Light-driven activation of carbon-halogen bonds by readily available amines for photocatalytic hydrodehalogenation

Article abstract of DOI:10.1016/S1872-2067(20)63582-3

A straightforward protocol using readily available aromatic amines, N,N,N′,N′-tetramethyl- p-phenylenediamine or N,N′,N′-tetramethylbenzidine, as photocatalysts was developed for the efficient hydrodehalogenation of organic halides, such as 4′-bromoacetophenone, polyfluoroarenes, cholorobenzene, and 2,2′,4,4′-tetrabromodiphenyl ether(a resistant and persistent organic pollutant). The strongly reducing singlet excited states of the amines enabled diffusion-controlled dissociative electron transfer to effectively cleave carbon-halogen bonds, followed by radical hydrogenation. Diisopropylethylamine served as the terminal electron/proton donor and regenerated the amine sensitizers.

Full text of DOI:10.1016/S1872-2067(20)63582-3

Chinese Journal of Catalysis 41 (2020) 1474–1479
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Communication
Light-driven activation of carbon-halogen bonds by readily
available amines for photocatalytic hydrodehalogenation
Di Meng a,b,†, Qian Zhu a,b,†,‡, Yan Wei a,b, ShengliZhenc, Ran Duan a, Chuncheng Chen a,b
Wenjing Song a,b,*, Jincai Zhaoa,b
,
a Key Laboratory of Photochemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing, 100190, China
b University of Chinese Academy of Sciences, Beijing, 100049, China
c Beijing GeoEnviron Engineering & Technology, lnc., Beijing, 100095, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A straightforward protocol using readily available aromatic amines, N,N,N,N-tetramethyl-
p-phenylenediamine or N,N,N,N-tetramethylbenzidine, as photocatalysts was developed for the
efficient hydrodehalogenation of organic halides, such as 4-bromoacetophenone, polyfluoroarenes,
cholorobenzene, and 2,2,4,4-tetrabromodiphenyl ether(a resistant and persistent organic pollu-
tant). The strongly reducing singlet excited states of the amines enabled diffusion-controlled disso-
ciative electron transfer to effectively cleave carbon-halogen bonds, followed by radical hydrogena-
tion. Diisopropylethylamine served as the terminal electron/proton donor and regenerated the
amine sensitizers.
Received 20 January 2020
Accepted 5 March 2020
Published 5 October 2020
Keywords:
Carbon-halogen bond activation
Photocatalysis
Halogenated organic pollutants
Reductive dehalogenation
Environmental remediation
© 2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Light-driven activation of carbon-halogen bonds (C–X) has
become a powerful approach for constructing new chemical
bonds in valuable pharmaceutical and industrial intermediates,
as well as for the elimination of polyhalogenated aromatic pol-
lutants [1‒8]. Essentially, the process relies on high-energy
excited states/intermediates that trigger electron transfer to
cleave carbon-halogen bonds (C–X + e‒ → C• + X‒, dissociative
electron transfer), affording carbon radicals that can engage in
diverse reactions, including hydrogenation and cross coupling
[9‒18]. For a thermodynamically favored process, energy levels
of the photogenerated species should locate above the an-
ti-bonding σ* or π* orbital of halogenated substrates. Promis-
ing results have been reported with classical photosensitizers,
Ru(bpy)3 and fac-Ir(ppy)3 (bpy is 2,2-bipyridine and ppy is
2+
2-phenylpyridine), where their triplet excited states (‒0.8 and
‒1.7 V vs. SCE) [1,19], or the reduced complexes (‒1.3 and ‒2.1
V vs. SCE) enable dissociative electron transfer. Recently, hex-
achlorocerate(III) (CeIIICl63‒), with a strongly reducing excited
state (‒3.45 V vs. Fc/Fc+), has been developed as a sensitizer
for the reduction of aryl chlorides under UVA irradiation
[20,21]. Copper complexes are likewise involved in conversions
of carbon-halogen bonds [22,23], e. g., C-F functionalization
* Corresponding author. Tel: +86-10-82615942; Fax: +86-10-82612075; E-mail: wsongunc@iccas.ac.cn
† These authors contributed equally to this work.
