Visible Light-Mediated Conversion of Alcohols to Bromides by a Benzothiadiazole-Containing Organic Photocatalyst

Article abstract of DOI:10.1002/adsc.201900416

The search for metal-free, stable and high effective photocatalysts with sufficient photo-redox potentials remains a key challenge for organic chemists. Here, we present a benzothiadiazole-containing molecular organic photocatalyst with redox potentials of ?1.30 V and +1.64 V vs. SCE. The singlet state lifetime is 13 ns. Direct conversion from aliphatic alcohols to bromides has been conducted with the designed organic photocatalyst under visible light irradiation with high efficiency and selectivity. The catalytic efficiency of the novel benzothiadiazole-based photocatalyst is comparable with the state-of-art metal and non-metal catalysts. Furthermore, advanced photophysical studies including time-resolved photoluminescence and transient absorption spectroscopy offer a powerful support for photo-induced electron transfer from photocatalyst to the reactive substrates. Lastly, no photo-bleaching effect is observed, demonstrating the high stability and recyclable of the designed organic photocatalyst. (Figure presented.).

Full text of DOI:10.1002/adsc.201900416

Title: Visible Light-mediated Conversion of Alcohols to Bromides by a
Benzothiadiazole-containing Organic Photocatalyst
Authors: Run Li, Dominik Gehrig, Charusheela Ramanan, Paul Blom,
Fabien Kohl, Manfred Wagner, Katharina Landfester, and Kai
Zhang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900416
Link to VoR: http://dx.doi.org/10.1002/adsc.201900416

10.1002/adsc.201900416
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Visible Light-mediated Conversion of Alcohols to Bromides by
a Benzothiadiazole-containing Organic Photocatalyst
Run Lia, b*, Dominik W. Gehrigb, Charusheela Ramananb, Paul W. M. Blomb, Fabien F.
Kohlb, Manfred Wagnerb, Katharina Landfesterb, and Kai A. I. Zhangb*
aCollege of Materials Science and Engineering, Hunan University, Changsha 410082, P. R. China
E-mail: lirun@hnu.edu.cn
bMax Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
E-mail: kai.zhang@mpip-mainz.mpg.de
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. The search for metal-free, stable and high Furthermore, advanced photophysical studies including
effective photocatalysts with sufficient photo-redox time-resolved photoluminescence and transient absorption
potentials remains a key challenge for organic chemists. spectroscopy offer a powerful support for photo-induced
Here, we present a benzothiadiazole-containing molecular electron transfer from photocatalyst to the reactive
organic photocatalyst with redox potentials of -1.30 V and substrates. Lastly, no photo-bleaching effect is observed,
+1.64 V vs. SCE. The singlet state lifetime is 13 ns. Direct demonstrating the high stability and recyclable of the
conversion from aliphatic alcohols to bromides has been designed organic photocatalyst.
conducted with the designed organic photocatalyst under
visible light irradiation with high efficiency and selectivity.
The catalytic efficiency of the novel benzothiadiazole-based
photocatalyst is comparable with the state-of-art metal and
non-metal catalysts.
Keywords: photocatalysis, bromination, photo-induced
electron transfer, organic photocatalyst, stability
acceptor-type photocatalysts was also achieved 9].
Nevertheless, further and insightful investigations for
design of highly efficient and stable organic
photocatalysts based on electron donor acceptor units
are still needed, especially for challenging
photoredox reactions.
[
Introduction
The employment of light-active photocatalysts
under sustainable and environmentally benign
reaction conditions has attracted much attention
among the organic and materials scientists in recent
years.[1] In the past decades, enormous effort has
been paid to investigate the influence of molecular
structure on the redox properties in order to obtain a
high efficient photocatalyst.[2] From the perspective
of the photocatalyst materials, most established
redox-active molecular photocatalysts are transition
Brominated compounds play an essential role in
the synthesis of various important intermediates,
pharmaceuticals, fine chemical, agrochemical and
polymers.[10] Molecular bromine and bromo-organic
reagents make
a
main contribution for the
bromination of most compounds.[11] In addition,
another approach, such as Appel reaction, have
proven to be an efficient strategy for providing
bromides from corresponding alcohols with PPh3 as
reductant and CBr4, diethyl bromomalonate or 1,2-
dihaloethanes etc. as electrophilic bromine source in
the mild reaction conditions.[12] However, due to the
hazardous nature of bromine, harsh synthesis
metal complexes, for example [Ru(bpy)3]Cl2 3], fac-
[
[Ir(ppy)3][4], Cu[5] and Cr[6] complexes. Currently,
aromatic, pure organic photocatalysts have emerged
as a promising alternative to the traditional metal-
containing photocatalysts due to their highly tunable
electronic properties. Recent research activities have
shown
a
number of small molecular[7] or
condition
of
bromo-organic
reagent,
and
macromolecular[8] systems containing organic donor
acceptor-type units as efficient photocatalysts for
stoichiometric waste as by-product, searching for
novel bromination methods has drawn much
visible light-promoted photoredox reactions. Recently, attentions and also witnessed an enormous growth in
we reported a structural design strategy of molecular
photocatalysts containing organic electron donor and
acceptor moieties for photoredox reaction.[7b] In
addition, the activation of sp2 carbon halogen bond
for C-C bond construction with organic donor
recent decades. Among them, visible light-promoted
bromination offer an environment friendly approach
for bromides preparation.[13] Recently, Stephenson
reports a novel way for halides obtainment via
nucleophilic substitution of alcohols under visible
1
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10.1002/adsc.201900416
Advanced Synthesis & Catalysis
light.[13a] However, the detail information for the
generation and transfer of electrons and radicals,
especially the photo-initiation process is still
unrevealed. An enhanced investigation for
mechanistic study is thus highly required.
