Rapid Access to Borylated Thiophenes Enabled by Visible Light

Article abstract of DOI:10.1021/acs.orglett.0c01076

Boron-containing thiophenes are important entities in organic/medicinal chemistry as well as in material science. In this Letter, a novel, straightforward, and fast procedure for their production employing visible light as an energy source at room temperature and ambient pressure is reported. All substrates are commercially available, and the process does not require the use of any external photocatalyst.

Full text of DOI:10.1021/acs.orglett.0c01076

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rapid Access to Borylated Thiophenes Enabled by Visible Light
,†
,†
Jorge C. Herrera-Luna,† Diego Sampedro,‡ M. Consuelo Jimenez,* and Raul Perez-Ruiz*
́
́
́
†
̀
̀
Departamento de Química, Universitat Politecnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain
‡
́
Departamento de Química, Centro de Investigacion en Síntesis Química (CISQ), Universidad de La Rioja, Madre de Dios, 53,
26006 Logrono, Spain
̃
S
* Supporting Information
ABSTRACT: Boron-containing thiophenes are important entities in organic/medicinal chemistry as well as in material science.
In this Letter, a novel, straightforward, and fast procedure for their production employing visible light as an energy source at
room temperature and ambient pressure is reported. All substrates are commercially available, and the process does not require
the use of any external photocatalyst.
Scheme 1. Strategies Related to the Borylation of
Thiophenes
rganoboron compounds play a key role as intermediate
Otargets in organic/medicinal chemistry and material
science1 due to their versatility and environmentally benign
and user-friendly characteristics. Sophisticated and elegant
methods have been reported for the insertion of boryl groups
into organic molecules, including recent advances in the
transition-metal-catalyzed or UV-photoinduced borylation of
alkanes, arenes, heteroarenes, fused arenes, olefins, and other
substrates.2 Among these products, borylated heteroarene rings
are considered important building blocks for subsequent
derivatization.3 Of particular interest is the fast production of
C−C bonds for the preparation of bioactive molecules.1b,4
In this context, boron-containing thiophenes are important
scaffolds, for example, in conjugated materials5 showing
organic electronic applications6 or the fabrication of light-
emitting diodes7 or even in antimicrobial activity.8 Early
reports on synthesizing 2-thiophenyl boronate esters used
stoichiometric or catalytic metalation protocols.9 In 1995,
Miyaura et al. published the Pd-catalyzed reaction between
tetraalkoxydiboron reagents with haloarenes to form arylbor-
onates, including the 3-iodobenzothiophene (Scheme 1a).10
Later, Li et al. reported an efficient alternative for producing
some arylboronic acids using a lithium−halogen exchange with
an in situ quench with borate (Scheme 1b).11 Regarding
iridium-catalyzed reactions, Sawamura et al. realized an ester-
directed borylation on a silica-supported iridium complex
(Scheme 1c, top reaction),12 whereas Smith et al. showed an
alternative strategy that included a sequential diborylation/
monodeborylation pathway to obtain boron-containing thio-
phenes (Scheme 1c, bottom reaction).13 Recently, Blum et al.
discovered a simple and transition-metal-free protocol to
exclusively produce 3-borylated thiophene derivatives (Scheme
1d).14 All of these methodologies somehow require mostly
Received: March 24, 2020
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.0c01076
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Searching for the Optimal Conditions
b
b
no.
solvent
ACN
ACN
ACN
ACN
base
time (h)
conv. (%)
2aa/3a
yield (%)
c
1
DIPEA
DIPEA
6
6
6
27
100
0
0
52
9
100
61
100
74
85
68
71
86
10
3
75/25
85/15
22
53
0
0
8
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
d
e
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
Et3N
overnight
6
6
MeOH
DMF
44/56
0
f
ACN-H2O
ACN-H2O
ACN-H2O
ACN-H2O
ACN-H2O
ACN-H2O
ACN-H2O
ACN-H2O
ACN-H2O
ACN-H2O
ACN-H2O
ACN-H2O
ACN-H2O
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
92/8
84 .