‡ Current Address: Beijing GeoEnviron Engineering & Technology, lnc., Beijing, 100095, P. R. China.
This work was supported by the National Key R&D Program of China (2018YFA0209302), National Natural Science Foundation of China (21590811,
21677148, 21827809, 21922609), the Key Research Program of Frontier Sciences (QYZDY-SSW-SLH028) of the Chinese Academy of Sciences.
DOI: 10.1016/S1872-2067(20)63582-3 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 10, October 2020

Di Meng et al. / Chinese Journal of Catalysis 41 (2020) 1474–1479
1475
excited states of the amine sensitizers enable diffu-
sion-controlled electron transfer to halogenated substrates,
followed by cleavage of the C–X to give an aryl radical; subse-
quent radical hydrogenation affords the dehalogenated product
(Scheme 1).
TMPD displays S0‒S1 absorption extended to 380 nm (Fig.
1(a)). In the presence of selected aryl halides
(4-bromoacetophenone, hexafluorobenzene and chloroben-
zene), TMPD absorption variations were negligible, excluding
the interaction of the TMPD ground state with these com-
pounds. The viability of TMPD in photo-induced activation of
carbon-halogen bonds was initially evaluated by steady-state
and time resolved emission spectroscopies. The emission in-
tensity of TMPD (0.1 mM, 360 nm excitation) decreased upon
introduction of all three substrates (2‒10 mM, Fig. 1(b) and Fig.
S1), which is consistent with an electron transfer from the sin-
glet excited states to these compounds. The emission lifetime
measured at 405 nm likewise decreased (Fig. 1(c)). The parallel
Stern-Volmer plot of emission intensity and lifetime (Fig. 1(d))
indicates an outer sphere electron transfer to hexafluoroben-
zene, with negligible contribution of static quenching via a
pre-associated ground state complex, in agreement with the
unchanged absorption spectra [30‒32]. The rate constants
calculated by Stern-Volmer analysis (Fig. S1) were 5.8 × 1010
M‒1 s‒1 for 4-bromoacetophenone, 4.4 × 1010 M‒1 s‒1 for hex-
afluorobenzene and 2.6 × 1010 M‒1 s‒1 for chlorobenzene (Fig.
S1). Notably, the observed electron transfer is diffusion-limited,
even in the case of exceptionally inert chlorobenzene [20]. For
TMB, the electron transfer to hexafluorobenzene was also like-
wise diffusion-limited (3.6 × 1010 M–1 s–1), but became activa-
Scheme 1. Working hypothesis of the photocatalytic hydrodehalogena-
tion.
under pulsed light-emitting diodes (LEDs) [24]. Organic sensi-
tizers, such as diaryl dihydrophenazines [25], phenylphenothi-
azine [26,27], perylene diimide[28] and pyrene [2], are also
efficient photosensitizers, producing active reducing species
(‒2.06 to ‒2.5 V vs. SCE) via photon upconversion or consecu-
tive two-photon absorption. These organic molecules are more
cost effective and less prone to deactivation (e.g., halogen coor-
dination in the case of transition metal catalysts) [3,26,27].
Halogen bonds or π‒π interaction between catalyst and sub-
strate may further facilitate bond activation via an inner sphere
electron transfer pathway, however synthetic efforts are re-
quired in order to modulate the intermolecular interactions
[15,29].
