Photo-induced thiolate catalytic activation of inert Caryl-hetero bonds for radical borylation

Article abstract of DOI:10.1016/j.chempr.2021.04.016

Substantial effort is currently being devoted to obtaining photoredox catalysts with high redox power. Yet, it remains challenging to apply the currently established methods to the activation of bonds with high bond dissociation energy and to substrates with high reduction potentials. Herein, we introduce a novel photocatalytic strategy for the activation of inert substituted arenes for aryl borylation by using thiolate as a catalyst. This catalytic system exhibits strong reducing ability and engages non-activated C_aryl–F, C_aryl–X, C_aryl–O, C_aryl–N, and C_aryl–S bonds in productive radical borylation reactions, thus expanding the available aryl radical precursor scope. Despite its high reducing power, the method has a broad substrate scope and good functional-group tolerance. Spectroscopic investigations and control experiments suggest the formation of a charge-transfer complex as the key step to activate the substrates.

Full text of DOI:10.1016/j.chempr.2021.04.016

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Photo-induced thiolate catalytic activation of
inert Caryl-hetero bonds for radical borylation
Shun Wang, Hua Wang,
Burkhard Konig
¨
burkhard.koenig@ur.de
Highlights
Highly reducing photocatalytic
system
Inexpensive organocatalyst
Borylation of strong C–F, C–O, C–
N, and C–S bonds
Various leaving groups and >50
examples
An unprecedented photo-induced thiolate-catalytic strategy for ipso-borylation of
strong Caryl-hetero bonds has been developed. This approach provides a platform
for activation of strong chemical bonds (C–F, C–O, C–X, C–N, and C–S) and their
transformation into synthetically valuable boronates for a broad substrate scope
with high functional-group compatibility. Mechanistic studies suggest that the
unique reactivity of the reaction stems from charge-transfer complex formation
between thiolate catalyst and substrates.
Wang et al., Chem 7, 1653–1665
June 10, 2021 ª 2021 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2021.04.016

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Photo-induced thiolate catalytic
activation of inert Caryl-hetero
bonds for radical borylation
Shun Wang,1 Hua Wang,1 and Burkhard Konig1,2,
¨
*
SUMMARY
The bigger picture
Cross-coupling reactions are
essential tools in the modern
synthesis of drugs, natural
Substantial effort is currently being devoted to obtaining photore-
dox catalysts with high redox power. Yet, it remains challenging to
apply the currently established methods to the activation of bonds
with high bond dissociation energy and to substrates with high
reduction potentials. Herein, we introduce a novel photocatalytic
strategy for the activation of inert substituted arenes for aryl boryla-
tion by using thiolate as a catalyst. This catalytic system exhibits
strong reducing ability and engages non-activated Caryl–F, Caryl–X,
products, and materials. The
recent developments in
photocatalytic radical generation
have improved and expanded the
classic metal-catalyzed cross-
coupling reactions even further.
However, sp2 cross-coupling
reactions require aryl halides or
related active leaving groups,
such as triflates. Substituted
arenes bearing strong C–X bonds
remain inert to current methods.
Now, we describe a new thiolate
photocatalysis for the activation of
inert substituted arenes in ipso-
borylation reactions. This catalytic
system exhibits strong reducing
power and allows the borylation of
stable Caryl–F, Caryl–O, Caryl–N,
and Caryl–S bonds, which are
considered chemically stable at
mild reaction conditions. Our
method considerably widens the
available substrate scope of aryl
radical precursors, and we
C
aryl–O, Caryl–N, and Caryl–S bonds in productive radical borylation
reactions, thus expanding the available aryl radical precursor scope.
Despite its high reducing power, the method has a broad substrate
scope and good functional-group tolerance. Spectroscopic investi-
gations and control experiments suggest the formation of a
charge-transfer complex as the key step to activate the substrates.
