Synthesis, characterization and initial evaluation of 5-nitro-1- (trifluoromethyl)-3H-1λ3,2-benziodaoxol-3-one

Article abstract of DOI:10.3762/bjoc.10.1

The synthesis of 5-nitro-1-(trifluoromethyl)-3H-1λ³,2- benziodaoxol-3-one (3), a hypervalent-iodine-based electrophilic trifluoromethylating reagent, is described. Whereas considerations based on cyclic voltammetry and X-ray structural properties would predict an inferior reactivity when compared to the non-nitrated derivative 2, ¹⁹F NMR kinetic studies showed that this new derivative is almost one order of magnitude more reactive. Furthermore, differential scanning calorimetry measurements indicated that, in addition, it is also safer to handle.

Full text of DOI:10.3762/bjoc.10.1

Synthesis, characterization and initial evaluation
of 5-nitro-1-(trifluoromethyl)-3H-
1λ3,2-benziodaoxol-3-one
Nico Santschi, Roman C. Sarott, Elisabeth Otth, Reinhard Kissner
and Antonio Togni*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1–6.
Department of Chemistry and Applied Biosciences, Swiss Federal
Institute of Technology (ETH Zürich), Wolfgang-Pauli-Str. 10,
CH-8093 Zürich, Switzerland
doi:10.3762/bjoc.10.1
Received: 23 September 2013
Accepted: 18 November 2013
Published: 02 January 2014
Email:
Antonio Togni* - atogni@ethz.ch
This article is part of the Thematic Series "Organo-fluorine chemistry III".
Guest Editor: D. O'Hagan
*
Corresponding author
Keywords:
cyclic voltammetry; differential scanning calorimetry; electrophilic
trifluoromethylation; 19F NMR kinetics; nitration; organo-fluorine
© 2014 Santschi et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The synthesis of 5-nitro-1-(trifluoromethyl)-3H-1λ3,2-benziodaoxol-3-one (3), a hypervalent-iodine-based electrophilic trifluoro-
methylating reagent, is described. Whereas considerations based on cyclic voltammetry and X-ray structural properties would
predict an inferior reactivity when compared to the non-nitrated derivative 2, 19F NMR kinetic studies showed that this new deriva-
tive is almost one order of magnitude more reactive. Furthermore, differential scanning calorimetry measurements indicated that, in
addition, it is also safer to handle.
Introduction
Since the advent of the hypervalent-iodine-based electrophilic
trifluoromethylation reagents 1 and 2 in 2006 they have found
widespread application in organic synthesis (Figure 1) [1-4].
Recently, increasing interest has been directed towards the
photocatalytic one-electron reduction of these compounds in
order to generate the highly electrophilic trifluoromethyl radical
Figure 1: Electrophilic trifluoromethylating agents 1 and 2.
[
5-7].
For example, Gouverneur et al. have highlighted the regioselec- also extensive electrochemical characterizations of various elec-
tive allylic trifluoromethylation employing reagent 2, trophilic trifluoromethylating agents are now available [8].
[
Ru(bpy)3]Cl2 and allylsilanes as substrates [7]. In this context, These studies reported cathodic peak potentials versus the
1

