# Chalcogen Bonding Catalysis: Tellurium, the Last Frontier?

Patrick Pale, Victor Mamane

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## To cite this version:

Patrick Pale, Victor Mamane. Chalcogen Bonding Catalysis: Tellurium, the Last Frontier?. Chemistry - A European Journal, In press, 10.1002/chem.202302755. hal-04234399

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# Chalcogen Bonding Catalysis: Tellurium, the Last Frontier?

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

<figure><img src="image_3.png" /></figure>
[a] Prof. P. Pale, Dr. V. Mamane
Institute of Chemistry of Strasbourg, UMR 7177-LASYROC
CNRS and Strasbourg University
4 rue Blaise Pascal, 67000 Strasbourg (France)
E-mail: ppale@unistra.fr; vmamane@unistra.fr

**Abstract:** Chalcogen bonding (ChB) is the non-covalent interaction occurring between chalcogen atoms as Lewis acid sites and atoms or groups of atoms able to behave as Lewis bases through their lone pair or π electrons. Analogously to its sister halogen bonding, the high directionality of this interaction was implemented for the precise structural organization in the solid state and in solution. Regarding catalysis, ChB is now accepted as a new mode of activation as demonstrated by the increased number of examples in the last five years. In the family of ChB catalysts, those based on tellurium rapidly appeared to overcome their lighter sulfur and selenium counterparts. In this review, we highlight the Lewis acid properties of tellurium-based derivatives in solution and summarize the start-of-the-art of their applications in catalysis.

# 1. Introduction

## 1.1. Importance of tellurium

Tellurium, the heaviest stable chalcogen [1], is an element which on Earth is mostly found in coinage metal ores, especially in volcanic associated sulfide deposits in which it can represent up to hundreds of ppm [2]. Although relatively rare but widely distributed on Earth [3], tellurium has become an important element because of its involvement in special alloys, notably for producing thermoelectric cooling devices, as well as in photovoltaic solar cells and in photoreceptors for copying machines/printers or telescopes. As such, tellurium is listed as 'critical metal' in the USA and European Union since 2010 [2,4]. It is also used as a vulcanization agent leading to stronger rubber, as a co-catalyst in ammonoxidation of alkenes or alkanes, especially towards acrylonitrile [5] and as a colouring agent in ceramics and glass (Fig. 1).

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Evolution of the various tellurium uses.</figcaption></figure>

## 1.2. Tellurium (bio)chemistry

Most of the applications mentioned above rely on the inorganic chemistry of tellurium [6]. In contrast, the organic chemistry of tellurium is so far less represented in these

applications. Pioneered by H. Rheinboldt in the mid-50's, this chemistry has slowly but gradually evolved up to the turn of the XXIst century with the production of dedicated reviews and books [7]. The scarcity of applications despite this rich chemistry may be due to the bad smelling reputation of tellurium stated by the early investigators, notably Wöhler and Heeren in 1850-60, who prepared the first dialkyltellurides, indeed stinky, volatile and highly toxic [8]. However, such nasty effects do not extent to other organotellurium derivatives, as shown by toxicology studies [9], although it is safer to take usual precautions when handling them. In some way but with an obvious delay, the organic chemistry of tellurium is facing the same problems as that of selenium at its beginning, up to the discovery that selenium is an essential trace element for numerous animals, including humans [10].

In contrast to selenium, tellurium does not seem to exhibit natural biological role. Nevertheless, it has been found that tellurium could be incorporated into some microorganisms as metalloproteins containing high level of tellurocysteine, tellurocystine, and telluromethionine [11], and in some plants growing respectively on tellurium-containing media or soils [12]. Furthermore, it has been shown that tellurium compounds present antitumoral, immunomodulatory and neuroprotective effects, mostly due to their ability to inhibit cysteine proteases, and antioxidant activity mimicking as such selenium derivatives (Fig. 2) [13]. Antibiotic and antiparasitic activity have also been reported and very recently reviewed [14]. These properties clearly provide strong bases for the development of innovative drugs and tools for biological imaging and diagnostic.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Selected examples of tellurium-based bioactive compounds.</figcaption></figure>

## 1.3. Chalcogen bonding

The growing importance of tellurium compounds is apparent from the above-mentioned aspects. In the last years, this importance increased even more with what can be seen as a
renaming of the concept of secondary bonding (intra- or intermolecular) interactions between chalcogens or chalcogen and other atoms [15] to the now so-called chalcogen bonds (ChB) [16]. This term was coined in obvious analogy with the common hydrogen bond (HB) and the more recently recognized halogen bond (XB) [17]. However, and in sharp contrast, XB and ChB exhibit higher directionality than HB, usually a linear one. Experimental and theoretical studies reveal that a combination of dispersion, electrostatic and orbital delocalization effects are responsible for such interactions and their directionality. Indeed, covalently-bonded electron-rich atoms exhibit anisotropic electronic densities, creating regions of lower density [18] along the extension of the σ bond, called σ-holes (blue regions in Fig. 3) [19]. Such a localized electrostatic potential, together with a concomitant empty σ* orbital can thus attractively interact with electron-rich negative sites (σ-hole acceptors noted B in Fig. 3) such as lone pairs, anions, π-electrons through the so-called σ-hole interaction [20-22]. Furthermore, the σ-hole nature depends on the atom polarizability and on the electronic property of the substituent bonded to this atom, and thus can be tuned [23].

Interestingly, the divalent nature of chalcogen atoms induces the presence of two σ-holes, and even three for chalconium [24], instead of one for halogen atom. More than one ChB can thus be established, and furthermore, the chalcogen atom can act both as Lewis acid through its σ-holes and as Lewis base due to its lone pairs (Fig. 3).

