# Chalcogen-Bond Catalysis: Telluronium-Catalyzed [4+2]-Cyclocondensation of (in situ Generated) Aryl Imines with Alkenes

Loic Groslambert, Andres Padilla-Hernandez, Robin Weiss, Patrick Pale, Victor Mamane

► To cite this version:

Loic Groslambert, Andres Padilla-Hernandez, Robin Weiss, Patrick Pale, Victor Mamane. Chalcogen-Bond Catalysis: Telluronium-Catalyzed [4+2]-Cyclocondensation of (in situ Generated) Aryl Imines with Alkenes. Chemistry - A European Journal, In press, 10.1002/chem.202203372. [hal-03982470](https://hal.science/hal-03982470)

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# Chalcogen-Bond Catalysis: Tellurium-Catalyzed [4+2]-Cyclocondensation of (in situ Generated) Aryl Imines with Alkenes

Loic Groslambert,[a] Andres Padilla-Hernandez,[a] Robin Weiss,[a] Patrick Pale,[*a] and Victor Mamane*[a]

**Abstract:** In the chalcogen series, tellurium species exhibit the strongest chalcogen bonding (ChB) interaction with electron-rich atom. This property explains the renewed interested toward tellurium-based derivatives and their use in different applications, such as organocatalysis. In this context, the catalytic activity of tellurium salts in the Povarov reaction is presented herein. Different dienophiles, as well as imines of variable electronic nature, efficiently react in the presence of catalytic amount of either diarylmeth- yltelluronium or triaryltelluronium salts. Both catalysts could also readily perform the three-component Povarov reaction

starting from aldehyde, aniline and dihydrofuran. The reactivity of telluriums towards imines and aldehydes was confirmed in the solid state by the ability of Te atom to interact through ChB with the oxygen carbonyl of acetone, and in solution with significant shift variations of the imine proton and of the tellurium atom in ¹H and ¹²⁵Te NMR spectroscopy. For the most active tellurium catalysts bearing CF₃ groups, association constants (K) with N-phenyl phenylmethanimine in the range 22–38 M⁻¹ were measured in dichloromethane.

## Introduction

Chalcogen bond (ChB) is a non-covalent interaction, identified as such relatively recently, analog to the now more common halogen bond (XB).[1] Both are due to attraction between a nucleophilic entity (Lewis base) and an electrophilic region, called σ-hole, present on a halogen or a chalcogen atom. Due to dissymmetric electron distribution around such atoms upon covalent bonding, the σ-hole region (blue area in Figure 1) is located at the antipode of the X– or Ch–R bond and spatially coincides with the LUMO orbital (σ*) of the X– or Ch–R bond.[2] In contrast to monovalent halogens, chalcogen atoms exhibit two σ-holes due to their divalent character (Figure 1).[3]

Although comparable to hydrogen bond (HB) in term of strength, XB and ChB are highly directional in sharp contrast to HB. This interesting property has initiated over the last decade for XB[4,5] and very recently for ChB their exploitation in crystal

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Location of σ-holes along covalent bond involving halogen or chalcogen.</figcaption></figure>

engineering,[6] supramolecular assemblies,[7] anion recognition and transport tools,[8] and even more recently in organocatalysis.[4a,9] In the latter area, the high directionality of XB over HB has led to amazing developments. Combining this directionality to the presence of at least two σ-holes on chalcogen atom should allow even greater precision in 3D control. Organocatalysis based on ChB should thus be even more powerful.