Beilstein J. Org. Chem. 2014, 10, 1–6.
Ag/Ag+ pair of −1.82 V and −1.10 V for 1 and 2, respectively. Inspection of the 13C NMR chemical shift of the quaternary
Therefore, we contemplated the possibility of enhancing the re- phenyl carbon attached to iodine (CAr-I) in 5 and comparison to
activity of reagent 2 by rendering its scaffold less electron-rich. literature data available for 2-iodobenzoic acid (4) indeed
This was to be achieved through the introduction of a strongly confirmed the installation of an electron-poor center [12].
electron-withdrawing substituent in para-position to the iodine Hence, in the parent acid 4 CAr-I gives rise to a signal at
atom, potentially also leading to a more positive standard reduc- 94.1 ppm and in 5 the corresponding carbon atom is deshielded
tion potential. Therefore, based on a recent report by Togo et al. by 9.6 ppm and observed at 103.7 ppm. The same observation
[
9], the nitration of the reagent scaffold to afford the new holds true for 6 and its unmodified analogue 7 (104.0 ppm
derivative 5-nitro-1-(trifluoromethyl)-3H-1λ3,2-benziodaoxol- versus 94.1 ppm, Δδ = 9.9 ppm) as well as for 3 and 2
3
-one (3) was taken into consideration.
(121.8 ppm versus 114.8, Δδ = 7 ppm). Furthermore, this com-
parison highlights that the replacement of the chlorine atom by
the CF3 group to a certain degree compensates the electronic
Results and Discussion
The direct nitration of 2-iodobenzoic acid (4) has been reported effects imposed by the NO2 group as indicated by the lessened
before and was carried out on a 10 gram scale in a mixture of extent of deshielding. Nitration also had a distinct effect on the
HNO3 and H2SO4 at 135 °C (Scheme 1) [10]. Reductive work- electronic spectrum of the final reagent. Solutions of 2 and 3 in
up using an aqueous acidic solution of potassium iodide DMSO/H2O 1:1 were studied at concentrations of 9.7 × 10−5 M
furnished the desired product 5 in 11–46% yield (29% average and 9.1 × 10−5 M, respectively. A graphic representation of the
yield of four experiments). Subsequent oxidation was centered molar absorptivity ε vs. the corresponding wavelengths is
on an improved synthesis of reagent 2 employing trichloroiso- presented in Figure 2. Parent reagent 2 features an absorbance
cyanuric acid (TCICA) as cheap, stoichiometric oxidant, as maximum at λmax = 237 nm (ε = 4.3 × 103 M−1 cm−1) and two
recently reported from our laboratory [11]. Although com-
pound 5 was readily converted to 6 in hot MeCN with TCICA
in 90% yield an alternative protocol was found advantageous in
terms of overall reaction time, solvent consumption and time
required to dry the product. Consequently, treatment of 5 in a
mixture of DCM/t-BuOH 9:1 at 0 °C with t-BuOCl under exclu-
sion of light led to the immediate precipitation of 1-chloro-5-
nitro-3H-1λ3,2,benziodaoxol-3-one (6) in 81–93% yield (89%
average yield of five experiments). This step was routinely
carried out on a 3 gram scale in less than 5 minutes. Finally,
introduction of the trifluoromethyl group was achieved by stir-
ring 6 and anhydrous KF in hot MeCN, followed by treatment
with Ruppert–Prakash reagent (TMSCF3) at 0 °C yielding
1
0–35% of the desired product 3 as a pale yellow solid. Unfor-
tunately, a slightly more economic route employing KOAc
instead of KF was not feasible. Additionally, the purity of the
product obtained was dependent on the grade of KF that was
used. Therefore, spray-dried KF (99%) was further flame-dried
under high vacuum immediately before use.
Figure 2: UV–vis spectra of reagents 2 (green) and 3 (blue) in
DMSO/H2O 1:1.
Scheme 1: Synthesis of 5-nitro-1-(trifluoromethyl)-3H-1λ3,2-benziodaoxol-3-one (3).
2

