Pentafluoro(aryl)-λ6-tellanes and Tetrafluoro(aryl)(trifluoromethyl)-λ6-tellanes: From SF5 to the TeF5 and TeF4CF3 Groups

Article abstract of DOI:10.1002/anie.201907359

The TeF₅ group is significantly underexplored as a highly fluorinated substituent on an organic framework, despite it being a larger congener of the acclaimed SF₅ group. In fact, only one aryl-TeF₅ compound (phenyl-TeF

Full text of DOI:10.1002/anie.201907359

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Accepted Article
Title: Pentafluoro(aryl)-λ6-tellanes and Tetrafluoro(aryl)(trifluorometh-
yl)-λ6-tellanes: From SF5 to the TeF5 and TeF4CF3 Groups
Authors: Dustin Bornemann, Cody Ross Pitts, Carmen J. Ziegler, Ewa
Pietrasiak, Nils Trapp, Sebastian Küng, Nico Santschi, and
Antonio Togni
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201907359
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l6
Pentafluoro(aryl)- -tellanes and Tetrafluoro(aryl)(trifluorometh-
l6
yl)- -tellanes: From SF5 to the TeF5 and TeF4CF3 Groups
Dustin Bornemann,† Cody Ross Pitts,†* Carmen J. Ziegler, Ewa Pietrasiak, Nils Trapp, Sebastian
Kueng, Nico Santschi,* and Antonio Togni*
Abstract: The TeF5 group is significantly underexplored as a highly
fluorinated substituent on an organic framework, despite it being a
larger congener of the acclaimed SF5 group. In fact, only one
aryl-TeF5 compound (phenyl-TeF5) has been reported in the literature
to date, synthesized using XeF2 – a testament to a clear lack of
synthetic accessibility. Herein, our recently developed mild TCICA/KF
approach to oxidative fluorination addresses this problem by providing
an affordable and scalable alternative to XeF2. Using this method, we
report a scope of extensively characterized aryl-TeF5 compounds,
along with the first SC-XRD data on this compound class. The
methodology was also extended to the synthesis and structural study
promising applications in medicinal chemistry, agrochemistry, and
materials, such as liquid crystals),[5] we decided to examine it
more closely. To the best of our knowledge, the only compound
containing an arene-based TeF5 group that has been reported to
date is phenyl-TeF5, which was synthesized on a small scale
using XeF2 and was never isolated.[6-10] Herein, we discuss the
development of a remarkably mild and inexpensive TCICA/KF
approach that, for the first time, generates a substrate scope and
demonstrates scalability of aryl-TeF5 compound synthesis.
Moreover, upon isolation of these compounds, we discovered that
the TeF5 group is less prone to degradation and slightly more
robust than formerly described. This finding has allowed us to
conduct the first extensive analysis of the molecular structure of
the TeF5 group on an arene (and subsequently, a novel TeF4CF3
group) in the solid state using X-ray diffraction data, as well as
study the stability and reactivity of this group. It was during these
studies that we also unveiled inconsistencies with the previous
literature on the "known" reactivity of phenyl-TeF5 and,
accordingly, we begin to address misconceptions about aryl-TeF5
compounds.
of heretofore unknown aryl-TeF4CF3 compounds.
Additionally,
preliminary reactivity studies unveiled some inconsistencies with
previous literature regarding phenyl-TeF5. Ultimately, although our
studies conclude that the arene-based TeF5 (and TeF4CF3) group is
not quite as robust as the SF5 group, we find that the TeF5 group is
arguably more stable than previous literature suggests, thus opening
a door to explore new applications of this motif.
