Diverse Reactions of Thiophenes, Selenophenes, and Tellurophenes with Strongly Oxidizing I(III) PhI(L)2 Reagents

Article abstract of DOI:10.1021/acs.inorgchem.6b02386

We report the outcomes of the reactions of aromatic group 16 thiophene, selenophene, and tellurophene rings with the I(III) oxidants PhI(OAc)(OTf) and [PhI(Pyr)₂][OTf]₂ (Pyr = pyridine). In all reactions, oxidative processes take place, with generation of PhI as the reduction product. However, with the exception of tellurophene with PhI(OAc)(OTf), +4 oxidation state complexes are not observed, but rather a variety of other processes occur. In general, where a C-H unit is available on the 5-membered ring, an electrophilic aromatic substitution reaction of either ?IPh or pyridine onto the ring occurs. When all positions are blocked, reactions with PhI(OAc)(OTf) give acetic and triflic anhydride as the identifiable oxidative byproducts, while [PhI(Pyr)₂][OTf]₂ gives pyridine electrophilic aromatic substitution onto the peripheral rings. Qualitative mechanistic studies indicate that the presence of the oxidizable heteroatom is required for pyridine to act as an electrophile in a substantial manner.

Full text of DOI:10.1021/acs.inorgchem.6b02386

Article
pubs.acs.org/IC
Diverse Reactions of Thiophenes, Selenophenes, and Tellurophenes
with Strongly Oxidizing I(III) PhI(L)2 Reagents
Sathsara Egalahewa, Mohammad Albayer, Antonino Aprile, and Jason L. Dutton*
Department of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Victoria, Australia,
3086
S
* Supporting Information
ABSTRACT: We report the outcomes of the reactions of aromatic group 16 thiophene, selenophene, and tellurophene rings
with the I(III) oxidants PhI(OAc)(OTf) and [PhI(Pyr)2][OTf]2 (Pyr = pyridine). In all reactions, oxidative processes take place,
with generation of PhI as the reduction product. However, with the exception of tellurophene with PhI(OAc)(OTf), +4
oxidation state complexes are not observed, but rather a variety of other processes occur. In general, where a C−H unit is
available on the 5-membered ring, an electrophilic aromatic substitution reaction of either −IPh or pyridine onto the ring occurs.
When all positions are blocked, reactions with PhI(OAc)(OTf) give acetic and triflic anhydride as the identifiable oxidative
byproducts, while [PhI(Pyr)2][OTf]2 gives pyridine electrophilic aromatic substitution onto the peripheral rings. Qualitative
mechanistic studies indicate that the presence of the oxidizable heteroatom is required for pyridine to act as an electrophile in a
substantial manner.
Te(IV) product 8; addition of pyridine to this species resulted
in the same EAS as with [PhI(4-DMAP)2]2+, indicating that the
EAS reaction likely proceeds through Te(IV) intermediate 5.6
These I(III) reagents (both PhI(OAc)(OTf) and [PhI-
(Pyr)2]2+ (Pyr = pyridine or related pyridine derivative)) are
finding increasing use as oxidants in organic chemistry as more
reactive analogues of commercially available PhI(OAc)2.
