Heterolysis of NH-indoles by bifunctional amido complexes and applications to carboxylation with carbon dioxide

Article abstract of DOI:10.1021/om500695a

Bifunctional amidoiridium complexes bearing C-N chelate ligands readily deprotonate unprotected NH-indole to give new κ¹-N-indolyl(amine) complexes, quantitatively. The resulting complexes were fully characterized by NMR, elemental analysis, and X-ray crystallography. The analogous Rh and Ru complexes were also synthesized by treatment of the precursory amine complexes with free indole in the presence of an equimolar amount of KO^tBu. The indolyl moiety bound to the Ir center exhibits nucleophilic character to react with benzoyl chloride and to give 1- and 3-acylated indoles in yields of 68% and 18%, respectively. A related amido-Ru complex derived from an N-sulfonylated 1,2-diphenylethylenediamine ligand promoted carboxylation of indoles with CO₂ under basic conditions to give indole-1-carboxylic acids.

Full text of DOI:10.1021/om500695a

Article
pubs.acs.org/Organometallics
Heterolysis of NH-Indoles by Bifunctional Amido Complexes and
Applications to Carboxylation with Carbon Dioxide
Atsushi Ueno, Yoshihito Kayaki, and Takao Ikariya*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama
2-12-1-E4-1, Meguro-ku, Tokyo 152-8552, Japan
S
* Supporting Information
ABSTRACT: Bifunctional amidoiridium complexes bearing C−N chelate ligands readily deprotonate unprotected NH-indole to
give new κ1-N-indolyl(amine) complexes, quantitatively. The resulting complexes were fully characterized by NMR, elemental
analysis, and X-ray crystallography. The analogous Rh and Ru complexes were also synthesized by treatment of the precursory
amine complexes with free indole in the presence of an equimolar amount of KOtBu. The indolyl moiety bound to the Ir center
exhibits nucleophilic character to react with benzoyl chloride and to give 1- and 3-acylated indoles in yields of 68% and 18%,
respectively. A related amido-Ru complex derived from an N-sulfonylated 1,2-diphenylethylenediamine ligand promoted
carboxylation of indoles with CO2 under basic conditions to give indole-1-carboxylic acids.
Scheme 1. Asymmetric C−C Bond Formation through
Interconversion of Bifunctional Amido and Amine
Complexes
INTRODUCTION
■
Group 8 and 9 metal complexes with a metal/NH bifunctional
effect have been developed as efficient catalysts for hydrogen
transfer and 1,4-addition reactions, in which both amido and
amine complexes are involved as real active intermediates
through proton exchange.1 The coordinatively unsaturated 16-
electron amido complexes, Ru(Ts-diamine)(η6-arene) and
Cp*M(Ts-diamine) (Ts-diamine = N-(p-toluenesulfonyl)-1,2-
diamine, Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl, M =
Rh, Ir), exhibit sufficient Brønsted basicity to form amine-
hydrido complexes by deprotonation of alcohols or formic acid
that is an essential process for the reduction of C−O double
bonds.2 The amido complexes can also cause deprotonation of
a range of acidic compounds such as malonates and
cyanoacetates to give the corresponding amine complexes
that serve as C-nucleophiles toward cyclic enones, nitroalkenes,
and azodicarboxylates in catalytic enantioselective C−C and
C−N bond formations (Scheme 1).3
The related C−N chelate amido complexes derived from
benzylic amines have also been successfully isolated as active
hydrogen transfer catalysts.4 Originating from strong σ-
donating properties of the C−N chelate ligand, the
amidoiridium complexes can deprotonate even acetone (pKa
= 26.5 in DMSO) to form the corresponding C-bound
acetonyl(amine) complexes. On the basis of equilibrium
constants in the deprotonation process, it has been suggested
that the amido-Ir complexes with C−N chelates have more
basic properties than those with N-sulfonylated diamine
ligands. Because the proton transfer from acidic molecules to
amido-metal moieties is a versatile trigger for nucleophilic
transformations, a series of bifunctional amido complexes will
allow for designing new catalysis based on these stoichiometric
reactions with pronucleophiles.
The objectives of this work are to investigate deprotonative
activation of indoles as nitrogen pronucleophiles5 and to
develop a new CO2 transformation by the bifunctional catalysts.
Indoles are one of the most attractive and useful heterocycles in
the field of medicinal and synthetic organic chemistry.6
Received: July 3, 2014
Published: August 25, 2014
© 2014 American Chemical Society
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Although characteristic features of indoles as N- and C-
nucleophiles have been applied to regioselective functionaliza-
tion using various acceptors, CO2-carboxylation of indoles has
been confined to stoichiometric reactions using a strong Lewis
acid7 or base.8 For example, dimethylaluminum chloride can
promote Friedel−Crafts type carboxylation7a of N-substituted
indoles at room temperature under 3.0 MPa of CO2 selectively
to give indole-3-carboxylic acids as the C-carboxylation product.
