Visible-Light-Induced Amination of Quinoline at the C8 Position via a Postcoordinated Interligand-Coupling Strategy under Mild Conditions

Article abstract of DOI:10.1021/acs.inorgchem.0c03026

The postcoordinated interligand-coupling strategy provides a useful and complementary protocol for synthesizing polydentate ligands. Herein, diastereoselective photoreactions of Λ-[Ir(pq)2(d-AA)] (Λ-d) and Λ-[Ir(pq)2(l-AA)] (Λ-l, where pq is 2-phenylquino

Full text of DOI:10.1021/acs.inorgchem.0c03026

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Article
Visible-Light-Induced Amination of Quinoline at the C8 Position via
a Postcoordinated Interligand-Coupling Strategy under Mild
Conditions
He-Long Peng, Yinwu Li, Xing-Yang Chen, Li-Ping Li, Zhuofeng Ke,* and Bao-Hui Ye*
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ABSTRACT: The postcoordinated interligand-coupling strategy provides a useful and
complementary protocol for synthesizing polydentate ligands. Herein, diastereose-
lective photoreactions of Λ-[Ir(pq)2(D-AA)] (Λ-D) and Λ-[Ir(pq)2(L-AA)] (Λ-L,
where pq is 2-phenylquinoline and AA is an amino acid) are reported in the presence of
O2 under mild conditions. Diastereomer Λ-D is dehydrogenatively oxidized into an
imino acid complex, while diastereomer Λ-L mainly occurs via interligand C−N cross-
dehydrogenative coupling between quinoline at the C8 position and AA ligands at
room temperature, affording Λ-[Ir(pq)(L-pq-AA)]. Furthermore, the photoreaction of
diastereomer Λ-L is temperature-dependent. Mechanistic experiments reveal the
ligand−radical intermediates may be involved in the reaction. Density functional theory
calculations were used to eluciate the origin of diastereoselectivity and temperature
dependence. This will provide a new protocol for the amination of quinoline at the C8
position via the postcoordinated interligand C−N cross-coupling strategy under mild
conditions.
Scheme 1b). The dehydrogenative oxidation and interligand
C−C coupling reactions may be involved in these processes.
Using the strategy of enhancing the acidity of α-CH in a
heterocycle ligand via coordination to the [Re(CO)3]+
fragment, a series of bidentate ligands, such as bipyridine
(bpy) and pyridylimidazole, have been synthesized by Riera’s
group from the simple imidazole and pyridine ligands via
deprotonation in the presence of KN(SiMe3)2 as a base,
followed by the oxidation reaction using Ag+ as an oxidant.9
Recently, a porphyrin analogue was synthesized in situ using a
bis(α,α′-bibromodipyrrin) Ni(II) complex and a phosphine
anion in the presence of a Pd(II) catalyst (see Scheme 1b).10
Compared to C−C coupling, interligand C−N coupling is
relatively scarce. A well-known instance is the interligand
coupling of the pyrazole−acetonitrile complex into the
pyrazolylamidino complex under mild conditions,11 in which
the electrophilic character of the acetonitrile ligand is
significantly enhanced by the coordination to a metal ion.
Moreover, a highly active metal−nitrene intermediate gen-
erated from organoazide was found to directly react with the
diamide diimine complex, producing two C−N bonds (see
INTRODUCTION
■
The classical metal-catalyzed cross-coupling methods play an
important role in ligand synthesis but often require
preactivated substrates or harsh reaction conditions.1 There-
fore, it is important to develop a facilely synthetic approach for
generating new multidentate ligands. Postcoordinated inter-
ligand coupling may provide a complementary strategy for
accessing this vision.2 The simple and easily available organic
ligands are first coordinated to a metal center via a classical
protocol and then couple to each other directly, affording new
polydentate ligands.3 Generally, heterocyclic ligands coordi-
nated to a metal ion would promote the acidity of α-CH,
resulting in the enhancement of the reactivity of the ligands.4
Furthermore, coordination to a metal ion also preorganizes the
substrates via a geometric control, facilitating their subsequent
coupling in regio- and stereoselectivity.5 In addition, highly
activated intermediates such as N and S radicals can be
stabilized by coordination to a metal ion via electron
delocalization.6 Indeed, the oxidation of metal−thiolate
complexes into metal−thiyl radical intermediates, affording
disulfides via S−S radical coupling, is well-documented (see
Scheme 1a).6a,7 The most interesting instance is interligand
C−C coupling, which provides a facile and straightforward
method for synthesizing polydentate ligands. In 2001, a
tetradentate ligand 2,2′-biphenanthroline (biphen), reported
by Gao’s group,8 was synthesized via a direct interligand
coupling of the [Co(phen)2]2+ fragment in the presence of
NH4VO3 and H3BO3 under hydrothermal conditions (see
Received: October 12, 2020
Published: January 4, 2021
https://dx.doi.org/10.1021/acs.inorgchem.0c03026
© 2021 American Chemical Society
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photolysis of the nickel−azide complex was also reported by
Lee’s group,13 affording a new polydentate ligand in situ.
However, these protocols are still restricted to specific
substrates, such as nitrile, organoazide, and azide. Therefore,
a general protocol for C−N bond formation via C−H and N−
H cross-coupling is still highly desirable.