Figure 1. (a) Structure of Ph-BT-Ph; (b) simulated electron
density of Ph-BT-Ph on its HOMO and LUMO levels; (c)
the photoredox potential comparison of Ph-BT-Ph with
other state-of-art organic and metal-based molecular
photocatalysts. Values of [Ru(bpy)3]2+, fac-[Ir(ppy)3],
Eosin Y and Acr+-Mes are taken from literature.[14] bpy =
2,2’-bipyridyl; ppy = phenylpyridyl.
In this work, we present a molecular organic
photocatalyst,
4,7-diphenyl-2,1,3-benzothiadiazole
(Ph-BT-Ph) as an efficient metal-free photocatalyst
for visible light-promoted conversion of alcohols to
bromides with high reaction yield and selectivity. The
photocatalytic efficiency of the organic photocatalyst
was comparable to that of the reported state-of-art
metal and non-metal photocatalysts. Remarkably, no
photo-bleaching effect of the organic photocatalyst
was observed. Furthermore, a detailed photophysical
study employing time-resolved photoluminescence
and transient absorption spectroscopy was conducted
to offer a deeper mechanistic insight for the electron
transfer between the catalyst and the substrates during
the photo-initiation process.
The UV/vis absorption spectrum of photocatalyst 1
showed an absorption range up to ca. 450 nm with a
maximum at 387 nm, while the emission area
between 400 and 650 nm with the maximum at ca.
505 nm was determined (Figure S1), which is
consistent with previous reports.[15] The optical
properties of photocatalyst 1 are similar to that of fac-
[Ir(ppy)3],
a
well-established transition metal
photocatalyst.[16] To further verify the intramolecular
charge transfer (ICT) properties, the solvatochromic
effect on the UV-Vis absorption and fluorescence
emission features of 1 was investigated (Figure S2-
S3). The UV-Vis absorption spectra of photocatalyst
1 appeared slight solvent-dependence, with the
maximum absorption corresponding to the ICT state
stabilized around 380 nm (Figure S2). However, the
maximum emission demonstrated an obvious solvent-
dependent behaviour, showing a bathochromic shift
as the polarity of solvent increased. The Lippert-
Matage plots showed a clearly higher gradient value
for polar solvents (6880 cm-1) than that of nonpolar
solvents (4620 cm-1) indicating a larger dipolar
change between the ground state and excited state in
polar solvents (Figure S3). This phenomenon is a
typical property for charge transfer compounds[17].
The cyclic voltammetry (CV) measurement revealed
the HOMO and LUMO of photocatalyst 1 at +1.64
eV and -1.30 eV vs. SCE (Figure S5), indicating its
possibly promising redox potentials. A comparison of
the photoredox potentials of 1 with selected state-of-
art transition metal complexes such as [Ru(bpy)3]Cl2,
fac-[Ir(ppy)3], or pure organic photocatalyts such as
Eosin Y,[14a] and 9-mesityl-10-methylacridinium
perchlorate (Acr+-Mes),[18] is shown in Figure 1c.
To demonstrate the feasibility of 1 as a high
effective photocatalyst, we test the direct bromination
of aliphatic alcohols. Brominated compounds play an
essential role in organic synthesis such as in the
formation reaction of carbon-carbon and carbon-
heteroatom bonds.[19] The screening and control
experiments of the model bromination reaction of 3-
phenylpropan-1-ol (4) into (3-bromopropyl)benzene
(5) were listed in Table 1. The model reaction was
carried out with a high conversion of 98% (entry 1),
and DMF was tested to be the best solvent (entries 2
and 3). The apparent quantum yield was calculated ca.