[PhI(Pyr)2]2+ was first reported by Weiss,7 and later used by
Zhdankin for preparation of phosphorane−iodonium reagents.8
We reported the first crystal structure of these dications in 2012
and found that they can be used for the oxidative coupling of
N-heterocyclic carbenes.9 Huber has shown that they are
effective reagents for the oxidative coupling of anilines.10 Very
recently Wengryniuk used these reactive dications for the
oxidative rearrangement of benzylic alcohols,11 and Liu and
Liang demonstrated their use in synthesizing labile I(III)
precursors for synthesizing radiofluorinated compounds.12
Wirth and co-workers used the PhI(OAc)(OTf) reagent
(among other related I(III) reagents) for stereoselective
INTRODUCTION
■
Recently Seferos and co-workers demonstrated a new type of
functionalization to modulate the properties of tellurophene
rings: oxidation of the Te center from Te(II) (e.g., 1) to
Te(IV) (2) using halogenating oxidizing agents, which was
found to be reversible under green light irradiation or heat.1−3
This is an interesting advance in potentially generating
switchable organic molecules, as tellurophenes are of increasing
interest in the organic electronics field.4 Klapotke and co-
̈
workers have demonstrated that dihalogenated Se(IV)
analogues of Se(II) benzoselenophenes (3 and 4) can be
isolated and characterized via oxidation using X2 sources for X
= F, Cl, Br.5
Our group has reported that use of the very strong I(III)
oxidizing agents PhI(OAc)(OTf) and [PhI(4-DMAP)2]2+ to
oxidize 2,5-diphenyltellurophene resulted in unusual reactivity
(Scheme 1). Use of [PhI(4-DMAP)2]2+ resulted in a reaction
where the oxidation state of Te remained at +2, but an
electrophilic aromatic substitution (EAS) reaction occurred at
the 3-position of the tellurophene via intermediates 5 and 6,
giving 7, where the pyridine nitrogen atom acts as the
electrophile. Oxidation with PhI(OAc)(OTf) gave a clean
Received: October 4, 2016
© XXXX American Chemical Society
A
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Scheme 1. Reactions of 2,5-Diphenyltellurophene with PhIL2 Reagents, Leading to Te Oxidation and Electrophilic Aromatic
Substitution of Pyridine onto the 3-Position
oxidations of aryl ene-ones.13 DiMagno used PhI(OAc)(OTf)
in the formation of aryl iodides.14 Very recently this highly
RESULTS AND DISCUSSION
■
Reactions with Tetraphenyltellurophene. We began
investigating reactions with tetraphenyltellurophene to examine
the effect of blocking the 3- and 4-positions on the EAS
reaction. The 1:1 stoichiometric reaction of tetraphenyltellur-
ophene and PhI(OAc)(OTf) in CDCl3 resulted in formation of
a brown solution within 10 min. After 30 min an aliquot was
reactive molecule had its solid state structure confirmed,
showing that the triflate is only weakly bound to the iodine
center, explaining the powerful nature of this oxidant as
essentially a [PhI(OAc)]+ source.15
1
In this context, we have investigated the reactivity of
[PhI(pyr)2]2+ and PhI(OAc)(OTf), as well as PhI(OAc)2
with selected thiophenes, selenophenes, and tellurophenes, as
well as control substrate anisole. The results show that EAS-
type reactions dominate, with either −IPh or pyridine acting as
the electrophile depending on the substrate. The results
indicate that for pyridine to act as the electrophile, the
presence of the oxidizable group 16 atom is required,
suggesting intermediacy of a Ch(IV) species to turn pyridine
into an electrophile.
removed for NMR analysis. The H NMR spectrum of the
mixture indicated the presence of iodobenzene and one other
species. The 125Te NMR showed a shift from 533 ppm for
tetraphenyltellurophene to 992 ppm. Tellurium-125 NMR
spectroscopy is useful in detecting changes of Te containing
compounds, especially in cases with a change in oxidation state
as this will result in a large shift in the signals.16 The change of
the 125Te NMR resonance from 533 to 992 indicates that the
Te in the tellurophene had likely been oxidized from +2 state to
+4 state. This data is consistent with compound 9 (Scheme 2)
having a OTf and an OAc attached to the Te(IV) of
tetraphenyltellurophene, similar to the Te(IV) compound (8)
B
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modest downfield shift from that of selenophene at 605.0 ppm,
Scheme 2. Reaction of PhI(OAc)(OTf) with
Tetraphenyltellurophene Giving 9
which given Klapotke’s work on compounds 3/4 is ambiguous
̈
if an oxidation of Se had taken place. In that study downfield
shifts of 150−300 ppm were seen in oxidations of
benzoselenophenes from Se(II) to Se(IV).5 The ESI mass
spectrum of the product in CH3CN solution had a peak with
the very distinctive isotope pattern for selenium at [m/z]+ =
335.0.
