Photocatalytic C-F Bond Borylation of Polyfluoroarenes with NHC-boranes

Article abstract of DOI:10.1021/acs.orglett.0c00020

The first photoredox-catalyzed defluoroborylation of polyfluoroarenes with NHC-BH₃ has been facilely achieved at room temperature via a single-electron-transfer (SET)/radical addition pathway. This new strategy makes full use of the advantage of photoredox catalysis to generate the key boryl radical via direct activation of a B-H bond. Good functional group tolerance and high regioselectivity offer this protocol incomparable advantages in preparing a wide array of valuable polyfluoroarylboron compounds. Moreover, both computational and experimental studies were performed to illustrate the reaction mechanism.

Full text of DOI:10.1021/acs.orglett.0c00020

pubs.acs.org/OrgLett
Letter
Photocatalytic C−F Bond Borylation of Polyfluoroarenes with NHC-
boranes
Peng-Ju Xia, Zhi-Peng Ye, Yuan-Zhuo Hu, Jun-An Xiao, Kai Chen,* Hao-Yue Xiang,* Xiao-Qing Chen,*
and Hua Yang*
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ABSTRACT: The first photoredox-catalyzed defluoroborylation of
polyfluoroarenes with NHC-BH3 has been facilely achieved at room
temperature via a single-electron-transfer (SET)/radical addition
pathway. This new strategy makes full use of the advantage of
photoredox catalysis to generate the key boryl radical via direct
activation of a B−H bond. Good functional group tolerance and high
regioselectivity offer this protocol incomparable advantages in
preparing a wide array of valuable polyfluoroarylboron compounds.
Moreover, both computational and experimental studies were
performed to illustrate the reaction mechanism.
olyfluoroarenes (ArFn) are important fluorinated aro-
matics that are ubiquitous in pharmaceuticals, materials,
P
agrochemicals, and electronic devices.1 Over the past years,
substantial efforts have been devoted to the development of
new methods to access such valuable scaffolds.2 In between,
transforming commercially available polyfluoroarenes into
polyfluoroaryl organoboron compounds could be one of the
most promising ways to achieve this goal. As is well-known,
organoboron compounds are versatile intermediates that
undergo Suzuki−Miyaura reactions, furnishing a series of
diversity-oriented targets.3 Nevertheless, because of the
robustness of the C−F bond and the lack of an efficient
catalytic system, direct C−F bond borylation of polyfluoroar-
enes still remains challenging.
In 2015, Zhang et al.4 developed a Rh-catalyzed ortho-
selective C−F bond borylation of N-heterocycle-substituted
polyfluoroarenes with bis(pinacolato)diboron (B2pin2) (Figure
1A), in which the N-heterocycle as a directing group was
necessary to secure the ortho selectivity. Subsequently, in 2016
Marder, Radius, and co-workers realized a thermal C−F
borylation of polyfluoroarenes with B2pin2 catalyzed by an N-
heterocyclic carbene (NHC) nickel complex.5 It is worth
noting that elevated temperature was necessary for this
transformation since a high energy barrier was involved in
the transmetalation step. While these reported protocols
heavily rely on thermal metal-assisted C−B bond formation,
the quest for new pathways to realize C−F borylation of
polyfluoroaromatics is persistently motivated in the journey.
With the rapid advance of photocatalysts, the field of
photocatalysis has provided a new dimension to expand the
potentials of synthetic methodology.6 Surprisingly, this newly
emerging synthetic tool has been relatively underdeveloped for
C−F bond borylation of polyfluoroarenes to date. In 2018,
Figure 1. Synthetic profiles for borylation of polyfluoroarenes.
Guo, Radius, Steffen, Marder, and co-workers documented an
elegant borylation of polyfluoroarenes with B2pin2 through a
well-designed dual catalytic system that included a nickel
Received: January 2, 2020
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© XXXX American Chemical Society
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catalyst to activate the C−F bond and a rhodium biphenyl
complex as the triplet sensitizer (Figure 1B).7 Similar to the
previously reported thermal methods, a transmetalation step
was involved in this process as well. At this point,
straightforward photocatalytic borylation of a C−F bond in
polyfluoroarenes is extremely appealing but challenging.
With regard to photoredox catalysis, various C−F bond
functionalizations, including arylation, alkylation, and hydro-
defluorination, have been accomplished in recent years.8 The
generally accepted mechanism for these transformations in the
selective activation of the C−F bond mainly depends on the
generation of a polyfluoroaryl radical via a photocatalytic single
electron transfer (SET) process, despite the high bond
dissociation energy (BDE) of the C−F bond.9 Thus, we
speculated that if a boryl radical could be formed
simultaneously, the corresponding radical−radical coupling
could take place to realize the borylation of polyfluoroarenes.
