Ceramic boron carbonitrides for unlocking organic halides with visible light

Article abstract of DOI:10.1039/d1sc01028j

Photochemistry provides a sustainable pathway for organic transformations by inducing radical intermediates from substrates through electron transfer process. However, progress is limited by heterogeneous photocatalysts that are required to be efficient, stable, and inexpensive for long-term operation with easy recyclability and product separation. Here, we report that boron carbonitride (BCN) ceramics are such a system and can reduce organic halides, including (het)aryl and alkyl halides, with visible light irradiation. Cross-coupling of halides to afford new C-H, C-C, and C-S bonds can proceed at ambient reaction conditions. Hydrogen, (het)aryl, and sulfonyl groups were introduced into the arenes and heteroarenes at the designed positions by means of mesolytic C-X (carbon-halogen) bond cleavage in the absence of any metal-based catalysts or ligands. BCN can be used not only for half reactions, like reduction reactions with a sacrificial agent, but also redox reactions through oxidative and reductive interfacial electron transfer. The BCN photocatalyst shows tolerance to different substituents and conserved activity after five recycles. The apparent metal-free system opens new opportunities for a wide range of organic catalysts using light energy and sustainable materials, which are metal-free, inexpensive and stable. This journal is

Full text of DOI:10.1039/d1sc01028j

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Ceramic boron carbonitrides for unlocking organic
halides with visible light†
Cite this: Chem. Sci., 2021, 12, 6323
a
a
b
a
*
*
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Tao Yuan, Meifang Zheng, Markus Antonietti
and Xinchen Wang
Photochemistry provides
a sustainable pathway for organic transformations by inducing radical
intermediates from substrates through electron transfer process. However, progress is limited by
heterogeneous photocatalysts that are required to be efficient, stable, and inexpensive for long-term
operation with easy recyclability and product separation. Here, we report that boron carbonitride (BCN)
ceramics are such a system and can reduce organic halides, including (het)aryl and alkyl halides, with
visible light irradiation. Cross-coupling of halides to afford new C–H, C–C, and C–S bonds can proceed
at ambient reaction conditions. Hydrogen, (het)aryl, and sulfonyl groups were introduced into the arenes
and heteroarenes at the designed positions by means of mesolytic C–X (carbon–halogen) bond
cleavage in the absence of any metal-based catalysts or ligands. BCN can be used not only for half
reactions, like reduction reactions with a sacrificial agent, but also redox reactions through oxidative and
reductive interfacial electron transfer. The BCN photocatalyst shows tolerance to different substituents
and conserved activity after five recycles. The apparent metal-free system opens new opportunities for
a wide range of organic catalysts using light energy and sustainable materials, which are metal-free,
inexpensive and stable.
Received 22nd February 2021
Accepted 22nd March 2021
DOI: 10.1039/d1sc01028j
rsc.li/chemical-science
In contrast to homogeneous catalysts, heterogeneous cata-
lysts feature the advantage of easy recyclability by simple
Introduction
ltration due to their insoluble nature, while still conserving
reactivity during prolonged photochemical operation. The
electron transfer mechanism of heterogeneous semiconductor
photocatalysts is otherwise similar to that of homogeneous
photosensitizers, as illustrated in Fig. 1.9 In the case of molec-
ular photosensitizers, one electron transfers to the LUMO
(lowest unoccupied molecular orbital) upon light excitation and
then transfers to the substrates.10 To close the catalytic cycle,
reductive or oxidative quenching is required to bring the
photosensitizer back to the ground state (Fig. 1a). For semi-
conductor materials, electron–hole pairs are produced under
light irradiation. Similarly to the frontier orbital theory, the CB
(conduction band) can donate electrons (like the LUMO),
forming reductive intermediates, and the VB (valence band) can
act as an electron acceptor (like the HOMO), forming oxidative
intermediates, which together participate in yielding the nal
products (Fig. 1b).11 When considering a semiconductor for
certain transformations thermodynamically, the light absorp-
tion and the related magnitude of the bandgap are important,
as well as the absolute redox potentials (the positions of the VB
and CB) with respect to those of the targeted substrates.12
Electron transfer to and from the substrate must be thermo-
dynamically allowed, otherwise the reaction cannot take place.
