Amide-Functionalized Naphthyridines on a RhII-RhII Platform: Effect of Steric Crowding, Hemilability, and Hydrogen-Bonding Interactions on the Structural Diversity and Catalytic Activity of Dirhodium(II) Complexes

Article abstract of DOI:10.1002/chem.201402936

Ferrocene-amide-functionalized 1,8-naphthyridine (NP) based ligands {[(5,7-dimethyl-1,8-naphthyridin-2-yl)amino]carbonyl}ferrocene (L¹H) and {[(3-phenyl-1,8-naphthyridin-2-yl)amino]carbonyl}ferrocene (L²H) have been synthesized. Room-temperature treatment of both the ligands with Rh₂(CH₃COO)₄ produced [Rh₂(CH₃COO)₃(L¹)] (1) and [Rh₂(CH₃COO)₃(L²)] (2) as neutral complexes in which the ligands were deprotonated and bound in a tridentate fashion. The steric effect of the ortho-methyl group in L¹H and the inertness of the bridging carboxylate groups prevented the incorporation of the second ligand on the {Rh^II-Rh^II} unit. The use of the more labile Rh₂(CF₃COO)₄ salt with L¹H produced a cis bis-adduct [Rh₂(CF₃COO)₄(L¹H)²] (3), whereas L²H resulted in a trans bis-adduct [Rh₂(CF₃COO)₃(L²)(L²H)] (4). Ligand L¹H exhibits chelate binding in 3 and L²H forms a bridge-chelate mode in 4. Hydrogen-bonding interactions between the amide hydrogen and carboxylate oxygen atoms play an important role in the formation of these complexes. In the absence of this hydrogen-bonding interaction, both ligands bind axially as evident from the X-ray structure of [Rh₂(CH₃COO)₂(CH₃CN)₄(L²H)₂](BF₄)₂ (6). However, the axial ligands reorganize at reflux into a bridge-chelate coordination mode and produce [Rh₂(CH₃COO)₂(CH₃CN)₂(L¹H)](BF₄)₂ (5) and [Rh₂(CH₃COO)₂(L²H)₂](BF₄)₂ (7). Judicious selection of the dirhodium(II) precursors, choice of ligand, and adaptation of the correct reaction conditions affords 7, which features hemilabile amide side arms that occupy sites trans to the Rh-Rh bond. Consequently, this compound exhibits higher catalytic activity for carbene insertion to the C-H bond of substituted indoles by using appropriate diazo compounds, whereas other compounds are far less reactive. Thus, this work demonstrates the utility of steric crowding, hemilability, and hydrogen-bonding functionalities to govern the structure and catalytic efficacyof dirhodium(II,II) compounds.

Full text of DOI:10.1002/chem.201402936

DOI: 10.1002/chem.201402936
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Dirhodium Complexes |Hot Paper|
Amide-Functionalized Naphthyridines on a RhII–RhII Platform:
Effect of Steric Crowding, Hemilability, and Hydrogen-Bonding
Interactions on the Structural Diversity and Catalytic Activity of
Dirhodium(II) Complexes
Mithun Sarkar, Prosenjit Daw, Tapas Ghatak, and Jitendra K. Bera*[a]
Abstract: Ferrocene-amide-functionalized 1,8-naphthyridine
(NP) based ligands {[(5,7-dimethyl-1,8-naphthyridin-2-yl)ami-
no]carbonyl}ferrocene (L1H) and {[(3-phenyl-1,8-naphthyridin-
2-yl)amino]carbonyl}ferrocene (L2H) have been synthesized.
Room-temperature treatment of both the ligands with
Rh2(CH3COO)4 produced [Rh2(CH3COO)3(L1)] (1) and
[Rh2(CH3COO)3(L2)] (2) as neutral complexes in which the li-
gands were deprotonated and bound in a tridentate fashion.
The steric effect of the ortho-methyl group in L1H and the in-
ertness of the bridging carboxylate groups prevented the in-
corporation of the second ligand on the {RhII–RhII} unit. The
use of the more labile Rh2(CF3COO)4 salt with L1H produced
a cis bis-adduct [Rh2(CF3COO)4(L1H)2] (3), whereas L2H result-
ed in a trans bis-adduct [Rh2(CF3COO)3(L2)(L2H)] (4). Ligand
L1H exhibits chelate binding in 3 and L2H forms a bridge-
chelate mode in 4. Hydrogen-bonding interactions between
the amide hydrogen and carboxylate oxygen atoms play an
important role in the formation of these complexes. In the
absence of this hydrogen-bonding interaction, both ligands
bind axially as evident from the X-ray structure of
[Rh2(CH3COO)2(CH3CN)4(L2H)2](BF4)2 (6). However, the axial
ligands reorganize at reflux into a bridge-chelate coordina-
tion mode and produce [Rh2(CH3COO)2(CH3CN)2(L1H)](BF4)2
(5) and [Rh2(CH3COO)2(L2H)2](BF4)2 (7). Judicious selection of
the dirhodium(II) precursors, choice of ligand, and adapta-
tion of the correct reaction conditions affords 7, which fea-
tures hemilabile amide side arms that occupy sites trans to
the Rh–Rh bond. Consequently, this compound exhibits
higher catalytic activity for carbene insertion to the CꢀH
bond of substituted indoles by using appropriate diazo com-
pounds, whereas other compounds are far less reactive.