While employment of n-BuLi or sec-BuLi to deprotonate NH-
indoles in N-carboxylation is a well-established method, it is
recently found that treatment with a large excess (5 equiv) of
LiOtBu at 100 °C in DMF resulted in C-carboxylation to afford
indole-3-carboxylic acids in high yields after hydrolysis.8f
Herein, we disclose the deprotonation of NH-indole (1) by
bifunctional amido complexes and catalytic application to the
carboxylation of indole under a CO2 atmosphere.
Figure 1. X-ray structures of indolyliridium(III) complexes 3a and 3b.
The hydrogen atoms except for the coordinating amine protons are
omitted for clarity, and the thermal ellipsoids represent the 50%
probability level.
The Rh and Ru variants (3c and 3d) were also isolated by
stoichiometric reactions of indole with the related C−N chelate
amine complex, Cp*RhCl[κ2(N,C)-(NH2CR2-2-C6H4)] [R =
C6H5 (4a)] or RuCl[κ2(N,C)-(NH2CR2-2-C6H4)](hmb) [R =
C6H5, hmb = hexamethylbenzene (4b)] in the presence of
KOtBu in the CH2Cl2 solution at room temperature, as
depicted in Scheme 3. Single crystals of 3c and 3d were
obtained in yields of 52% and 62%, respectively, after slow
diffusion of pentane into the CH2Cl2 solution. The analogous
molecular structure determined by single-crystal X-ray
diffraction is shown in Figure 2.
Compared with the structures between Ir and Rh complexes
(3b and 3c), the bond lengths around the Rh center, M−NH2
(2.094(3) Å), M−Nindolyl (2.078(3) Å), and M−C (2.023(3)
Å), were slightly shorter than those of the Ir complex. As listed
in Table 1, a marked elongation of the Ir−Namine bond or Ir−
Nindolyl bond in 3d implies greater steric hindrance of the
Ru(hmb) system than the Cp*Rh.
RESULTS AND DISCUSSION
■
Reaction of Cyclometalated Amido-Ir Complexes with
Indole. The isolable 16-electron amido complex, Cp*Ir-
[κ2(N,C)-(NHCR2-2-C6H4)] [R = CH3 (2a), R = C6H5
(2b)] derived from cumylamine or tritylamine,4 was found to
react readily with an equimolar amount of indole (1a) in
CH2Cl2 at room temperature, leading to the 18-electron N-
coordinated indolyl complex, Cp*Ir[κ1(N)-indolyl][κ2(N,C)-
(NH2CR2-2-C6H4)] (3a and 3b), quantitatively. After slow
diffusion of diethyl ether into the solution in dichloromethane,
orange crystals were isolated in yields of 48% and 56%,
respectively (Scheme 2). These structures were unambiguously
Scheme 2. Reaction of Indole with C−N Chelate Amido
Complexes
Hererolysis of the N−H bond by the 16e amido complex was
strongly influenced by the structure of indoles. While a
sterically congested indole, 7-methylindole (1b), did not
become deprotonated in a CD2Cl2 solution containing an
equimolar amount of 2a, an equilibrium between the amido
complex and indolyl(amine) complex (3e) was confirmed by
NMR experiments for the case of 2-methylindole (1c). In a
separate experiment, 3e was successfully isolated as a yellow
powder, which was fully characterized by NMR analysis. The
indolyliridium complex 3e proved to be labile in solution and
gradually converted into the amido complex 2a and free 1c
above −10 °C. From the equilibrium constant (Keq)
determined by the 1H NMR monitoring of an equimolar
mixture of 2a with 1c in CD2Cl2 in a temperature range of −20
to 30 °C, the van’t Hoff plot of ln Keq versus 1/T can be fit
linearly to determine ΔH = −43.2 kJ mol−1 and ΔS = −126.6 J
mol−1 K−1 (see the Supporting Information).
Reactivity of Indolyl-Ir Complexes with Electrophiles.
The σ-coordination of the indolyl unit to metals can promote
nucleophilic transformation. The isolable indolyliridium com-
plex 3a was found to react with 2 mol amounts of benzoyl
chloride in CD2Cl2 at 50 °C in a sealed NMR tube to give the
corresponding N- and C-acylated products in 68% and 17%
yields, respectively, along with quantitative formation of a well-
defined chloro(amine)iridium complex, Cp*IrCl[κ2(N,C)-
{NH2C(CH3)2-2-C6H4}] (Scheme 4).4a In a related report by
Hartwig and co-workers, a κ1(N)-pyrrolyl-Ir(I) complex
synthesized from a Vaska complex and lithium pyrrolide can
be converted into the corresponding N-acylated pyrrole in 17%
yield by treatment with benzoyl chloride.10
characterized by NMR spectroscopy, elemental analysis, and X-
ray crystallography. The H NMR spectra of 3a and 3b in
1
CD2Cl2 show signals due to vinylic protons at 6.13 and 6.96
ppm for 3a, and at 6.23 and 7.09 ppm for 3b. While two
independent peaks assigned to NH2 protons on the C−N
chelate ligands were displayed at 3.99 and 4.14 ppm for 3a and
at 5.06 and 5.30 ppm for 3b, no peaks due to the NH proton
on the indole motif appeared in these spectra.
