Synthesis and Characterization of Fluorinated Hypervalent Tellurium Derivatives

Article abstract of DOI:10.1021/acs.organomet.7b00535

The synthesis of tellurium derivatives bearing fluorinated groups, similar in structure to their hypervalent iodine congeners, is reported. Thus, a series of CF₃, CF₂H, and C₆F₅ aryltellurium(II) species bearing various functional groups interacting with the Te atom was obtained. The installation of the various fluorinated groups relies on the use of the corresponding trimethylsilyl precursors. The hypervalent nature of the products is discussed on the basis of their NMR spectroscopic and X-ray crystallographic characterization.

Full text of DOI:10.1021/acs.organomet.7b00535

Article
pubs.acs.org/Organometallics
Synthesis and Characterization of Fluorinated Hypervalent Tellurium
Derivatives
Ewa Pietrasiak and Antonio Togni*
Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, ETH Zurich, Vladimir-Prelog-Weg 2,
̈
CH-8093 Zurich, Switzerland
̈
S
* Supporting Information
ABSTRACT: The synthesis of tellurium derivatives bearing fluorinated
groups, similar in structure to their hypervalent iodine congeners, is
reported. Thus, a series of CF3, CF2H, and C6F5 aryltellurium(II) species
bearing various functional groups interacting with the Te atom was
obtained. The installation of the various fluorinated groups relies on the
use of the corresponding trimethylsilyl precursors. The hypervalent nature
of the products is discussed on the basis of their NMR spectroscopic and
X-ray crystallographic characterization.
compounds 1 and 2, in which six electrons account for bonding
INTRODUCTION
Since their introduction in 2006, hypervalent iodine reagents 1
and 2 (Figure 1) have found numerous applications in
■
and two free electron pairs are assigned to the iodine(III) center,
giving a total of 10 valence electrons. The situation is somehow
more complex in case of organotellurium compounds, since the
oxidation state III is not stable for tellurium.
However, a hypervalent tellurium structure can be obtained if a
tellurium(II) center with two substituents connected by single
bonds interacts with a lone pair of a proximal heteroatom
(structure A in Figure 1).7 The total number of valence electrons
for tellurium is 10 in this case, and the resulting molecule is
isolectronic with that of the known hypervalent iodine reagents.
Interactions of this type are common for chalcogens and have
received considerable attention in the literature. Depending on
the publication, they are denoted as hypervalent bonds, three-
center−four-electron (3c-4e), charge-transfer, or secondary
bonding interactions (SBIs). However, in each case the
fundamental description of the bonding is the same. The
distinction is based either on the strength of the interaction or
simply on the publication context.
Reported models being considered as relevant for the work
presented in this paper are shown in Figure 2. In the work by
Woollins and co-workers8−16 and in the early work by Nakanishi
and co-workers17−21 systems derived from 1,8-disubstituted
naphthalenes (type B) were analyzed using X-ray diffraction and
solid-state and solution NMR spectroscopy, as well as quantum
mechanical calculations (e.g., determination of interaction
energies, natural bond orbital analysis, and topological analysis
of electron densities). It was demonstrated that structures with Y,
X, and Z atoms aligned are preferred. These structures are
Figure 1. Hypervalent iodine reagents for electrophilic trifluoromethy-
lation and a structurally related tellurium-centered analogue. Y =
heteroatom, RF = CnFm.
electrophilic trifluoromethylation reactions.1 In the process of
designing more broadly applicable reagents, we reported
modifications of the parent structures, including functionaliza-
tions of the aromatic ring,2 modifications of the five-membered
ring incorporating the iodine atom,3 exchange of the heteroatom
bound to the iodine center,4 and introduction of perfluoroalkyl
substituents replacing the CF3 group.5 However, a formal
replacement of the element iodine by its neighbor chalcogen
tellurium has not yet been attempted. Therefore, we decided to
embark on a detailed study of tellurium compounds structurally
analogous to 1 and 2 with the aim of possibly broadening our
fundamental knowledge of the reactivity of compounds of these
heavy main-group elements as viewed in comparison to each
other.