Thus, this work demonstrates the utility of steric crowding,
hemilability, and hydrogen-bonding functionalities to govern
the structure and catalytic efficacyof dirhodium(II,II) com-
pounds.
ate groups possess higher Lewis acidity[7] and exhibit en-
hanced catalytic activity.[8] A series of complexes with various
anionic bridging ligands, such as carboxylates,[9] amidinates,[10]
triazenides,[11] and orthometalated phosphanes,[12] have been
synthesized and their catalytic activities explored. Chiral
modification of the bridging ligands has afforded a new class
of catalyst for asymmetric synthesis.[1a–d,2a,13]
Introduction
Dirhodium catalysts are widely known for the decomposition
of diazo compounds and subsequent carbene transfer to vari-
ous organic substrates.[1] The majority of the complexes are de-
rived from Rh2(CH3COO)4 and retain a paddlewheel geometry
around the {Rh–Rh} unit.[1b, 2] Axial and bridging ligands control
the stereoelectronic properties,[3] thus influencing the catalytic
process.[3b,4] Strong s/p-donor ligands at the equatorial sites
increase electron density on the rhodium center relative to
Rh2(CH3COO)4,[5] therefore decreasing the electrophilicity of the
metal carbene formed after diazo decomposition and provid-
ing a higher level of thermodynamic control of the selectivity.[6]
Dirhodium complexes that contain electron-deficient carboxyl-
The incorporation of neutral ligands makes the complexes
cationic and further increases the electrophilicity of both the
metal center and the carbenoid.[14] 1,8-Naphthyridine (NP) and
its derivatives have been employed as potential bi-, tri-, and
tetradentate ligands (Scheme 1). Kaska and co-workers, with
others, have reported several cationic dirhodium complexes
with NP, 2-(2-pyridyl)-1,8-naphthyridine (pyNP), and 2,7-bis-(2-
pyridyl)-1,8-naphthyridine (bpNP) ligands.[15] The unique syn,
[a] M. Sarkar, P. Daw, T. Ghatak, Prof. J. K. Bera
Department of Chemistry
Center for Environmental Science and Engineering
Indian Institute of Technology Kanpur, Kanpur 208016 (India)
Fax: (+91)512-2597436
E-mail: jbera@iitk.ac.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402936.
Scheme 1. Different coordination modes of 1,8-naphthyridine-derived li-
gands
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syn-bridging coordination mode of the NP ligands makes them
a suitable alternative to bridging carboxylate ligands.[16] The
rigid skeleton holds two metal centers in proximity, thus offer-
ing synergistic action between the metal centers, and the func-
tionalized side arms provide additional stability by chelation.
Although these complexes have long been known, their cata-
lytic utilities have not been evaluated. We proposed to synthe-
size cationic dirhodium complexes by incorporating suitably
designed NP ligands at the equatorial sites and use them as
catalysts for a carbene-transfer reaction. Because most of the
catalytic reactions of dirhodium complexes occur at the axial
sites, these positions need to be accessible for substrate bind-
ing. Thus, multidentate naphthyridine ligands with strong axial
donors make the complexes catalytically ineffective; however,
functionalization of the naphthyridine side arm with an amide
group alleviates this problem. The carbonyl oxygen atom
binds at the axial site and allows substrate coordination during
the catalytic cycle. This type of hemilabile behavior of amide-
functionalized naphthyridine has been reported earlier for
PdI–PdI and RuI–Ru1 platforms by our group.[17]
13C NMR spectrum. The IR spectrum shows two strong bands
at n˜ =1678 and 1662 cmꢀ1 for the CO stretching and NꢀH
bending vibrations, respectively. The NꢀH stretching vibration
appears at n˜ =3449 cmꢀ1. The ESI-MS spectrum shows a signal
at m/z 434.09, which corresponds to the [M+H]+ ion.
The molecular structure of L2H (Figure 1) reveals that the
amide unit is not planar with the naphthyridine ring, as reflect-
ed from the dihedral angle N2-C8-N3-C15 f=58.9(3)8, as op-
Herein, we demonstrate the effect of steric crowding,
hemilability, and hydrogen-bonding functionalities of L1H and
L2H ligands to control the structures and catalytic activities of
dirhodium(II) compounds (Scheme 2).