Herein, we establish a metal- and additive-free photocata-
lytic system based on a N,N,N,N-tetramethyl-p-phenylene-
diamine (TMPD), or N,N,N,N-tetramethylbenzidine (TMB)
sensitizer featuring strongly reducing excited singlet states and
diisopropylethylamine (DIPEA) as an electron/proton donor,
for hydrodehalogenation of a variety of haloarenes under UV
irradiation ( > 360 nm). In this approach, photogenerated
(a)
(b)
Hexafluorobenzene
TMPD
TMPD,10 mM Hexafluorobenzene
1.0
1.0
1.0
0 mM
0.5
0.8
0.6
0.4
0.2
0.0
0.8
0.0
0.6
10 mM
300
350
400
Wavelength (nm)
0.4
0.2
0.0
300
400
500
600
350
400
450
500
550
600
Wavelength (nm)
Wavelength(nm)
1.6
1.5
1.4
1.3
1.2
1.1
1.0
(c)
(d)
3000
2500
2000
1500
1000
500
I0/I
τ0/τ
0 mM
10 mM
0
2
4
6
0
2
4
6
8
10
Time (ns)
[Q]/ mM
Fig. 1. (a) Absorption (solid line) and emission (dashed line) spectra of TMPD; (b) Changes in the emission intensity of TMPD upon addition of hex-
afluorobenzene. Note that the absorbance at the excitation wavelength did not change (inset); (c) Changes in the emission lifetime of TMPD; (d) Plots
of the relative emission intensities (black) and lifetimes of TMPD (red) obtained from (b) and (c) against hexafluorobenzene concentration. All meas-
urements were conducted in acetonitrile at room temperature.

1476
Di Meng et al. / Chinese Journal of Catalysis 41 (2020) 1474–1479
tion-controlled in the case of chlorobenzene (6.2 × 109 M–1 s–1,
Fig. S1). The slower electron transfer observed for TMB could
be attributed to the fact that its excited state energy is lower
than that of TMPD (–3.48 and –3.56 vs. Fc/Fc+, respectively)
[33]. These results established a kinetically feasible electron
transfer, the initial step for hydrodehalogenation [6].
In the initial test, where 4′-bromoacetophenone (1 mM) was
used as the model substrate with a stoichiometric amount of
TMPD (2:1 according to the 2e/1H+ process) as the photore-
ductant, a conversion of 86% with a selectivity of 90% for hy-
drodebromination was observed after 6 h of irradiation (Table
1, entry 1). The quantitative production of bromide was addi-
tionally confirmed by ionic chromatography (Fig. S2). Involve-
ment of aryl radical intermediates via the initial electron trans-
fer was supported by the generation of C–C coupled products in
the presence of N-methyl pyrrole (Fig. S3) [3,34]. In situ elec-
tron paramagnetic resonance (EPR) experiments and the sim-
ulated EPR spectra also suggested the generation of TMPD+•
and an acetophenone radical (via C–Br bond cleavage of the
4′-bromoacetophenone radical anion) (Fig. S4 and Table S1).
Next, we conducted the hydrodehalogenation under cata-
lytic conditions with 5% TMPD, using DIPEA (15 equivalents)
as the electron/hydrogen donor for radical intermediates.
DIPEA also reduces TMPD+• to recover the photosensitizer (Fig.
S5). The results are summarized in Table 2 and Fig. S6. Gratify-
ingly, the conversion of 4′-bromoacetophenone occurred
smoothly, with a hydrodebromination yield of 90% (entry 1),
corresponding to a turnover number (TON) of 18. The hydro-
debromination yield of bromobenzene was 37% after 6 h of
irradiation (entry 2). Hydrodefluorination yields above 70%
were obtained for hexafluorobenzene and pentafluoropyridine
(enter 3 and 4). Notably, the yields of pentafluorobenzene
(25%)
and
the
di-hydrodefluorination
product,
1,2,4,5-tetrafluorobenzene (50%, Fig. S7) were significantly
higher than those obtained under stoichiometric conditions,
presumably due to
a more efficient hydrogenation by
DIPEA/DIPEA+•. In the case of chlorobenzene, the hydrodechlo-
rination yield approached 40% after 2 h of irradiation, but did
not increase with extended reaction time (entry 5). The dechlo-
rination could be resumed by the addition of TMPD (0.1 equiv-
alent, Fig. S6). The hydrogenation yield of methyl
4-chlorobenzoate reached 72% after 7.5 h of irradiation (entry
6). In addition, the catalytic system demonstrated compatibility
with halogenated heteroarenes (entries 7 and 8), affording
The aryl radical then abstracts
a hydrogen atom from
TMPD/TMPD+• to give the hydrogenated product. TMB exhib-
ited comparable activity under these conditions, with a yield of
88% (Table S2). Control experiments confirmed that pho-
toirradiation and amines were essential for efficient hy-
drodehalogenation. The highest hydrodebromination yield was
obtained in acetonitrile, while significantly inferior yields were
observed with dimethylformamide (27%), methanol (20%),
dichloromethane (17%), and n-hexane (10%) (Table S2). For
hexafluorobenzene (Table 1, entry 2), both mono- and
di-defluorination products were observed after 5 h of reaction
time, albeit in relatively low yields (pentafluorobenzene, 26%,
1,2,4,5-tetrafluorobenzene, 17%). It is possible that the fluori-
nated aryl radical engages in other processes due to the lack of
efficient H donors. Chlorobenzene also demonstrated satisfac-
tory conversion (63% at 5 h) and selectivity (80%, Table 1,
entry 3).