INTRODUCTION
Photoredox catalysis has emerged as a powerful technique in organic synthesis over the
pastdecadethrough convertingtheenergyofphotonsintoredox potentialsforchemical
processes. This empowers organic chemists to realize a number of unprecedented
chemical transformations under mild conditions for organic synthesis.1–6 However, the
range of attainable redox potentials in a typical photocatalysis system is governed by
the energy of the absorbed photon. Intersystem crossing and non-radiative pathways
diminished this energy inevitably.7 Thus, it is challenging to activate high redox power
demanding substrates by visible-light irradiation. For instance, reductive transforma-
tions, particularly those that require highly reducing power, remain an underdeveloped
field. To this end, considerable efforts have been devoted toward the development of
new photocatalytic systems with a high reduction ability over the past few years. To over-
come the thermodynamic limits, several approaches to accumulate the energy of two or
more photons enhancing the reducing power of a photocatalyst have been estab-
lished.7–10 Wehavereportedaconsecutivephoto-inducedelectrontransfer(conPET)uti-
lizingtwo photons viaexcitation ofphotogeneratedradical anions(Scheme1A, equation
1).11–13 Likewise, photoexcitation of electrochemically generated radical anions was
shown to be capable of generating high reduction power.14,15 Nicewicz et al. showed
that the acridine radical ACR, could act as an extremely potent photoreductant upon
excitation with light (Scheme 1A, equation 2).16 Moreover, direct photoexcitation of an
organo-anionic species has been demonstrated to provide exciting opportunities to ac-
cess highly reducing reactive intermediates (Scheme 1B).10,17,18
anticipate that this report will
inspire new chemistry based on
inert chemical bond activation.
Arylboronates are recognized as essential building blocks in organic synthesis, ma-
terial science, and drug discovery.19,20 Transition-metal-catalyzed Miyaura
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A
B
C
D
Scheme 1. Photocatalytic strategies for accessing high reduction power for SET activation of
substituted arenes
borylation of aryl halides has been developed as one of the most efficient methods
for synthesizing arylboron reagents.21,22 In recent years, photocatalytic borylation
has attracted considerable research interest and opened a new avenue to access ar-
ylboronates.23–27 A number of photo-induced systems that allow for the borylation
of aryl-X bonds from a wide range of substrates, such as aryldiazonium salts,28–30 aryl
ammonium salts,31,32 aryl (pseudo) halides,15,31–41 and redox-active esters,42 have
been developed. However, most established photo-induced borylation methods
are limited to substrates with labile aryl-hetero bonds or low reduction potentials.
1Institute of Organic Chemistry, Faculty of
Chemistry and Pharmacy, University of
Regensburg, 93040 Regensburg, Germany
2Lead contact
*Correspondence: burkhard.koenig@ur.de
https://doi.org/10.1016/j.chempr.2021.04.016
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Thus, it remains highly challenging to apply the current methods to the borylation of
inert bonds, such as unactivated Caryl–F, Caryl–O, Caryl–N, and Caryl–S bonds,
because of their high bond dissociation energy and high reduction potentials of sub-
strates. The development of a strategy for the borylation of these bonds would not
only greatly widen the available substrate types to access arylboronates but also
might represent a significant step toward utilizing unreactive bonds as functional
groups in cross-coupling reactions.
Continuing our research in exploring the reducing ability of anionic species in photo-
redox catalysis,43,44 we wondered whether we could generate aryl radicals from
inert bonds (Caryl–F, Caryl–O, Caryl–N, and Caryl–S bonds) for borylation by using a sul-
fur-centered anion (e.g., thiolate) as a photocatalyst. The proposed catalytic system
was inspired by our previous finding wherein thiolate could efficiently shuttle elec-
trons from the boronate radical anion, a species that is produced in radical boryla-
tion processes, to the photocatalytic system.45 It was therefore hypothesized that us-
ing thiolate directly as a photocatalyst might offer an exciting opportunity for the
activation of inert Caryl–X bonds to generate aryl radicals. In the anticipated activa-
tion of substituted arenes by thiolate photocatalysis, two possible reaction modes
might be involved as shown in Scheme 1C: (1) direct interaction of substituted arenes
with the excited state of thiolate and (2) the formation of a charge-transfer complex
between thiolate catalyst and substituted arenes.46–58 We report herein the success-
ful application of thiolate catalysis for the photo-induced borylation of substituted
arenes through the cleavage of non-activated C(sp2)–F, C(sp2)–O, C(sp2)–N, and
C(sp2)–S bonds (Scheme 1D).