Semiconductor photocatalysts have been broadly investi-
gated and applied to the water splitting reaction aiming to
mitigate the issues of the impending energy shortage and the
The renaissance of chemistry with radicals in organic synthesis
is also related to the revival of photochemistry.1 The growth of
the eld comes from the direct conversion of solar energy to
chemical energy, thus enabling mild conversions, even uphill
reactions, as well as efficient access to open shell reactive
intermediates.2 Photocatalysts, which can be classied into
transition metal-based complexes, organic dyes and semi-
conductors, have all been widely applied in a range of organic
transformations with light irradiation.3 Most previous contri-
butions used homogeneous photosensitizers, for example,
Ru(ppy)3Cl2,1 fac-Ir(ppy)3,3 eosin Y,4 and acridinium salts.5
Previous reports on homogeneous molecular photosensitizers
are inevitably limited by photo-corrosion as well as catalyst
deactivation due to the strong interaction with strong
nucleophiles/electrophiles and reactive radical intermedi-
ates.5–8 Besides photobleaching, there is also a general concern
about the sustainability and price of widely-used metal
complexes containing precious metals, such as Ru and Ir, as
well as related health risks.
aState Key Laboratory of Photocatalysis on Energy and Environment, College of
Chemistry, Fuzhou University, Fuzhou 350116, China. E-mail: mfzheng@fzu.edu.cn;
xcwang@fzu.edu.cn
bMax-Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry,
Research Campus Golm, 14424 Potsdam, Germany
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1sc01028j
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Fig. 1 Schematic illustrations of molecular and semiconductor photoredox catalytic electron transfer modes in the activation of substrates and
the specific process of photochemical organic transformations.
utilization of light energy.13–16 However, the developed metal- transformations have been made with BCN. We herein report
based semiconductors, such as metal oxides, (oxy)nitrides and the application of ceramic BCN as heterogeneous photoredox
(oxy)suldes, are either visible light-irresponsive or photo- catalysts for the radical coupling of organic halides using visible
corrosive, and the involved precious metals compromise the light. Over the past decades, signicant advances in the use of
sustainability. Metal-free semiconductors, for example, conju- halides in cross-coupling have been made in organic chemistry,
gated microporous polymers (CMPs),14 covalent organic usually under the involvement of transition metal complexes,
frameworks (COFs),17,18 and graphitic carbon nitrides (g-CN),15,19 enabling the conversion of C–X (carbon–halogen) into carbon–
are the emerging, superior heterogeneous photocatalysts for carbon, –nitrogen, –sulfur, and –oxygen bonds, such as the
light energy conversion and environmental remediation. The Suzuki, Ullman, and Kumada reactions.33–35 The interest in
ceramic semiconductor boron carbonitride (BCN) has just developing mild conditions has propelled new advances in the
recently been developed but is already well-known in the area of transformation of halides and has made photocatalytic tech-
articial photosynthesis.20–22 BCN materials are inorganic niques an attractive alternative.36–38 Reports exemplied fac-
semiconductor photocatalysts, which in principle feature Ir(ppy)3 as a photosensitizer, which converted non-activated C–I
a lower binding energy of excitons and faster charge migration into C–H bonds via photoinduced electrons or energy transfer
properties than polymeric photocatalysts.23 Analogous to processes to substrates under visible light irradiation.39 The
hexagonal boron nitride (h-BN), BCN exhibits excellent MacMillan group reported that combination of iridium
adsorption properties for organic molecules because of the complexes and nickel organocatalysts can drive the coupling of
existence of the polar B–N bond and the high specic surface C(sp2/sp3)–Br with C(sp2/sp3)–H (Fig. 1c).36,40,41 Despite the
area.24–26 BCN ceramics are visible-light-responsive semi- widespread success of transition-metal-catalyzed coupling
conductors with an adjustable band gap energy and energy methodologies, “metal residue” is a concern in the synthesis of
levels depending on the amount of carbon incorporated into the medicinally important or biologically active compounds, and
boron nitride (BN) frameworks.27–29 The ternary BCN materials realizing a metal-free methodology has aroused extensive
allow a tunable band gap energy in a wide range of 0–5.5 eV by research interest. To get rid of “metal residues” in the nal
varying the concentration of B, C and N, which indeed enables products, reduction of electron-decient aryl halides has been
extended use of light irradiation to run the organic reac- achieved with PDI (perylene-diimide),42 Rh-6G,43 phenolate
tions.30–32 In addition, the cheap and simple preparation also anion,44 phenothiazine45 etc. under visible light illumination in
makes BCN (a few euros per kilogram) a promising alternative homogeneous systems. Here we show that the ceramic BCN
as
a
metal-free heterogeneous photocatalyst for organic semiconductors can efficiently catalyze the general coupling of
halides in heterogenous and metal-free systems under mild
photosynthesis.