0.71% for the model reaction (details in SI). As
expected, the photocatalytic bromination reaction
could not be initiated without either light, or
photocatalyst 1 (entries 6 and 7). In addition, the
photo-generated electron/hole pair of 1 played a
crucial role during the catalytic cycle. Using KI as
hole scavenger led to only trace conversion (entry 8).
Results and Discussion
The organic photocatalyst 4,7-diphenyl-2,1,3-
benzothiadiazole (Ph-BT-Ph, photovcatalyst 1)
(Figure 1a) was synthesized via a simple Suzuki-
coupling procedure between 4,7-dibromo-2,1,3-
benzothiadiazole and phenylboronic acid, as
previously reported.[13c] The synthetic and
characterization details are described in the
experimental section and supporting information (SI).
Theoretical calculations using the density functional
theory (DFT) at B3LYP/6-31G* level revealed that
the electron densities are mainly focused on the
electron-donating
phenyl
and
-withdrawing
benzothiadiazole moieties on the highest occupied
molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) for 1, respectively (Figure
1b). This demonstrates that the donor-acceptor-type
(D-A-type) molecular design of 1 could enhance the
photo-induced electron/hole charge separation and
thereby facilitate its photocatalytic activity.
2
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10.1002/adsc.201900416
Advanced Synthesis & Catalysis
By adding benzoquinone as electron scavenger, a
decreased conversion of 29% was obtained (entry 9).
To study the origin of the bromine source, we then
carried out the model reaction without CBr4, 8%
conversion was determined (entry 10). Without using
NaBr as additive, a decreased conversion of 80%
could be obtained (entry 11). Furthermore, in order to
investigate the influence on reduction potential of
photocatalyst on catalytic efficiency. Another two
oxidation of DMF followed by nucleophilic addition
between alcohol 4 and imine intermediates as
mentioned in previous report.[20] By employing
[Ru(bpy)3]Cl2 and fac-[Ir(ppy)3] as photocatalysts,
reaction conversions of 96% and 98% were observed
(entry 14 and 15). This confirmed the high photo-
activity of 1 as promising photocatalyst
To understand the reaction mechanism, we further
studied the role of CBr4. A control experiment
conducted in deuterated DMF for 4 h showed that
only deuterated formate ester was formed (Figure S6,
SI). This indicates that DMF was involved in the
bromination reaction. According to previous reports
by Léonel[21] and Stephenson[13a], there are actually
two radical pathways in the formation of the
brominated product based on either the formation of a
carbene (:CBr2, 8) or radical •CBr3 (7) as the
oxidatively quenched intermediates. To precisely
identify this step, we then conducted an additional
experiment with α-methylstyrene (15) as carbene 8 or
radical 7 trapping agent. As shown in Figure S4, both
dibromo (17, SI) and tribromo (16, SI) products were
determined by GC-MS with a ratio of 1/8.8,
demonstrating that both carbene 8 and radical 7
existed in the reaction mixture, which is slight
photocatlysts
4,7-bis(4-methoxyphenyl)
(2) and 4,4'-
benzo[c][1,2,5]thiadiazole
(benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzonitrile
(3) were also prepared with either electron-donating
or –withdrawing groups on the phenyl rings of the
basic structure of photocatalyst 1 (see structures in
the experimental part). It appeared that photocatalyst
2 (entry 4) with electron-donating groups showed a
comparable efficiency with conversion of 97% to 1.
Photocatalyst 3 (entry 5) with electron-withdrawing
groups showed a considerably lower efficiency with
conversion of 47% than 1 and 2.
Table 1. Screening and control experiments of the
2+
different from Ru(bpy)3 photocatalyst reported by
Stephenson where no carbine 8 was generated[13a]. In
addition, the radical 7 played a major role for the
formation of the final product, with the amount which
photocatalytic bromination reaction with model substrate.a
Entry Reaction condition variations
Conv. (%)b
98
1
standard conditions
THF as solvent
acetonitrile as solvent
photocatalyst 2
photocatalyst 3
no catalyst
2
trace
40
3
4
97
5
47
6
trace
trace
trace
29
7
no light
8c
9d
10
11
12
13
14
15
hole scavenger
electron scavenger
no CBr4
8
no NaBr
80
Eosin Y as catalyst
Acr+-Mes as catalyst
[Ru(bpy)3]Cl2 as catalyst
fac-[Ir(ppy)3] as catalyst
3
is 8.8 times that of carbene 8 (Figure S7).
trace
96
Figure 2. Proposed reaction mechanism of photocatalytic
direct conversion of alkyl alcohols to bromides.
98
a)Reaction conditions: alcohol [4] = 0.05 M, [CBr4] = 0.1
M, [NaBr] = 0.1 M, dry DMF 10 mL, photocatalyst [1] = 1
mg/ml (6.5 mol%), blue LED lamp (460 nm, 0.16 W/cm2),
room temperature, 12 h. b)Conversion determined by GC-
MS. c)KI as hole scavenger. d)Benzoquinone as electron
scavenger.
