Halogen bonding matters: Visible light-induced photoredox catalyst-free aryl radical formation and its applications

Article abstract of DOI:10.1039/d0cp00946f

Photo-involving aryl halide activation plays a pivotal role in organic synthesis and materials science. Revealing the mechanism and understanding the photophysical and photochemical processes in the activation is of great importance. Here, we found that a

Full text of DOI:10.1039/d0cp00946f

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H.-P. Loock et al.
Determination of the thermal, oxidative and photochemical
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ARTICLE
Halogen bonding matters: visible light-induced photoredox
catalyst-free aryl radical formation and its applications
Received 00th January 20xx,
Accepted 00th January 20xx
Rong Miao,a Dan Wang,a Jianliang Xiao,a Jiani Ma,b Dong Xue,*a Fengyi Liu*a and Yu Fang*a
DOI: 10.1039/x0xx00000x
Photo-involving aryl halide activation plays a pivotal role in organic synthesis and materials science. Revealing the
mechanism and understanding the photophysical and photochemical processes in the activation is of great importance.
Here, we found that aryl halide could be directly activated to form aryl radical via halogen bonding under visible light
irradiation without using photocatalysts or high power light. Interaction between aryl halide and Lewis base (Et3N) as well
as triplet state formation play crucial roles. Halogen bonding between aryl halide and Et3N facilitates intersystem crossing
(ISC) and leads to abundance of triplet aryl halide/Et3N complex, where the carbon-halogen bond is much lengthened and
prone to fracture. Therefore, visible light-driven photoredox catalyst-free C-C coupling reaction and radical-initiated
polymerization were achieved. Avoid of using catalyst not only brings convenience for low-cost operation, but also facilitates
further purification. Meanwhile, wide scope of aryl halides tolerance provides opportunities for chemical synthesis and
polymerization.
has also been realized in visible light-induced aryl halide
13, 14
activation with the help of KOtBu (in DMSO)].
So far, an
1. Introduction
acknowledged explanation for BHAS reactions is that the dimsyl
anion from DMSO is the key ingredient, which further reacts
with aryl halides via single electron transfer (SET) to produce
aryl halide radical anion (top right in Scheme 1). 15 Though there
is much difference between these methods in reaction
condition, Lewis base is indispensable and formation of aryl
halide radical anion is a crucial step in all the cases (topside of
Scheme 1).
Studying the underlying photochemical process and developing
feasible method for visible-light-driven synthesis is one of the
noble missions of chemistry. Accurate understanding on a
reaction is not only helpful for new system design but also
beneficial for process optimization in related research fields.
Aryl halides, one of the most frequently used reactants in
transition metal catalyzed C-C bond-forming reactions, are also
favorable candidates for light-driven synthesis.1-4 With rapid
development in photochemistry, remarkable progress has been
achieved in aryl halide-based photosynthesis.5-7 In the early
years, aryl halides are usually activated under high power UV
light via a hemolytic process or radical anion-mediated process,
where the aryl halide changes into an aryl halide radical anion
when it gets an electron from a Lewis base.8, 9 In recent years,
visible light-driven aryl halide activation has been reported,
where photoredox catalyst is indispensible. 10, 11 It is proposed
that electron transfer happens between the excited photoredox
catalyst and a base; and the formed ionic complex (usually
photoredox catalyst radical anion) then serves as reductive
species to react with aryl halide to produce aryl halide radical
anion (top left in Scheme 1).10-12 Aroused from heat-induced
base-promoted homolytic aromatic substitution (BHAS), BHAS
Scheme 1. Comparison of different mechanisms for aryl halide activation. PC:
photoredox catalyst; Ar-X: aryl halide; ET: electron transfer.
Intermolecular halogen bonding, which is often formed
between electron-deficient halogen substituents and halogen
bond acceptor atoms such as nitrogen, is an electrostatically
17
driven, noncovalent interaction.16, It is well recognized as
aKey Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education,
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an
710062, People’s Republic of China.
bCollege of Chemistry and Materials Science, Northwest University, Xi’an 710127,
People’s Republic of China.
important interactions in biological systems, supramolecular
chemistry, and molecular crystals.18-20 Crucially, halogen
bonding is also an effective strategy to control the spin state
and reactivity of some species, such as aryl halides and
†
Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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carbenes.21, 22 However, to the best of our knowledge, role of
halogen bonding in aryl halide activation remains unexplored.
