Rhodium-Catalyzed C?H Activation Enabled by an Indium Metalloligand

Article abstract of DOI:10.1002/anie.201910197

Rhodium complexes with an indium metalloligand were successfully synthesized by utilizing a pyridine-tethered cyclopentadienyl ligand as a support for an In?Rh bond. The indium metalloligand dramatically changes the electronic and redox properties of the

Full text of DOI:10.1002/anie.201910197

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Accepted Article
Title: Awakening Rhodium Catalysis for C–H Activation with an In-
Metalloligand
Authors: Ryosuke Yamada, Nobuharu Iwasawa, and Jun Takaya
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10.1002/anie.201910197
Angewandte Chemie International Edition
COMMUNICATION
Awakening Rhodium Catalysis for C–H Activation with an In-
Metalloligand
Ryosuke Yamada[a], Nobuharu Iwasawa[a], and Jun Takaya*[a][b]
Abstract: Rhodium complexes having an In-metalloligand are
successfully synthesized utilizing pyridine-tethered
properties of the rhodium metal, awakening its catalysis for an
sp2C–H bond activation reaction.
a
cyclopentadienyl ligand as a support for an In–Rh bond. The In-
metalloligand dramatically changes the electronic and redox
properties of the rhodium metal, awakening its catalysis for an sp2C–
H bond activation reaction.
a) Z-type ligands with phosphine-based supports
Cl
2+
L
Ph2P
PPh2
Pd
N
R2P
Ni
N
Cl
M
Ph2P
PPh2
E
N
N
N
E
N
N
E
E
Cl
PCy2
P
N
R2P
N
Pd
PCy2
E = Li, Cu, Zn
Z-type ligands have recently emerged as a new tool for
tuning reactivity of transition metal complexes.[1] They are
usually main group Lewis acids and serve as a σ-acceptor via
dative bonding from a transition metal, which is in sharp contrast
to the case of commonly employed σ-donor, L-type ligands such
as phosphines. Such a reverse electronic perturbation is highly
promising for unique reactivity and catalysis. In particular,
rationally designed supported Z-type ligands are beneficial to
stabilize the M–Z bond (Z = Z-type ligand), thus enabling
systematic synthesis of the M–Z complexes and their use as a
catalyst in synthetic reactions.[2,3] For examples, an ortho-
phosphinophenyl-linker has been widely utilized for Z-type
ligands of group 13 elements and antimony, and the catalysis of
their metal complexes were often demonstrated in electrophilic
activation of alkynes utilizing the electron-accepting nature of the
Z-type ligands (Figure 1-a).[1c,4,5] Lu, Tauchert, and our group
developed N,P-multidentate ligands as suitable supports for M–
Z bonds (Z = group 13 metals, Li, Cu, Zn), in which the Z-type
ligands substantially affect the reactivity of group 10 metals for
hydrogenation and hydrosilylation reactions.[6-8] However,
previous reports mostly focused on phosphine-based supports
for M–Z bonds to give metal complexes having PZP-pincer type
or ZP3-tetradentate tripodal ligands, and their catalysis were
rather limited to activation of alkynes and E–H bonds (E = H,
Si).[9] Therefore, development of novel M–Z complexes that
enable new modes of catalysis control by Z-type ligands is
necessary in order to extend its usage in synthetic chemistry.
N
N
M = Au, Pt, Fe
E = BR, SbXn
E = Al, Ga, In
E = Al, Ga, In
Tuning catalysis in alkyne activation, hydrogenation, and hydrosilylation reactions
b) This work
+
• CpRh complexes with a supported Z-type
In-metalloligand
• Awakening Rh-catalysis for sp2C–H activation
N
Rh
In
Cl
Cl
by changing electronic and redox properties
Figure 1. Z-type metalloligands in organic synthesis
Cp-metal complexes are one of the most important classes
of transition metal catalysts in synthetic chemistry because of
their various reactivity and utility. However, tuning their catalysis
with Z-type ligands has rarely been investigated partly due to the
difficulty of designing
a multidentate support with the Cp
structure.[10,11] We chose a tetramethyl(2-pyridyl)cyclopentadienyl
ligand as a suitable support for an M–Z bond and examined
synthesis of the corresponding rhodium complexes. The reaction
of potassium tetramethyl(2-pyridyl)cyclopentadienylide with
[RhCl(cod)]2 afforded a rhodium complex 1 having a pyridine-