X-ray diffraction studies were done on a single crystal
obtained from crystals grown in CH3CN at −35 °C, which
confirmed the product as iodonium derivative 10 (Scheme 3,
from our earlier study.6 Consistent with our previous work,
attempts to isolate a bulk powder from the reaction mixture
resulted in decomposition, and in this case we were unable to
grow single crystals from solutions stored at low temperatures
to confirm the compound’s identity. However, the NMR data is
consistent with compound 9.
Scheme 3. Synthesis of 10 from Selenophene and
PhI(OAc)(OTf)
The 1:1 stoichiometric reaction of tetraphenyltellurophene
and [PhI(Pyr)2][OTf]2 in CDCl3 resulted in a pale yellow
1
solution after stirring overnight. Examination of the H NMR
spectrum revealed that the reaction mixture contained
iodobenzene, tetraphenyltellurophene, and one other pyridine
containing species, with a chemical shift downfield from that of
free pyridine. Tellurium-125 NMR spectroscopy confirmed the
presence of tetraphenyltellurophene with only one resonance
observed at 533 ppm. The presence of iodobenzene in the
reaction mixture indicates that an oxidation reaction had
occurred, but had apparently left the tetraphenyltellurophene
unchanged. The 19F NMR spectrum of the reaction mixture
had only one resonance at −78.1 ppm, indicating that the
triflate anion is intact. This leaves the pyridine ligands as the
likely oxidation products. To explore this we repeated the
reaction where the [PhI(Pyr)2][OTf]2 had been generated
from labeled d-5 pyridine. The ESI mass spectra of the labeled
and unlabeled reaction were compared, but unfortunately there
were no signals that increased in apparent mass by 2.5, 5, or 10
mass units.
Figure 1. Solid-state structure of the selenophene dication in
compound 10. Thermal ellipsoids are drawn to the 50% probability
level; triflate anion and hydrogen atoms are omitted. The major
component of the disordered selenophene ring is shown.
To investigate whether the reaction of tetraphenyltellur-
ophene with [PhI(Pyr)2][OTf]2 potentially goes through a
Te(IV) intermediate, 2 equiv of pyridine was added to 9. This
experiment resulted in the 125Te NMR chemical shift
immediately reverting to 533 ppm from 992 ppm, indicating
that 9 is reduced back to tetraphenyltellurophene upon
addition of pyridine. The unidentified pyridine byproduct was
identical to the direct reaction with [PhI(Pyr)2][OTf]2. Acetic
Figure 1), an unreported derivative of selenophene. Unfortu-
nately severe disorder between the 3-position carbon and the
selenium atom in the crystal structure prevents any
interpretation of the structure beyond simple identification.
When selenophene was reacted with PhI(OAc)2, it resulted in
no reaction.
The treatment of selenophene with [PhI(Pyr)2][OTf]2 in
CD3CN resulted in a color change to brown within 5 min. The
1H NMR spectrum of the reaction mixture indicated that there
was a complex mixture of products present. The 77Se NMR
spectrum had several peaks in the 550−750 ppm region,
indicating six selenophene containing products in the mixture,
all being likely in the Se(II) oxidation state given the result
observed for 10 (Scheme 4).
1
acid and acetic anhydride could also be observed in the H
NMR spectrum, and intact triflate was observed in the 19F
NMR spectrum. Based on the available data, the pyridine is
likely oxidized somehow, but we have been unable to identify
the fate of the pyridine. Clearly, however, even with the 3- and
4-positions of the tellurophene ring blocked, the target pyridine
stabilized dication analogous to 5 is an unviable species.
Reactions with Selenophene and Substituted Seleno-
phene Derivatives. The reaction of the parent selenophene
with PhI(OAc)OTf in CD3CN resulted in a color change to
The positive ESI-MS detection had signals with the
distinctive isotope pattern for selenium containing complexes
that could be identified at [m/z]+ = 334.9 corresponding to 10
and [m/z]+ = 210.0 corresponding to 11.
