Magnesiation of Aryl Fluorides Catalyzed by a Rhodium-Aluminum Complex

Article abstract of DOI:10.1021/jacs.0c04905

We report the magnesiation of aryl fluorides catalyzed by an Al-Rh heterobimetallic complex. We show that the complex is highly reactive to cleave the C-F bonds across the polarized Al-Rh bond under mild conditions. The reaction allows the use of an easy-to-handle magnesium powder to generate a range of arylmagnesium reagents from aryl fluorides, which are conventionally inert to such metalation compared with other aryl halides.

Full text of DOI:10.1021/jacs.0c04905

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Magnesiation of Aryl Fluorides Catalyzed by a Rhodium-Aluminum Complex
Ikuya Fujii, Kazuhiko Semba, Qiao-Zhi Li, Shigeyoshi Sakaki, and Yoshiaki Nakao
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c04905 • Publication Date (Web): 09 Jun 2020
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Magnesiation of Aryl Fluorides Catalyzed by a Rhodium–Aluminum
Complex
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†,∥
†,∥
‡
,‡
,†
Ikuya Fujii, Kazuhiko Semba, Qiao-Zhi Li, Shigeyoshi Sakaki,* Yoshiaki Nakao*
†Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510,
Japan
‡Fukui Institute for Fundamental Chemistry, Kyoto University, Sakyo-ku, Kyoto 606-8103, Japan
Supporting Information Placeholder
ABSTRACT: We report the magnesiation of aryl
fluorides catalyzed by an Al–Rh heterobimetallic
complex. We show that the complex is highly reactive to
cleave the C–F bonds across the polarized Al–Rh bond
under mild conditions. The reaction allows the use of an
easy-to-handle magnesium powder to generate a range of
arylmagnesium reagents from aryl fluorides, which are
conventionally inert to such metalation compared with
other aryl halides.
(Me
B–Rh bond in (Me
(Me Si)Rh(PMe
.8b We thus expected reactivity of the
2
Al)Rh(PMe
3
)
2
was higher than those containing the
B)Rh(PMe and the Si–Rh bond in
)
3 2
2
)
3 2
3
Al–Rh bonds toward the C–F bond activation higher than
that of B–Rh and Si–Rh bonds based on these results.
Herein, we report the activation of C–F bonds in
fluoroarenes by means of an Al–Rh bond under very mild
conditions, which results in an unprecedented catalytic
magnesiation of fluoroarenes using easy-to-handle Mg
9
powder.
The reaction of Al–Rh complex 1a, which was
8
a
generated in situ by the reduction of Al–Rh complex 2a
with KC , and fluorobenzene (3a) under N resulted in the
Complexes that contain metal–metal bonds have
received considerable attention in organometallic
chemistry due to their unique electronic properties, which
differ significantly from those of single-metal systems,
especially with respect to achieving novel organic
8
2
activation of the C–F bond at the Al–Rh bond to afford
complex 4a in 95% yield (Scheme 1). The solid-state
structure of 4a was determined unequivocally by single-
crystal x-ray diffraction analysis. In 4a, the Al(III) moiety
coordinates to the Rh center as an electron-accepting Z-
1
transformations. Among complexes with metal–metal
bonds, heterobimetallic metal–metal bonding motifs are
particularly reactive towards the cleavage of
conventionally inert bonds due to their inherent
polarization. In a pioneering study, Bergman showed that
1
0
type ligand with trigonal bipyramidal geometry. The Rh
center adopts a square-pyramidal geometry stabilized by
an end-on N ligand. It is noteworthy that the C–F-bond
2
activation proceeds even at –30 °C (for details, see the
Supporting Information). To the best of our knowledge,
these are the mildest hitherto reported conditions for the
activation of unactivated C–F bonds by a homogeneous
Zr–Ir complexes can activate
a
variety of
thermodynamically strong bonds including O–H and N–
2
3
4
H bonds. B–Rh and Si–Rh manifolds have been
5
reported to effectively activate C–F bonds in
3
,4,5,11
metal complexes.
perfluorinated
organic
molecules
such
as
pentafluoropyridine and hexafluorobenzene. Moreover, a
Zr–Co complex realizes the challenging activation of the
C=O bond in benzophenone to afford a cobalt carbene
Ph
P
P
N
N
F
Me
Rh
F
N
N
N
Al
+
N
Al Rh
N
6
THF
complex. However, catalytic transformations that
P
P
N
–78 °C to r.t., 12 h
under N2
Me
include these bond-activation events remain elusive.