Patrick Pale obtained his PhD in 1982 from the University of Champagne-Ardenne, France, with Prof. J. P. Pete, J. Muzart. After an industrial stay in a pharmaceutical company, he joined Prof. L. Ghosez in Belgium. He was successively appointed as CNRS Researcher at the University of Champagne-Ardenne, where he obtained a "Doctorat d'Etat" (Habilitation) in 1988, as Research Associate at Harvard University with Prof. G. Whitesides (1988-1990) and as Professor of Organic Chemistry at the University of Strasbourg (1995-present). He was awarded Professor at the Institut Universitaire de France (1996-2001). His scientific interests include total synthesis, green chemistry and catalysis.

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

Victor Mamane obtained his Master (1998 with Prof. H. B. Kagan) and his PhD (2001 with Prof. O. Riant) in organic chemistry from the University of Orsay, France. From 2001 to 2004, he successively worked as Max-Planck Research Fellow in the groups of Prof. A. Fürstner (Mülheim/Ruhr) and Prof. H. Waldmann (Dortmund). He was appointed CNRS Researcher in 2004 at University of Nancy and in 2013, he moved to the University of Strasbourg where he became CNRS Director in 2023. His research interests mainly focus on the study of sigma-hole interactions in solution and application to catalysis.

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

<figure><img src="image_8.png" /><figcaption>**Figure 3.** Electrostatic surface potential maps of chalcogen atoms highlighting their σ-holes (blue areas) acting as Lewis acid. Left: the resulting ChB with σ-hole acceptors *B* (or Lewis bases); Right: a tellurium cation with its 3 σ-holes highlighted (black arrows).</figcaption></figure>

With such properties, ChBs have very recently raised increasing interest with its application to crystal engineering [25], following its initial identification in solid state [26], but also more recently to organic chemistry, especially in supramolecular chemistry and in anion recognition and transport [27]. Even more recently, ChB-driven organocatalysis has emerged, and the few reports have already been reviewed [28].

## 1.4. Chalcogen bonding in solution involving tellurium

In the chalcogen series, tellurium is the largest [29], the less electronegative [30], the more polarizable and ionizable, and thus tellurium species exhibit the strongest ChB, compared to their sulfur and selenium analogues [15]. Tellurium derivatives are thus the most promising for the above-mentioned applications, especially catalysis. Nevertheless, catalysis based on tellurium really is in its infancy.

In contrast to crystal engineering, catalysis is usually performed in a solvent. However, the solvent nature is a critical issue for successful ChB-driven catalysis, because solvent molecules could act as Lewis base and thus compete with the reagent supposedly able to establish the expected ChBs. Such competition could lower the catalytic rate or even inhibit the catalyst. In the initial works, trials and errors were the usual way, but now, several studies have been performed to better understand the ChB behavior and strength in solution (Fig. 4) [31].

Data regarding the thermodynamics of the ChB in solution and the solvent influence are sparse. In 2010, Gabbaï et al. designed a bidendate Lewis acid containing a tellurium cation as a recognition site for fluoride (Fig. 4A) [32]. Structural and computational studies showed that the unusual high fluoride affinity of the tellurium resulted from a strong donor–acceptor interaction from F lone-pair to the Te–C σ* orbital. Moreover, on the basis of the association constant reported for a control receptor, the contribution of the ChB to the free energy of fluoride binding in methanol may have been as much as 12.7 kJ/mol. Later, Taylor et al. conducted a detailed solution study with the aim to monitor the spectroscopic modifications induced by the ChB formation between benzotelluradiazoles and Lewis bases (*B*) in different solvents (Fig. 4B) [33]. The association constants determined by UV-visible absorption showed that the ChB strength was clearly influenced by the solvent coordination ability.

In regard to the increasing importance of ChB these last years, several solution studies involving tellurium-based derivatives were recently reported. In a series of papers, our group evaluated the ChB performance of compounds based on tellurium at
different oxidation states. Te(+I)-based diarylditellurides and phosphine oxides could form very weak ChBs detectable by NMR only when deuterated non-coordinating cyclohexane was used as a solvent (Fig. 4C) [34]. Te(+II)-based dibenzo-1,5-ditellurocine showed no intramolecular ChB in solution but instead, a Te•••H HB could be detected [35]. However, theoretical calculations suggested that intermolecular ChBs with the solvent could explain the preference of the dibenzotellurocine towards the boat conformer in solution (Fig. 4D). An intramolecular Te•••F ChB could be detected and quantified by solution NMR in fluorinated Te(+II)-diaryltellurides (Fig. 4E) [36]. The highest energy of interaction of 11 kJ mol⁻¹ was obtained when the Te σ-holes were activated by the presence of strong electron withdrawing substituents (R = CF₃). Finally, the strongest ChBs were formed between Te(+IV)-based telluriums and different Lewis bases B, as demonstrated by NMR and Isothermal Titration Calorimetry (ITC) experiments (Fig. 4F) [24, 37]. For instance, enthalpies of first association of triphenylphosphine oxide with tellurium salts could reach up to -16 kJ/mol in 1,2-dichloroethane.

A Pt•••Te ChB was recently identified by Kukushkin et al. on the basis of XRD studies and theoretical calculations of the adduct between a highly electron-poor telluride and a platinum complex (Fig. 4G) [38]. The association of the formed adduct in CDCl₃ solutions was studied by NMR and ITC revealing a 1:1 association.

