Visible Light Induced Rhodium(I)-Catalyzed C?H Borylation

Article abstract of DOI:10.1002/anie.201905924

An efficient visible light induced rhodium(I)-catalyzed regioselective borylation of aromatic C?H bonds is reported. The photocatalytic system is based on a single NHC?Rh^I complex capable of both harvesting visible light and enabling the bond breaking/forming at room temperature. The chelating nature of the NHC-carboxylate ligand was critical to ensure the stability of the Rh^I complex and to provide excellent photocatalytic activities. Experimental mechanistic studies evidenced a photooxidative ortho C?H bond addition upon irradiation with blue LEDs, leading to a cyclometalated Rh^III-hydride intermediate.

Full text of DOI:10.1002/anie.201905924

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Accepted Article
Title: Visible Light Induced Rhodium(I)-Catalyzed Directed C-H
Borylation
Authors: Olivier Baslé, Jompol Thongpaen, Romane Manguin, Vincent
Dorcet, carine Duhayon, Marc Mauduit, and Thomas Vives
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201905924
Angew. Chem. 10.1002/ange.201905924
Link to VoR: http://dx.doi.org/10.1002/anie.201905924
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10.1002/anie.201905924
Angewandte Chemie International Edition
COMMUNICATION
-
Visible Light Induced Rhodium(I)-Catalyzed Directed C H
Borylation.
Jompol Thongpaen, Romane Manguin, Vincent Dorcet, Thomas Vives, Carine Duhayon, Marc Mauduit
and Olivier Baslé*
Abstract: An efficient visible light-induced rhodium(I)-catalyzed
regioselective borylation of aromatic C-H bonds is reported. The
photocatalytic system is based on a single NHC-Rh(I) complex
capable of both harvesting visible light and enabling the bond
breaking/forming at room temperature. The chelating nature of the
NHC-carboxylate ligand was critical to ensure the stability of the Rh(I)
complexes and to provide excellent photocatalytic activities.
Experimental mechanistic studies evidenced a photooxidative ortho
C-H bond addition upon irradiation with blue LEDs that led to a
cyclometalated Rh(III) hydride intermediate.
borylation of arenes is arguably the most straightforward
approach to access these useful building blocks.12 Despite the
recent advances made in TM-catalyzed site selective C-H
borylation,13 only poor regioselectivities have been observed with
the few photocatalytic systems that have been reported and that
rely on the use of less attractive high energy UV-light (Figure
1c).14 We report here an unprecedented visible light-induced
rhodium(I)-catalyzed regioselective borylation of aromatic C-H
bonds at room temperature (Figure 1d).
Previous work:
hν (visible light)
hν (visible light)
C-FG
a)
c)
b) C-H
C-FG
C-H
PC TM
PC
The direct functionalization of inert C-H bonds has emerged over
the past two decades as an increasingly important synthetic tool.1
hν (UV light)
Bpin
H
In
particular,
transition
metal
(TM)-catalyzed
C-H
Ar
R
Ar
mixture
(o:m:p)
R
TM
functionalization has witnessed continuing improvements in
performance,2 allowing to expend the toolbox available for organic
material synthesis,3 natural products synthesis4 and drug
discovery programs.5 In spite of this success, there is still a need
for the development of C-H transformations that can efficiently
operate under mild conditions with broad functional group
tolerance and high selectivity. The recent enthusiasm for visible
light-driven reactions has resulted in a rise in the number of
innovative catalytic systems that enable C-H transformations
under mild conditions.6 Most of these methodologies involve the
photosensitization of a photoredox catalyst (PC) that promotes
the transformation either via hydrogen atom transfer (HAT) or
single electron transfer (SET) (Figure 1 a).7 More recently, by
merging photoredox catalysis and TM-catalysis, this dual
catalysis strategy has allowed the installation of unique functional
groups with efficient selectivity control (Figure 1 b).8 An ideal
alternative, would be to use a single TM catalyst capable of both
harvesting visible light and enabling the bond breaking/forming
within the coordination sphere of the metal center.9 While this
strategy employing unconventional TM photocatalysts has
notably led to attractive reactivities in coupling reactions,10 the
field of visible light-induced TM-catalyzed C-H functionalization
still requires fine catalyst engineering to address the various
challenges associated with the selective transformation of
unactivated C-H bonds.11
This work:
d)
DG
Ar
νmax = 460 nm
DG
Ar
H
Bpin
Rh
unactivated
selective o-borylation
Figure 1. Light-driven catalytic C-H functionalization strategies.
