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# Telluronium-Catalyzed Halogenation Reactions: Chalcogen-Bond Activation of N-Halosuccinimides and Catalysis

Loic Groslambert, Patrick Pale, Victor Mamane

► To cite this version:

Loic Groslambert, Patrick Pale, Victor Mamane. Telluronium-Catalyzed Halogenation Reactions: Chalcogen-Bond Activation of N-Halosuccinimides and Catalysis. Chemistry - A European Journal, 2024, 10.1002/chem.202401650. [https://doi.org/10.1002/chem.202401652](https://doi.org/10.1002/chem.202401652)

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# Tellurium-Catalyzed Halogenation Reactions: Chalcogen-Bond Activation of N-Halosuccinimides and Catalysis

Loic Groslambert,[a] Patrick Pale,[a] and Victor Mamane*[a]

The ability of triaryltellurium salts to interact with N-halosuccinimides (NXS) through chalcogen bonding (ChB) in the solid state and in solution is demonstrated herein. Cocrystals of the triaryltellurium bearing two $CF_3$ electron-withdrawing groups per aryl ring with N-chloro-, N-bromo- and N-iodosuccinimide (respectively NCS, NBS and NIS) were analyzed by X-ray diffraction, evidencing a ChB between tellurium and the carbonyl group of NXS. This ChB was confirmed in solution by NMR spectroscopy, especially by $^{125}$Te

NMR titration experiment, which allowed the determination of the association constant ($K_a$) between the tellurium and NBS. The so-obtained $K_a$ value of 17.3 ± 0.6 M⁻¹ indicated a moderate interaction in solution because of the competitive role of the solvent. The strength of the Te...O ChB was however sufficient enough to promote the catalytic halofunctionalization of aromatics and of alkenes such as the intra- and intermolecular haloalkoxylation and haloesterification of alkenes.

## Introduction

When linked to another atom, halogen and chalcogen atoms exhibit electropositive region(s) located at the outermost end of the σ-bond(s) including them. These electropositive regions, coined σ-hole as they spatially coincide with the corresponding σ* orbital, can thus interact with an electron donor atom (Lewis base).[1] Such σ-hole based interactions, defined as halogen or chalcogen bond (XB or ChB),[2,3] have attracted interest in various applications,[4] including noncovalent organocatalysis.[5] In this regard, organocatalysis based on XB was first introduced in 2008[6] whereas its ChB counterpart followed about ten years later[7] and ChB-based organocatalysis is currently burgeoning.[8]

As the largest, the less electronegative and the more polarizable of the stable chalcogens, tellurium exhibits the strongest ChB, especially when bonded to electron-withdrawing atoms or groups. Tellurium derivatives have therefore emerged as the most promising in ChB catalysis.[9] Being involved these last years in the study and application of ChB in solution,[10] we recently explored the catalytic properties of cationic tellurium salts,[11] where the trivalent tellurium(IV) exhibits three σ-holes and can establish three ChBs.[12] In particular, the methyldiaryl-telluronium 1 (Scheme 1b) was able to catalyze the bromination of anisole and the intramolecular bromolactonization of some ω-unsaturated carboxylic acids in the presence of N-bromosuccinimide (NBS).[11a]

a) Usual modes of activation of NXS

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

b) This work

<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Halogenation of alkenes with N-halosuccinimides (NXS) catalyzed by tellurium derivatives (BArF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate).</figcaption></figure>

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Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202401650

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© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

N-halosuccinimides NXS (X=Cl, Br, I) are versatile and widely used sources of halogens in electrophilic halogenation reactions. Depending on the partner nucleophilicity, they may require activation. Such activation could be achieved either by adding Brønsted acids, Lewis acids or Lewis bases. Among these approaches, one of the most successful for the halofunctionalization of alkenes and aromatics relies on the use of organocatalysts.[13] NXS can be activated by the interaction of

<footnote>[a] L. Groslambert, P. Pale, V. Mamane
Institute of Chemistry of Strasbourg, UMR 7177, CNRS and Strasbourg
University, 4 rue Blaise Pascal, 67000 Strasbourg, France
E-mail: vmamane@unistra.fr
ppale@unistra.fr</footnote>
<figure><img src="image_8.png" /></figure>

the organocatalyst either with the carbonyl oxygen or with the X atom. Lewis acid catalysts preferably interact through hydrogen bond (HB) with the oxygen[14] whereas Lewis base catalysts undergo addition on the X atom through a transient XB adduct.[15] A combination of both activation modes (HB and XB) was also proposed with bifunctional catalysts[16] (Scheme 1a). In contrast, the involvement of ChB in NXS activation was rare. Very recently, selenoniums,[17] telluroniums,[11a] and sulfoniums[18] were indeed shown to activate NBS and to catalyze the bromination of arenes and the bromolactonization of alkenes. Although suggested, ChB involvement was not demonstrated. Nevertheless, a theoretical investigation of anisole bromination in the presence of telluronium cations suggested that a Te···O ChB between the NBS carbonyl and Te σ-hole is first established and precedes the bromine transfer from NBS to anisole.[19] However, no experimental evidence of ChB involving NBS or any of its analog, namely N-chlorosuccinimide (NCS) or N-iodosuccinimide (NIS), has been reported.