51
58
31
45
56
63
63
10
0
g
89/11
84/16
80/20
88/12
87/13
92/8
h
i
j
DIPA
DBU
90/10
85/15
DABCO
Ph3N
pyridine
DIPEA
DIPEA
0
100
100
0
80
76
k
18
19
85/15
85/15
l
a
b
1a (0.08 mmol), B2pin2 (1.6 mmol), and base (0.12 mmol) in 3 mL of N2/solvent; irradiation (cold-white LED), 23 °C. GC-FID yields of 2aa.
c
d
e
f
g
h
i
With DCA (10 mol %). Heating (50 °C) in dark. ACN−H2O (9:1 v/v). Isolated yield. DIPEA (0.08 mmol). DIPEA (0.16 mmol). B2pin2
j
k
l
(0.4 mmol). B2pin2 (0.8 mmol). Using 2-acetyl-5-bromothiophene (0.08 mmol). Using 2-acetyl-5-iodothiophene (0.08 mmol).
complex conditions, such as the performance of the reaction in
a glovebox, the employment of high temperatures or iridium
precatalysts, the preliminary synthesis of the starting materials,
or sometimes the need for prolonged reaction times. To
reduce costs and unwarranted residues in the final products,
the development of new and “greener” strategies with milder
conditions is highly desired.
Herein we report as a proof of concept a straightforward,
rapid, and photocatalyst-free process to fabricate borylated
thiophenes using visible light under mild conditions (Scheme
1e). This reaction implies the in situ generation of a ground-
state complex that is capable of absorbing light in the visible
region, initiating the process.
min of irradiation, leading to product 2aa in excellent yield and
with high selectivity (Table 1, entry 7). Varying the amount of
DIPEA or B2pin2 did not gain better outputs of the reaction in
terms of the conversion and yield (Table 1, entries 8−11).
Changing the base under optimal conditions presented good
results when triethylamine (Et3N), diisopropylamine (DIPA),
1,8-diazabicyclo(5.4.0)undec-7-ene (DBU), or 1,4-
diazabicyclo[2.2.2]octane (DABCO) was used (Table 1,
entries 12−15), whereas the reaction did not occur with
triphenylamine (Ph3N) or pyridine (Table 1, entries 16 and
17). Finally, the procedure was applied to the thioaryl bromide
or iodide derivative, and excellent results were also obtained
(Table 1, entries 18 and 19).
In view of the successful merging of visible-light excitation
for chemical transformations15 and inspired by our previous
experience,16 we commenced with a reaction of 2-acetyl-5-
chlorothiophene (1a) and bis(pinacolato)diboron (B2pin2) in
To better understand this methodology, absorptivity
measurements of different mixtures under reaction conditions
were carried out (Figure 1). Before irradiation, new bands at
ca. 400 and 550 nm were clearly observed when the three
ingredients (1a + B2pin2 + DIPEA) were present, and, indeed,
the solution turned red (Figure 1, inset). This fact, together
with the colorless mixtures to the naked eye for the rest of
combinations (Figure 1, inset), pointed to the formation of a
complex in the ground state with remarkable absorbance in the
visible region, which allowed the initiation of the investigated
reaction.
the presence of Hunig’s base (diisopropylethylamine, DIPEA)
̈
and catalytic amounts of 9,10-dicyanoanthracene (DCA) using
cold-white LEDs (Table 1, entry 1). The expected product 2aa
was obtained in moderate yield after the incomplete
conversion of 1a. Surprisingly, a control experiment in the
absence of DCA afforded the complete conversion of 1a
together with adduct 2aa in good yield (Table 1, entry 2). No
product formation was detected in the absence of base or after
heating the reaction mixture to 50 °C overnight in the dark
(Table 1, entries 3 and 4), demonstrating that both
components (base and light) were essential for this photo-
chemical protocol. Changes in solvent conditions did not
improve the results (Table 1, entries 5 and 6).
To shed light on the nature of this complex, NMR
experiments were conducted (Figure 2). First, the 11B NMR
spectrum of B2pin2 showed its characteristic δ at 31 ppm.
However, a new species was observed (δ 22.5 and 20) when
DIPEA or DIPEA/1a was added, which was safely ascribed to
the sp3−sp2 diboron A (Figure 2a).2n,17,18 Thus the treatment
On the basis of the literature data,17 we performed the
reaction using an acetonitrile (ACN)/water mixture as the
solvent, anticipating that the process could be accelerated.
Indeed, the complete conversion of 1a was observed after 30
of B2pin2 in water with Hunig’s base gave rise to A and the
̈
cationic protonated DIPEA. Furthermore, the interaction
between B2pin2 and 1a did not take place, and 2aa was not
detected in the three-component mixture (Figure 2a),
B
DOI: 10.1021/acs.orglett.0c01076
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Organic Letters
Letter
and A could induce some effects on aromatic substituents, the
most relevant part of the 1H NMR spectra (Figure 2c) showed
a weak shift of the CH resonances of the thiophene core. The
foregoing experimental results pointed out the formation of a
three-component complex in the ground state, where the
carbonyl group played a pivotal role in this type of processes.