Table 2
Substrate scope of photocatalytic hydrodehalogenation.
Substrate
Time (h)
Product
Yield (%)
90
Entry
1
4
2
6
37
3a
24
25, 50
,
4
5
6
6
4
70
41
72
Table 1
Photochemical reduction results of various halogenated substrates:
conversion and selectivity.
7.5
Conversion Selectivity
Entry
1
Substrate
Product
7
18
25
20
41
60
15
(%)
(%)
86
90
8
9b
2 a
3 b
91
63
26,17
80
,
10c
4
90
Conditions: 1 mM substrates and 2 mM TMPD in 20 mL of acetonitrile
solution, irradiated by a xenon lamp with a 360 nm cutoff filter (the
light intensity was 110 mW cm–2). Reaction was conducted under Ar
atmosphere at 298 K. Yields were determined by high-performance
liquid chromatography (HPLC) and ionic chromatography (IC).
Conditions: 1 mM of substrate, 0.05 equivalents of amine sensitizer, and
15 equivalents of DIPEA in 20 mL of acetonitrile solution, irradiated by
a xenon lamp with a 360 nm cut of filter (the light intensity was 180
mW cm–2). Reaction was conducted under Ar atmosphere at 298 K.
Yields were determined by HPLC and GC.
a The products were identified by 19F NMR and the yields were deter-
mined by GC. b TMB was used as the photocatalyst. c 4,4',4''-(1,3,5-tri-
azine-2,4,6-triyl)trianiline (TTA) was used as the photocatalyst.
a 4 mM TMPD. The products were identified by 19F NMR and the yields
b
were determined by GC. Product yields were determined by gas
chromatography.

Di Meng et al. / Chinese Journal of Catalysis 41 (2020) 1474–1479
1477
moderate to good product yields. The TON of these substrates
with TMPD ranged from 7.4 to 14.4. When TMB was employed
as the photocatalyst, a low dechlorinated product yield of 15%
was observed under the same conditions, despite extended
reaction times of 20 h (entry 9). This result could be attributed
to the weaker electron transfer driving force from TMB excited
states to the halogenated substrates, compared to TMPD, as
indicated by emission quenching dynamics. 4,4',4''-(1,3,5-tri-
azine-2,4,6-triyl)-tri-aniline (TTA), which demonstrates similar
absorption to TMPD, was also tested for its photocatalytic abil-
ity in the hydrodehalogenation of 4′-bromoacetophenone (Fig.
S8). The product yield after 4 h of irradiation reached 90%
(entry 10). These results demonstrate that amine-based sensi-
tizers are promising photocatalysts in light-driven organic
transformations.
We further investigated the performance of TMPD as a
photocatalyst in the debromination of 2,2,4,4-tetrabromo-
diphenyl ether (BDE47), a typical persistent organic pollutant
that is resistant to conventional reductive treatments
[10,35,36]. The removal of bromines in polybrominated di-
phenyl ethers with low bromine substitution is challenging, as
debromination becomes less thermodynamically favorable
with decreasing bromine atom numbers. Under photocatalytic
conditions, Pd cocatalysts are usually required to provide high
degrees of debromiantion [10,37,38]. It was impressive to find
that with 1 equivalent of catalyst and 2.5 v% DIPEA, BDE47
underwent facial debromination and 83% debromination effi-
ciency was realized after 30 h, producing the completely
debrominated product, diphenyl ether in 40% yield (Fig. S9).
