Lewis Acidic Telluronium Cations: Enhanced Chalcogen-Bond Donor Properties and Application to Transfer Hydrogenation Catalysis

Article abstract of DOI:10.1021/acs.organomet.1c00279

We describe the synthesis and structures of o-C6F4(TeMes)2 (1) and o-C6F4(TeArF)2 (2, ArF = 3,5-(CF3)2C6H3)), two new bifunctional tellurides featuring an electron-withdrawing backbone. While 2 resisted methylation, 1 reacted with Me3O·BF4 in CH2Cl2 to afford o-C6F4(TeMes)(TeMeMes) ([3]+), a mixed-valent telluride/telluronium cation isolated as a tetrafluoroborate salt. Although attempts to methylate the second telluride have been unsuccessful, [3]+ readily catalyzes the hydrogenation of 2-phenyl-quinoline with Hantzsch ester. Comparison with simple telluronium cations including [ArF2TeMe]+ and [MesArFTeMe]+ confirms that the catalytic activity of these compounds originates from the presence of a tetravalent, cationic tellurium center.

Full text of DOI:10.1021/acs.organomet.1c00279

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Lewis Acidic Telluronium Cations: Enhanced Chalcogen-Bond Donor
Properties and Application to Transfer Hydrogenation Catalysis
Benyu Zhou and Franco̧ is P. Gabbaï*
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ABSTRACT: We describe the synthesis and structures of o-
C6F4(TeMes)2 (1) and o-C6F4(TeArF)2 (2, ArF = 3,5-
(CF3)2C6H3)), two new bifunctional tellurides featuring an
electron-withdrawing backbone. While 2 resisted methylation, 1
reacted with Me3O·BF4 in CH2Cl2 to afford o-C6F4(TeMes)-
(TeMeMes) ([3]+), a mixed-valent telluride/telluronium cation
isolated as a tetrafluoroborate salt. Although attempts to methylate
the second telluride have been unsuccessful, [3]+ readily catalyzes
the hydrogenation of 2-phenyl-quinoline with Hantzsch ester.
Comparison with simple telluronium cations including
[ArF TeMe]+ and [MesArFTeMe]+ confirms that the catalytic activity of these compounds originates from the presence of a
2
tetravalent, cationic tellurium center.
ver the past decades, the field of Lewis acid chemistry
has witnessed the emergence of saturated acceptors that
ment of the electron acceptors within supporting molecular
scaffolds can be adjusted by simple synthesis variations.
This atypical, chalcogen-centered, hypervalent bonding
mode has drawn increasing attention, especially in the areas
of anion sensing3 and organocatalysis.2g,h To our knowledge,
all applications considered by the community favor derivatives
where the Te atom is in the divalent state. On the basis of the
understanding that increasing the charge and valence of a
saturated acceptor can enhance its Lewis acidity, we made the
hypothesis that the D: → Te interactions could be
strengthened by oxidation of the chalcogen atom into a
tetravalent cation, as in complexes of type B (Chart 1).4 This
hypothesis is based on the notion that the increased valence
and charge of the chalcogen atom would (i) lower the energy
of the accepting σ* orbitals and (ii) result in a deeper σ-hole
(Chart 1). While we have obtained validation for our approach
in the realm of anion sequestration and transport,4 we have not
investigated the use of the cationic chalcogen bond donors as
promoters of organic reactions. We were also intrigued by the
possibility of accessing bifunctional systems, reasoning that the
acceptor properties of such systems might be enhanced by
cooperation, as we have recently observed for bifunctional
antimony Lewis acids.5
O
interact with Lewis basic partners through the formation of
secondary interactions.1 One of the most potent classes of such
saturated acceptors is based on tellurium (Te) derivatives that
interact with donors to form “chalcogen-bonded” complexes of
type A (Chart 1).1,2 The donor−acceptor interaction formed
Chart 1. Schematic Representation of “Chalcogen-Bonded”
Complexes of Types A and B and Origin of the Increase in
Lewis Acidity upon Oxidation of a Telluride into a
a
Telluronium
a
D: = electron donor.
in these complexes displays a stability greater than that of
typical halogen bonds involving electron-deficient iodides
because of the greater Lewis acidity of Te derivatives.
