Photoinduced Miyaura Borylation by a Rare-Earth-Metal Photoreductant: The Hexachlorocerate(III) Anion

Article abstract of DOI:10.1002/anie.201804022

The first photoinduced carbon(sp²)–heteroatom bond forming reaction by a rare-earth-metal photoreductant, a Miyaura borylation, has been achieved. This simple, scalable, and novel borylation method that makes use of the hexachlorocerate(III) anion ([Ce^IIICl₆]^3?, derived from CeCl₃) has a broad substrate scope and functional-group tolerance and can be conducted at room temperature. Combined with Suzuki–Miyaura cross-coupling, the method is applicable to the synthesis of various biaryl products, including through the use of aryl chloride substrates.

Full text of DOI:10.1002/anie.201804022

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Accepted Article
Title: Photoinduced Miyaura Borylation by a Rare Earth
Photoreductant: the Hexachlorocerate(III) Anion
Authors: Yusen Qiao, Qiaomu Yang, and Eric Schelter
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201804022
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Photoinduced Miyaura Borylation by a Rare Earth Photoreductant:
the Hexachlorocerate(III) Anion
Yusen Qiao, Qiaomu Yang, and Eric J. Schelter*
Abstract: The first photoinduced sp2 carbon-heteroatom bond
forming reaction by a rare earth photoreductant, a Miyaura borylation,
has been achieved. This simple, scalable, and novel borylation
method that makes use of the hexachlorocerate(III) anion, [CeIIICl6]3−,
has a broad substrate scope and functional group tolerance and can
be conducted at room temperature. Combined with Suzuki-Miyaura
cross-coupling, the methodology is applicable to the synthesis of
various biaryl products, including through the use of aryl chloride
substrates.
Photocatalysis is an expanding area of productive and green
systems for synthetic chemistry.[1] Among established
photocatalysts, rare earth complexes are an emerging class of
potent, energy efficient, cheap, and readily available molecular
sensitizers.[2] Replacing Ru/Ir noble-metal catalysts by redox
Scheme 1. The established strategies (a and b) and our strategy (c) for
photoinduced borylation of aryl substrates.
active lanthanide compounds is attractive and beneficial in terms
of the lower cost and larger earth abundancy.[2e-h, 3] Our group has
to generate alkoxy radicals.[2e, 2f] Based on the previous findings,
we proposed that the aryl radical generated in situ by [CeIIICl6]3−
could mediate further organic transformations, for example,
photoinduced Miyaura borylation.[5]
been developing molecular cerium(III) photocatalysts whose
excitation and emission result not from 4f→4f transitions, but from
4f→5d excitations, to mediate organic transformations.[2b-d] These
doublet-to-doublet, metal-centered transitions avoid energy
losses through a spin-state change, which commonly occur in
metal-to-ligand charge transfer (MLCT) transitions of transition-
metal photocatalysts. After photoexcitation of the redox active
Ce3+ cation into its long-lived (ns) 2D excited states, it reacts as a
highly reactive metalloradical that participates in single electron
transfer (SET) reactions and reduction of organic substrates. Our
group has discovered that a simple Ce(III) chloride complex, the
hexachlorocerate(III) anion ([CeIIICl6]3−), which can be generated
in situ from CeCl3 and NEt4Cl in acetonitrile, is a potent
photoreductant (E1/2* ≈ −3.45 V versus Cp2Fe0/+) that can reduce
unactivated aryl chlorides.[2d] The anion also exhibited a fast
Arylboronic acids and esters are important building blocks in
synthetic chemistry.[6] Transition-metal catalyzed Miyaura
borylation of aryl halides (or pseudo halides) has been developed
during the past two decades and is one of the most efficient
methods to synthesize arylboron reagents.[7] Borylation of aryl
halides through SET pathway was also reported.[7j, 7k, 8] However,
these reactions required strong base or high temperatures to
activate aryl chlorides. Recently, Larionov and Li independently
reported a metal-free, scalable, and efficient photoinduced
borylation of aryl halides and arylammonium salts under
ultraviolet C (UVC) light (100−280 nm) and continuous-flow
conditions (Scheme 1a).[9] This photoinduced borylation method
can also be applied to aryl triflate substrates.[10] The method is a
complementary approach for transition-metal catalyzed borylation
pathway. However, these reactions required the use of
carcinogenic UVC light and quartz reaction vessels. Using a more
benign energy source and standard laboratory glassware is more
convenient. Moreover, visible-light-induced borylation of aryl
substrates was also reported (Scheme 1b).[11] However, these
methods were not applicable to aryl chlorides and may require
non-commercially available substrates or expensive iridium
catalysts. Herein, we incorporated [CeIIICl6]3− into the first
example of photoinduced Miyaura borylation by a rare earth metal
compound (Scheme 1c). Our approach is applicable to both
aryl/heteroaryl chlorides and bromides with structural and
electronic variations, and various diboron reagents, and
proceeded under room temperature without the use of UVC light,
quartz reaction vessels, non-commercially available substrates,
or expensive transition-metal catalysts.