As defined by Musher in 1969,6 hypervalent compounds are
molecular species containing an atom of group 13−18 of the
periodic system displaying more than eight electrons in its
valence shell. This condition is clearly fulfilled in the case of
Received: July 15, 2017
© XXXX American Chemical Society
A
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were developed as analogues of reagents 1 and 2, this interaction
is denoted as a hypervalent bond.
Transformations involving tellurium compounds are well
established in organic synthesis and material science.32 Over the
past few decades, tellurium(II) compounds referred to as
hypervalent kept appearing in the literature. The first examples
were reported during the 1970s, including the seminal work of
Piette (Figure 3).33−36 More recently, the scope has been
expanded by Singh37,38 and Engman.39
Figure 2. (left) Selection of chalcogen compounds studied in the
literature as models for hypervalent interactions. X = chalcogen, Y =
chalcogen, halogen, Z = alkyl, aryl, or halogen. Atoms participating in
this interaction are marked blue. (right) Orbital interaction relevant for
hypervalent bonding.
stabilized by a 3c-4e interaction originating from donation of
electron density from a lone pair on Y into the σ* orbital on X (σ-
type n(Y)···σ*(X-Z) 3c-4e interaction and n(Y)→σ*(X-Z)
charge transfer; Figure 2, right). The stabilization increases
from the lighter to the heavier chalcogens and reaches its
maximum for X = Te, due to its larger size and higher
polarizability in comparison to the lighter chalcogens.
By applying atoms in molecules dual functional analysis to type
C compounds, Nakanishi and co-workers classified the 3c-4e
interaction in these molecules as a regular closed-shell
interaction, meaning a covalent interaction between atoms
fulfilling the octet rule.22 That fact that the interaction falls in the
covalent regime means that contributions from orbital mixing
dominate over the electrostatic counterpart and supports the 3c-
4e bond model.
Figure 3. Selection of hypervalent tellurium compounds reported in the
literature. Z = halogen, pseudohalogen, R = alkyl, aryl.
Although tellurium compounds directly involving a CF3 group
attached to tellurium atom are known, with the simplest example
being Te(CF3)2,40 reports on hypervalent CF3 tellurium
compounds are rare. The only structurally related molecule
originates from the work of Umemoto and co-workers41 and is a
synthetic intermediate in Umemoto’s reagent synthesis (Figure
4). Umemoto’s reagents are known as electrophilic trifluor-
In the work by Vargas-Baca and co-workers,23−28 intermo-
lecular interactions of this kind (denoted SBIs) were studied,
originally in the context of polymer formation from structures of
type D. Evidence for dimer formation in the gas phase was
provided using UVLDI-MS in concert with DFT calculations.24
Furthermore, numerous examples of solid-state Y···Te−Z SBIs
available from the Cambridge Crystallographic Database were
analyzed.26 The shortest Y···Te−Z distances were found when
the electronegativity of both Y and Z substituents was high. The
influence of electronegativity of the Z substituent can be
rationalized by MO considerations: the σ*(Te−Z) orbital
becomes more localized on Te and its energy decreases as the
electronegativity of Z increases; this in turn lowers the energy gap
between σ*(Te−Z) and n(Y) and allows for more efficient
mixing of these orbitals.
Figure 4. Synthesis of Umemoto’s reagents.
omethylation agents in organic syntheses. The vast majority of
literature examples report the use of a sulfur-centered
compound, due to its shelf stability and large substrate scope.
Its less reactive tellurium analogue did not receive significant
attention though.
However, a different model is necessary to explain the effect of
electronegativity of the X substituent. Gleiter and co-work-
ers29−31 applied symmetry-adapted perturbation theory (SAPT)
as one of the tools to analyze the bonding in type E compounds.