The distribution of debrominated intermediates, e.g., the 4:1
ratio of 2,4,4′-tribromodiphenyl ether (BDE28) to
2,2′,4-tribromodiphenyl ether (BDE17), indicated a debromin-
ation preference for the ortho position, which is in accordance
with an electron transfer-triggered debromination [39,40].
Emission quenching kinetics and the results of photochem-
ical/photocatalytic hydrodehalogenation are consistent with
the cleavage of carbon halogen bonds via an electron transfer
from singlet excited states of TMPD or TMB. Control experi-
ments indicated that DIPEA had a negligible effect on the emis-
sion of TMPD (Fig. S10), which is different from the activation
of carbon-halogen bonds by the pre-photoreduced sensitizer in
pyrene derivatives, and fac-Ir(ppy)3 catalyzed hydrodehalo-
genation [1,16]. Analogous pathways have been reported for
3–
phenylphenothiazine and CeIIICl6 under near-UV or UVA irra-
diation [20]. The aromatic amines feature strongly reducing
excited states, and their electron transfer kinetics are one order
of magnitude faster than the activation-limited transfer from
CeIIICl6* to 4-F-C6H4X (X = Br, Cl, F, 0.9–3.0 × 109 M–1 s–1). Nota-
bly, the overall conversion is additionally limited by the compe-
tition of carbon-halogen bond cleavage of the aryl halide radical
anion intermediates, and the non-productive thermal back
electron transfer [6]. Nonetheless, the activation of C–Cl and
C–F bonds with readily available amines is an attractive alter-
native for selective hydrodehalogenation under mild condi-
tions, and in a sustainable manner. The development of related
compounds with suitable excited state energies and
HOMO-LUMO levels are expected to achieve visible light activi-
ty, rapid catalyst turnover and enhanced activity. These amine
sensitizers, in combination with a sencond light absorber could
be integrated into tandem photocatalytic systems, in order to
inhibit the back electron transfer, thus favoring the cleavage of
carbon-halogen bonds.
In summary, we demonstrated that excited states of amine
sensitizers enable diffusion-controlled electron transfer to ef-
fectively cleave a number of highly challenging carbon-halogen
bonds. Successful light-driven hydrodehalogenation of various
aromatic halides and organic pollutants was achieved in the
absence of hazardous reagents. This method is a practical and
cost-effective strategy, enabling exceptionally high activity.
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Graphical Abstract
Chin. J. Catal., 2020, 41: 1474–1479 doi: 10.1016/S1872-2067(20)63582-3
Light-driven activation of carbon-halogen bonds by readily
available amines for photocatalytic hydrodehalogenation
DiMeng, QianZhu, Yan Wei, ShengliZhen, Ran Duan,
Chuncheng Chen, WenjingSong *, Jincai Zhao
Institute of Chemistry, Chinese Academy of Sciences;
University of Chinese Academy of Sciences
Photocatalytic system based on readily available amine sensitizers
offers efficient hydrogenation of a variety of aromatic halides.

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基于芳香胺化合物的光催化脱卤加氢
孟 涤a,b,†, 朱 倩a,b,†,‡, 魏 燕a,b, 甄胜利c, 段 苒a, 陈春城a,b, 宋文静a,b,*, 赵进才a,b
a中国科学院化学研究所光化学重点实验室, 中国科学院分子科学科教融合卓越中心, 北京100190
b中国科学院大学, 北京100049
c北京高能时代环境技术股份有限公司, 北京100095
摘要: 近年来, 光催化活化碳卤键已经成为构筑新化学键的有力方法. 其基本原理在于利用光激发产生的高活性激发态或
中间物种, 经由电子转移过程实现碳卤键的断裂产生碳自由基和卤离子(C–X + e → C• + X–). 碳自由基通过加氢或者或
亲电进攻等途径完成反应, 实现碳卤键到碳氢、碳碳等化学键的转化. 目前已报道的用于活化碳卤键的光催化剂包括过渡
金属(如钌、铱)配合物, 过渡金属盐(CeIIICl63–)、有机小分子光(二氢苯二嗪、苯基吩噻嗪、苝酰亚胺以及芘类衍生物分子)
等.