Despite possessing obvious advantages including easy reaction conditions, with good to excellent selectivity and yields
separation, high photo/thermal/chemical stability, and recy- in a gram-scalable manner (Fig. 1d).
clable features, only
a
few explorations of organic
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determined to be +1.58 V vs. SCE, see ESI, Fig. S7†). iPrOH acts as
both the hydrogen and electron donor (Eox ¼ +1.27 V vs. SCE, see
ESI, Fig. S13†), which was conrmed by the deuteration experiment
(Fig. S22†). The aryl radical intermediate could be captured by
TEMPO, demonstrating the existence of a single-electron-transfer
(SET) process in our metal-free heterogeneous system (Fig. S23†).
Aryl bromides with methoxy and methyl substituents at different
positions provided the corresponding arenes in good to excellent
yields (2a–2f) (Table 1a). A 58% yield of 2g was isolated, revealing
methylthio substituent tolerance. 4-Bromobiphenyl was reduced to
biphenyl (2h) in 95% yield, and the substituted naphthalenes
showed excellent yields (2i–2j) (Table 1a). Besides, ketone, ester,
and carboxyl functionalities were maintained with good to excel-
lent yields (2k–2q) (Table 1a). Moreover, heteroarenes such as
benzothiophene (2r), carbazole (2s), quinoline (2t), and 1,4-benzo-
dioxan (2u) can be achieved from the reduction of the corre-
sponding bromides with good to excellent yields (Table 1a). In
addition, the system is also feasible for dechlorination, which is
a thermodynamically challenging process. Pleasingly, hydro-
dechlorination of 4-chloroanisole occurred smoothly leading to an
87% yield of 2v. An 82% yield of 2w was isolated using 2-bromo-1,4-
Results
Hydrodehalogenations of (het)aryl and alkyl halides
Hydrodehalogenation is a kind of reaction in which a halogen
atom is formally substituted by a hydrogen atom.46 It not only
serves as a deprotection toolkit in the multistep synthesis of
complex natural products, but also applies to the degradation of
articial and environmentally toxic halogen-containing organic
pollutants, such as residual pesticides, hard-metabolizable phar-
maceuticals, and re retardants.47 Herein, BCN was chosen as
a dehalogenation photocatalyst because of its high surface area, its
morphology of porous, two-dimensional nanosheets, and its ability
for light absorption.20 Control experiments demonstrated that the
presence of light and BCN are crucial for the reaction to occur, as
no product 2a was detected in the absence of light or photocatalyst
(Table S1,† entries 9–10). Other metal-free semiconductors, such as
polymeric carbon nitride (CNU and mpg-CN), have been previously
reported for some oxidation reactions19,48 and the C–N cross-
coupling of aryl bromides with the Ni complex cocatalyst,49
however, they were not able to photocatalyze the hydro-
dehalogenation reactions successfully with visible light (Table S1,†
entries 12–13).
dimethoxybenzene
as
the
substrate.
2-Hydroxy-
benzoicacimethylester (2x), a natural perfume, was obtained in
70% isolated yield under a prolonged irradiation time (Table 1b).
