Metathetical redox reaction of (diacetoxyiodo)arenes and iodoarenes

Article abstract of DOI:10.3390/molecules201219874

The oxidation of iodoarenes is central to the field of hypervalent iodine chemistry. It was found that the metathetical redox reaction between (diacetoxyiodo)arenes and iodoarenes is possible in the presence of a catalytic amount of Lewis acid. This discovery opens a new strategy to access (diacetoxyiodo)arenes. A computational study is provided to rationalize the results observed.

Full text of DOI:10.3390/molecules201219874

Article
Metathetical Redox Reaction of
(Diacetoxyiodo)arenes and Iodoarenes
Antoine Jobin-Des Lauriers and Claude Y. Legault *
Received: 25 November 2015 ; Accepted: 15 December 2015 ; Published: 17 December 2015
Academic Editors: Wesley Moran and Arantxa Rodríguez
Centre in Green Chemistry and Catalysis, Department of Chemistry, University of Sherbrooke,
2500 boul. de l’Université, Sherbrooke, QC J1K 2R1, Canada; Antoine.Jobin-Des.Lauriers@USherbrooke.ca
*
Correspondence: claude.legault@usherbrooke.ca; Tel./Fax: +1-819-821-8017
Abstract: The oxidation of iodoarenes is central to the field of hypervalent iodine chemistry. It was
found that the metathetical redox reaction between (diacetoxyiodo)arenes and iodoarenes is possible
in the presence of a catalytic amount of Lewis acid. This discovery opens a new strategy to access
(diacetoxyiodo)arenes. A computational study is provided to rationalize the results observed.
Keywords: hypervalent iodine; (diacetoxyiodo)arenes; metathesis
1. Introduction
In the recent years, the development of hypervalent iodine-mediated synthetic transformations
has been receiving growing attention [1–5]. This is not surprising considering the importance of
improving oxidative reactions and reducing their environmental impact. Hypervalent iodine reagents
are a great alternative to toxic heavy metals often used to effect similar transformations [6–10].
Among the plethora of iodine(III) compounds [11], it is undeniable that the (diacetoxyiodo)arenes
remain the most common and used ones [12]. They are usually stable solids and can serve as
precursors to numerous other iodanes, such as other (diacyloxyiodo)arenes and [hydroxy(tosyloxy)
iodo]arenes [13,14]. Access to (diacetoxyiodo)arenes is, of course, usually accomplished by the
oxidation of the corresponding iodoarene substrate. The traditional method involves peracetic
acid oxidation in acetic acid [15]. One unexploited strategy to access these specific reagents is the
metathetical redox reaction between (diacetoxyiodo)arenes and related iodoarenes (Scheme 1a).
While it is widely accepted that the ligands on iodane species are typically easily exchanged, the
interconversion of oxidation states between λ3-iodane and iodoarenes is much rarer. The first example
of such a metathetical redox reaction was reported by Koser et al., using [hydroxy(tosyloxy)-iodo]
benzene (HTIB) as the oxidant, enabling transfer to a variety of iodoarenes (Scheme 1b) [16].
Two applications of this initial method were subsequently reported, including one example using
[bis(trifluoroacetoxy)iodo]benzene (BTI) [17,18]. More recently, Ciufolini et al. have reported the
iodonium salt metathesis reaction with iodoarenes (Scheme 1c) [19].
To the best of our knowledge, the metathetical redox reaction between (diacetoxyiodo)arenes and
iodoarenes has never been reported. Expanding the scope of iodane metathesis to these commonly
used reagents would be of great interest as an alternative synthetic method. Herein we report a
protocol to achieve such a metathesis of (diacetoxyiodo)benzene and iodoarenes. We also provide a
computational study to determine the origin of the observed reactivity.
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Molecules 2015, 20, 22635–22644
Scheme 1. Concept and examples of metathetical redox reactions of iodanes and iodoarenes.
2. Results and Discussion
The reactivity of HTIB and (diacetoxyiodo)benzene (DIB) toward iodoarenes was evaluated,
using conditions analogous to Koser’s successful iodane metathesis with HTIB [16]. We submitted
p-methyliodobenzene and p-bromoiodobenzene to iodane metathesis with HTIB (Scheme 2a) and DIB
at room temperature (Scheme 2b). In accordance with the results reported by Koser, equilibrium is
achieved for the reaction with HTIB within 96 h. In contrast, no metathesis was observed after 72 h
with DIB.