The use of N-Heterocyclic Carbene (NHC) ligands has
allowed for the preparation of TM complexes that are highly
valuable materials to access efficient and selective homogeneous
catalysis,15 including photocatalysis.16 Nevertheless, the use of
such ligands in TM-catalyzed C-H bond borylation is scarce.17 We
recently developed a new class of bidentate carboxylate NHC
ligands,18 which proved to be beneficial for Cp*Rh(III)-based
catalysts in aromatic and aliphatic C-H bond borylations under
thermal conditions.19 These bidentate NHC-carboxylate ligands
have also been recently used in the preparation of particularly
robust luminescent octahedral Ir(III) complexes, which were
shown to display some interesting photophysical properties useful
in the context of visible light-induced catalysis.20 Therefore, with
the objective to achieve borylation under the mildest conditions
possible, we hypothesized that the incorporation on a Rh(I)
complex of a strong electron-donating bidentate NHC-carboxylate
ligand in association with a relatively labile 1,5-cyclooctadiene
(cod) ligand should afford simple visible-light absorbing
complexes that fit the requirements to achieve the pursued
photocatalytic activity. To this end, a series of amino acid-based
NHC ligand precursors 1 were converted to their corresponding
silver-intermediates and then transmetalated to [Rh(cod)Cl]2 to
afford the desired Rh(I) complexes in good yields (Figure 2). X-
ray diffraction analysis unambiguously confirmed the structure of
complex 2a, which exhibits a slightly distorted square planar
geometry (Figure 2).21 Importantly, UV-vis spectra of complexes
1 evidenced the expected visible light absorption (Figure S2)
suitable for excitation with blue light emitting diodes (LEDs).
Organoboron compounds are versatile intermediates for
medicinal chemistry and materials science and the direct C-H
[*]
J. Thongpaen, R. Manguin, T. Vives, Dr. M. Mauduit, Dr. O. Baslé
Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes,
CNRS, ISCR – UMR 6226, F-35000 Rennes, France.
E-mail: olivier.basle@ensc-rennes.fr
J. Thongpaen, Dr. C. Duhayon, Dr. O. Baslé
LCC-CNRS, Université de Toulouse, CNRS, Toulouse, France
E-mail : olivier.basle@lcc-toulouse.fr
V. Dorcet
Univ Rennes, CNRS, ISCR – UMR 6226, F-35000 Rennes, France.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201905924
Angewandte Chemie International Edition
COMMUNICATION
Table 1. Optimization of the reaction conditions.[a]
Cl
R
R
N
N
N
N
R
R
O
O
HO
Rh
O
1
N
N
1) Ag2O
[Rh] (5 mol%)
B2pin2
H
Bpin
+
2) [Rh(cod)Cl]2
Solvent, 25 °C
R = iBu, 2a, 95% yield
R = iPr, 2b, 93% yield
R = Me, 2c, 82% yield
Blue LEDs, 16 h, Ar
4a (R = H)
4b (R = Me)
3a (R = H)
3b (R = Me)
2a
Entry
Catalyst
R
Solvent
Yield (%)[b]
Figure 2. (NHC)Rh(cod) complexes 2 (left) and X-ray crystal structure of 2a
(right). Most hydrogen atoms are omitted for clarity. For complex 2a: distances
between Ccarbenic and Rh = 2.04 Å, between Rh and O = 2.08 Å. Angle for
Ccarbenic-Rh-O = 85.2°. cod =1,5-cyclooctadiene
1
2
3
4
5
2a
2a
2a
2a
2a
H
THF
50
>99
85
<5
0
Me
Me
Me
Me
THF
C6H6
With the new complexes in hand, we decided to investigate
their photocatalytic activity in aromatic C-H borylation directed by
CH2Cl2
CH3CN
a
pyridine group using B2pin2 [bis(pinacolato)diboron] as
6[c]
7
2a
Me
Me
Me
Me
Me
Me
Me
THF
THF
THF
THF
THF
THF
THF
51
91
76
<5
<5
0
borylating reagent (Table 1). The functionalization of 2-
phenylpyridine (3a) in tetrahydrofuran (THF) in the presence of