This theoretical study was published while we were working on the mechanism of our tellurium-catalyzed bromination and bromolactonization mentioned above.[11a] We are now able to provide herein evidence for NXS activation by ChB in solid state and in solution, and we have demonstrated their role in triaryltellurium-catalyzed intra- and intermolecular halofunctionalizations of alkenes (Scheme 1b).

## Results and Discussion

As already shown,[10,11] important information on ChB interaction could be gained both in the solid state by X-ray diffraction (XRD) analysis if tellurium/NXS cocrystals could be obtained, and with NMR spectroscopy study of such mixtures in solution.

A CSD survey revealed that only a few cocrystals with NXS have been reported, but they mostly correspond to NXS and Lewis bases such as polyamines,[20] pyridines,[21] bipyridines,[22] and pyridine N-oxides,[23] and they only revealed XB as the main interaction between them. As we were able to isolate cocrystals of tellurium with acetone and characterize a highly directional ChB between tellurium and oxygen,[11b] we attempted cocrystallizing NXS derivatives with tellurium salts.

Two types of tellurium salts were recently reported by our group, the methyldiaryltelluroniums (Ar₂MeTe⁺X⁻) and the triaryltelluroniums (Ar₃Te⁺X⁻). In both series, the BArF salts 1 and 2 bearing CF₃ groups in 3,3'-positions of the aryl groups have shown the best catalytic properties.[11] In order to check the ability of tellurium salts 1 and 2 to bind through ChB to NBS, NCS and NIS in the solid state, we systematically engaged these halide donors with telluroniums 1 and 2, and attempted crystallizing such mixtures. Among these BArF salts, tellurium 2 showed the exceptional ability to cocrystallize with each NXS (X=Cl, Br, I) (see e.g. Figure 1 and S.I for structures and details). The pairs 2-NCS, 2-NBS and 2-NIS crystallized in the same P-1 space group with one tellurium cation, one BArF anion, one NXS molecule and one molecule of 1,2-dichloroethane (1,2-DCE) from the recrystallization solvent (Table 1). As shown in Figure 1a for cocrystal 2-NBS, Te atom interacts through its

<figure><img src="image_9.png" /><figcaption>**Figure 1.** X-Ray diffraction structure of 2-NBS (BArF anion and hydrogens were omitted for clarity): a) distances (Å) between atoms involved in ChBs and XBs; b) angles (°) of the interactions (the NBS located in the back is shown in light grey); c) distance (Å) between two planes containing NBS. Color codes: Te (orange), Br (brown), O (red), N (blue), Cl (green), C (grey), F (yellow).</figcaption></figure>

<table><caption>**Table 1.** Key parameters for ChBs and XBs observed in cocrystals of 2 with NBS, NCS and NIS.</caption><thead><tr><th>Entry</th><th>Data</th><th>2-NCS<sup>[b]</sup></th><th>2-NBS<sup>[b]</sup></th><th>2-NIS<sup>[b]</sup></th></tr></thead><tbody><tr><td>1</td><td>d<sub>Te-01</sub> (Å)</td><td>2.878(6)</td><td>2.874(5)</td><td>2.870(6)</td></tr><tr><td>2</td><td>RR<sub>ChB1</sub><sup>[a]</sup></td><td>0.80</td><td>0.80</td><td>0.80</td></tr><tr><td>3</td><td>θ<sub>1</sub> (°)</td><td>175.6(2)</td><td>175.5(2)</td><td>175.6(2)</td></tr><tr><td>4</td><td>d<sub>Te-02</sub> (Å)</td><td>3.145(3)</td><td>3.123(3)</td><td>3.104(4)</td></tr><tr><td>5</td><td>RR<sub>ChB2</sub><sup>[a]</sup></td><td>0.88</td><td>0.87</td><td>0.87</td></tr><tr><td>6</td><td>θ<sub>2</sub> (°)</td><td>173.6(2)</td><td>172.2(2)</td><td>170.2(2)</td></tr><tr><td>7</td><td>d<sub>Te-Cl</sub> (Å)</td><td>3.72(1)</td><td>3.57(1)</td><td>3.57(1)</td></tr><tr><td>8</td><td>RR<sub>ChB3</sub><sup>[a]</sup></td><td>0.97</td><td>0.94</td><td>0.94</td></tr><tr><td>9</td><td>θ<sub>1</sub> (°)</td><td>170.4(2)</td><td>177.6(2)</td><td>165.4(3)</td></tr><tr><td>10</td><td>d<sub>X-Cl</sub> (Å)</td><td>3.28(1)</td><td>3.114(2)</td><td>3.163(3)</td></tr><tr><td>11</td><td>RR<sub>XB</sub><sup>[a]</sup></td><td>0.94</td><td>0.87</td><td>0.85</td></tr><tr><td>12</td><td>θ<sub>1</sub> (°)</td><td>163.6(3)</td><td>174.8(1)</td><td>173.7(2)</td></tr><tr><td>13</td><td>d<sub>N-X</sub> (Å)</td><td>1.680(5)</td><td>1.841(4)</td><td>2.041(6)</td></tr></tbody></table>