Scheme 2. Evaluation of the reactivity of DIB toward the metathetical redox process.
These results demonstrate the very different reactivity profiles of DIB and HTIB. The origin of
this reactivity dichotomy was investigated. In a recent computational study, we have suggested
that reaction with HTIB would involve, at room temperature, iodonium intermediates, through
a ligand dissociation mechanism [20]. Due to the large difference in effective electronegativity
between the hydroxy and tosyloxy groups on HTIB, ionization in solution is achievable at room
temperature [21,22]. In contrast, (diacetoxyiodo)arenes have identical ligands involved in the
three-center four-electron bonding, and thus dissociation is expected to be difficult [23]. We have
evaluated the energetic of dissociation for HTIB, DIB, and diphenyliodonium salt 4 (see the
computational details in Section 4), the results are illustrated in Scheme 3.
Scheme 3. Energetics of ligand dissociation on DIB, 4, and HTIB (energies reported in kcal/mol).
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Molecules 2015, 20, 22635–22644
As suspected, the dissociation of the tosyloxy group is calculated to be endergonic, but feasible.
Dissociation of the triflate anion in the diphenyliodonium salt 4 is predicted to be even more facile at
room temperature. In contrast, the calculations suggest that dissociation of an acetoxy group on DIB
at room temperature would be too difficult.
The dissociation energies seem to be correlated to the intrinsic reactivity of these iodanes toward
metathesis. We postulated that the key to the metathetical redox process might occur by the formation
of an iodonium intermediate. If it is the case, this could be achieved with DIB by promoting the
dissociation of an acetoxy group through the use of a Lewis acid. We elected to use BF3¨ OEt2,
as it was shown in numerous methodologies to be a proficient and compatible Lewis acid with
DIB [24,25]. DFT Calculations, using BF3¨ OMe2 as a model, suggest that this Lewis acid would permit
the formation of an iodonium intermediate at room temperature, as illustrated in Scheme 4.
Scheme 4. Energetics of ligand dissociation on DIB with BF3¨ OMe2 (energies reported in kcal/mol).
With this in mind, we tested the iodane metathesis between DIB and p-bromoiodobenzene (1b)
in the presence of 5 mol % of BF3¨ OEt2 (Equation (1)). To our delight, the metathetical redox process
proceeded cleanly to attain equilibrium within 67 h. The calculated free energy of reaction was
predicted to be +0.5 kcal/mol, which is in good agreement with the experimentally measured value
of +0.24 kcal/mol. The reaction did also proceed with p-methyliodobenzene (1a), but formation
of a suspension pointed to a Friedel-Crafts side reaction of 1a with activated DIB, and was not
investigated further.
(1)
With these promising results in hand, the reaction mechanism was investigated using DFT
calculations (see computational details in Section 4). A single electron transfer (SET) redox process
between the iodonium intermediate and the iodoarene was considered (Equation (2)). The transfer
was found to be energetically too costly to occur readily at room temperature. In accordance with the
calculations, it was found that light had no effect on the rate of reaction in the conditions described in
Equation (1), further suggesting that the process does not involve a SET.
(2)
An ionic mechanism was thus considered. The intermediates evaluated are illustrated in
Scheme 5. This mechanism has similarities to the one proposed by Ciufolini et al. for diaryliodonium
salt metathesis [19], with the exception of the transferred group being easily dissociated. For the sake
of simplicity, the aryl (Ar) group in the intermediates is a simple phenyl group and the initial iodane is
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Molecules 2015, 20, 22635–22644
DIB. The transition states leading to the different intermediates described were not studied, as it was
assumed that these processes of association and dissociation do not require large activation energies.
Scheme 5. Reaction mechanism evaluated using DFT calculations (Ar = Ph for simplicity).
The energetic properties of all these intermediates were calculated and are illustrated in an
energy diagram (Scheme 6). As described in Scheme 6, formation of intermediate AIB+, following
complexation by BF3¨ OMe2, was found to be an endergonic but feasible process at room temperature.
At this point, the iodonium intermediate can act as a Lewis acid toward an iodoarene (in this
´
case iodobenzene) to form adduct AIB+¨ ArI. Association of the AcOBF3 counterion led to Int1.
