Visible-Light-Mediated Alkenylation of Alkyl Boronic Acids without an External Lewis Base as an Activator

Article abstract of DOI:10.1021/acs.orglett.1c00399

Herein we report a protocol for the direct visible-light-mediated alkenylation of alkyl boronic acids at room temperature without an external Lewis base as an activator, and we propose a mechanism involving benzenesulfinate activation of the alkyl boronic acids. The protocol permits the efficient functionalization of a broad range of cyclic and acyclic primary and secondary alkyl boronic acids with various alkenyl sulfones. We demonstrated its utility by preparing or functionalizing several pharmaceuticals and natural products.

Full text of DOI:10.1021/acs.orglett.1c00399

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Letter
Visible-Light-Mediated Alkenylation of Alkyl Boronic Acids without
an External Lewis Base as an Activator
Fuyang Yue, Jianyang Dong, Yuxiu Liu, and Qingmin Wang*
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ABSTRACT: Herein we report a protocol for the direct visible-
light-mediated alkenylation of alkyl boronic acids at room
temperature without an external Lewis base as an activator, and
we propose a mechanism involving benzenesulfinate activation of
the alkyl boronic acids. The protocol permits the efficient
functionalization of a broad range of cyclic and acyclic primary
and secondary alkyl boronic acids with various alkenyl sulfones. We demonstrated its utility by preparing or functionalizing several
pharmaceuticals and natural products.
Scheme 1. Photoredox-Mediated Alkenylation Reactions
ecause of their versatility, alkenes are widely used in
synthetic chemistry and materials science.1 Consequently,
B
tremendous research effort has been devoted to developing
methods for alkene synthesis, such as Heck-type,2 Wittig-type,3
and Corey−Winter-type reactions4 as well as many others.5 Of
the known methods, the transition-metal-catalyzed cross-
coupling of alkenes with preactivated/functionalized alkanes
is the best, most straightforward tool for the synthesis of these
compounds with predictable chemo- and regioselectivity.5
Nevertheless, the currently available methods have some
drawbacks, such as requiring the use of organometallic reagents
(which are usually sensitive to water and air), strong oxidants,
or high temperatures.6
With the rapidly increasing interest in the use of photoredox
catalysis in organic chemistry,7 photoredox alkenylation
reactions have begun to emerge in recent years.8 For example,
visible-light-mediated alkenylation reactions that use alkenyl
sulfones as radical acceptors and aliphatic carboxylic acids,8a
alkyl bromides,9 or Katritzky pyridinium salts10 as radical
precursors have been reported (Scheme 1A). In addition, the
use of vinyl potassium trifluoroborates11 and alkenylboronic
acids12 instead of vinyl sulfones has been reported. Although
these reactions are operationally simpler and use fewer
oxidants than previously reported methods, most of them
have a limited substrate scope, and some of them require high
temperature.
SCE for secondary boronic acids), they cannot efficiently
quench the excited states of metal or organic photocatalysts to
generate alkyl radicals,15 and their synthetic utility has
therefore been limited. However, it is worth noting that Ley
and coworkers16 overcame this problem by using a Lewis base
to decrease the oxidation potentials of alkyl boronic acids so
that they could efficiently quench excited-state photocatalysts.
Chen and coworkers17 used hypervalent iodine activated
In 2016, Molander and coworkers13 described a protocol for
visible-light-mediated alkenylation that utilizes potassium α-
pyrrolidinyltrifluoroborate as an alkyl radical source in
reactions involving alkenyl sulfones (Scheme 1B); however,
the preparation of these alkyl fluoroborate salts needed three
harsh steps, and the substrate scope was limited in potassium
α-pyrrolidinyltrifluoroborate.
Received: February 2, 2021
Published: March 12, 2021
We speculated that readily available alkyl boronic acids
could serve as an alternative radical source.14 Because alkyl
boronic acids have high oxidation potentials (Ered = +2.5 V vs
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boronic acids as an alkyl radical source. We hypothesized that
benzenesulfinates, which can be obtained from alkenyl
sulfones, could effectively activate boronic acids and thus
avoid the need for an additional Lewis base. Herein we now
report that we have indeed succeeded in developing a protocol
for mild, visible-light-mediated alkenylation reactions of a wide
array of boronic acids directly, without the need for an external
activator (Scheme 1C).