As for XB, ChB strength depends on the polarizability of the chalcogen atom (S < Se < Te) and thus, chalcogen interactions involving tellurium are the strongest.[8a,10] Tellurium derivatives are thus becoming the most promising for applications based on σ-hole interactions, with a few emerging in anion recognition[9] and binding[11] and in catalysis.[12] Nevertheless, those applications remain scarce, especially in catalysis, in comparison to the lighter chalcogen-based derivatives.[13]

Since we and others recently demonstrated that some tellurium and tellurium derivatives 1–4 indeed act as efficient

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

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

<footnote>[a] L. Groslambert, A. Padilla-Hernandez, Dr. R. Weiss, Prof. Dr. 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
Homepage: www.asymhole.cnrs.fr
www.lasyroc.chimie.unistra.fr</footnote>
<figure><img src="image_7.png" /></figure>

catalysts in a few cases (Figure 2),[12] we embarked on a program exploring the possibility and limit of such catalysts in organic synthesis.

To implement this goal, we reported here the synthesis of di- and triaryl tellurium derivatives 4–5 and their use as efficient catalysts in [4 + 2]-cycloadditions, namely the Povarov reaction (Scheme 1).[14] While we were working on this reaction,[15] Huber et al. reported that derivatives of type 2 were also efficient catalysts in the Povarov reaction between imines and 1,2-dihydrofuran.[16]

This cycloaddition was selected for its synthetic interest, as it provides (hydro)quinoline derivatives. The latter is a common

<figure><img src="image_8.png" /><figcaption>**Figure 2.** Few known ChB-based Te catalysts and the one proposed here.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Scheme 1.** Telluronium-catalyzed Povarov reaction.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 3.** Selected examples of bioactive compounds containing a hydroquinoline motif.</figcaption></figure>

structural motif in bioactive compounds, either natural or not, some having ended up as drugs (Figure 3), and as such have initiated various syntheses.[17]

## Results and Discussion

### Telluronium synthesis

To tune the tellurium σ-holes and thus enhance stronger interactions, triaryltellurium derivatives carrying electron-withdrawing substituents or groups (EWG) 5b were prepared and compared to those without such EWG 5a and to the analogous methyldiaryltellurium salts 4a–b (Scheme 2). The latter were obtained in three steps from the corresponding diaryl ditellurides 7a–b. While 7a is commercially available, its trifluoromethyl analog 7b has to be prepared from aryl bromide 6.[12e] Its metalation through halogen-lithium exchange in the presence of t-butyllithium provided an aryl metal species, which could then be converted to diaryl ditelluride 7b[18] by addition of tellurium powder at low temperature, ethanolysis and air oxidation. 7a–b were then very efficiently converted to the corresponding diaryltellurides 8a–b in the presence of powdered copper.[19] Alkylation with methyl iodide in the presence of silver triflate gave the corresponding diarylmethyltellurium triflate 4a–b<sub>OTf</sub> with excellent yields. Because

<figure><img src="image_11.png" /><figcaption>**Scheme 2.** Di- and triaryltellurium syntheses.</figcaption></figure>
<figure><img src="image_12.png" /></figure>

triflate anion is known to act as weak Lewis base towards
several elements,[20] the non-coordinating anion tetrakis[3,5-
bis(trifluoromethyl)phenyl]borates (BArF) was also introduced to
minimize any competition in solution. Anion metathesis readily
occurred in the presence of sodium BArF salt and provided the
telluronium BArF 4a–b*BArF* in very high yields. A shorter one- or
two-step route was used for the preparation of the triaryltel-
luronium salts 5a–b. Triphenyltellurium chloride[21] 5a*Cl* was
directly obtained from addition of PhLi to TeCl₄. For the
fluorinated aryl derivatives, addition of Mg turnings to 6
generated the Grignard reagent to which was added TeCl₄ at
low temperature. Upon warming, the tellurate intermediate
evolved to the corresponding triaryltellurium bromide 5b*Br*.
X-ray analysis revealed that the counter-ion was a bromine in
this case. As for the above mentioned methyldiaryltelluroniums
4a–b, anion metathesis could be applied to triaryltelluroniums
5a*Cl* and 5b*Br* with either AgOTf, AgBF₄ or NaBArF. The
corresponding tellurium salts 5a–b*OTf*, 5a–b*BF₄* and 5a–b*BArF*
were obtained in near quantitative yields.