By internal Lewis acid migration, Int2 would be accessed. This intermediate can then lead to
AIAr+¨ PhI adduct, which can liberate iodobenzene and finally provide, by transfer of BF3 to another
DIB molecule, the newly formed (diacetoxyiodo)arene (DIAr). The calculations thus suggest an ionic
mechanism relying on the asymmetry or activation of the iodane moiety to initiate the metathetical
redox process, through a key iodonium intermediate.
Int1
Int2
G
16.2
16.2
AIAr+
11.7
AIB+
11.7
next
catalytic
cycle
11.6
11.6
AIB+•ArI
AIAr+•PhI
DIB + BF3
0.0
-2.6
DIB•BF3
-2.6
DIAr•BF3
-2.6
DIAr
+
DIB•BF3
Scheme 6. Energy diagram of the proposed mechanism (energies reported in kcal/mol).
Passage through a non-symmetric iodane intermediate could explain the metathesis reported by
Koser et al., in which iodoarene 5 is converted to its corresponding cyclic iodane 6 by the action of
BTI, without the need of a catalyst (Equation (3)) [17].
(3)
In this reaction, rapid ligand exchange would lead to intermediate Int-A (Scheme 7). Due to
the large difference in effective electronegativity between the alkoxy and trifluoroacetoxy groups,
dissociation of the latter to furnish the iodonium intermediate Int-B is conceivable. Furthermore,
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Molecules 2015, 20, 22635–22644
the fact that Int-B binds together both I(I) and I(III) partners might facilitate the metathetical
redox process.
Scheme 7. Proposed intermediate to rationalize reactivity of BTI toward 5.
Calculations using t-BuOH as a model alcohol support this proposal, as described by the
energetic values illustrated in Scheme 8. While dissociation of a trifluoroacetate group on the
symmetric BTI was calculated to be too costly to occur at room temperature. In contrast, due to
the lower effective electronegativity of the t-butoxy compared to the trifluoroacetoxy group, the
dissociation on the non-symmetric tBTFIB is facilitated.
Scheme 8. Alcohol-promoted formation of an iodonium intermediate.
This alternative activation mechanism raises an important point concerning the presence
of water in these reactions. As a protic solvent similar to an alcohol, water might act as an
activator to promote acyloxy group dissociation. The energetic values of water exchange and
possible acyloxy dissociation were computed for both DIB and BTI; the results are illustrated in
Scheme 9. In the case of DIB, even the intermediate species [hydroxy(acetyloxy)iodo]benzene (HAIB)
would not permit dissociation of the acetyloxy group to form the iodonium intermediate at room
temperature. In contrast, the calculations suggest that dissociation of the trifluoroacetoxy group
on the analogous [hydroxy(trifluoroacetoxy)iodo]benzene (HTFIB) intermediate might be possible
at room temperature. In essence, even if BTI is found to be more stable than HTFIB, the latter would
possess reactivity similar to Koser’s reagent (HTIB). The main difference between DIB, BTI, and HTIB
´
´
´
can simply be explained by the better nucleofugality of TsO and CF3CO2 vs. AcO .
Scheme 9. Water-promoted formation of iodonium intermediates.
These calculations support the non-reactivity observed for DIB even in non-anhydrous solvents.
In contrast, we could expect reactivity from BTI even at room temperature under non-anhydrous
conditions. To test this hypothesis, BTI was let to react with p-bromoiodobenzene at room
temperature without strict exclusion of water. For comparison, a similar experiment involving
5 mol % of BF3¨ OEt2 was also performed. The results are illustrated in Scheme 10.
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Molecules 2015, 20, 22635–22644
F3CCO2
I
O2CCF3
F3CCO2
I
O2CCF3
I
I
Additive
CDCl3 (1.0 M)
rt, time
Br
Br
BTI
1b
7
Grxn (kcal/mol)
found +1.2
calculated +0.6
Additive
time (h) conv. (%)
none
19
41
19
41
6
13
25
26
BF3•OEt2 (5 mol%)
Scheme 10. Evaluation of BTI toward uncatalyzed and catalyzed iodane metathesis.