1
green within 10 min. The H NMR spectrum of the mixture
The reaction of 2,5-diphenylselenophene with PhI(OAc)-
(OTf) in CD3CN resulted in the immediate formation of a
brown solution. In the H NMR spectrum, iodobenzene was
present (indicating that an oxidation had occurred), but intact
2,5-diphenylselenophene remained, which was confirmed via
77Se NMR spectroscopy. The 19F NMR spectrum displayed
peaks at −72.2 and −73.5 ppm, as well as a larger signal at −77
contained a downfield proton at ∼11 ppm and a methyl
resonance consistent with the presence of acetic acid (HOAc)
and one other species, with resonances consistent with a
substituted selenophene. The addition of diethyl ether resulted
1
1
in a green precipitate, which the H NMR spectrum indicated
was a substituted selenophene bearing a phenyl group. The 77Se
NMR shift of the green product was 727 ppm, which is a
C
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Scheme 4. Reaction of Selenophene with [PhI(Pyr)2][OTf]2
ppm, all upfield from ionic triflate. The peak at −72.2 ppm was
assigned to triflic anhydride based on comparison with the
chemical shift of a commercial sample. The region of −72 to
−73 ppm is also consistent with the chemical shifts of aryl
triflates.17 In the mass spectrum of the reaction mixture a signal
at m/z = 102.1 was present, which is consistent with acetic
[m/z]+ = 514.1 as the most abundant peak and [m/z]+ = 296.1.
The peak at 296.1 corresponds to a dicationic species as the
distance between the isotopic peaks is 0.5 mass unit giving a
mass of 592 for the dicationic compound. The difference
between 592 and 514 is consistent with loss of a pyridine unit.
To investigate the presence of pyridine containing species the
experiment was repeated using deuterated pyridine by reacting
tetraphenylselenophene and d-5 pyridine labeled [PhI(Pyr)2]-
[OTf]2. The positive ESI-MS had two peaks with the selenium
isotope pattern at [m/z]+ = 519.1 and [m/z]+ = 301.1. Both
peaks are five mass units higher than the previous peaks.
Therefore, the dicationic species would be ten mass units
higher, indicating the presence of two pyridine units in this
species.
1
anhydride, which was also observed in the H NMR spectrum.
Triflic anhydride and acetic anhydride are in an oxidized state
compared to ionic triflate and acetate; therefore it appears that
the Se(IV) intermediate analogous to 8 and 9 is unstable and
Se returns to the +2 oxidation state via oxidizing triflate and
acetate fragments. Triflic anhydride has also been observed to
be generated from high oxidation state triarylphosphorus(V)−
triflate species in conjunction with the PhI(OAc)(OTf) reagent
and other systems.9,18
X-ray diffraction studies were performed on a single crystal
obtained from yellow crystals grown in CDCl3 solution, at −35
°C, and confirmed the new product as dicationic 12, with
pyridines having substituted at the para positions of the phenyl
groups at the 2- and 5-positions of the selenophene (Figure 2).
The reaction of 2,5-diphenylselenophene with one stoichio-
metric equivalent of [PhI(Pyr)2][OTf]2 in CD3CN resulted in
1
the formation of a brown solution. The H NMR spectrum of
the reaction solution showed a mixture of products, with
multiple pyridine containing species, as well as starting material.
The 19F NMR spectrum in this case showed largely ionic
triflate, with only a trace of triflic anhydride present. A mass
spectrum of the reaction mixture revealed a signal associated
with starting 2,5-diphenylselenophene, as well as a signal
consistent with 7Se, the analogue of compound 7 that is
quantitatively generated in the reaction with 2,5-diphenyltellur-
ophene. The presence of this compound was confirmed by
performing the reaction with [PhI(Pyr)2][OTf]2 generated
from d-5 pyridine, which gave a corresponding increase of 5
mass units to the signal arising from 7Se. A small amount of
acetic anhydride was also present in the mass spectrum of the
reaction mixture. No single compound could be isolated from
the reaction mixture.
Figure 2. Solid-state structure of the selenophene dication in
compound 12. Thermal ellipsoids are drawn to the 50% probability
level; triflate anions and hydrogen atoms are omitted.