Telluronium structures and interactions

To look for σ-hole interactions in the solid state, numerous
attempts to get suitable crystals out of the diarylmethyl- or
triaryltelluronium salts having coordinating ions were pursued.
Among them, 5a<sub>BF<sub>4</sub></sub> and 5a<sub>BArF</sub> were successfully crystallized
from a dichloromethane solution in the dark by slow diffusion
of cyclohexane and their solid-state molecular structures were
determined by single crystal X-ray diffraction (XRD) (See § III in
S.I.). 5a<sub>BF<sub>4</sub></sub> structure exhibits close contacts between three
fluorine atoms from one anion and three different tellurium
atoms. Lengths and angles strongly suggest the presence of σ-
hole interactions. Indeed, the Te...F distances d<sub>Te-F</sub> are all below
the sum of van der Waals radii and the C<sub>Ar</sub>-Te...F angles are in
the range 166-178°, showing the high linearity of the
interaction. It is worth noticing that although dichloromethane
molecule was found in the XRD structure, no interaction of the
chlorine atoms with tellurium atom could be observed.

Due to these results, attempts to co-crystallize triaryltelluro-
nium salts in the presence of Lewis bases were undertaken.
Rewardingly, 5a<sub>BF4</sub>, 5b<sub>Br</sub> and 5b<sub>BArF</sub> gave crystals from mixture of
dichloromethane and acetone, and their XRD structures con-
tained an acetone molecule in close contact with one σ-hole of
the tellurium atom (Figure 4). In the crystal structure of adduct
[5a<sub>BF4</sub>.CH3COCH3], the Te...O distance d<sub>Te-O</sub> is 2.996 Å, far below
the sum of van der Waals radii (r<sub>vdwTe</sub> = 2.08 Å, r<sub>vdwO</sub> = 1.52 Å, RR
(reduction ratio) = (r<sub>vdwTe</sub> + r<sub>vdwF</sub>)/d<sub>Te-O</sub> = 0.83) and the C<sub>Ar</sub>-Te...O
angle (θ) is 177.0°. In agreement with the higher Te σ-hole
activation in the trifluoromethylated tellurium 5b, shorter
d<sub>Te-O</sub> values of 2.851 Å (RR = 0.79) and 2.714 Å (RR = 0.75) were
noted in the crystal structures of [5b<sub>Br</sub>.CH3COCH3] and
[5b<sub>BArF</sub>.CH3COCH3], respectively.

As expected from the lower coordinating ability of the BArF
anion,[20] the Te...O interaction in the BArF salt is much stronger
than in the bromide one. The two geometrical parameters
(RR<1 and θ in the range 160-180°) confirm the Te...O ChB

<figure><img src="image_13.png" /><figcaption>**Figure 4.** X-ray structures of the triaryltellurium 5a<sub>BF4</sub> (up, left), 5b<sub>Br</sub> (up, right) and 5b<sub>BArF</sub> (bottom) co-crystallized with acetone. Hydrogen atoms are omitted for clarity. Color code: carbon in gray, tellurium in orange, oxygen in red, boron in pink, fluorine in green, bromine in brown.</figcaption></figure>

between telluriums 5a/5b and acetone. It is worth noting
that in the structures of [5a<sub>BF4</sub>.CH3COCH3] and [5b<sub>Br</sub>.CH3COCH3],
the two other Te σ-holes respectively interact with BF4⁻ and Br⁻
anions through strong Te...F and Te...Br ChBs leading to the
formation of anion bridges between two tellurium-acetone
adducts (See § III in S.I.).