Obviously, seeking suitable sources of boryl radicals is crucial
to the success of this hypothesis. It was disclosed that NHC-
boranes could serve as reliable sources of boryl radicals
through photocatalytic hydrogen atom transfer (HAT)
processes.10 On the basis of the oxidation potential of NHC-
borane and its low BDE for the B−H bond (74.5 kcal/mol),11
we speculated that the B−H bond of NHC-borane might be
directly scissored via a SET process to give the corresponding
boryl radical. Thus, we rationalized that the direct borylation of
polyfluoroarenes with NHC-boranes via a photoinduced SET
process could be feasible. As part of our ongoing interest in
photocatalysis,12 we were delighted to achieve the first
transformation of polyfluoroarenes into NHC-boryl polyfluor-
oarenes under photoredox conditions (Figure 1C). More
importantly, mechanistic studies indicated that this borylation
reaction presumably proceeds through an unexpected radical
addition pathway.
(dtbpy)][PF6] and [Ir(ppy)2(dtbbpy)][PF6] were able to
facilitate the transformation to give the desired borylated
perfluoropyridine effectively (entries 1 and 2). Ir-
(ppy)2(dtbbpy)PF6 was proven to be the optimum photo-
catalyst, furnishing the desired product 3a (CCDC 1966819)
in 94% yield. Expectedly, both light and the photocatalyst were
enssential to this transformation (entries 5 and 6). K2HPO4
acted as a base to neutralize the HF byproduct, thus improving
the chemical yield (entry 7). Further exploration showed that
increasing the concentrations of the reactants appropriately
could further improve the reaction efficiency (entry 8).
However, reducing the amount of 2a led to a prolonged
reaction time (entry 9).
With the established reaction conditions, we sought to
define the generality of the methodology by exploring the
scope of polyfluoroarenes. As illustrated in Scheme 1, a variety
of polyfluoroarylboron compounds were prepared through this
photocatalytic protocol in good to excellent yields. Fluoroar-
enes with different degrees of fluorination were well-tolerated.
For 1,2,3,4,5-pentafluorobenzenes, functional groups including
N(Bz)2, MeO, CH2CN, Cl, CF3, CN, esters, ketone, and amide
were compatible with this photoredox-catalyzed system with
good reaction efficiencies (3b−n). It is worth mentioning that
the nature of the substituent on the 1,2,3,4,5-pentafluor-
obenzene significantly impacted the regioslectivity. Specifically,
electron-donating groups, such as MeO and alkyl groups,
resulted in lower regioslectivity, while strong electron-with-
drawing substituents consistently provided good to excellent
regioslectivities. Unexpectedly, bis(perfluorophenyl)dimethyl-
silane (2q) underwent a desilylation process to afford 3b in
82% yield. Interestingly, monoborylated tris(perfluorophenyl)-
phosphane 3r was successfully obtained in excellent yield
(92%). Unfortunately, the substrates bearing OH, dicyano, or
NO2 (2u−x) were unable to deliver the desired product under
the standard reaction conditions. Pleasingly, the scaled-up
reaction of 1a and 2a smoothly delivered 3a in 86% yield
simply through a single run of filtration and recrystallization.
Meanwhile, treatment of polyfluoroarylboron compound 3a
with Selectfluor furnished NHC-difluoroborane 3aa, which
served as a bench-stable organoboron reagent with potential
for synthetic utilization.3a
With the survey of radical receptors established, we next
shifted our attention to other NHC-boryl radical precursors,
using perfluoropyridine as the model radical receptor (Scheme
2). The substituents on the NHC framework slightly interfered
with the reaction efficiency, smoothly furnishing the desired
products 4a−e. Meanwhile, other types of Lewis base-based
boranes (LB-BH3) were also evaluated. It was found that
pyridine−borane derivatives worked under these conditions as
well, affording the corresponding products 4f and 4g in good
yields. However, no desired products were detected for Et3N−
BH3 and Ph3P−BH3, likely because the B−H bonds in these
LB-BH3 could not be oxidized directly by the photoexcited
catalyst.
To gain insights into the reaction mechanism, we carried out
combined computational and experimental studies (Figure 2).
On the basis of our studies, a plausible reaction pathway
through radical addition is proposed as illustrated in Figure 2a.
The photocatalytic transformation begins with the photo-
excitation of [IrIII]+ to *[IrIII]+, followed by SET with 1a to
produce a reducing [IrII] species and NHC-boryl radical I,
along with a proton transfer from NHC-BH3 to [HPO4]2−.