[a] $RR = d/\Sigma r_{vdw}$ ($r_{vdwTe} = 2.06$ Å, $r_{vdwO} = 1.52$ Å, $r_{vdwCl} = 1.75$ Å, $r_{vdwBr} = 1.83$ Å, $r_{vdwI} = 1.98$ Å)

[b] For comparison purposes, literature data from X-ray structures of NCS,[25] NBS[26] and NIS[27] are the following: $d_{N-Cl} = 1.682(4)$ Å, $d_{N-Br} = 1.839(1)$ Å, $d_{N-I} = 2.060$ Å.

three σ-holes with two molecules of NBS via their carbonyl oxygen ($d_{Te-01} = 2.874$ Å and $d_{Te-02} = 3.123$ Å), and with one molecule of 1,2-DCE via one of its chlorine ($d_{Te-Cl} = 3.567$ Å). Interestingly, the other 1,2-DCE chlorine also interacts through XB with the bromine atom of NBS ($d_{Br-Cl} = 3.114$ Å). For each interaction, *d* is far below the sum of van der Waals radii[24] and, as expected for ChB and XB, these interactions are almost linear with angles (C<sub>Ar</sub>–Te···O and N–Br···Cl) in the range 172–176° (Figure 1b). Interestingly, the two molecules of NBS interact in an antisymmetric way through their other oxygen atoms with another Te atom, resulting in the anti-parallel arrangement of the two NBS molecules with an interplanar distance of 3.439 Å (Figure 1c). This organization (Figure 1a) is surrounded by the BArF anions through multiple CH–F interactions (see S.I. for details).
<figure><img src="image_10.png" /></figure>

In order to compare the ChB and XB strength in the three cocrystals, the reduction ratio (RR) parameter defined as $d/\Sigma r_{vdw}$ was employed, where d is the distance between the two interacting atoms and $\Sigma r_{vdw}$ the sum of their van der Waals radii (Table 1). The values of $RR_{ChB1}$ for the strongest ChB and $RR_{ChB2}$ for the weaker one show that the nature of the X atom has no influence on the strength of these two Te···O ChBs (entries 2 and 5). The third ChB between Te and the chlorine atom of 1,2-DCE is much weaker (see $RR_{ChB3}$ values of entry 8) but the observation of this ChB revealed the probable competing role of such solvent in solution with respect to a Lewis base. However, in the solid state, the 1,2-DCE molecule seems to increase the crystal cohesion by forming an additional XB with one NXS molecule. In this case and as expected, the XB strength between Cl and the X atom followed the usual trend $Cl < Br < I$ (see $RR_{XB}$ values of entry 11). Almost all the observed interactions show high linearity ($178^{\circ} > \theta > 170^{\circ}$, see entries 3, 6, 9 and 12), except two that involve the chlorine atom of the 1,2-DCE (entry 9 for NIS and entry 12 for NCS). In these cases, the values of $163-165^{\circ}$ still stand in the range of angles generally observed for $\sigma$-hole interactions ($160-180^{\circ}$).

The Te···O ChBs are expected to induce the elongation of the N–X bond. It is worth noting that the measured N–Br distance in cocrystal **2-NBS** is 1.841 Å (entry 13), exactly the same value calculated by Chen, Wang and co-workers in the optimized geometry of telluronium **1** with NBS.[19] The authors noted that this bond was elongated to 1.843 Å when the Te-bonded aryl groups were substituted by more electron-withdrawing groups. The comparison between the N–X distance in the telluronium cocrystals and in NXS molecules alone was difficult because in the reported structural analysis of NCS,[25] NBS[26] and NIS,[27] an intermolecular X···O XB was always present

that could also lead to the elongation of the N–X bond. In particular, the N–I bond was significantly longer in NIS alone than in **2-NIS** (2.060 vs 2.041 Å) whereas the N–Br bond was slightly longer in **2-NBS** than in NBS alone (1.841 vs 1.839 Å).