You et al. studied the influence of the intramolecular Te•••O/N ChB on the conformational preference of benzaldehydes and associated imines bearing a tellurium in *ortho* position (Fig. 4H) [39]. The ChB was shown to favour the imine formation by significantly decelerating the exchange kinetics and thermodynamically stabilizing the imines.

<figure><img src="image_9.png" /><figcaption>**Figure 4.** Studies of the ChB in solution with tellurium-containing derivatives.</figcaption></figure>

From these physicochemical studies, it is now possible to establish a more rational and better control of ChB involving tellurium, and as such, to better design of tellurium-based catalysts.

## 1.5. Aim of the review

Within this context, it seems timely to look at tellurium-based catalysis. The aim of this review is to cover the various approaches in which tellurium derivatives have been used as catalyst in various organic transformations, and to look for future developments in this area.

## 2. Catalysis with tellurium derivatives

The importance of the ChB in catalysis was highlighted in 2017 by Matile [40] and Huber [41] with sulfur- and selenium-based ChB donors. Before these important findings, the known examples in catalysis focused exclusively on ChB as a tool to rigidify structural motifs [42] but tellurium was never used as catalyst *per se*.

The first example of ChB intermolecular catalysis involving tellurium was reported by Matile in 2018 [43]. In this study, the authors compared the catalytic activity of pentafluorophenyl-substituted halogen (Br, I), chalcogen (Se, Te) and pnictogen (P, As, Sb) derivatives in two reactions involving chloride abstraction. Increased reaction rates were observed with C₆F₅I, (C₆F₅)₂Te (1-Te) and (C₆F₅)₃Sb whereas the lighter analogues (Br, Se, P and As) were almost inactive. Although a superior catalytic activity of the antimony catalyst was found in this series, 1-Te furnished the functionalized products 1 and 2 in moderate yields (Scheme 1). These results thus revealed the potential of tellurium in ChB catalysis, but also highlighted the fact that improvement of the tellurium catalyst structures would be necessary to achieve higher catalytic activity.

Since this seminal work, different authors tried to improve the catalytic activity of tellurium-based compounds with the aim to widen the scope of catalysed reactions, in particular by using neutral compounds which form adduct of weaker strength with the catalyst. In this regard, two types of highly active tellurium catalysts were recently developed in the literature, the dicationic bidentate derivatives where the tellurium atom is attached to a positively charged triazolium heterocycle and the tellurium salts where the positive charge is directly borne by the tellurium atom.

<figure><img src="image_10.png" /><figcaption>**Scheme 1.** The first examples of ChB catalysis involving a tellurium σ-hole donor.</figcaption></figure>
## 2.1. Bis-triazolium tellurium catalysts

The bis-triazolium skeleton has been introduced in XB catalysis by Huber in 2012 [44] and since then, catalysts based on this skeleton have received increasing attention. The popularity of these catalysts relied on its bidendate nature and on the triazolium positive charge which increase the σ-hole depth of iodine. More recently, Huber reported the chalcogen-based analogues and as-expected the tellurium derivatives **2-Te₂** performed better in all tested catalytic reactions. Their synthesis was realized in few steps from the known 1,3-bis(triazolyl)benzene derivative **3** (Scheme 2A) [45].

<figure><img src="image_11.png" /><figcaption>A] Synthesis of bis-triazolium tellurium catalysts **2-Te₂(X)**</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>B] Fluorinated and chiral bis-triazolium tellurium catalysts</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>C] Iodine- and selenium-based analogues</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Scheme 2.** Tellurium-containing bis-triazolium catalysts (A, B) and the structures of their iodine and selenium analogues used for comparison (C).</figcaption></figure>

Deprotonation of **3** with lithium diisopropylamide (LDA) in the presence of diphenylditelluride provided the expected bis-tellurated product **4** as well as the mono-tellurated analogue in about 1:1 ratio. Forming the triazolium was achieved by alkylation of **4** with $Me_3OBF_4$, $MeOTf$ and $MeNTf_2$, which provided respectively **2-Te₂(BF₄)**, **2-Te₂(OTf)** and **2-Te₂(NTf₂)** in good yields. The X-ray structure obtained from single crystals of the bis-triflate **2-Te₂(OTf)** surprisingly revealed the presence of two types of Te···O interactions, one with the triflate anion and one with (adventitious) water. Distances (3.274 and 3.046 Å, respectively) below the sum of the van der Waals radii of the two interacting atoms (O 1.52 Å, Te 2.06 Å) as well as angles close to 180° confirmed the ChB formation with both Te atoms.

The less-coordinating BArF anion was introduced through anion metathesis starting from **2-Te₂(BF₄)** to give **2-Te₂(BArF)** in moderate yield. This synthetic sequence further allowed two structural modifications of the catalysts: the introduction of a fluorine atom on the central aryl ring (catalyst **3-Te₂**) with the aim to study the catalyst stability by ¹⁹F NMR [46] and the installation of chiral groups as the second Te substituents (catalyst **4-Te₂**) with the objective to use these tellurium derivatives in asymmetric catalysis (Scheme 2B) [47].

Catalysts **2-Te₂** were first employed in a nitro-Michael reaction between 5-methoxyindole **5** and E-β-nitrostyrene **6** to give the 3-substituted indole **7** (Scheme 3) [45]. The formation of indole **7** was demonstrated to be catalysed through ChB activation of **6**. Indeed, various reference compounds were tested in order to exclude other modes of activation or hidden acid catalysis. The best catalysts in this reaction were found to be **2-Te₂(BF₄)** and **2-Te₂(BArF)** which delivered **7** with respectively 78% and 81% yield after 48 h by using 20 mol% catalyst loading whereas the selenium-based analogues **2-Se₂(X)** (Scheme 2C) remained inactive under the same reaction conditions. As expected from the coordinating ability of the anion, the rate acceleration was about three times higher with **Te₂(BArF)** compared to **2-Te₂(BF₄)**. The twofold coordination of the substrate nitro group by the catalyst was proved by comparing the performance of **2-Te₂(BF₄)** and its mono-tellurated analogue. Theoretical investigations of this catalytic reaction revealed the critical role of two-water molecules, that ensure proton transfer from the oxygen of NO₂ to the adjacent carbon (Scheme 3, right) [48].