tethered Cp ligand in good yield (Scheme 1). Four alkenyl
protons on the cod moiety were observed equivalently as a
1
broad singlet in H NMR, thus suggesting the pyridine side arm
does not coordinate to rhodium.[12] Screening of various Lewis
acidic metals as an additive to 1 revealed that some metal
halides, in particular InCl3, can efficiently work as a Z-type
metalloligand for the rhodium.[13] The reaction of 2 equivalents of
InCl3 with 1 in toluene at 80 ˚C afforded a cationic In–Rh
bimetallic complex [2][InCl4] having an –InCl4 as a counter anion
in 64% isolated yield.[14] Treatment of [2][InCl4] with sodium
Herein we report the first synthesis and catalysis of
cyclopentadienylrhodium (CpRh) complexes having a Z-type In-
metalloligand supported by a pyridine-tethered Cp ligand. The
In-metalloligand dramatically changes the electronic and redox
[a]
[b]
Ryosuke Yamada, Prof. Dr. Nobuharu Iwasawa, Dr. Jun Takaya
Department of Chemistry, School of Science
Tokyo Institute of Technology
O-okayama, Meguro-ku, Tokyo 152-8551, Japan
E-mail:takayajun@chem.titech.ac.jp
Dr. Jun Takaya
tetra(3,5-bis(trifluoromethyl)phenyl)borate
gave
the
corresponding BArF complex [2][BArF]. 1H NMR of both
complexes displayed two sets of alkenyl protons of the cod
moiety, indicating the coordinated cod became asymmetric due
to the formation of an In–Rh bond. No decomposition of [2][InCl4]
was observed even in coordinating solvent CD3CN at 80 ˚C,
indicating sufficient stability of the supported In–Rh bond. The
structures of [2][InCl4] and [2][BArF] were fully characterized by
JST, PRESTO
Honcho, Kawaguchi, Saitama, 332-0012, Japan
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201910197
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X-ray analyses, and the ORTEP of [2][BArF] was depicted in
Figure 2. The In–Rh bond length is 2.6170(7) Å, which is shorter
than the sum of the covalent radii of In and Rh (2.84 Å).[15] The
covalent ratio (r = 0.92) is in a range of typical M–Z distances,[1b]
thus supporting Z-type ligation of the InCl2 moiety. The geometry
around In is trigonal bipyramidal with a vacancy and pyridine at
apical and Cl and Rh at equatorial positions. The average
distance between Rh and alkenyl carbons of cod is 2.220 Å,
which is longer compared to that of the reported CpRhI(cod)
complex (2.115 Å).[16] This reflects weaker π-back donation of
Rh in [2][BArF] caused by strong electron-accepting nature of the
In-metalloligand.
(Figure 3). Molecular orbital analysis also demonstrated that
MOs of [2]+ are composed of those of a neutral CpRhI and a
cationic InIIICl2 via Z-type ligation (see Figure S6). The orbital
energy of LUMO was calculated to be –0.094 a.u. by DFT, which
is lower than that of Cp*Rh(cod) and comparable to that of
Cp*RhCl2 (Figure 4).[17] The lowered LUMO level of [2]+ was
further confirmed by cyclic voltammetry (Figure 5). Monovalent
rhodium complexes, 1 and Cp*Rh(cod), were electrochemically
inert in a range from –1.0 to –1.7 V. On the other hand, the In–
Rh complex [2][InCl4] exhibited an irreversible reduction peak at
Epc = –1.41 V vs. Fc+/Fc, which was a similar value to that of
[Cp*RhCl2]2 (Epc = –1.38 V vs. Fc+/Fc). These results strongly
support that the In–Rh complex [2]+ has similar electronic and
redox properties to those of trivalent Cp*RhIII complexes rather
K+
than the original monovalent RhI complex
complexation with the Z-type In-metalloligand.
1
through
N
[RhCl(cod)]2 (0.5 equiv)
THF, rt, 12 h
N
–
Rh
1 85%
+
a)
X–
N
InCl3 (2.0 equiv)
Rh
In
Cl
Cl
toluene, 80 ˚C, 6.5 h
NaBArF (1.0 equiv)
CH2Cl2, rt
[2][InCl4] (X = InCl4) 64%
[2][BArF] (X = BArF4) 71%
Scheme 1. Synthesis of In–Rh complexes
LP2(Rh, d), occ = 1.86
LP4(Rh, d), occ = 1.63
b)
Donor
Acceptor
E [kcal/mol]
31.2
C2
C3
LP2(Rh) LV1(In)
LP4(Rh) LV1(In)
20.7
C1
C5
C4
N
E = stabilization energy
LV1(In, s 95%, p 5%), occ = 0.79
Rh
Cl1
C7
In
Figure 3. NBO analysis of [2]+. a) Donor and acceptor orbitals of [2]+ for the
In–Rh bond. LP denotes a lone pair orbital, LV denotes a lone vacancy
(unoccupied valence orbital), and occ denotes the electron occupancy. b)
Stabilization energies by the Donor-Acceptor interaction.