Monitoring by NMR spectroscopy was then performed to detect if a ChB was also occurring in solution between telluronium **2** and the three NXS compounds (Table 2) (For data involving telluronium **1**, see SI). Successfully used to detect ChB in solution,[10f–g,11a] $^{125}$Te NMR was applied together with $^{1}$H and $^{13}$C NMR spectroscopy. Upon mixing the telluronium **2** with one equiv. of NXS in deuterated dichloromethane ($CD_{2}Cl_{2}$), significant chemical shift variations ($\Delta\delta$) were observed in $^{13}$C NMR spectra for the carbonyl group of NXS and in the $^{125}$Te NMR spectra for the telluronium. For the carbonyl group, a similar variation of ($\Delta\delta = +0.8 \pm 0.1$ ppm) was measured for NCS and NBS whereas a higher variation was observed for NIS ($\Delta\delta = +2.2$ ppm). The most significant $\Delta\delta$ (from +11.7 to +17.4 ppm) were observed in the $^{125}$Te NMR spectrum of **2**, as expected from the higher sensitivity of this nucleus and because of the direct involvement of Te in the ChB. Interestingly, high variations were also noted for the *ipso* carbon $C_{1}$ of **2**. This result is in accordance with our recent analysis of the distribution of Bader charges in a telluronium-triphenylphosphine oxide (OPPh$_{3}$) adduct showing that a large part of the electron density is transferred from OPPh$_{3}$ to the *ipso* carbon of the telluronium.[12] In contrast, low $\Delta\delta$ were observed in both the $^{1}$H and $^{13}$C NMR spectra for the 2- and 4-positions of the aryl groups of telluronium **2**.

Since the more pronounced effects occurred in $^{125}$Te NMR spectroscopy, the latter was employed for titration measurements in order to determine association constants ($K_{a}$) between **2** and NBS. Unfortunately, the low solubility of NCS and NIS in

<table><caption>**Table 2.** Chemical shifts ($\delta$) and variations ($\Delta\delta$) in $^{1}$H, $^{13}$C and $^{125}$Te NMR spectra upon addition of 1 equiv. of NXS to telluronium **2** in $CD_{2}Cl_{2}$ and $K_{a}$ value obtained by titration.</caption><thead><tr><th colspan="2">2</th><th colspan="6">Ar = 3,5-(CF<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub></th></tr><tr><th></th><th></th><th colspan="3">δ and Δδ (ppm)</th><th colspan="3"></th></tr><tr><th></th><th></th><th>2/NCS</th><th>Δδ</th><th>2/NBS</th><th>Δδ</th><th>2/NIS</th><th>Δδ</th></tr></thead><tbody><tr><td>2<sup>[a]</sup></td><td>NXS<sup>[b]</sup></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>H<sub>2</sub></td><td>-</td><td>8.14</td><td>+0.05</td><td>8.12</td><td>+0.03</td><td>8.14</td><td>+0.05</td></tr><tr><td>H<sub>4</sub></td><td>-</td><td>8.43</td><td>-0.01</td><td>8.41</td><td>-0.01</td><td>8.42</td><td>-0.02</td></tr><tr><td>C<sub>1</sub></td><td>-</td><td>122.5</td><td>+1.2</td><td>122.4</td><td>+1.1</td><td>122.8</td><td>+1.5</td></tr><tr><td>C<sub>2</sub></td><td>-</td><td>134.8</td><td>+0.1</td><td>134.8</td><td>+0.1</td><td>134.9</td><td>+0.2</td></tr><tr><td>C<sub>4</sub></td><td>-</td><td>130.0</td><td>-0.4</td><td>130.0</td><td>-0.3</td><td>129.9</td><td>-0.5</td></tr><tr><td>Te</td><td>-</td><td>783.9</td><td>+11.7</td><td>785.9</td><td>+13.7</td><td>789.6</td><td>+17.4</td></tr><tr><td>-</td><td>C=O</td><td>172.3</td><td>+0.7</td><td>174.5</td><td>+0.9</td><td>180.1</td><td>+2.2</td></tr><tr><td>K<sub>a</sub> (M<sup>-1</sup>)<sup>[c,d]</sup></td><td></td><td>n.d.<sup>[e]</sup></td><td></td><td>17.3 ± 0.6</td><td></td><td>n.d.<sup>[e]</sup></td><td></td></tr></tbody><tfoot><tr><td colspan="8"><sup>[a]</sup> Chemical shifts (ppm) for 2 in $CD_{2}Cl_{2}$: $\delta_{H2}$ = 8.09, $\delta_{H4}$ = 8.44, $\delta_{C1}$ = 121.3, $\delta_{C2}$ = 134.7, $\delta_{C4}$ = 130.4, $\delta_{Te}$ = 772.2</td></tr><tr><td colspan="8"><sup>[b]</sup> Chemical shifts (ppm) in $CD_{2}Cl_{2}$ for C=O: $\delta_{NCS}$ = 171.6, $\delta_{NBS}$ = 173.6, $\delta_{NIS}$ = 177.9</td></tr><tr><td colspan="8"><sup>[c]</sup> monitoring by $^{125}$Te NMR spectroscopy</td></tr><tr><td colspan="8"><sup>[d]</sup> Value for the 1:1 association (errors of 20% are generally assumed for $K_{a}$ measured by titration)</td></tr><tr><td colspan="8"><sup>[e]</sup> not determined because of the low solubility of NCS and NIS in $CD_{2}Cl_{2}$.</td></tr></tfoot></table>
<figure><img src="image_11.png" /></figure>