<figure><img src="image_15.png" /><figcaption>**Scheme 3.** ChB-catalysed nitro-Michael reaction. Reproduced from ref. [48] Copyright (2022), with permission from American Chemical Society.</figcaption></figure>

The Michael addition reaction between 1-methylindole **8** and E-crotophenone **9** was also efficiently catalysed by **2-Te₂**.
catalysts (Scheme 4) [49]. Whereas the Michael adduct **10** was quantitatively produced within 4 h with **2-Te₂(BArF)**, the reaction was slower with **2-Te₂(BF₄)** and **2-Te₂(OTf)** which needed 24 h to drive the reaction to completion. Compared to the iodinated analogue **2-I₂(BArF)** (Scheme 2C), the reaction rates were accelerated by a factor of 150 and 1000 by using **2-Se₂(BArF)** and **2-Te₂(BArF)**, respectively. With the latter catalyst, only 10 mol% were necessary and even 7.5 mol% still produced a good yield of 82% within 4 h, whereas with 5 mol%, the yield dropped significantly to 14%. Nevertheless, under the latter conditions, a yield of 61% could be obtained upon longer reaction time (24 h). Even unprotected indoles **11**, with electron-rich/poor groups as well as sterically demanding ones, could be engaged in reactions with the best catalyst **2-Te₂(BArF)** giving products **12** with yields in the range 64-99%.

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>**Scheme 4.** ChB-catalysed Michael reaction.</figcaption></figure>

The chloride abstraction from 1-chloroisochroman was also catalysed by the dicationic tellurium derivatives, resulting in its transformation to product **2** (Scheme 1) [46]. The best catalytic activity was again observed with the BArF catalyst **2-Te₂(BArF)**, followed by **2-Te₂(OTf)** whereas the product was not formed when **2-Te₂(BF₄)** was used as catalyst. In the latter case, decomposition of BF₄⁻ occurred to give BF₃ and F⁻ which reacted with tert-butyldimethylsilyl (TBS) to produce TBSF (observed by ¹⁹F NMR). This degradation finally resulted in the formation of the non-catalytically active chloride salt **2-Te₂(Cl)**.

**2-Te₂** catalysts were also successfully employed in the Povarov reaction, i.e. the [4+2] condensation of aromatic imines **13** with 2,3-dihydrofuran to produce dihydroquinolines **14** (Scheme 5) [47]. The reaction was shown to be exclusively catalysed through ChB. Then, by using different counterions (BArF⁻, BF₄⁻ and TfO⁻), the order of activation determined from the kinetic profiles followed the expected trend, that is **2-Te₂(BArF)** > **2-Te₂(BF₄)** > **2-Te₂(OTf)**. After 5 h of reaction with 5 mol% loading, **2-Te₂(BArF)** and **2-Te₂(BF₄)** furnished **14** (R¹ = R² = H) with a comparable yield of about 80% yield but with a different diastereoselectivity, higher with the BF₄⁻ salt (endo:exo ratio of 68:32 vs 47:53 for the BArF salt). The yield with **2-Te₂(OTf)** dropped to 64% but with a similar endo:exo ratio than with **2-Te₂(BF₄)** catalyst. Much lower yields, around 25%, were obtained after 5 h of reaction with the selenium-based catalysts **2-**

**Se₂(BArF)** and **2-Se₂(BF₄)**. However, unexpected results were obtained with the triflate salts of the selenium- and sulfur-analogues **2-Se₂(OTf)** and **2-S₂(OTf)**, which delivered **14** with exactly the same yield and diastereoisomeric ratio than **2-Te₂(OTf)**. These results remained unexplained but it is obvious that the triflate anion actively takes place in the catalytic cycle. The nature of the solvent was also evaluated in this reaction with **2-Te₂(BArF)** catalyst, showing that chlorinated solvents were the best, as usually observed in XB- and ChB-catalysis. However, moderate yields of 63% and 49% were obtained in acetone and THF, respectively. The scope of the imines was studied with the BArF catalyst, showing that imines with electron-neutral and electron-withdrawing groups were more efficiently converted to the dihydroquinoline products than the electron-donating ones. Nevertheless, the endo:exo ratio did not significantly change, whatever the functional group on the aryl groups.

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

<figure><img src="image_19.png" /><figcaption>**Scheme 5.** ChB-catalysed Povarov reaction.</figcaption></figure>

Finally, the chiral catalysts **4-Te₂** (Scheme 2B) were applied to this reaction. However, only racemic mixtures of dihydroquinoline **14** were obtained probably because of the high flexibility of the chiral substituents borne by the tellurium atoms.