The study showed that, for chalcogens heavier than O, the
dispersive contribution to the interaction energy is dominant.29
Since dispersion can only be taken into account in models
considering electron correlation, MO diagrams, including those
applied for 3c-4e bonds, are not suitable to describe these types
of interactions since they are derived from single-electron
models. Indeed, the effect of the electronegativity of Y on the
interaction strength is correctly predicted by the SAPT model.
On the other hand, it is also shown that the simple MO model
often provides correct predictions of trends and molecular
structures.
SYNTHESIS
■
The targeted structures of type A were accessed starting from the
ditellurides that were prepared following conditions reported by
Engman and co-workers.39 Thus, a lithium halogen exchange of
an aryl bromide, followed by quenching of the resulting lithiated
species with tellurium metal and subsequent air oxidation,
afforded the corresponding ditellurides (Scheme 1). However, in
our hands direct quenching of the intermediate tellurium-
centered anion with an electrophile failed, presumably due to the
low nucleophilicity of the corresponding lithium aryl telluride.
Originally, acetals were chosen as substrates for ditelluride
synthesis. The acetal group is compatible with the necessary
In this work, NMR correlation experiments and X-ray
diffraction measurements were used to characterize the
interaction between Te and Y. Since the compounds presented
B
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Scheme 1. Synthesis of Ditellurides Following Literature
Conditions
Scheme 2. Installation of CF3 Group on Tellurium by Use of
CF3I
a
a
The proposed mechanism of side product formation is shown.
weakens the C−O bond, effectively promoting the C−O bond
cleavage. Consequently, nucleophilic attack of a hydride leads to
a formal reduction at carbon, yielding an ether.
It is worth mentioning that the C−O bond cleavage in an
acetal is not a straightforward transformation and usually requires
harsh conditions in order to proceed. In an early example
provided by Akhmatdinov and co-workers,42 a strong acid,
HSO3F, was used to promote this reactivity pattern and yield the
corresponding carbocation. More recent examples, summarized
by Larson et al.,43 utilize organosilanes in combination with
Lewis acids or transition-metal complexes. A reaction of TiF4
with an acetal also yields the corresponding ether as one of the
products.44
In order to avoid this competing side reaction, ditelluride 3b
containing a cyclic acetal protecting group was employed.
Surprisingly, the starting material or the corresponding product
(4b in Scheme 4) undergoes transacetalization under reaction
conditions, yielding compound 4a (Scheme 3).
reaction conditions (use of highly reactive tBuLi followed by air
oxidation in water), and the oxygen atom in the acetal group is
capable of coordination to the tellurium center, as illustrated by
the general structure A. Moreover, an acetal can be converted to a
carbonyl group that can also participate in hypervalent bond
formation or be subjected to further transformations.
The method reported by Engman is suitable for acetals 3a−c.
The corresponding products were obtained in up to quantitative
yields as yellow solids or oils. No decomposition was observed
over months upon storing at low temperature.
Next we focused our attention on esters. The ester functional
group is also capable of coordination to the tellurium center. In
addition, an ester can be hydrolyzed to the corresponding
carboxylic acid. In the presence of a suitable base, such acids may
form salts with a formally negatively charged tellurium atom and
hence the concept illustrated in structure A could be extended to
ionic compounds.
Scheme 3. Unexpected Transacetalization Encountered
during Reduction of 3b
Unfortunately, inferior results were obtained for ester
functional groups, presumably due to their base-sensitive nature.
When 2-(trimethylsilyl)ethyl 2-bromobenzoate (a protected
form of 2-bromobenzoic acid that can be deprotected in the
presence of a fluoride source) was subjected to standard reaction
conditions, only decomposition of the starting material was
observed. On the other hand, the hindered tert-butoxycarbonyl
substrate can be converted to the corresponding ditelluride 3d,
albeit in low yield.