Herein, we demonstrate that halogen bonding is of great
importance in aryl halide activation. Theoretical calculations reveal
that structure of the halogen bonding complex (aryl halide/Et3N)
changes much when it is excited into triplet state via intersystem
crossing. In triplet state, carbon-halogen bond in a halogen bonding
complex is prone to break. Thus, aryl halide can be directly activated
under visible light in the presence of a Lewis base (downside of
Scheme 1). Moreover, the activated aryl halides can be used in both
C-C coupling reaction and radical initiated polymerization under mild
condition, where only a proper amount of Et3N is needed and neither
photoredox catalyst nor strong base is required. It is expected that
the proposed mechanism as well as the presented reactions would
facilitate further studies in related areas.
3. Results and discussion
Referring to reported work, it is speculated that there would be
halogen bonding between aryl halide and Et3N.23,
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DOI: 10.1039/D0CP00946F
24
14N NMR
technique was used to study the interaction between aryl halide and
Et3N (Figure 1). 2-chlorobenzonitrile was chosen for 14N NMR study,
as the N atom in –CN group would be used as a reference for
chemical shift comparison. Compared with the 14N NMR spectrum of
individual 2-chlorobenzonitrileor Et3N, the peak around 1 ppm
belongs to the N atom from Et3N; the peak around 225 ppm belongs
to the N atom in the –CN of 2-chlorobenzonitrile.25 It is noted that
the N atom form Et3N shifted to lower field in the mixture (2-
chlorobenzonitrile and Et3N), while no obvious change was found for
the N atom in the –CN group. Based on the 14N NMR results, it is
reasonable to conclude that there must be interaction between the
halogen atom in the aryl halide and the N atom in Et3N, which is the
expected halogen bonding.
3.1 Theoretical calculations
2. Experimental and computation methods
Halogen bond between aryl halide was also confirmed by theoretical
calculations. To study the interaction between aryl halide and Et3N,
4-I-COMe and 4-Cl-COMe were used. Similar results were obtained
for both the aryl halides tested. When Et3N is introduced, the aryl
halide and the Lewis base were loosely combined as a complex (S0-
min) by a halogen bond formed between I/Cl and N atoms (left part
in Figure 2A).26-28
The reaction was done on a homemade photochemical reactor
(Figure S1) with 400 nm LED (Figure S2). In a typical experiment, a 5
mL glass vial was charged with acetonitrile (2 mL) and 4’-
iodoacetophenone (4-I-COMe, 0.1 mmol). The solution was degassed
via a syringe needle for 20 min, and Et3N (30 µL) was added during
the process. N-methylpyrrole (2.4 mmol, 24 eqv.) was added to the
deoxygenized mixture and the vial was sealed up and the reaction
mixture was irradiated through the bottom side of the vial using a
homemade reactor with LED light of 400 nm for 12 h. The reaction
was monitored by thin-layer chromatography (TLC) analysis. When
the reaction is completed, the reaction mixture was transferred into
a 25 mL round-bottom flask and then concentrated in vacuum at 30
ºC. Purification of the crude product was achieved by column
chromatography using petrol ether/ethyl acetate (v/v=10:1) on silica
gel column. The isolated compounds were characterized by high
resolution ESI as well as 1H and 13C NMR spectroscopies.
The geometry optimizations were done at the M062X-D3 level
of theory with a def2-SVPD basis set and SMD solvation model using
acetonitrile as solvent (ε=35.69). The time-dependent (TD) density
functional was used for the S1 state. The MS-CASPT2 singlet-point
calculations coupled with RASSI(SO) algorithm were carried out on
top of the DFT-optimized PECs in order to obtain corrected energies
as well as to evaluate the spin-orbit coupling effects.
