Photochemical Dehalogenation of Aryl Halides: Importance of Halogen Bonding

Article abstract of DOI:10.1021/acs.jpca.9b06716

Upon UVA irradiation, aryl halides can undergo dehalogenation in the presence of bases and methanol as a hydrogen donor. This catalyst-free photochemical dehalogenation is furnished through a facile radical chain reaction under mild conditions. The chain reaction follows UVA irradiation of the reaction mixture in a transition-metal-free environment. Mechanistic studies support a chain mechanism in which initiation involves absorption by a methoxide-bromoarene complex facilitated by halogen-bonding interactions. The methoxide-bromine interaction leads to a weakened Br-C bond that is prone to facile cleavage during the initiation and propagation steps.

Full text of DOI:10.1021/acs.jpca.9b06716

Article
pubs.acs.org/JPCA
Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX
Photochemical Dehalogenation of Aryl Halides: Importance of
Halogen Bonding
Ayda Elhage, Paolo Costa, Amrah Nasim, Anabel E. Lanterna,* and Juan C. Scaiano*
Department of Chemistry and Biomolecular Science, Centre for Advanced Materials Research (CAMaR), University of Ottawa, 10
Marie Curie, Ottawa, ON K1N 6N5, Canada
S
* Supporting Information
ABSTRACT: Upon UVA irradiation, aryl halides can undergo dehalogenation in the
presence of bases and methanol as a hydrogen donor. This catalyst-free photochemical
dehalogenation is furnished through a facile radical chain reaction under mild conditions.
The chain reaction follows UVA irradiation of the reaction mixture in a transition-metal-
free environment. Mechanistic studies support a chain mechanism in which initiation
involves absorption by a methoxide−bromoarene complex facilitated by halogen-bonding
interactions. The methoxide−bromine interaction leads to a weakened Br−C bond that is
prone to facile cleavage during the initiation and propagation steps.
Scheme 1. Photoinduced Ar−X Bond Dissociation: Solvent-
Mediated Photoinitiation19 and Base-Mediated
Photoinitiation
INTRODUCTION
■
The transformation of haloarenes into the corresponding
arenes has great relevance to organic synthesis, industrial
processes, and environmental remediation.1 Conventional
methods for dehalogenation of aryl halides usually involve
high H2 pressures, reduction by tin or silicon hydrides,2 or
catalytic systems including transition metals like Pd,3−7 Rh,8
Ru,9,10 Mo,11 Co,12 and Ni.5,13 More recent examples study the
use of light to catalyze the reaction in the presence of organic14
or inorganic15 photocatalysts. Although some recent examples
suggest concerted mechanisms through hydrogen-bonding
formation,16 the majority of the photosensitized mechanisms
are based on the aryl radical formation.17−21 Many have
suggested that the presence of a strong base, usually KOtBu in
DMSO, is involved in the photoinduced Ar−X bond
dissociation.22 However, a 2017 landmark study by Rossi et
al.19 showed that light-induced electron transfer (eT)
reactionswhere none of the isolated reagents can absorb
excitation lightcan be photoinitiated by the formation of a
dimsyl anion (deprotonation of DMSO in a highly basic
medium, such as in the presence of KOtBu). They
demonstrated that this anion absorbs light into the visible
region and can initiate the eT process with aryl and alkyl
halides via the photoejection mechanism for the initial proton-
electron-transfer (PET) path. As they state, “dimsyl anion or
its derivatives, rather than the −OtBu or −H anion, may be
responsible for starting the arylation reactions by photo-
stimulated base-promoted homolytic aromatic substitution
reaction (BHAS)”. Here, we show that aryl halides can also
be photochemically dehalogenated (under mild UV excitation)
in the presence of a strong base, that is, methoxide, without
solvent assistance (Scheme 1). The key discovery in this
contribution results from the initiation step where the halogen-
bond formation activates the reagents toward long-wavelength
light absorption and the propagation step where halogen
bonding weakens the C−X bond promoting dissociation. The
product is furnished through hydrogen abstraction during an
electron-transfer-catalyzed chain process (proton-coupled-
electron transfer, PCET).23 This contrasts with the reaction
under thermal conditions in the dark where the same substrate
undergoes a nucleophilic aromatic substitution (SNAr). The
reaction is most favored with very strong bases and electron-
withdrawing group (EWG) substituents, such as in Scheme 2,
which exemplifies the type of system examined here,
illustrating the contrasting pathways observed thermally and
photochemically. These findings constitute a new path to
understand the photoinduced dehalogenation process where
the base−halogen interaction can play a crucial role.