CD₂Cl₂ did not allow to measure the corresponding Kₐ values with tellurium 2. Considering that three σ-holes are present on the Te atom of telluriums,[¹²] the association of 2 with NBS should lead to three new species in solution: 2-(NBS)₁, 2-(NBS)₂ and 2-(NBS)₃. In contrast to the strong Lewis bases employed in our previous study (phosphine oxides, dimethyl sulfoxide and pyridine N-oxide),[¹¹a,¹²] no signal in Isothermal Titration Calorimetry (ITC) was detected after NBS addition to 2, indicating that this weak Lewis base did not strongly bind to the tellurium. For this reason, only the species 2-(NBS)₁ was considered in the present study for Kₐ determination. For a 1:1 association, a value of Kₐ = 17.3 M⁻¹ was obtained with tellurium 2, slightly higher but close to the value of Kₐ = 16.4 M⁻¹ determined for the association with tellurium 1 (see SI for details). Based on these values and on the higher stability of tellurium 2 in solution, the latter was preferred as catalyst for the halogenation reactions (see below).

Having demonstrated the interaction of **2** with NXS compounds through ChB in the solid state and more importantly in solution, this tellurium was then used as catalyst in typical halogenation reactions that employ NXS as reagent.

For comparison purposes, the classical aromatic halogenation was first evaluated (Scheme 2). As already reported,[¹¹a] the electron-rich anisole was readily and selectively brominated to give **3a** within minutes in the presence of 1 or 5 mol% of tellurium salt **1** (respectively 96 or 97% after 5 min). Similar results were achieved with catalyst **2**, although with slightly

longer reaction time (quantitative with 1 or 5 mol% of **2** after 3 or 0.5 h). Nevertheless, the higher stability of the triaryltelluronium **2** compared to the methyldiaryltelluronium **1** led us to select the former for this study and the following. In contrast, chlorination did not proceed under the same conditions and the starting material was recovered. Iodination proved more effective and a mixture was obtained, in which the expected 4-iodoanisole **3b** was the major product, with an estimated 69% yield, together with some of the bis-iodinated product and the starting anisole. After 2 h, their proportion were respectively of 73, 7 and 20%.

Interestingly, control experiments showed that no reaction occurred without catalyst, revealing the crucial role of the tellurium catalyst in such reaction.

Less activated substrates proved far less reactive, as shown by the lack of reaction with benzene, toluene and even with tert-butylbenzene. Interestingly, phenol and aniline could also be brominated under these conditions, without the need of protecting groups. 2,6-Dimethylphenol quantitatively gave its 4-bromo analog **3c** within minutes, while 4-bromoaniline gave a mixture of mono- and dibromo derivatives. With 1.3 equiv. of NBS, a mixture of the starting 4-bromoaniline, 2,4-dibromoaniline and 2,4,6-tribromoaniline was obtained in a ratio of respectively 42:24:33. However, using 2.1 equiv. of NBS allowed the unique and again quantitative formation of the expected 2,4,6-tribromoaniline **3d**. Heteroaromatics and polyaromatics also readily reacted with NBS in the presence of **2**, but depending on their reactivity, they could be mono-, di- or polybrominated. For example, 2-methylthiophene or α-methoxynaphthalene could be quantitatively monobrominated within minutes, providing **3h** and **3e** respectively. In contrast, anthracene and pyrene provided a mixture of brominated derivatives; however, 9,10-dibromoanthracene **3f** could be selectively obtained by using 2.1 equivalents of NBS, while a mixture of brominated pyrenes **3g** was obtained, even with an NBS excess.