With the optimized conditions in hand, we explored the
scope of the reaction with respect to the alkenyl sulfone
(Scheme 2). Reactions between cyclohexylboronic acid and
Scheme 2. Scope of the Reaction with Respect to the
a
Alkenyl Sulfone
To optimize the reaction conditions, we used phenyl trans-
styryl sulfone (1, 1.0 equiv) and cyclohexylboronic acid (2, 2.0
equiv) as model substrates (Table 1). First, several different
a
Table 1. Optimization of Conditions
b
entry
solvent
base
yield (%)
1
2
3
4
DCM
K2HPO4
K2HPO4
K2HPO4
K2CO3
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
no base
K3PO4
44
51
57
60
67
75
89 (80 )
78
62
NR
NR
trace
42
DCM/EA = 2:1
DCM/EA = 2:1
DCM/EA = 2:1
DCM/EA = 2:1
DCM/EA = 2:1
DCM/EA = 2:1
DCM/EA = 2:1 (0.13 M)
DCM/EA = 2:1 (0.2 M)
DCM/EA = 2:1
DCM/EA = 2:1
DCM/EA = 2:1
DCM/EA = 2:1
5
c
6
cd
,
e
7
8
cdf
, ,
cdg
, ,
9
c dh
, ,
10
11
12
13
c di
,
,
c dj
,
,
c dk
,
,
a
Reaction conditions, unless otherwise noted: 1 (0.2 mmol), 2 (0.4
mmol), Ir[dF(CF3)ppy]2(dtbbpy)PF6 (0.004 mmol), base (0.4
b
mmol), solvent (2 mL), rt, Ar atmosphere, 24 h. Yields were
1
determined by H NMR spectroscopy with dibromomethane as an
c
a
internal standard. NR = no reaction. 2 (4 equiv., 0.8 mmol), 36 h.
Reactions were performed on a 0.2 mmol scale, and isolated yields
d
e
f
b
Base (1.5 equiv., 0.3 mmol). Isolated yield. Solvent (1.5 mL, 0.13
are given. Reactions were performed under 450 nm LED irradiation.
g
h
i
j
M). Solvent (1 mL, 0.2 M). No photocatalyst. No light. No base.
k
Air atmosphere.
various sulfones with electron-donating or electron-with-
drawing substituents all gave moderate yields (52−87%) of
the desired products (4−9 and 10−13, respectively).
Fluorinated and brominated sulfones (14 and 16) gave higher
yields than a chlorinated sulfone (15), and para substitution
(14) resulted in a higher yield than ortho substitution (17).
Naphthyl, pyridyl, and 1,1-disubstituted alkenyl compounds
were also suitable, affording the desired products 18−20 in
moderate yields (38−63%). In addition, we were delighted to
find that ((2-phenylallyl)sulfonyl)benzene and 1-chloro-4-(3-
(phenylsulfonyl)prop-1-en-2-yl)benzene could also be used as
radical acceptors; when they were subjected to the alkenylation
conditions, the desired products 21 and 22 were obtained in
moderate yields. We continued to explore other vinyl
compounds, such as thiophene (23) and benzothiazole (24),
all of which were obtained in moderate yields (59 and 65%).
We were pleasantly surprised to find that when we used alkynyl
sulfone as a free-radical acceptor, the products (25 and 26)
were still obtained in moderate yields (74 and 70%). As for Z/
E geometry,18 unfortunately, in our reaction system, we could
not increase its ratio. (See the SI.)
solvents were screened with Ir[dF(CF3)ppy]2(dtbbpy)PF6 as
the photocatalyst and K2HPO4 as the base under irradiation
with a 26 W blue LED for 24 h at room temperature (entries
1−3). We were pleased to find that the desired product 3
could be obtained in 57% yield from the reaction in 2:1 (v/v)
dichloromethane/ethyl acetate. When we varied the base, we
discovered that K3PO4 gave a slightly higher yield than
K2HPO4 and K2CO3 (compare entries 3−5). When we
increased the amount of 2 from 2.0 to 4.0 equiv and the
reaction time from 24 to 36 h, the yield further increased to
75% (entry 6). Simultaneously reducing the amount of base
from 2 to 1.5 equiv afforded the highest yield (89% by NMR,
80% isolated; entry 7). Increasing the concentration of the
reactants (entries 8 and 9) was detrimental. Control
experiments showed that the reaction failed to proceed in
the absence of light, base, or photocatalyst (entries 10−12).