RESULTS AND DISCUSSION
Investigation of reaction conditions
We commenced the study by evaluating the defluoroborylation reaction of fluoro-
benzene 1a with B2pin2 (bis(catecholato)diboron). As a result of the high bond disso-
ciation energy (BDE) of Ar–F bonds (e.g., the BDE of Ph–F bond is 526 kJ$mol–1), only
a few examples are available for the borylation of unactivated aryl fluoride under
transition-metal-free conditions at present.31,37,59 To our delight, the desired bory-
lated product 2a was obtained in 33% yield at room temperature by irradiation with a
385–390 nm LED with CsF as a base and sodium 3-methyl-phenyl thiolate (30% mol)
as the catalyst (Table 1, entry 1; Figure S1). Studies with other thiolates revealed that
pyridine-2-thiolate (2-PySNa) is the optimal catalyst yielding the product in 56%
yield (entries 2–4). It is worth mentioning that sodium cyclohexanethiolate (CySNa)
and sodium 1-adamantanethiolate (1-AdSNa) showed comparably good catalytic
performances affording the product, whereas pyridine as a catalyst did not give
the product (entry 5), thus indicating that in our reaction the reactivity stems from
the thiolate under photocatalytic conditions. Changing the solvent from MeCN to
dimethyl sulfoxide (DMSO) or tetrahydrofuran (THF) gave either a poor yield or trace
amount of the product (entry 6 and 7). To further increase the yield, we examined a
variety of bases. Tetramethylammonium fluoride (TMAF) turned out to be the
optimal base, resulting in the formation of 2a in 74% yield; the increased reactivity
was most likely a result of its superior solubility in MeCN (entry 8). Using other bases,
such as KF, Cs2CO3, and CsOAc, failed to improve the yield (entries 9–11). In the
absence of base, the reaction afforded 2a in only 13% yield (entry 12). To our delight,
the yield was further improved to 81% when slight excess amounts of B2pin2 and
TMAF were employed (entry 13). The use of the more Lewis-acidic diboron source
B2cat2 afforded only a trace amount of compound 2a (entry 14). Control experiments
showed that both thiolate catalyst 2-PySNa and light irradiation (385–390 nm) were
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Table 1. Evaluation of reaction conditions for the borylation of fluorobenzene with B2pin2
Entry
Base
Catalyst
Solvent
Yielda
1
CsF
3-Me-PhSNa
CySNa
MeCN
MeCN
MeCN
MeCN
MeCN
DMSO
THF
33%
48%
40%
56%
n.d.
2
CsF
3
CsF
1-AdSNa
2-PySNa
pyridine
2-PySNa
2-PySNa
2-PySNa
2-PySNa
2-PySNa
2-PySNa
2-PySNa
2-PySNa
2-PySNa
2-PySNa
2-PySNa
2-PySNa
4
CsF
5
CsF
6
CsF
24%
trace
74%
19%
33%
27%
13%
7
CsF
8
TMAF
KF
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
9
10
11
12
13b
14e
15g
16h
17i
Cs2CO3
CsOAc
–
TMAF
TMAF
TMAF
TMAF
TMAF
81% (78%),c 70%d
tracef
n.d.
trace
10%
Reaction conditions: 1a (0.2 mmol), B2pin2 (0.4 mmol), base (0.4 mmol), and catalyst (30 mol %) in 2 mL of solvent under irradiation with a 385–390 nm LED (3.8 W)
at 30ꢀC–35ꢀC (internal temperature) for 24 h under N2 atmosphere. n.d., not detected; TMAF, tetramethylammonium fluoride; B2pin2, bis(pinacolato)diboron;
CySNa, sodium cyclohexanethiolate; 1-AdSNa, sodium 1-adamantanethiolate; 2-PySNa, pyridine-2-thiolate; B2cat2, bis(catecholato)diboron.
aYields were determined by GC-FID analysis of the crude reaction mixture with n-dodecane as an internal standard.
bB2pin2 (3.0 equiv), TMAF (3.0 equiv).
cIsolated yield.
d6 mmol scale.
eB2cat2 in place of B2pin2.
fWork up with Et3N (0.5 mL) and pinacol (0.8 mmol).
g455 nm LED.
hIn the dark.
iNo 2-PySNa.
essential for this reaction (entries 15–17). This method is easily scalable such that
boronic ester 2a can be obtained in 70% yield on a 6 mmol scale (Figure S2).