These results in the solid state anticipate the fact that
carbonyl groups and thus related imines can be activated in
solution through ChB by tellurium catalysts. This was
confirmed by conducting NMR experiments of a 1:1 CD2Cl2
mixture between the simplest non-functionalized imine, i.e. the
N-phenyl phenylmethanimine, and telluriums 4b<sub>BArF</sub> and
5b<sub>BArF</sub> (see § VII in S. I.). In ¹H NMR, the signal of the imine
proton at 8.474 ppm was deshielded to 8.498 ppm and
8.480 ppm upon addition of 4b<sub>BArF</sub> and 5b<sub>BArF</sub>, respectively.
Accordingly, variations were also observed in ¹²⁵Te NMR, with a
shift from 655.08 ppm for 4b<sub>BArF</sub> to 663.77 ppm in the presence
of imine (Δδ 8.69 ppm), and with a larger shift for 5b<sub>BArF</sub>, from
774.94 to 787.14 ppm in the presence of imine (Δδ 12.2 ppm).
The capacity of telluriums 4b<sub>BArF</sub> and 5b<sub>BArF</sub> to interact with
N-phenyl phenylmethanimine could be compared by determin-
ing their association constants K through titration experiments
(see Figures S7 and S8 in Supporting Information). Thus, the
chemical shift variations for 4b<sub>BArF</sub> and 5b<sub>BArF</sub> upon addition of
increasing amount of the imine were monitored by ¹²⁵Te NMR.
The so-obtained K values revealed a slightly higher interaction
of 4b<sub>BArF</sub> with the imine (K(4b<sub>BArF</sub>/imine) = 38 ± 5 M⁻¹ vs. K-
(5b<sub>BArF</sub>/imine) = 22 ± 4 M⁻¹).

It is worth noting that a slow decomposition of 4b<sub>BArF</sub> to the parent tellane 8b occurred upon prolonged contact times as
<figure><img src="image_14.png" /></figure>

evidenced by NMR. After 16 h at room temperature, ¹⁹F NMR indicated the formation of about 50% of **8b**. Such decomposition could not be detected with **5b<sub>BARF</sub>**.

## Telluriums as catalysts in Povarov reaction

As expected from a ChB-driven reaction, a preliminary solvent survey revealed that coordinating or (Lewis) basic solvents hamper the Povarov reaction (See Table S1 in S.I.). Only the chlorinated solvents dichloromethane or 1,2-dichloroethane allowed the formation of the expected tetrahydroquinoline adducts. These results are in line with the absence of interaction between dichloromethane and tellurium in XRD structure, while clear interactions with co-crystallized acetone do exist in other XRD structures (see above).

The performances of the tellurium catalysts were first evaluated in the reaction between *N*-phenyl phenylmethanimine, and 2,3-dihydrofuran, known for its high activity in Povarov reaction (Table 1). While no reaction occurred without catalyst, even after 24 h (entry 1), the simple diphenylmethyltellurium BA<sub>F</sub>R 4a<sub>BARF</sub> was able to catalyze this model reaction at room temperature within a few hours (entry 2). Moderate conversion and yield were nevertheless observed after 5 h without significant selectivity for one or the other endo or exo stereoisomers. As expected, the trifluoromethylated analog 4b<sub>BARF</sub> proved more efficient and allowed to get 9 in high yield, but still without much selectivity (entry 3). The corresponding triflate 4b<sub>OTF</sub> provided similar results, but with a selectivity in favor of the endo isomer (entry 4). As noted from NMR studies, a small amount of tellane 8b was observed at the end of the reaction. To attest that the reaction was only catalyzed by the tellurium, the catalytic activity of 8b was also evaluated and no conversion was observed (entry 5).

In the triaryltellurium series, the triphenyltellurium BA<sub>F</sub>R salt 5a<sub>BARF</sub> only led to slow reactions at room temperature. Upon slight heating, it nevertheless proved as efficient as its diphenylmethyl analog 4a<sub>BARF</sub>, leading to similar results both in term of yields and selectivity (entry 6 vs. 2). The corresponding triflate 5a<sub>OTF</sub> afforded the expected adducts in similar yields but with a higher selectivity and without heating (entry 7 vs. 6). Under the same conditions, the tetrafluoroborate 5a<sub>BF4</sub> exhibited a slightly lower reactivity, but induced a slightly higher selectivity in favor of the endo isomer (entry 8 vs. 7 vs. 6). Interestingly, the trifluoromethylated derivative 5b<sub>BARF</sub> exhibited a much higher efficacy as catalyst but with the same lack of selectivity (entry 9 vs. 6). Here again, shifting from BA<sub>F</sub>R as counter-ion to triflate or tetrafluoroborate provided similar yields with better selectivity, especially with the triflate derivative 5b<sub>OTF</sub> (entry 9 vs. 10 vs. 11). This salt effect on the diastereoselectivity was also observed by Huber et al. but could not be clearly rationalized.[16] The results in our hands did not allow to get more insight.