Under air, 3 was obtained in 42% yield (entry 13). Then, we
screened the photocatalyst. The desired product 3 was
obtained in excellent yield when 2 mol % of Ir[dF(CF3)-
ppy]2(dtbbpy)PF6 was used as the photocatalyst. (See the SI.)
Then, the light source was screened. All light sources could
provide products in excellent yield (see the SI), and the yield
would increase slightly when we used 450 nm light.
Next we explored the scope of the reaction with respect to
the alkyl boronic acid by using methyl (E)-4-(2-
(phenylsulfonyl)vinyl)benzoate or (E)-4-(2-(phenylsulfonyl)-
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vinyl)-1,1′-biphenyl as the alkenyl sulfone (Scheme 3). We
Scheme 4. Applications of the Alkenylation Protocol: (A)
Alkenylation of Natural Products and Drug Molecules and
(B) Gram-Scale Experiment
found that a wide range of primary alkyl boronic acids reacted
a
Scheme 3. Scope of the Reaction with Respect to the Alkyl
a
Boronic Acid
a
Reactions were performed on a 0.2 mmol scale, and isolated yields
are given.
Having explored the substrate scope and the utility of the
reaction, we turned our attention to its mechanism (Scheme
5). First, we found that the reaction of 1 and 2 was prevented
Scheme 5. Mechanistic Experiments: (A) Free-Radical
a
Inhibition Experiment and (B) Control Experiment
a
Reactions were performed on a 0.2 mmol scale, and isolated yields
b
are given. Reactions were performed under 450 nm LED irradiation.
c
1
t = 48 h, Yields were determined by H NMR spectroscopy with
dibromomethane as an internal standard.
with methyl (E)-4-(2-(phenylsulfonyl)vinyl)benzoate to afford
alkenylated products (27−30) in 27−40% yields. Owing to the
lower nucleophilicity and stability of primary radicals, the
alkenylation of primary alkyl boronic acids is much more
challenging than the alkylation of secondary and tertiary
compounds.19 Therefore, we tested some secondary boronic
acids in our alkenylation reaction under the standard
conditions and found that they afforded the corresponding
alkenylated products (31−36) in much higher yields than
those obtained from the primary alkyl boronic acids. We were
pleasantly surprised to find that when we used more complex
boronic acids, the products (37 and 38) could be obtained in
moderate yields (47 and 52%). When we used a 450 nm light
source and extended the reaction time to 48 h, the yield of the
reaction was slightly improved (see the SI), but for tertiary
boronic acid, the yield was extremely poor.20
Next, we subjected some natural products and drug
molecules to the standard alkenylation conditions to test the
utility of the protocol for functionalized molecules. We were
pleased to find that L-menthol-, ibuprofen-, and fenbufen-
derived (39−41) alkenylation products (42−44) could be
obtained in excellent yields (Scheme 4A). In addition, a gram-
scale reaction between 45 and 2 under the standard conditions
gave the corresponding alkenylated product (11) in 78% yield
(Scheme 4B).
a
Reactions were performed on a 0.2 mmol scale. Yields were
1
determined by H NMR spectroscopy with dibromomethane as an
internal standard.
when a radical scavenger such as TEMPO (2,2,6,6-tetramethyl-
1-piperidinyloxy) or BHT (butylated hydroxytoluene) was
present in the reaction mixture (Scheme 5A). Furthermore, the
radical trapping product 1-(cyclohexyloxy)-2,2,6,6-tetramethyl-
piperidine (46) was detected by high-resolution mass
spectrometry. This experiment clearly points to a radical
pathway. A light/dark experiment showed that 3 formed only
under light irradiation (see the SI), indicating that light was
essential for product formation and that any chain-propagation
process was short-lived. To determine whether the benzene-
sulfinate derived from the alkenyl sulfone could effectively
activate the boronic acid, we measured the 1H NMR spectra of
mixtures of phenethylboronic acid and benzenesulfinate at
various concentrations in DMSO-d6, and we observed that
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gradually increasing the total concentration of benzenesulfinate
in the NMR tube shielded the phenethylboronic acid α-CH2
signal. (See the SI.) In addition, we performed cyclic
voltammetry measurements to study the effect of the
benzenesulfinate on the oxidation potential of cyclohexylbor-
onic acid. When we added benzenesulfinate and cyclo-
hexylboronic acid to acetonitrile, we detected a new, low-
potential oxidation peak (see the SI). We also studied the
emission quenching experiments. (See the SI.) Therefore, we
assumed that the oxidation potential of cyclopentane boronic
acid was markedly reduced by the coordination of
benzenesulfinate.