Figure 1. Molecular structure of ligand L2H, showing intermolecular hydro-
gen bonding with the important atoms labeled. All the hydrogen atoms,
except for NꢀH, have been omitted for the sake of clarity. The ferrocene
rings of the second ligand at C41 have been removed for clarity. Thermal
ellipsoids are drawn at the 40% probability level.
posed to the near-planar configuration in L1H.[18] This differ-
ence is caused by the substituent phenyl group on the NP
ring. The C15ꢀO1 and C15ꢀN3 distances are 1.222(3) and
1.369(3) ꢁ, respectively. The crystal structure of L2H shows
a double hydrogen-bonded dimer that involves an amide
hydrogen atom and a proximal nitrogen atom of the naphthyr-
idine unit.
Scheme 2. Line drawings of ligands L1H and L2H.
Results and Discussion
L1H and L2H ligands
Ferrocene-amide-functionalized naphthyridine-based ligands
L1H and L2H were synthesized by a reaction between 5,7-di-
methyl-2-amino-1,8-naphthyridine and 2-amino-3-phenyl-1,8-
naphthyridine, respectively, with freshly prepared ferrocenoyl
chloride in THF in the presence of Et3N (Scheme 3). A detailed
structural description and spectroscopic data for L1H has been
provided earlier.[18] The 1H NMR spectrum of L2H exhibits a char-
acteristic singlet for the amide proton at d=9.05 ppm, where-
as the amide carbon atom resonates at d=171.2 ppm in the
Dirhodium complexes
Reaction with Rh2(CH3COO)4
Reaction of
a
solution of L1H in dichloromethane with
Rh2(CH3COO)4 at room temperature in a ratio of 1:1 afforded
the complex [Rh2(CH3COO)3(L1)] (1) in high yield (80%;
Scheme 4). The ligand was deprotonated and only one ligand
could be incorporated, even after the use of excess ligand and
carrying out the reaction at a higher temperature for a longer
1
time. The absence of the NꢀH proton in the H NMR spectrum
(see Figure S3 in the Supporting Information) indicates depro-
tonation. Three NP protons appear in the range d=7.13–
8.02 ppm. Two methyl groups resonate at d=3.84 and
2.59 ppm, whereas signals at d=5.14, 4.48, and 4.20 ppm in an
intensity ratio of 2:2:5 correspond to the cyclopentadienyl
(Cp)-ring protons. The ESI-MS spectrum reveals a signal at m/
z 767.95 attributed to the [M+H]+ ion. The IR absorption of
the amide carbonyl group appears at n˜ =1635 cmꢀ1, a differ-
ence of 43 cmꢀ1 relative to the free ligand.[18]
Scheme 3. Synthesis of ligands L1H and L2H.
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Scheme 4. Syntheses of dirhodium complexes 1–7. 1,2-DCE=1,2-dichloroethane.
Reaction of Rh2(CH3COO)4 with the L2H ligand under identi-
cal reaction conditions produced a red precipitate. The poor
solubility of the product in common organic solvents prevent-
ed reliable characterization. However, the ESI-MS signal at
m/z 815.91 (see Figure S9 in the Supporting Information) in
acetonitrile suggests the formation of [Rh2(CH3COO)3(L2)] (2),
which is analogous to complex 1 (Scheme 4).
The molecular structure of 1 (Figure 2) reveals a paddlewheel
structure that consists of three bridging acetate groups and
one L1 ligand, which spans the dimetal unit with deprotonated
amide oxygen atoms that occupy one of the axial sites. The
second axial site remains blocked by ortho-methyl protons
1
(Rh1···H19A/H19C=2.802(1)/2.798(1) ꢁ). The H NMR spectrum
reveals a large downfield shift (Dd=1.12 ppm) for the ortho-
methyl protons relative to the free ligand, thus reflecting Rh···H
preagostic interactions.[19] The Rh1ꢀRh2 distance is 2.386(1) ꢁ.
The N3ꢀC21 distance (1.329(6) ꢁ) is shorter and the C21ꢀO7
distance (1.271(5) ꢁ) is longer than in L1H by 0.046 and
0.048 ꢁ, respectively, thus suggesting delocalization of the
negative charge throughout the amide skeleton.
Figure 2. Molecular structure of 1 with the important atoms labeled. All the
hydrogen atoms, except for those in the ortho-methyl group, have been
omitted for the sake of clarity. Thermal ellipsoids are drawn at the 40%
probability level.
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Reaction with Rh2(CF3COO)4
Rh2(CF3COO)4, on reaction with L1H in a ratio of 1:2, gave the
adduct [Rh2(CF3COO)4(L1H)2] (3) as a dark-red solid in 76% yield
under identical conditions as for 1 (Scheme 4). The labile tri-
fluoroacetate ions allowed the incorporation of two neutral li-
gands on the {Rh2} unit. An attempt to isolate a 1:1 adduct by
reaction of equimolar amounts of Rh2(CF3COO)4 and L1H only
gave 3, but in decreased yields and starting-material
Rh2(CF3COO)4 was recovered.