Each crystallographic structure of 3a or 3b, illustrated in
Figure 1, indicates a monomeric complex with a three-legged
piano-stool structure bearing an indolyl, Cp*, and C−N chelate
ligands. The sum of the angles around the indolyl nitrogen are
almost 360°, and the Ir−Nindolyl bond lengths are 2.089(4) and
2.093(3) Å (Table 1), which are similar to those for the
reported κ1(N)-indolyliridium(I) complex.9 The bicyclic indole
scaffold is virtually planar and involves a C−C double bond
between 2- and 3-positions. These structural data suggest that
an sp2-nitrogen on the indolyl ligand coordinates in a κ1 fashion
to the Ir(III) center.
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Table 1. Structural Data for Indolyl Complexes 3a−3d
complex
M−Namine (Å)
M−Nindolyl (Å)
Cp*centroid−M (Å)
around Nindolyl (deg)
3a
3b
3c
3d
2.127(5)
2.115(3)
2.094(3)
2.137(3)
2.089(4)
2.093(3)
2.078(3)
2.115(2)
1.831(3)
1.826(2)
1.825(2)
359.2
360.0
359.9
359.9
a
1.723(1)
a
Distance between a centroid of hmb and the Ru center.
Scheme 3. Synthesis of Indolyl-Rh and Indolyl-Ru
Complexes with C−N Chelate Ligand Derived from
Triphenylmethylamine
Scheme 5. Reaction of 2a with Indole-1-carboxylic Acid or
Indole-3-carboxylic Acid
Scheme 6. Decarboxylation from Indole-3-carboxylato
Complex 6
Figure 2. X-ray structures of indolyl complexes 3c and 3d. The
hydrogen atoms except for the coordinating amine protons are
omitted for clarity, and the thermal ellipsoids represent the 50%
probability level.
Scheme 7. Formation of Amido Complex from 5 and 6
under Basic Conditions
Scheme 4. Reaction of 3a with Benzoyl Chloride
In further attempts on carboxylation of the indolyl fragment,
we tested a reaction of 3a with pressurized CO2 up to 10.0 MPa
at 50 °C in THF, but unfortunately, no CO2-incorporated
1
products were detected by H NMR spectroscopy. To realize
the stability of carboxylated products in the presence of the
bifunctional catalysts, the amidoiridium complex 2a was treated
with indolecarboxylic acids in THF at room temperature. As
shown in Scheme 5, both of indole-1-carboxylic acid and
indole-3-carboxylic acid were smoothly deprotonated to afford
the corresponding carbamato and carboxylato complexes (5
and 6) in almost quantitative yields. The structures were fully
characterized by NMR and IR spectroscopies as well as
elemental analysis. The carbamato group in 5 remained even
under vacuum at 50 °C in THF-d8, whereas 6 derived from
indole-3-carboxylic acid gradually decomposed under the same
conditions with release of CO2 to give the stable indolyliridium
complex 3a, quantitatively (Scheme 6). On the other hand, as
shown in Scheme 7, the carbamato complex 5 and carboxylato
complex 6 were convertible to the parent amido-Ir complex 2a
by addition of KOtBu at room temperature. The quantitative
1
release of indolecarboxylates was also confirmed by H NMR
spectroscopy after treatment of the reaction mixture with 1 N
HCl.
Carboxylation of Indoles Using Bifunctional Catalysts.
On the basis of new findings on the isolable indolyl complexes
3 discussed above, we examined carboxylation of indoles in the
presence of catalytic amounts of bifunctional catalysts. The
reaction of NH-indole 1a was conducted under atmospheric
pressure of CO2 at 50 °C in THF containing a series of the
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a b c
, ,
amido or precursory amine complexes with a substrate/catalyst
(S/C) ratio of 50. The results are summarized in Table 2.
Table 3. Catalytic Carboxylation of NH-Indole Using 8
Table 2. Catalytic Carboxylation of 1a Using Bifunctional
a
Complexes
b
entry
catalyst
base
% yield
1
2
3
4
5
6
7
8
9
2a
2a
3a
2b
4a
4b
0
28
29
20
17
32
78
25
52
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
NaOtBu
LiOtBu
8, Ru[(S,S)-PMsdpen] (hmb)
8
8
a
Reaction conditions: The reaction was carried out with indole (1.0
mmol) and catalyst (0.02 mmol) in the presence of base (1.0 mmol)
b
1
in THF (3.0 mL) under CO2 (1 atm) at 50 °C. Determined by H
NMR.