In the last year, our laboratory developed a mild, gas-reagent-
free approach to the synthesis of aryl-SF4Cl compounds using
trichloroisocyanuric acid (TCICA), potassium fluoride (KF), and
catalytic acid.[1] Under identical conditions, we also observed that
diaryl diselenides display divergent behavior, providing aryl-SeF3
compounds as opposed to the corresponding SeVI species.[1,2]
Initially out of sheer academic curiosity, our focus more recently
drifted further down the chalcogen series, and we asked: how
would diaryl ditellurides react? Surprisingly, we found that similar
TCICA/KF oxidative fluorination conditions allow direct access to
pentafluoro(aryl)-l6-tellanes (aryl-TeF5 compounds), i.e. with the
Te atom in a different oxidation state and/or with a different
degree of fluorination than S or Se in the aforementioned products
of diaryl disulfide and diaryl diselenide oxidative fluorination
(Scheme 1). This curiosity evolved into an unexpected foray into
organotellurium chemistry.[3,4]
previous applications of TCICA/KF approach:
S in +6 oxidation state, chlorofluorination observed
Se in +4 oxidation state, only fluorination observed
X = S
Ar SF4Cl
O
Cl
O
Cl
O
N
N
X = Se
X
Ar
Ar SeF3
N
Ar
X
Cl
acid cat.
KF
X = Te
new application:
Ar TeF5
Te in +6 oxidation state, only fluorination observed
mild scalable
increased scope
allows detailed structural and reactivity studies
Considering the TeF5 group is ostensibly a larger, greasier,
underexplored congener of the SF5 group (which has shown
Scheme 1. Discovery of a new application of the TCICA/KF
approach vis-à-vis mild aryl-TeF5 synthesis, whereby diaryl
ditellurides display divergent behavior from their disulfide and
diselenide congeners under similar reaction conditions.
[*]
D. Bornemann,† Dr. C. R. Pitts,†* C. J. Ziegler, E. Pietrasiak, Dr. N.
Trapp, S. Küng, Dr. N. Santschi,* Prof. Dr. A. Togni*
Department of Chemistry and Applied Biosciences
ETH Zürich
Regarding reaction optimization, it was determined that
consistent, high yields of aryl-TeF5 compounds can be obtained
by stirring the corresponding diaryl ditelluride substrates with 6.0
equiv. TCICA, 24 equiv. KF, and 10 mol % TFA in MeCN at
ambient temperature overnight (ca. 16 h). See Table S1 in the
Supporting Information for more details.
Vladimir-Prelog-Weg 2, 8093 Zürich (Switzerland)
E-mail: cpitts@ethz.ch
nicosantschi@gmail.com
atogni@ethz.ch
†These authors contributed equally.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201907359
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COMMUNICATION
Table 1. Substrate scope of aryl-TeF5 formation.
or alkyl groups with benzylic sites, known background reactions
with TCICA (e.g. ring or benzylic chlorination) may become
competitive.[14] For instance, we found that an unsubstituted
biphenyl ditelluride is electron-rich enough for unselective ring
chlorination to be problematic (however, note that a crystal
structure was obtained for one of the chlorinated isomers).
Conversely, biphenyl ditellurides substituted with electron-
TCICA
KF
Te
Ar
Ar-TeF5
Ar
Te
cat. TFA
MeCN, rt, 16 h
TeF5
TeF5
86%
83%
(56%)
withdrawing groups, e.g.
a CF3 group, convert to their
(78%)
[gram scale]
F
corresponding aryl-TeF5 compounds selectively (11). In addition,
we were able to access the slightly more complex compound 12,
albeit in a lower yield, whose overall structure is reminiscent of a
liquid crystal.
1
3
5
7
2
4
6
F
TeF5
TeF5
TeF5
TeF5
TeF5
TeF5
TeF5
58%
(45%)
83%
(68%)
Cl
F3CO
tBu
Table 2. Proof-of-concept for novel aryl-TeF4CF3 formation.
78%
(75%)
78%
(68%)
TCICA
KF
Br
Ar-TeCF3
Ar-TeF4CF3
cat. TFA
MeCN, rt, 16 h
75%a
67%
(59%)
TeF4CF3
TeF4CF3
8
89%
98%
O
(80%)
(65%)
Cl
84%
(81%)
80%
(65%)
13
14
Me
19F NMR yields reported. Isolated yields in parentheses. Crystal structure
of compound 14 determined from single-crystal X-ray diffraction
(displacement ellipsoids given at 50% probability level).