To support this hypothesis, several carbonyl-free thiophenes
were submitted to visible-light irradiation under the standard
conditions. As expected, no conversion of the starting material
was observed, even after 6 h of reaction. (See the details in the
SI.)
With these data in hand, we proposed the following reaction
mechanism (Figure 2d). After the formation of intermediate A,
a hydrogen-bonding interaction with 1a occurs, affording a
new complex B that might tentatively be responsible for the
red color of the solution. Hence, under visible-light irradiation,
complex B is promoted to its excited state (B*). This species is
now capable of oxidizing DIPEA (this pathway is feasible based
on theoretical calculations; data not shown), and, after a fast
splitting of the C−Cl bond, the thiophene radical intermediate
C is generated, which was confirmed by PhSSPh19 trapping
(Scheme 2). Now, C would react with A to lead to the desired
Figure 1. UV−vis absorption spectra. Inset: photographs of different
solutions. Conditions: 1a (0.08 mmol), B2pin2 (0.16 mmol), and
DIPEA (0.12 mmol); solvent: ACN/H2O (9/1 v/v) in a total volume
of 3 mL under an air atmosphere.
supporting the notion that light was a crucial parameter for the
reaction (see Table 1, entries 3 and 4).
Besides, the 13C NMR spectroscopic analysis of the starting
material 1a in the absence/presence of B2pin2 + DIPEA was
achieved (Figure 2b). Interestingly, in addition to the carbonyl
signal (δ 192), a new weakly shifted peak appeared in the
mixture, suggesting the possibility of a hydrogen-bonding
equilibrium between A and the CO group of 1a. Moreover,
the quaternary carbon in the α position and the CH in the β
position related to the CO group were somehow affected
due to this presumably noncovalent interaction (Figure 2b).
Assuming that this hydrogen-bonding interaction between 1a
Scheme 2. Trapping of Thiophene Radical
Figure 2. (a) 11B NMR spectra in CD3CN/D2O (9/1 v/v), (b) 13C NMR spectra of 1a (top) and 1a + B2pin2 + DIPEA (bottom) in CD3CN/D2O
(9/1 v/v), (c) 1H NMR spectra of 1a (blue) and 1a + B2pin2 + DIPEA (red) in CD3CN/D2O (9/1 v/v), and (d) proposed reaction mechanism.
C
DOI: 10.1021/acs.orglett.0c01076
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Letter
2aa and the boryl radical anion D.18b,20 Finally, one single
electron transfer (SET) from D to the DIPEA radical cation
would provide OH-Bpin as the byproduct,2n which hydrolyzes
to volatile products upon workup.21 Importantly, the role of
DIPEA could be two-fold because it can act as (i) a base and
(ii) a photoreductant in the electron transfer process.
a
Scheme 3. Substrate Scope for Borylated Thiophenes
To further rationalize the proposed reaction mechanism,
computational studies under the framework of the density
functional of theory (DFT) were performed. (See the SI for
computational details.) Thus different alternatives for complex
B were first computed. (See Figure S1.) From this preliminary
screening, the proposed structure of B including a hydrogen
bond between A and 1a was found to be the predominant one
that was fully in agreement with the experimental data (NMR
and UV−vis). Then, the electronic absorption spectrum of B
was calculated using 6-31+G** as the basis set together with a
variety of functionals. (See the SI.) Although a similar
qualitative trend was found, the B3LYP functional showed
the best theoretical−experimental relationship. As depicted in
Figure 3a, the absorption spectrum of B in acetonitrile was
a
1a (1 equiv), (OR)2B−B(OR)2 (20 equiv), and DIPEA (1.5 equiv).
b
Full conversion of 1a in all cases; GC-FID yields of products. 0.5 h of
irradiation time. 1 h of irradiation time. 2 h of irradiation time.
c
d
conditions. To strongly support this proof of concept and
demonstrate the versatility of the protocol, we proceeded to
investigate the reaction, combining several commercially
available thiophene derivatives and the diboron compounds
previously employed (Scheme 4). Gratifyingly, all tested
Scheme 4. Versatility Screening for Photocatalyst-free
a
Visible-Light Thiophene Borylations
Figure 3. (a) Absorption spectrum and (b) detail of B computed
using B3LYP/6-31+G** in acetonitrile. Orbitals involved in the
transition at 423 nm (c) LUMO and (d) HOMO−1.
dominated by the thiophene-type transition at 292 nm that
thoroughly matches with the computed value for 1a (292 nm;
experimental value: 289 nm). Interestingly, two additional
small absorption bands at longer wavelengths appeared for B
(Figure 3b) at 543 and 423 nm, with an oscillator strength of f
= 0.00001 and 0.0005, respectively; this finding differed from
the 1a absorption spectrum where these two bands were not
found. Therefore, a good correlation could be established
between the results from the UV−vis measurements and the
computational data. In addition, the electronic transition at
423 nm might imply the movement of an electron from the
B2pin2 moiety to the thiophene unit (Figure 3c,d).