Figure 2. Theoretical study on the halogen complex of aryl halide and Et3N.
(A) Structures of S0 and T1 minima of halogen bonding complexes (gray:
carbon, white: hydrogen, green: chloride, purple: iodine); 4-Cl-COMe/Et3N
(top) and 4-I-COMe/Et3N (down). (B) S1/T1/S0 potential energy curves
optimized at (TD)M062X-D3/def2-SVPD(ECP) level with MS-CASPT2/ANO-
RCC-VDZP energy corrections. The optimized minima on the MEPs are
emphasized by purple circles.
Figure 1. 14N NMR spectra of 2-chlorobenzonitrile (blue trace), Et3N (black
trace) and the mixture (red trace). Solvent: DMSO-d6. Concentration of 2-
chlorobenzonitrile was 0.1 M and 2 equiv. Et3N was added.
2 | J. Name., 2012, 00, 1-3
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The excited complexes are crucial for a light-induced synthesis,
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DOI: 10.1039/D0CP00946F
as they are key intermediates in the photochemical reaction.
Therefore, structure of the halogen bonding complex (aryl
halide/Et3N) in excited state was optimized and studied.
Multireference ab initio calculations were conducted and the results
showed that the excited aryl halide existed in both singlet (S1) and
triplet state (T1). Compared with the ground state, structure of the
complex changed little when they were in S1, however, structure of
the complex changed much when they were in triplet state. The most
obvious difference is the extremely lengthened halogen bond
compared with the singlet state (Figure 2A), indicating a much
weaker and more fragile carbon-halogen bond. Systematic
calculation in Figure S3 revealed that during the photo-cleavage of C-
I bonds in 4-I-COMe, the repulsive singlet and triplet πσ* excited
states roughly decrease from the Franck-Condon region to longer C-
I distance. With the elongation of C-I bond, the downhill πσ* states
and uphill ground state become close in energy, which finally leads
to a nearly degeneracy situation at the dissociative structure (i.e., dC-I
> 4.0 Å).
Figure 3. Luminescence behavior of 4-I-COMe in the absence/presence of
Et3N. (A) Fluorescence spectra of 4-I-COMe in the absence (black) and in the
presence (red) of Et3N (λex= 360 nm). The dashed blue line is the excitation
spectrum. (B) Phosphorescence spectra of 4-I-COMe in the absence (black)
and in the presence (red) of Et3N (λex= 360 nm). The inset picture shows the
normalized phosphorescence spectra of B. Concentration of 4-I-COMe was
0.1 M and 2 equiv. Et3N was added. The fluorescence spectrum was recorded
under room temperature (25 ℃), but the phosphorescence measurements
were done by freezing the 4-I-COMe solution under liquid nitrogen for 5 min.
Both fluorescence (Figure 3A) and phosphorescence (Figure 3B
and Figure S8) of 4-I-COMe were enhanced after the addition of Et3N,
The interaction between aryl halide and Et3N effectively tunes which is in accordance with the results from solid state studies.34
the electronic structure of 4-I-COMe and, as expected, alters the Meanwhile, maximum emission wavelength of phosphorescence
excited state PECs of C-I cleavage. Figure 2B illustrates the optimized showed an obvious blue shift (inset picture of Figure 3B) and
minima on S0 and T1 states as well as the PECs obtained at the MS- phosphorescence lifetime of 4-I-COMe increased after Et3N addition
CASPT2//TD-M062X-D3 level of theory. On the T1 PEC, a shallow (Figure S8). These results confirmed the interaction, probably
minimum (T1-min) emerges at CI distance of 3.835 Å.29-31 The halogen bonding, between 4-I-COMe and Et3N. It is worth
considerably elongated CI distance and shortened IN distance in mentioning that phosphorescence of 4-I-COMe increased nearly 10
T1-min, with respect to those in S0-min, suggest that the T1-min is times after interacting with Et3N (Figure 3B), suggesting the halogen
composed with two different fragments (Et3NIaryl group) from S0- bonding complex (4-I-COMe/Et3N) had higher ISC efficiency than 4-I-
min. Natural bonding orbitals (NBO) charge distribution analyses COMe.