METHODS
■
Experimental Section. All reagents and materials were
purchased from Sigma Aldrich, unless otherwise stated, and
used as received. Irgacure-2959 (I-2959) was a generous gift
Received: July 18, 2019
Revised: October 26, 2019
Published: October 29, 2019
© XXXX American Chemical Society
A
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The Journal of Physical Chemistry A
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solvent = methanol). For the B3LYP functional, D3 dispersion
corrections were included.29 All DFT computations were
carried out with the Gaussian 09 software package.30 All the
computed energies presented in this work include corrections
for the zero-point vibrational energy unless stated otherwise.
Scheme 2. Contrasting Thermal and Photochemical
Products in the Reaction of 4-Bromomethylbenzoate with
Methoxide in Methanol
from BASF. The formaldehyde test used was the MQuant
colorimetric test from Sigma.
Figure 1. Formaldehyde test.
UV−vis spectroscopy was carried out in an Agilent Cary 60
using quartz cuvettes. Photoproduct yields were measured with
a PerkinElmer Clarus gas chromatographer (DB-5 column, 30
m length, 0.320 mm diameter, 0.25 μm film) coupled to a
flame ionization detector (GC-FID) with helium carrier gas
and dodecane as an external standard or an Agilent gas
chromatographer coupled with mass spectrometer (GC−MS)
and t-butylbenzene as an external standard. Light-emitting
diodes from LED Engin were used for photochemical reactions
in the UV (centered at 368 nm) and visible regions (centered
at 465 nm).
Photochemical Dehalogenation. In a dry and clean quartz
tube, 0.2 mmol of 4-methyl-bromobenzoate and 0.6 mmol of
the base were suspended in 5 mL of HPLC methanol. The
reaction mixture was sonicated for 15 min and then purged
with argon for 10 min prior to irradiation with 368 nm LED
working at 0.33 W cm−2 under continuous stirring.
Quantification of the product was done by GC-FID using
dodecane as the external standard.
Formaldehyde Test. The formation of formaldehyde was
detected by using the MQuant test. Briefly, 0.5 mL of reaction
mixture after UV irradiation was mixed together with 4 mL of
MilliQ water and 10 drops of the dye. Then the strip was
immersed in the solution, and after 60 s, the color change was
evaluated. The test was repeated at least three times reaching
saturation in all cases. Control tests were performed using the
reaction mixture before UV exposure.
Quantum Yield. Detailed calculation of the quantum yield
can be found in the Supporting Information. The experimental
procedure was as follows: Ferrioxalate (6 mM) was prepared
by dissolving 60 mg of potassium ferrioxalate in 20 mL of 0.05
M H2SO4, and phenanthroline 0.1% buffered solution was
prepared by mixing 20 mg of phenanthroline, 4.5 g of
CH3COONa·3H2O in 20 mL of 0.5 M H2SO4. Both solutions
were kept in brown bottles. For determination of the quantum
yield, 4 mL of ferrioxalate solution was irradiated for 15 min in
a photoreactor equipped with 14 UVA lamps prior to
complexation with phenanthroline for determination of
moles of ferrous ions produced in the solution. The
photochemical dehalogenation was determined by mixing 0.2
mmol of 4-methyl-bromobenzoate and 3 equiv. of K2CO3 in 4
mL of HPLC methanol in a clean dry quartz tube and then
purging with argon prior to irradiation for 1 h in a
photoreactor equipped with 14 UVA lamps.