To explore the role of the base, we did a control experiment.
We found that in the absence of base, we detected the
formation of benzenesulfonic acid in the reaction system
(Scheme 5B), and the fluorescence quenching experiment
proved that the mixture of base and boronic acid could not
quench the photocatalyst. The solubility of benzenesulfinate in
our reaction solvent is very poor. Therefore, we think that if
there is no base and our reaction system is acidic and
benzenesulfinate will capture hydrogen protons, then the
ability to activate boronic acid by benzenesulfinate will be
significantly reduced.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00399.
■
sı
Experimental procedure and characterization for all new
compounds, copies of NMR spectra, and cyclic
voltammograms (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Qingmin Wang − State Key Laboratory of Elemento-Organic
Chemistry, Research Institute of Elemento-Organic Chemistry,
College of Chemistry, Nankai University, Tianjin 300071,
People’s Republic of China; Collaborative Innovation Center
of Chemical Science and Engineering (Tianjin), Tianjin
300071, People’s Republic of China; orcid.org/0000-
0002-6062-3766; Email: wangqm@nankai.edu.cn
Authors
Fuyang Yue − State Key Laboratory of Elemento-Organic
Chemistry, Research Institute of Elemento-Organic Chemistry,
College of Chemistry, Nankai University, Tianjin 300071,
People’s Republic of China
Jianyang Dong − State Key Laboratory of Elemento-Organic
Chemistry, Research Institute of Elemento-Organic Chemistry,
College of Chemistry, Nankai University, Tianjin 300071,
People’s Republic of China
On the basis of our experimental observations and literature
reports,8−12,17 we proposed the mechanism depicted in
Scheme 6.21 First, the Ir3+ photocatalyst is excited by visible
Scheme 6. Proposed Mechanism
Yuxiu Liu − State Key Laboratory of Elemento-Organic
Chemistry, Research Institute of Elemento-Organic Chemistry,
College of Chemistry, Nankai University, Tianjin 300071,
People’s Republic of China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00399
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21732002, 22077071) and the Tianjin Research
Innovation Project for Postgraduate Students (2019YJSB085)
for generous financial support for our programs. Dedicated to
the 100th anniversary of Chemistry at Nankai University.
light; then, the excited state is quenched by benzenesulfinate-
activated alkyl boronic acid 48 to form a nucleophilic alkyl
radical and species 49, along with a highly reducible Ir2+
species. Then, the alkyl radical adds to the alkenyl sulfone to
afford benzyl radical 50. Single-electron reduction of 50 by Ir2+
generates benzyl anion 51, and the subsequent departure of a
sulfone group gives the product and completes the photoredox
cycle. Attack of solvent-derived water on 49 completes the
benzenesulfinate activation cycle.
In conclusion, we have developed a mild protocol for visible-
light-mediated alkenylation reactions of alkyl boronic acids
without the need for an external Lewis base as an activator, and
we propose that the reactions proceed via a mechanism
involving benzenesulfinate-activated alkyl boronic acids. The
mildness of the protocol makes it suitable for the late-stage
functionalization of natural products and drug molecules. Our
findings offer a new route to active alkyl boronic acids, leading
to the generation of alkyl radicals under mild photoredox
conditions.
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(20) When tert-butyl boric acid is used, only a <10% yield can be
detected from the NMR yield.
(21) As for the initiation of the reaction, we guessed in our reaction
that alkenyl sulfones would generate trace benzenesulfinate under
irradiation and a photocatalyst.
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Org. Lett. 2021, 23, 2477−2481