Scope of leaving group for ipso-borylation
Encouraged by these results, we next turned our attention to test the feasibility of
this catalytic system with arene substrates bearing different leaving groups
(Scheme 2). When the same reaction was attempted with chlorobenzene 1b (Ep
À2.78 V versus saturated calomel electrode [SCE]) and bromobenzene 1c (Ep
=
=
red
red
À2.44 V versus SCE), borylation product 2a was obtained in 74% and 78% yield,
respectively. We then focused on the borylation of C–O bonds of phenol derivatives
considering that phenols have emerged as versatile and cost-efficient alternatives to
aryl halides.25,60,61 It is worthwhile to note that, unlike the use of reactive aryl sulfo-
nates such as aryl triflates, mesylates, or tosylates, only a few methods are known for
the catalytic radical borylation of less or even non-activated aryl esters as coupling
partners via C–O bond cleavage.25,32,40 In particular, using slightly modified reac-
tion conditions (Table S2) borylated O-Boc-protected phenol 1d efficiently to give
product 2a in 65% yield. To the best of our knowledge, this is the first example of
transition-metal-free borylation of aryl carbonates via C–O bond cleavage.
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LG
Bpin
B2pin2 (2-3 eq.), base (2-3 eq.)
RSNa (15-40 mol%), MeCN
385-390 nm LED, 24-36h
1
2a
OR
X
NMe3X
1d, R = Boc, 65%a
1a, X = F, 81%a
1b, X = Cl, 74%a
1c, X = Br, 78%a
1e, R = SO2NMe2, 51%b
1f, R = SO2CF3, 60%c
1g, R = P(O)(OEt)2, 48%b
1h, R = C(O)NMe2, 46%b
1i, R = Ph, 20%b
1j, X = Br, 76%d
1k, X = Cl, 71%d
SO2R
S
R
O
S
R
1l, R = Ph, 96%e
1m, R = Et, 38%e
1n, R = Bn, 32%e
1o, R = vinyl, 30%e
1r, R = Ph, 70%g
1s, R = vinyl, 30%g
1t, R = Me, 13%g
1p, R = Ph, 61%f
1q, R = Me, 23%f
Scheme 2. Leaving-group scope for photo-induced ipso-borylation of substituted arenes
Reaction condition: 1 (0.2 mmol), B2pin2 (3.0 equiv), base (3.0 equiv), and 2-PySNa (15%–40% mol) in
2 mL of MeCN under irradiation with a 385–390 nm LED (3.8 W) at 30–35ꢀC for 24 h under N2
atmosphere. Yields were determined by GC-FID using n-dodecane as an internal standard.aCsF
(3.0 equiv), 2-PySNa (40 mol %), 36 h.bTMAF (3.0 equiv), MeCN (1.5 mL), 36 h.cCsOAc (3.0 equiv),
AcSK (20 mol %).dKOAc (3.0 equiv), 2-PySNa (15 mol %), MeCN (1.5 mL).eCsF (3.0 equiv), 1-AdSNa
(30 mol %).fB2pin2 (2.0 equiv), CsF (2.0 equiv), CySNa (30 mol %).gB2pin2 (2.0 equiv), CsF (2.0 equiv),
1-AdSNa (30 mol %).
Furthermore, we found that the reaction system was feasible for sulfamate 1e, triflate
1f, phosphate 1g, and carbamate 1h. Interestingly, even the C–O bond of dipheny-
lether showed a certain degree of reactivity in the reaction. Arylammonium salts 1j
and 1k were also viable for this transformation, leading to C–N borylated products
in 76% and 71% yield, respectively.
Given that organosulfur compounds are widely present in natural products, drugs,
and proteins, our protocol was briefly examined as a viable route for the C–S
bond borylation of sulfur-containing molecules, which allows modification of their
structures.62–64 Indeed, the C–S bond of diphenyl sulfone 1l (Epred = À2.06 V versus
SCE) could be borylated to give product 2a in excellent yield. When alkyl phenyl sul-
fones (1m and 1n) and vinyl phenyl sulfone 1o were used, the borylation occurred
selectively at the phenyl-SO2 bonds to give aryl boronic ester product 2a, whereas
the alkyl boronic esters were merely observed in trace amounts. The reactivity is in
contrast to Nambo and Crudden’s observation, wherein the borylation of alkyl
phenyl sulfones occurred predominately to afford alkyl boronic esters in a pyridine
catalytic system.65 Other than sulfones, our strategy permitted the use of diphenyl
sulfoxide 1p (Ep = À2.42 V versus SCE) and methyl phenyl sulfoxide 1q (Ep
=
red
red
À2.58 V versus SCE) as the phenyl radical precursors for borylation, albeit the reac-
tion in the latter case proceeds with lower efficiency. Subsequently, we were glad to
find that this methodology was applicable to convert aryl sulfides to aryl boronic es-
ters through C–S cleavage. Specifically, diphenyl sulfide 1r (Ep
= À2.8 V versus
red
SCE) reacted smoothly to give 2a in a 70% yield, whereas vinyl phenyl sulfide
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X
Bpin
B2Pin2 (2-3 eq.), base (2-3 eq.)