Although the amidoiridium complex 2a did not exhibit
catalytic activity without base, an equimolar addition of KOtBu
to indole resulted in formation of indole-1-carboxylic acid (7a)
as a sole product in 28% yield after quenching by aqueous 1 N
HCl (entries 1 and 2). Catalytic use of the isolated indolyl
complex 3a showed a comparable activity (29% yield) with the
amido complex 2a, indicating that N−H heterolysis by the
bifunctional amido catalyst is involved in the catalytic
mechanism (vide infra). The preferential N-carboxylation is
presumably ascribable to the thermally stable nature of the
carbamato complex generated from 3a and CO2, compared to
the carboxylate complex via a possible C-carboxylation as
mentioned above. Among the C−N chelate complexes (2 or 4),
the Ru analogue 4b displayed better catalytic performance in a
yield of 32% (entries 2 and 4−6). Notably, N−N chelate amido
complexes, which have a good track record in the C−C and C−
N bond formation,3 were also applicable to the catalytic
carboxylation. A further increase in the yield to 78% was
attained by employment of Ru(PMsdpen)(hmb) (8)
(PMsdpen = N-pentamethylbenzenesulfonyl-1,2-diphenyl-
ethylenediamido) (entry 7).3c,j The sterically congested
environment for 8 rather than the C−N chelate complexes
may facilitate nucleophilic attack of the indolyl ligand to CO2.
Unsatisfactory results on switching the base from KOtBu to
NaOtBu or LiOtBu indicate that the choice of base is crucial for
furnishing 7a (entries 7−9).
The carboxylation of 1a under the optimized reaction
conditions using 2 mol % of the Ru catalyst 8 and KOtBu under
a CO2 atmosphere and subsequent treatment with CH3I
provided an esterification product (7a′) in 85% yield (Table 3,
entry 1). As listed in Table 3, various NH-indoles were
efficiently converted into methyl esters of indole-1-carboxylic
acids, although sterically hindered indoles, 1b and 1c, were
completely recovered under the identical conditions (entries 2
and 3). Incorporation of electron-donating groups, such as
CH3, OCH3 and OSi(CH3)C(CH3)3, did not affect or spoil the
a
Reaction conditions: The reaction was carried out with indoles 1 (1.0
mmol) and catalyst 8 (0.02 mmol) in the presence of KOtBu (1.0
b
mmol) in THF (4.0 mL) under CO2 (1 atm) at 50 °C. Isolated yield.
c
DMF was used as a cosolvent.
catalytic activity to give the corresponding methyl ester (7d′−
7f′) in high yields (entries 4−6). For the reaction of less
soluble substrates having OCH3 or p-methoxyphenyl sub-
stituents, addition of DMF as a cosolvent proved to be effective
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according to the literature. Carbon dioxide (99.999%) and other
reagents were used as delivered. Most measurements, including
1H(399.78 MHz) and 13C(100.53 MHz) NMR, IR, and mass spectra,
were conducted with standard commercial instruments. The NMR
chemical shifts were referenced to SiMe4 by using residual protio
impurities in the deuterated solvent. Abbreviations for 1H NMR are as
follows: s = singlet, d = doublet, t = triplet, q = quartet or m =
multiplet.
(entries 5 and 7). Notably, halogen or pseudohalogen (OTf)
substituted indoles (1h−1l) were also applicable to this
carboxylation in moderate to good yields (45−85%, entries
8−12) without affording dehalogenated products. Regardless of
basic conditions, the ester group on 1m remained intact
throughout the carboxylation to afford diester (7m′) in 65%
yield (entry 13).
To account for participation of the bifunctional metal/NH
moiety in activation of indoles, a possible catalytic mechanism
is shown in Scheme 8. Although an indolyl complex was not
RuCl[κ1(N,C)-NH2C(C6H5)2-2-C6H4](hmb) (4d). A mixture of
Ru(OAc)2(hmb) (160.2 mg, 0.42 mmol) and the triphenylmethyl-
amine (0.42 mmol) in CH3CN (5 mL) was stirred at room
temperature for 24 h. The solvent was removed under reduced
pressure. After it was washed with dry Et2O (10 mL × 2), the mixture
was evaporated under reduced pressure to give a ruthenacycle product
Ru(OAc)[κ2(N,C)-NH2C(C6H5)2-2-C6H4](hmb) as an orange pow-
der. To a mixture of ruthenacycle and CH2Cl2 (10 mL) was added
excess KCl (742 mg, 10 mmol), and the mixture was stirred for 24 h at
room temperature. After the resulting insoluble solid was removed by
filtration through a pad of Celite under an Ar atmosphere, 4d was
obtained as an orange powder after removal of the solvent under
reduced pressure. Orange plate crystals of 4d were collected after slow
diffusion of hexane into the solution in CH2Cl2 (140.4 mg, 0.25 mmol,
Scheme 8. Proposed Mechanism of Carboxylation of Indoles
Catalyzed by Amido Complexes
1
60% yield). H NMR (CD2Cl2, 399.8 MHz,): δ 1.74 (s, 18H, hmb),
2
2
4.10 (br, JHH = 10.4 Hz; NH), 5.29 (br, JHH = 10.4 Hz; NH), 6.04
3
4
3
(dd, 1H, JHH = 7.6 Hz, JHH = 1.2 Hz), 6.68 (dd, 1H, JHH = 7.6 Hz,
4JHH = 1.2 Hz) 7.04−7.06 (m, 2H), 7.14−7.21 (m, 5H), 7.34−7.38 (m,
3H), 7.73 (dd, 1H, JHH = 7.6 Hz, JHH = 1.2 Hz). 13C{1H} NMR
(CD2Cl2, 100.5 MHz): δ 15.2, 77.9, 92.4, 121.4, 125.9, 126.2, 126.9,
128.1, 128.3, 128.4, 128.7, 129.6, 137.4, 145.1, 148.7, 152.3, 176.6.