To efficiently introduce the trifluoromethyl group on the
tellurium atom, we were successful when applying conditions
originally reported by Umemoto and co-workers.41 Thus, a
ditelluride was reduced with NaBH4 in ethanolic solution and the
resulting anionic intermediate reacted with liquified CF3I
(Scheme 2). This pathway allowed access to compound 4a,
but a competing side reaction leading to compound 5 was
additionally operating. Compounds 4a and 5 were formed in a
88:12 ratio, as indicated by 19F NMR analysis of the crude
material, and could not be separated.
In a control experiment, 2-(2-bromophenyl)-1,3-dioxolane
was stirred in ethanol in the presence of NaBH4 overnight.
However, only traces of the corresponding transacetalization
product were observed after this time. Moreover, no acyclic
acetal formation was observed when ditelluride 3b was
recrystallized from ethanol. These observations suggest that
the transacetalization leading to 4a takes place after the
installation of the CF3 group and is most likely promoted by
the electron-withdrawing properties of this substituent.
Ostensibly, the mechanism of ditelluride reductions includes
nucleophilic attack of a hydride on one of the tellurium atoms
followed by the displacement of an anionic tellurium species as a
leaving group. This interpretation, together with a literature
report by Irgolic in which a ditelluride reaction with a Grignard
Hence, it was reasoned that the presence of an electron-
withdrawing CF3 group on the tellurium atom renders this center
Lewis acidic. This in turn strengthens the O−Te bond and
C
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reagent was proposed,45 prompted us to study another strategy
for the installation of a CF3 group. Thus, the alternative pathway
included Te−Te bond scission with a nucleophilic CF3 source
(Scheme 4). Although this method cannot be considered to be
Scheme 6. Synthesis of Hypervalent CF2H Tellurium
Compound
Scheme 4. Installation of CF3 Group on Tellurium by Use of
the Ruppert−Prakash Reagent
high pressure. Difluoro(chalcogen)methylation reported more
recently by Hu and co-workers49 uses the same approach for the
synthesis of CF2H aryltellurides. Since the synthesis of CF2H
analogues of reagents 1 or 2 using TMSCF2H under conditions
developed for their original preparation of has been unsuccessful
so far, compound 7 is of particular interest, especially as a
potential electrophilic or radical CF2H source.
In order to further demonstrate the generality of this method,
we chose the installation of the perfluorophenyl group on
tellurium as our next goal. The synthesis of a related compound,
Te(C6F5)2, was reported in 1968 starting from Hg(C6F5)2 and
elemental tellurium.50 Thus, when (pentafluorophenyl)-
trimethylsilane was reacted with 3b under reaction conditions
developed for the CF2H transfer, two new compounds could be
isolated from the reaction mixture (Scheme 7). Apart from the
target product 8a, the side product 8b was formed. This
compound results from nucleophilic aromatic substitution on 8a.
The aryltelluride anion formed from 3b upon nucleophilic attack
of fluoride-activated TMSC6F5 acts as a nucleophile in this
reaction.
atom economical by virtue of utilizing half of the ditelluride
molecule as a nucleofuge, it does allow access to a variety of
structures that cannot be obtained with the CF3I method. For
example, compound 4b could be isolated in acceptable yield.
Moreover, the synthesis is operationally simpler, since it does not
require the use of expensive and difficult to handle CF3I gas.
Interestingly, when KF was replaced by CsF in the
aforementioned procedure, an unexpected compound was
isolated (Scheme 5). Analysis of the isolated material revealed
Electrophilic aromatic substitution on the perfluorophenyl
ring has been reported previously, e.g. as a side pathway in the
reaction between dimethyl disulfide and Li(C6F5).51 Addition-
ally, a study covering reactions of different pentafluorobenzenes
with NaSMe and NaOH revealed that the para-substituted
product is formed preferentially.52
Scheme 5. Unexpected Product Formation in the Presence of
CsF
The nature of the counterion and/or the solvent used for the
reaction in Scheme 7 seems critical for the formation of 8b. When
Scheme 7. Reaction of Ditelluride 3b with
(Pentafluorophenyl)trimethylsilane
a compound having structure 6. The mechanism of its formation
includes deprotonation of acetonitrile by anhydrous and highly
basic fluoride.46 Nucleophilic attack of the resulting anion on the
ditelluride yields compound 6.