The molecular structure of 3 (Figure 3) shows a four-mem-
bered chelate coordination of two L1H ligands that involve
both naphthyridine nitrogen atoms and the Rh centers at
Figure 4. Molecular structure of 4 with the important atoms labeled. All the
hydrogen atoms, except for those in the NꢀH and the ortho-NPꢀH bonds,
have been omitted for the sake of clarity. Thermal ellipsoids are drawn at
the 40% probability level.
ture of 4 comprises of two trans-oriented NP ligands, among
which one is deprotonated (Figure 4). Two bridging trifluoro-
acetate moieties and one axially bound trifluoroacetate unit
complete the coordination around the {Rh2} unit. Two naph-
thyridine ligands are arranged in a head-to-head fashion[15g]
and are stabilized by a hydrogen-bonding interaction between
the NꢀH group of the neutral L2H ligand and the axially bound
amide oxygen atom of the deprotonated ligand. Similarly, the
ortho-hydrogen atom of the two naphthyridine ligands engag-
es in a hydrogen-bonding interaction with the axially coordi-
nated trifluoroacetate moiety. The Rh1ꢀRh2 distance is
2.409(1) ꢁ. The overall structure deviates significantly from
a paddlewheel geometry, as indicated from the relevant metri-
cal parameters (f=18.5(2) and 18.1(2)8 for N1-Rh1-Rh2-N2 and
N4-Rh1-Rh2-N5, respectively).
Figure 3. Molecular structure of 3 with the important atoms labeled. All the
hydrogen atoms, except for NꢀH, have been omitted for the sake of clarity.
Thermal ellipsoids are drawn at the 40% probability level.
equatorial sites (aN1-Rh1-N2 and aN4-Rh2-N5=65.0(1)8).
Two trifluoroacetate groups bridge the dirhodium unit and the
remaining two groups occupy two axial sites. The amide NꢀH
hydrogen atoms engage in hydrogen-bonding interactions
with the unbound oxygen atoms of the axial trifluoroacetate
groups, therefore stabilizing the rare coordination mode of
naphthyridine. Careful examination of the crystal structure re-
veals that two chelate-bound NP rings are in a syn-periplanar
conformation around the RhꢀRh bond (the corresponding di-
hedral angles N1-Rh1-Rh2-N5 and N2-Rh1-Rh2-N4: f=21.5(1)8)
and are disposed in a head-to-tail fashion. Interestingly, the
Rh1ꢀRh2 distance is quite long, 2.551(1) ꢁ) relative to the
other carboxylate-bridged dirhodium complexes in this series.
Reaction with [Rh2(CH3COO)2(CH3CN)6](BF4)2
The reaction of equimolar amounts of [Rh2(CH3COO)2(CH3CN)6]
(BF4)2 and L1H in 1,2-dichloroethane at reflux produced the
bridge-chelate complex [Rh2(CH3COO)2(L1H)(CH3CN)2(BF4)2] (5)
as a red solid within 24 hours (Scheme 4). The use of excess
ligand did not lead to the incorporation of the second ligand
due to the steric effect of the ortho-methyl group at one of
the axial sites. The appearance of the NꢀH proton at d=
9.54 ppm and carbonyl carbon atom at d=174.6 ppm in the
1H and 13C NMR spectra, respectively, indicates no deprotona-
tion and weak coordination through the carbonyl oxygen
atom to the metal center. A weak IR band at n˜ =3240 cmꢀ1
(NꢀH stretching) and strong band at n˜ =1638 cmꢀ1 (CO
stretching) also substantiate this fact.
1
The singular nature of the H and 13C NMR spectra (see Fig-
ure S4 in the Supporting Information) in CD2Cl2 suggests a sym-
metrical structure in solution. The NꢀH proton appears at d=
10.17 ppm in the 1H NMR spectrum and the amide carbon
atom appears at d=170.2 ppm in the 13C NMR spectrum. The
IR spectrum also shows uncoordinated carbonyl stretching
frequencies at n˜ =1678 and 1672 cmꢀ1. The ESI-MS spectrum
reveals a signal at m/z 1314.92, which corresponds to the
[MꢀCF3COO]+ ion.
The molecular structure of 5 was further characterized by X-
ray crystallography studies, and the dicationic unit of 5 is
shown in Figure 5. This unit consists of two cis-bound bridging
acetate moieties, two acetonitrile molecules at equatorial sites,
The use of L2H under similar conditions afforded the com-
plex [Rh2(CF3COO)3(L2)(L2H)] (4; Scheme 4). The molecular struc-
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to that of the free ligand. The ESI-MS spectrum exhibits signal
at m/z 1277, which corresponds to the [Rh2(L2H)2(BF4)]+ ion.