Scheme 1. Selective Examples of Interligand-Coupling
Reactions and Amidation of Quinoline N-Oxide at the C8
Position via C−H Functionalization
Quinoline is an important structural motif in natural
products and functional materials.14,15 In particular, 8-amino-
quinolines are important motifs in biological activities,16−18
ligands used in coordination chemistry19 and catalysis in C−H
functionalization.20 However, the traditional synthesis protocol
of 8-aminoquinolines is nitration of quinoline under strongly
acidic conditions. In 2014, Chang’s group first developed the
C8 amidation of quinoline N-oxides using an Ir(III) catalyst
with the sulfinyl azides as the amide source in the presence of
AcOH as an additive under mild conditions (see Scheme
1d).21a Following this strategy, they extended the amide source
to 1,4,2-dioxazol-5-ones.21b Two years later, an alternative
strategy based on hypervalent iodine reagents as an amidation
source was developed to directly amidate C8 of quinoline N-
oxides in the presence of a Rh(III) catalyst (see Scheme 1d).22
In these protocols, the specific amide sources, such as sulfinyl
azides, 1,4,2-dioxazol-5-ones, and amidobenziodoxolones, are
required to generate metal−nitrene intermediates in situ.
Therefore, a facile and general approach to direct amination of
quinoline at the C8 position using commercially available
primary and secondary amines as a nitrogen source is still
unexploited.
Visible-light-induced photoreaction has recently received a
great deal of attention.23 This process relies on the ability of
the photosensitizer to engage in the transfer of an electron
and/or energy to substrates under visible-light irradiation to
produce high-energy and reactive intermediates. Polypyridyl
Ru(II) and Ir(III) complexes have been widely applied in
photocatalytic reactions as photosensitizers, because of their
high photostability and long-lived photoexcited states as well as
their tunable photophysical properties.24 However, reports of
self-sensitized photoreaction remain scarce.25 This process may
be employed in postcoordination synthesis to produce new
ligands and new complexes in situ, which sometimes can work
via the classical protocol,26 because the reactivity of the ligand
is highly enhanced by coordination to a metal ion.4a,27 In light
of these challenges, we have observed the self-sensitized
photoreaction of [Ru(bpy)2(TMBiimH2)](ClO4)2
(TMBiimH2 is 4,5,4′,5′-tetramethyl-2,2′-biimidazole) in the
presence of O2.28 Under visible-light irradiation, the Ru(II)
complex was oxidized by singlet oxygen (1O2), producing
[Ru(bpy)2(DMDAImH)](ClO4)2 [DMDAImH is 4,5-dimeth-
yl-2-(N,N-diacetyl)carboximidamide-1H-imidazole]. More-
over, a protocol of postcoordinated asymmetric oxidation of
thioether complexes into sulfoxide complexes was also
developed by our group using a chiral-at-metal strategy to
generate enantiomeric sulfoxide ligands in situ.29 As part of our
ongoing study, we observe chiral Ir(III) complexes because
they are highly photostable and widely used in asymmetric
synthesis,30,31 molecular recognition,32 and chiral resolution.33
For the α-amino acid (AA) complexes, the photophysical
properties of Δ-[Ir(pq)2(L-pro)] (Δ-L) and Δ-[Ir(pq)2(D-
pro)] (Δ-D) (where pq is 2-phenylquinoline and pro is
proline) diastereomers are different.34 More importantly, only
the pro ligand in the Δ-L diastereomer was oxidized into an
imino acid under a natural condition.35 We are wondering
what will happen in the Λ-L or Δ-D diastereomer under similar
Scheme 1c).12 More recently, an interligand C−H function-
alization via a nickel−nitridyl intermediate produced by
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conditions. This also inspires us to observe the reaction in
detail. We found that the photoreactions of AA complexes are
diastereoselective in the presence of O2; namely, the Λ-D or Δ-
L diastereomer is oxidized to the corresponding imino acid
complex, while the Λ-L or Δ-D diastereomer occurs mainly via
interligand C−N cross-dehydrogenative coupling between
quinoline and AA ligands at room temperature (see Scheme
1e). More interestingly, the photoreaction of the Λ-L or Δ-D
diastereomer is temperature-dependent. Furthermore, density
functional theory (DFT) calculations were conducted to
elucidate the diastereoselectivity and temperature dependence.
This will provide a new protocol for amination of quinoline at
the C8 position via the postcoordinated interligand C−N
cross-dehydrogenative coupling reaction under mild condi-
tions.
Figure 1. 1H NMR spectra of Λ-[Ir(pq)2(L-ala)], Λ-[Ir(pq)2(D-ala)],
Λ-[Ir(pq)2(ala-2H)], and Λ-[Ir(pq)(L-pqa)] in CDCl3.
EXPERIMENTAL SECTION
The general information about the experiments and the instrumenta-
tion is given in the Supporting Information.
■
Synthesis of Λ-[Ir(pq)2(L-AA)] and Λ-[Ir(pq)2(D-AA)] Com-
plexes. Λ-[Ir(pq)2(MeCN)2](PF6) (1.0 mmol) was added to 25 mL
of a methanol solution containing L-AA or D-AA (1.0 mmol) and
MeONa (1.0 mmol). The solution was stirred for 12 h at room
temperature in the dark. Upon removal of the methanol solvent, 30
mL of water was added to dissolve the residue. The resulting solution
was extracted with DCM (3 × 20 mL). The extracted solution was
collected and dried with Na2SO4. The corresponding amino acid
Ir(III) complex was obtained upon removal of the solvent. The yield
and characterization of the synthesized complexes are given in the
Supporting Information.
Synthesis of the Λ-[Ir(pq)2(ala-2H)] Complex by the
Dehydrogenative Oxidation Reaction. The Λ-[Ir(pq)2(D-ala)]
(0.05 mmol, ala is alanine) complex was dissolved in 100 mL of
ethanol. The solution was stirred at 30 °C under an O2 atmosphere
under 10 W blue light-emitting diode (LEDs) (λ = 450−470 nm)
irradiation for 13 h. Upon removal of the solvent, the residue was
purified by silica gel column chromatography using a DCM/MeOH
mixture [from 100:0.5 to 100:4 (v/v)] as the eluent. Yield, 32 mg,
93%. The characterization data are given in the Supporting
Information.