Subsequently, control experiments to assess the role of σ-hole interactions in the reaction were performed with the BA<sub>F</sub>R catalyst 5b<sub>BARF</sub>. Known for its strong interaction with σ-holes, two equivalents of chloride salt (tetrabutylammonium chloride)

<table><caption>**Table 1.** Tellurium-catalyzed Povarov reaction starting from pre-formed imines: catalyst screening.[a]</caption><thead><tr><th>Entry</th><th>Tellurium structure</th><th>Catalyst</th><th>Yield (%)<br>(endo:exo)<sup>[b]</sup></th></tr></thead><tbody><tr><td>1</td><td>-</td><td>None</td><td>0</td></tr><tr><td>2</td><td><img src="image_0_0.png" alt="Chemical structure of 2-ethyl-1,3-dithiazol-2-yl tellurium cation"></td><td>4a<sub>BARF</sub></td><td>53<sup>[c,d]</sup><br>(55:45)</td></tr><tr><td>3</td><td><img src="image_0_1.png" alt="Chemical structure of 2-ethyl-1,3-dithiazol-2-yl tellurium cation"></td><td>4b<sub>BARF</sub></td><td>88<sup>[d]</sup><br>(48:52)</td></tr><tr><td>4</td><td><img src="image_0_2.png" alt="Chemical structure of 2-ethyl-1,3-dithiazol-2-yl tellurium cation"></td><td>4b<sub>OTF</sub></td><td>95<sup>[d]</sup><br>(68:32)</td></tr><tr><td>5</td><td><img src="image_0_3.png" alt="Chemical structure of 2-ethyl-1,3-dithiazol-2-yl tellurium cation"></td><td>8b</td><td>0</td></tr><tr><td>6</td><td><img src="image_0_4.png" alt="Chemical structure of 2-ethyl-1,3-dithiazol-2-yl tellurium cation"></td><td>5a<sub>BARF</sub></td><td>49<sup>[c,e]</sup><br>(49:51)</td></tr><tr><td>7</td><td><img src="image_0_5.png" alt="Chemical structure of 2-ethyl-1,3-dithiazol-2-yl tellurium cation"></td><td>5a<sub>OTF</sub></td><td>47<sup>[c]</sup><br>(68:32)</td></tr><tr><td>8</td><td><img src="image_0_6.png" alt="Chemical structure of 2-ethyl-1,3-dithiazol-2-yl tellurium cation"></td><td>5a<sub>BF4</sub></td><td>39<sup>[c]</sup><br>(73:27)</td></tr><tr><td>9</td><td><img src="image_0_7.png" alt="Chemical structure of 2-ethyl-1,3-dithiazol-2-yl tellurium cation"></td><td>5b<sub>BARF</sub></td><td>85<br>(45:55)</td></tr><tr><td>10</td><td><img src="image_0_8.png" alt="Chemical structure of 2-ethyl-1,3-dithiazol-2-yl tellurium cation"></td><td>5b<sub>OTF</sub></td><td>98<br>(67:33)</td></tr><tr><td>11</td><td><img src="image_0_9.png" alt="Chemical structure of 2-ethyl-1,3-dithiazol-2-yl tellurium cation"></td><td>5b<sub>BF4</sub></td><td>92<br>(61:39)</td></tr><tr><td>12</td><td><img src="image_0_10.png" alt="Chemical structure of 2-ethyl-1,3-dithiazol-2-yl tellurium cation"></td><td>5b<sub>BARF</sub></td><td>0<sup>[f]</sup></td></tr><tr><td>13</td><td><img src="image_0_11.png" alt="Chemical structure of 2-ethyl-1,3-dithiazol-2-yl tellurium cation"></td><td>5b<sub>BARF</sub></td><td>84<sup>[g]</sup><br>(48:52)</td></tr></tbody></table>