Tetraphenylselenophene with all four positions of seleno-
phene blocked was reacted with PhI(OAc)OTf in CDCl3,
which resulted in a color change to brown within 5 min. The
results of the reaction were similar to that found from 2,5-
diphenylselenophene, with iodobenzene, tetraphenylseleno-
phene (also observed in the 77Se NMR), and acetic anhydride
The reaction was driven to completion when performed
using two stoichiometric equivalents of [PhI(Pyr)2][OTf]2,
tracked by having one peak at 628 ppm in the 77Se NMR
spectrum of the reaction mixture, and 12 could be isolated in an
∼80% yield (Scheme 5). It should be noted here that
reinspection of the mass spectrum of the reaction mixture
generated from tetraphenyltellurophene and [PhI(Pyr)2]-
[OTf]2 did not show any evidence for the presence of a
complex analogous to 12.
Reactions with Thiophenes. The 1:1 stoichiometric
reaction of thiophene with PhI(OAc)OTf in CDCl3 resulted
in a color change from an initially colorless solution to purple
and then on to brown within 10 min. A proton NMR spectrum
of the mixture indicated the presence of acetic acid (HOAc)
and one other compound. The addition of n-hexane resulted in
a dark green precipitate. Proton NMR spectroscopy of the
filtered, washed, and dried product indicated that the thiophene
was substituted at the 2-position with a phenyl group attached
to it.
1
being present in the H NMR spectrum, and triflic anhydride
observed in the 19F NMR spectrum. Again, these results
indicate that the Se(IV) analogue of 8 and 9 is likely an
unstable species.
The reaction of tetraphenylselenophene and [PhI(Pyr)2]-
[OTf]2 in a 1:1 stoichiometric ratio in CD3CN resulted in a
1
color change to orange after stirring overnight. The H NMR
spectrum of the reaction mixture revealed the presence of
tetraphenylselenophene, protonated pyridine, and one other
pyridine and one other selenophene containing compound in
the reaction mixture. The 77Se NMR spectrum had two
resonances at 620 and 628 ppm for the starting material and
the new compound, respectively.
The cationic ESI mass spectrum had two peaks with the
distinctive isotope pattern for selenium containing complexes at
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Scheme 5. Synthesis of 12
The positive ESI-MS of the mixture gave [m/z]+ = 286.9 as
the base peak. This indicated the presence of 13 with an
iodobenzene group attached to the thiophene, consistent with
NMR data. Colorless crystals were grown from CH2Cl2
solution at −35 °C, and X-ray diffraction studies were done
on a single crystal to confirm the product 13. An examination
of the literature indicated that this compound was known and
in fact had been synthesized via the reaction of thiophene with
a related hypervalent I(III) compound, commercially available
Koser’s reagent PhI(OTs)(OH), arising from direct EAS from
the thiophene ring onto the iodine center.19
to the product 13 as the base peak, [m/z]+ = 160.0 for
compound 16, with a phenyl substituted, and [m/z]+ = 162.0
for compound 15, with pyridine substituted (Scheme 7). The
similar solubilities of these products prevented their isolation.
The 1:1 stoichiometric reaction of 2,5-dimethylthiophene
with [PhI(Pyr)2][OTf]2 in CDCl3 resulted in an orange
1
solution after overnight stirring at room temperature. The H
NMR spectrum of the reaction mixture again showed several
species in solution. The positive ESI-MS detection of the
mixture indicated the presence of protonated pyridine by [m/
z]+ = 80.1, [m/z]+ = 315.0 corresponding to the product 14 as
the most abundant peak, [m/z]+ = 188.0 for compound 17, and
[m/z]+ = 190.0 for compound 18 in a similar result to the
parent thiophene (Scheme 7). The similar solubilities of these
products again prevented their isolation as single components.
For tetraphenylthiophene with all four positions blocked,
reaction with PhI(OAc)(OTf) gave a similar result to the
reaction with the analogous selenophene, with iodobenzene,
triflic anhydride, and/or aryl triflates and acetic anhydride being
identified through a combination of NMR and mass spectral
studies. In this case however free tetraphenylthiophene could
not be identified in the 1H NMR spectrum among the products
of a rather messy reaction.