RESULTS AND DISCUSSION
■
For the system shown in Scheme 1, the thermal reaction is
completed within a few hours under reflux in methanol, while
under a modest light intensity (<1 W cm−2, T < 40 °C) the
photochemical product is obtained quantitatively in less than 3
h (Table 1). This result is particularly interesting given that the
Table 1. Reaction Yields under Standard Conditions and
with Changes Designed To Test the Reaction Mechanism
a
b
entry
changes to standard conditions
none
% yield
1
98
52
79
70
98
traces
0
0
0
0
0
2
1 h irradiation
3
4
5
6
1 h irradiation, traces of Br2
1 h irradiation, 0.71 W cm−2
NaOCH3 and no K2CO3
no base
7
dark
8
9
465 nm LED irradiation
air
10
11
12
13
3 equiv. of TEMPO
1 equiv. of nitrobenzene
no base, traces of Br2
no base, 2 equiv. of photoinitiatorc
0
0
a
For the use of other bases, see Tables S1 and S2. See Figure 4 for the
b
reaction dependence on light intensity. Conversions and yields
determined by GC-FID using dodecane as an external standard. I-
2959.
c
initial reagents (base or benzyl halides) do not absorb light in
the emission region of the light source. Naturally, light
absorption is a requirement in the system, and therefore, we
decided to investigate the reaction in more detail, starting by
the analysis of the absorption profile of the reaction mixture,
that is, methyl-4-bromobenzoate, various bases, and methanol
as the solvent. As can be seen in Figure 2, the reaction mixture
between methyl-4-bromobenzoate and a base shows a red-
shifted absorption with a new small band around 350 nm,
which is responsible for the absorption of the LED light
centered at 368 nm. All bases show similar behavior (see the
Supporting Information), which is interpreted as all being
simple methoxide sources. Moreover, the addition of base
Computational Details. The DFT calculations presented
in this work were performed using B3LYP and M06-2X density
functionals24−27 using the def2-TZVP basis set.28 Further, the
continuum solvation model was implemented, and methanol
was used as the solvent via keywords SCRF = (IEFPCM,
B
DOI: 10.1021/acs.jpca.9b06716
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Article
involves a σ-hole interaction releasing 4.4 kcal mol−1 at the
B3LYP-D3/Def2-TZVP level of theory. Similar interaction is
predicted for chloro and iodo benzoates with binding energies
of 1.4 and 10.5 kcal mol−1, respectively. It is also important to
notice that the C−X bond is slightly elongated after the
interaction with the methoxide is in place (Figure 3 and Figure
S3). Along with the halogen-bond complex, other three local
minima structures have been computed between methyl-4-
bromobenzoate and methoxide (Figure S6). Among them,
only structure b has higher binding energy than the halogen-
bonding complex at both levels of theory (M06-2X and
B3LYP-D3). Such a structure corresponds to the anionic
tetrahedral intermediate of the base-promoted hydrolysis of
carboxylic esters; however, this intermediate would not furnish
the photochemical product found. Indeed, the theoretical
absorption spectrum of the tetrahedral intermediate is blue-
shifted compared to that of 4-methyl-bromobenzoate (Figure
S7).
Figure 2. Absorption spectra for the reaction mixture in the absence
of base (black solid line) and in the presence of 3 equiv. of NaOCH3
(red solid line). Dashed lines represent the theoretical absorption
spectra computed at the M06-2X/Def2TZVP, SCRF = (IEFPCM,
solvent = CH3OH) level of theory; calculated spectra also suggest a
significant red-shift.
It is important to note that, if no UV irradiation is applied to
the system, the reagent can be easily recovered, that is, by
means of chromatography. Thus, the changes that occur upon
mixing and lead to absorption at 350 nm must reflect a
reversible equilibrium.