Moreover, the presence of two substituents in Te(II)
derivatives provides access to a broader chemical space in
which the strength of the donor−acceptor interactions, their
compatibility with different environments, and the arrange-
Received: May 6, 2021
Published: July 2, 2021
https://doi.org/10.1021/acs.organomet.1c00279
© 2021 American Chemical Society
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Figure 1. (A) Synthesis of 1, 2, and [3]BF4] (route a: Mes2Te2, THF, −78 °C; route b: (3,5-(CF3)2C6H3)2Te2, Et2O, −78 °C). Inset shows the
crystal structure of 1. (B) Crystal structure of [3]+ as the BF4− salt. The ESP map of [3]+ (color scale is at a.u.), its LUMO contour plot (isovalue =
0.05), and the contour plot of the NBOs (isovalue = 0.05) involved in the Te(II) → Te(IV) interactions are also included. When shown, the
thermal ellipsoids in the crystal structures are set at the 50% probability level, and the H atoms are omitted.
We first synthesized o-C6F4(TeMes)2 (1) and o-
C6F4(TeArF)2 (2, ArF = 3,5-(CF3)2C6H3) by reaction of o-
Li2C6F4 with the corresponding ditelluride at −78 °C (Figure
1). 19F NMR spectroscopy confirmed the presence of a
tetrafluoro-o-phenylene unit in both products as indicated by
the detection of resonances at −106.8 and −153.0 ppm for 1
and −105.3 and −148.2 ppm for 2. In the case of 2, the ArF
groups give rise to another 19F NMR resonance corresponding
to the CF3 groups. These compounds are air-stable. Their
solid-state structure confirms the presence of two Te atoms
spaced by 3.736(1) Å in 1 (Figure 1) and 3.579(1) Å in 2 (see
the Supporting Information). Having confirmed the identity of
these compounds, we investigated their methylation. While 2
failed to react with Me3O·BF4 due to its electron deficiency
and steric encumbrance, we found that a reaction occurred in
the case of the more electron-rich derivative, 1, leading to the
formation of [o-C6F4(TeMes)(TeMeMes)]+ ([3]+), a mixed-
valent telluride/telluronium cation (Figure 1). The structure of
[3][BF4] shows a shortening of the Te−Te distance from
3.736(1) Å in 1 to 3.515 (1) Å in [3]+, indicative of Te(II) →
Te(IV) bonding as confirmed by natural bond orbital (NBO)
analysis (Figure 1). This interaction tames the nucleophilicity
of the Te(II) center as indicated by the reluctance of [3]+ to
reduction of quinolines using Hantzsch ester (Scheme 1). This
simple reaction is often used to benchmark the catalytic
Scheme 1. Transfer Hydrogenation of 2-Phenyl-Quinoline
and Structures of Additional Te-Based Catalysts
activity of saturated acceptors based on Group 15 and 16
elements.5b,7,8 We first tested the neutral bifunctional tellurides
1 and 2 (Table 1, entries 1 and 2). We only observed
conversion in the case of 2, in line with its more electron-
deficient structure and lower steric crowding around the Te
atoms. However, the reaction proved to be slow, only reaching
8% after 3 h when the reaction was carried out in CDCl3 with a
1
undergo further methylation. In the H NMR spectrum, the
methyl group appears as a doublet (JH−F = 3.5 Hz), displaying
the expected 125Te satellites spaced by JTe−H = 33.5 Hz.4b
2
Consistent with the presence of a Te(II) → Te(IV)
interaction, we note that the two 125Te NMR resonances of
[3]+ are flanked by satellites separated by JTe−Te = 527 Hz. This
value is smaller than that measured for [1-(PhTe)-8-PhMeTe)-
naphthalene]+ (1093 Hz) and [1-(PhTe)-8-PhMeTe)-ace-
naphthalene]+ (946 Hz), whose backbones support shorter
Table 1. Compilation of the Results Obtained for the
Transfer Hydrogenation of 2-Phenyl-Quinoline Using
Hantzsch Ester
6
−
Te−Te interactions. In the structure, the BF4 counteranion
forms a secondary interaction or chalcogen bond with the
telluronium ion, confirming the increased Lewis acidity of the
Te(IV) center. DFT calculations followed by a visualization of
the electrostatic potential (ESP) map show that oxidation leads
to an accentuated σ-hole on the Te(IV) center (Figure 1). As
expected, this σ-hole coincides with the LUMO of the
compound which has Te−C σ* parentage.
entry
catalyst ([Te])
time
conv.