further prove a homolytic pattern of C-I bond cleavage. Thus, in the
In addition, nanosecond transient absorption (Ns-TA) technique
presence of Et3N, the photo-induced cleavage of 4-I-COMe generates was used to study triplet absorption of 4-I-COMe without or with
an aryl radical that is protected by a loosely bonded Et3NI moiety, Et3N (Figure S9). It was observed that spectra of 4-I-COMe/Et3N
supposing the intersystem crossing (ISC) between S1 and T1 is complex showed higher OD value than that of 4-I-COMe under the
efficient. Spin-orbit treatments at the MS-CASPT2/RASSI-SO level same concentration, inferring more triplet species ratio in the
showed that the S1 and T1 state indeed have large magnitude of spin- complex. Both Ns-TA spectra of the complex and 4-I-COMe showed
orbit interaction matrix elements, and the spin orbit coupling (SOC) negative absorption around 469 nm, which should be aroused from
S1 state has significant contributions from spin-free T1 in the range of fluorescence emission of the system (c.f. Figure 3A). Existence of aryl
2.6 ~ 3.0 Å.32, 33 Those in both suggest an efficient S1→T1 ISC process. halide in triplet state was also verified by using 4-I-COMe as triplet
More detailed discussion and supplementary data (Figure S3-7, Table photosensitizer. A commonly used singlet oxygen (1O2) scavenger,
S1) on theoretical calculation are provided in the supporting 1,5-dihydroxyl napthalene (DHN), was chosen. Upon irradiated
information.
under visible light (400 nm LED), characteristic absorption of DHN at
Overall, theoretical calculations reveal that carbon-halogen 301 nm decreased and a new peak at 427 nm belonging to juglone
bond probably goes cleavage through triplet state, which is in appeared, which is due to oxidation of DHN by 1O2 (Figure S10a).35, 36
accordance with the well-known heavy atom effect. Meanwhile, Addition of 4-I-COMe in DHN solution could accelerate the oxidation
theoretical calculations suggest that Lewis base (Et3N) not only plays process (Figure S10b), inferring that 4-I-COMe can be used as triplet
a role in photo-activation of C-I bond, but also serves as a protection sensitizer. In the control experiment, no juglone was found under
group to stabilize the metastable aryl radical, which indicates the dark, suggesting light is indispensable. All these photophysical study
crucial role of halogen bonding in the activation process.
results provided solid proof not only for the interaction between 4-I-
COMe and Et3N but also for the suggestion that halogen bonding
would facilitate ISC process. Combined with theoretical calculation,
it is rational to speculate that the aryl radical would be obtained
directly via halogen bonding-facilitated triplet state rupture under
light irradiation.
3.2 Photophysical study
To verify the validity of the results from theoretical calculations,
experimental studies were carried out. 4-I-COMe was chosen as a
model molecule, as it showed superior photophysical properties,
which would facilitate further mechanism study. First, fluorescence
and phosphorescence behaviors of the 4-I-COMe before and after
addition of Et3N were investigated (Figure 3). To ensure reliable
intensity comparison, all the parameters were kept the same during
the whole measurement.
3.3 Application of the activated aryl halide radical
To testify the speculation of the halogen bonding-facilitated direct
aryl radical formation mechanism, 2,2,6,6-tetramethyl piperidinoxyl
(TEMPO) was used as trapping agent for aryl radical. As expected,
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TEMPO-trapped aryl radical was found when TEMPO was added into
the mixture of 4-I-COMe and Et3N under visible light irradiation
(Figure S11). Meanwhile, abundance of dehalogenation product
(acetophenone) was obtained after the mixture of 4-I-COMe and
Et3N was irradiated for 12 h (Figure S12), while little acetophenone
was found in the reaction without Et3N. These results certified the
formation of aryl radical and provide solid proof for the proposed
halogen bonding activation process deduced from theoretical
calculation and experimental results. What’s more, success in aryl
radical acquisition paved way for further application, as aryl radical
are highly reactive species and they are important intermediates in
organic synthesis and materials science.