(MeONa) does not alter the absorption spectrum of methyl
benzoate (Figure S2), which further proves that the interaction
of the base and the halogen atom is responsible for the spectral
change of the reaction mixture. Intrigued by the formation of
this intermediate, we use time-dependent DFT (TD-DFT)
calculations using the M06-2X level of theory31−34 to simulate
the absorption spectra of each methyl-4-halobenzoate before
and after addition of CH3O− (Figure 2 and Figure S3). It is
commonly accepted that halogen atoms in organohalides
present anisotropic electron density when they are covalently
bonded to other atoms. Thus, the electrostatic potential
around the covalent bond is highly negative, whereas a
positively charge region is formed over the halogen atom,
known as the σ-hole.5,35,36 This particular structural feature has
attracted much attention5,37,38 because it can interact with lone
pairs (i.e., nucleophiles) forming halogen bonding. These
halogen bonds (XB) can influence conformation, binding, and
reactivity in the solution phase,37 including playing an
important role in catalysis.38 As shown in Figure 3, the
complex between methyl-4-bromobenzoate and methoxide
In order to determine the nature of the photoinduced
dehalogenation, we performed different experiments summar-
ized in Table 1 (Tables S1−S3). The photoinduced
dehalogenation reaction works using UVA irradiation under
an inert atmosphere, atmospheric pressure, room temperature,
and in the presence of bases (concentrations from 1 to 3 equiv.
show increased reaction yields (Table S2, entries ii−iv)), and
methanol as a hydrogen donor. The reaction does not proceed
in the absence of base, in the dark, or under visible-light
excitation. The solvent also plays an important role; the
reaction works well in methanol, likely due to the in situ
formation of methoxide. Ethanol and isopropanol also work,
but yields are significantly lower (see the Supporting
Information). The reaction can work in toluene only in the
presence of methanol and base, showing that more soluble
TMAOH can improve the reaction yields compared to K2CO3
(Table S1). As shown in Table 1, the reaction is sensitive to
the presence of radical quenchers, such as oxygen, TEMPO, or
nitrobenzene (entries 9−11), and is accelerated in the
presence of bromine (entry 3), a known radical initiator.39
However, the reaction does not proceed in the absence of base
and even using other radical initiators such as Br2 or Irgacure-
2959 (Scheme S1) as shown in entries 12−13. It appears as if
the only way to break the C−Br bond is after it is debilitated
by, for instance, the formation of halogen bonds, as shown in
Figure 3. Finally, the quantum yield of the reaction was
calculated to be 43, using a ferrioxalate actinometer.40,41 Based
on all these pieces of evidence, we can conclude that the
dehalogenation reaction proceeds as a radical chain reaction.
The chain reaction proposed in order to rationalize a
quantum yield of 43 is illustrated in Scheme 3 where the
initiation step is favored by the XB formation between the base
and the aryl halide. Further calculations suggest that the XB
weakens the C−X bond favoring the dissociation in 50.1, 40.2,
and 20.9 kcal mol−1 for Cl, Br, and I, respectively (Table S4).
Note that the propagation is expected to yield formaldehyde in
stoichiometric amounts relative to ArH. Indeed, using a
commercially available formaldehyde test, we could detect
amounts higher than 100 mg L−1 formaldehyde formed after
the reaction is completed (see Figure 1). As mentioned above,
Figure 3. Optimized structure of methyl-4-bromobenzoate with and
without methoxide at the M06-2X/Def2-TZVP, SCRF = (IEFPCM,
solvent = CH3OH) level of theory. The C−Br bond length is
highlighted in red. Values in brackets were computed at the B3LYP-
D3/Def2-TZVP, SCRF = (IEFPCM, solvent = CH3OH) level of
theory. ΔE represents the energy of the halogen-bonding interaction.
A plot of the electrostatic potential (isosurface value 0.02) is shown
on the right (blue: positive potential; red: negative potential).