Another advantage of this method is the opportunity to replace
TMSCF3 by another silane-protected nucleophile, as it provides
a general, convenient route to tellurium derivatives carrying
fluorinated substituents on the tellurium atom, as shown in the
general structure A. Hence, under conditions developed for
difluoromethylation of disulfides,47 the CF2H tellurium com-
pound 7 could be prepared in moderate yield (Scheme 6). The
synthesis of CF2H tellurium compounds was reported more than
30 years ago,48 but the procedure required Freon 22 gas under
the reaction is conducted in diethyl ether using LiC6F5 as a
nucleophile, this side product is not detected. Unfortunately,
under these conditions target product 8a is formed only in trace
amounts, probably due to the poor solubility of the starting
material in ethereal solvents. In addition, literature studies53 of
tellurium compounds bearing the pentafluorophenyl group
installed using LiC6F5 do not mention SNAr reaction pathways.
D
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The functional group coordinated to the tellurium atom in
tellurium CF3 derivatives described so far can be modified, as
illustrated by acetal cleavage54−56 and imine formation (Scheme
8).57 In the case of 4a, deprotection proceeds readily upon
Scheme 8. Modifications of Functional Group Interacting
with the Tellurium Atom
Figure 5. (top) ORTEP views of the single-crystal structures of
tellurium compounds 4b, 9, and 10. Thermal ellipsoids are set to 30%
probability, and hydrogen atoms are omitted for clarity. (bottom left)
Known molecules chosen for comparison of selected bond lengths and
angles. (bottom right) General description of compounds compared in
Table 1.
standing in chloroform solution. For cyclic acetals 4c and 8a
longer reaction times and higher temperatures are necessary. In
each case the target carbonyl compound could be isolated after
column chromatography in high yields as a yellow or orange
solid. Furthermore, aldehyde 9 could be converted to imine 10.
The reaction is carried out in refluxing toluene; therefore, the
high yield reflects the thermal stability of both compounds.
CF3 group, Te atom, and heteroatom differs from the theoretical
value of 180° by about −12°. This difference is larger but still
similar to that observed in iodine reagent 2 (−9.5(1)°). A larger
deviation is measured for 4b, in which the acetal ring is tilted with
respect to the aromatic ring. This tilt is quantified by selected
torsion angles and most likely occurs due to steric interactions.
In the previously reported compound 13,58 the Te−Br bond is
longer than the Te−O distance. This trend is reversed in the case
of Te−CF3 compounds in that the Te−CF3 bond is shorter than
the Te−heteroatom bond (by 0.699(2), 0.382(6), and 0.275(11)
Å for 4b, 9, and 10, respectively). This is also in contrast to iodine
compound 2, for which the lengths of the corresponding bonds
are similar (difference of 0.064(4) Å). This observation indicates
that the Te−CF3 interaction is stronger than the Te−heteroatom
SOLID-STATE STRUCTURES
■
Upon standing in a refrigerator, compounds 4b, 9, and 10
crystallized in the form of needles suitable for X-ray diffraction
measurements. Representations of the molecular structures are
shown in Figure 5, and selected parameters (bond lengths and
angles) are compared in Table 1. In each case, the heteroatom
(oxygen in 4b and 9, nitrogen in 10) is pointing toward the
tellurium center. For molecules 9 and 10 the angle defined by the
a
Table 1. Selected Bond Lengths and Angles Measured for Hypervalent Tellurium Compounds in the Solid State
param
2
13
4b
9
10
X−Y distance (Å)
X−Z distance (Å)
X−C1 distance (Å)
Z−X−Y angle (deg)
C1−X−Z angle (deg)
C1−X−Y angle (deg)
C2−C1−X−Y torsion (deg)
C6−C1−X−Y torsion (deg)
2.283(2)
2.219(4)
2.113(4)
170.5(1)
93.8(1)
76.8(1)
0.3(2)
2.309
2.618
2.097
168.5
94.6
74.0
0.1
2.868(2)
2.169(2)
2.123(2)
159.9(7)
93.9(9)
70.1(7)
19.8(2)
34.9(2)
2.570(3)
2.188(5)
2.104(5)
168.3(2)
94.7(2)
73.6(2)
0.5(4)
2.481(6)
2.206(9)
2.119(7)
167.4(3)
93.8(3)
73.7(2)
2.1(5)
0.7(3)
3.8
0.7(5)
5.7(7)
a
Atom numbering as in Figure 4.