Anal. Calcd for C31H34NClRu: C 66.83, H 6.15, N 2.51. Found: C
66.72, H 6.42, N 2.56.
3
4
Cp*Ir[κ1(N)-indolyl][κ2(N,C)-NH2C(CH3)2-2-C6H4] (3a). Indole
(1a; 11.7 mg, 0.1 mmol) was added to a solution of Cp*Ir[κ2(N,C)-
NHC(CH3)2-2-C6H4] (2a) (47 mg, 0.1 mmol) in dry CH2Cl2, and the
reaction mixture was stirred for 24 h at room temperature. The
solution was concentrated in vacuo, and the residue was washed with
hexane (5 mL × 2). After drying under vacuum, Cp*Ir[κ1(N)-
indolyl][κ2(N,C)-NH2C(CH3)2-2-C6H4] (3a) was obtained as a
yellow powder. Orange plate crystals of 3a were collected by
crystallization from CH2Cl2/Et2O (32 mg, 0.055 mmol, 56% yield).
1H NMR (CD2Cl2, 399.8 MHz): δ 1.21 (s, 3H; C(CH3)2), 1.22 (s,
3H; C(CH3)2), 1.59 (s, 15H; Cp*), 4.00−4.15 (d, 2H, NH2), 6.13
(dd, 1H, 3JHH = 2.6 Hz, 4JHH = 0.6 Hz; IrNCHCH), 6.67 (dd, 1H, 3JHH
identified by the reaction of the Ru(PMsdpen)(hmb) complex
8 and 1a, the heterolysis of the N−H bond seems to be
promoted similar to the C−N chelate Ir complexes 2.
Therefore, the catalytically active amido complex 8 deproto-
nates NH-indoles to form κ1(N)-indolyl complexes, and the
following addition to CO2 will generate the corresponding
carbamato complexes. As demonstrated in the model reaction
of 5 with KOtBu (Scheme 7), the successive irreversible release
of the indole-1-carboxylate salt and tBuOH with regeneration of
8 with the aid of the base should be responsible for the
carboxylation. The sterically demanding structure of the most
effective catalyst bearing hmb and PMsdpen will enhance
dissociation of the carboxylated products from the amine
complex to drive the catalytic cycle.
In conclusion, we have demonstrated the facile N−H
addition of indoles toward bifunctional amido complexes to
provide new indolyl(amine) complexes 3. The nucleophilic
nature of the indolyl complex was realized by reaction with
benzoyl chloride to give the corresponding acylated products.
On the basis of the reactivities of bifunctional indolyl and
indolecarboxylato complexes, the selective N-carboxylation of
NH-indoles under a CO2 atmosphere was successfully
demonstrated by catalytic use of the bifunctional Ru catalyst 8.
4
= 7.6 Hz, JHH = 0.6 Hz), 6.78 (m, 1H; indolyl), 6.89 (m, 1H; CN
ligand), 6.92 (m, 1H, indolyl), 6.96 (d, 1H; IrNCH), 6.97 (m, 1H;
3
N(CH)2CCH), 7.10 (ddd, 1H; CN ligand), 7.45 (d, 1H, JHH = 7.6
3
4
Hz; IrNCCH), 7.78 (dd, 1H, JHH = 7.7 Hz, JHH = 0.6 Hz; IrCCH;
CN ligand). 13C{1H} NMR (CD2Cl2, 100.5 MHz): δ 9.2, 30.0, 31.8,
66.7, 87.3, 99.4, 112.9, 116.2, 117.6, 119.8, 121.0, 122.3, 127.0, 132.5,
136.2, 137.0, 142.3, 153.1, 154.8. Anal. Calcd for C27H33N2Ir: C 56.13,
H 5.76, N 4.85. Found: C 56.40, H 5.85, N 5.02.
Cp*Ir[κ1(N)-indolyl][κ2(N,C)-NH2C(C6H5)2-2-C6H4] (3b). 1a (10
mg, 0.085 mmol) was added to a solution of Cp*Ir[κ2(N,C)-
{NHC(C6H5)2-2-C6H4}] (2b, 45 mg, 0.085 mmol) in dry CH2Cl2,
and the reaction mixture was stirred for 24 h at room temperature.