Based on the observations, a plausible reaction
mechanism was proposed as illustrated in Figure 2. In
the major pathway, the radical 7 was generated via
one electron transfer from the LUMO level of Ph-BT-
Ph to CBr4. The radical was trapped by DMF to form
its radical intermediate (radical 7), which then
released an electron and form the iminium
intermediate 11. The iminium intermediate 11 was
attacked by the bromide ion to form the Vilsmeier-
Haack agent 12, which reacted with the alcohol to
obtain the final product.[22] In the minor reaction
pathway, the carbene 8 species could be generated
In direct comparison with other state-of-art organic
photocatalysts, Eosin Y and Acr+-Mes only led to
trace or no conversion (entries 12 and 13). Here, an
O,N-aminal product (6) was determined with both
organic photocatalysts (SI), corresponding the
3
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10.1002/adsc.201900416
Advanced Synthesis & Catalysis
through stepwise reducing CBr4 by the photocatalyst
process between the photocatalyst 1 and reactive
substrates. As shown in Figure 4, the signal peaking
at 660 nm represented the photo-induced absorption
(PIA) of 1 in its excited state. The lifetime of the
singlet excited state of 1 in a N2-purged solution was
1.[21, 23] The formed carbene 8 then reacted with
DMF,
leading
to
CO
and
(dibromomethyl)dimethylamine intermediate (10)
which was in equilibrium with Vilsmeier-Haack
agent 12. The facile combination between photo-
generated radicals and DMF have been frequently
reported, which offered an excellent platform for
getting Vilsmeier-Haack intermediates and thus
Table 2. Excited state lifetime of the measured mixtures.
Fluorescence
lifetime (ns)
Transient
lifetime (ns)
Entry Sample name
producing
various
desired
and
halides[13a],
anhydrides[24],
nitriles[25]
α-formyloxy
1
photocataltst 1
13.0
13.1
compounds[26] in DMF solvent. In addition, while as
a minor pathway, carbene 8 could also directly
inserted into OH bond of alcohols to generate α,α-
dibromo ether intermediate which would afterwards
decompose into the final bromides in the other
solvent such as acetonitrile.[23, 27]
photocataltst 1 +
CBr4
2
9.5
7.2
photocataltst 1+
alcohol 4
3
12.9
9.2
10.8
7.7
photocataltst 1 +
CBr4 + alcohol 4
4
The
steady-state
fluorescence
quenching
experiments showed that the fluorescence of 1 could
be quenched dramatically by CBr4 with the calculated
rate constants of 154 [M]-1, indicating a photo-
induced electron transfer from the photocatalyst to
CBr4 (Figure S8-S9). In order to further identify the
inside quenching process, a time-resolved Stern-
Volmer plot was conducted as shown in Figure S10-
S11. A linearly decreased fluorescence lifetime the
calculated rate constants of 31 [M]-1 was observed,
indicating the presence of dynamic quenching.
Therefore, both static quenching and collisional
quenching happens between the photocatalyst 1 and
CBr4, which is commonly found in previous
reports[26, 28]. In addition, the high percent of static
quenching demonstrate that the formation of non-
emission complexes between 1* and CBr4 make a
major contribution for the electron transfer process.
To gain a deeper insight and elucidate the electron
transfer pathways, we further used time-resolved
photoluminescence (TRPL) and transient absorption
(TA) spectroscopy to characterize the electron
transfer between the photocatalyst and substrates
during the bromination process. As shown in Figure 3
and Table 2, the fluorescence lifetime of 1 in DMF
after purging with N2 was determined to be 13.0 ns,
which was comparable to the well-applied organic
photocatalysts, such as Acr+-Mes (15 ns)[13b] and
Eosin Y (6 ns).[29] In the presence of CBr4, the
excited state of 1 could be quenched obviously with
decreased fluorescence lifetime of 9.5 ns. This
indicated a possible electron migration from the
LUMO of 1 to CBr4, which is indeed the initiation
step for the bromination reaction, as described in the
aforementioned control experiments. In comparison,
the fluorescence lifetime of 1 did not change by
adding alcohol 4, indicating no interaction between
the alcohol 4 and the photocatalyst 1. By employing
the reaction mixture containing CBr4, alcohol 4 and 1,
a similar fluorescence lifetime delay was recorded as
only adding CBr4, confirming that the reaction
between CBr4 and 1 was indeed the initial step in the
catalytic cycle of the bromination reaction.
photocatalyst 1
1 + CBr4
100
10-1
10-2
10-3
1 + alcohol 4
1 +CBr4 + alcohol 4
0
10
20
30
40
50
60
70
Time [ns]
Figure 3. Fluorescence decay spectra of 1 (black), mixture
of 1 with CBr4 (red), with alcohol 4 (green), and with both
CBr4 and alcohol 4 (blue), the concentration of 1, CBr4 and
alcohol 4 were 20 mg/L, 8 mM and 4 mM in DMF,
respectively. Excitation wavelength at 400 nm.