The experimental outcome of the iodane metathesis reactions with BTI is in agreement with the
calculation results. Metathesis occurs slowly in the absence of a Lewis acid; equilibrium could not be
achieved, even after 41 h. As with DIB, the Lewis acid has a drastic effect on the rate of metathesis, as
equilibrium is attained within 19 h. Again, the calculated free energy of reaction is in good agreement
with the experimentally observed conversion (Scheme 10).
As the iodane metathesis reactions are under thermodynamic control, we selected a substrate
that would favor a complete metathetical redox process. We performed the iodane metathesis
using DIB and o-iodobenzoic acid (o-I-BzOH) as the iodoarene (Scheme 11). The thermodynamic
drive (i.e., entropic gain) of the reaction was predicted to lead to a complete conversion
(∆Grxn = ´11.8 kcal/mol). Additionally, o-I-BzOH enables the evaluation of the proximity effect of
the iodine(I) and iodine(III) partners on the rate of the metathetical redox reaction. Indeed, o-I-BzOH
can serve as an acyloxy ligand. We thus performed AcO/o-I-BzO exchange on DIB in the absence
of a Lewis acid. Treatment of an equimolar mixture of DIB and o-I-BzOH in chloroform, followed
by AcOH removal through hexane azeotrope, resulted in a clean mixture of DIB, 8, and 9 in a
1:2:1 ratio, indicative of an almost statistical equilibrium. It is important to note that no iodobenzene
was observed at this point. The proximity effect of the I(I) and I(III) partners did not facilitate in
any way the metathetical redox process in the absence of a Lewis acid. Treatment of this statistical
mixture of [bis(acyloxy)iodo]benzenes with a catalytic quantity of BF3¨ OEt2 resulted in the smooth
and clean conversion to the single cyclic iodane 10 and iodobenzene. The same efficient metathetical
redox process could be done in one-pot by treating an equimolar quantity of DIB and o-I-BzOH with
5 mol % of BF3¨ OEt2 for 4 h in dichloromethane. Compound 10 could be obtained in essentially
quantitative yield by simple removal of PhI by trituration with hexane.
Scheme 11. Ligand exchange between DIB and o-I-BzOH followed by iodane metathesis.
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Molecules 2015, 20, 22635–22644
There is a need to reconcile the fact that ligand exchange on DIB is facile at room temperature
without the need for any activation, while the metathetical redox process is not possible. The nature
of the exchange processes needs to be considered. While the calculations suggest a mandatory
dissociative process through an iodonium intermediate to achieve metathesis, an associative
mechanism could still be considered for ligand exchange (Scheme 12). It was calculated that cis-DIB,
a T-shaped isomer of DIB in which both acetoxy ligands are cis to each other, would be energetically
accessible at room temperature. This intermediate neutral iodane would react readily with a
carboxylic acid (AcOH was used for simplicity), through proton transfer, to afford in return DIB
in its most stable conformation, bearing its two most electron withdrawing groups in trans relation.
Addition of iodobenzene on cis-DIB cannot occur readily, as it is not a strong enough donor to stabilize
expulsion of the acetoxy group.
Scheme 12. Plausible mechanism for ligand exchange (energies reported in kcal/mol).
In summary, we have demonstrated that, with the simple action of a Lewis acid, it is
possible to achieve the metathetical redox reaction of (diacetoxyiodo)arenes and iodoarenes. Both the
experimental results and computational insights draw a clearer picture of what is needed for
these metathetical redox reactions to proceed. The results presented herein open the way for
further exploration.
3. Experimental Section
3.1. General Information
All glassware was stored in the oven and/or was flame dried prior to use under an
inert atmosphere of gas. CDCl3 was dried over anhydrous K2CO3 but not thoroughly dried.
Analytical thin-layer chromatography (TLC) was performed on precoated, glass-backed silica gel
(Merck 60 F254). Visualization of the developed chromatogram was performed by UV absorbance,
aqueous cerium molybdate, ethanolic phosphomolybdic acid, iodine, or aqueous potassium
permanganate. Nuclear magnetic resonance spectra (1H, 13C, DEPT) were recorded either on an
Avance III HD 300 (Bruker, Billerica, MA, USA) or Mercury+ 400 (Agilent Technology, Santa Clara,
1
CA, USA) spectrometers. Chemical shifts for H-NMR spectra are recorded in parts per million
from tetramethylsilane with the solvent resonance as the internal standard. Data are reported as
follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, qn = quintet,
sext = sextuplet, m = multiplet and br = broad), coupling constant in Hz, integration. Chemical shifts
for 13C-NMR spectra are recorded in parts per million from tetramethylsilane with the solvent
resonance as the internal standard. All spectra were obtained with complete proton decoupling.