C6
C9
Cl2
C8
+
Figure 2. ORTEP drawing of [2][BArF] at 50% probability level. The counter
anion BArF and hydrogen atoms are omitted for clarity. Selected bond lengths
(Å) and angles (deg): In–Rh = 2.6170(7), In–Cl1 = 2.376(2), In–Cl2 = 2.357(2),
Rh–C6 = 2.246(6), Rh–C7 = 2.196(6), Rh–C8 = 2.241(7), Rh–C9 = 2.197(8),
Cl1–In–Cl2 = 107.29(7), Cl2–In–Rh = 122.71(5), Rh–In–Cl1 = 121.42(5).
N
Rh
Rh
Rh
In
Cl
Cl
Cl Cl
Cp*RhCl2
Cp*Rh(cod)
[2]+
/ a.u.
/ a.u.
–0.010
–0.17
–0.12
–0.24
–0.094
–0.25
LUMO :
HOMO :
We performed theoretical calculations to gain further insights
into bonding situation and electronic nature of [2]+. The Z-type
ligation of the In-metalloligand to Rh is clearly supported by the
natural bond orbital (NBO) analyses. The In–Rh bond mainly
consists of two donor-acceptor interactions between occupied d
orbitals on Rh and a vacant orbital on In; one is a donation from
LP2(Rh) and the other is from LP4(Rh) to LV1(In), which
resulted in 31.2 and 20.7 kcal/mol stabilization, respectively
Figure 4. Energy levels of frontier molecular orbitals of [2]+ and standard
Cp*Rh complexes.
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10.1002/anie.201910197
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first example of tuning transition metal catalysis for C–H
activation by
a Z-type metalloligand. Detailed mechanistic
studies are ongoing in our group and will be reported in due
course.
0.0E+00
-2.0E-06
-4.0E-06
-6.0E-06
-8.0E-06
-1.0E-05
-1.2E-05
-1.4E-05
-1.6E-05
-1.8E-05
Table 1. C–H amidation catalyzed by In-Rh 2
catalyst (5 mol%)
+
TsN3
Cp*Rh(cod)
1
ClCH2CH2Cl
85 ˚C, 6 h
N
N
TsHN
(1.5 equiv)
[Cp*RhCl2]2
[2][InCl4]
3
Entry
Catalyst
Yield of 3[a] [%]
-1.8
-1.6
-1.4
-1.2
-1
Potential vs Fc+/Fc [V]
1
[2][InCl4]
[2][BArF]
1
66 (61)[b]
2
66
0
Figure 5. Cyclic voltammetry of 1, [2][InCl4], Cp*Rh(cod), and [Cp*RhCl2]2 in
CH2Cl2 (0.1 M NBu4PF6, 1 mM substrate, 100 mV/sec).
3
4
Cp*Rh(cod)
[Cp*RhCl2]2
Trace
0
The unique electronic and redox properties of the In–Rh
complex invoked us to test its catalytic reactivity for the
chelation-assisted oxidative sp2C–H bond functionalization
reactions of arenes, which are usually catalyzed by Cp*RhIII
complexes.[18] After extensive screening, the In–Rh complex
[2][InCl4] was found to catalyze the sp2C–H amidation reaction of
benzo[h]quinoline with 1.5 equiv. of TsN3 to afford an N-aryl
tosylamide derivative 3 in good yield (Entries 1, Table 1).[19]
Counter anions did not affect the reactivity (Entry 2). CpRhI
complexes have rarely been reported to be active for this type of
reactions,[20] and indeed, the In-free, pyridine-tethered 1 and
Cp*Rh(cod) did not catalyze the reaction at all (Entries 3 and 4).
Importantly, in situ formation of the In–Rh complex by mixing 1
and InCl3 was also effective (Entry 7), and the catalytic activity
was on the same level with that of the standard catalyst system
for this reaction, Cp*RhCl2/AgSbF6 (Entry 6).[19a]. This method
enables easy screening of Z-type metalloligands for optimization
of reaction conditions. Addition of AlCl3, GaCl3, or
trifluoromethanesulfonic acid (TfOH) resulted in low yield of 3 or
[c]
5
[d]
6
[Cp*RhCl2]2/AgSbF6
81
73
30
0
[e]
7
1 + InCl3
[e]
8
1 + AlCl3
[e]
9
1 + GaCl3
10
11
1 + TfOH[e]
0
[e]
Cp*Rh(cod) + InCl3
7
[a] NMR yield. [b] Isolated yield. [c] 2.5 mol%. [d] 2.5 mol% [Cp*RhCl2]2 and 10
mol% AgSbF6. [e] 10 mol%.