Figure 4. Transient absorption spectra of (a) pure
photocatalyst 1, (b) with CBr4 and (c) with alcohol 2 at in
DMF at various timescale. (d) Time profiles of absorbance
Transient absorption (TA) spectra were recorded to
further investigate the concrete electron transfer
4
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10.1002/adsc.201900416
Advanced Synthesis & Catalysis
decay at 670 nm of 1 (black, square), 1 with CBr4 (red,
circle), 1 with alcohol 4 (green, up triangle) and 1 with
CBr4 and alcohol 4 (blue, down triangle). Concentrations
of 1, CBr4 and alcohol 4 were 20 mg/L, 8 mM, and 4 mM,
respectively. Excitation at 355 nm.
with bromide product being trace amount (Figure 5),
indicating the potential reactive iminium intermediate
generated from the reaction between alcohol and
Vilsmeier-Haack reagent, which is similar to previous
report.[13a] Lastly, it is worth to mention that no
photobleaching effect of photocatalyst
1 was
determined after the reaction, as shown the UV-Vis
spectra (Figure S12). While it is an unavoidable
problem for most reported state-of-art photocatalysts,
our photocatalyst 1 demonstrate an excellent stability
during the whole photocatalytic cycle.
determined to be 13.1 ns, which was consistent to that
obtained from TRPL (Table 2). The addition of CBr4
did not change the shape of the PIA curves of 1
(Figure 4b). The possible reason could be that the
signals of the possibly formed intermediates such as
the radical 7 and carbene 8 were located less than 500
nm,[30] which are outside the spectral window. No
interaction was found between the excited state 1 and
4 (Figure 4c), indicating that no electron and hole
transfer between the photocatalyst 1 and alcohol 4
occurred during the catalytic cycle. This partially
supports our hypothesis that the brominated product
should be only obtained via the reaction between the
alcohol and reactive intermediate. Although the
present of CBr4 had limited influence on the PIA
curve, it obviously still could accelerate the decay
profile of the excited singlet state of 1 from 13.1 ns to
7.7 ns (Table 2), and thereby revealing the electron
transfer in the excited state. To note, the decay
profiles of the TA were consistent with that of the
TRPL (Figure S13).
Conclusion
In summary,
a
small molecule organic
photocatalyst,
4,7-diphenyl-2,1,3-benzothiadiazole
was presented as an pure organic, highly efficient
photocatalyst for direct conversion from alcohols to
bromides. The photo-catalytic efficiency was
comparable with the state-of-art organic and
transition metal photocatalysts. No photo-bleaching
effect was observed, demonstrating the excellent
stability of the photocatalyst. Furthermore, a deeper
mechanistic insight of the photocatalytic process was
investigated using advanced photophysical studies.
Electron transfer between the photocatalyst and
substrates, which involves the photo-induced electron
transfer during the catalytic processes, was
illuminated and confirmed. With its sufficient
photoredox potentials, long photo-generated exciton
lifetime and high efficiency, we believe that 4,7-
diphenyl-2,1,3-benzothiadiazole can be a promising
photocatalyst in a broader range of photoredox
reactions.
Experimental Section
General Information. All chemicals and solvents were
purchased from commercial sources and used as received
unless otherwise noted. The reactions conversion was
monitored by GC-MS. Reaction yield refer to pure
compound after being purified via column chromatography.
Column flash chromatography was conducted over silica
60 (0.063-0.2 mm).
General Analytical Information. UV-Vis absorption and
fluorescence spectra were recorded on a Perkin Elmer
Lambda 100 spectrophotometer and J&M TIDAS
spectrofluorometer at ambient temperature, respectively.
Electron paramagnetic resonance (EPR) spectrum was
Figure 5. Scope of the photocatalytic reactions of alkyl
alcohols to bromides using 1 as photocatalyst with isolated
yields.
conducted on
spectrometer. Cyclic voltammetry measurement was
carried out on Metrohm Autolab PGSTAT204
a
Magnettech Miniscope MS200
a
potentiostat/galvanostat with a three-electrode-cell system:
glassy carbon electrode as the working electrode,
Hg/HgCl2 electrode as the reference electrode, platinum
wire as the counter electrode, and Bu4NPF6 (0.1 M in
acetonitrile) as supporting electrolyte with a scan rate of
100 mV s-1 in the range of -2 eV to 2 eV. The redox
potential of catalyst was obtained by dissolving compound
into acetonitrile directly with a concentration of 1 mg/ml.