complex 2a irradiated overnight with 20W blue LEDs allowed 90%
conversion of 3a with modest selectivity and gave the desired o-
borylated product in 50% yield.22 Pleasantly, the regioselectivity
outcome of the borylation reaction could be efficiently improved
using 4-methylpyridine as directing group to afford the borylated
product 4b in nearly quantitative yield (entry 2). The use of other
solvents dramatically impacted the catalytic activities (entries 3-
5). When THF was replaced by benzene, the yield of 4b
decreased to 85%. Only traces of product were obtained in
dichloromethane and no detectable amount of 4b could be
observed when performing the reaction in acetonitrile. On the
other hand, the use of pinacolborane (HBpin) instead of B2pin2
gave the desired product in modest 51% yield (entry 6). The
influence of the chelating side arm of the NHC ligand was also
investigated (entries 7-10). While similar efficiency was obtained
using 2b, a noticeable decreased of the borylation efficiency was
observed with the alanine-based complex 1c. The importance of
the chelating NHC ligand was further evidenced with the very poor
catalytic activities observed using [Rh(cod)Cl]2 (entry 9), or
monodentate NHC-based (IMes)Rh(cod)Cl (entry 10). Finally, a
series of control experiments revealed that both the rhodium
catalyst and the light are essential for the reaction to proceed
(entries 11 and 12).
2b
8
2c
9
[Rh(cod)Cl]2
(IMes)Rh(cod)Cl
2a
10[d]
11[e]
12
none
0
[a] Reaction conditions: 3 (0.1 mmol), B2pin2 (0.2 mmol), [Rh] (0.005 mmol),
solvent (0.1M), 20W Blue LEDs (lmax = 460 nm), 16 h under Ar. [b] NMR
yield using 1,3,5-trimethoxybenzene as an internal standard. [c] HBpin
instead of B2Pin2. [d] IMes = 1,3-Bis(2,4,6-trimethylphenyl)imidazolidene. [e]
Reaction performed in the dark from 25 to 80 °C.
(3m-3q). No reaction could be observed for the prochiral
ferrocenyl substrate 3r. This lack of reactivity may originate from
the strong visible light absorbance of the substrate (l ~ 460 nm)
that can interfere with the catalyst activation (Figure S3). On the
other hand, the site-selective C(sp3)-H borylation of substrate 3s
could be achieved with high efficiency and low enantio-
induction.24 Finally, substrate 3t was engaged in a pyridine-
directed desymmetrization,25 to afford the desired product 5t in
good 67% isolated yields and with similar 14% enantiomeric
excess.
To probe the mechanism of the visible-light induced C-H
borylation process, we performed a series of mechanistic
experiments. First, the catalytic borylation of substrate 3b
conducted in the presence of butylated hydroxytoluene (BHT) or
TEMPO [2,2,6,6-tetramethylpiperidine-1-oxyl] proceeded without
noticeable decrease of catalytic activities, which suggested that
processes involving radicals are unlikely to be operative (Scheme
S10). Then, the on/off experiment was combined with the kinetic
isotope effect (KIE) cross-experiment (Scheme 1). When the
LEDs were switched off, the reaction did not proceed in the dark,
which indicated that constant photoexcitation is essential for the
transformation. In addition, the KIE value of 2.1 determined by
comparing the two reaction rate constants independently
measured using either 3b or its pentadeuterated analogue d5-3b
suggested that the C-H bond breaking occurs during the rate-
determining step.26 Further insights into the mechanism of the
C-H breaking process were gained through the reaction of
stoichiometric amount of complex 2a and substrate 3b in d8-THF.