R
R
RSNa (30-40 mol%)
MeCN, 385-390 nm LED
3
4
C-F bonds
Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
N
PMP
Ph
MeO
Bn
Et
4c, 62%
4d, 80%
4g, 52%
4e, 61%
4b, 68%
4f, 53%
Bpin
MeOOC
Me
Bpin
Bpin
Bpin
Bpin
Bpin
N
N
N
Bpin
H2N
BocHN
4l, 79%a,35%b
4m, X=Cl, 53%
4j, 52%
4k, 50%
4h, 49%
Bpin
4i, 62%
Me
Ph
Bpin
Me
Bpin
Bpin
Bpin
Bpin
MeOOC
N
NC
O
Bpin
4r, 41%
4n, X=Cl, 56%
4p, 34%
4q, 57%
4o, 36%
4s, <20%
Bpin
Bpin
MeO
Bpin
Bpin
N
Bpin
H
OPh
Me
4t, 58%
OMe
4w, 33%
NH2
4v, 36%
4u, 48%
4x, X=Cl, 57%
C-O bonds
(X = OBoc)
Bpin
Bpin
Bpin
BocHN
Bpin
Bpin
Bpin
MeOOC
BocHN
Bpin
PhO
Ph
Me
4ab, 38%c
Bpin
4aa, 42%c
4y, 50%c
4z, 47%c
4k, 43%c
4e, 51%c
Me
MeO
Bpin
O
O
Bpin
O
H
Me
H
Me
Me
Me
from tocopherol
4af, 65%d
4ac, 48%c
4ae, 44%c
4ad, 57%c
O
O
C-S bonds
H
Bpin
Bpin
H
H
Bpin
MeO
Bpin
4d, from diaryl sulfone, 72%e
from diaryl sulfoxide, 62%f
from diaryl sulfide, 55%g
Ph
4e, from diaryl sulfone, 72%e
Me
from estrone
4ag, 45%c
4y, from diarylsulfone, 64%e
Scheme 3. Substrate scope for photo-induced ipso-borylation
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Scheme 3. Continued
Reaction conditions: 3 (0.2 mmol), B2pin2 (3.0 equiv), TMAF (3.0 equiv), and 2-PySNa (30% mol) in 2 mL of MeCN under irradiation with a 385–390 nm LED
(3.8 W) at 30ꢀC–35ꢀC for 24 h under N2 atmosphere. Isolated yields are shown.aCsF (3.0 equiv), B2pin2 (3.0 equiv).bFrom 1,4-difluorobenzene, TMAF (4.0
equiv), B2pin2 (4.0 equiv), mono-borylation product was obtained in 25% yield.cCsF (3.0 equiv), 2-PySNa (40 mol %), 36 h.dX = OSO2NMe2, 2-PySNa (40%
mol), MeCN (1.5 mL).eCsF (3.0 equiv), 1-AdSNa (30 mol %).fCsF (2.0 equiv), B2pin2 (2.0 equiv), CySNa (30 mol %).gCsF (2.0 equiv), B2pin2 (2.0 equiv), 1-
AdSNa (30 mol %).