GC-MS measurement was performed on Shimadzu GC-
2010 plus gas chromatography and QP2010 ultra mass
spectrometer with fused silica column (Phenomenx,
The scope of various alcohols demonstrated that
both primary and secondary alcohols could be
transformed into the corresponding bromides using 1
as photocatalyst (Figure 5). And the reactions of
primary alcohols exhibited slightly higher conversion
than that of the secondary alcohols. It could be
observed that formate ester was the common minor
side product for most of the alcohols. Additionally,
for the reaction with cyclododecanol, cyclododecyl
formate (26) was determined as the main product
1
Zebron 5-ms nonpolar) and flame ionization detector. H
and 13C NMR spectra for all compounds were measured
5
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10.1002/adsc.201900416
Advanced Synthesis & Catalysis
using Bruker AVANCE 300 MR (at 300 MHz) and are
referenced to 0.00 ppm and 0.0 ppm for SiMe4.
Preparation of photocatalyst 3:[32]
To
a
solution of 2,1,3-
Time-Resolved
Photoluminescence:
Time-resolved
benzothiadiazole-4,7-bis
(boronic acid pinacol ester)
photoluminescence (TRPL) spectra were taken with a
C4742 Hamamatsu streak camera system in slow sweep
mode. Excitation pulses at 400 nm were provided by
(200 mg, 0.52 mmol) and 4-
bromobenzonitrile (280 mg,
frequency doubling the output of
femtosecond amplifier laser system (Coherent LIBRA-HE). 51.9 μmol) and aqueous 2M NaCO3 (4 ml), the mixture
a
commercial
1.54 mmol) in 10 ml dioxane was added Pd(PPh3)4 (60 mg,
was heated at 120 0C under Argon for 24 hours. The
reaction was quenched by addition of Milli Q water (30
Transient
Absorption
Spectroscopy:
Transient
ml). The formed solid was collected by filtration and
washed thoroughly with water, methanol and
dichloromethane, respectively. The crude product was
purified though silica column using dichloromethane as
elute. Finally, the titled product was obtained as light
yellow powder after removal of the solvent (127.15mg,
absorption (TA) measurements were performed with a
home-built pump-probe setup. To generate white light in
the visible the output of commercial titanium: sapphire
amplifier (Coherent LIBRA-HE, 3.5 mJ, 1 kHz, 100 fs)
was used to pump a commercial optical parametric
amplifier (Coherent OPerA Solo). The parametric
amplifier (OPA) was used to generate the seed beam of
1300 nm for white-light generation. The 1300 nm seed of a
few µJ was focused into a cut 3 mm thick sapphire window
for white-light generation. Mostly reflective elements were
used to guide the probe beam to the sample to minimize
chirp. The excitation pulse was provided by an actively Q-
switched Nd:YVO4 laser (AOT Ltd. MOPA) at 355 nm
with a resolution of 600 ps. The delay between pump and
probe was controlled by an electronic delay generator
(Stanford Research Systems DG535). The white-light
pulses were dispersed onto a linear silicon photodiode
array, which was read out at 1 kHz by home-built
electronics. Adjacent diode readings corresponding to the
transmission of the sample after an excitation pulse and
without an excitation pulse were used to calculate ΔT/T.
TA measurements were performed at room temperature.
1
69%), H NMR (300 MHz, CDCl3) δ 8.06 (d, J = 8.1 Hz,
4H), δ 7.81 (d, J = 2.4 Hz, 4H), δ 7.77 (s, 2H). 13C NMR
(300 MHz, CDCl3) δ 153.5, 141.3, 132.5, 132.4, 129.9,
128.6, 118.7, 112.3. MS m/z calculated for C20H14N4S ([M
+ H]+) 339.4, found 339.4.
General Experimental Procedure for Photocata-lytic
Bromination Reaction of Alcohols: A flame-dried 25 mL
schlenk flask with a magnetic stir bar was added with Ph-
BT-Ph (10 mg, 0.065 equiv), corresponding alcohol (0.50
mmol, 1.0 equiv), sodium bromide (1.00 mmol, 2.0 equiv),
carbon tetrabromide (1.00 mmol, 2.0 equiv) and dry DMF
(10 ml). Then the flask was capped with rubber septum and
degassed with nitrogen. The mixture was bubbled with
nitrogen through freeze-thaw process for
3 times.
Afterwards, the flask was placed under the irradiation of a
blue LEDs lamp (0.16 W/cm2). The reaction mixture was
stirred at room temperature until it was completely finished
which was determined by GC-MS measurement.
Subsequently, the mixture was poured into a separatory
funnel containing the mixture of Et2O (25 ml) and H2O (25
ml). The organic layers were separated and extracted with
Et2O (25 ml) for 2 more times. The combined organic
layers were washed with saturated Na2S2O3, brine, and
then dried over anhydrous MgSO4. After removing the
solvent with rotary evaporator, a crude product was
obtained. The crude product was then purified with column
chromatography on silica to afford the pure compound.