O
O
TeF5
9
10
TeF5
TeF5
77%
(74%)
54%
(25%)
Beyond diaryl ditelluride substrates, we became interested in
the oxidative fluorination of aryl-TeCF3 compounds. Syntheses of
aryl-TeCF3 compounds have been developed by Umemoto,[15]
and more recently in our laboratory[16] and by Schönebeck.[17] To
our satisfaction, under TCICA/KF conditions, we were able to
convert phenyl-TeCF3 to trans-phenyl-TeF4CF3 (13), obtaining the
product in 80% isolated yield as an oil (Table 2). As this is, to our
knowledge, the first time this TeF4CF3 group has been observed,
we also synthesized para-chloro-derivative 14 in order to obtain
X-ray diffraction data (discussed in more detail below).
The TeF5 and TeF4CF3 groups display interesting
spectroscopic differences. For one, the 19F NMR spectrum of 1
displays a doublet (2JF-F ≈ 151 Hz) at -53.39 ppm that corresponds
to the four equatorial fluorine atoms on the TeF5 group, whereas
the equatorial fluorine atoms of the TeF4CF3 group (quartet, 3JF-F
≈ 22 Hz) are markedly shifted upfield to -68.75 ppm.[18] Slight shift
differences also manifest in the 125Te NMR spectra, i.e. the 125Te
signal of 1 appears at ca. 709 ppm vs. 13 at ca. 757 ppm. We
also found that both compounds exhibit broad, intense
asymmetric Te-F stretch signals in the IR spectra, though the
Te-F stretch signal for 13 (n = 625 cm-1) is red-shifted from that of
1 (n = 655 cm-1), indicating a weaker Te-F bond. Lastly, we should
note that although there is a possibility for stereoisomers for
aryl-TeF4CF3 compounds, we observe formation of the trans-
isomers exclusively. In part, this may be due to the fact that the
trans-isomer is calculated to be -5.6 kcal/mol more stable at
wB97XD/aug-cc-pVTZ, with an aug-cc-pVTZ-PP basis set used
for the Te atom.[19-22]
F3C
11
C5H11
12
19F NMR yields reported. Isolated yields in parentheses for compounds
isolable via extraction. aUnable to obtain pure isolated product.
Under these conditions, the unsubstituted phenyl-TeF5 (1)
was formed in 86% yield by 19F NMR. Immediately thereafter, we
also found that 1 can be synthesized on a 2-gram scale and
isolated by extraction (i.e. without chromatography) in 78% yield.
Interestingly, however, the material could also be subjected to
flash chromatographic separation on silica without undergoing
complete decomposition. Subsequently, we explored the reaction
scope (Table 1). Notably, diphenyl ditelluride is the only
commercially available starting material, to our knowledge. All
other ditelluride starting materials were synthesized with varying
degrees of success using the methods of Engman,[11] Stefani,[12]
and Zhou.[13]
This reaction tolerates standard electron-withdrawing groups
well in the meta- and para-positions, such as halogens (2-5) and
a trifluoromethoxy substituent (6). An initial oxidative fluorination
attempt using a substrate with an ortho-fluoro-substitution pattern
resulted in an unclear and complicated 19F NMR spectrum; it is
possible that the TeF5 group is too large to form selectively in the
presence of ortho-substituents, though this may not be purely a
steric effect. Additionally, mild electron-donating groups are
tolerated in the form of cyclopropyl (7), tert-butyl (8), and acetal
(9) substituents. (In the last case, note that ketones do not
necessarily require protection as acetals, e.g. benzophenone
derivative 10 was formed in good yield under TCICA/KF
conditions.) When employing stronger electron-donating groups
Another comparison lies in the TeF5 group versus the SF5
group. By 19F NMR, there are drastic chemical shift differences
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4
5
7
9
10
15
Figure 1. Crystal structures, from left to right, of 4, 5, 7, 9, 10, and 15 determined from single-crystal X-ray diffraction (displacement
ellipsoids depicted at 50% probability level).
in both the equatorial and axial fluorine atoms on phenyl-TeF5 (ca.
