Iron-Catalyzed Borylation of Aryl Ethers via Cleavage of C-O Bonds

Article abstract of DOI:10.1021/acs.orglett.0c00679

Herein, we report the iron-catalyzed borylation of aryl ethers and aryl amines via cleavage of C-O and C-N bonds. This protocol does not require the use of Grignard reagents and displays a broad substrate scope, which allows the late-stage borylation. It also provides facile access to multisubstituted arenes through C-H functionalization using 2-pyridyloxy as the directing group.

Full text of DOI:10.1021/acs.orglett.0c00679

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Letter
Iron-Catalyzed Borylation of Aryl Ethers via Cleavage of C−O Bonds
Xiaoqin Zeng, Yuxuan Zhang, Zhengli Liu, Shasha Geng, Yun He, and Zhang Feng*
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ABSTRACT: Herein, we report the iron-catalyzed borylation of aryl ethers and aryl amines via cleavage of C−O and C−N bonds.
This protocol does not require the use of Grignard reagents and displays a broad substrate scope, which allows the late-stage
borylation. It also provides facile access to multisubstituted arenes through C−H functionalization using 2-pyridyloxy as the directing
group.
examples were reported using other transition metals.5 Iron-
he transformation of inert chemical bonds is an attractive
1
Tand challenging task in synthetic chemistry. It provides a catalyzed cross-coupling reactions have received great attention
due to their nontoxicity and abundance.6 Although the iron-
catalyzed reactions have been studied for several decades, the
great advantage for orthogonal diversification of complex
compounds via the late-stage functionalization.1 Due to the
exploration of inert bonds catalyzed by iron has been rarely
reported.7 Hence, it is of great interest to develop efficient
iron-catalyzed transformations of inert bonds.
ubiquitous existence of anilines and phenols, and their valuable
applications as synthetic building blocks, numerous protocols
have been developed for the diversification of inert C−N2 and
C−O bonds3 over the past decades (Scheme 1). This area is
dominated by nickel catalysis at present,4 and only a few
Organoboron compounds have been widely employed as
intermediates in organic synthesis.8 Cook, Bedford, and
Nakamura reported the iron-catalyzed syntheses of organo-
boron compounds,9 in which the halides were used as
substrates.9a,b,e Moreover, to facilitate the oxidative addition,
Grignard reagents were sometimes required to generate the
low-valent iron species via in situ reductive elimination.9b,c In
continuation of our interest in transition-metal catalysis,10 we
intended to develop the method for the formation of C−B
bond through activation of inert C−N and C−O bonds via
iron catalysis, which is operationally simple and without the
use of Grignard reagents.
Scheme 1. Iron-Catalyzed Functionalization of C−N and
C−O Bonds
To explore the iron-catalyzed borylation of inert C−N bond,
we synthesized a number of potential nitrogen-based electro-
philes which were well studied in the nickel-catalyzed
transformation of C−N bonds.4 When P(Cy)3, P(t-Bu)3, and
Received: February 21, 2020
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© XXXX American Chemical Society
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A