Synthesis of Organic Photocatalyst Ph-BT-Ph.[13c] A
250 ml three-necked flask was charged with 4, 7-dibromo-
2, 1, 3-benzothiadiazole (1.47 g, 5 mmol), phenylboric acid
(2.68 g, 22 mmol) and toluene (25 mL). Then, a solution of
potassium carbonate (2.76 g, 20 mmol) in H2O (10 mL)
were added into the flask. After degassing for 30 min,
Pd(PPh3)4 (0.047 g, 0.04 mmol) was added, and the
reaction mixture was heated to 90 °C and reacted for
overnight under N2 atmosphere. After cooling to room
temperature, the mixture was poured into water and
extracted with dichloromethane. The organic layer was
washed with water and dried over anhydrous MgSO4. After
concentrated with rotary evaporator, the crude product was
purified via column chromatography on silica with
dichloromethane as eluent. For further purification, the
crude product was recrystallized with methanol to afford
Ph-BT-Ph as yellow needles. Yield: 1.17 g (82%). 1H
NMR (300 MHz, CDCl3, δ): 8.00 (m, 4H), 7.81 (s, 2H, Ar
H), 7.58 (m, 4H), 7.49 (t, J = 9, 2H); 13C NMR (75 MHz,
CDCl3, δ): 128.13, 128.38, 128.63, 129.25, 133.39, 137.44,
154.11. MS (APCI) m/z calculated for C18H12N2S ([M +
H]+) 289.1, found 289.1
(3-bromopropyl)benzene (Figure 5, 18, 91 mg, 92%,
colorless oil):[13d] Rf (hexane/dichloromethane 5/1): 0.5;
1H NMR (CDCl3, 300 MHz): δ 7.22 (m, 2H), 7.14 (m, 3H),
3.32 (t, J=7.5 Hz, 2H), 2.71 (t, J=7.5 Hz, 2H), 2.10 (p, J=6
Hz, 2H); 13C NMR (CDCl3, 75 MHz): δ 140.5, 128.6,
128.5, 126.2, 34.2, 34.0, 33.1. MS (APCI) m/z calculated
for C9H8Br ([M + H]+) 198.0, found 198.0
1-(3-bromopropyl)-4-methoxybenzene (Figure 5, 19, 94
mg, 82%, colorless oil):[13a] Rf (hexane/dichloromethane
6/1): 0.35; 1H NMR (CDCl3, 300 MHz): δ 7.15 (d, J=6 Hz,
2H), 6.87 (d, J=6 Hz, 2H), 3.83 (s, 3H), 3.42 (t, J=6 Hz,
2H), 2.76 (t, J=6 Hz, 2 Hz), 2.17 (p, J=6 Hz, 2H); 13C
NMR (CDCl3, 75 MHz): δ 158.1, 132.6, 129.5, 113.9, 55.3,
34.4, 33.2, 33.1. MS (APCI) m/z calculated for C10H13BrO
([M + H]+) 228.1, found 228.1
Preparation
of
photocatalyst 2:[31]
An oven-dried flask was
charged with CsF (113.6 mg,
0.748
mmol),
(4-
methoxyphenyl)boronic acid (113.7 mg, 0.748 mmol), 4,
7-dibromo-2, 1, 3-benzothiadiazole (100 mg, 0.34 mmol)
and 1,4-dioxane (5 mL). After degassing for 20 min,
Pd(PPh3)4 (6.4 mg, 0.0054 mmol) was added and degassed
again for 20 min. Then, the reaction mixture was stirred at
4-(3-bromopropyl)phenol (Figure 5, 20, 97 mg, 91%,
colorless solid):[13a] Rf (hexane/ethyl acetate 5/1): 0.35; 1H
NMR (CDCl3, 300 MHz): δ 7.07 (d, J=9 Hz, 2H), 6.79 (d,
J=9 Hz, 2H), 3.41 (t, J=6 Hz, 2H), 2.73 (t, J=6 Hz, 2H),
2.14 (p, J=6 Hz, 2H); 13C NMR (CDCl3, 75 MHz): δ 153.8,
132.8, 129.7, 115.4, 34.4, 33.2, 33.0. MS (APCI) m/z
calculated for C9H11BrO ([M + H]+) 214.1, found 214.1
0
130 C for 18 h. After cool down to room temperature and
solvent vaporisation a crude material was obtained. The
final product was chromatographed directly on silica gel
1
with Et2O as red solid, Yield: 79.63 mg (84%). H NMR
(3-bromobutyl)benzene (Figure 5, 21, 96 mg, 90%,
1
colorless oil):[13a] Rf (hexane): 0.45; H NMR (CDCl3,
(300 MHz, CDCl3, δ): 7.88 (d, J = 8.4 Hz, 4H), 7.66 (s,
2H), 7.03 (d, J = 8.7 Hz, 4H), 3.83 (s, 6H); 13C NMR (75
MHz, CDCl3, δ): 128.13, 128.38, 128.63, 129.25, 133.39,
137.44, 154.11. MS (APCI) m/z calculated for
C20H16N2SO2 ([M + H]+) 349.4, found 349.4
300 MHz): δ 7.35 (m, 2H), 7.26 (m, 3H), 4.14 (m, 1H),
2.93 (m, 1H), 2.82 (m, 1H), 2.19 (m, 1H), 2.11 (m, 1H),
1.79 (d, J=6 Hz, 3H); 13C NMR (CDCl3, 75 MHz): δ 140.9,
128.6, 128.5, 126.1, 50.9, 42.7, 34.0, 26.6. MS (APCI) m/z
calculated for C10H13Br ([M + H]+) 212.0, found 212.0
6
This article is protected by copyright. All rights reserved.