quenching kinetics (kq
organohalogens.[2d]
=
~
109–1010 s−1) toward
We have employed [CeIIICl6]3− to dehalogenation reactions of
aryl chlorides and bromides through an aryl radical mechanism.[2d]
Our system serves as a complementary method for the homolytic
cleavage of sp2 C−Cl bonds under photolysis conditions to
generate aryl radicals, which then participate in the subsequent
formation of sp2 carbon-carbon and carbon-heteroatom bonds.[4]
Recently, cerium(III) chloride complexes have also been applied
[*]
Y. Qiao, Q. Yang, Prof. E. J. Schelter
Department of Chemistry
University of Pennsylvania
231 S. 34th St., Philadelphia, PA 19104 (USA)
E-mail: schelter@sas.upenn.edu
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201804022
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internal standard. c16 % yield of fluorobenzene was detected. dIrradiated 6 d.
e57% conversion of the starting material.
In pursuit of expanded functionalization of aryl chlorides by
[CeIIICl6]3−, we examined a similar reaction condition to our
previous dechlorination protocol[2d] in the presence of a catalytic
With the optimized reaction conditions, we next evaluated the
substrate scope with respect to the aryl chlorides and bromides
(Table 2). Stern-Volmer experiments of [CeIIICl6]3− demonstrated
fast quenching rates (kq = ~ 109 s−1) of aryl and heteroaryl
chlorides with structural and electronic variations (Figure 1a).
amount of CeCl3 (10 mol%) and 1 equiv B2pin2 (B2pin2
=
Bis(pinacolato)diboron) under black light irradiation (entry 1 in
Table 1). Excess NEt4Cl (0.1 M in CH3CN, 75 mol%) was applied
not only to generate [CeIIICl6]3− but also to avoid the formation of
the photochemically inactive [Ce2Cl9]3–. Adding less than 75 mol%
NEt4Cl was not sufficient for the formation of pure [CeIIICl6]3− in
the reaction conditions (Figure S18 and Table S6).[2d] This initial
attempt gave 72% conversion of 2-(4-fluorophenyl)-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane, as indicated by 19F NMR
spectroscopy using p-fluorotoluene as an internal standard.
Control reactions verified that CeCl3, NEt4Cl, and black light
irradiation were all essential for this transformation (Tables 1 and
S1-S2, Figures S4-S9). Using less powerful black light CFLs (23
W) or hydrated CeCl3 slowed the reaction and decreased the
conversion of the starting materials (entries 4 and 6). Notably,
16% conversion of the dehalogenation side product,
fluorobenzene, was observed in entry 1. To eliminate the
undesired byproduct, we set out to screen solvent and
concentration of the starting materials. The solvent screen
revealed that CH3CN was uniquely effective (entries 7 and 8, and
Table S3). Trace product was detected in other common solvents
in photochemical synthesis, such as THF, DME, acetone, and
ethyl acetate due to the low solubility of the cerium catalyst. No
luminescent cerium complex was formed in polar solvents such
as DMF, presumably due to the decomposition of [CeIIICl6]3−
(Figure S19). Next, we decreased the volume of CH3CN to
suppress the dehalogenation reaction (entry 9 and Table S4).
Incomplete dissolution of NEt4Cl and B2pin2 was observed in entry
11, thus 1.5 mL of CH3CN was the optimal amount of solvent.