The concerted proton-coupled electron transfer (PCET) step
We commenced our survey by evaluating the reaction
between NHC-BH3 (1a) and perfluoropyridine (2a) in the
presence of the photocatalyst and K2HPO4 as the base in
MeCN under irradiation by a 30 W blue LED at room
temperature (Table 1). Pleasingly, both [Ir(dFCF3ppy)2-
Table 1. Optimization of the Photocatalyzed Reaction
a
Conditions
entry
cat
base
time yield (%)
1
2
3
4
5
[Ir(dFCF3ppy)2(dtbpy)][PF6]
[Ir(ppy)2(dtbbpy)][PF6]
[Ru(bpy)3][PF6]
Ir(ppy)3
−
[Ir(ppy)2(dtbbpy)][PF6]
[Ir(ppy)2(dtbbpy)][PF6]
[Ir(ppy)2(dtbbpy)][PF6]
[Ir(ppy)2(dtbbpy)][PF6]
K2HPO4
K2HPO4
K2HPO4
K2HPO4
K2HPO4
K2HPO4
−
24
24
24
24
24
24
24
24
36
32
94
nr
trace
nr
trace
32
96
b
6
7
c
8
d
K2HPO4
K2HPO4
9
85
a
Reaction conditions: 1a (0.2 mmol, 1.0 equiv), 2a (0.3 mmol, 1.5
equiv), K2HPO4 (0.3 mmol, 1.5 equiv), catalyst (2 mmol %), MeCN
b
(2 mL), 30 W blue LED, argon atmosphere, rt, 24 h. Without LED.
c
d
Performed on a 0.4 mmol scale of 1a with MeCN (2 mL). 2a (0.24
mmol, 1.2 equiv).
B
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a b
,
a b
,
Scheme 1. Scope with Respect to Polyfluoroarenes
Scheme 2. Scope with Respect to LB-BH3
a
Standard conditions: 1 (0.4 mmol, 1.0 equiv), 2a (0.6 mmol, 1.5
equiv), K2HPO4 (0.6 mmol, 1.5 equiv), Ir(ppy)2(dtbbpy)PF6 (2
mmol %), MeCN (2 mL), 30 W blue LED, argon atmosphere, rt.
b
Isolated yields of products are shown.
species donates one electron to addition intermediate II to
afford anion III, and the fluoride extrusion occurs easily
because of the weaker C−F bond (1.58 vs 1.35 Å), giving the
final product 3a. We envisioned perfluoropyridine to be a
possible competent acceptor to receive one electron from
[IrII], after which radical−radical coupling of multifluoroaryl
radical and NHC-boryl radical I could also form product 3a.
red
1/2
However, the redox potential of perfluoropyridine (E
=
−2.12 V vs SCE)13 is lower than that of the IrII species
(E(IrII/III) = −1.51 V vs SCE),14 suggesting that the favorable
electrostatic stabilization between positive IrIII and negative
perfluoropyridine radical anion could facilitate this endergonic
electron transfer.13 Because of the entropy loss due to
complexation and the large dielectric constant of acetonitrile
(ε = 35.688), this favorable interaction is still not enough to
compensate for this large potential difference (0.6 eV = 14.1
kcal/mol; see the Supporting Information for details). As
expected, the calculated redox potential of radical addition
intermediate II is −0.24 V (Figure 2b), making it an ideal
oxidant to accept one electron from [IrII], which supports our
radical addition mechanism. Meanwhile, experimental studies
were designed to further understand the reaction mechanism.
Stern−Volmer experiments revealed that the excited photo-
catalyst could be effectively quenched by NHC-BH3 and
DMAP-BH3 but was unaffected by Ph3P−BH3. In addition, the
corresponding product could be formed only during irradiation
in light/dark experiments. These results support the first SET
process between *[IrIII]+ and 1a. Radical addition intermediate
II could be detected by HRMS. With the addition of the spin-
trapping agent 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO),
the reactions were significantly inhibited under the standard
a
Standard conditions: 1a (0.4 mmol, 1.0 equiv), 2 (0.6 mmol, 1.5
equiv), K2HPO4 (0.6 mmol, 1.5 equiv), Ir(ppy)2(dtbbpy)PF6 (2
mmol %), MeCN (2 mL), 30 W blue LED, argon atmosphere, rt.
b
c
Isolated yields of the products are shown. Isolated yield of
inseparable regioisomers. The ratio of regioisomers was determined
by 19F NMR analysis.
is highly exergonic by −49.0 kcal/mol, which is preferred to
the stepwise HAT/SET or SET/HAT pathway (see the
Supporting Information for details). Then radical intermediate
I can react with perfluoropyridine via TS1, resulting in the
generation of radical addition species II with an activation free
energy barrier of about 12.8 kcal/mol. Subsequently, the [IrII]
C
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Letter
of this radical transformation of NHC-BH3 with polyfluoroar-
enes would significantly facilitate the exploration of efficient
C−B bond formation in the syntheses of valuable fluorinated
scaffolds.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00020.