10.1002/adsc.201900416
Advanced Synthesis & Catalysis
Methyl-2-bromo-2-phenylacetate (Figure 5, 22, 93 mg,
81%, colorless oil):[13a] Rf (hexane/ethyl acetate 10/1):
0.44; 1H NMR (CDCl3, 300 MHz): δ 7.46 (m, 2H), 7.28 (m,
3H), 5.29 (s, 1H), 3.71 (s, 3H); 13C NMR (CDCl3, 75
MHz): δ 168.8, 135.7, 129.3, 128.9, 128.7, 53.4, 46.5. MS
(APCI) m/z calculated for C9H9BrO2 ([M + H]+) 227.9,
found 227.9
Hernandez-Perez, S. K. Collins, Acc. Chem. Res. 2016,
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(2-bromoethyl)benzene (Figure 5, 23, 82 mg, 89%,
colorless oil):[33] Rf (hexane/dichloromethane 5/1): 0.56;
1H NMR (CDCl3, 300 MHz): δ 7.37 (m, 3H), 7.29 (m, 2H),
3.63 (t, J=7.7 Hz, 2H), 3.23 (t, J=7.6 Hz, 2H); 13C NMR
(CDCl3, 75 MHz): δ 138.9, 128.70, 128.66, 127.0, 39.5,
33.0. MS (APCI) m/z calculated for C8H9Br ([M + H]+)
184.0, found 184.0
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1H NMR (CDCl3, 300 MHz): δ 7.32 (m, 3H), 7.24 (m, 2H),
4.34 (m, 1H), 3.24 (m, 1H), 3.13 (m, 1H), 1.73 (d, J=6.6
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128.4, 126.8, 50.5, 47.5, 25.7. MS (APCI) m/z calculated
for C9H11Br ([M + H]+) 198.0, found 198.0
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1
colorless oil):[13d] Rf (hexane/ethyl acetate 5/1): 0.53; H
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NMR (CDCl3, 300 MHz): δ 7.09 (d, J=8.5 Hz, 2H), 6.83 (d,
J=8.5 Hz, 2H), 5.38 (bs, 1H), 3.55 (t, J=7.7 Hz, 2H), 3.11
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for C8H9BrO ([M + H]+) 200.1, found 200.1
Cyclododecyl formate (Figure 5, 26, 95 mg, 90%, colorless
oil):[13a] Rf (hexane/dichloromethane 40/1): 0.12; 1H
NMR (CDCl3, 300 MHz): δ 7.98 (s, 1H), 5.07 (m, 1H),
1.66 (m, 2H), 1.48 (m, 2H), 1.28 (m, 18H); 13C NMR
(CDCl3, 75 MHz): δ 161.0, 72.2, 29.1, 24.0, 23.8, 23.3,
23.2, 20.8. MS (APCI) m/z calculated for C13H24O2 ([M +
H]+) 212.3, found 212.3
Acknowledgements
The authors acknowledge the Max Planck Society for the
financial support and R.L. acknowledge the support from the
Fundamental Research Funds for the Central Universities. R.L.
also thank the China Scholarship Council (CSC) for the graduate
scholarship. Stefan Schuhmacher is acknowledged for technical
support.
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Advanced Synthesis & Catalysis
FULL PAPER
Visible Light-mediated Conversion of Alcohols to
Bromides by a Benzothiadiazole-containing
Organic Photocatalyst
Adv. Synth. Catal. Year, Volume, Page – Page
R. Li*, D. W. Gehrig, C. Ramanan, P. W. M.
Blom, F. F. Kohl, M. Wagner, K. Landfester, and
K. A. I. Zhang*
9
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