[a] Reaction conditions: imine (0.055 mmol), 2,3-dihydrofurane (0.110 mmol) in 1,2-DCE (0.4 mL) at 25 °C for 5 h; [b] Yield and endo/exo ratio were estimated by ¹H NMR by using 1,4-trimethylsilylbenzene as internal standard; [c] incomplete conversion; [d] Tellane 8a or 8b was observed at the end of the reaction; [e] reaction performed at 40 °C; [f] Addition of 10 mol% of tetrabutylammonium chloride; [g] Addition of 5 mol% of 2,6-ditertbutyl-4-methylpyridine.

were added relative to the tellurium salt. This presence totally suppressed the reaction (entry 12 vs. 9). To ensure that no acid trace could catalyze the reaction, another experiment was performed in the presence of a base, hindered to minimize competition for ChB. A stoichiometric amount of 2,6-ditertbutyl-4-methylpyridine relative to the tellurium salt was thus added and the cyclization proceeded as without it (entry 13 vs.
<figure><img src="image_15.png" /></figure>

reaction with enol ethers (entries 4–5 vs. 3, 2 vs. 1 and 7–8 vs. 6).

It is worth mentioning that this telluronium-catalyzed Povarov reaction compared well with other versions catalyzed by more classical catalysts, which usually required either higher catalyst loading and/or longer reaction time and sometimes higher temperatures<sup>[22,23]</sup> (See Table S2 in Supporting Information).

To further exemplify the present telluronium-catalyzed Povarov reaction and in attempts to shed light on the possible mechanisms,<sup>[24]</sup> we prepared and investigated a series of imines carrying a *N*-phenyl group tuned with electron-withdrawing or donating substituents (EWG or EDG) at each *ortho*-, *meta*-, *para*-position (Scheme 3).

Submitted to the more electrophilic trifluoromethylated triphenyltelluronium BArf salt 5b<sub>BArf</sub>, these imines provided the expected hydroquinolines usually in high and nearly quantitative yields (see **14-m**, **15-o-m-p**, **16-o**). Except the strongly electron-withdrawing nitro group, which slowed the reaction (see **17-o-m-p**), no clear electronic effects seem to affect the reaction (see **16-o** vs. **15-o**). Nevertheless, and surprisingly, the *ortho*- and *para*-methoxy substituted derivatives were produced with lower yields than the *meta* isomer (see **14-o-p** vs. **14-m**) and than the methyl substituted derivatives (see **14-o-p** vs. **15-o-p**).

Looking for a truly multi-component one-pot reaction, we performed the same reactions starting from the imine precursors, i.e. aniline and benzaldehyde (Table 3). The latter in the presence of dihydrofuran and catalytic amount of methyl or aryl telluronium BArf salts gave the expected adducts usually in the same or similar yields and stereoselectivity compared to the reaction starting from pre-formed imines (entries 1–4 and 6).