The solution was concentrated in vacuo, and the residue was washed
with Et2O (5 mL × 2). After evaporation under vacuum, Cp*Ir[κ1(N)-
indolyl][κ2(N,C)-NH2C(CH3)2-2-C6H4] (3b) was obtained as a
brown powder. Orange plate crystals of 3b were collected after slow
diffusion of Et2O into the solution in CH2Cl2 (29 mg, 0.041 mmol,
48% yield). 1H NMR (CD2Cl2, 399.8 MHz): δ 1.39 (s, 15H), 5.08 (d,
1H, 2JHH = 10.1 Hz, NH), 5.34 (d, 1H, 2JHH = 10.2 Hz, NH), 5.99 (d,
2H, 3JHH = 7.3 Hz), 6.06 (dd, 1H, 3JHH = 7.4 Hz, 4JHH = 1.2 Hz), 6.23
3
EXPERIMENTAL SECTION
(1H, m, NCHCH), 6.76−6.89 (m, 4H), 6.90 (dt, 1H, JHH = 7.3 Hz,
■
4JHH = 1.2 Hz; IrC(CH)2), 6.97 (m, 1H), 7.02 (m, 1H), 7.08−7.12 (m,
General Procedure. Solvents were dried by refluxing over sodium
benzophenone ketyl (toluene, THF, ether) or P2O5 (CH2Cl2), and
distilled under argon. DMF was used after distillation of a
commercially available dehydrated reagent. The C−N chelate Ir and
Rh complexes, 2a,4a 2b,4a 4a,4b and Ru(OAc)2(hmb),11 were prepared
3
4
3H), 7.15 (dt, 1H, JHH = 7.3 Hz, JHH = 1.2 Hz, IrC(CH)2), 7.35−
3
3
7.39 (m, 3H), 7.54 (d, 1H, JHH = 7.6 Hz), 7.90 (dd, 1H, JHH = 7.4
Hz, JHH = 1.1 Hz; IrCCH). 13C{1H} NMR (CD2Cl2, 100.5 MHz): δ
4
8.6, 80.0, 87.3, 99.4, 112.8, 116.3, 117.9, 120.1, 121.9, 126.4, 127.0,
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127.2, 127.7, 128.2, 128.4, 128.7, 129.1, 132.6, 136.4, 137.7, 141.7,
144.9, 147.8, 153.5, 156.5. Anal. Calcd for C37H37N2Ir: C 63.31, H
5.31, N 3.99. Found: C 62.90, H 5.60, N 4.12.
Cp*Ir(indole-3-carboxylato)[κ2(N,C)-NH2C(CH3)2-2-C6H4] (6).
Indole-3-carboxylic acid (5.4 mg, 0.033 mmol) was added to a
solution of Cp*Ir[κ2(N,C)-NHC(CH3)2-2-C6H4] (2a) (15.4 mg,
0.033 mmol) in dry THF (2 mL), and the reaction mixture was
stirred for 24 h at room temperature. The solution was concentrated in
vacuo, and the residue was washed with hexane (5 mL × 2). After
evaporation under vacuum, Cp*Ir(3-indolecarboxylato)[κ2(N,C)-
NH2C(CH3)2-2-C6H4] (6) was obtained as a brown powder (20.4
mg, 0.033 mmol, >99% yield). 1H NMR (CD2Cl2, 399.8 MHz): δ 1.18
(s, 3H; C(CH3)2), 1.27 (s, 3H; C(CH3)2), 1.55 (s, 15H, Cp*), 3.73
(d, 1H, NH2), 5.73 (d, 1H, NH2), 6.61−7.98 (m, 9H, indole and
C6H4), 10.2 (br, 1H; indole NH). 13C{1H} NMR (100.5 MHz,
CD2Cl2): δ 9.2, 30.1, 31.9, 67.0, 87.6, 105.4, 114.1, 119.8, 119.9, 121.1,
121.3, 122.9, 127.5, 131.1, 136.0, 144.1, 146.2, 152.2, 154.8, 170.2.
Anal. Calcd for C28H33N2O2Ir: C 54.09, H 5.35, N 4.51. Found: C
54.10, H 5.48, N 4.40. IR (KBr, cm−1): 1680 (vs, νCO).
Cp*Rh[κ1(N)-indolyl][κ2(N,C)-NH2C(C6H5)2-2-C6H4] (3c). KOtBu
(9.0 mg, 0.080 mmol) and indole (1a; 45 mg, 0.085 mmol) were
added to a solution of Cp*RhCl[κ2(N,C)-NHC(C6H5)2-2-C6H4] (4a,
45 mg, 0.076 mmol) in dry CH2Cl2, and the reaction mixture was
stirred for 24 h at room temperature. The solution was concentrated in
vacuo, and the residue was washed with Et2O (5 mL × 2). After
evaporation under vacuum, Cp*Rh[κ1(N)-indolyl][κ2(N,C)-NH2C-
(C6H5)2-2-C6H4] (3c) was obtained as a brown powder. Orange plate
crystals of 3c were collected after slow diffusion of pentane into the
solution in CH2Cl2 (25 mg, 0.041 mmol, 48% yield). 1H NMR
(CD2Cl2, 399.8 MHz): δ 1.34 (s, 15H; C5(CH3)5), 4.31 (d, 1H, 2JHH
=
2
10.4 Hz; NH), 4.77 (d, 1H, JHH = 10.4 Hz; NH), 5.97 (d, 2H, Ph2-
3
3
4
para-H, JHH = 7.3 Hz), 6.03 (dd, 1H, JHH = 7.6 Hz, JHH = 1.5 Hz;
3
RhCCCH), 6.31 (1H, d, JHH = 2.2 Hz; NCHCH), 6.82−6.86 (m,
4H), 6.90 (t, 1H, JHH = 7.0 Hz; RhC(CH)2), 6.98 (m, 1H), 7.04 (d,
Catalytic Reactions. A 20 mL Schlenk flask was charged with 1a
(117 mg, 1.0 mmol), catalysts (13.7 mg, 0.02 mmol), and KOtBu (112
mg, 1.0 mmol) in THF (4.0 mL) under an Ar atmosphere. The flask
was evacuated, and gaseous CO2 was quickly introduced into the
reaction mixture for several times. After stirring the solution under the
required conditions for 24 h, THF was evaporated under reduced
pressure. DMF (2.0 mL) and MeI (284 mg, 2.0 mmol) were
subsequently added to the residue, and the resulting mixture was
stirred at 70 °C for 3 h. After removal of organic solvents, the resulting
yellow oil was purified by silica gel column chromatography using
hexane/acetone (4/1) mixtures and acetone as the eluent to give
methylindole-1-carboxylate as a white powder (148.9 mg, 0.85 mmol,
85% yield).