Ir(III) centers are the main determinant. Cyclic voltammetry
was used to estimate the oxidation potentials (Ir4+/Ir3+ redox
couple) of the diastereomers and revealed a single quasi-
reversible oxidation wave at E1/2 (vs FeCp2 /FeCp2) values of
0.746 and 0.787 V for Λ-[Ir(pq)2(L-ala)] and Λ-[Ir(pq)2(D-
ala)], respectively (see Figure S7).
+
Absorption and emission spectra of the diastereomers in the
MeOH solution were recorded at room temperature. The
absorption spectra of the diastereomers are similar to those of
the known Δ-[Ir(pq)2(D-pro)] complex.34 The intense
absorption peak at 273 nm can be assigned to the spin-
allowed π−π* transitions (LC) of the pq ligand. The weaker
band at 462 nm could be assigned to the metal-to-ligand
(3MLCT) transitions.36,37 Moreover, the moderately intense
peak at 344 nm is assigned to the spin-allowed metal-to-ligand
charge transfer (1MLCT).38 The emission spectra of the
diastereomers exhibit a band at 604 nm, which can be assigned
34
3
to a mixed MLCT/LC state (see Figure S8). The emission
lifetimes (τ) and quantum yields (Φr) of Λ-[Ir(pq)2(D-ala)]
(164 ns and 4.4%, respectively) are larger than those of Λ-
[Ir(pq)2(L-ala)] (138 ns and 2.1%, respectively) (see Figure
S9), indicating that the photophysical properties of the
diastereomers are distinguishable.34
Procedure for the C−N Cross-Coupling Reaction. Λ-
[Ir(pq)2(L-AA)] (0.05 mmol) was added to 100 mL of ethanol.
The solution was stirred at 60 °C under an O2 atmosphere under 10
W blue LED (λ = 450−470 nm) irradiation for an appropriate time.
Upon removal of the solvent, the residue was purified by silica gel
column chromatography using a DCM/MeOH mixture [from 100:0.5
to 100:4 (v/v)] as an eluent. The yield and characterization data of
the synthesized complexes are given in the Supporting Information.
To gain a better understanding of the structure, the single-
crystal structure of Λ-[Ir(pq)2(L-ala)] was determined (see
Table S1). It crystallizes in space group P41. The absolute
configurations at the metal center and ala ligand are Λ and L,
respectively. Each Ir(III) ion is coordinated by two carbon
atoms from pq ligands in cis positions and two nitrogen atoms
in a trans fashion as well as an oxygen atom and a nitrogen
atom from an ala ligand, resulting in a slightly distorted
octahedron, as shown in Figure 2. The Ir(III) ion slightly
departs from the equatorial plane that consists of two carbon
atoms of pq ligands and nitrogen and oxygen atoms from the
ala ligand. The Ir−N [2.084(13) and 2.082(11) Å] and Ir−C
[2.003(15) and 1.991(16) Å] distances of the pq ligands are
consistent with those reported for pq Ir(III) complexes (see
Table S2).34,35 The Ir−N [2.218(12) Å] distance of the ala
ligand is longer than that of the pq ligands, but consistent with
the reported value [2.211(14) Å] of [Ir(ptpy)2(L-ala)] [ptpy is
2-(p-tolyl)pyridinato].39 The oxygen atom of the carboxylate
forms intramolecular hydrogen bonds with C14−H (C14−H···
O1 = 2.975 Å, ∠C14−H−O1 = 142.9°) of the pq ligand. This
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Λ-[Ir(pq)2(L-ala)]
and Λ-[Ir(pq)2(D-ala)] Complexes. Diastereomers Λ-[Ir-
(pq)2(L-ala)] and Λ-[Ir(pq)2(D-ala)] were synthesized by the
reaction of Λ-[Ir(pq)2(MeCN)2](PF6) with L-ala and D-ala
using NaOMe as a base in a MeOH solution, respectively.
Their structures were determined by NMR and MS techniques
(see Figures S1−S5). Their photophysical properties are
similar to those of the Λ-[Ir(pq)2(L-pro)] and Λ-[Ir(pq)2(D-
pro)] complexes.34 The diastereomers can be distinguished in
the resonance peaks of α-H of the pq ring at 8.81 ppm for Λ-
[Ir(pq)2(L-ala)] and 8.52 ppm for Λ-[Ir(pq)2(D-ala)], as
shown in Figure 1. The diastereomeric ratio was estimated to
be 100:1 via integration of these peaks. CD spectra showed Λ-
[Ir(pq)2(L-ala)] and Λ-[Ir(pq)2(D-ala)] diastereomers have
similar profiles and identical Cotton effects at 273, 308, 354,
and 467 nm (see Figure S6), suggesting the configurations at
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Scheme 2. Photoreaction of Λ-[Ir(pq)2(L-ala)] and Λ-
[Ir(pq)2(D-ala)] Diastereomers in the Presence of O2 at
Various Temperatures
Figure 2. Molecular structures of Λ-[Ir(pq)2(L-ala)] (left) and Λ-
[Ir(pq)(L-pqa)] (right) with 50% probability ellipsoids. Hydrogen
atoms have been omitted for the sake of clarity.
is consistent with the observation of the resonance peak of
H14 in the pq ligand shifting to low field (see Figure 1).