C
DOI: 10.1021/acs.jpca.9b06716
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Scheme 3. Radical Chain Reaction Proposed for the
Dehalogenation of p-Substituted Halobenzenes
Table 2. Screening of the Reaction with Different 4-
a
Substituted Bromobenzenes
Hammett substituent
constants43
%
%
yield
b
b
entry
R
conversion
i
ii
iii
−CN
−C(O)OCH3
−OCH3
+0.660
+0.450
−0.268
97
>99
14
89
>98
c
13
a
Reaction conditions: 0.2 mmol of 4-substituted bromobenzene, 3
equiv. of MeONa, 5 mL of MeOH, 368 nm LED, Ar atmosphere.
b
Conversions and yields determined by GC−MS using t-butylben-
c
zene as an external standard. 24 h of reaction, nothing detected after
6 h of irradiation.
the reaction does not proceed in the absence of base, which
could also support the role of the base as a proton scavenger in
the propagation step. Notice that the products of the initiation
reaction are very minor and overlap with normal propagation
products. Additionally, the chain length is expected to be much
larger than 43, as the quantum yield of photoinitiation is not
expected to equal 1.42 Further confirmation of the chain
process is obtained from the negative curvature of the yield
versus irradiance plot (Figure 4), as in the case of chain
reactions the anticipated second-order termination causes
exceptionally mild, facile, and based on reagents that are
transition-metal-free and have excellent atom efficiency. It is
also an illustration of how bond weakening induced by σ-holes
and halogen bonding can play a major role in organic methods.
In summary, we have described how the photochemical
behavior of aryl halides in the presence of a nucleophile can
furnish the dehalogenation product quantitatively through a
radical chain reaction. The mechanism is surprisingly different
from the SNAr taking place under dark conditions. Exper-
imental and computational analyses of the absorption profile of
the reaction mixture, together with mechanistic studies, suggest
the reversible formation of a complex species, more likely
stabilized by XB formation, which weakens the C−X bond
facilitating the radical reaction initiation. Additionally, the base
acts as a proton scavenger that can also assist in the
propagation step, as the reaction can be regarded as a PCET
process.
reaction rates to depend upon I1/2
.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpca.9b06716.
■
S
More experimental and computational details (PDF)
Figure 4. Dependence of the reaction yield with light intensity (368
nm) after irradiation for 45 min under otherwise standard conditions
(see Table 1).
AUTHOR INFORMATION
Corresponding Authors
*E-mail: alantern@uottawa.ca (A.E.L.).
*E-mail: jscaiano@uottawa.ca (J.C.S.).
■
To further understand this base−halogen interaction, we
also evaluated replacing the fourth position substituent
(−C(O)OCH3, σ = +0.4543) with a more EWG (−CN, σ =
+0.6643) or electron-donor group, EDG, (−OCH3, σ =
−0.26843). As shown in Table 2, the presence of an
electron-deficient group favors the reaction. Figure S4 clearly
shows that the base−halogen interaction produces a greater
red-shift of the aryl bromide absorption spectrum in the case of
EWG; thus, it is not surprising that the reaction is more
favored in those cases. We have also calculated the NBO
charge distribution of these derivatives and found that the
halogen-bonding interaction is less favored for EDG (Figure
S5), in agreement with our empirical findings.
ORCID
Ayda Elhage: 0000-0001-5241-7375
Juan C. Scaiano: 0000-0002-4838-7123
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Natural Sciences and
Engineering Research Council of Canada, the Canada
Foundation for Innovation, the Canada Research Chairs
Program, Canada’s International Development Research
Centre (IDRC), and the German National Academy of
Sciences Leopoldina (Grant No. LPDS 2017-15 to P.C.).
The authors are grateful to Dr. Victoria Benavente Llorente for
her generous assistance in performing electrochemical
measurements.
CONCLUSIONS
■
While numerous strategies are known for reductive hydro-
dehalogenation,2 we believe that the one reported here is
D
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