Recently, we have developed an Al–Rh heterobimetallic
1a
3a, 0.60 mmol
4a, 95%
generated in situ
7
complex in which an X-type Al moiety is ligated to the
KC8 (4.2 eq.)
THF
Rh center, and demonstrated its reactivity in the
P
Rh
8a
alkylation of pyridines. An NBO analysis suggested a
+
–
Cl Cl
P
N
N
polarized Al –Rh bond, and this result prompted us to
test the reactivity of such Al–Rh bonds in the context of
cleaving -bonds, which usually exhibit high bond-
dissociation energies. We previously reported that the
energy level of the orbital containing the Al–Rh bond in
Me
N
Al Rh
N
Al
N
P
N
P
F
N
2
2
a, 0.30 mmol
N
P = P(i-Pr)2
Scheme 1. C–F Bond Activation by Al–Rh Complex
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F
Rh
P
N
ANl
P
N
F
Rh
TS'
4.2
P
2
N
ANl
C–F bond activation at Al–Rh
C–F bond activation at Rh
P
N
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AD2
.4
3.7
3
0
.0
0
.9
Rh
F
P
F
Rh
F
Rh
N
ANl
–46.6
P
P
P
N
ANl
N
ANl
N
P
P
–63.5
N
N
Ph
P
P
N
1
AD1
Rh
1
AD2'
TS
F
Rh
N
P
N
F
N
ANl
N
Al
P
N
N
Me
P = P(i-Pr)2
4b
4a
Figure 1. Energy diagram of the C–F bond activation of 3a by 1a.
To gain mechanistic insights into the C–F-bond
activation by 1a, the reaction pathway was theoretically
modelled by DFT calculations (Figure 1; for details, see
Ar–F bonds. The reduction potential of Mg metal should
be sufficiently high to reduce the Al center of 4 (Mg /Mg
= –2.4 V; Al /Al = –1.7 V vs SHE). In contrast to the
magnesiation of Ar–X bonds (X = I, Br, Cl), that of Ar–
F bonds is extremely difficult, even for highly dispersed
Mg. For example, the magnesiation of p-fluorotoluene
proceeds only in moderate efficiency, even when a large
excess of Rieke magnesium is used in refluxing THF.
Although the magnesiation of the C–F bond with Mg(I)–
2
+
3
+
12
2
3
13
the Supporting Information). The C =C bond of
fluorobenzene coordinates to the Rh atom and the F atom
occupies the position close to the Al atom in Al–Rh -
complex 1AD1. The C–F bond of 1AD1 then changes its
orientation to give adduct 1AD2. The activation of the C–
F bond occurs at the Al–Rh -bond in a cooperative
fashion to afford Rh–phenyl complex 4b via transition
state TS. After isomerization, which includes a positional
change of the Ph group, followed by coordination of N2,
another Rh–phenyl complex 4a is generated. The Gibbs
1
4
1
4a
5
h–k
Mg(I) complexes has recently been reported,
precedents of the magnesiation of C–F bonds using
readily available and easy-to-handle Mg powder remain
elusive.
‡
energy of activation (Gº ) and the Gibbs energy of
–
1
reaction (Gº) were estimated to be 3.7 kcal mol and –
–
1
63.5 kcal mol , respectively (blue line in Figure 1). The
very small Gº‡ stands in sharp contrast to the
‡
considerably large Gº required for the activation of the
C–F bond by Rh alone (24.2 kcal mol ; red lines in
Figure 1). The small Gº value obtained for the
cooperative activation of the C–F bond matches well with
the experimentally observed reaction, which proceeds at
low temperature.
–1
‡
Next, we turned our attention to developing new
catalytic transformations based on the unique reactivity
of this Al–Rh manifold toward the activation of C–F
bonds. Our working hypothesis is shown in Scheme 2.