Photoreaction of Λ-[Ir(pq)2(D-ala)] and Λ-[Ir(pq)2(L-
ala)] Complexes. To the best of our knowledge, the different
reactivities between the stereoisomers remain unexploited. In
1983, Keene et al. reported that Δ-[Ru(bpy)2(R-Meampy)]2+
and Λ-[Ru(bpy)2(S-Meampy)]2+ [Meampy is 2-(1-
aminoethy1)pyridine] diastereomers have different oxidation
rates under chemo- and electro-oxidation conditions.40 Twenty
years later, Kurosaki et al. found that only the mer-[Fe(2-
DPA)(CN)3]− [2-DPA is bis(2-pyridylmethyl)amine] isomer
was oxidized into the imine complex in the presence of
ammonium peroxodisulflate as an oxidant.41 Inspired by these
findings, we have observed the photoreaction of Δ-[Ir(pq)2(D-
pro)] and Δ-[Ir(pq)2(L-pro)] diastereomers and found the
oxidation dehydrogenation rate of Δ-[Ir(pq)2(L-pro)] is ∼60
times faster than that of Δ-[Ir(pq)2(D-pro)].35 Therefore, only
the Δ-[Ir(pq)2(L-pro)] diastereomer was oxidized into the
corresponding imino acid complex Δ-[Ir(pq)2(pro-2H)] when
it crystallized under natural conditions. When we further
observed the photoreaction of the Δ-[Ir(pq)2(D-pro)]
diastereomer, we found the reaction is too slow to optimize
the reaction conditions. Therefore, we turned to the primary
AA complexes and selected Λ-[Ir(pq)2(L-ala)] and Λ-
[Ir(pq)2(D-ala)] as models to observe the reaction in detail
(see Scheme 2). First, DFT calculation showed the Gibbs free
energy of Λ-[Ir(pq)2(D-ala)] is ∼1.1 kcal/mol higher than that
of Λ-[Ir(pq)2(L-ala)] at the wB97XD/def2-TZVP, SMD
(EtOH)//wB97XD/def2-SVP level of theory, indicating that
the Λ-L diastereomer is thermodynamically stable (see Figure
S10). NMR analysis showed the characteristic peak at 2.37
ppm assigned to the α proton of ala completely disappeared
upon irradiation of the ethanol solution of Λ-[Ir(pq)2(D-ala)]
with a blue LED light under an O2 atmosphere at 30 °C for 13
h. Meanwhile, the multiple peaks of the coordinated amine
group of ala at 3.29 and 2.72 ppm were found to shift to 9.89
ppm in a singlet peak, which is a characteristic peak of the
imine group, as shown in Figure 1. Furthermore, the α-carbon
of ala at 54.14 ppm was also found to shift to 169.62 ppm (see
Figures S1 and S11). These all support the dehydrogenative
oxidation of Λ-[Ir(pq)2(D-ala)] into Λ-[Ir(pq)2(ala-2H)]. HR-
MS analysis of Λ-[Ir(pq)2(ala-2H)] showed an intense band at
m/z 688.15670 ([M + H]+), which is consistent with the
calculated isotope pattern for the molecular formula
[C33H24IrN3O2 + H]+ (m/z 688.15705) (see Figures S5 and
S12). Additionally, the photooxidation dehydrogenative
products Δ-[Ir(pq)2(pro-2H)] and Δ-[Ir(pq)2(ala-2H)] have
been confirmed by single-crystal structure analysis in our
previous studies.35,42 We also observed the photoreaction at 60
°C to determine whether the reaction is temperature-
dependent and found only product Λ-[Ir(pq)2(ala-2H)] was
obtained quantitatively but the reaction time decreased to 10
h, indicating that a higher temperature increases the rate of the
photoreaction.
On the basis of our investigation of the unstable
diastereomer Λ-D or Δ-L that is oxidized into the
corresponding imino acid complex, we became interested in
whether the stable diastereomer Λ-L or Δ-D would be able to
react in the presence of O2 under visible-light irradiation.
Indeed, when the Λ-[Ir(pq)2(L-ala)] diastereomer was
irradiated under identical conditions (see Scheme 2), the
resonance peaks of the α-H of ala at 2.96 ppm and the
coordinated amine group of ala at 3.53 and 2.12 ppm
completely disappeared in 24 h (see Figure 1), suggesting that
the reaction material was completely consumed. This
stimulates us to isolate and identify the products. Gratifyingly,
two products were isolated in a ratio of 1:2 by silica gel column
chromatography. Structural analyses show one is Λ-[Ir-
(pq)2(ala-2H)] with a yield of 32%, the same as the
dehydrogenative oxidation product from Λ-[Ir(pq)2(D-ala)].
The other was analyzed by HR-MS spectrometry and also
showed a molecular ion peak at m/z 688.15452 with a strong
isotopic cluster (see Figure S15) that is consistent with the
calculated isotope pattern for the molecular formula
[C33H24IrN3O2 + H]+ (m/z 688.15705), indicating that it is
also a dehydrogenative product of the substrate with a yield of
68%. However, their NMR spectra are different (see Figure 1).
Further analysis shows the peak of the α-H of ala shifted to
3.97 ppm and the peak at 7.98 ppm assigned to H29 of the pq
ring completely disappeared. Furthermore, the peak of the α-
carbon of ala at 51.48 ppm shifted to 67.29 ppm (see Figure
S16), implying the new product may be involved in pq and ala
ligands. This was further confirmed by single-crystal structural
analysis, as shown in Figure 2. It is an interligand C−N
coupling product between pq and L-ala ligands, namely Λ-
[Ir(pq)(L-pqa)] [pqa is N-(2-phenylquinolin-8-yl)alanine].
This is a new reaction for the aromatic amination of the
quinoline ring at C8 via C−H and N−H direct functionaliza-
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a
tion under mild conditions, which has never before been
reported,43 affording a new chiral tetradentate ligand.