X-ray Structure Determinations of 3a−3d. All measurements
were made on a Rigaku Saturn CCD area detector equipped with
graphite-monochromated Mo-Kα radiation [λ = 0.71075 Å (3a, 3d),
0.71070 Å (3b, 3c)] under a nitrogen stream at 123 K (3a, 3c) or 93 K
(3b, 3d). Indexing was performed from 18 images. The crystal-to-
detector distance was 75.11 mm (2a), 44.90 mm (3b), 44.95 mm (3c),
and 45.01 mm (3d). The data were collected to a maximum 2θ value
of 55.0°. A total of 1440 (3a) or 720 (3b, 3c, and 3d) oscillation
images were collected. A sweep of data was carried out using ω scans
from −120.0 to 60.0° (3a) or from −110.0 to 70.0° (3b, 3c, 3d) in
0.5° steps, at χ = 45.0° and ϕ = 0.0°. A second sweep was performed
using ω scans from −120.0 to 60.0° (3a) or from −110.0 to 70.0° (3b,
3c, 3d) in 0.5° steps, at χ = 45.0° and ϕ = 90.0°. Intensity data were
collected for Lorentz-polarization effects as well as absorption.
Structure solution and refinements were performed with the
CrystalStructure program package. The heavy atom positions were
determined by Direct methods [SIR97 (3a) or SIR2002 (3b, 3c, 3d)],
and the remaining non-hydrogen atoms were found by subsequent
Fourier techniques. An empirical absorption correction based on
equivalent reflections was applied to all data. All non-hydrogen atoms
other than solvent molecules were refined anisotropically by full-matrix
least-squares techniques based on F2. All hydrogen atoms were
constrained to ride on their parent atom. Relevant crystallographic
data are compiled in the Supporting Information (Table S1).
3
3
3
1H, JHH = 2.2 Hz; NCH), 7.15−7.19 (m, 3H), 7.25 (dt, 1H, JHH
=
4
7.6 Hz, JHH = 1.5 Hz, RhC(CH)2), 7.38−7.42 (m, 3H), 7.57 (d, 1H,
3JHH = 7.6 Hz), 7.99 (d, 1H, 3JHH = 7.6 Hz; RhCCH). 13C{1H} NMR
(CD2Cl2, 100.5 MHz): δ 8.8, 77.4, 94.75 (d JRh−C = 5.8 Hz), 99.8,
112.9, 115.9, 117.6, 120.1, 122.3, 126.6, 126.9, 127.3, 127.5, 128.1,
128.4, 128.7, 129.0, 136.6, 138.8, 145.3, 147.8, 148.1, 153.3, 171.45 (d
JRh−C = 7.6 Hz). Anal. Calcd for C37H37N2Rh: C 72.54, H 6.09, N 4.57.
Found: C 72.10, H 6.19, N 5.02.
Ru[κ1(N)-indolyl][κ2(N,C)-NH2C(C6H5)2-2-C6H4](hmb) (3d).
KOtBu (35 mg, 0.31 mmol) and indole (1a; 38 mg, 0.33 mmol)
were added to a solution of RuCl[κ2(N,C)-NH2C(C6H5)2-2-C6H4]-
(hmb) (4d, 159 mg, 0.29 mmol) in dry toluene, and the reaction
mixture was stirred for 24 h at room temperature. The solution was
filtered through a Celite pad under an Ar atmosphere and
concentrated in vacuo. The residue was washed with hexane (5 mL
× 2). After evaporation under vacuum, Ru[κ1(N)-indolyl][κ2(N,C)-
NH2C(C6H5)2-2-C6H4](hmb) (3d) was isolated as a brown powder.