The analyzed cases of BCN photocatalytic dehalogenation
are shown in Table 1 (the valence band position of BCN was
Table 1 Scope of the photocatalytic hydrodehalogenations by means of reductive radical processa
a
The reaction was conducted with 10 mg BCN, 0.2 mmol halide 1 and 6 mL iPrOH at 40 ꢀC under N2 atmosphere, the isolated yield and reaction
time are given in parentheses. b 0.3 mmol of 2-bromo-1-phenylethanone was added. c 20 mg of BCN was used. d 2 equivalents of K2CO3 were added.
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Aryl chloride derivatives with electron-withdrawing groups also cyclization of inactive o-brominated alkylaryl ethers proceeded
went through the reduction reaction successfully with excellent smoothly, and dihydrobenzofuran (3a) and dihydrobenzopyran
yields of 2y (87%), 2z (93%), and 2aa (96%), respectively (Table 1b). (3b) were obtained with satisfying isolated yields in 72 h
1-Chloro-4-phenylbutane could be reduced to 2bb with a 36% yield (Fig. 2a). This success of the intramolecular coupling under
of 2bb in this system (Fig. S12†). However, trace amounts of photoreductive conditions inspired the idea of exploring other
quinoline (<5%, 2cc) were detected aer 15 h of illumination cross-coupling reactions.
(Table 1b).
Biaryl structures are ubiquitous motifs in pharmaceutical
molecules, biochemistry, and polymeric materials.50–52 In the
previous dehalogenation examples, the photo-generated holes
were consumed and removed from the reaction by sacricial
agents. To get full utilization of all charge carriers, arenes were
C(sp2)–H (het)arylations with C(sp2)–Br
The metal-free BCN photocatalyst was efficient for the coupling
of C(sp2)–H and C(sp2)–Br as well (Fig. 2a). Intramolecular
Fig. 2 Photoredox coupling of organic halides with (het)arenes and sulfinates to afford new C–C and C–S bonds using the ceramic BCN
photocatalyst. Detailed reaction conditions can be found in Methods (see General procedure 2–4 in this article). It should be noted that the yields
of 5x–5x0 and 7s–7v were determined by GC-MS.
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introduced into the system, leading to the formation of heter- to optimize the reaction conditions (Table S3†). The reaction is
oarene cation radicals, resulting in the C–C cross-coupling thermodynamically favorable due to the valence band (VB)
biaryl motifs. To get rid of the hydrogenation products, the position of BCN (+1.58 V vs. SCE) outdistancing the oxidation
aprotic solvent DMSO was used instead of protic solvents. The potential of sodium benzenesulnate (+0.37 V vs. SCE, Table
valence band (VB) of BCN is +1.58 V vs. SCE (Fig. S7†), which is S4†). According to the results, BCN had a product yield of 51%
thermodynamically capable of oxidizing 1,3,5-trimethox- at 24 h under 3 W blue LED irradiation. Polymeric carbon
ybenzene (4a, E4a/4ac+ ¼ +1.34 V vs. SCE, Table S4†) to generate nitrides (CNU, mpg-CN, and CCN) as reference metal-free
an arene cation radical. With the optimized reaction condition, photocatalysts did not give the desire sulfonyl product (Table
a 76% yield of 5a was obtained (Table S2†). Examples of direct S3†). With prolonged illumination time, a satisfactory yield of
C(sp2)–H (het)arylations using C(sp2)–Br as nucleophilic agents 7a (78%) was obtained using BCN as the photocatalyst under
are displayed in Fig. 2b. Aryl bromide derivatives with cyano or mild conditions.
triuoromethyl substituents, coupling with 1,3,5-trimethox-
Examples of C(sp2/sp3)–Br sulfonylations using sulnates as
ybenzene (4a), afforded good to excellent yields (5a–5c). The coupling reagents are depicted in Fig. 2c. Diverse sulfone motifs
mild and free-of-base-and-metal system shed light on the were obtained via BCN photoredox catalysis. For aryl bromides,
maintenance of the aldehyde, ester, and amido groups, and the protocol could tolerate a variety of functional groups,
moderate yields of products (5d–5g) were obtained (Fig. 2b). including cyano (7a, 7b), triuoromethyl (7c), aldehyde (7d),
Bromobenzene can couple with C(sp2)–H with BCN as the and ester (7e) groups (Fig. 2c). Bromobenzene can be the
photocatalyst (5h). Electron-rich aryl halides proceeded with coupling partner with a 39% yield of sulfonyldibenzene (7f), as
relatively low yields, as shown in Fig. 2d (5x–5x0, Fig. S14–S17†). well as a p-bromopyridine motif (7g). Moreover, the C(sp3)-
Except for the arenes, heteroaromatic derivatives are reactive as centered reactant gave the corresponding product with an
well, and the corresponding products were achieved in good excellent yield of 93% (7h). Electron-rich aryl bromides resulted
yields (5i and 5j) (Fig. 2b).