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P(Ad)3, common electron-rich ligands, were employed in the
presence of Fe(OAc)2, neither aromatic amine 1a bearing the
directing group nor 1b containing a traceless directing pyridyl
group produced the corresponding borylated product. With
enhanced reactivity of the C−N bond, the amine 1c containing
an electron-deficient Boc group generated the desired
compound in a 9% yield (Scheme 2). Different cyclic amides
Scheme 3. Representative Results for the Optimization of
the Iron-Catalyzed Borylation of 2e
a
Scheme 2. Optimization of the Reaction of Aryl Amine with
a
B2pin2
a
Reaction conditions (unless otherwise specified): Aryl ether (0.2
mmol), B2pin2 (0.4 mmol), [Fe] (5 mol %), Ligand (10 mol %), Base
b
(2.5 equiv), Solvent (2.0 mL), 120 °C, 12 h. NMR yield using
mesitylene as an internal standard. The isolated yield is shown in
parentheses.
a
Reaction conditions: for details, please see Supporting Information.
were investigated employing P(t-Bu)3 as the ligand. The five-
member ring amide 1f showed the highest reactivity, resulting
in the corresponding product in 21% yield, which was in
agreement with those previously reported works (Scheme 2).4
Installing two electron-deficient groups onto the amine led to
the decomposition of substrates 1d and 1h, and only trace
amounts of the desired products were obtained (Scheme 2).
These results suggest that electron-rich ligands could promote
the borylation reaction when relatively reactive amides 1c and
1f were used as substrates, albeit with low efficiency (Scheme
2; for details, see Supporting Information). It confirms that the
borylation of C−N bonds catalyzed by iron is challenging in
the absence of Grignard reagents, as the C−N bonds are
challenging to undergo oxidative addition owing to the high
bond dissociation energy.11
Encouraged by these results, we envisioned that the
borylation of C−O bonds via iron catalysis should be more
feasible. Therefore, we turned our attention to the readily
available aryl ethers. The nickel-catalyzed transformation of
enol ethers to olefins was demonstrated by the Wenkert group
in 1979.12 To our knowledge, the construction of the C−
heteroatom bond from aryl ethers catalyzed by iron was not
reported. An important example of reductive cleavage of ether
C−O bond by iron catalyst was reported by the Wang group,13
in which a high temperature of 140−180 °C was required.
Inspired by the studies from the Kakiuchi5 and Snieckus
groups,14 the aryl ethers bearing a directing group, 2a, 2b, and
2c, were first investigated. No desired products were observed
after evaluation of some electron-rich phosphine ligands, such
as P(Cy)3, P(t-Bu)3, and P(Ad)3 in the presence of Fe(OAc)2
(Scheme 3). When the pyridyl group was used as a directing
group,15 2d did not undergo the borylation (Scheme 3). The
2-pyridyloxy group has been commonly used in the C−H bond
functionalization reactions,16 but there are no efficient
methods to remove the pyridine ring,17 thus limiting its
practical applications. The removal of the pyridine ring from
the 2-pyridyloxy moiety usually requires two steps: (i) N-
methylation with a strong methylating reagent and (ii)
cleavage of the C−O bond using a strong base. Therefore,
the development of an efficient method for direct borylation of
2-pyridyloxy via iron catalysis would be highly appealing.18 To
our delight, 2-pyridyl ether 2e indeed underwent the
borylation, providing the corresponding product in 32% yield
in the presence of P(Cy)3 (Scheme 3, entry 1). Other iron
sources were also tested. Fe(OAc)2 and Fe(acac)3 could
provide the product in moderate to good yields (Scheme 3,
entries 2−3). This reaction was very sensitive to inorganic
bases, and only strong bases, such as t-BuOLi and t-BuONa,
could efficiently promote this transformation (for details, see
Supporting Information). Other solvents, such as diisopropyl
ether and dioxane, also provided the corresponding product in
moderate yields, but toluene stood out as the best (Scheme 3,
entries 6−8). After investigating various ligands, 88% isolated
yield was provided with t-BuXPhos (Scheme 3, entry 9).
Control experiments demonstrated the importance of both
ligand and iron species. No desired product was observed
without the use of an iron catalyst or ligand (Scheme 3, entries
10−11).
We then set out to examine the scope of the reaction (Table
1). When naphthyl ethers were employed, the transformation
proceeded smoothly and provided the borylated products in
moderate to good yields (2ee−13ee, 47%−88%). Functional
groups, such as methoxy, amine, morpholyl, and fluoro, were
well-tolerated (3ee, 7ee−10ee). Substrate 4e containing an
electron-rich methoxy group at the 4-position on the ring
B
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a
Table 1. Scope of the Iron-Catalyzed Borylation of Aryl Ethers
a
Reaction conditions: Aryl ether (0.2 mmol), B2pin2 (0.4 mmol), [Fe] (5 mol %), Ligand (10 mol %), Base (2.5 equiv), toluene (2.0 mL), 120 °C,
b
12 h (for details, please see Supporting Information). Aryl ether (0.2 mmol), B2pin2 (0.4 mmol), Fe(OAc)2 (10 mol %), i-Pr NHC (20 mol %), t-
BuOK (3.0 equiv), toluene (2.5 mL), 128 °C, 15 h. Aryl ether (0.2 mmol), B2pin2 (0.6 mmol), Fe(OAc)2 (10 mol %), i-Pr NHC (20 mol %), t-
BuOK (4.0 equiv), MTBE (3.2 mL), 128 °C, 15 h.
c
bearing a 2-pyridyloxy group furnished 4ee in 47% yield, which
is in sharp contrast to the borylation of substrate 3e (3ee,
88%). Heteroarene 11e bearing a furanyl group also reacted
well, delivering borylated product in good yield. N-Hetero-
cyclic carbazole substrate performed well, leading to product
14ee in moderate yield. Polycyclic aryl ethers are also suitable
substrates for this transformation, delivering the borylated
products in moderate yields (15ee−18ee, 52%−68%). The
biphenyl and monophenyl substrates (19e−35e) were
relatively inert reaction partners, and the reaction proceeded
sluggishly. To our delight, when the phosphine ligand was
switched to a more electron-rich ligand, N-heterocyclic