Beilstein J. Org. Chem. 2014, 10, 1–6.
resolved sidebands at 272 nm (ε = 1.1 × 103 M−1 cm−1) and Both structures are stabilized by intermolecular hydrogen bonds
2
80 nm (ε = 1.0 × 103 M−1 cm−1), respectively. In contrast, the (NO2···H–Ar) [16] and by short intermolecular I–O2 contacts,
nitrated product 3 exhibits a maximum λmax = 236 nm as typical for this class of compounds (see Supporting Informa-
ε = 8.9 × 103 M−1 cm−1) and features a broad sideband ranging tion File 1). These structural findings also account for the slow
(
from 260 nm (ε = 8.0 × 103 M−1 cm−1) up to 350 nm, hence dissolution kinetics observed.
accounting for its pale yellow coloration. Generally, these
observations are in line with data reported on the analogous pair Generally, the introduction of the NO2 functionality is asso-
of iodobenzene and 1-iodo-4-nitrobenzene [13,14]. However, ciated with a shortening of the I–X bond (Δ = 0.0201 Å for
whereas the broad sideband in 1-iodo-4-nitrobenzene is usually X = Cl; Δ = 0.019 Å for X = CF3) and concomitant elongation
observed around 300 nm, a strong hypsochromic shift of of the I–O1 bond (Δ = 0.057 Å for X = Cl; Δ = 0.023 Å for X =
roughly 40 nm is observed for 3. This can be explained by the CF3). Furthermore, the O1–I–X angles are comparable for all
installation of the hypervalent bond and the presence of the four structures being 1 to 1.5° smaller for the nitro-substituted
strongly electron-withdrawing CF3 group, as also indicated by compounds. When comparing the same unit I–X within the
the carbon chemical shift analysis.
analogous compounds 2 and 3, a shorter bond would intuitively
correspond to a stronger bond. Therefore, based on simple
Compounds 6 and 3 were also characterized by means of X-ray geometrical parameters, reagent 3 was expected to be slightly
crystallography. Due to the very poor solubility of 6 in most less reactive compared to 2, meaning that reductive cleavage of
common solvents, suitable single crystals for solid-state the I–CF3 bond should display a higher activation barrier.
analysis were grown from a saturated solution in hot MeCN that
was slowly cooled to −20 °C. Similarly, suitable crystals of Recently, differential scanning calorimetry (DSC) measure-
reagent 3 were obtained from a boiling solution in bromoben- ments for reagent 2 and its precursors were published indi-
zene. Whereas 6 crystallizes in plain sheets, 3 forms zig-zag- cating a decomposition energy of 159 kJ/mol (502 J/g) for 2
shaped layers. ORTEP representations of compounds 6 and 3 [17]. Furthermore, a Koenen test suggested explosiveness.
are given in Figure 3. Selected bond lengths and angles of 6, 3 Consequently in light of these results, DSC measurements of 2
and the parent compounds 7, 2 are summarized in Table 1.
and 3 were carried out in order to assess the thermal stability of
the new derivative. These measurements were performed in
punctured Al2O3 pans with a heating rate of 10 K/min. For
reagent 2, a decomposition energy of 138 kJ/mol (438 J/g,
slightly lower than reported) with a maximum heat flow of
3
.7 kW/mol (11.9 W/g) was found. Intriguingly, compound 3
features a significantly lower decomposition energy of only
9 kJ/mol (273 J/g) and a maximum heat flow of 2.2 kW/mol
6.2 W/g), therefore indicating that it should be safer to handle
9
(
than parent compound 2 (Figure 4). The higher temperature of
decomposition of 25 °C for 3, as measured by the difference of
the corresponding temperatures at peak heat flow, is believed to
directly result from the additional stabilization of 3 by intermol-
ecular H-bonding (vide supra).
Figure 3: ORTEP representation of X-ray structures 6 and 3. Thermal
ellipsoids set to 30% probability. Hydrogen atoms omitted for clarity.
Additionally, in order to test the initial hypothesis that nitration
may lead to a more positive reduction potential, cyclic voltam-
metry (CV) measurements were carried out. These experiments
were conducted under an atmosphere of nitrogen in anhydrous
MeCN with Bu4NBF4 (0.1 M) as supporting electrolyte. A plat-
inum working electrode and a Ag/Ag+ (0.1 M AgBF4 in MeCN)
reference electrode were employed with a scan rate of 0.1 V/s.
Analysis of the resulting cyclic voltammogram of a 1 mM solu-
tion of 3 revealed three quasi-reversible processes with cathodic
peak potentials (Epc) of −1.2, −1.7 and −1.8 V, respectively
(Figure 5). Subsequently, through addition of methyl 2-iodo-5-
nitrobenzoate (1 mM), the first wave could be unambiguously
Table 1: Selected bond lengths and angles for compounds 2, 3, 6, 7.
Entry
I–X [Å]
I–O1 [Å]
O1–I–X [°]
X = Cl, CF3
X = Cl, CF3
7
6
2
3
a
2.461(1)
2.4409(11)
2.219(4)
2.200(6)
2.091(3)
2.148(3)
2.283(2)
2.306(4)
171.96(8)
170.52(8)
170.49(12)
169.01(17)
b
aReference [15]. bReference [1].
3