Table 1. Optimization of the Reaction Conditions and
Control Experiments
Λ-[Ir(pq)(L-pqa)] crystallizes in space group P212121. The
metal center and α-carbon of ala are in Λ and L configurations,
respectively. These are consistent with configurations of the
precursor Λ-[Ir(pq)2(L-ala)], indicating the configurations at
the Ir(III) center and carbon center of ala persist during the
reaction. The N3−C29 bond is indeed formed with a length of
1.452(11) Å, comparable to the N3−C32 bond length
[1.489(12) Å]. The Ir(III) ion is coordinated by pq and pqa
ligands in a slightly distorted octahedral geometry. The Ir−N
[2.083(7), 1.965(7), and 2.245(7) Å], Ir−C [2.037(10) and
1.995(9) Å], and Ir−O [2.176(6) Å] distances are consistent
with those of Λ-[Ir(pq)2(L-ala)]. The C1−Ir1−O1
[170.1(3)°], N3−Ir1−C16 [159.4(4)°], and N2−Ir1−N3
[81.4(3)°] bond angles are significantly smaller than those of
the corresponding bond angles [174.3(5)°, 165.3(6)°, and
97.2(4)°, respectively] in Λ-[Ir(pq)2(L-ala)] because of the
coupling of N3 and C29 (see Table S2).
The CD spectrum of Λ-[Ir(pq)(L-pqa)] displays the Cotton
effect at 261, 290, 313, 353, and 463 nm (see Figure S13). The
absorption spectrum of Λ-[Ir(pq)(L-pqa)] shows an intense
band at 278 nm and a moderately intense band around 352
nm, which are slightly bathochromically shifted compared to
those of Λ-[Ir(pq)2(L-ala)]. The weaker absorption band at
438 nm is significantly blue-shifted relative to the precursor.
The emission maximum slightly red shifts to 612 nm, with a
longer emission lifetime (242 ns) and a larger quantum yield
(5.8%) compared with those of 138 ns and 2.1% for Λ-
[Ir(pq)2(L-ala)] and 12 ns and 0.5% for Λ-[Ir(pq)2(ala-2H)],
respectively (see Figure S14).
Optimization the C−N Coupling Conditions. The
finding of C−N coupling reaction encourages us to further
optimize the photoreaction conditions by monitoring the
characteristic resonance peaks of H14 of the pq ligand at 8.81
ppm for Λ-[Ir(pq)2(L-ala)], 8.59 ppm for Λ-[Ir(pq)2(ala-2H)],
and 9.21 ppm for Λ-[Ir(pq)(L-pqa)] (see Figure 1). The effect
of temperature on the reaction was first observed (see entries
1−4 in Table 1). When the reaction temperature was increased
to 60 °C, the substrate Λ-[Ir(pq)2(L-ala)] was completely
consumed in 18 h, indicating the increase in the temperature
favors the conversion rate. NMR analysis showed that the ratio
of Λ-[Ir(pq)(L-pqa)] to Λ-[Ir(pq)2(ala-2H)] increased to 96:4
from 68:32 at room temperature, meaning that the C−N
coupling and dehydrogenative oxidation reactions are com-
petitive, and the C−N coupling reaction is predominant under
these conditions. When the reaction temperature is continually
increased to 70 °C, the effect on the conversion rate and yield
can be neglected. In contrast, the yields of Λ-[Ir(pq)(L-pqa)]
and Λ-[Ir(pq)2(ala-2H)] changed to 29% and 71%,
respectively, in 36 h when the reaction temperature was
decreased to 0 °C, indicating that the decreasing temperature
favors conversion into the imino acid complex. Therefore, the
reaction is dependent on temperature. It is well-known that the
photoreaction significantly depends on the reaction solvent.
Thus, the photoreaction was then estimated in various solvents
(see entries 5−8 in Table 1). When polar solvents such as
MeOH and MeCN were used instead of EtOH, the
conversions decreased to 28% and 25%, respectively. More-
over, no product was detected when nonpolar solvents such as
toluene and DCE were used, indicating that EtOH is the more
efficient solvent in this reaction. In addition, the effect of the
base on the coupling reaction was also observed (see entries
conversion
A
B
b
c
c
c
entry solvent
DFSC
(%)
(%)
(%)
1
2
3
4
5
6
7
8
9
10
EtOH
EtOH
EtOH
EtOH
MeOH
MeCN
toluene
DCE
30 °C, 24 h
100
100
100
100
28
68
96
96
29
28
25
trace
trace
72
32
4
−
70 °C
4
0 °C, 36 h
71
−
−
−
−
trace
trace
trace
trace
3
25
trace
trace
75
EtOH
EtOH
12 h
3 equiv of Na2CO3,
12 h
3 equiv of NaOMe,
12 h
50 equiv of NaOMe,
12 h
75
74
1
11
12
EtOH
EtOH
76
84
74
8
2
76
13
14
15
16
17
18
19
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
Ar
dark
trace
trace
88
2
93
trace
trace
88
trace
83
trace
trace
trace
2
10
76
5 equiv of NaN3
5 equiv of BQ
30 °C, 5 equiv of NaN3
30 °C, 5 equiv of BQ
3 equiv of H2O2, dark
76
trace
trace
trace
trace
a
Reaction conditions: Λ-[Ir(pq)2(L-ala)] (0.005 mmol) in 10 mL of
solvent with an O2 balloon under a 10 W blue light at 60 °C for 18 h.
b
c
DFSC is the deviation from standard conditions. The conversion
1
and yield were determined by H NMR spectroscopy.
9−12 in Table 1). When 3 equiv of NaOMe or Na2CO3 was
added to reaction mixture, the effects on conversion and yield
could be neglected. However, when a large excess of base (50
equiv of NaOMe) was used, the yield of the imino acid
product significantly increased to 76% in 12 h, indicating that
the excess strong base favors the α-C−H dehydrogenation of
AA (vide infra).