carbene (NHC), and the catalyst loading was increased, this
reaction could proceed smoothly and yielded the borylated
products in moderate to good yields (19ee−35ee, 50%−75%).
Additionally, the borylation reaction could be conducted on a
gram scale, and the product 2ee was obtained in 65% yield.
To further explore the utility of our method, we set out to
investigate the late-stage functionalization of biorelevant
molecules, such as estrone and vitamin E (Scheme 4). The
borylated products were obtained in moderate yields (36ee−
37ee, 51%−52%). The inherent utilities of this protocol were
further demonstrated by ortho- and meta-arylation with the aid
of 2-pyridyloxy, followed by iron-catalyzed borylation. The
ortho-arylated substrates could be easily obtained through a
palladium-catalyzed cross-coupling reaction17 and underwent
the borylation reaction smoothly, delivering the corresponding
products in moderate yields (38ee−42ee, 48%−61%). More-
over, a meta group could be introduced in the aryl ring,19 and
the corresponding borylated products could be obtained in
48%−60% yields (43ee−46ee). These results suggest that this
protocol could provide a means for orthogonal transformations
of aryl ethers and facile access to ortho- or meta-substituted
arenes.
To explore the mechanism of the reaction, the following
experiments were conducted. The radical inhibitor BHT or
radical scavenger TEMPO was used as the additive. This
reaction was completely shut down. Moreover, no adduct of
the aryl radical with TEMPO was observed (Scheme 5A).
However, a radical signal was indeed observed in this catalytic
system by the EPR experiment (Scheme 5A). Furthermore, a
radical clock experiment was carried out. Interestingly, β-
carbon elimination product 47ee was obtained instead of the
common radical ring-opening product 48ee (Scheme 5B).20
These results demonstrated that the radical pathway was
involved in this reaction. However, it might not be a boron
centered radical species. To confirm whether the nitrogen
atom on the pyridine ring played a directing role, the control
experiments were conducted. As depicted in Scheme 5C,
increasing the steric hindrance of the substituent on the 6-
position of the pyridine ring diminished yield (Scheme 5C, 2f),
and no desired product was observed when 2g with a bulky
C
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on the pyridine ring may act as an iron-coordinating group and
facilitate the oxidative-addition process.
Scheme 4. Functionalization of Biomolecules and
Orthogonal Transformation of Aryl Ethers
To gain further insight into the nature of iron species in this
reaction, the reaction mixture of Fe(OAc)2, t-BuONa, t-
BuXphos, and diboron was analyzed by the X-ray photo-
electron spectroscopy (XPS) (for details, see Supporting
Information, Figures S2−S3). A peak corresponding to FeII
was observed with the binding energy at 710.3 eV (compared
with FeO).21 Moreover, when the mixture under standard
reaction conditions was analyzed by XPS, both FeII and FeIII
were found with the binding energy at 710.3 eV (compared
with FeO) and 712.8 eV (compared with FePO4).22 These
results suggest that an FeII/FeIII catalytic cycle might be
involved in this reaction, but the FeII/FeIV pathway could not
be excluded.
Based on the preliminary results, a probable mechanistic
pathway was proposed (Scheme 6). An ate iron alkoxide
Scheme 6. Proposed Mechanistic Pathway
Scheme 5. Mechanistic Studies
complex I was first generated through the reaction of t-BuONa
with Fe(OAc)2.23 Subsequent transmetalation of the resulting
iron species I with diboron afforded complex II with high
reduction potential,24 which then reacted with aryl ether
through a single electron transfer pathway, delivering an
iron(III) intermediate III. Finally, the desired product was
provided through the reductive elimination, and the iron(II)
species IV was regenerated.
In conclusion, we have developed a protocol for iron-
catalyzed borylation of aryl ether via C−O bond cleavage,
which offers late-stage borylation of complex molecules, and
also provides a good opportunity to access multisubstituted
arenes through the C−H bond functionalization using 2-
pyridyloxy as the directing group. Preliminary mechanistic
studies suggest that there may be an FeII/FeIII catalytic cycle in
this reaction.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00679.
Detailed experimental procedures, and characterization
data for new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Zhang Feng − School of Pharmaceutical Sciences, Chongqing
University, Chongqing 401331, P. R. China; School of
Preclinical Medicine, North Sichuan Medical College, Sichuan
Key Laboratory of Medical Imaging & Department of
isopropyl group was used as substrate. Moreover, the 4-
hydroxy pyridine group could not promote this transformation
(Scheme 5C, 2h). Thus, the nitrogen atom adjacent to oxygen
D
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Chemistry, Sichuan 637000, China; orcid.org/0000-0001-
7776-8200; Email: fengzh@cqu.edu.cn
Authors
Xiaoqin Zeng − School of Pharmaceutical Sciences, Chongqing
University, Chongqing 401331, P. R. China
Yuxuan Zhang − School of Pharmaceutical Sciences, Chongqing
University, Chongqing 401331, P. R. China
Zhengli Liu − School of Pharmaceutical Sciences, Chongqing
University, Chongqing 401331, P. R. China
Shasha Geng − School of Pharmaceutical Sciences, Chongqing
University, Chongqing 401331, P. R. China
Yun He − School of Pharmaceutical Sciences, Chongqing
University, Chongqing 401331, P. R. China; orcid.org/
0000-0002-5322-7300
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00679
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NSFC (No. 21801029),
Graduate Scientific Research and Innovation Foundation of
Chongqing (No. CYS18046), Natural Science Foundation of
Chongqing (No. cstc2019jcyjmsxmX0048), Chongqing Post-
doctoral Science Foundation (No. cstc2019jcyj-bshx0057),
and Sichuan Key Laboratory of Medical Imaging (North
Sichuan Medical College, No. SKLMI201901).
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