Very recently, Huber reported a mixed catalyst **5-Te,I** combining tellurium and iodine as the ChB and XB donor centres, respectively. In this unsymmetrical structure, tellurium was carried by a triazolium unit such as in catalysts **2-Te₂** (Scheme 2A) and iodine was placed on a benzimidazolium heterocycle [50]. The synthesis of **5-Te,I** was performed in several steps from benzimidazole **15** (Scheme 6A). After a Sonogashira reaction with trimethylsilylacetylene, iodination of the benzimidazole was realized by deprotonation and I₂ quenching to give **16** in good yield. Deprotection of the alkyne and introduction of the phenyltelluride group furnished **17** that was then subjected to the copper-mediated click-reaction with n-octylazide to produce **18**. The latter was alkylated with Me₃OBF₄ and the resulting salt **5-Te,I(BF₄)** was transformed to **5-Te,I(BArF)** via anion metathesis. The X-ray structural analysis of **5-Te,I(BF₄)** confirmed the mixed ChB/XB through a concomitant interaction of iodine and tellurium
with a fluorine atom of the BF₄ anion. The geometrical parameters of both interactions suggested that the XB is stronger than the ChB, even though the latter is slightly more linear. It is worth noting that the anion chelation observed herein was not present in the crystal structure of 2-Te₂(OTf), probably because of the presence of a coordinated water molecule (see Scheme 2A).

The mixed catalyst 5-Te,I(BArF) was employed in the Diels-Alder reaction between cyclopentadiene and methylvinylketone and its catalytic activity was compared to the symmetrical ditellurium 2-Te₂(BArF) and diiodine 19-I₂(BArF) catalysts (Scheme 6B). They all catalysed the reaction with more or less the same reactivity, 2-Te₂(BArF) nevertheless proved slightly more active than the two others as shown from the turnover frequency number (TOF) determined after 20 min of reaction. The cooperative effect of iodine and tellurium in the catalysis was also supported by DFT calculations.

### A] Synthesis of catalyst 5-Te,I

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

### B] Diels-Alder reaction

<figure><img src="image_22.png" /><figcaption>**Scheme 6.** The catalysed Diels-Alder reaction by a mixed ChB/XB catalyst.</figcaption></figure>

A photoswitchable tellurium-based bis-triazolium catalyst 6-Te₂ was recently used in catalysis by the group of Han and Zhang [51]. This catalyst in its E configuration 6-Te₂(E) was prepared in four steps from the bis-alkyne 20. The bis-alkynyl silver derivative 21 was first generated before reaction with bromoaryltelluride to give the bis-tellurated alkyne 22. The latter was subjected to a copper-catalysed click reaction with benzyl azide to produce the bis-triazole product 23 in an overall 15% yield. Finally, 6-Te₂(E) was obtained as BF₄ salt after alkylation with Me₃OBF₄ in 29% yield (Scheme 7).

<figure><img src="image_23.png" /><figcaption>**Scheme 7.** Synthesis and photoisomerisation of 6-Te₂ catalyst.</figcaption></figure>

The geometric positioning arrangement of the two Te moieties could be regulated upon irradiation at respectively 590 nm for the E to Z isomerization and at 430/420 nm for the Z to E isomerization. As anticipated, the catalyst under Z configuration 6-Te₂(Z) induced higher association constants with halides (Cl- and Br-) compared to is E isomer 6-Te₂(E). This property was then applied to the photo-controlled Friedel-Crafts alkylation of 1,3,5-trimethoxybenzene 24 with benzhydral chloride 25. After 24 h at 20°C in CD₃CN, 6-Te₂(E) furnished product 26 in only 9% yield. Interestingly, a higher yield of 34% was obtained with 6-Te₂(Z) as the result of the cooperativity of the two tellurium atoms in the chloride abstraction. Because of a fast Z to E isomerization occurring under the reaction conditions, the reaction was performed under continuous irradiation of the reaction mixture at 590 nm, resulting in a yield increase from 34% to 58% (Scheme 8).
<figure><img src="image_24.png" /><figcaption>**Scheme 8.** ChB-induced Friedel-Crafts alkylation of 1,3,5-trimethoxybenzene.</figcaption></figure>

## 3.2. Tellurium catalysts

Telluroniums are defined as trivalent positively charged tellurium (IV) derivatives. Early studies have pointed that these compounds can form secondary interactions in the solid-state between the tellurium atom and the counterion [52], currently commonly described as ChBs. However, the first application regarding the ability of telluroniums to drive ChB in solution was reported only in 2010 by Gabbaï for fluoride recognition [32]. The same group recently investigated the aptitude of telluronium salts to transport anions across phospholipid bilayers [53]. The ChB catalysis involving tellurium salts was concomitantly reported in 2021 by our group [37] and the group of Gabbaï [54] which both highlighted their high catalytic activity [55].

Two types of telluroniums were mainly used as ChB catalysts, the methyldiaryltelluroniums **7-TeR(X)** and the triaryltelluroniums **8-TeR(X)** (Scheme 9). Telluroniums **7-TeR(X)** were efficiently prepared by nucleophilic reaction between diaryltellurides **27-TeR** and iodomethane in the presence of silver salt AgX [37, 53]. Interestingly, variation of AgX structure allowed accessing to a multitude of telluroniums with counterions of different nature. The preparation of telluroniums **7-TeR(BArF)** carrying low-coordinating BArF anions was completed through anion metathesis between **7-TeR(OTf)** and NaBArF (Scheme 9A). Whereas the above methodology allowed smooth reactions and high yields, harsher conditions were necessary for the methylation of bis-telluride **28** where the tellurium atoms were substituted by the strong electron-withdrawing tetrafluoroaryl group and the bulky mesityl group. In this case, the use of Me₃OBF₄ in refluxing CH₂Cl₂ allowed the methylation of only one tellurium atom, generating the mixed-valent telluride/telluronium salt **9-Te(BF₄)** in a moderate yield of 53% (Scheme 9B) [56]. The synthesis of triaryltelluroniums **8-TeR(X)** bearing three identical aryl groups was performed in one step by reaction of an excess aryllithium or arylmagnesium species with TeCl₄. Different counterions could then be quantitatively introduced through anion metathesis (Scheme 9C) [57]. Alternatively, tellurium **10-Te(BArF)** was prepared in 79% overall yield from phenoxatellurine **29** by difluorination with CsF/NFSI (N-fluorobenzenesulfonimide) to generate **30** followed by reaction with PhB(OH)₂ in the presence of BF₃·Et₂O and final salt metathesis with NaBArF. The bis-telluronium **11-Te₂(BArF)** was obtained from **30** in 69% yield under analogous conditions using boronic acid **31** (Scheme 9D) [54].