Slightly excess of B2pin2 (2 equiv) led to quantitative conversion
of the aryl chloride and improved the conversion of the arylboronic
ester to 95% (entry 10 and Table S4). Finally, applying a lower
catalyst loading (5 mol%) did not significantly impact this
transformation (entry 12 and Table S5).
Table 2. Scope of haloarenesa
Table 1. Optimization of the reaction conditions.
entry
1
2
3
4
5
6
7
variations from standard conditionsa
none
yield(%)b
72c
0
aChloroarenes were used unless specified otherwise. Standard conditions: 0.2
mmol substrate, 0.4 mmol B2pin2, 0.01 mmol (5 mol %) CeCl3, 0.15 mmol
NEt4Cl, 1.5 mL CH3CN, irradiated by ten 8 W black light lamps in a Rayonet
photochemical reactor at room temperature, 24 h. bIsolated yield for all
substrates. cIrradiated for 48 h. dIrradiated for 72 h.
in the dark
blue LEDs
black light CFLs (23 W)
no CeCl3 or NEt4Cl
CeCl3•7H2O instead of CeCl3
DMF instead of CH3CN
0
49d
0
43e
0
Chloroarenes with electron-donating, -neutral, and -withdrawing
groups at the para-, meta-, or ortho-positions--including amino,
methoxy, allyloxy, cyano, ester, phenyl, fluorine, and
trifluoromethyl groups were all converted to the corresponding
arylboronic esters in moderate-to-excellent yields. Moreover, the
borylation method could be carried out at a gram scale (3a). The
borylation of chloroarenes with electron-donating groups at the
para-positions (3i−3k) was more sluggish than that of the
substrates with electron-withdrawing groups (3b−3g). The
sluggish reactivity was due to the relatively slow electron-transfer
8
9
10
11
12
DME instead of CH3CN
< 5
81
95
53
95
1.5 mL CH3CN instead of 2 mL CH3CN
1.5 mL CH3CN and 2 equiv. of B2pin2
1 mL CH3CN and 2 equiv. of B2pin2
entry 10 + 5 mol% CeCl3 instead of 10 mol%
aInitial conditions: 0.2 mmol substrate, 0.2 mmol (1 equiv) B2pin2, 0.02 mmol (10
mol%) CeCl3, 0.2 mmol NEt4Cl, 2 mL CH3CN, irradiated by ten 8 W black light
lamps in a Rayonet photochemical reactor at room temperature, 24 h. bYield
(conversion) determined by 19F NMR spectroscopy using p-fluorotoluene as an
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10.1002/anie.201804022
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rates between electron-rich chloroarenes and [CeIIICl6]3− (Figure
1d). Using the corresponding, more readily reduced bromoarenes
(3k and 3l) resulted in improved yields. The electronic effects from
the meta-positions (3m−3o) and ortho-positions (3p−3r) were
less significant and good yields were achieved. The yield of the
sterically hindered arylboronic ester (3s) was observed to be
lower. Irradiating the reaction for longer time (72 h) improved the
yield of 3s from 24 to 46%. In addition, our borylation conditions
were successfully applied to heteroaryl chlorides and bromides,
and resulted in moderate-to-good yields of the heteroaryl boronic
esters (3u−3ab). The conversion of heteroaryl halides was nearly
quantitative because of the fast quenching rates between the
substrates and [CeIIICl6]3− (Figures 1a and S10). However, the
borylation of heteroaryl halides produced other, insoluble, as yet
unidentified by-products resulting in the low yields of the
heteroaryl boronic esters. Substrates containing pyridine and
quinoline groups, such as 3-bromopyridine and 3-bromoquinoline,
gave low yields (15%) presumably due to the coordination of the
substrates to the cerium cation which quenched the catalyst.
borylation, we simply removed the solvent by evacuation to give
the crude arylboronic ester, which was directly used in the next
Suzuki-Miyaura cross-coupling reaction without any purification.
This protocol afforded the coupling product in 68% isolated yield.
Scheme 2. Demonstration of sequential borylation and Pd-catalyzed cross-
coupling reaction.
Next, mechanistic studies were performed. The formation of
an aryl radical intermediate was supported by radical trapping
experiments (Figure 1b). Adding 9,10-dihydroanthracene (DHA)
as a hydrogen atom donor significantly suppressed the borylation
pathway and led to the formation of fluorobenzene and 14%
conversion of anthracene. In addition, the aryl radical was also
trapped by benzene and led to biaryl formation.