Beilstein J. Org. Chem. 2014, 10, 1–6.
reported for reagent 2 (Epc = −1.1 V vs Ag/Ag+, scan rate
0
.2 V/s, MeCN) [8]. Therefore, nitration did not influence the
standard reduction potential significantly and 3 should behave
in a comparable manner as 2. These electrochemical findings
are in line with the expectations of reactivity based on the X-ray
structural considerations.
Finally, the decomposition kinetics of both reagents in a model
system were studied providing insights into their relative reac-
tivities. To this end, the reaction of 2 and 3 with p-toluenesul-
fonic acid monohydrate to 4 and 5, respectively, was monitored
in MeCN by means of 19F NMR spectroscopy (Figure 6). Limi-
tations arose from the poor solubility of 3 (1 mg in 0.25 mL
MeCN) and, in the case of 2, from line broadening resulting
after the addition of the substrate, hence necessitating recording
of 6 kinetic traces per reagent. Reagent 3 exhibits an exponen-
tial decay whereas the concentration of 2, on the same time
scale, decreases linearly. Therefore, rate data for 3 were
obtained by curve fitting according to an exponential rate law
and the method of initial rates was applied to reagent 2. Subse-
Figure 4: DSC traces obtained for 2 (green) and 3 (blue).
identified as the reduction of the hypervalent bond. Therefore, quently, the initial rates were transformed to the corresponding
the more negative waves most likely correspond to the reduc- first-order rate constants under the assumption that for both
tion of the nitro group and/or further reduction of iodine as
reported in CV studies carried out on bis(acetoxy)iodobenzene
[
18].
Figure 5: Cyclic voltammetry of 3 (1 mM, black) and of a mixture of 3
(
1 mM) and methyl 2-iodo-5-nitrobenzoate (1 mM, green) in anhy-
drous MeCN + 0.1 M Bu4NBF4, platinum electrode, scan rate =
.1 V/s.
0
Figure 6: Sample kinetic traces for the decomposition of 2 (green) and
(blue) to 4 and 5, respectively, upon reaction with p-TsOH in MeCN.
Solid lines represent the average decay (kdecomp) based on 6 indi-
vidual measurements and dashed lines are decays based on
kdecomp ± σ.
3
For the first wave Epc1 = −1.17 and Epa1 = −0.97 were deter-
mined, indicating a standard reduction potential E01/2 = −1.1 V
vs the Ag/Ag+ pair. These values are comparable to the ones
4