### A] Methyldiaryltellurium salts

<figure><img src="image_25.png" /></figure>

### B] Mixed valent telluride/telluronium salt

<figure><img src="image_26.png" /></figure>

### C] Triaryltelluronium salts

<figure><img src="image_27.png" /></figure>

### D] Bis-telluronium salt

<figure><img src="image_28.png" /></figure>

### E] Selenonium salts

<figure><img src="image_29.png" /><figcaption>**Scheme 9.** Synthesis of tellurium catalysts (A-D) and the structure of selenonium salts used for comparison (E).</figcaption></figure>

The tellurium salts tended to easily crystallize allowing structural analysis by X-ray diffraction. For instance, the structures of **7-TeCF₃(OTf)** and **7-TeCF₃(NO₃)** revealed the presence of three ChBs between each Te σ-hole and the oxygen atoms from the counterion [37]. Indeed, nearly linear arrangements of the TfO/NO₃•••Te-C bonds (166 to 175°) and distances (2.831 to 3.048 Å) below the sum of the van der Waals radii of the two interacting atoms (O 1.52 Å, Te 2.06 Å) were observed in each case (Fig. 7A). Comparison of the geometrical parameters of the Te•••O ChB in the crystal structures of the triflate salts **2-Te₂(OTf)** and **7-TeCF₃(OTf)** indicated a much stronger interaction in the latter (3.274 vs 2.909 Å and similar linearity).
Interestingly, co-crystals of triaryltellurium salts 8-Te<sub>H</sub>(BF<sub>4</sub>) or 8-Te<sub>CF3</sub>(Br) with acetone showed a ChB between Te and O (d<sub>Te-O</sub> = 2.996 and 2.851 Å, respectively) [57]. The shorter distance observed with 8-Te<sub>CF3</sub>(Br) clearly results from the higher activation of the Te σ-holes induced by the CF<sub>3</sub> electron-withdrawing groups (Fig. 5B).

Finally, the chelating ability of bis-tellurium 11-Te<sub>2</sub> was demonstrated from the crystal structure of 11-Te<sub>2</sub>(BF<sub>4</sub>) where two fluorine atoms of the same BF<sub>4</sub> anion clearly interact with the two tellurium atoms with distances and angles corresponding to Te•••F ChBs (Fig. 5C) [54]. Interestingly, the Te-F distances are all below 3 Å, much shorter than the one observed in 5-Te,I(BF<sub>4</sub>) (d = 3.288 Å). These observations in the crystal state of 11-Te<sub>2</sub>(BF<sub>4</sub>) represented useful preliminary indications that interesting ChB properties could be expected in solution with tellurium salts, in particular in catalysis.

A] Selected ChBs in Methyldiaryltelluroniums

<figure><img src="image_30.png" /></figure>

B] Selected ChBs in triaryltelluronium/acetone adducts

<figure><img src="image_31.png" /></figure>

C] Selected ChBs in the bis-telluronium salt

<figure><img src="image_32.png" /><figcaption>Figure 5. Crystal structures of telluriums with selected ChBs involving Te.</figcaption></figure>

The performance of methyldiaryltelluroniums 7-Te<sub>R</sub>(X) as ChB catalysts was demonstrated in several reactions by our group (Scheme 10) [37, 57]. The bromination of anisole 32 through N-

bromosuccinimide (NBS) activation was first studied in order to analyse the influence of the aryl substituents and the nature of the counterion on the catalytic activity of the tellurium catalysts (Scheme 10A). High yields of the brominated product 33 were obtained in each considered case after 5 min in CH<sub>2</sub>Cl<sub>2</sub> at room temperature in the presence of 5 mol% catalyst. Nevertheless, and as expected from the NMR study in solution, the tellurium BArF salt bearing the CF<sub>3</sub> group 7-Te<sub>CF3</sub>(BArF) showed the highest catalytic activity, giving 33 in almost quantitative yield. Importantly, no yield decrease was observed upon reducing the catalyst loading down to 0.5 mol%. As NBS activation is also required for the transformation of ω-unsaturated carboxylic acids 34 to bromolactones 35, the best catalyst 7-Te<sub>CF3</sub>(BArF) was also tested in this bromolactonisation reaction. 5 mol% catalyst loading was generally necessary to produce medium-ring bromolactones 35 in good yields within 30 min, whereas larger rings were obtained in lower yields (Scheme 10B). These results show that the methyldiaryltellurium catalysts are by far more efficient in both reactions than the similar selenium catalysts 12-Se(BF<sub>4</sub>) and 12-Se(BArF) bearing only one CF<sub>3</sub> group per aryl substituent [58]. Indeed, for the bromination of anisole, only 46% and 60% yields of 33 were obtained respectively with 12-Se(BF<sub>4</sub>) and 12-Se(BArF) after 24 h of reaction. Comparable high yields of bromolactone 35 (n = 1) were obtained with 7-Te<sub>CF3</sub>(BArF) and 12-Se(BF<sub>4</sub>), although a longer reaction time (24 h) and higher loading (10 mol%) were needed with the selenium catalyst.