in low yields, as shown in Fig. 2d (7s–7v, Fig. S18–S21†). A wide
Next, the scope of C(sp2)–H was explored. When 1,3-dime- range of structurally diverse sulfonyl reagents were tolerant for
thoxybenzene was used as an aromatic substrate, the coupling this transformation. Sodium sulnates with phenyl and alkyl
products at the a and b-position (5k) were obtained in a 73% groups went through the sulfonyl process smoothly with good
yield with 20 : 13 of the mixture (Fig. 2b). Anisole and 1,3,5- yields (7i–7k). Noteworthily, sodium triuoromethanesulnate
trimethylbenzene can also react with 4-bromobenzonitrile (1b), can retain the sulfonyl group, and an 80% yield of sulfonylation
affording biaryl compounds 5l and 5m with moderate yields of product was formed (7l). Besides, haloarenes were compatible
51% and 47%. Moreover, a- and b-position substituted aryl with the reaction conditions, leading to the corresponding
naphthalenes were successfully synthesized with the present products (7m–7o) in moderate to good yields. Alkyl and
metal-free system (5n). Among them, the cyano-containing heterocyclic sulfones were also afforded (7p and 7q). In addi-
compound 5n-a is a direct precursor for the one-pot synthesis tion, this protocol was applicable to aryl chloride with a 56%
of 2,4,6-tris(4(naphthalen-1-yl)phenyl)-1,3,5-triazine (TNPT), yield (7r).
which is valuable as an excellent hole blocking layer (HBL)
material in long lifetime organic light emitting diodes
(OLEDs).53 Several heteroarenes went through cross-coupling
Discussion
successfully, and heterocyclic C(sp2)–H arylation products For the mechanistic studies, a series of monochromatic light
were isolated with general yields of 33–62% (5o–5s). Biologically experiments with different wavelengths were carried out.
relevant pyrrole derivatives were also compatible with the Apparently, the trend of the wavelength-dependent yields of 5a
reaction conditions, producing the het-aryl and het–het biaryl matches well with the optical absorption intensity of BCN,
motifs in generally excellent yields (5t and 5u). These unique supporting the photo-induced nature of the process (Fig. 3a).55
structures could serve as the backbone units of drug mole- The variation of the concentration of reactant 1b, C–H arylation
cules.54 Polycyclic biaryl structures could be acquired with product 5a, and hydrodehalogenation by-product during the
moderate yields as well (5v and 5w).
photoredox reaction is shown in Fig. 3b. The concentration of
1b declined gradually while the arylation product 5a steadily
increased. Aer 24 h irradiation, 1b was completely consumed.