When ambiguous, proton and carbon assignments were established using COSY, NOESY, HMQC
and DEPT experiments. High resolution mass spectra were recorded on a Maxis ESI-Q-Tof (Bruker,
Billerica, MA, USA) at the Université de Sherbrooke.
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Molecules 2015, 20, 22635–22644
3.2. Metathetical Redox Reaction of HTIB and p-Methyliodobenzene
To a round-bottom flask were added [hydroxy(tosyloxy)iodo]benzene (HTIB) (333.4 mg,
0.850 mmol), p-methyliodobenzene (185.3 mg, 0.850 mmol) and dichloromethane (1.7 mL, 0.5 M).
The white suspension was stirred at room temperature. Aliquots (5–10 µL) were taken from clear
dichloromethane supernatant after allowing the suspension to settle and diluted in HPLC-grade
hexanes (500–750 µL) at various moments. Conversions were determined by GC-MS by integration
of signals of iodobenzene and p-methyliodobenzene.
3.3. Lewis-Acid-Catalyzed Metathetical Redox Reaction of DIB and p-Bromoiodobenzene
To a flame-dried vial equipped with a septum, under an argon atmosphere, were added
DIB (372.1 mg, 1.155 mmol), p-bromoiodobenzene (325.6 mg, 1.155 mmol) and CDCl3 (442 µL).
Diluted BF3¨ Et2O (713 µL of a freshly made solution of BF3¨ Et2O [100 µL BF3¨ Et2O in 10 mL CDCl3],
0.058 mmol) was added for a total volume of 1.16 mL of CDCl3 (1 M). The solution was stirred at room
temperature under inert atmosphere. Conversions were determined by integration of well-defined
1H-NMR signals (5 s relaxation time) of the two (diacetoxyiodo)arenes present in solution. The free
energies of reaction (Equation (1) and Scheme 10) were determined with the measured conversions
(see Supplementary Materials for more details).
3.4. One-Pot Metathetical Redox Reaction of DIB and o-Iodobenzoic Acid
To a flame-dried vial, under argon, were added o-iodobenzoic acid (247.7 mg, 1.00 mmol), DIB
(322.1 mg, 1.00 mmol) and a solution of BF3¨ Et2O (1.22 mL, [100 µL BF3¨ Et2O in 10 mL CH2Cl2],
0.012 mmol). The final reaction concentration is 0.8 M. The solution was allowed to stir at room
temperature for 4 h. The solvent was evaporated and the crude residue washed twice with hexane.
The crude product was dried under vacuum to afford compound 10 in quantitative yield. The spectral
data is consistent with the reported one in the literature [26].
4. Computational Details
The geometry optimizations were done using the Gaussian 09 software package [27] with the
M06-2X [28] density functional in combination with the 6-31+G(d,p) basis set [29,30] for all atoms
except iodine, for which LANL2DZdp + ECP was used [31,32]. The structures were optimized with
a solvation model (SMD) for chloroform [33]. Unless otherwise stated, a fine grid density was used
for numerical integration in the calculations. Conformational searches were done on all the species
described throughout the mechanistic pathways. Harmonic vibrational frequencies were computed
for all optimized structures to verify that they were either minima or transition states, possessing
zero or one imaginary frequency, respectively. All the free energies are reported in kcal/mol and
incorporate unscaled free energy corrections based on the vibrational analyses and temperature of
298 K.
Supplementary Materials: Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/20/
12/19874/s1. Cartesian coordinates, electronic and zero-point vibrational energies.
Acknowledgments: This work was supported by the National Science and Engineering Research Council
(NSERC) of Canada, the Canada Foundation for Innovation (CFI), the FRQNT Centre in Green Chemistry and
Catalysis (CGCC), and the Université de Sherbrooke.
Author Contributions: A.J.-D.L. and C.Y.L. conceived and designed the experiments; A.J.-D.L. performed the
experiments; C.Y.L. performed the computational calculations; C.Y.L. wrote the paper; A.J.-D.L. helped with the
correction of the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Not available.
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