本工作中我们开发了基于小分子芳香胺, 包括N,N,N’,N’-四甲基对苯二胺（TMPD）与N,N,N’,N’-四甲基联苯胺（TMB）
等作为光催化剂实现高惰性芳香卤代化合物脱卤加氢的催化体系. 荧光强度/寿命测试表明芳香胺的强还原性单重激发态
可通过扩散控制的电子转移实现惰性卤代底物（氯苯, 六氟苯等）中碳卤键的解离；并且原位顺磁共振直接观察到了这一
步骤产生的芳基自由基以及TMPD正离子自由基; 自由基捕获实验也为解离电子转移活化碳卤键提供了进一步的支持.
芳香胺分子在计量反应条件下可同时作为光敏剂和电子/氢给体可在紫外光照下(λ > 360 nm)实现芳香卤代化合物的
脱卤加氢, 并表现出较高的转化率和选择性:溴苯乙酮(86%, 90%)、六氟苯(91%, 五氟苯26%/1,2,4,5-四氟苯17%)、氯苯(63%,
80%). 引入N,N二异丙基乙二胺(DIPEA)作为电子给体, 能够还原芳香胺正离子自由基完成催化剂循环, DIPEA同时作为

Di Meng et al. / Chinese Journal of Catalysis 41 (2020) 1474–1479
1479
氢给体参与芳基自由基加氢. 我们在催化反应条件下(5 mol%芳香胺)调研了脱卤加氢的卤代底物范畴. 以TMPD为例, 对
溴苯乙酮光照4小时后可得到90%脱溴产物; 相对难还原的溴苯经过6小时光反应脱卤达到37%. 六氟苯经过24小时反应后
可以脱去1‒2个氟, 生成五氟苯(25%)和1,2,4,5-四氟苯(50%). 对氯苯甲酸甲酯在7.5小时反应后得到了72%的加氢产物；而
对于更难还原的氯苯两小时光照可生成约40%的脱氯产物(增加TMPD用量可进一步提高脱氯效率至63%). TMPD催化杂
环卤代化合物脱卤的效果也较好, 如3-溴噻吩(18 h, 41%)和3-溴-4甲基吡啶(25 h, 60%). 另外TMB和4,4',4''-(1,3,5-三嗪
-2,4,6-三基)三苯胺作为光催化剂也可实现脱卤加氢.
我们进一步研究了典型持久性有机污染物2,2,4,4-四溴联苯醚(BDE47)的光催化脱溴反应, 30小时光照后脱溴效率可
达83%, 并且产生了40%的全脱溴产物联苯醚. 脱溴中间产物分布表明邻位溴脱除速率高于对位溴. 这一结果与解离电子
转移活化碳卤键的机理一致.
总而言之, 我们的工作表明小分子芳香胺光敏剂的激发态能够通过扩散控制的电子转移活化碳卤键. 它们作为光催
化剂可实现多种有机卤化物的脱卤加氢.
关键词: 碳卤键活化; 光催化; 脱卤加氢; 卤代有机污染物; 环境修复
收稿日期: 2020-01-20. 接受日期: 2020-03-05. 出版日期: 2020-10-05.
*通讯联系人. 电话: (010)82615942; 传真: (010)82612075; 电子信箱: wsongunc@iccas.ac.cn
†相同贡献作者.
‡现地址: 北京高能时代环境技术股份有限公司, 北京100095
基金来源: 国家重点研发计划 (2018YFA0209302); 国家自然科学基金(21590811, 21677148, 21827809, 21922609); 中国科学院
前沿科学重点研究计划(QYZDY-SSW-SLH028).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).