slightly lower than expected due to the volatility of the ester products. Substitution at the *meta* and *para* positions of the aromatic ring did not significantly affect the yields. Thanks to the mild reaction conditions, other carbonyl-containing groups such as aldehyde **(11t)**, ketone **(11u)** and ester **(11v)** were well tolerated (70%, 65%, 64% yield respectively). Particularly interesting was the behavior of *bis-amide 1x*: under the standard conditions, the pyrrolidine amide was selectively converted into the ester, while the benzyl amide remained mostly untouched, resulting in 52% yield of monoester **11x** and only 11% of the diester product. Next, polysubstituted aromatic amides were evaluated. 2,2,2-Trifluoroethyl 1-naphthoate **11y** was isolated in 72% yield, which increased to 89% yield when the reaction was scaled up to 2.5 mmol. Other aromatic frameworks also performed well under the optimized conditions, affording corresponding phenanthrene ester **11aa** and fluorenone ester **11ab** in very good yields. Most combinations of polysubstitution were well tolerated, with the notable exception of *di-ortho*-chlorinated amide **1ae**, which failed to react probably because of excessive steric hindrance. Despite giving only moderate yields, some heteroaromatic scaffolds, including pyrrole and furan, were successfully transformed into their corresponding esters both in 48% yield. Encouragingly, the method also proved effective for non-aromatic amides. α,β-Unsaturated amides reacted smoothly, giving the desired cinnamic ester derivatives. In the case of phenylpropiolic amide **1an**, an interesting 70% NMR yield was observed; however, the isolated yield was lower due to the presence of a difficult-to-separate impurity. Promisingly, although generally less reactive, even aliphatic amides were amenable to the transformation, affording the desired esters in moderate yields.
<figure><img src="image_10.png" /><figcaption>**Scheme 3. Scope of the Esterification Reaction<sup>a,b</sup>**</figcaption></figure>

<sup>a</sup>General reaction conditions: **1** (0.3 mmol, 1 equiv), **10c** (2 equiv) in CH<sub>2</sub>Cl<sub>2</sub> (1.5 mL) at 20 °C for 20 h *then* THF (5 mL) and HCl<sub>aq</sub> (3M, 5 mL) added, stirred at 20 °C for 2 h. <sup>b</sup>Isolated yields were reported. <sup>c</sup>Volatile products. <sup>d</sup>11% of diester **11x'** was isolated. <sup>e</sup>NMR yield of product **11an** determined using 1,3,5-trimethoxybenzene as the internal standard.

Given that the reaction likely proceeds through electrophilic activation of the amide *via* an imidate salt intermediate, we explored the possibility of intercepting this intermediate with nucleophiles other than water to access new functional groups (Scheme 4). First, after activation of amide **1q** with trifluoroethyliodonium salt **10c**, the addition of simple alcohols such as methanol or ethanol afforded the corresponding esters in good yields, 48% and 60% respectively. The addition of alcohols to the imidate requires a slightly elevated temperature (60 °C), as the formation of fluoroester **11q** was observed at room temperature. Notably, bis-nucleophiles could also be successfully employed. The use of 2-aminophenols led directly to the formation of benzoxazoles **14q** and **15q**, showcasing the potential of this strategy for heterocycle synthesis.

<figure><img src="image_11.png" /><figcaption>**Scheme 4. Addition of Different Nucleophiles<sup>a,b</sup>**</figcaption></figure>

<sup>a</sup>General reaction conditions: **1q** (0.3 mmol, 1 equiv), **10c** (2 equiv) in CH<sub>2</sub>Cl<sub>2</sub> (1.5 mL) at 20 °C for 20 h *then* THF (5 mL) and alcohols (1.5 mL) or 2-aminophenols (5 equiv) added, stirred at 60 °C for 16 h *then* HCl<sub>aq</sub> (3M, 5 mL). <sup>b</sup>Isolated yields were reported.

Among all many utilities of amides in organic chemistry, one of the most prominent is their role as directing groups in C–H functionalization reactions. In particular, pyrrolidine amides have been successfully employed for this purpose.<sup>15</sup> To demonstrate the synthetic utility of our method, we used a rhodium-catalyzed addition of benzamide **1i** to nitrostyrene, developed by Ellman’s group (Scheme 5).<sup>15b</sup> Resulting *ortho*-alkylated product **1aq** was then subjected to our esterification conditions, affording fluoroester **11aq** in an excellent 89% yield.

<figure><img src="image_12.png" /><figcaption>**Scheme 5. Application with C–H Functionalization**</figcaption></figure>

In light of previous studies involving 2,2,2-trifluoroethyl(mesityl)iodonium triflate as an electrophile,<sup>16</sup> and supported by our experimental observations (see the Supporting information, paragraph 7), a plausible reaction mechanism is proposed in Scheme 6. First, amide **1** reacts with the iodonium triflate to form imidate triflate **I**, which was identified by NMR and HRMS analyses following removal of the dichloromethane solvent. Subsequently, water attacks intermediate **I**, leading to hydrolysis of the imidate moiety and formation of corresponding fluoroester **11**.

<figure><img src="image_13.png" /><figcaption>**Scheme 6. Plausible Mechanism**</figcaption></figure>

In summary, we have developed a novel amide activation strategy using 2,2,2-trifluoroethyl(mesityl)iodonium triflate as an electrophilic activator. This method enables the efficient conversion of a broad range of amides into esters and benzoxazoles under mild conditions. The high functional group tolerance and ability to selectively differentiate between two amide moieties underscore the synthetic utility of this protocol. Furthermore, the incorporation of various nucleophiles at the imidate intermediate stage expands the scope and versatility of the transformation. Extending this strategy to other classes of iodonium salts is currently in progress in our laboratory.
# ASSOCIATED CONTENT

## Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

## Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Experimental details and characterization data for all new compounds, including NMR spectra (PDF).

# AUTHOR INFORMATION

## Corresponding Author

Sébastien Prévost – Laboratoire de Synthèse Organique (LSO, UMR 7652), CNRS, Ecole Polytechnique, ENSTA, Institut Polytechnique de Paris, 91120 Palaiseau, France; Email: sebastien.prevost@ensta.fr

## Author Contributions

The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

### Notes

The authors declare no competing financial interest.

# ACKNOWLEDGMENT

This work was supported by the ANR JCJC grant NaFMeTS (ANR-20-CE07-0017). The CNRS, ENSTA and Ecole Polytechnique are acknowledged for financial support. T. Z. thanks the China Scholarship Council for his Ph.D. fellowship. The authors thank Sophie Bourcier (LCM - CNRS, Ecole Polytechnique, Institut Polytechnique de Paris), Marianne Guillard (LSO - CNRS, Ecole Polytechnique, ENSTA, Institut Polytechnique de Paris) and Pinar Yildirim (LSO - CNRS, Ecole Polytechnique, ENSTA, Institut Polytechnique de Paris) for HRMS analyzes. The ResoMag facility (CNRS/Institut Polytechnique de Paris) is acknowledge for the support with NMR facility.

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