-53 and -37 ppm, respectively) from phenyl-SF5 (ca. +63 and +85
ppm), though JF-F-values are nearly identical at ca. 150 Hz.
Y
O
Z
2
Cl
Y = SF5 (17) vs. Z = TeF5 (4) vs.
Additionally, in the IR spectra, the Te-F asymmetric stretch
frequencies of 1 are 176 cm-1 red-shifted from the corresponding
S-F stretches of phenyl-SF5 (n = 831 cm-1).
X
Cl
X = SF5 (16) vs.
TeF5 (10)
TeF5 (15) TeF4CF3 (14)
As 1) the aryl-TeF5 compounds in Table 1 were unexpectedly
stable in air and easy to isolate and 2) the solid-state structure of
aryl-TeF5 is heretofore unknown, we grew several single crystals
that proved suitable for X-ray diffraction measurements (Figure 1).
The TeF5 group exhibits a slightly distorted octahedral geometry,
consistent with the 19F NMR data. Analysis of the bond lengths
about the Te atom in these six structures indicates average
d(Cipso–Te) = 2.068 Å, average d(Te–Fax) = 1.838 Å, and average
d(Te–Feq) = 1.856 Å. Here, note that the average lengths of the
Te–Feq bonds are greater than the Te–Fax bonds. Additionally,
the average qC-Te-Fax = 179.4° does not deviate significantly from
linearity; however, an average qC-Te-Feq = 94.4° indicates that the
four equatorial fluorine atoms are puckered away from the arene.
To put these aspects into perspective, we examined
molecular structures of 10 and 15 in juxtaposition with their SF5
congeners 16 and 17 (Figure 2). It is evident in both cases that
Figure 2. (Top) Scaffolds used to compare SF5, TeF5, and
TeF4CF3 in the solid state. (Bottom) Superpositions of 10 and 16
(left) and 4 and 14 (right), with hydrogen atoms omitted for clarity
(displacement ellipsoids depicted at 50% probability level).
As a final point in our structural analysis, we assessed the
relative volume (VM) of the TeF5 group in 10 to the SF5 group in
16 using our X-ray data (inspired by reports that have compared
the volume of the SF5 group to the CF3 group[23,24]). Considering
there are many ways to interpret and to derive VM, we provide
results from three different analyses in Table 3.[25] Just as SF5 is
ca. 1.5-1.6 times larger in volume than CF3, TeF5 is ca. 1.3-1.5
larger than SF5. To take it one step further, a comparative
analysis between the molecular structures of 4 and 14 revealed
that the TeF4CF3 group, in its "highly fluorinated glory," is 1.4-1.5
times larger in volume than the TeF5 group.
qC-Te-Fax ≈ qC-S-Fax, but qC-Te-Feq > qC-S-Feq, indicating that equatorial
fluorine atoms in the TeF5 group deviate from 90° to a greater
extent than in the SF5 group. Unsurprisingly, the Te–C and Te–F
bonds are longer than the S–C and S–F bonds, but an interesting
comparison may lie the ratio of the equatorial:axial chalcogen–
fluorine bonds. As mentioned above, the Te–Feq bonds are longer
than the Te–Fax bonds on average in every aryl-TeF5 compound
discussed thus far, but this is not necessarily consistent in aryl-
SF5 compounds (i.e. there appears to be more variation in this
ratio). For instance, in 16, average d(S–Feq) = 1.562 Å is
approximately equal to d(S–Fax) = 1.569 Å whereas in 17, average
d(S–Feq) = 1.590 Å is greater than d(S–Fax) = 1.549 Å.