View Article Online
DOI: 10.1039/D0CP00946F
Scheme 2. The model reaction in this work.
Table 1. Photoredox catalyst -free C-C coupling reaction under different light.
Entry
Light
365 nm
400 nm
465 nm
520 nm
White light
Yield (%)
1
2
3
4
5
80
99
54
0
19
Reaction time: 12 h; 2equiv. Et3N; and the yields were determined by 1H
NMR analysis of crude reaction mixture with standard after the reaction.
3.3.1 Visible light-driven C-C coupling reaction. Based on the
revealed mechanism, light-induced photoredox catalyst-free C-C
coupling reaction was explored by choosing 4-iodoacetophone (4-I-
COMe) and a well-known trapping agent, N-methylpyrrole (Scheme
2).12, 37, 38 Proper amount of Et3N was introduced and the reactions
were done under light of different wavelengths (LED, 365 nm, 400
nm, 465 nm, 520 nm and white light). As is shown in Table 1, C-C
coupling products were obtained for most tests. It is worth to note
that excellent yields (99%) were obtained for 400 nm light
irradiation.
Except for the green light (LED, wavelength of 520 nm), all the
lights used with different wavelengths are all capable to induce
reaction, though the coupling yields are different (entry 1-5, Table 1).
After 12 hour’s reaction, the UV light (LED, 365 nm) irradiation gave
a yield of 84%, the purple light (LED, 400 nm) gave a yield of 99%, the
blue light (LED, 465 nm) gave a yield of 54% and the white light led
to a yield of 19%, which demonstrates that purple light (Table 1)
irradiation is more efficient than others. Therefore, 400 nm LED light
was chosen as a preferable light source in further study.
Control experiment was done without light and no product was
found (entry 6, Table 1), which reveals that light irradiation is
necessary for the reaction. Although similar reactions have been
done by Kӧnig et al., photoredox catalysts (organic fluorophores)
must be used in their systems. However, benefiting from the re-
understanding of the photophysical and photochemical processes,
no photoredox catalyst is needed in the as-reported work.To further
test the reliability of the proposed mechanism, control experiments
were done and different reactions were conducted under varied
conditions. First, effect of base was investigated and no product was
found without base, indicating crucial role of the interaction
between aryl halide and base (entry 6, Table S2). In addition, amount
of base also matters, sufficient base is needed but too much base is
not advisable (entry 5, 7, 8, Table S2). These results indicate that base
is indispensable for the reaction indicating the crucial role of halogen
bonding in the reaction.
Figure 4. Substrate scope for photoredox catalyst-free C-C coupling reaction. Reaction conditions are the same as in Table 1, entry 2 except for N-phenylpyrrole
derivatives, where 4 equiv. of N-phenylpyrrole was used.
4 | J. Name., 2012, 00, 1-3
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ARTICLE
Second, different solvents were chosen and it turned out that
polar solvents, such as MeCN and DMF, are more favorable for the
reaction (Table S3), which is similar to some reported work.12 Third,
amount of N-methylpyrrole was also varied and it showed that
coupling yield increased when concentration of N-methylpyrrole
increased (entry 1-5, Table S4). The coupling yield reached to the
highest when 24 equivalent of N-methyl pyrrole was employed
(entry 4, Table S4), which is similar to the reported work where
excessive N-methylpyrrole was used.38, 39 In addition, it was found
that deoxygenization is helpful for the reaction and the yield was
lower when the reaction was done with air (closed system without
deoxygenization), which infers that there must be oxygen-sensitive
intermediates in the reaction system. All these control experiments
show that light and base are indispensable in the reaction, which
implies the importance of halogen bonding and ISC and further
confirms the proposed mechanism. Subsequently, the scope of the
reaction was examined by choosing different aryl halides and
pyrroles (Figure 4). Not only aryl iodides, but also aryl bromides and
chlorides are allowed with excellent yields. It was found that the
method is more suitable for aryl halides with electron withdrawing
groups, such as COOMe, COMe, CHO and CN. Corresponding
arylation products were obtained with good to excellent yield when
the three pyrrole derivatives (N-H, N-Me, N-Ph) were used,
suggesting good tolerance of the reaction.