We anticipated that the C–F-bond activation would afford
Scheme 2. A Plausible Catalytic Cycle for the
Magnesiation of Aryl Fluorides using Al–Rh
Complexes such as 2
III
I
Z-type Al –Rh complex 4, which could potentially be
reduced with Mg to realize a catalytic magnesiation of
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F
CO H
2
catalyst
CO2
1
2
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The proposed catalytic magnesiation of 1-
fluoronapthalene (3b, 0.30 mmol) with Mg powder (0.90
mmol), which was pre-activated upon treatment with 1,2-
dibromoethane (5.0 mol%), was attempted in the
presence of various catalysts in THF at room temperature
then H+
Mg (0.90 mmol)
BrCH CH Br (5.0 mol%)
2
2
3b, 0.30 mmol
5a
THF, r.t., 22 h
yield of 5a (conv. of 3b)
Al–Rh complexes (5.0 mol% Rh)
(Scheme 3). Gratifyingly, 1-napthoic acid (5a) was
Cl Cl
Cl Cl
Me
Me
obtained in 88% NMR yield using 2a (5.0 mol% Rh) after
none
N
N
Al Rh
P(i-Pr)
N
Al Rh
quenching of the reaction with CO (1 atm), followed by
2
N
PPh
2
2
acidic work-up using 3 M HCl. Al–Rh complex 2b, which
bears phenyl groups on the phosphorus atoms instead of
isopropyl groups, also furnished 5a in high yield. The
reaction did not proceed in the absence of an Al–Rh
P(i-Pr)2
PPh2
N
N
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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0
2
2
2a
2b (@–15 °C)
89% (>95%)
88% (>95%)
<5% (<5%)
complex. Catalytic systems based on [RhCl(nbd)] (5.0
2
in-situ-generated catalysts
[RhCl(nbd)]2 (5.0 mol% Rh)/P ligand (10 mol% P)/Et AlCl (20 mol%)
mol% Rh; nbd = 2,5-norbornadiene)/phosphorus ligands
2
(
10 mol% P)/Et AlCl (20 mol%) did not afford 5a.
2
Adding LiCl, which facilitates the magnesiation of C–Br
Ph
1
5
PPh2
bonds, or adding a catalytic amount of anthracene, to
P
P
PPh2
1
6
Ph
Ph
generate magnesium anthracene, did not show any
effect. Using i-PrMgClLiCl led to the magnesiation of 3b
to afford 5a in low yield, albeit that this approach is
<5% (<5%)
<5% (<5%)
<5% (<5%)
1
7
unsuitable for simple aryl fluorides such as 3a.
Accordingly, it can be concluded that only Al–Rh
complexes 2a and 2b catalyze the magnesiation of aryl
fluorides.
O
Ph P
PPh2
2
Ph P
PPh2
2
<
5% (<5%)
<5% (<5%)
others
Scheme 3. Screening Catalysts for the Catalytic
Magnesiation of 3b
LiCl
0.90 mmol)
i-PrMgCl (0.30 mmol)
LiCl (0.90 mmol)
(
(10 mol%)
<5% (<5%)
<5% (<5%)
10% (12%)
nbd = 2,5-norbornadiene
The magnesiation of various aryl fluorides 3 was
examined at the 0.50-mmol-scale under the optimized
conditions (Scheme 4). Under these conditions, 1-
naphthoic acid (5a) was obtained from 1-
fluoronaphthalene (3b) and isolated in 99% yield, while
benzoic acid (5b) was isolated in 86% yield from the
reaction involving fluorobenzene (3a). 5a could be also
prepared in 82% yield on a 10 mmol-scale. Electron-
donating substituents at the para- or meta-position of
fluorobenzene were tolerated to afford the corresponding
benzoic acids (5c–5g) in good to high yield, whereas the
reaction efficiency was decreased with 4-fluorobiphenyl
3h). It should also be noted here that the C–S bond of 4-
fluorothioanisole (3f), which is easily functionalized by
palladium or nickel catalysts, was tolerated in this
transformation. Sterically demanding 2-fluorotoluene (3i)
and 2,6-dimethylfluorobenzene (3j) furnished the
corresponding carboxylic acids (5i and 5j) in 92% and
(
1
8
4
8% yield, respectively. D O, B(Oi-Pr) , and N-methoxy-
2 3
1
9
N-methylbenzamide can also serve as quenching
electrophiles to generate the corresponding deuterated,
borylated, and acylated products (5k–5m).
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diphenylmagnesium was implied by H and 13C NMR
spectroscopies of the crude mixture upon magnesiation of
1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Scheme 4. Scope of the Reaction with Respect to Aryl
Fluorides 3
3
a under the optimized conditions (for details, see the
20
Supporting Information).
Cl Cl
Me
N
Al Rh
N
PPh
2
PPh2
N
2
E+
2b (5.0 mol% Rh)
Ar
F
Ar E
0
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2
3
4
5
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9
0
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2
3
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0
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9
0
1
2
3
4
5
6
7
8
9
0
Mg (1.5 mmol)
BrCH2CH2Br (5.0 mol%)
THF, –30 °C, 22 h
3
, 0.50 mmol
5
E+ = CO2; H+
CO2H
CO2H
Me
CO2H
CO2H
CO2H
CO2H
Bn
5
a
5b
5c
5d
9
9%a
2% (1.4 g)a, b
86%
86%
99%
8
CO2H
CO2H Me
MeO
MeS
Ph
5e
5f
5g
5h
In conclusion, we have developed a catalytic
91%
79%
90%
83%
magnesiation of Ar–F bonds through the C–F bond
activation across the X-type heterobimetallic Al–Rh
center. It is worth mentioning that the cooperative
activation allows the functionalization of C–F bonds in
unactivated fluoroarenes under very mild conditions. This
is a rare example of successful applications of
heterobimetallic catalysis. Further developments of
catalytic functionalization of other strong polar -bonds
by the heterobimetallic systems are currently under
investigation in our laboratories.