The activation of imines 36 was then studied with catalyst 7-Te<sub>CF3</sub>(BArF) in two different cycloaddition reactions: the aza-Diels-Alder-type reaction with the Danishefsky diene 37 and the Povarov reaction with electron-rich alkenes 38. The aza-Diels-Alder reaction could be catalysed with only 1 mol% catalyst loading affording cycloadducts 39 in good to high yields within 2 h at room temperature (Scheme 10C). The Povarov reaction was studied in more detail, in particular by the screening of different dienophiles. The reaction of *N*-phenyl phenylmethanimine with 2,3-dihydrofuran in the presence of 5 mol% 7-Te<sub>CF3</sub>(BArF) provided the dihydroquinoline product 40 in 88% yield and 46:54 *endo*: *exo* after 5 h at room temperature. It is worth noticing that the tellurium catalyst and the dicationic triazolium catalyst 2-Te<sub>2</sub> performed similarly in this reaction, both in term of yield and diastereoselectivity (Scheme 5). Regarding the nature of the dienophile, high yields of dihydroquinoline products 40 and variable diastereoselectivity were observed, the best *endo*: *exo* ratio was obtained with norbornene, probably because of its rigid nature (Scheme 10D). The Povarov reaction was also considered with the triaryltellurium catalysts 8-Te<sub>R</sub>(X). As expected, catalyst 8-Te<sub>CF3</sub>(BArF) bearing the CF<sub>3</sub> substituent and carrying the BArF anion performed better compared to 8-Te<sub>H</sub>(BArF) and to the other tellurium salts. Whereas 8-Te<sub>CF3</sub>(BArF) and the methyltellurium analogue 7-Te<sub>CF3</sub>(BArF) performed similarly with 2,3-dihydrofuran and ethyl vinyl ether dienophiles, 7-Te<sub>CF3</sub>(BArF) was the best catalyst with dihydropyran, cyclopentadiene and norbornene. However, 8-Te<sub>CF3</sub>(BArF) proved to be more stable in the reaction mixture, allowing the efficient one-pot Povarov reaction directly starting from aldehydes, aniline and 2,3-dihydrofuran [57].
### A) Bromination of anisole

<figure><img src="image_33.png" /></figure>

### B) Bromolactonisation of olefinic acids

<figure><img src="image_34.png" /></figure>

### C) Aza-Diels-Alder reaction

<figure><img src="image_35.png" /></figure>

### D) Povarov reaction

<figure><img src="image_36.png" /><figcaption>**Scheme 10.** ChB-catalysed reactions with methyldiaryl- and triaryltelluroniums.</figcaption></figure>

The performance of the mixed-valent catalyst **9-Te(BF₄)** was evaluated in the catalytic reduction of 2-phenylquinoline **41** by the Hantzsch ester by Gabbaï [56]. A complete conversion was obtained with 10 mol% catalyst after 3 h in CDCl₃ at room temperature, producing tetrahydroquinoline **42** in 96% yield. The primary role of the telluronium cation moiety of **9-Te(BF₄)** in the catalysis was confirmed by using the simplest telluronium **7-TeCF₃(BF₄)** bearing only one tellurium atom. Indeed, a similar yield of 91% was obtained, whereas the tellurium(II) analogue **27-TeCF₃** remained almost inactive (Scheme 11A).

The chelating property of bis-telluronium dication **11-Te₂(BArF)** toward chloride ion was reported by Gabbaï [54]. UV-visible monitoring of Ph₃CCl ionization in the presence of **11-Te₂(BArF)** and its mono-tellurium analogue **10-Te(BArF)** afforded equilibrium constants K of 14.5 and 1.58 x 10⁻³, respectively. The higher affinity of the dicationic telluronium to chloride was ascribed to its ability to form a chelate with this ion. This result was then transposed to the metal-chloride bond activation as a way to promote cycloisomerisation reactions. The effect of telluronium addition in the gold-catalysed cycloisomerisation of propargyl amide **43** to dihydrooxazole **44** was first considered. As expected, a higher activity was observed with the bidendate catalyst **11-Te₂(BArF)** in combination with

Ph₃AuCl complex in a 1:1 ratio (Scheme 11B). The favourable cooperative effect for chloride abstraction allowed by **11-Te₂(BArF)** catalyst was also demonstrated in the course of the platinum-catalysed transformation of 1,6-enyne **45** to diene **46**. In this case however, the dichloride nature of cis-(PPh₃)₂PtCl₂ catalyst implied that two equivalents of **11-Te₂(BArF)** per platinum were necessary to achieve high conversion (Scheme 11C).

### A) Transfer hydrogenation of 2-phenylquinoline

<figure><img src="image_37.png" /></figure>

### B) Propargyl amide cycloisomerisation

<figure><img src="image_38.png" /></figure>

### C) Enyne cycloisomerisation

<figure><img src="image_39.png" /><figcaption>**Scheme 11.** ChB-catalysed quinoline reduction and cycloisomerisation reactions.</figcaption></figure>

Takagi reported that hydroxy diaryltelluronium **12-Te(OTf)**, formed *in situ* by hydrolysis of ditelluroxane **13-Te₂(OTf)** could promote the Ritter reaction of benzhydryl bromide **47** [59]. The use of 1 equivalent of **12-Te(OTf)** with respect to **47** furnished quantitatively amide **48** after 96 h at room temperature. DFT calculations suggested that three non-bonding interactions, namely ChB, HB and cation/π could be involved in the activation of the carbon-halogen bond (Scheme 12A).