In comparison, the reported diarylmethyl- or triarylselenonium salts at a 5 mol% loading were able to promote the NBS-induced formation of 4-bromoanisole **3a** with modest yields (20–60%) after long reaction time (24 h), while the analogs sulfonium salts were even less effective as catalyst (traces-15%).[¹⁷a] In the case of aromatics **3c-3f**, selenonium salts provided a similar reactivity but after long reaction times (24–72 h),[¹⁷a] highlighting the increased efficacy of tellurium salts.

With these encouraging results in hands, we further explored other classical halogenation such as the bromocyclization of alkenyl alcohols and acids (Scheme 3).

In the presence of catalyst **2** (5 mol%) and of a slight excess of NBS, alkenyl alcohols provided the expected cyclic bromoethers **4a-c** in very high yields (92–95%) within 1–2 h. As usual in such cyclization,[²⁸] *exo* or *endo* products were formed depending on the chain length and the alkene substitution. So, 4-phenylbut-3-en-1-ol only led to the *endo* product **4a**, while pent-4-en-1-ol provided only the *exo* product **4b**. However, its methyl substituted analog led to a mixture of *exo* and *endo* isomers **4c** (82:18). This preference for **5-endo-cyclization** as well as the partial formation of **6-endo** product when the alkene

<figure><img src="image_12.png" /><figcaption>**Scheme 2.** Tellurium-catalyzed aromatic halogenation with NXS reagents. The reactions were carried out with aromatic (0.2 mmol), catalyst and NBS (0.26 mmol) in CD₂Cl₂ at 25 °C (nr: no reaction). The yields were determined by ¹H NMR spectroscopy with dimethyl terephthalate as the standard (see SI for details). a) a complex mixture was obtained; b) performed with 2.1 equiv. of NBS; c) isolated yield; d) performed with 4.2 equiv. of NBS.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Scheme 3.** Telluronium-catalyzed bromo-alkoxylation and -lactonization of alkenes. The reactions were carried out with unsaturated alcohol or carboxylic acid (0.2 mmol), catalyst and NBS (0.21 mmol) in CD₂Cl₂ at 25 °C. The yields were determined by ¹H NMR spectroscopy with dimethyl terephthalate as the standard (see SI for details). <sup>a</sup> Isolated yield.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Scheme 4.** Telluronium-catalyzed halo-alkoxylation and -esterification of alkenes. The reactions were carried out with alkene (0.2 mmol), alcohol or carboxylic acid (0.2 mmol), catalyst (0.01 mmol) and NXS (0.21 mmol) in CD₂Cl₂ at 0 °C. The yields were determined by ¹H NMR spectroscopy with dimethyl terephthalate as the standard (see SI for details). <sup>a</sup> Isolated yield.</figcaption></figure>

carries a terminal (conjugating) substituent is typical of halocyclization reactions.[28]

The comparison with other catalysts in this bromoetherification is again beneficial to telluronium as catalyst. Indeed, selenonium salts could promote the NBS cyclization of pent-4-en-1-ol with the same yield (92%), but starting from its trimethylsilyl derivative and with 10 mol% of catalyst after 24 h, instead of 2 h with only 5 mol% for **2** as catalyst.[17a]

Similarly, the analog but-3-enoic, pent-4-enoic and hex-5-enoic acids could be readily cyclized to the expected bromolactones **5a-e**, but with longer reaction times as expected with the less nucleophilic acid moiety and with half catalyst loading. These acids when unsubstituted at the alkene moiety only provided the exo products **5a**, **5d-e**, in modest to high yields depending on the chain length. With a methyl or phenyl group on the alkene moiety, only the *endo* products **5b-c** were obtained, in good to high yields.[28] As expected, doubling the amount of catalyst led to an approximative half decrease in reaction time to reach the same yield (see e.g. **5d**). Alternatively, none of the above-reported reactions occurred without catalyst, and the starting materials were recovered.

In comparison, selenium-activated or selenonium-catalyzed halocyclizations gave similar yields, but with higher catalyst loading or the requirement of cocatalyst (e.g. DMAP) in longer reaction times.[17a,29]

In a third set of experiments, we evaluated the capacity of tellurium catalyst in the more challenging intermolecular halo-alkoxylation and -esterification of alkenes (Scheme 4). A series of simple alkenes, cyclic or not, were engaged together with common alcohols or acids in the presence of NXS and of catalytic amount of tellurium **2**.

Cyclohexene readily reacted with methanol or benzyl alcohol and NBS to furnish the corresponding *trans* 1-alkoxy-2-bromocyclohexanes **6a-c**. Only moderate yields (~60%) were achieved despite various attempts to tune the conditions, mostly due to the fact that polymerization occurred as the main side-reaction.[30] In the presence of carboxylic acids as nucleophile, the reaction became less efficient due to lower nucleophilicity, but still provided the expected *trans* product **6c**. With the more reactive cyclic enol ether analog, the reaction provided better yield of the bromo acid adduct **7b**, but the simple bromoalkoxylation became more sluggish and the product **7a** could not be isolated.