<figure><img src="image_16.png" /><figcaption>**Scheme 3.** Telluronium-catalyzed Povarov reaction starting from pre-formed imines: screening of the imine. Reaction conditions: imine (0.055 mmol), 2,3-dihydrofurane (0.110 mmol) in 1,2-DCE (0.4 mL) at 25 °C; Yield and endo:exo ratio were estimated by ¹H NMR by using 1,4-trimethylsilylbenzene as internal standard; [a] Incomplete conversion.</figcaption></figure>

<table><caption>**Table 3.** Telluronium-catalyzed Povarov reaction from in situ formed imines.</caption><thead><tr><th>Entry</th><th>Catalyst</th><th>Ar</th><th>Yield<sup>[b]</sup> (%)<br>(endo:exo)</th><th>Product</th></tr></thead><tbody><tr><td>1</td><td>4b<sub>BArf</sub></td><td>C<sub>6</sub>H<sub>5</sub></td><td>80<sup>[c]</sup><br>(49:51)</td><td>9</td></tr><tr><td>2</td><td>5b<sub>BArf</sub></td><td>C<sub>6</sub>H<sub>5</sub></td><td>84<br>(51:49)</td><td>9</td></tr><tr><td>3</td><td>5b<sub>BArf</sub></td><td>p-ClC<sub>6</sub>H<sub>4</sub></td><td>91<br>(51:49)</td><td>16-p</td></tr><tr><td>4</td><td>5b<sub>BArf</sub></td><td>m-ClC<sub>6</sub>H<sub>4</sub></td><td>80<br>(51:49)</td><td>16-m</td></tr><tr><td>5</td><td>5b<sub>BArf</sub></td><td>o-ClC<sub>6</sub>H<sub>4</sub></td><td>66<br>(53:47)</td><td>16-o</td></tr><tr><td>6</td><td>5b<sub>BArf</sub></td><td>m-MeC<sub>6</sub>H<sub>4</sub></td><td>98<br>(49:51)</td><td>15-m</td></tr></tbody></table>

[a] Reaction conditions: aldehyde (0.055 mmol), aniline (0.0558 mmol), 2,3-dihydrofurane (0.110 mmol) in 1,2-DCE (0.4 mL) at 25 °C; [b] Yield and endo:exo ratio were estimated by ¹H NMR by using 1,4-trimethylsilylbenzene as internal standard; [c] Tellane 8b was observed at the end of the reaction.

However, the one-pot reaction with the *ortho*-chlorobenzaldehyde provided the adducts with a significantly lower yield, probably linked to the slower in situ imine formation (entry 5).

## Conclusion

The synthesis of telluronium salts and their catalytic activity in the Povarov reaction involving imines, or their components, and alkenes was reported. The ChB ability of the telluronium salts was first attested in the solid state by co-crystallization with acetone. The geometrical parameters of Te···O contacts (distances and angles) clearly correspond to a σ-hole based interaction. Moreover, the strength of the Te···O ChB could be correlated to the electron-withdrawing ability of Te substituents (*m*,*m*-(CF<sub>3</sub>)<sub>2</sub>-C<sub>6</sub>H<sub>3</sub> vs. C<sub>6</sub>H<sub>5</sub>) and to the coordinating ability of the counter-ion (BArf vs. Br). In solution, addition of a stoichiometric amount of the telluronium BArf salts to the imine induced significant chemical shifts variations in ¹H and ¹²⁵Te NMR as a result of the Te···N ChB, allowing to determine association constants. Subsequently, the catalytic performances of the telluronium BArf salts and its generality were demonstrated through various Povarov reactions involving either different dienophiles or imines. Finally, a one-pot protocol for the
<figure><img src="image_17.png" /></figure>

Povarov reaction was developed, starting from various aldehydes and aniline.

Further work to expand the scope of chemical functions that could be activated by the tellurium salts are underway in our group.

## Experimental Section

**General procedure for the Povarov reaction:** In a 1 mL sealed tube containing the imine (0.055 mmol), catalyst (5 mol%) in 0.4 mL of freshly distilled and degased 1,2-DCE was added the dienophile (0.110 mmol). The reaction mixture was shaken at 25 °C with a vortex. At the appropriate time, the standard (1,4-trimethylsilylbenzene, 200 µL, 80 mM in acetone) was added to the reaction mixture. The solvent was removed under reduced pressure to remove volatile dienophile and solvent. Deuterated solvent was added and the resulting mixture was monitored by ¹H NMR.