The hydrodehalogenation by-product was minor but unavoid-
C(sp2/sp3)–Br sulfonylations using sulnates
The introduction of sulfonyl groups could enhance the surface able. In addition, the relative reactivities of different para-
hydrophilicity and/or decrease the rates of enzymatic metabo- substituted aryl bromides toward C(sp2)–H arylation were
lization, improving the pharmacokinetic properties of lead examined (Fig. 3c and Table S5†). A positive linear correlation
candidates in drug discovery.55 Several reports employed between the calculated log(kR/kH) values and sp constants was
homogeneous nickel/photoredox dual catalysis to introduce observed from the Hammett plot analysis, disclosing that the
sulfonyl groups by cross-coupling of sodium sulfonates with reaction proceeds through nucleophilic aromatic substitution.58
halides.56,57 It is mild and odorous thiols and strong oxidizing Besides, the recovered BCN could be used for the corresponding
reagents can be avoided. Herein, a metal-free photocatalytic reactions for ve repeats, with slightly decreased yields. This
system was introduced to install sulfonyl motifs into aryls. illustrates the photo and chemical stability of the photocatalyst
Solvents and a series of metal-free catalysts were screened rst (Fig. 3d), which was further conrmed by structural
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Fig. 3 (a) Wavelength-dependent yields of C(sp2)–H arylation product (5a), the reaction time was 12 h. (b) Variation of the amount of 5a, the
hydrodehalogenation by-product and reactant 1b during the C(sp2)–H arylation reaction. (c) Hammett plots for the arylation of C(sp2)–H with
substituted aryl bromides. Hammett plots were obtained from a ratio of the yield with a reaction time of 24 h. (d) Evaluation of catalysts recycling
for different reactions (dehalogenation, C–H arylation, and C–Br sulfonylation performed in a sequence yielding 2a, 5a, and 7a, respectively)
under standard conditions, respectively. Upscaling experiments, gram-scale preparation of 7c using ceramic BCN as the photocatalyst are
displayed in (e–g): (e) the required reagents and reaction flask; (f) the photochemical reaction device under blue LED irradiation; and (g) the
recovered BCN and isolated product 7c.
characterizations before and aer the reaction (Fig. S30 and and the gram-scale reaction for the preparation of 7c proceeded
S31†), essentially reecting no change. Thus, the photochem- smoothly with an isolated yield of 91% (Fig. 3e–g, also see
ical stabilities of BCN allows easy scaling to gram quantities, Fig. S9†).
Fig. 4 The mechanistic insights of C(sp2)–H arylations with C(sp2)–Br. (a) The intermolecular KIE experiment. EPR measurements of DMSO
solution of DMPO and BCN with addition of (b) 4a and (c) 1b/4a mixture upon 3 min visible light irradiation. (d) EPR spectrum of measurement
results.
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Fig. 5 The proposed mechanism for (a) hydrodehalogenation, (b) intramolecular cyclization, (c) C–H (het)arylation, and (d) C–Br sulfonylation to
form new C–H, C–C, and C–S bonds by BCN under visible light.
Kinetic isotope effect (KIE) and radical capture experiments addition of 1,1-diphenylethylene, the sulfonylation process was
were conducted to further characterize the metal-free C(sp2)–H prohibited, meanwhile, the sulfonyl and aryl radicals were
arylation process. As shown in Fig. 4a and S24,† the value of KIE captured by 1,1-diphenylethylene, with molecular weights of
was 1.25 (KH : KD), suggesting that the C–H bonds cleavage of 258 and 281, respectively (Fig. S27c and S28†). The results
arenes is not the rate-determining step. In addition, the yield of indicated the involvement of both photogenerated-hole oxida-
5a declined sharply when two equivalents of 2,2,6,6-tetrame- tion and photogenerated-electron reduction.
thylpiperidinoxyl (TEMPO) were added as the radical scavenger,
Based on the above experimental results, we put forward
and the radical intermediate from 4a was trapped by TEMPO a possible reaction mechanism for C–X couplings, as illustrated
with the molecular weight of the adduct being detected by GC- in Fig. 5. Upon visible light irradiation, BCN is excited to
MS (Fig. S25†), proving the oxidation step of arenes by BCN. generate photo-induced electrons at the conduction band (CB,
Thereaer, EPR study using 5,5-dimethyl-1-pyrroline N-oxide ꢁ1.09 V vs. SCE) and holes at the valence band (VB, +1.58 V vs.