The reaction of 2,5-dimethylthiophene, having the 2- and 5-
positions blocked, with PhI(OAc)(OTf) also resulted in
iodobenzene EAS at the 3-position (14), as has also been
reported using Koser’s reagent (Scheme 6). The reaction of
Scheme 6. Reaction of Thiophene and 2,5-
Dimethylthiophene with PhI(OAc)(OTf)
Reaction with [PhI(Pyr)2][OTf]2 also gave the same result as
with the selenium analogue (Scheme 8), with dication 19 being
clearly apparent as the major product in the mass spectrum of
the reaction if two stoichiometric equivalents of [PhI-
(Pyr)2[OTf]2 was used. Solids isolated from the reaction gave
1H NMR resonances consistent with 19 via comparison to Se
analogue 12. Single crystals were grown from CH3CN solution
at −35 °C. While the crystals were of relatively poor quality, we
were able to obtain an X-ray structural solution confirming the
synthesis of 19 (Figure 3).
Qualitative Mechanistic Studies. The results obtained for
thiophene and selenophene, with −IPh being substituted onto
the rings, raise questions about the mechanism we proposed for
the electrophilic aromatic substitution of pyridine onto
tellurophenes using [PhI(Pyr)2][OTf]2, which was hypothe-
PhI(OAc)2 with thiophene and 2,5-dimethylthiophene resulted
in no reaction, indicating that this reagent is not electrophilic
enough to perform the substitution reaction.
The 1:1 stoichiometric reaction of thiophene with [PhI-
(Pyr)2][OTf]2 in CDCl3 resulted in a brown solution after
overnight stirring at room temperature. The 1H NMR spectrum
of the reaction solution indicated that it contained a mixture of
products including protonated pyridine. The positive ESI-MS
detection of the mixture indicated the presence of protonated
pyridine by [m/z]+ = 80.1 and [m/z]+ = 286.9 corresponding
Scheme 7. Reaction of Thiophene and 2,5-Dimethylthiophene with [PhI(Pyr)2][OTf]2
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Scheme 8. Reaction of Tetraphenylthiophene with [PhI(Pyr)2][OTf]2 Giving 19
iodine and oxidation of the S/Se atoms followed EAS at the
pyridine nitrogen. To test the validity of this hypothesis
PhI(OAc)(OTf) and [PhI(Pyr)2][OTf]2 were reacted with
anisole as a model EAS substrate without an oxidizable
heteroatom. The reaction of PhI(OAc)(OTf) with anisole in
1
CDCl3 resulted in an instant color change to blue. The H
NMR spectrum of the reaction showed the presence of acetic
acid, iodobenzene, and one other species with para substitution
on the anisole, and a new phenyl containing product. The ESI-
MS of a sample had signals at [m/z]+ = 311.1 and [m/z]+ =
184.2 as the most abundant peak corresponding to product 20
(Scheme 10), which has been previously reported by Kita and
co-workers using Koser’s reagent.20
Figure 3. Solid-state structure of the thiophene dication in compound
19. Thermal ellipsoids are drawn to the 50% probability level; triflate
anions and hydrogen atoms are omitted.
Scheme 10. Outcome of the Reaction of PhI(OAc)(OTf)
and [PhI(Pyr)2][OTf]2 with Anisole
sized to go via a Te(IV) intermediate.6 The isolation of 10, 13,
and 14 suggests an alternate possibility, where the reaction
results in a cationic −IPh substituted product via direct EAS,
and the liberated pyridine reacts with this product in a
subsequent nucleophilic aromatic substitution. To investigate
this possibility, pyridine or 4-DMAP was added to samples of
1
10 and 14 in CH3CN solvent and monitored via H NMR
spectroscopy (Scheme 9). No reaction was observed, indicating
Scheme 9. Null Reaction on Adding Pyridine or 4-DMAP to
Compounds 10 and 14
The reaction of anisole with [PhI(Pyr)2][OTf]2 (14)
1
resulted in a yellow solution after stirring overnight. The H
NMR spectrum of the reaction showed 20 as the major
product, which was also apparent in the mass spectrum. In the
mass spectrum a signal consistent with 21 was also observed,
but only a very small amount. This result indicates that pyridine
substitution is possible, but very suppressed in this case, in
contrast to the systems containing potentially oxidizable group
16 atoms.
that this alternate nucleophilic substitution pathway is not
occurring. For the syntheses of 12 and 19, if the formation was
via a para −IPh substituted cation/dication generated via direct
electrophilic aromatic substitution, it would be expected that
such a species would be observed as a product in the reactions
with PhI(OAc)(OTf), analogous to the isolation of 10, 13, and
14. However, close inspection of the mass spectra shows no
evidence of mass of cations or dications consistent with single
or double substitution of −IPh onto the tetraphenylthiophene
or selenophene in reactions with PhI(OAc)(OTf).