E
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interaction and most likely reflects a weak three-center−four-
electron interaction.
Among the three tellurium compounds, 9 displays geometrical
features very similar to those of iodine compound 2. The
resemblance is illustrated by overlaying the two crystal structures
(Figure 6). Although the tellurium−oxygen distance is longer
Figure 6. Overlay of crystal structures obtained for 259 (blue) and for 9
(red).
than the iodine−oxygen bond, the tellurium−carbon and
iodine−carbon bond lengths are almost the same (0.031(6) Å
shorter in 9 than in 2).
NMR ANALYSIS
Figure 7. Chemical shifts of tellurium compounds obtained in this work.
CDCl3 was used as a solvent, and the measurements were done at room
temperature. All shifts are reported with respect to Me2Te.
■
The element tellurium has two spin 1/2 nuclei, 125Te and 123Te;
therefore, tellurium compounds can be characterized using NMR
spectroscopy. Due to its higher natural abundance (7%), 125Te is
the isotope of choice. The chemical shift range for this isotope
spans from about −1400 to 3400 ppm, assuming that Me2Te
resonates at 0 ppm.60
Compounds carrying the perfluorophenyl fragment on the
tellurium atom (8a,b, 12) follow this trendthey appear in the
range 428−580 ppm, which is intermediate between alkyl and
CF3 tellurium compounds. The perfluoroalkyl substituent is
expected to be less electrophilic than the CF3 group, because
tellurium and fluorine are separated by three bonds (in
comparison to the to two-bond separation in the case of the
CF3 group). As in the case of CF3 compounds, a molecule with an
aldehyde group (12) has the highest shift.
CF2H compound 7 resonates at 741 ppm, suggesting that in
addition to the number of bonds separating fluorine and
tellurium the total number of fluorine atoms has an effect on
increasing the chemical shift. Finally, ditellurides resonate
around 300−400 ppm, in accordance with literature data.61
Both findings are in agreement with the trend.
Although it is possible to measure 125Te NMR directly, we
decided to use 1H−125Te and 19F−125Te correlation techniques
because measurement times are shorter and additional structural
information on a chosen compound is gained from a 2D
spectrum.
The results are summarized in Figure 7. Some known
hypervalent tellurium compounds (3e, 4e, 14, 15) were also
prepared, and the corresponding shifts are provided for
comparison.
It is evident that the chemical shift range for hypervalent CF3
tellurium compound lies within 844 and 1014 ppm. The lowest
chemical shift in this group is observed for compound 10, in
which a nitrogen atom is coordinated to the tellurium center.
Higher shifts are observed for compounds with oxygen
coordinated, with aldehyde 9 having the highest value.
Umemoto’s intermediate 4e also falls in this range.
In contrast, alkyl aryl tellurides resonate at significantly lower
frequency. In both analyzed examples (6 and 14) negative
chemical shifts were obtained. It can therefore be concluded that
the CF3 group has a pronounced deshielding effect on the 125Te
chemical shift. In addition, compound 15, in which a chlorine
atom is directly bound to tellurium, has the highest chemical shift
of all synthesized compounds. Therefore, it can be concluded
that the 125Te chemical shift increases with the electronegativity
of the substituent attached to tellurium. On the other hand, it has
to be kept in mind that the chemical shifts of nuclei different from
1H are determined by additional factors, including energies of
excited states, and not only electronegativity of the substituent.