Scheme 3. Proposed reaction mechanism for catalyst-free visible-light-driven
reaction.
Clearly, all the results from the reaction study are in accordance
with the presented mechanism. Therefore, a plausible mechanism is
illustrated in Scheme 3 to give a clear image of the reaction process.
Conclusions
Though deep understanding on a reaction mechanism is a great
challenge, especially for photo-induced reactions, we would benefit
a lot when we have a clear image of the reaction process. We tried
to involve halogen bonding in understanding the photophysical and
photochemical processes in aryl halide activation. Both experimental
studies and theoretical calculations reveal that halogen bonding play
3.3.2 Visible light-driven polymerization. According to the
interesting findings, attempt was made to use the reported system
in light-induced polymerization, which was considered as revolution
in polymer science.40, 41 Different from the reported work for light-
induced polymerization,42, 43 where only limited initiators (such as 2-
cyanopropyl iodide) were used, we found that abundance of aryl
halides with electron withdrawing groups could serve as initiators in
the presence of Et3N. Meanwhile, it was noticed that molecular
weight of the obtained polymers goes in the order of iodo- < bromo-
< chloro- when aryl halides with the same EWG was used as initiators,
which indicates that molecular weight of the polymers can be easily
tuned by halogen substitution (Table 2).
a
crucial role in the process. Different from the reported
mechanisms, where aryl halide radical anion is the key intermediate,
we found that the carbon-halogen bond in a halogen bonding
complex can be directly decomposed under visible light irradiation.
The halogen bonding complex can be easily excited to triplet state
via intersystem crossing, where the carbon-halogen bond is highly
activated for rupture. Based on these findings, we have successfully
demonstrated photoredox catalyst-free visible-light-driven aryl
halide activation for C-C coupling reaction and radical initiated
polymerization. Avoid of using catalyst not only brings convenience
for low-cost operation, but also facilitates further purification and
brings in excellent coupling yields. Meanwhile, wide scope of aryl
halide tolerance provides plenty of opportunities for radical initiated
polymerization, which would contribute to functionalization of
polymers and thus facilitate further applications. We anticipate that
the revealed photophysical and photochemical process for aryl
halide activation as well as the reactions will provide new strategies
and opportunities in organic chemistry, materials science and
photochemistry.
Table 2. Photoredox catalyst-free light-induced polymerization.
Aryl halide Monomer Con (%)
T (h)
Mn
PDI
4-I-COMe
4-I-CN
4-I-COMe
4-Br-COMe BuAC
4-Cl-COMe BuAC
2-I-CN
MMA
MMA
BuAC
85
82
89
78
72
94
3
3
2
2
2
2
76702 2.32
47954 1.84
26314 1.75
47803 1.92
91574 1.80
56193 2.01
BuAC
Aryl halide: 0.1 mmol (1 eqv.); monomer: 100 eqv.; Et3N: 2 eqv; MMA: Methyl
methacrylate; BuAC: Butyl acrylate. Mn and PDI was determined by gel
permeation chromatography (GPC) with a multiangle laser light-scattering
(MALLS) detector.
Conflicts of interest
There are no conflicts to declare.
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30 Y. Zhao and D. Truhlar, Theor Chem Acc, 2007, 120, 215.
Acknowledgements
31 S. Grimme, Wiley I nterdiscip. Rev. Comput. MolV.iSewciA.,rt2ic0le1O1n,lin1e,
DOI: 10.1039/D0CP00946F
211.
We acknowledge funding from the Natural Science Foundation
of China (21603139, 21527802, 21673133, 21473107), the 111
project (B14041), the China Postdoctoral Science Foundation
(1202040102), and the Program for Changjiang Scholars and
Innovative Research Team in University (IRT-14R33). The
authors also thank Gang Li and Liu Yang et al. for duplication
experiments to verify the reaction yields.
32 P. Malmqvist, B. Roos and B. Schimmelpfennig, Chem Phys
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