demonstrated diminished reactivity. An attempt to borylate thioanisole 1t was un-
successful in that it afforded the target product 2a in 13% yield. These results are
particularly noteworthy given that thiolate anions are rarely used as leaving groups
in radical arylation reactions and that sulfides possess more negative reduction po-
tentials, which are caused by the electron-donating property of SR groups.8,66
Substrate scope for the borylation of C–X, C–O, and C–S bonds
After surveying the scope of phenyl radical precursors, we next sought to study the
preparative scope of our reaction by utilizing substituted aryl fluorides and chlo-
rides, O-Boc-protected phenols, and organosulfur compounds (Scheme 3). We
were pleased to see a wide variety of aryl fluorides bearing para-(3b–3p), ortho-
(3q and 3r), and meta-(3t–3v) substituents reacting smoothly to afford the corre-
sponding boronic esters. A broad range of substituents at the para-position—
including strongly electron-donating groups, such as –OMe (3d), piperidinyl (3i),
and –NH2 (3j), and electron-neutral ethyl (3b), benzyl (3c), and boronic ester (3l)
groups—were all tolerated and provided the desired products in good yields. The
presence of acidic protons in amine (3j and 3v) and amide (3k) did not interfere
with the reaction. Aromatic substituents, such as aryl (3e and 3f), pyrrole (3g), and
pyrazole (3h), were compatible with the reaction conditions. The use of aryl fluorides
bearing electron-withdrawing groups (EWGs) (e.g., acetyl, CN, and CF3) afforded
the products in low yields, although the starting materials were completely
consumed under the reaction conditions. However, corresponding arylchlorides
bearing strong EWGs (e.g., COOMe and CN) were tolerated in the reaction system
and delivered the desired products 4m and 4n in moderate yields. This observation
might be attributed to the strong electron-withdrawing property of fluoro substitu-
ents, which leads to the over-reduction of substrates bearing other EWGs on the ar-
omatic ring. Substrates bearing functional groups such as morpholine and ester on
the alkyl chains were tolerated (3o and 3p). ortho-Methyl-substituted aryl fluorides
3q and 3r reacted smoothly to provide the products in 57% and 41% yields, respec-
tively, whereas more sterically hindered phenyl groups at the ortho-position in-
hibited the reactivity such that the hydrodefluorination product was detected as
the major byproduct. Meta-substituted aryl fluorides bearing various functional
groups, such as phenoxy and amine, showed good reactivity in the reaction condi-
tions. Aryl fluoride bearing two methoxy groups also reacted to furnish the corre-
sponding product (4w) in 33% yield, highlighting the high reactivity of the catalytic
system. In addition, an unprotected indole functionality was preserved in the bory-
lation process. It should be noted that hydrodehalogenation products were de-
tected as the main side products in most cases as a result of a competing hydrogen
atom transfer (HAT) pathway (Schemes S1 and S2).
Prompted by these results, we proceeded to explore the scope of the ipso-borylation of
Ar–OBoc and Ar–S bonds. We were delighted to find that many synthetically useful
functional groups, including alkyl (4y and 4ad), phenyl (4e), phenoxy (4z), amide (4k),
and alkoxy (4ac and 4ae) substituents on the phenyl ring of Boc-protected phenols,
could be tolerated and provided the products in moderate yields. These results are
noteworthy considering that the electron-rich Boc-protected phenols should possess
very negative reduction potentials. Moreover, amide 4aa and ester 4ab functionalities
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on the alkyl chain of the phenolic compounds remained untouched in the reaction con-
ditions. The synthetic utility of this chemistry was further demonstrated by the successful
borylation of a d-tocopherol and estrone derivatives, affording the products 4af and 4ag
in 65% and 45% yields, respectively. Likewise, we observed hydrodefunctionalized
products as the major side products for the borylation of Boc-protected phenols. In
these cases, we observed lower conversions than with the fluoroarenes results (Scheme
S3). Finally, the borylation of diarylsulfones bearing methyl, methoxy, and phenyl groups
was fruitful in giving 64%–72% product yields of the isolated boronic esters (4y, 4d, and
4e). Moreover, methoxy-substituted diaryl sulfoxide and sulfide were also suitable sub-
strates for borylation.