Without further reaction optimization, we next proceeded to
expand the scope of the diboron reagents (Table 3). Interestingly,
the phenylboronic acid product was isolated from the reaction
between B2eg2 (B2eg2
=
2,2'-bi(1,3,2-dioxaborolane) and
chlorobenzene (4a). GC-MS data indicated the formation of
phenylboronic ester in the reaction mixture (Figure S23), thus we
inferred that the corresponding phenylboronic ester was first
generated in situ, then hydrolysed into phenylboronic acid during
work-up procedures. This reaction scope was expanded to
several chloroarenes with various substitution patterns and
resulted in good yields of the arylboronic acid (4b−4d). Moreover,
other diboron esters delivered the corresponding product in high
yields (4e−4g). Using bromobenzene instead of chlorobenzene
significantly improved the yields by 10−20% (Table S8).
Table 3. Scope of diboron reagentsa
aStandard conditions: 0.2 mmol chlorobenzene, 0.4 mmol diboron reagent, 0.01
mmol (5 mol %) CeCl3, 0.15 mmol NEt4Cl, 1.5 mL CH3CN, irradiated by ten 8
W black light lamps in a Rayonet photochemical reactor at room temperature,
24 h. bIsolated yield for all substrates. cB2eg2 (2,2'-bi(1,3,2-dioxaborolane)) was
used as the diboron reagent.
We also demonstrated the practicality of our borylation
method in the sequential borylation and subsequent Pd-catalyzed
cross coupling reaction (Scheme 2, see the supporting
information for experimental details). The sequential synthetic
procedure avoids the tedious and problematic purification
processes of the arylboronates.[12] Upon the completion of
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absorption spectrum of [CeIVCl6]2− in CH3CN was observed when
adding 10 equiv NEt4Br to the solution (Figure S20), therefore the
bromide anion generated from the reduction of bromoarene
substrates cannot reduce [CeIVCl6]2−. For a radical chain pathway,
the boryl radical could be presumably trapped by aryl chloride to
form the boronic ester and the Cl2•−, which reacted with excess
B2pin2 to re-generate the boryl radical and propagate the chain.[8,
9c]
•−
The Cl2 could be quenched by the boryl radical to terminate
the chain (Figure S12).[9a, 9c, 9d, 10]
In summary, we have demonstrated the first practical,
efficient, and scalable photoinduced Miyaura borylation of aryl
chlorides and bromides by a rare earth metal catalyst ([CeIIICl6]3−).
The reaction conditions accommodated a wide range of functional
groups and diboron reagents, and afforded moderated-to-
excellent yields of the arylboronates. This borylation method is
expected to find applications in organic synthesis. We are
currently exploring the applications of our cerium catalyst in other
carbon-heteroatom bond forming chemistries.
Acknowledgements
We gratefully acknowledge the University of Pennsylvania and
the American Chemical Society Petroleum Research Fund (PRF
No. 56128-ND3) for financial support. The Petersson, Chenoweth,
and Park groups at the University of Pennsylvania are thanked for
use of their fluorometers. The Smith group is thanked for use of
their photochemical reactor. Prof. Di Qiu (Tianjin Normal Univ.),
Dr. Shuguang Zhang (UPenn), and Yifan Deng (UPenn) are
acknowledged for helpful discussions
Keywords: aryl boronates • borylation • cerium • photocatalysis
• photochemistry
Figure 1. Mechanistic studies. (a) Stern-Volmer experiments. Quenching rates
(kq) were given with standard deviations. (b) Radical trapping experiments. Yield
(conversion) was determined by 19F NMR using p-fluorotoluene as an internal
standard. (c) Proposed chain and closed catalytic mechanisms.
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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Yusen Qiao, Qiaomu Yang, Eric J.
Schelter*
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Photoinduced Miyaura Borylation by
a Rare Earth Photoreductant: the
Hexachlorocerate(III) Anion
Photoinduced borylation of aryl halides by Ce(III): Reported herein is the first
photoinduced Miyaura borylation by a rare earth photoreductant (the
hexachlorocerate(III) anion, [CeIIICl6]3−). This operationally simple, scalable, and
practical borylation method has a broad substrate scope and functional group
tolerance.
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