**X-Ray Diffraction:** Single crystals of 5a<sub>BF4</sub> and 5a<sub>BArf</sub> were obtained by slow diffusion at 10 °C of cyclohexane into a solution of the tellurium in dichloromethane. Single crystals of [5a<sub>BF4</sub>.CH<sub>3</sub>COCH<sub>3</sub>], [5b<sub>Br</sub>.CH<sub>3</sub>COCH<sub>3</sub>] and [5b<sub>BArf</sub>.CH<sub>3</sub>COCH<sub>3</sub>] were obtained by slow diffusion at 4 °C of cyclohexane into a solution of the tellurium in acetone. The crystals were placed in oil, and a single crystal was selected, mounted on a glass fiber and placed in a low-temperature N<sub>2</sub> stream. X-ray diffraction data collection was carried out on a Bruker PHOTON III DUO CPAD diffractometer equipped with an Oxford Cryosystem liquid N<sub>2</sub> device, using Mo-Kα radiation (λ = 0.71073 Å). The crystal-detector distance was 37 mm. The cell parameters were determined (APEX3 software)<sup>[25]</sup> from reflections taken from 1 set of 180 frames at 1 s exposure. The structures were solved using the program SHELXT-2014.<sup>[26]</sup> 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; transmission factors: T<sub>min</sub>/T<sub>max</sub> = 0.7091/0.7456 for 5a<sub>BF4</sub>; T<sub>min</sub>/T<sub>max</sub> = 0.7264/0.7456 for 5a<sub>BArf</sub>; T<sub>min</sub>/T<sub>max</sub> = 0.6819/0.74650 for [5a<sub>BF4</sub>.CH<sub>3</sub>COCH<sub>3</sub>], T<sub>min</sub>/T<sub>max</sub> = 0.6876/0.7460 for [5b<sub>Br</sub>.CH<sub>3</sub>COCH<sub>3</sub>] and T<sub>min</sub>/T<sub>max</sub> = 0.7187/0.7456 for [5b<sub>BArf</sub>.CH<sub>3</sub>COCH<sub>3</sub>].

Depositions Numbers 2212565 (for 5a<sub>BF4</sub>), 2212566 (for 5a<sub>BArf</sub>), 2212567 (for 5a<sub>BF4</sub>.CH<sub>3</sub>COCH<sub>3</sub>), 2212568 (for [5b<sub>Br</sub>.CH<sub>3</sub>COCH<sub>3</sub>]) and 2212569 (for [5b<sub>BArf</sub>.CH<sub>3</sub>COCH<sub>3</sub>]) contain 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.

## Acknowledgements

This research was funded by the International Center Frontier Research in Chemistry (icFRC), the LabEx CSC (ANR-10-LABX-0026 CSC) and French National Research Agency (ANR-21-CE07-0014). RW thanks the LabEx CSC, Strasbourg, for a PhD fellowship. LG thanks the ANR for a PhD fellowship and AP-H thanks the French Ministry of Research for a PhD fellowship.

## Conflict of Interest

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 bonds · cycloaddition · σ-holes · organocatalysis · tellurium

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Manuscript received: October 31, 2022

Accepted manuscript online: December 16, 2022

Version of record online: ■■, ■■
**Telluronium salts** bearing electron-withdrawing groups were prepared, and their ability to drive chalcogen bonds with ketones and imines was demonstrated in the solid state and in solution. These strong α-hole bond donors were able to efficiently catalyze the Povarov reaction through chalcogen bond activation of imines and subsequent reaction with alkenes. The title reaction was found to be as efficient with pre-formed or in situ generated imines.

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

L. Groslambert, A. Padilla-Hernandez, Dr. R. Weiss, Prof. Dr. P. Pale*, Dr. V. Mamane*

1-9

**Chalcogen-Bond Catalysis: Telluronium-Catalyzed [4+2]-Cyclocondensation of (in situ Generated) Aryl Imines with Alkenes**

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

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