Scope of the C−N Coupling Reaction. With the optimal
conditions in hand, the scope and limitation of the photo-
reaction were also investigated (see Scheme 3). First, the
difference between the enantiomers was observed. When Δ-
[Ir(pq)2(D-ala)] was used instead of Λ-[Ir(pq)2(L-ala)] under
identical conditions, we found the photoreaction proceeded
smoothly, affording Δ-[Ir(pq)(D-pqa)] with a yield of 88% in
18 h. This demonstrated no difference existed between the
enantiomers in the photoreaction. Second, various AAs were
used to evaluate the reactivity. When the primary AAs, such as
valine (val), serine (ser), and phenylalanine (pal), were used
instead of ala, the coupling reaction proceeded smoothly,
affording the corresponding products Δ-[Ir(pq)(D-pqv)] [pqv
is N-(2-phenylquinolin-8-yl)valine], Δ-[Ir(pq)(D-pqs)] [pqs is
N-(2-phenylquinolin-8-yl)serine], and Δ-[Ir(pq)(D-pqpa)]
[pqpa is N-(2-phenylquinolin-8-yl)phenylalanine] in yields of
83% (60 h), 53% (60 h), and 62% (72 h), respectively (see
Figures S17−S34). The longer reaction time required for the
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a
S51). When an electron-deficient group, chloro or bromo, was
introduced into the pq at the C7 position, the desired product
Λ-[Ir(Cpq)(L-Cpqa)] [Cpq is 7-chloro-2-phenylquinoline, and
Cpqa is N-(7-chloro-2-phenylquinolin-8-yl)alanine (see Fig-
ures S52−S57)] or Λ-[Ir(Bpq)(L-Bpqa)] [Bpq is 7-bromo-2-
phenylquinoline, and Bpqa is N-(7-bromo-2-phenylquinolin-8-
yl)alanine (see Figures S58−S63)] was obtained in a yield of
89% in 28 h or 86% in 27 h, respectively, indicating that an
electron-donating or electron-withdrawing group at the
quinoline ring is tolerated. The longer reaction time required
for the quinoline ligands with the electron-deficient group
indicates that the electronic effect of the substituents at the C7
position is sensitive to C−N bond formation and the electron-
donating groups appear to be beneficial. These are consistent
with the radical aromatic addition pathway.44 Furthermore,
both chloro and bromo substituents were tolerated under the
reaction conditions, which enable further functionalization at
this position.
Scheme 3. Scope of the C−N Coupling Reaction
Mechanistic Studies for the C−N Coupling Reaction.
The previous study has demonstrated that singlet oxygen
(1O2) is the main reactive oxygen species (ROS) for the
conversion of the Δ-L diastereomer into the imino acid
complex under an O2 atmosphere with light irradiation and a
ligand radical may be involved in the reaction.35,42 To probe
the photoreaction mechanism of the C−N coupling reaction,
complex Λ-[Ir(pq)2(L-ala)] was selected as a model for
observation of the reaction under the optimal conditions.
The control experiments showed that both blue light and O2
are indispensable for C−N bond formation (see entries 13 and
14 in Table 1). Furthermore, no described products were
observed when free L-ala was added to the solution of Λ-
[Ir(pq)2(MeCN)2](PF6) under an O2 atmosphere with blue-
light irradiation, indicating that the reactivity of ala is
significantly promoted via coordination to the Ir(III) ion.27
a
Reaction conditions: Ir(III) complexes (0.005 mmol) in 10 mL of
EtOH with an O2 balloon under a 10 W blue light at 60 °C. Reported
b
yields are those of the isolated products. The A:B ratio was
c
determined by 1H NMR spectroscopy. In the presence of 3 equiv of
NaOMe.
conversion indicates the steric effect of the α-AA is sensitive to
the photoreaction. We also found the electronic effects of the
substituents at α-carbon significantly impact C−N bond
formation. For the ser complex, an electron-withdrawing
hydroxymethyl group at the α-carbon significantly increases
the acidity of the α-hydrogen of ser, leading to a decrease in
the yield of the C−N coupling product to 53% relative to a
yield of 83% for Δ-[Ir(pq)(D-pqv)]. In contrast, the yield of
imino acid product Δ-[Ir(pq)2(ser-2H)] increases to 33%
relative to a yield of 1% for Δ-[Ir(pq)2(val-2H)]. A similar
situation was also observed in Δ-[Ir(pq)2(D-pal)], where the
yields of Δ-[Ir(pq)(D-pqpa)] and Δ-[Ir(pq)2(pal-2H)] were
67% and 28%, respectively. This is also consistent with the
observation of the large excess of a strong base that favors the
formation of the imino acid complex, also indicating the
dehydrogenative oxidation and C−N coupling reactions are
competitive. Moreover, when the secondary AA pro was used,
the corresponding C−N coupling product Δ-[Ir(pq)(D-pqp)]
[pqp is N-(2-phenylquinolin-8-yl)proline] was also afforded in
a yield of 72% (see Figures S34−S40), indicating that the
primary and secondary amines are suitable nitrogen sources for
the amination of quinoline at the C8 position. In addition, the
achiral AA glycine (gly) was also used to examine the reaction.
Indeed, the C−N coupling product Δ-[Ir(pq)(pqg)] [pqg is
N-(2-phenylquinolin-8-yl)glycine] was obtained in a yield of
68% in 60 h when Δ-[Ir(pq)2(gly)] was employed as the
substrate under identical conditions (see Figures S41−S45),
indicating the reaction is compatible with diverse AAs.
1
Two main ROS, such as O2 and superoxide radical (O2•−),
have been observed in the photoreaction under an O2
atmosphere. To demonstrate which is the main ROS, the
inhibition experiments were carried out under the optimal
conditions (see entries 15 and 16 in Table 1). When 5 equiv of
1
NaN3, a known O2 scavenger, was added to the reaction
solution, the C−N coupling product was still produced in a
yield of 88%. In contrast, when 5 equiv of benzoquinone (BQ),
a scavenger for O2•−, was added to the reaction solution, only a
•−
trace of the product was detected, indicating that O2 is a
main ROS in the reaction. This is inconsistent with the
previous observation for the dehydrogenative oxidation of the
unstable Δ-L diastereomer into the imino acid complex at
35,42
1
room temperature, where O2 is the predominant ROS.