Using styrene derivatives allowed the double addition to occur with high yields, but the temperature must be maintained at 0 °C to limit polymerization. The simple styrene rapidly provided almost quantitatively the bromo benzyloxy adduct **8b**, while the bromo methoxy adduct **8a** was only isolated with moderate yield among a mixture (97 vs 45%). Interestingly, the more stable *α*-methyl styrene furnished both adducts **8c** and **8e** in high yields. Furthermore, the reaction could also be performed as effectively with NIS, providing the iodo methoxy adduct **8d** in similar yield. However, using benzoic acid as nucleophile only led to a mixture, in which the expected product **8f** could barely be detected, due to polymerization. As a more rigid analog, indene proved more stable than styrene and a quantitative yield of the bromo benzyloxy adduct **9b** could be obtained, while the bromo methoxy adduct **9a** was only isolated with moderate yield.
<figure><img src="image_15.png" /></figure>

It is worth noticing that diaryl selenoethers carrying an *ortho*-carbonyl group have recently been described as efficient catalysts at low loading for such intermolecular halo esterification. They nevertheless required longer reaction time.[31]

It is worth also mentioning that in the reactions detailed above, no degradation of the catalyst was observed, as checked by ¹⁹F NMR spectroscopy on the crudes after reaction.

The reactivity study described above, combined with our investigations in the solid state and in solution, clearly show that in these reactions, the tellurium catalyst act as a Lewis acid and activate NXS through a ChB. Furthermore, the spatial arrangement around the tellurium atom should leave enough room for the aromatic or the alkene to interact with the halogen atom of the so-activated NXS, favoring the reaction (Scheme 5a).

Due to the presence of three σ-holes on the Te atom, it could be envisaged to have the nucleophile also bound to one of the σ-holes, similarly to 1,2-DCE in the **2-NXS** crystal structures (see Figure 1a and b). As such, the catalyst would serve as a template preorganizing the different partners, and thus facilitating their reaction, in a way close to how enzymes are acting (Scheme 5b). Indeed, analysis by NMR spectroscopy of mixtures containing tellurium **2** and pent-4-enoic acid showed significant chemical shift variations of the acid carbonyl group and of the tellurium in respectively ¹³C NMR and ¹²⁵Te NMR spectra (see SI for details).

Further work is clearly required to gain more details about the actual mechanism of these tellurium-catalyzed halogenation reactions.

## Conclusions

Chalcogen bonds between a triaryltellurium salt and N-halosuccinimides (NXS, X=Cl, Br, I) have been investigated and then applied to catalysis. XRD analysis clearly showed that interaction occurred between the tellurium Te atom and the carbonyl oxygen of each NXS. Interestingly, the three Te σ-holes were involved in ChBs, two with the carbonyl groups of two NXS, and one with one of the chlorine atoms from 1,2-DCE,

used as cocrystallization solvent. Multinuclear NMR investigations revealed that in solution, addition of a stoichiometric amount of NXS to the tellurium salt induced significant chemical shifts variations in ¹³C and ¹²⁵Te NMR spectra as a result of the Te...O ChB. Subsequently, the ability of the tellurium salt to activate NBS and to catalyze halogenation reactions was evaluated. Good to high catalytic activity was found in several reactions, including aromatic bromination, intramolecular bromoetherification and bromolactonization of alkenes. More importantly, the tellurium salt could also catalyze the intermolecular bromo- or iodoetherification and bromoesterification of alkenes, in the presence of respectively alcohol and carboxylic acid nucleophiles.

Further studies combining ChB catalysis and cocrystallization experiments with triaryltellurium salts as a tool to elucidate ChB-catalyzed reactions are underway in our group.

## Experimental Section

**X-Ray Diffraction.** Single crystals of **2-NCS**, **2-NBS** and **2-NIS** were obtained at -30 °C by slow vapor diffusion of heptane into a solution containing the tellurium and NXS (ratio 1:3) in 1,2-DCE. The crystals were placed in oil, and a single crystal was selected, mounted on a glass fiber and placed in a low-temperature N₂ stream. X-ray diffraction data collection was carried out on a Bruker PHOTON III DUO CPAD diffractometer equipped with an Oxford Cryosystem liquid N₂ device, using Mo-Kα radiation (λ = 0.71073 Å). The crystal-detector distance was 37 mm. The cell parameters were determined (APEX3 software)[32] from reflections taken from 1 set of 180 frames at 1 s exposure. The structures were solved using the program SHELXT-2014.[33] The H-atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F2. A semi-empirical absorption correction was applied using SADABS in APEX3.