Our key supposition in the EAS reaction where pyridine is
turned into an electrophile at nitrogen is the intermediacy of a
Ch(IV) species with pyridine bound to the chalcogen center.
We hypothesized that the mixture of products observed in the
reaction of [PhI(Pyr)2][OTf]2 with thiophenes and seleno-
phenes arises from competing processes of direct EAS at the
CONCLUSIONS
■
Our study sheds light on the reactions of the oxidations of
thiophenes, selenophenes, and tellurophenes using the PhIL2
class of oxidants. With the exception of Te(IV) compounds 8
and 9, generated from PhI(OAc)(OTf), the Ch(IV) species are
clearly unstable species and undergo further reactions. The
initial target of a pyridine stabilized Ch(IV) dication is clearly
unviable for all rings investigated. However, there is evidence
for Ch(IV) intermediates in these reactions, indicating that the
+4 species can be fleetingly obtained.
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In the case of reactions with either the parent or
disubstituted thiophenes and selenophenes when using [PhI-
(Pyr)2][OTf]2, competing reactions are observed, resulting in
substitution of either −IPh or pyridine. With PhI(OAc)(OTf),
−IPh substitution is observed when there are available C−H
positions on the chalcogen containing ring, giving known
thiophene derivatives, but a new derivative of selenophene.
These −IPh substituted compounds were shown not to be
intermediates in the generation of pyridine substituted
compounds, as addition of pyridine to these cations does not
result in displacement of iodobenzene. We believe that the two
competing pathways are direct electrophilic aromatic sub-
stitution when −IPh is substituted and Ch(IV) mediated
electrophilic aromatic substitution when pyridine is the
substrate. This is supported by the generation of 12 and 19,
where reactions with PhI(OAc)(OTf) with the tetraphenylth-
iophene and tetraphenylselenophene return starting material
and mixed byproducts including acetic and triflic anhydride
with no evidence for −IPh substitution, but [PhI(Pyr)2][OTf]2
gives relatively clean substitution onto the para position of the
phenyl rings at the 2- and 5-positions. We hypothesize that the
observed oxidative byproducts of acetic and triflic anhydride in
the reactions with PhI(OAc)(OTf) are generated from unstable
Ch(IV) intermediates, which undergo reduction back to the
stable aromatic Ch(II) starting compounds.
Also supportive of the hypothesis of Ch(IV) involvement in
turning pyridine into an electrophile is the dominant
substitution of −IPh onto the para position of anisole, where
no oxidizable heteroatom is present, with reactions between
anisole and [PhI(Pyr)2][OTf]2 only giving a trace of pyridine
substituted compound.
A key outcome of this study, given the increased use of these
oxidants, is that care should be taken when using PhI(OAc)-
(OTf) or [PhI(Pyr)2]2+ as these are highly active toward
electrophilic aromatic substitution reactions and can give
unpredictable results when they do occur. The substrates
used in concert with these reagents should be designed so that
undesirable EAS side reactions are unlikely.
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■
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02386.
Experimental details, 1H, 13C, 77Se, and 125Te NMR
spectra, and mass spectra (PDF)
X-ray crystallographic details (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: j.dutton@latrobe.edu.au.
■
ORCID
Jason L. Dutton: 0000-0002-0361-4441
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank The La Trobe Institute for Molecular Science for
their generous funding of this project. This work was also
supported by ARC DECRA and Future Fellowships (J.L.D.,
DE130100186, FT160100007).
G
DOI: 10.1021/acs.inorgchem.6b02386
Inorg. Chem. XXXX, XXX, XXX−XXX