Table 2. Generality of substrates
TsN3
X
N
X
N
[2][BArF] (5 mol%)
+
no reaction, thus demonstrating crucial role of InCl2 as a
+
or
O
Y
H
Y
NHR
Toluene
120 ˚C, 4 h
metalloligand (Entries 8-10). The combination of Cp*Rh(cod)
and InCl3 was also not effective, giving the amidation product 3
in poor yield (Entry 11). These results clearly demonstrated that
Ph
O
N O
5 (X = CH, Y = C, R = Ts)[a]
6 (X = CH, Y = C, R = Bz)[b]
7 (X = N, Y = C or N, R = Ts)[a]
4
+
both InCl2 and the pyridine-tethered cyclopentadienyl ligand
(1.5 equiv)
were necessary to turn on the C–H activation catalysis of the
rhodium metal in 1.
N
N
Preliminary investigation on substrate generality disclosed
that the In-Rh catalyst system was also applicable to the
reaction of various 2-arylpyridines (5a-c, Table 2). Not only TsN3,
but also 3-phenyl-1,4,2-dioxazole-5-one 4 was employable as a
nitrene source to give N-aryl benzamide derivatives 6a-c in
moderate to good yield. Furthermore, pyrimidine was also
effective as a directing group for this reaction. N-tosyl amidation
of N-pyrimidyl indole proceeded selectively at 2-position to afford
7a. Selective double amidation at both ortho-positions of 2-
arylpyrimidine was also achieved using 3 equiv. of TsN3 to give
7b in good yield. Although the reaction mechanism is not clear
yet (see the SI for some preliminary investigations), this is the
R =
H (5a) 57%
R = H (6a) 77%[c]
OMe (5b) 40%
CF3 (5c) 38%
OMe (6b) 73%
CF3 (6c) 54%[c]
NHTs
NHBz
NHTs
R
R
N
N
N
MeO
N
N
NHTs
NHTs
7b 74%[e]
7a 53% (81%)[d]
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COMMUNICATION
[8]
[9]
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1017–1022.
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added. [d] NMR yield. [e] 3.0 equiv. of TsN3 was used.
Pincer-type transition metal complexes having an anionic boryl or
aluminyl ligand as an X-type ligand were reported to catalyze
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a) E. Kwan, Y. Kawai, S. Kamakura, M. Yamashita, Dalton Trans. 2016,
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In conclusion, we have developed new CpRh complexes
having
a supported Z-type In-metalloligand that turns on
rhodium catalysis for sp2C–H bond activation. This study
demonstrated powerful utility of the Z-type metalloligand for
tuning transition metal catalysis by seemingly changing the
oxidation state of the transition metal. Application of this strategy
to other reactions and Cp metal complexes is in progress toward
further development of organometallic and synthetic chemistry.
[10] There are a few examples of synthesis of bimetallic complexes utilizing
an aminoethyl-functionalized Cp ligand. See: a) R. Fischer, S. Nlate, H.
Hoffmann, E. Herdtweck, J. Blümel, Organometallics 1996, 15, 5746–
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Acknowledgements
This research was supported by JSPS KAKENHI Grant
Numbers 15H05800, 17H03019 (Gran-in-Aid for Scientific
Research (B)), 18H04646 (Hybrid Catalysis), and JST, PRESTO
Grant Number JY290145, Japan.
[11] Synthesis and reactions of Cp-metal complexes having ambiphilic
phosphine ligands are reported, in which the Lewis acidic metal moiety
activates substrates or substituents as a pendant ligand. See: a) F.-G.
Fontaine, D. Zargarian, J. Am. Chem. Soc. 2004, 126, 8786–8794. b) J.
Boudreau, F.-G. Fontaine, Organometallics 2011, 30, 511–519. c) T.
Ostapowicz, C. Merkens, M. Hölscher, J. Klankermayer, W. Leitner, J.
Am. Chem. Soc. 2013, 135, 2104–2107.
Keywords: Z-type ligand • Metalloligand • C–H activation •
Rhodium • Indium
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[14] Among metal complexes examined, GaCl3 and AlCl3 also formed
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were low judged by 1H NMR. See the Supporting Information for details.
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amidation of 2-phenylpyridine with TsN3 in the ref 18a.
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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Text for Table of Contents
Author(s), Corresponding Author(s)*
Page No. – Page No.
Title
Layout 2:
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Ryosuke Yamada, Nobuharu Iwasawa,
Jun Takaya*
+
N
Awakening C–H activation catalysis
with a Z-type metalloligand
Rh
In
Page No. – Page No.
Cl
Cl
Awakening Rhodium Catalysis for C–
H Activation with an In-Metalloligand
Rhodium complexes having an In-metalloligand are successfully synthesized
utilizing a pyridine-tethered cyclopentadienyl ligand as a support for an In–Rh bond.
The In-metalloligand dramatically changes the electronic and redox properties of
the rhodium metal, awakening its catalysis for an sp2C–H bond activation reaction.
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