Orange plate crystals of 3d were collected after slow diffusion of
pentane into the solution in CH2Cl2 (114.7 mg, 0.18 mmol, 62%
1
yield). H NMR (CD2Cl2, 399.8 MHz): δ 1.72 (s, 18H; C(CH3)6),
2
1.59 (s, 15H; Cp*), 4.37 (d, 1H, JH3H = 11.0 Hz; NH), 5.01 (d, 1H,
2JHH = 11.0 Hz; NH), 5.68 (d, 2H, JHH = 7.3 Hz; NH2CCH), 5.94
(dd, 1H, 3JHH = 7.6 Hz, 4JHH = 1.2 Hz; RuCCCH), 6.23 (dd, 1H, 3JHH
4
3
= 3.3 Hz, JHH = 1.5 Hz; RuNCHCH), 6.69 (t, 2H, JHH = 7.9 Hz;
NH2CCH), 6.74 (dd, 1H, 3JHH = 7.3 Hz, 4JHH = 1.2 Hz; RuCCHCH),
6.81 (m, 1H; NCCH(CH)2-indolyl), 6.85−6.93 (m, 2H), 6.94 (d, 1H,
3JHH = 3.3; RuNCH), 7.12−7.19 (m, 2H), 7.39−7.41 (m, 3H), 7.56 (d,
1H, 3JHH = 7.6 Hz; RuNCCH-indolyl), 8.11(dd, 1H, 7.7 Hz, 4JHH = 1.2
Hz; RuCCH; CN ligand). 13C{1H} NMR (CD2Cl2, 100.5 MHz,): δ
15.5, 78.0, 94.0, 99.4, 113.2, 115.5, 117.3, 120.2, 121.4, 126.1, 126.2,
126.6, 127.4, 127.9, 128.3, 128.5, 129.5, 133.2, 138.0, 139.5, 142.6,
145.0, 149.1, 152.7, 177.4. Anal. Calcd for C39H40N2Ru: C 73.44, H
6.32, N 4.39. Found: C 73.10, H 6.19, N 4.72.
Cp*Ir(indole-1-carboxylato)[κ2(N,C)-NH2C(CH3)2-2-C6H4] (5).
Indole-1-carboxylic acid (5.4 mg, 0.034 mmol) was added to a
solution of Cp*Ir[κ2(N,C)-NHC(CH3)2-2-C6H4] (2a) (15.4 mg,
0.033 mmol) in dry THF (2 mL), and the reaction mixture was
stirred for 24 h at room temperature. The solution was concentrated in
vacuo, and the residue was washed with hexane (5 mL × 2). After
evaporation under vacuum, Cp*Ir(1-indole-carboxylato)[κ2(N,C)-
NH2C(CH3)2-2-C6H4] (5) was obtained as a gray powder,
quantitatively (20.3 mg, 0.033 mmol, 99% yield). 1H NMR
(CD2Cl2, 399.8 MHz): δ 1.23 (s, 3H; C(CH3)2), 1.24 (s, 3H;
C(CH3)2), 1.61 (s, 15H, Cp*), 4.01 (d, 1H, NH2), 5.73 (d, 1H, NH2),
6.12 (d, 1H, 3JHH = 2.3 Hz; IrNCHCH), 6.67 (d, 1H, 3JHH = 7.3 Hz),
ASSOCIATED CONTENT
■
S
* Supporting Information
Texts, figures, and CIF files giving characterization data for 7a
and 7a′−7m′; NMR spectra of 3a−3e, 4d, 5, and 6; details of
determination of thermodynamic parameters in the formation
of 3e; and crystallographic data for 3a−3d. This material is
available free of charge via the Internet at http://pubs.acs.org.
3
3
6.81 (t, 1H, JHH = 6.9 Hz), 6.90−7.01 (m, 4H), 7.13 (t, 1H, JHH
=
=
AUTHOR INFORMATION
7.3 Hz), 7.47 (d, 1H, 3JHH = 7.8 Hz; IrCCHCH), 7.81 (d, 1H, 3JHH
■
7.8 Hz; IrCCH; CN ligand). 13C{1H} NMR (100.5 MHz, CD2Cl2): δ
9.5, 30.4, 32.1, 67.1, 87.6, 99.7, 113.2, 116.5, 118.0, 120.2, 121.4, 122.6,
127.4, 132.8, 136.6, 137.4, 142.6, 153.4, 155.1. Anal. Calcd for
C28H33N2O2Ir: C 54.09, H 5.35, N 4.51. Found: C 54.30, H 5.50, N
4.60. IR (KBr, cm−1): 1720 (vs, νCO).
Corresponding Author
*E-mail: tikariya@apc.titech.ac.jp (T.I.).
Notes
The authors declare no competing financial interest.
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dx.doi.org/10.1021/om500695a | Organometallics 2014, 33, 4479−4485

Organometallics
Article
ACKNOWLEDGMENTS
■
This work was financially supported by JSPS KAKENHI (Grant
Numbers 22225004, 24350079, and 26620143) and partially
supported by the GCOE Program. The authors thank the
Material Analysis Suzukake-dai Center, Technical Department,
Tokyo Institute of Technology, for HRMS analysis.
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