The same group recently reported the polymerization of *p*-methoxystyrene **49** by using benzyl alcohol **50** as initiator and methyltelluronium **14-Te(OTf)** as catalyst [60]. With the appropriate ratio between [50]/[14-Te(OTf)]/[49] (i.e. 10:5:500) in CH₂Cl₂ at 0°C, the polymer was obtained with an average molar mass of Mn = 6930 and a relatively narrow molecular weight distribution (Mw/Mn = 1.47). Interestingly, the catalyst could be recovered after the polymerization without decomposition, as shown by ¹²⁵Te NMR. From a mechanistic point of view, the catalyst **14-Te(OTf)** reacted with **50** to generate the *p*-methoxybenzyl cation **51** inducing the living polymerization of **49** (Scheme 12B).
### A) Ritter reaction induced by hydroxy tellurium 12-Te(OTf)

<figure><img src="image_40.png" /></figure>

<figure><img src="image_41.png" /></figure>

### B) Tellurium-catalysed polymerisation of *p*-methoxystyrene

<figure><img src="image_42.png" /><figcaption>**Scheme 12.** Ritter reaction and polymerisation of *p*-methoxystyrene induced by telluriums.</figcaption></figure>

The relative catalytic activity of different chalconiums, including telluriums **15-Te**, was compared with haloniums and pnictoniums through theoretical calculations [61]. This computational study presented some limitations because it focused on two model reactions with very simple substrates, the hydrolysis of methylchloride and the addition of ammonia to acetone. Moreover, the pre-association processes involving the catalyst was not considered in the calculations despite its important role in catalysis, as recently demonstrated by the same authors [62]. Nevertheless, this study emphasized the role of side-interactions in chalconiums and pnictoniums which cooperate with the *σ*-hole interaction for the stabilization of the reactions transition states (Scheme 13).

<figure><img src="image_43.png" /><figcaption>**Scheme 13.** Stabilization of the transition states through several interaction types in two ChB-catalysed model reactions.</figcaption></figure>

### 3.3. Miscellaneous tellurium catalysts

The catalytic activity of 2-benzamide tellurenyl iodide **16-Te** was very recently described by the group of Kumar [63]. This compound was prepared in high yield through the copper-catalysed tellurium insertion in the carbon-iodine bond of 2-iodo-benzamide **52** (Scheme 14A). The structure of **16-Te** was stabilized by an intramolecular ChB between Te and the carbonyl oxygen, as demonstrated from X-ray diffraction analysis. Compound **16-Te** was shown to catalyse the reactions of CO₂ with epoxides **53** to form carbonates **54** with high structural diversity in very good yields (Scheme 14B). The analysis of the mixture of **16-Te** with cyclohexene oxide by ¹²⁵Te NMR and HRMS revealed that the actual catalyst in this reaction is a tellurium analogue of ebselen, ebttellur **17-Te**, which activate the epoxide through ChB and allow it to react with the iodide co-catalyst to form the iodoalcoolate. The latter further reacted with CO₂ to produce the carbonate products **54**. Ebtellur **17-Te**, formed *in situ* from the reaction of **16-Te** and styrene oxide, was also able to catalyse the synthesis of ureas **56** from anilines **55**, probably through ChB between tellurium and the aniline nitrogen followed by an unclear CO₂ insertion (Scheme 14C).

#### A) Synthesis of 2-benzamide tellurenyl iodide 16-Te

<figure><img src="image_44.png" /></figure>

#### B) Cycloaddition of CO₂ with epoxides

<figure><img src="image_45.png" /></figure>

<figure><img src="image_46.png" /></figure>

#### C) Synthesis of 1,3-diarylureas

<figure><img src="image_47.png" /><figcaption>**Scheme 14.** Synthesis and ChB catalysed reactions involving ebttellur **17-Te**.</figcaption></figure>

### 7. Summary and Outlook

With a delay of about five years compared to XB, ChB catalysis is currently burgeoning, maturing as a whole field. In this respect, catalysts based on tellurium combine the *σ*-hole properties of iodine and the improved tunability allowed by higher valency. The reported structures were up-to-now limited to tellurium and bis-triazolium tellurium derivatives, probably in part due to the lability of the C-Te bond. However, the renewed interest for tellurium
chemistry should allow in the near future to expand the number of tellurium-based ChB catalysts, as shown by a very recent report [63]. Another aspect that will attract much interest is asymmetric catalysis [64]. In this regard, the first chiral tellurium catalyst was just described in the literature (see Scheme 1: catalyst 4-Te2), although it was not efficient in an asymmetric version of the Povarov reaction [46]. As noted by the authors, the use of less flexible chiral chains on tellurium should be beneficial for better asymmetric induction. Telluronium salts also offer some possibilities for ChB asymmetric catalysis, by using either chiral counterions or the central chirality of the tellurium atom [65]. More chiral tellurium-based structures are thus expected to appear in the literature very soon, together with the corresponding applications.

## Acknowledgements

We thank the University of Strasbourg, the CNRS and the French National Research Agency (ANR-21-CE07-0014) for financial support.

**Keywords:** catalysis • chalcogen bond • non-covalent interaction • σ-hole • tellurium

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This review highlights the importance of chalcogen bonding in organic synthesis through a compilation of tellurium derivatives used up-to-now in catalysis. These powerful chalcogen bond donors were shown to activate numerous kind of substrates by non-covalent interaction with functional groups such as chlorides, carbonyls, imines, epoxides, alcohols and nitro derivatives.

Institute and/or researcher Twitter usernames: @mamane_victor