Experimental procedures, spectral data for all new
compounds, and crystallographic data for 3a, 3f, 3g,
3aa, and 4f (PDF)
Accession Codes
CCDC 1966818−1966820, 1967106, and 1972312 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44
1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Kai Chen − College of Chemistry and Chemical Engineering,
Central South University, Changsha 410083, P. R. China; State
Key Laboratory of Chemical Oncogenomics, Peking University
Shenzhen Graduate School, Shenzhen 518055, P. R. China;
Email: kaichen@csu.edu.cn
Hao-Yue Xiang − College of Chemistry and Chemical
Engineering, Central South University, Changsha 410083, P. R.
China; orcid.org/0000-0002-7404-4247;
Email: xianghaoyue@csu.edu.cn
Xiao-Qing Chen − College of Chemistry and Chemical
Engineering, Central South University, Changsha 410083, P. R.
China; Guangxi Key Laboratory of Natural Polymer Chemistry
and Physics, Nanning Normal University, Nanning 530001, P.
R. China; orcid.org/0000-0002-8768-8965;
Figure 2. Computational and experimental support for the reaction
mechanism. (a) Calculated potential energy surface of the plausible
reaction pathway. (b) Calculated and experimental one-electron
reduction potentials. (c) Important adducts detected. (d) Kinetic
isotope effect (KIE) for this reaction. Energies are given in kcal/mol
and distances in angstroms. [IrIII]+ and [IrII] are short for
[Ir(ppy)2(dtbbpy)]+ and [Ir(ppy)2(dtbbpy)], respectively.
Email: xqchen@csu.edu.cn
Hua Yang − College of Chemistry and Chemical Engineering,
Central South University, Changsha 410083, P. R. China;
Guangxi Key Laboratory of Natural Polymer Chemistry and
Physics, Nanning Normal University, Nanning 530001, P. R.
China; orcid.org/0000-0002-5518-5255;
Email: hyangchem@csu.edu.cn
reaction conditions, and NHC-boryl radical I could be trapped,
as detected by HRMS (Figure 2c). A significant intermolecular
kinetic isotope effect (KIE) (kH/kD ≈ 3.3) between 1a and its
deuterated analogue 1a′ was observed (Figure 2d), suggesting
that the B−H bond cleavage is the rate-limiting step. This is in
full agreement with our calculations, as the concerted PCET
step from 1a to I is rate-determining, as shown by the potential
energy surface (Figure 2a).
Authors
Peng-Ju Xia − College of Chemistry and Chemical Engineering,
Central South University, Changsha 410083, P. R. China
Zhi-Peng Ye − College of Chemistry and Chemical Engineering,
Central South University, Changsha 410083, P. R. China
Yuan-Zhuo Hu − Key Laboratory of Hunan Province for Water
Environment and Agriculture Product Safety, Central South
University, Changsha 410083, P. R. China
In conclusion, by virtue of the redox feature of NHC-BH3, a
photocatalytic C−F borylation was accomplished via a
photoinduced SET process. The method is featured by its
broad scope without sacrificing its practicality and efficiency.
The resulting polyfluoroarylboron compounds have potential
applications in synthetic and medicinal chemistry. Experimen-
tal observations, kinetic data, and DFT calculations synergeti-
cally support a radical addition pathway involving an NHC-
boryl radical. We believe that a full mechanistic understanding
Jun-An Xiao − Guangxi Key Laboratory of Natural Polymer
Chemistry and Physics, Nanning Normal University, Nanning
530001, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00020
Notes
The authors declare no competing financial interest.
D
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Letter
Intermediates. Acc. Chem. Res. 2016, 49, 2273−2283. (b) Kikushima,
K.; Grellier, M.; Ohashi, M.; Ogoshi, S. Transition-Metal-Free
Catalytic Hydrodefluorination of Polyfluoroarenes by Concerted
Nucleophilic Aromatic Substitution with a Hydrosilicate. Angew.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (21576296,
21776318, 21676302, and 81703365), the Natural Science
Foundation of Hunan Province (2017JJ3401 and 2018JJ3868),
and the Fundamental Research Funds for Central South
University (2019zzts222).
̈
Yao, C.-J.; Konig, B. Metal-Free Perfluoroarylation by Visible Light
Photoredox Catalysis. ACS Catal. 2016, 6, 369−375. (d) Senaweera,
S.; Weaver, J. D. Dual C-F, C-H Functionalization via Photocatalysis:
Access to Multifluorinated Biaryls. J. Am. Chem. Soc. 2016, 138,
2520−2523. (e) Priya, S.; Weaver, J. D. Prenyl Praxis: A Method for
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