(DMPO) as the radical scavenger further proved the formation SCE). Then the photogenerated electron–hole pair separates
of a carbon-centered radical of 4a in the mixture, and the signal and migrates to the surface of the photocatalyst. For the
values (aN ¼ 15.2, aH ¼ 24.9) were nearly identical to the re- hydrogenation or cyclization process, the hole oxidizes sacri-
ported values of the carbon-centered radical (Fig. 4b–d).59
cial agent 1A with the release of a proton and electron to afford
Several control experiments were also carried out for the C– 1B, while the electron on the CB reduces the C–X substrate (1C
Br sulfonylation reaction. The reaction was restrained with or 2C) to form the active intermediate 1D or 2D. Subsequently,
a product yield of 14% under the open air (Fig. S27a†), indi- mesolytic C–X cleavage generates a halogen anion and (het)aryl
cating that the presence of oxygen is detrimental to the reaction. radical (1E or 2E). The radical (1E or 2E) then accepts one
Additionally, the yield declined with the loading amount of electron and one proton with the formation of a hydrogenation
TEMPO (the yield declined to minor with 5 equivalents of or cyclization product (Fig. 5a and b). Moreover, the reaction
TEMPO, Fig. S27b†), suggesting a radical pathway. With the might be further thermodynamically favored through a proton-
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coupled electron transfer (PCET) process that overcomes the mg), iPrOH (6 mL) and halide (Br or Cl, 0.2 mmol), then the tube
disfavored energetic limitation.60,61 For the construction of C–C was degassed in vacuo and relled with N2 5 times. With the
and C–S bonds, the sacricial agent is replaced with the corre- purple LED (420 nm, 15 W) switched on, the reaction mixture
sponding synthons – (het)arene and sulnate (3A and 4A). was stirred for a certain time (noted in the text) at 40 ꢀC until the
Aerward, the oxidative electron transfer involves these reactant was fully consumed which was determined by GC-MS.
substrates (3A and 4A) with the formation of an arene cation Aer the reaction, the mixture was ltered and extracted by
radical (3B) or sulfonyl radical (4B0). Aryl bromide 3C then CH2Cl2, the solvent was removed under reduced pressure, and
couples with 3B to form the intermediate 3D followed by the the residue was puried by column chromatography on silica
subsequent reduction by the electron on the CB, nally gel using petroleum ether/ethyl acetate and pentane/diethyl
releasing the bromide anion and C–C coupling product ether as the eluent to afford the nal hydrodehalogenation
(Fig. 5c). Alternative possible mechanisms for C–H (het)aryla- product.
tions are initiated with reductive cleavage of C–Br bond
(Fig. S26†). Different from C–C coupling, the C–X substrate (4C)
General procedure 2
accepts an electron and develops to an activated radical anion
Intramolecular cyclizations of inactive o-alkenyl bromo-
(4D) through reductive electron transfer. The coupling of two
benzenes. Typically, to a Schlenk tube containing a stirring bar
was added BCN (20 mg), K2CO3 (55 mg, 2 equiv.), iPrOH (6 mL)
and reactant o-alkenyl bromobenzene (0.2 mmol), then the tube
was degassed in vacuo and relled with N2 5 times. With the
radicals (4D and 4B0) affords the nal C–S coupling product
(Fig. 5d).
Conclusions
purple LED (420 nm, 15 W) switched on, the reaction mixture
was stirred for 72 h at 40 ꢀC, and the reaction progress was
The ceramic BCN semiconductor was introduced as a visible-
monitored by TLC or GC-MS. Aer the reaction, the mixture was
light-responsive photocatalyst for the coupling of halides with
ltered and extracted by CH2Cl2, the solvent was removed under
various synthons. A wide range of organic halides including
reduced pressure, and the residue was puried by column
aryl-, alkyl-, bromides and chlorides can be effectively reduced
chromatography on silica gel using pure petroleum ether as the
to carbon-centered radicals in good to excellent yields and with
eluent to afford the nal intramolecular cyclized product.
good substituent tolerance. The good performance is due to the
accessible photocatalytic sites of BCN. In this system, hydro-
General procedure 3
genations take advantage of electron reduction and coupling
reactions orchestrate oxidative and reductive electron transfer.
All these are achieved free of any transition-metal catalysts or
ligands. The protocol provides an ideal alternative for the
construction of C–H, C–C, and C–S bonds to classical transition
metal catalysts and organic dyes. We believe that the generality
of this methodology will be useful also for other organic
coupling chemistry. The protocol provides an avenue to organic
synthesis in an environment-friendly, atom-economic, and
sustainable manner, which may be valuable for large scale
synthesis or even industrial production.