This also inspired us to further observe the inhibition
experiment at room temperature (see entries 17 and 18 in
Table 1). The control experiment showed O2•− is still the main
participant in C−N bond formation at room temperature.
To probe the radical mechanism, 2 equiv of 2,2,6,6-
tetramethyl-1-piperidinyl-oxy (TEMPO)45 was added to the
reaction solution under the standard conditions for 18 h (see
Scheme 4a). HR-MS analysis showed that a new species at m/z
845.30099 was observed (see Figure S64) in addition to the
peak at m/z 688.15705 assigned to the dehydrogenative
products Λ-[Ir(pq)(L-pqa)] and Λ-[Ir(pq)2(ala-2H)]. This
new peak can be assigned to the trapping products Λ-
[Ir(pq)(pq-TEMPO)(L-ala)] or/and Λ-[Ir(pq)2(L-ala-
TEMPO)] according to the calculated isotope pattern for
the molecular formula [C42H43IrN4O3 + H]+ (m/z
Then, we turned our attention to the quinoline substrate.
When 7-methyl-2-phenylquinoline (Mpq), bearing an electron-
rich methyl group at the C7 position of pq, was used instead of
the pq ligand, the reaction proceeded smoothly, affording Λ-
[Ir(Mpq)(L-Mpqa)] [Mpqa is N-(7-methyl-2-phenylquinolin-
8-yl)alanine] with a yield of 89% in 22 h (see Figures S46−
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oxidation of I− into I2 by H2O2.49 Because H2O2 could be a
potential oxidant for the oxidative dehydrogenative reaction,
the control experiment was conducted under the optimal
conditions. When 3 equiv of H2O2 was added to a solution of
Λ-[Ir(pq)2(L-ala)] (see entry 19 in Table 1), no desired
product was detected. Thus, H2O2 as an oxidant in the reaction
would be excluded.
Scheme 4. Mechanistic Experiments
On the basis of the fact that there are two competitive
pathways derived from the intermediate, which is mediated by
the reaction conditions, such as the effect of temperature and
electron on the α-carbon of the AA ligand, and the literature
evidence,50 a possible photoreaction mechanism for the
conversion of Λ-[Ir(pq)2(L-ala)] into Λ-[Ir(pq)(L-pqa)] and
Λ-[Ir(pq)2(ala-2H)] in the presence of O2 was proposed, as
shown in Scheme 5.51 The ground state Ir(III) complex is
Scheme 5. Proposed Mechanism
845.30372), indicating that the reaction may proceed through
a radical process. We tried to separate the products to assign
the exact structure. However, the major product was Λ-
[Ir(pq)(L-pqa)] and the amount of the trapping species was
too small to measure the NMR spectrum. This may be
attributed to the intramolecular radical reaction resulting in
C−N formation being faster than the intermolecular reaction.
Moreover, a nitrogen-centered radical intermediate on the
coordinated AA ligand, namely metal aminyl,46 may be
involved in C−N bond formation. Thus, 2 equiv of 2,6-di-
tert-butyl-4-methylphenol (BHT), a nitrogen radical trapping
reagent,47 was added to the reaction solution under the
standard conditions for 18 h. Unfortunately, no trapping
product was detected and the C−N coupling product was
produced in a yield of 98% (determined by NMR). This may
also be related to the quick intramolecular reaction, resulting in
suppression of the intermolecular reaction between the
nitrogen radical and BHT.
Furthermore, the ratios of the two products that are
significantly mediated by temperature also strongly indicate
that the imine product may be an intermediate in the reaction.
To verify this hypothesis, the imino acid complex Λ-
[Ir(pq)2(ala-2H)] was used as a substrate instead of Λ-
[Ir(pq)2(L-ala)] under the standard reaction conditions (see
Scheme 4c). However, no C−N coupling product was detected
and the substrate was recovered in almost quantitative yield.48
Therefore, imine as an intermediate can be ruled out.
excited into a high-energy excited singlet state 1[Ir(III)]*
under light irradiation, and then the singlet state undergoes
3
intersystem crossing to a long-lived excited triplet state [Ir-
(III)]* quickly. The excited triplet state exhibits high activity
in oxidation and reduction reactions.23a It undergoes a facile
disproportionation to [Ir(II)] and [Ir(IV)] that have been
widely accepted in the dehydrogenative oxidation of amine and
AA complexes.27,52 Subsequent electron transfer from [Ir(II)]
to O2 regenerates [Ir(III)] and forms O2 ROS.23a The
•−
•−
generated O2
radical anion undergoes protonation to
produce the HOO• radical and metal aminyl intermediate
(I).53,54 This process may be involved in the deprotonation of
an amine, followed by intramolecular single-electron transfer
from the ligand to the metal center, forming a ligand radical
species. Although the N-radical intermediate is highly reactive,
it can be stabilized by delocalization of the electron to the
coordination metal ion.47 Furthermore, the N−H bond
dissociation free energy of AA is decreased by coordination
to a metal.55 This also facilitates the cleavage of the N−H bond
and formation of I. The aminyl radical undergoes a radical
electrophilic addition to the quinoline ring, generating a new
C−N bond and carbon-centered radical (II) that was indeed
trapped by TEMPO. The fact that an electron-deficient group
at the C7 position of the quinoline ring is not favored for C−N
coupling also supports the electrophilic addition mechanism.44
Subsequently, the HOO• species abstracts a hydrogen atom
from II, affording the C−N coupling product and H2O2 as a
byproduct. On the contrary, the O2•− radical anion abstracts a
hydrogen from the α-carbon to generate an α-amino radical
(III) that was indeed trapped by TEMPO, followed by
deprotonation to form the imine (CN) product and H2O2
as described in the literature.56 This is also consistent with the
To evaluate the reaction product, 3 equiv of KI was added to
the resulting solution in 2 h. The ultraviolet−visible absorption
showed the characteristic peaks of the I3− anion at 292 and 351
nm became intense (see Figure S65). This indicates the
−
formation of the I3 anion, which may be generated by the
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fact that the electron-withdrawing groups at the α-carbon favor
the formation of imino acid products.57
Furthermore, DFT calculations were conducted to probe the
validity of the proposed mechanism. After light excitation, the
reactions are exothermic (see Figure S66). On the C−N
coupling pathway, intermediate I attacks the quinoline ring,
generating a new C−N bond and carbon-centered radical (II;
ΔG = −18.7 kcal/mol) via transition state Λ-L-TS1 (ΔG = 3.5
kcal/mol). II reacts with HOO• species via transition state Λ-
L-TS2 (ΔG = −6.5 kcal/mol), affording the C−N coupling
product. On the dehydrogenative oxidation pathway, inter-
mediate III reacts with HOO• species to afford an imine (C
N) product. Although the C−N coupling reaction goes
through the transition state with an energy barrier of 18.2
kcal/mol, the overall free energy of the C−N coupling pathway
is lower than that of the C−N dehydrogenative oxidation by
5.5 kcal/mol, as shown in Scheme 6. Therefore, the C−N
Figure 3. Free energy calculation for transition state models of the
C−N coupling of Λ-[Ir(pq)2(D-ala)] (left) and Λ-[Ir(pq)2(L-ala)]
(right) at the wB97XD/def2-TZVP, SMD (EtOH)//wB97XD/def2-
SVP level of theory.