1
2
3
4
5
6
7
1
2
3 h
3 h
3 h
3 h
3 h
3 h
3 h
<1%
8%
96%
<1%
95%
6%
[3][BF4]
4
[5][BF4]
6
Returning to one of the primary intents of this study, we
tested the activity of the above-mentioned derivatives for the
[7][BF4]
91%
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catalyst loading of 10 mol %. By contrast, we found that
[3][BF4] smoothly promoted this reaction suggesting that its
enhanced chalcogen bond donor properties or synonymously
its higher Lewis acidity boosts its catalytic activity in this
reaction. To more firmly correlate the enhancement in catalytic
activity to the oxidation of the Te center, we endeavored to
compare the activity of [3][BF4] to that of less structurally
complex telluride and telluronium cations. We selected 4 and
[5][BF4] as well as 6 and [7][BF4], which were prepared using
protocols that we recently described for electron-deficient
tellurides and telluronium cations.4b These four new
derivatives, which have been characterized using conventional
means (see the Supporting Information), are air-stable and can
be stored on the benchtop for extended periods of time. Our
survey of these four additional compounds shows that
telluronium cations [5][BF4] and [7][BF4] are active catalysts
(entries 5 and 7), while 4 and 6 induced hardly any conversion
within the 3 h time frame chosen in this study (entries 4 and
6). The higher activities of [5][BF4] and [7][BF4] fortify the
notion that the oxidative methylation of the Te center is
responsible for the higher catalytic activity of these salts. It has
been recently proposed that Lewis acid catalysts may acidify
Hantzsch ester, thus promoting protonation of the quinoline in
the rate-determining step of the reaction.9 Another mechanistic
suggestion involves activation of the quinoline electrophile by
coordination of the nitrogen to the Lewis acid.8 Since both
pathways would benefit from a more potent Lewis acid, we
propose that the lower σ*-orbitals and deeper σ-holes of the
telluronium cations are at the origin of the rate acceleration
observed with the oxidized derivatives.
derivatives displaying short Te−Te separations and electron-
withdrawing ligands.6b,10,11
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00279.
■
sı
Additional experimental and computational details,
1
crystal structures, 125Te, H, 13C{1H}, and 19F NMR
spectra, natural bond orbital output data, and electro-
static potential maps (PDF)
Optimized structures (XYZ)
Accession Codes
CCDC 2081831−2081833 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Franco̧ is P. Gabbaï − Department of Chemistry, Texas A&M
University, College Station, Texas 77843-3255, United States
of America; orcid.org/0000-0003-4788-2998;
Email: francois@tamu.edu
Author
Benyu Zhou − Department of Chemistry, Texas A&M
University, College Station, Texas 77843-3255, United
States of America
To conclude this study we decided to survey additional
substrates using [5][BF4] as a catalyst. When used in CDCl3
with a loading of 10 mol %, we found that quinoline was
readily hydrogenated, reaching a conversion of 87% within 1 h
(Scheme 2). The CC bond of trans-cinnamaldehyde proved
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00279
Author Contributions
Scheme 2. Transfer Hydrogenation of Quinoline and trans-
Cinnamaldehyde Using [5][BF4] as the Catalyst
B.Z. carried out all experiments and drafted the manuscript.
B.Z. and F.P.G conceived the study.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Welch Foundation (Grant A-1423), the
National Science Foundation (Grant CHE-1856453) and
Texas A&M University (Arthur E. Martell Chair of Chemistry)
for funding.
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