Me
CO2H
Me
CO2H
Me
5i
2%a
5j
48%c
9
D
B(pin)2
O
Ph
Me2N
5
k
5l
67%a, d
5m
22%a
O
99%a
ASSOCIATED CONTENT
E+
=
2
D O
B(Oi-Pr)3
MeO
N
Ph
Me
The Supporting Information is available free of charge on
the ACS Publications website.
Crystallographic data for 2b (CIF)
a
Reaction was carried out at room temperature. bReaction was run on a 10
mmol scale. Reaction was carried out at 50 °C. dThe pinacol ester was
c
isolated after reacting with pinacol. pin = pinacolato
Crystallographic data for 4a (CIF)
Detailed experimental procedures and computational details,
as well as spectroscopic and analytical data (PDF)
Optimized structures (XYZ)
CCDC 2000480 (2b) and 1999655 (4a) contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html.
To evaluate the plausible catalytic cycle shown
in Scheme 2, stoichiometric reactions were conducted. To
gain insights into the reduction of 2 to 1 in Scheme 2, the
reaction of 2a (30 mol) with Mg powder (0.60 mmol) in
the presence of nbd (90 mol) was examined. It
proceeded at 60 °C to afford nbd-coordinated Al–Rh
complex 1a-nbd in high yield (Eq 1). The catalytic
magnesiation of p-fluorotoluene (3c, 0.25 mmol) with Mg
powder (0.75 mmol) in the presence of 4a (20 mol%, 50
AUTHOR INFORMATION
Corresponding Authors
mol) afforded p-toluic acid (5c) in 70% yield (0.18
*sakaki.shigeyoshi.47e@st.kyoto-u.ac.jp
*nakao.yoshiaki.8n@kyoto-u.ac.jp
mmol) based on 3c under concomitant formation of
benzoic acid (5b) in 48% yield (24 mol) based on 4a (Eq
ORCID
2
). These results support the generation of
Kazuhiko Semba: 0000-0001-7903-4091
Shigeyoshi Sakaki: 0000-0002-1783-3282
Yoshiaki Nakao: 0000-0003-4864-3761
phenylmagnesium species from 4a under the applied
conditions, and thus corroborate the catalytic cycle
proposed in Scheme 2. In fact, the formation of
Author Contributions
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Journal of the American Chemical Society
∥I.F. and K.S. contributed equally to this work.
Raza, A. L.; Braun, T. Consecutive C–F bond activation and C–F bond
formation of heteroaromatics at rhodium: The peculiar role of
1
2
3
4
5
6
7
8
9
Notes
FSi(OEt) . Chem. Sci. 2015, 6, 4255−4260.
3
(
5) (a) Liu, X.-W.; Echavarren, J.; Zarate, C.; Martin, R. Ni-Catalyzed
Borylation of Aryl Fluorides via C–F Cleavage. J. Am. Chem. Soc.
015, 137, 12470–12473. (b) Yoshikai, N.; Matsuda, H.; Nakamura, E.
The authors declare no competing financial interests.
2
ACKNOWLEDGMENTS
Hydroxyphosphine Ligand for Nickel-Catalyzed Cross-Coupling
through Nickel/Magnesium Bimetallic Cooperation. J. Am. Chem. Soc.
009, 131, 9590−9599. (c) Nova, A.; Reinhold, M.; Perutz, R. N.;
Macgregor, S. A.; McGrady, J. E. Selective Activation of the ortho C–F
Bond in Pentafluoropyridine by Zerovalent Nickel: Reaction via a
Metallophosphorane Intermediate Stabilized by Neighboring Group
Assistance from the Pyridyl Nitrogen. Organometallics 2010, 29,
This work was supported by the JST CREST program (grant
JPMJCR14L3) via the “Establishment of Molecular
Technology towards the Creation of New Functions” project
and by JSPS KAKENHI grant JP20H00376. We thank Dr.
Hiroyasu Sato (Rigaku Corporation) for x-ray
crystallographic analyses and Naofumi Hara (Kyoto
University) for the characterization of 2b.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
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