Subsequently, we compared molecular structures of 4 and 14
for a closer look at TeF5 vs. TeF4CF3 (Figure 2). Compound 14
exhibits a similar distorted octahedral geometry, although with
qC-Te-CF3 = 176.8° (deviating from linearity more significantly) and
an average qC-Te-Feq value of 92.8° (closer to 90° than observed
for the TeF5 group). Moreover, the Cipso–Te bond distances of 4
and 14 are similar, but the Te–Feq bonds in 14 are notably longer
(i.e. average d(1.889) = Å), which is consistent with the observed
red shift of 14 in our IR data. It is also interesting to note that the
F3C–Te bond in 14 is ca. 6% longer than the Cipso–Te bond.
Further detailed comparisons can be made from the X-ray data
tabulated in the Supporting Information.
Beyond synthesis and structure, we further studied the
reactivity of the TeF5 group. According to literature precedent,
phenyl-TeF5 was used as a putative reagent in the vicinal
difluorination of olefins.[8,9]
First, it was synthesized from
combining diphenyl ditelluride and stoichiometric XeF2 in CH2Cl2
in an NMR tube, and later, an olefin was introduced directly to this
mixture (i.e. phenyl-TeF5 did not appear to be isolated or purified
prior to the reaction). However, when we exposed several olefins
to phenyl-TeF5 1 that was synthesized and isolated using our
TCICA/KF protocol, no reactivity was observed in any case.
Reactions were attempted in both CH2Cl2 and MeCN, at
temperatures ranging from rt to 81 oC, and were monitored from
hours to days (see SI for details).
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Table 3. Relative volumes (VM) of CF3, SF5, TeF5, and TeF4CF3
vessels that were open to air from days to months, with virtually
no observable degradation. However, when dissolved in MeCN
and exposed to excess H2O, 1 and 13 underwent rapid and
virtually complete conversion to cis-phenyl-TeF4OH (18) and
putative eq-phenyl-TeF3(CF3)OH (19), respectively, in 5 min
(Figure 3). The 19F NMR spectrum of 18 is consistent with what
was reported by Janzen and co-workers,[31] and we can now
characterize 19 by 19F NMR: -49.99 ppm (1F, tq, J = 59.2, 28.5
Hz), -59.74 (3F, dt, J = 28.5, 19.6 Hz), -73.24 (2F, dq, J = 59.2,
19.6 Hz). In addition, we note that the 125Te NMR chemical shift
groups determined using X-ray diffraction data.[26-28]
Substituent V1/Z1-V2/Z2 (Å3)a Hirshfeld (Å3)b Promolecule (Å3)b
CF3
SF5
39.2c
62.6c
95.1c
138.1d
47.9
75.7
44.0
66.5
TeF5
92.6e
125.0
82.5e
112.2
TeF4CF3
difference between
1 (709 ppm) and its corresponding
aReferenced to unsubstituted benzophenone molecular structure to obtain initial
value for CF3, assuming a spherical hydrogen atom with VvdW = 7.2 Å3. bCalculated
using Crystal Explorer. cDetermined by comparing molecular structures with same
benzophenone core. dDetermined by comparing molecular structures with same
para-chloro-phenyl core. eDetermined for benzophenone derivative.
monohydrolysis product 18 (735 ppm) is more pronounced than
the shift difference between 13 (757 ppm) and its corresponding
monohydrolysis product 19 (762 ppm). It was also observed that
exchange of additional fluorine atoms with OH groups will
continue to happen more slowly over time.[32]
We found this quite peculiar; therefore, we conducted a short
series of control experiments. First, using styrene as a substrate,
we were able to reproduce the literature procedure (albeit
synthesizing (1,2-difluoroethyl)benzene in a significantly lower
F
F
F
F
F
F
CF3
F
Te
Te
yield
than
previously
reported).[8,9]
Notably, (1,2-
Ph
Ph
OH
OH
difluoroethyl)benzene (and all other reported products of putative
difluorination with phenyl-TeF5) would also be the expected
products of a reaction with XeF2. [29] Indeed, we confirmed that
this product is also formed in 70% yield using only stoichiometric
XeF2. We then wondered if unreacted XeF2 could initiate a chain
reaction with phenyl-TeF5, but doping a reaction mixture of 1 and
styrene with 30 mol % XeF2 only resulted in 14% yield of the
difluoride product, with no degradation of 1 by 19F NMR. In light
of the fact that product formation was only observed in reaction
mixtures where XeF2 could have been present, XeF2 may be the
real actor in difluorination under previously reported conditions.