Further analysis beyond the scope of this work would be
necessary to identify the various contributions.
In addition to being a tool for characterization, NMR
spectroscopy provides evidence indicative of the hypervalent
nature of the compounds obtained. For example, in case of 9 a
cross peak between the aldehyde proton and one of the aromatic
protons is observed in the NOESY spectrum. This observation
strongly suggests that the aldehyde oxygen is pointing toward the
tellurium center, since only in this conformation both protons
1
are close in space. Furthermore, in the H−125Te correlation
spectrum couplings between the tellurium center and both
aromatic and aldehyde protons are observed when sufficiently
long delay times are applied. The latter correlation may result
from a coupling through the scaffold in that the nuclei (tellurium
and aldehyde proton) are separated by four bonds. On the other
hand, it may also indicate the presence of a hypervalent bond,
since the nuclei are separated by three bonds in this model. A
coupling through three bonds is more likely to be observed than
that through four. In addition, cross peaks between the tellurium
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Figure 8. 1H−125Te correlation spectrum of molecule 10. The 1H NMR spectrum of 10 is shown as an external projection.
center and aromatic protons other than those in ortho positions
were not detected.
CONCLUSION
■
In summary, a series of new hypervalent CF3 tellurium
compounds were successfully prepared. X-ray analysis revealed
a close structural resemblance between these molecules and
hypervalent iodine derivatives known as reagents for perfluor-
oalkylation. In addition, CF2H and C6F5 groups could be
installed on tellurium, providing structures so far unknown for
iodine analogues. Further studies concerning the use of these
molecules as perfuoroalkyl transfer agents are currently being
carried out in our laboratory and will be reported in due course.
In the case of molecule 10, the presence of methyl groups on
the mesidine fragment allows us to observe an additional effect.
1
In the H−125Te correlation spectrum, a coupling between the
mesidine ortho methyl protons and the tellurium center is
observed (Figure 8). This is an indication of the A-type structure
in which these protons and tellurium are close enough in space to
allow for orbital overlap between tellurium lone pairs and the
hydrogen s orbital. If the coupling between these protons and the
tellurium nucleus were through the scaffold, it would have to be
mediated by eight bonds. Such a long-range coupling is unlikely
to be observed.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00535.
■
S
DFT CALCULATIONS
■
The interaction between the oxygen atom and the tellurium
center in 9 was further characterized by density functional theory
at the B3LYP/cc-pVDZ-PP level. A similar analysis of
structurally related organotellurium compounds has been
reported previously.62 According to the calculations in the gas
phase, conformer 9 is 8.3 kcal/mol lower in energy in
comparison to conformer 9a, in which the oxygen is pointing
away from tellurium (Figure 9). The strength of the interaction
Experimental procedures, characterization of compounds,
NMR and IR spectra, crystallographic data for compounds
4b, 9, and 10, and computational details (PDF)
Accession Codes
CCDC 1548965−1548967 contain the supplementary crystallo-
graphic 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
*E-mail for A.T.: atogni@ethz.ch.
■
Figure 9. Possible conformers of aldehyde 9.
ORCID
Antonio Togni: 0000-0003-3868-1799
Notes
The authors declare no competing financial interest.
between the oxygen lone pair and Te−CF3 antibonding orbital is
13.42 kcal/mol, as determined by natural bond orbital second-
order perturbation analysis. Therefore, structure 9 is the
preferred conformation, in accordance with observations made
in the solid state.
ACKNOWLEDGMENTS
■
The authors are grateful to the Swiss National Science
Foundation for a research grant (200021_156989).
G
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DOI: 10.1021/acs.organomet.7b00535
Organometallics XXXX, XXX, XXX−XXX