Instead, when the reaction was carried out at reflux in 1,2-di-
chloroethane, a bis-bridge chelate complex [Rh2(CH3COO)2
(L2H)2(BF4)2] (7) was isolated as a red solid in 73% yield after
24 hours (Scheme 4). The molecular structure of the dicationic
unit of 7 (Figure 7) consists of a dirhodium unit spanned by
Figure 5. Molecular structure of the dicationic unit [Rh2(L1H)(m-CH3COO)2
(CH3CN)2] in 5 with the important atoms labeled. All the hydrogen atoms,
except for those in the ortho-methyl group and NꢀH bond, have been omit-
ted for the sake of clarity. Thermal ellipsoids are drawn at the 40% probabili-
ty level.
and a neutral L1H ligand that spans the dirhodium unit in a tri-
dentate fashion. The Rh1ꢀRh2 distance is 2.430(1) ꢁ and N2-
C18-N3-C21 dihedral angle is 9.7(1)8, which shows little devia-
tion of the amide plane from the NP ring. The N3ꢀC21 and
C21ꢀO5 bond lengths are 1.376(7) and 1.237(7) ꢁ, respectively,
which are comparable to the free ligand.
Figure 7. Molecular structure of the cationic unit [Rh2(L2H)}2(m-CH3COO)2] in
7 with the important atoms labeled. All the hydrogen atoms, except for Nꢀ
H, have been omitted for the sake of clarity. Thermal ellipsoids are drawn at
the 40% probability level.
The reaction of L2H with [Rh2(CH3COO)2(CH3CN)6](BF4)2 in
a ratio of 2:1 at room temperature afforded the simple bis-
axial adduct [Rh2(CH3COO)2(L2H)2(CH3CN)4(BF4)2] (6), which was
confirmed by X-ray crystallography studies (Figure 6). The ob-
tained structure shows that both the ligands coordinate to the
metal center only through the distal nitrogen atom of naph-
two cis-L2H ligands. The N-C-N units of the naphthyridine li-
gands bridge between two Rh centers, and the sites trans to
the RhꢀRh bond are occupied by amide oxygen atoms. Thus,
two L2H ligands and two cis-bridging acetates complete the
coordination around the {Rh2} unit.
1
The H NMR spectrum of 7 shows two closely separated sin-
1
thyridine, therefore exhibiting similar H and 13C NMR spectra
glets for NꢀH protons at d=9.35 and 9.34 ppm, whereas the
NP, Ph, and Cp protons of two ligands appear in the range
7.60–8.95 ppm (see Figure S7 in the Supporting Information).
Interestingly, nine Cp protons of two Cp rings resonate with an
intensity ratio of 1:1:1:5:1, unlike other cases in which the Cp
protons show intensity ratios of 2:2:5. One of the ortho-hydro-
gen atoms in the substituted ferrocene ring falls in the shield-
ed region of the phenyl ring and appears in the upfield region.
The 13C NMR signal at d=173.8 ppm and IR stretching frequen-
cy at n˜ =1653 cmꢀ1 for the amide carbonyl bond signifies
weak coordination to the metal center. The ESI-MS spectrum
exhibits at a signal at m/z 1277, which is attributed to the
[MꢀBF4]+ ion.
Effect of steric crowding, lability, and hydrogen-bonding
interactions
From the above discussion, it is evident that the formation of
complexes 1–7 is governed by 1) the steric effect of the ortho-
methyl group in the L1H ligand, 2) the basicity/lability of the
bridging carboxylate moieties, 3) the structural rigidity of the
carboxylate-bridged dirhodium complexes, and 4) hydrogen-
bonding interactions between the amide proton and the car-
Figure 6. Molecular structure of the dicationic unit [Rh2(L2H)2(m-CH3COO)2-
(CH3CN)4] in 6 with the important atoms labeled. All the hydrogen atoms,
except for NꢀH, have been omitted for the sake of clarity. Thermal ellipsoids
are drawn at the 40% probability level.
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boxylate oxygen atom. The crystal structures of complexes
1 and 5 show that the ortho-methyl group engages the axial
site of the {Rh2} core, thus preventing another ligand from ap-
proaching. Furthermore, the bridging carboxylate units render
a planar structure that involve the {Rh2} unit and NP ring,
which is evident from the dihedral angles of f=1.3(1) and
1.1(1)8 for N1-Rh1-Rh2-N2 in 1 and 5, respectively. This finding
is unlike the RuIꢀRuI complex [Ru2(L1H)2(CO)4(BF4)2],[17] in which
the NP rings significantly deviate from planarity due to permit-
ted rotation around the RuꢀRu bond (the corresponding
angles are f=33.0(2) and 33.6(2)8 for N1-Ru1-Ru2-N2 and N4-
Ru2-Ru1-N5, respectively; Figure 8). Therefore, the ortho-
methyl groups in the RuIꢀRuI complex remain away from the
axial sites of the {Ru2} core and allow the incorpora-
tion of the second L1H ligand. This case does not
occur for complexes 1 and 5.
in 3 due to steric crowding between the ortho-methyl groups.