Indeed, while no reaction was observed in the dark, blue LEDs
irradiation of a solution of the rhodium complex 2a with 3b allowed
Subsequently, the scope of the borylation reaction with
respect to 2-arylpyridine based substrate 3 was studied (Table 2).
The optimized conditions were utilized to examine steric and
electronic influences on the reaction efficiency. The yield of the
ortho-borylated product
4
was determined by 1H NMR
spectroscopy and the crude reaction mixture was then treated
with Oxone® to allow the efficient isolation of the corresponding
hydroxylated product 5.19 Aryl groups bearing a substituent at
para-(3c, 3d), meta-(3e) and ortho-(3f) were well tolerated
affording the functionalized products (5c-5f) in good isolated
yields. When comparing with the result obtained with the electron-
rich 3g, a slight decrease of reactivity was observed with electron-
deficient 3h and 3i. 2-phenylpyridine bearing a trifluoromethyl (3j)
and a phenyl (3k) substituent on the pyridyl group are also
amenable substrates. Borylation of the 1-naphthyl fragment of the
sterically congested substrate 3l was highly efficient and allowed
the isolation of 5l in a racemic form.23 The isoquinoline-group
were also amenable to direct the borylation in ortho-position of
electron-rich and electron-deficient substituted aryl fragments
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10.1002/anie.201905924
Angewandte Chemie International Edition
COMMUNICATION
Table 2. Scope of the visible-light induced C-H borylation. [a]
2a (5 mol%)
N
N
+ / or 3b
d5
+ / or 4b
B2pin2, d8-THF, 25 °C
blue LEDs, Ar
KIE = kH/kD = 2.3 (competition)
R
R
R
Bpin
N
N
N
d4
2a (5 mol%)
H
Bpin
OH
B2pin2 (2 equiv.)
THF, 25 °C, Ar
Blue LEDs, 16 h
d5-3b
d4-4b
Aqueous
Oxone
KIE = kH/kD = 2.1 (parallel reactions)
Ar
Ar
Ar
35
4, (Yield)[b]
5, Yield[c]
3
30
25
20
15
10
5
N
N
OH
N
N
OH
R
OH
OH
5b, 89% (98%)
R = Me, 5c, 70% (85%)
R = Ph, 5d, 71 (91%)
5e, 69% (>98%)
5f, 64% (>98%)
CF3
Ph
N
N
N
N
OH
EtO
0
0
OH
OH
OH
R = OEt, 5g, 82% (>98%)
R = CO2Et, 5h, 54% (67%)
R = CF3, 5i, 68% (79%)
30
60
90
120
Time (min)
150
180
210
240
270
300
5j, 49% (60%)
5k, 65% (77%)
5l, 80% (86%)
C-H borylation
C-D borylation
Scheme 1. Time profile of the isotope cross-experiment in light and dark.
N
Bpin
N
N
N
N
OH
Fe
R
OH
Bpin
Dark, 25 °C
No reaction
d8-THF
R = H, 5m, 52% (61%)
R = OEt, 5n, 68% (70%)
R = OMe, 5o, 50% (58%)
R = Ph, 5p, 40 % (53%)
R = tBu, 5q, 41% (51%)
4r, no reaction
4s[d], 73% (88%)
12% ee[f]
5t[e], 67% (75%)
14% ee[f]
N
N
2a
+
3b
(1 equiv.)
O
(III)
O
O
blue LEDs, 25 °C
Rh
d8
d8-THF
[a] Reactions conditions: 3 (0.2 mmol), B2pin2 (0.4 mmol), THF (0.1M), 20W Blue
LEDs (lmax = 460 nm), 16 h under Ar. [b] NMR yield of the borylated product 4.
[c] Isolated yield of the hydroxylated product 5. [d] 48 h reaction. [e] 2a (10 mol%).
[f] Enantiomeric excess determined by HPLC analyses.