Mechanistic insights
To obtain mechanistic insights into the reaction mechanism, we carried out a number
of experimental studies. At the outset, the effect of radical scavengers on the current
borylation system was examined (Scheme S5; Figure 1A). The presence of 1,1-DPE
(1,1-diphenylethylene) inhibited the reaction completely, and the corresponding trap-
ping adducts were formed in 5% yield. When aryl fluoride 3ah with an ortho-allyloxy
side chain was subjected to the standard conditions, cyclic alkylboronate 4ah derived
from a radical addition-borylation sequence was detected in 13% yield. These results
collectively suggest the intermediacy of the phenyl radical in the reaction. To elucidate
the radical generation process, we conducted further control experiments. Under light
irradiation (385–390 nm), fluorobenzene 1a reacted with 2-PySNa to give benzene, di-
sulfide, and sulfide, albeit in low conversion (16%), whereas no reaction could be
observed in the dark (Figure 1B, equations 1 and 2). These results imply that phenyl
radical and thiyl radical could be generated through direct photo-induced electron
transfer between the thiolate and fluorobenzene with low efficiency.
Next, we performed UV-visible (UV-vis) spectroscopic measurements of reaction
components to explore the electron transfer process between 2-PySNa and 1a. It
turned out that absorption spectra of B2pin2, substrate 1a, and TMAF showed bands
exclusively in the UV region (<300 nm) (Figure S10). However, 2-PySNa was observed
to absorb light at 390 nm, and the addition of B2pin2 (1.0 equiv) led to a clear redshift
by ~50 nm in absorption. A further significant redshift appeared when TMAF and 1a
were added to the mixture of 2-PySNa and B2pin2, whereas this signal change was
not observed in the absence of TMAF (Figure 1C). Likewise, obvious redshifts
were observed in the mixture of 2-PySNa, B2pin2, and TMAF (or CsF) with other types
of substrates (1b–1h) (Figures S11–S17). In contrast, new absorption bands were
directly observed when 1j, 1l, 1p, or 1r was mixed with the corresponding optimal
thiolate (Figures S18–S21). These results might support the formation of charge-
transfer complexes in the reaction processes.67,68
Moreover, we were intrigued by the redshift in the UV-vis spectrum of 2-PySNa after
the addition of B2pin2. 11B NMR spectroscopic analysis revealed that a new peak
with a chemical shift of 8.8 ppm appeared when B2pin2 was mixed with 2-PySNa, sug-
gesting the formation of an anionic diboron species (Figure S24). Meanwhile, both the
1H NMR and 13C NMR signals of 2-PySNa were shifted after 1.0 equiv of B2pin2 was
added, confirming the formation of a new boryl species (Figures S25 and S26). Consid-
ering the ease with which diboronate esters can react with pyridine type bases to form
the corresponding Lewis base adducts,42,69–71 we postulated that a Lewis acid-base
adduct formed between 2-PySÀ and B2pin2 was responsible for the observed redshift.
To study the photo-induced electron-transfer process between substrates and thio-
late catalysts, we conducted a series of control experiments using substrates 1 and
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A
B
Figure 1. Mechanistic studies
(A) Radical trapping experiments.
(B) Control experiments to elucidate the mechanism.
(C) UV-vis spectra of reaction components.
(D) 19F NMR spectra of 1a in the presence of B2pin2 and/or TMAF.
thiolates as reactants (Schemes S7–S18). First, thiolates could react with substrates
1j, 1p, and 1r, which can directly form an electron donor-acceptor (EDA) complex
with them, to afford benzene in high yields under light irradiation (Schemes S15,
S17, and S18). Subsequent control experiments demonstrated that in the absence
of fluoride, a low conversion of 1a (24%) and only 8% borylation product 2a were
observed with 1.0 equiv of 2-PySNa (Figure 1B, equation 3). Similarly, reactions of
1b–1h and 1l with 2-PySNa (1.0 equiv) gave low conversions (Schemes S8–S14 and
S16) under light irradiation. We thus suspect that it is very likely that the activation
of substrates by the co-existence of fluoride and B2pin2 could facilitate the single-
electron transfer (SET) between thiolate and 1a–1h.
Subsequently, we chose fluorobenzene 1a as the model substrate to study the po-
tential activation effect by fluoride and B2pin2. We first observed a C–F chemical shift
moving upfield from À114.51 to À114.84 ppm after mixing 1a with 2.0 equiv of
B2pin2 and TMAF, whereas the C–F chemical shift was not affected by B2pin2 or
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A
B
C
D
E
Scheme 4. Mechanistic proposal
TMAF separately (Figures 1D and S28). On the other hand, we observed the
formation of a sp2–sp3 fluoride diborane adduct between TMAF and B2pin2 by 19F
NMR measurements (Figure S28).39,72,73 We thus propose that an in-situ-generated
nucleophilic boryl anion species [B2pin2-FÀ] could activate fluorobenzene 1a, facili-
tating the single electron transfer and subsequent fluoride leaving process. We
speculate that this activation effect is also essential for the photo-induced elec-
tron-transfer event between substrates 1b–1h or 1l and the catalyst to occur.