Instead, two ligands bind in a rare bidentate chelate mode and
are disposed in a head-to-tail fashion to minimize steric repul-
sion between them.
Complexes 1, 2, and 4 reveal ligand deprotonation in the
final structures. Deprotonation under mild conditions in the
absence of an external reagent invokes the possibility of hy-
drogen-bonding interactions between the amide hydrogen
and carboxylate oxygen atoms[20] prior to deprotonation. It is
assumed that the naphthyridine ligand equilibrates from N8
(distal) to N2 (proximal) coordination to engage in hydrogen-
bonding interaction of the amide hydrogen atom with the car-
boxylate oxygen atom (Scheme 5). This behavior is followed by
The use of more labile trifluoroacetate moieties
allows us to incorporate two ligands into complexes
3 and 4. The crystal structures reveal the different co-
ordination modes of L1H and L2H. Due to the absence
of any ortho-methyl group in L2H; two ligands are
trans-oriented and span the dirhodium core in the
most stable bridge-chelate mode in 4. However, such
a trans bis-adduct formation is not possible with L1H
Scheme 5. Hydrogen-bonding-assisted rearrangement of the naphthyridine ligand from
axial to bridge-chelate coordination at room temperature.
hydrogen abstraction and subsequent rearrangement from the
axial to bridge-chelate coordination. In the absence of hydro-
gen-bonding interactions, the ligands remain coordinated at
the axial sites, which is evident from complex 6. The bridge-
chelate form 7 is obtained only at reflux.
Electronic-absorption spectroscopic analysis and electro-
chemical studies
The electronic-absorption data and electrochemical-potential
values for L1H, L2H, and 1–7 are summarized in Tables 1 and 2,
respectively. The ligands exhibit multiple p!p* transitions be-
tween l=230 and 338 nm. Accordingly, the absorptions ob-
served in the UV region for 1–7 correspond to ligand-centered
p!p* transitions.[3d,18] The visible regions in the spectra of all
the complexes show two intense metal–ligand charge-transfer
Table 1. Electronic-absorption data for L1H, L2H, and 1–7 in CH2Cl2 at
room temperature.
Comp.
Absorption data
lmax [nm] (eꢂ10ꢀ3 [molꢀ1 dm3 cmꢀ1])
L1H
L2H
1
2
3
4
5
6
7
230 (46.5), 327 (16), 336 (15.7), 436 (1.2)
231 (54.6), 303 (10.5), 338 (13.5), 484 (1.1)
230 (43.5), 314 (19.8), 358 (15.6), 377 (19.5), 436 (6.1), 506 (5.6)
230 (51.2), 310 (22.6), 354 (18.3), 376 (21.5), 438 (5.9), 512 (4.8)
229 (53.8), 302 (41.5), 332 (34.1), 408 (5.7), 506 (4.5)
231 (71.4), 311 (27.2), 351 (20.8), 440 (6.5), 536 (3.6)
230 (64.7), 303 (35.7), 341 (29.5), 428 (7.1), 527 (4.7)
231 (64.2), 303 (26.8), 341 (20), 440 (3), 516 (2.2)
235 (66), 312 (37.3), 336 (24), 349 (22.5), 450 (7.0), 523 (6.4)
Figure 8. POV-ray diagram of the dicationic unit of a) [Ru2(CO)4(L1H)2](BF4)2
(RuI–RuI) and b) [Rh2(OAc)2(L1H)(CH3CN)2](BF4)2 (5) that shows the deforma-
tion of the NP planes. Fc=ferrocene, N4=N5=CH3CN in 5. Dihedral angles
[8]: N1-Ru1-Ru2-N2=33.0(2), N4-Ru2-Ru1-N5=33.6(2).