Accordingly, the behavior of phenyl-TeF5 as a "difluorination
reagent" may be a misconception, as pure phenyl-TeF5 in our
hands was completely unreactive toward styrene and several
other olefins (as well as alkynes) under conditions outlined in the
SI. Or, at the very least, phenyl-TeF5 requires some form of
activation for such a reaction to occur; this remains unclear.
On the other hand, we were able to reproduce a number of
the reactions of phenyl-TeF5 with nucleophiles (such as
alcohols,[7] secondary amines,[7] and azides[30] – see SI for details)
that have been reported in the last few decades. Primarily cis-
isomers were formed in each case (by substitution of an
equatorial fluorine atom with a nucleophile), which is consistent
with our observation in the X-ray data that Te-Feq bonds are, on
average, longer than Te-Fax bonds. In an attempt to expand upon
these reports, we examined reactions of 1 with a variety of
additional nucleophiles (e.g. KCN, tBuNC, AgSCF3, KSCN, PhLi,
and MeLi), only to find that 1 stays completely intact. Additionally,
we explored the reactivity of 1 with a number of substrates under
300 nm irradiation, in both the presence and absence of
sensitizers. Again, in all cases, 1 is ostensibly unreactive. From
another perspective, we also examined the behavior of 1 in the
presence of TMS-X reagents (e.g. X = CF3, CF2H, CF2CF3, CN,
and acetylide) – with and without CsF – and observed no reactivity.
Lastly, we investigated the hydrolytic stability of 1 and 13. A
sample of each was stored in a borosilicate vial and kept in a
refrigerator for ca. 1 year with virtually no degradation by 19F NMR
analysis. Moreover, most of the single crystals were grown in
18
19
Figure 3. Monohydrolysis products derived from 1 (left) and 13
(right) characterized by 19F NMR.
Yet, overall, the TeF5 and TeF4CF3 groups on arenes are
surprisingly more robust than previous literature implies. Leading
up to this finding, the divergent behavior of Te-based substrates
from previously reported S- and Se-based substrates using our
newly-developed TCICA/KF oxidative fluorination approach has
enabled us 1) to synthesize several new aryl-TeF5 and aryl-
TeF4CF3 compounds, 2) to analyze their solid-state structures in
detail, and 3) to investigate (and begin to address some
misconceptions regarding) reactivity.
Given the observed
decomposition of aryl-TeF5 and aryl-TeF4CF3 compounds on
direct contact with water in solution, these compounds may less
likely be of interest to the medicinal chemistry and agrochemistry
communities; rather, their otherwise relative stability, size, and
properties may have a more realistic appeal to applications in
certain materials, such as liquid crystals.[33] Investigations are
currently ongoing in our group.
Acknowledgements
We thank the ETH transfer office for support in filing a patent
application on this work, in which C.R.P., N.S., and A.T. are listed
as inventors, as well as Dr. René Verel (ETH Zürich) for his help.
MoBiAS (ETH) is acknowledged for assistance with HRMS
analyses. Financial support was provided by ETH Zürich and the
ETH Postdoctoral Fellowship Program (C.R.P.).
Conflict of Interest
The authors declare no conflict of interest.
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Keywords: fluorine chemistry • structural chemistry • oxidative
fluorination • trichloroisocyanuric acid • tellurium
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