C(sp2)–H (het)arylations with C(sp2)–Br. To a Schlenk tube
containing a stirring bar was added BCN (20 mg), (het)aryl
bromide (0.2 mmol), and (het)arenes (20 equiv.), then DMSO (2
mL) was added to the mixture. Aer that, the tube was degassed
in vacuo and relled with N2 for 5 times. With the purple LED
(420 nm, 15 W) switched on, the reaction mixture was stirred for
24 h at 40 ꢀC, and the reaction progress was monitored by TLC
or GC-MS. Aer the reaction, the mixture was ltered and
extracted with ethyl acetate several times, the organic layers
were combined and dried over anhydrous Na2SO4, the solvent
was removed under reduced pressure, and the residue was
puried by column chromatography on silica gel using petro-
leum ether/ethyl acetate as the eluent to afford the nal C–C
cross-coupling product.
Methods
Synthesis of ceramic BCN
Typically, urea (1.6 g), boric acid (0.8 g), and glucose (5.6 g) were
ground fully with an agate mortar for 20 min. Then, 3 g KCl was
poured into the mortar and the mixture was ground again for
10 min. Aerwards, the mixed precursor was put into a hori-
zontal tube furnace. Ammonia was pumped into the tube for
20 min to expel air before heating up. Then the mixture was
heated to 1250 ꢀC for 5 h at a heating rate of 5 ꢀC minꢁ1 under
a ow of ammonia (200 mL minꢁ1). The obtained product was
washed with 200 mL water 5 times and dried overnight at 80 ꢀC
in a vacuum oven. The resulting nal sample was denoted BCN.
General procedure 4
C(sp2/sp3)–Br sulfonylations using sulnates. To a Schlenk tube
containing a stirring bar was added BCN (10 mg), (het)aryl
halide (Br or Cl, 0.2 mmol), and sulnate (1 mmol, 5 equiv.),
then DMSO (2 mL) was added to the mixture. Aer that, the tube
was degassed in vacuo and relled with N2 5 times. With the
blue LED (455 nm, 3 W) switched on, the reaction mixture was
ꢀ
stirred for 72 h at 25 C and the reaction progress was moni-
tored by TLC or GC-MS. Aer the reactions, the mixture was
ltered and extracted with ethyl acetate several times, the
organic layers were combined and dried over anhydrous
General procedure 1
Hydrodehalogenation of (het)aryl and alkyl halides. To Na2SO4, the solvent was removed under reduced pressure, and
a Schlenk tube containing a stirring bar was added BCN (10 the residue was puried by column chromatography on silica
6330 | Chem. Sci., 2021, 12, 6323–6332
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gel using petroleum ether/ethyl acetate as the eluent to afford 13 C. M. Wolff, P. D. Frischmann, M. Schulze, B. J. Bohn,
the nal C–S cross-coupling product.
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Author contributions
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H. Lu, D. Wang, L. C. da Silva, R. Li, L. Wang,
K. Landfester and K. A. I. Zhang, Angew. Chem., Int. Ed.,
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X. W. and M. Z. conceived and directed the project. M. Z. and T.
Y. designed the experiments. T. Y. performed and analyzed the
experiments. T. Y., M. Z., M. A. and X. W. prepared the manu-
script. All authors contributed to the analysis and interpretation
of the data and commented on the nal dra of the manuscript.
16 Y. X. Fang, Y. Zheng, T. Fang, Y. Chen, Y. D. Zhu, Q. Liang,
H. Sheng, Z. S. Li, C. C. Chen and X. C. Wang, Sci. China:
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Conflicts of interest
There are no competing interests.
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (U1905214, 21961142019,
22071026, 22032002, 21702030, 21761132002, 21861130353,
21902031, and 21425309), the National Key Technologies R & D
Program of China (2018YFA0209301), the National Basic
Research Program of China (2013CB632405), the Chang Jiang
Scholars Program of China (T2016147), and the 111 Project
(D16008).
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6332 | Chem. Sci., 2021, 12, 6323–6332
© 2021 The Author(s). Published by the Royal Society of Chemistry