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been made to functionalize the C3 or C2 positions of substitut-
ed indoles selectively by applying this strategy.[27] Wenkert
et al. first reported the C3 functionalization of N-methylindole
with ethyl diazoacetate with Cu bronze as a catalyst, and ethyl
2-(1-methyl-1H-indol-3-yl)acetate was obtained in 27% yield.[28]
A recent report on the same procedure provided tert-butyl 2-
(1-methyl-1H-indol-3-yl)acetate in 11% yield by using tert-butyl
diazoacetate as the carbenoid source and a CuO/SiO2 mixture
as the catalyst.[29] Although these catalyst systems and the ac-
ceptor carbenoid showed disappointed results, a major break-
through occurred by using the donor/acceptor diazo com-
pound as the carbenoid source with various dirhodium carbox-
ylate derivatives as catalysts.[30] Muthusamy et al. reported the
regiospecific intermolecular CꢀH insertion of 3-diazooxindoles
with various substituted indoles in the presence of Rh2(OAc)4
in high yield (up to 99%).[31] Fox and co-workers demonstrated
the enantioselective CꢀH functionalization of indoles with
ethyl a-alkyl-a-diazoacetates with 0.5 mol% Rh2(S-NTTL)4
(NTTL=N-(1,8-naphthaloyl)-tert-leucinate) and [Rh2(S-PTTL)3TPA
Table 2. Electrochemical potentials for L1H, L2H, and 1–7 in CH3CN at
room temperature.[a]
Compound
Oxidation [V]
Reduction [V]
L1H
L2H
1
2
3
4
5
6
7
0.64 (76)[b]
ꢀ0.84[d], ꢀ1.57[d]
0.62 (78)[b]
ꢀ0.83[d], ꢀ1.51[d]
0.57[c], 1.24[c], 1.54[c]
0.54[c], 1.22[c], 1.58[c]
0.72 (66)[b]
ꢀ1.02[d], ꢀ1.53[d]
ꢀ1.01[d], ꢀ1.42[d]
ꢀ0.57[d], ꢀ1.08[d], ꢀ1.62[d]
ꢀ0.85[d], ꢀ1.30[d], ꢀ1.71[d]
ꢀ0.56[d], ꢀ1.12[d], ꢀ1.57[d]
ꢀ0.57[d], ꢀ1.23[d], ꢀ1.58[d]
ꢀ0.73[d], ꢀ0.95[d], ꢀ1.48[d]
0.63 (143)[b]
0.65 (109)[b]
0.58 (75)[b]
0.80 (81)[b]
[a] Potentials were measured at a Pt disk with Ag/AgCl as the reference
electrode, Pt wire as an auxiliary electrode, and 0.1m TBAPF6 as the sup-
porting electrolyte at a scan rate of 100 mVSꢀ1. [b] Half-wave potentials
evaluated from cyclic voltammetry as E1/2 =(Ep,a +Ep,c)/2; peak potential
differences [mV] are given in parentheses. [c] Peak potentials Ep,a for irre-
versible oxidation processes. [d] Peak potentials Ep,c for irreversible reduc-
tion processes.
(PTTL=N-phthaloyl-tert-leucinate, TPA=triphenylacetate) in
32]
(MLCT) bands assigned to {Rh2}(p*)!NP(p*).[3d,15i] The higher
intensities of the MLCT bands overshadow the much weaker
d–d transitions for the ferrocenyl unit[21] and the metal-cen-
tered {Rh2}(p*)!{Rh2}(s*) transitions observed in the parent
dirhodium complexes.[3d,15b,22]
82–96% yield with 79–99% enantioselectivity.[9c,
Lian and
Davies developed the enantioselective CꢀH functionalization
of indoles by using vinylcarbenoids in the presence of a catalyt-
ic amount (2 mol%) of Rh2(S-biTISP)2 (TISP=5,5’-(1,3-phenyle-
ne)bis[1-(2,4,6-triisopropylphenylsulfonyl)pyrrolidine-2-carboxyl-
ate] in 64–86% yield with 86–95% enantioselectivity.[33] Several
other metal catalysts, including copper,[34] ruthenium,[35] and
iron,[36] have also been used toward this goal. The metal carbe-
noids flanked by electron-withdrawing and electron-donating
groups exhibit greater stability and selectivity over the tradi-
tional acceptor rhodium carbenoids (e.g., ethyl diazoacetate),
in which the carbene carbon atom lacks donor-group stabiliza-
tion, and are therefore highly reactive.[14] Furthermore, by in-
corporating various donor functionalities into the diazo com-
pound, it is possible to obtain a wide range of functionalized
products with a particular dirhodium catalyst.
The electrochemical studies of both ligands show one rever-
sible ferrocenyl oxidation at around +0.6 V and two irreversi-
ble reductions at around Ep,c =ꢀ0.8 and ꢀ1.5 V. In contrast,
only one reduction process is observed for NP, pyNP, and bpNP
ligands (at ꢀ1.88, ꢀ1.68, and ꢀ1.56 V, respectively).[3d,15i] Al-
though complexes 1 and 2 show multiple oxidations, com-
plexes 3–7 show multiple reductions in their respective cyclic
voltammograms. All the complexes show ferrocene-based oxi-
dation below +0.81 V, whereas two additional oxidations
occur above +1.2 V for complexes 1 and 2. These processes
are considered to be rhodium centered.[15b] Complexes 1 and 2
exhibit two irreversible reduction processes, whereas an addi-
tional reduction below ꢀ0.86 V is observed for complexes 3–7.