On the basis of the aforementioned results and mechanistic pathways previously re-
ported in the literature, we propose the mechanism depicted in Schemes 4 and S19–
S21. The reaction sequence is initiated by a photo-induced electron-transfer process
between boryl-anion-activated 1 (LG = X, O, or N) (A) and 2-PySNa/B2pin2 adduct B
to afford the thiyl radical and radical anion of 1.74 In the case of 1j, 1p, and 1r, EDA
complexes were directly formed between substrates and thiolates (Schemes S19 and
S20). Upon photoexcitation of the EDA complexes, an inner-sphere electron transfer
occurs to arrive at the thiyl radical and radical anion of 1.75 The resulting radical anion
undergoes cleavage of the C–LG bond to form the phenyl radical, which then reacts
with a sp3–sp2 diboron species [F-B2pin2]À to give the desired borylation product 2a
and boryl radical anion E. Finally, the thiyl radical C is reduced by boryl radical anion
E to regenerate thiolate, closing the catalytic cycle.
Conclusion
In summary, we have developed a new photocatalytic strategy for the ipso-boryla-
tion of substituted arenes by using thiolate as a catalyst. This strategy realized the
borylation of a very broad range of inert C–X bonds, including non-activated C–F
bonds, C–O bonds of phenol derivatives (carbonate, sulfamate, phosphate, and
carbamate), C–N bonds of ammonium salts, and C–S bonds (sulfone, sulfoxide,
and sulfide), with very negative reduction potentials, which are challenging via exist-
ing photoredox activation. In this manner, this system allows the utilization of a range
of unconventional leaving groups for radical borylation reactions, thus expanding
available substrate types to access arylboronates. Despite the generated high
reducing power, this reaction displays broad functional-group tolerance and fur-
nishes borylated products in moderate to excellent yields. We propose the forma-
tion of an EDA complex between thiolate/B2pin2 and boryl-anion-activated sub-
strates as the key step in obtaining the reactivity, which is supported by UV-vis
1662 Chem 7, 1653–1665, June 10, 2021

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measurements, NMR analysis, and control experiments. This combination of photo-
chemistry with thiolate catalysis constitutes a unique activation mode of substituted
arenes in cross-coupling reactions. Further studies on the detailed reaction mecha-
nism and the extension of this catalytic system to other coupling partners are
currently underway in our laboratory.
EXPERIMENTAL PROCEDURES
Resource availability
Lead contact
Further information and requests for resources should be directed to and will be ful-
filled by the lead contact, Burkhard Konig (burkhard.koenig@ur.de).
¨
Materials availability
Unique and stable reagents generated in this study will be made available on
request, but we might require a payment and/or a completed materials transfer
agreement if there is potential for commercial application.
Data and code availability
There is no dataset or code associated with the paper.
Methods
Full experimental procedures are provided in the supplemental information.
SUPPLEMENTAL INFORMATION
Supplemental information can be found online at https://doi.org/10.1016/j.chempr.
2021.04.016.
ACKNOWLEDGMENTS
This project received funding from the European Research Council under the Euro-
pean Union’s Horizon 2020 Research And Innovation Programme (grant agreement
no. 741623). S.W. thanks the China Scholarship Council (CSC) for a predoctoral
fellowship (CSC student no. 201606280052). We are grateful to Prof. Armido Studer
(University of Munster) for helpful discussions. We further thank Dr. Rudolf Vasold
¨
(University of Regensburg) for his assistance in GC-MS measurements and Ms. Re-
gina Hoheisel (University of Regensburg) for her assistance in cyclic voltammetry
measurements.
AUTHOR CONTRIBUTIONS
B.K. supervised the project; S.W. developed the catalytic system, conducted the ex-
periments, and analyzed the data; S.W. and H.W. conducted the mechanistic
studies; and S.W. and B.K. wrote the manuscript with input from all of the authors.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: December 1, 2020
Revised: February 24, 2021
Accepted: April 29, 2021
Published: May 28, 2021
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