N
H
- 1,3-cod
6
δ -21,7 (d, JRhH = 36.9 Hz)
(1H,13C) HMBC correlations
Scheme 2. Stochiometric photooxidative C-H bond addition experiment
the formation of a new species with a characteristic Rh-H
resonance at d -21.7 (d, JRhH = 36.9 Hz, Scheme 2) that is
concomitant with release of the 1,5-cod ligand isomerized to 1,3-
cod (Figure S10).27 In line with the solvent effect observed during
the optimization of the catalytic process (Table 1), the
stoichiometric reaction between 2a and 3b upon blue LEDs
irradiation in C6D6 revealed similar results, while no such Rh-H
signal could be detected in CD3CN. Additional thorough NMR
spectroscopy analyses, including 2D (1H,13C)-HMBC at 213 K in
d8-THF, revealed strong correlations between the hydride ligand,
the Ccarbenic and the ortho-Cphenyl atoms that allowed us to propose
the expected cyclometalated Rh(III)-hydride species 6 resulting
from the oxidative addition of the ortho-C-H bond of 3b (Scheme 2
and Figure S13). Despite unsuccessful isolation of 6 due to
pronounced instability, structural characterization of the
corresponding Rh-(µ-OH) species arising from hydrolysis could
further confirm the octahedral geometry of the postulated
cyclometalated Rh(III) intermediate (Scheme S12). Furthermore, it
is important to note that no such hydride species were detected
when 3b was subsequently added in the dark to a pre-irradiated
solution of complex 2a in d8-THF which confirmed that light induced
the C-H activation process. In analogy with the mechanistic study
of Gonzalez-Herrero regarding C-H activation in the triplet excited
state of Pt(II) complex,28 we postulate that a metal to ligand charge
transfer (MLCT) promotes oxidative ortho C-H addition leading to
the cyclometalated Rh(III)-hydride intermediate 6.29 Although, we
are unsure at this point of the exact nature of the species involved
in the subsequent C-B formation event,30 we propose that 6 reacts
with B2pin2 to produce HBpin and the corresponding Rh(III)-boryl
species,31 which undergo reductive elimination of the desired
ortho-borylated product 4.32
In summary, rhodium-catalyzed regioselective borylation of
aromatic C-H bonds can be induced by visible-light. We
discovered a photocatalytic process that involves the direct
visible-light absorption of a NHC-Rh(I) complex capable of
breaking Csp2-H bonds and forming C-B bonds at room
temperature. This mild approach to activate C-H bonds is
anticipated to find applications in other functionalization
reactions, including asymmetric transformations.
Acknowledgements
We acknowledge the CNRS; the ENSCR and the MESRI (grant
to RM). This work was supported by the ANR (ANR-15-CE07-
0012-01 “CHADOC” grant to OB, JT), We thanks Dr. Christian
Bijani (LCC) for NMR analysis of the Rh-H intermediate.
Keywords: Visible-light• borylation • NHC • rhodium • C-H
activation
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COMMUNICATION
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A
proposed mechanism is shown in Figure S18. For alternative
mechanisms, see reference 12.
This article is protected by copyright. All rights reserved.

10.1002/anie.201905924
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
An efficient visible light-induced
J. Thongpaen, R. Manguin, V. Dorcet, T.
Vives, C. Duhayon, M. Mauduit,
O. Baslé*
B2pin2
Photoexcited
NHC-Rh(I) catalyst
rhodium(I)-catalyzed
regioselective
borylation of aromatic C-H bonds is
reported. The photocatalytic system is
based on a single NHC-Rh(I) complex
capable of both harvesting visible light
N
N
Page No. – Page No.
O
Rh
O
Visible Light-Induced Rhodium(I)-
Catalyzed Directed C-H Borylation
and
enabling
the
bond
R
R
breaking/forming at room temperature.
The bidentate structure of the NHC
ligand was demonstrated to be critical
both for the stability and the reactivity
of Rh(I) complexes.
N
N
Visible light
Directed o-borylation
Photooxidative C-H addition
H
Bpin
Ar
Ar
This article is protected by copyright. All rights reserved.