Scheme 6. DFT Free Energy (in kilocalories per mole)
Calculation for Two Reaction Pathways at the wB97XD/
def2-TZVP, SMD (EtOH)//wB97XD/def2-SVP Level of
Theory
the Λ-D or Δ-L diastereomer is dehydrogenatively oxidized into
the imino acid complex, while the Λ-L or Δ-D diastereomer
mainly participates in intramolecular C−N coupling at C8 of
the quinoline ring at room temperature. It is quite remarkable
that the photoreaction of the Λ-L or Δ-D diastereomer is
temperature-dependent. Our results demonstrate that the
reaction is compatible with various amine groups, such as
chiral and achiral AAs and primary and secondary amines. This
is the first example of amination of quinoline at the C8
position using commercially available primary and secondary
amines as a nitrogen source. Furthermore, the configurations at
metal and carbon centers remain during the C−N coupling
reaction. Mechanistic studies reveal ligand−radical intermedi-
ates may be involved in the reaction. This will provide a new
and complementary approach for amination of quinoline at the
C8 position via the postcoordinated interligand C−N coupling
reaction under mild conditions.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03026.
■
sı
dehydrogenative oxidation is kinetically favored, while the C−
N coupling is thermodynamically preferred. This also explains
why the C−N coupling pathway is favored at high reaction
temperatures.
Experimental procedures, characterization of the com-
pounds, and DFT calculations (PDF)
To elucidate the origin of the diastereoselectivity in the C−
N coupling reaction, the two putative transition states Λ-D-
TS1 and Λ-L-TS1 that are generated from Λ-[Ir(pq)2(D-ala)]
and Λ-[Ir(pq)2(L-ala)] diastereomers, respectively, are pro-
posed according to the reaction mechanism in Scheme 6. DFT
calculations showed that the Λ-L-TS1 configuration is favored
by 2.3 kcal/mol. In Λ-D-TS1, the methyl group of ala points to
the quinoline ring, which causes the repulsive interaction. On
the contrary, there is no such repulsion in Λ-L-TS1. Therefore,
the energy barrier to undergo an electrophilic addition,
producing the C−N coupling product, should be lower. This
may be the reason why no C−N coupling reaction took place
in the Λ-[Ir(pq)2(D-ala)] diastereomer (Figure 3).
Accession Codes
CCDC 2019904−2019908 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Zhuofeng Ke − School of Materials Science & Engineering,
Sun Yat-sen University, Guangzhou 510275, Guangdong,
China; orcid.org/0000-0001-9064-8051;
Email: kezhf3@mail.sysu.edu.cn
CONCLUSIONS
Bao-Hui Ye − MOE Key Laboratory of Bioinorganic and
Synthetic Chemistry, School of Chemistry, Sun Yat-sen
University, Guangzhou 510275, Guangdong, China;
■
In conclusion, we have found the photoreactions of Ir(III) AA
complexes are diastereoselective in the presence of O2, where
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orcid.org/0000-0003-0990-6661; Email: cesybh@
pubs.acs.org/IC
Article
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Ed. 2013, 52, 12510−12529. (d) Xiong, T.; Zhang, Q. New amination
strategies based on nitrogen-centered radical chemistry. Chem. Soc.
Rev. 2016, 45, 3069−3087.
mail.sysu.edu.cn
Authors
He-Long Peng − MOE Key Laboratory of Bioinorganic and
Synthetic Chemistry, School of Chemistry, Sun Yat-sen
University, Guangzhou 510275, Guangdong, China
Yinwu Li − School of Materials Science & Engineering, Sun
Yat-sen University, Guangzhou 510275, Guangdong, China
Xing-Yang Chen − MOE Key Laboratory of Bioinorganic and
Synthetic Chemistry, School of Chemistry, Sun Yat-sen
University, Guangzhou 510275, Guangdong, China
Li-Ping Li − MOE Key Laboratory of Bioinorganic and
Synthetic Chemistry, School of Chemistry, Sun Yat-sen
University, Guangzhou 510275, Guangdong, China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03026
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge the financial support from the
National Natural Science Foundation of China (Grants
21971266 and 21673301).
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