Having established the optimal conditions and understood
the method, we next conducted the substrate scope using
different diboron derivatives (Scheme 3). After cold-white
LED irradiation, full conversion of the starting thiophene
material 1a was effectively observed in all cases, and high yields
of products (2aa−ad) were obtained. It is noteworthy that the
borylation reactions magnificently occurred using other
diboron esters as counterparts such as 1,1,3-trimethylethylene
glycol (Bhex), 1,1,4,4,-tetramethylethylene glycol (Boct), and
neopentyl glycol (Bneo), delivering the corresponding
thiophene boronic esters in high yield under very mild
a
1 (1 equiv), (OR)2B−B(OR)2 (20 equiv), and DIPEA (1.5 equiv).
b
Full conversion of 1 in all cases; GC-FID yields of products. 0.5 h of
irradiation time. 1 h of irradiation time. 2 h of irradiation time.
c
d
reactions worked perfectly under the optimal conditions, and
the desired products (2ba−ed) were obtained in high yield
and with high selectivities. Therefore, these types of
thiophene−boronates can be easily prepared at shorter
irradiation times (ranging from 0.5 to 2 h), making this
protocol feasible for a late-stage functionalization of boron-
based compounds for the synthesis of more complex organic
molecules.
D
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Letter
advances in C−B bond formation through a free radical pathway. Adv.
Synth. Catal. 2018, 360, 1040−1053. (e) Cuenca, A. B.; Shishido, R.;
In conclusion, fast access to borylated-like thiophenes by
means of visible light (cold-white LEDs) as an energy source
has been reported. This photocatalyst-free reaction is proposed
to proceed via the in situ formation of a ground-state complex
that, after visible-light absorption, evolves to the formation of
the corresponding borylation of the thiophene core. The
mechanistic aspects have been demonstrated by spectroscopic
measurements and theoretical calculations, and the scope and
the versatility of the procedure have been successfully proven.
This work constitutes a new, mild strategy for affording boron-
containing thiophenes that should be of potential application
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01076.
Experimental details, general procedure, optimization of
reaction conditions, GC-FID chromatograms, character-
ization of products, computational information, and
spectroscopic data of all compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
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Suzuki−Miyaura Cross-Couplings. J. Am. Chem. Soc. 2017, 139,
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Ghozati, K.; Unsworth, P. J.; Nambo, M.; Voth, S.; Hutchinson, M.;
Laberge, V. S.; Maekawa, Y.; Imao, D. Iterative protecting group-free
cross-coupling leading to chiral multiply arylated structures. Nat.
Commun. 2016, 7, 11065. (c) Close, A. J.; Kemmitt, P.; Mark Roe, S.;
Spencer, J. Regioselective routes to orthogonally-substituted aromatic
MIDA boronates. Org. Biomol. Chem. 2016, 14, 6751−6756. (d) Li, J.;
Ballmer, S. G.; Gillis, E. P.; Fujii, S.; Schmidt, M. J.; Palazzolo, A. M.
E.; Lehmann, J. W.; Morehouse, G. F.; Burke, M. D. Synthesis of
many different types of organic small molecules using one automated
process. Science 2015, 347, 1221−1226. (e) Xu, L.; Zhang, S.; Li, P.
Boron-selective reactions as powerful tools for modular synthesis of
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reagents for Suzuki−Miyaura coupling. Chem. Soc. Rev. 2014, 43,
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*Email: mcjimene@qim.upv.es (M.C.J.).
*Email: raupreru@qim.upv.es (R.P.-R.).
ORCID
Diego Sampedro: 0000-0003-2772-6453
́
M. Consuelo Jimenez: 0000-0002-8057-4316
́
́
Raul Perez-Ruiz: 0000-0003-1136-3598
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Generalitat Valenciana (CIDE-
GENT/2018/044) and the Spanish Government (CTQ2016-
78875-P, CTQ2017-87372-P, and BES-2017-080215) is grate-
fully acknowledged.
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DOI: 10.1021/acs.orglett.0c01076
Org. Lett. XXXX, XXX, XXX−XXX