Beilstein J. Org. Chem. 2014, 10, 1–6.
reagents the same mechanism of decay is operational. Indeed, 1 equiv) in DCM/t-BuOH 9:1 (12 mL) at 0 °C t-BuOCl
the data imply that 3 (kdecomp = 4.43 ± 2.15 × 10−3 s−1) is less (0.42 mL, 3.7 mmol, 1.1 equiv) was added in one portion,
stable under the reaction conditions chosen and decays almost taking care to shield this reagent from light. Stirring was
one order of magnitude faster than reagent 2 (kdecomp = continued for 20 min, the suspension was filtered and the
5
.46 ± 2.23 × 10−4 s−1).
filtrate washed with DCM to afford pure product (0.95 g, 86%)
as a yellow solid. X-ray quality crystals were obtained by
recrystallization from boiling MeCN. Mp 225 °C (dec); 1H
Conclusion
In conclusion, the new derivative 3 having an additional nitro NMR (400 MHz, DMSO-d6) δ 8.38 (d, J = 2.1 Hz, 1H), 8.26 (d,
functionality as compared to reagent 2 was successfully J = 8.6 Hz, 1H), 7.99 (dd, J = 8.5, 2.2 Hz, 1H); 13C NMR (101
prepared and characterized. The resulting electronic modifica- MHz, DMSO-d6) δ 166.3 (CO2), 147.3 (CNO2), 142.5 (CH
tion was studied by analyzing carbon chemical shifts, UV–vis ortho to CI), 137.9 (CCO2), 126.2 (CH para to CCO2), 124.2
spectra and cyclic voltammetry. Whereas the first two methods (CH ortho to CCO2), 104.1 (CI); HRMS–EI (m/z): [M]+ calcd
clearly indicated the installation of an electron-poor center adja- for C7H3ClINO4, 326.8795; found, 326.8775; Anal. calcd for
cent to iodine, cyclic voltammetry delivered a standard reduc- C7H3ClINO4: C, 25.68; H, 0.92; N, 4.28; found: C, 25.42; H,
tion potential comparable to the one of 2. Notably, differential 1.02; N, 4.28.
scanning calorimetric measurements indicate that the new
reagent is thermally more stable. In presence of a proton source, 5-Nitro-1-(trifluoromethyl)-3H-1λ3,2-benziodaoxol-3-one
however, increased lability of 3 was noted such that it decayed (3): Spray-dried KF (0.61 g, 10.3 mmol, 3 equiv) was flame-
almost one order of magnitude faster than 2 under similar dried under high vacuum, then suspended with 1-chloro-5-nitro-
conditions. Therefore, the incorporation of an electron-with- 3H-1λ3,2-benziodaoxol-3-one (6, 1 g, 3.1 mmol, 1 equiv) in dry
drawing group into the reagent scaffold leads to an enhanced MeCN (125 mL) under argon and stirred at 75 °C for 3.5 h,
thermal stability that is paralleled by increased lability of the leading to the precipitation of a white solid. The suspension was
I–CF3 bond under acidic conditions. However, the major draw- cooled to 0 °C, TMSCF3 (0.55 mL, 3.7 mmol, 1.2 equiv) was
back of the new modification is its low solubility, a problem added and stirring continued for 2 h. The reaction mixture was
that is currently being addressed in our group.
filtered through a Celite pad. Subsequently, the organic layer
was washed with brine (50% sat), dried over Na2SO4, and
concentrated under reduced pressure to turbidity. Filtration and
Experimental
2
-Iodo-5-nitrobenzoic acid (5): During this preparation up to washing with MeCN afforded the pure product (0.35 g, 32%) as
2
5 mmol I2 are formed. Therefore, the synthesis has to be a pale yellow solid. If impure product is obtained, recrystalliza-
conducted in a well-ventilated fume cupboard. Prepared tion form MeCN may be applied. X-ray quality crystals were
according to literature procedure [10]. 2-Iodobenzoic acid (4, obtained by recrystallization from boiling bromobenzene. Mp
1
0 g, 40.3 mmol, 1 equiv) was added to a mixture of HNO3 163 °C (dec); 1H NMR (300 MHz, DMSO-d6) δ 8.68 (d, J = 2.6
(
35 mL, 65%) and H2SO4 (85 mL, 95%). The solution was Hz, 1H), 8.64 (dd, J = 8.9, 2.7 Hz, 1H), 8.08 (d, J = 8.8 Hz,
heated to 135 °C and stirred for 1 h. Subsequently, the resulting 1H); 13C NMR (75 MHz, DMSO-d6) δ 164.2 (CO2), 149.9
brown slurry was poured onto ice and the grey precipitate was (CNO2), 134.8 (CCO2), 130.0 (q, JCF = 3.7 Hz, CH ortho to
filtered off and washed with copious amounts of water. Then, CI), 129.3 (CH para to CCO2), 125.7 (CH ortho to CCO2),
the filtrate was suspended in water (100 mL), heated to 100 °C 121.8 (CI), 106.6 (q, JCF = 383.0 Hz, CF3); 19F NMR (282
and treated with a solution of KI (8.5 g, 51.2 mmol, 1.3 equiv) MHz, DMSO-d6) δ −32.6; HRMS–EI (m/z): [M + H]+ calcd for
and H2SO4 (5 drops) in water (10 mL) over the course of 1 h. C8H3F3INNaO4, 383.8951; found, 383.8961; Anal calcd for
Finally, the brown suspension was filtered hot and washed with C8H3F3INO4: C, 26.62; H, 0.84; N, 3.88; found: C, 26.67; H,
water to afford the pure product (5 g, 42%) as a brown solid. If 0.93; N, 4.08.
impure product is obtained, boiling with water followed by hot
filtration can be applied. Mp 195–198 °C; 1H NMR (300 MHz,
Supporting Information
DMSO-d6) δ 8.40 (d, J = 2.7 Hz, 1H), 8.29 (d, J = 8.6 Hz, 1H),
8
.01 (dd, J = 8.6, 2.7 Hz, 1H), 4.00 (s, 1H); 13C NMR (75 MHz,
Supporting Information File 1
DMSO-d6) δ 166.5 (CO2), 147.3 (CNO2), 142.4 (CH ortho to
CI), 138.2 (CCO2), 126.1 (CH para to CCO2), 124.1 (CH ortho
to CCO2), 103.7 (CI).
Experimental details, crystallographic data (selected
intermolecular bond lengths) as well as a description of the
1
9F NMR monitoring experiments and data analysis.
http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-1-S1.pdf]
[
1
-Chloro-5-nitro-3H-1λ3,2-benziodaoxol-3-one (6): To a
solution of 2-iodo-5-nitrobenzoic acid (5, 1 g, 3.4 mmol,
5

Beilstein J. Org. Chem. 2014, 10, 1–6.
Acknowledgements
This work was supported by ETH Zürich. C. Mensing is grate-
License and Terms
fully acknowledged for recording the DSC traces.
This is an Open Access article under the terms of the
Creative Commons Attribution License
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