Depositions Number(s) 2348710 (for **2-NCS**), 2348709 (for **2-NBS**), and 2348708 (for **2-NIS**) contain(s) the supplementary crystallographic data for this paper. These data are provided by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.

## Catalytic Experiments

**Aromatic bromination (Scheme 2).** To an NMR tube containing the aromatic compound (0.2 mmol) and NBS (0.26 mmol for the monobromination, 0.42 mmol for the dibromination) in CD₂Cl₂, was added a solution of the catalyst **2** (20 mM in CD₂Cl₂, 1 or 5 mol%) to reach a total volume of 0.5 mL CD₂Cl₂. The mixture was stirred in the dark at 25 °C for the indicated time. The reaction was monitored by ¹H NMR spectroscopy after 5 min, 30 min, 1 h and then each hour up to completion. The stability of the catalyst was controlled by ¹⁹F NMR spectroscopy after reaction completion. After completion of the reaction, the standard (dimethyl terephthalate) was added to the NMR tube to determine the yields by ¹H NMR spectroscopy.

**Intramolecular bromo-etherification and lactonization (Scheme 3).** To an NMR tube containing the unsaturated alcohol or acid (0.2 mmol) and NBS (0.21 mmol) in CD₂Cl₂, was added a solution of the catalyst **2** (20 mM in CD₂Cl₂, 5 mol%) to reach a total volume of 0.5 mL CD₂Cl₂. The mixture was stirred in the dark at 25 °C for the indicated

<figure><img src="image_16.png" /><figcaption>**Scheme 5.** Mechanistic proposal for substrate activation in the tellurium-catalyzed halogenation of alkenes: the example of pent-4-enoic acid. a) Activation of the substrate through XB with NBS; b) Concomitant activation of the substrate through XB with NBS and ChB with the tellurium.</figcaption></figure>
time. The reaction was monitored by ¹H NMR spectroscopy after 5 min, 30 min, 1 h, 2 h, 6 h, 10 h, 16 h and 24 h. The stability of the catalyst was controlled by ¹⁹F NMR spectroscopy after reaction completion. After completion of the reaction, the standard (dimethyl terephthalate) was added to the NMR tube to determine the yields by ¹H NMR spectroscopy.

Intermolecular bromo-etherification and esterification (Scheme 4). To an NMR tube containing NBS (0.21 mmol) in CD₂Cl₂, was added a solution of the catalyst 2 (20 mM in CD₂Cl₂, 5 mol%) to reach a total volume of 0.5 mL CD₂Cl₂. Then, the alkene derivative (0.2 mmol) was added to the NMR tube followed by the alcohol or the carboxylic acid nucleophile (0.20 mmol). The mixture was stirred in the dark at 0 °C for 1 h then at 25 °C for 5 min. The reaction was monitored by ¹H NMR spectroscopy and the stability of the catalyst was controlled by ¹⁹F NMR spectroscopy. After completion of the reaction, the standard (dimethyl terephthalate) was added to the NMR tube to determine the yields by ¹H NMR spectroscopy.

## Supporting Information

X-ray diffraction data of cocrystals, NMR spectroscopy details (chemical shift variation data and titration experiments) and full details on catalysis experiments are available in the Supporting Information. The authors have cited additional references within the Supporting Information.[³⁴⁻⁴⁷]

## Acknowledgements

This research was funded by the French National Research Agency (ANR-21-CE07-0014). LG thanks the ANR for a PhD fellowship. Centre national de la Recherche Scientifique (CNRS) and the University of Strasbourg are thanked for their support.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords:** Chalcogen bonding • Catalysis • Halogenation • σ-Hole • Tellurium

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Manuscript received: April 26, 2024

Accepted manuscript online: May 24, 2024

Version of record online: ■■, ■■
RESEARCH ARTICLE

The chalcogen bond (ChB) involving the tellurium atom of CF₃-functionalized triaryltelluronium salt and the carbonyl oxygen of N-chloro-, N-bromo- and N-iodosuccinimide (NCS, NBS and NIS) was evidenced in the solid state by X-ray diffraction and in solution by ¹²⁵Te NMR. This ChB was strong enough in solution to allow catalytic halogenation reactions to occur at room temperature and even at 0°C with activated substrates.

<figure><img src="image_18.png" /></figure>

L. Groslambert, P. Pale*, V. Mamane*

1 – 9

Telluronium-Catalyzed Halogenation
Reactions: Chalcogen-Bond Activa-
tion of N-Halosuccinimides and
Catalysis

<figure><img src="image_19.png" /></figure>