These reductions at lower potentials are attributed to be metal
centered.[15b] The occurrence of additional oxidation and reduc-
tion processes for complexes 1, 2, and 3–7 can be explained
on the basis of their ligand compositions. The NP ligands in
1 and 2 are anionic, which increases the electron density at
On this backdrop, and with well-characterized dirhodium
complexes in hand, we performed a CꢀH functionalization of
indoles. Comparative catalytic studies were carried out for all
complexes 1–7 and Rh2(OAc)4 by using three types of carbe-
noid source, namely, an acceptor (ethyl diazoacetate), a donor/
acceptor (methyl phenyldiazoacetate), and an acceptor/accept-
or (dimethyl diazomalonate). We rationalized that complex 7
would be the best choice among 1–7 because it has hemilabile
amide groups at both the axial sites, whereas the axial sites in
other complexes are less accessible due to strong axial coordi-
nation of the ligands (see below). Accordingly, 1-methyl-1H-
indole, ethyl diazoacetate (acceptor), and Rh2(OAc)4 underwent
a reaction in a ratio of 1:1.2:0.05 in 1,2-dichloroethane at room
temperature with one equivalent of dodecane as an internal
standard. The reaction was monitored by using GC-MS, and
a maximum yield of 56% was obtained within 24 hours
(Figure 9). A longer reaction time did not increase the product
formation. Next, complex 7 was employed under identical con-
ditions, and only 22% product was obtained, even after
36 hours. Subsequently, the reaction was carried out at higher
temperature (608C), and we observed 58% product formation
4+
5+
the Rh center, thus favoring the oxidation of Rh2 !Rh2
!
6+ [23,15b]
Rh2
.
Complexes 3–7 are either cationic and contain
neutral NP ligands or consist of more electron-withdrawing tri-
fluoroacetate groups. Therefore, in addition to two ligand-cen-
4+
3+
tered reductions, a metal-centered Rh2 !Rh2 reduction is
also accessible within the potential range.[15b,24]
Catalytic CꢀH functionalization of indoles
The transition-metal-catalyzed decomposition of diazo com-
pounds provides one of the most powerful approaches to the
functionalization of CꢀH bonds.[1d,25] Because indole derivatives
are important structural motifs in many naturally occurring al-
kaloids and pharmaceutically active compounds,[26] efforts have
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1 mol% Rh2(OAc)4 and complex 7 independently (the results
are summarized in Table 4). At room temperature, 90% yield of
methyl 2-(1-methyl-1H-indol-3-yl)-2-phenylacetate was isolated
within 12 hours for Rh2(OAc)4, whereas complex 7 produced
a marginally increased yield (92%) under identical conditions.
Table 4. Optimization of the reaction conditions and catalysts.[a]
Entry
Catalyst
[mol%]
Yield [%][b]
1
2
3
4
5
6
7
8
9
Rh2(OAc)4
Rh2(OAc)4
1
0.5
1
0.5
1
1
1
1
1
1
90
76
92
72
22
16
35
18
78
55
Figure 9. Time- and temperature-dependence curve for CꢀH functionaliza-
tion of 1-methyl-1H-indole with ethyl diazoacetate and 5 mol% dirhodium
catalyst. a) Rh2(OAc)4 at room temperature, b) complex 7 at room tempera-
ture, and c) complex 7 at 608C.
7
7
1
2
3
4
5
6
within 24 hours (Figure 9). A longer reaction time (36 h) and
carrying the reaction out at reflux (858C) only led to a marginal
increase in the yield (60%), whereas Rh2(OAc)4 lead to a lower
yield due to catalyst decomposition at this temperature.
10
[a] Conditions: Catalyst=0.5–1 mol%), 1-methyl-1H-indole (1 mmol), slow
addition of methyl phenyldiazoacetate (1.2 mmol) over 30 min, and stir-
ring at room temperature for 12 h in a nitrogen atmosphere. [b] Yield of
the isolated product.
Lowering the catalyst loading from 5 to 2 mol% did not
affect the product formation by much for both Rh2(OAc)4 and
7, but further lowering the catalyst loading to 1 mol% gave
much lower yields (Table 3). Subsequently, complexes 1–6
were tested with 2 mol% of the complexes at 608C for
24 hours. Although complex 5 resulted in 45% yield, far less
conversion was obtained for complexes 1–4 and 6 (Table 3).
Methyl phenyldiazoacetate (donor/acceptor) was used next
and the reaction was carried out for N-methylindole with
As in the previous case, a decreased catalyst loading and the
use of other dirhodium complexes 1–6 produced much lower
yields (Table 4). Finally, dimethyl diazomalonate (acceptor/ac-
ceptor) was used as the carbenoid source. Although it was
readily decomposed by Rh2(OAc)4 at room temperature,[37] no
decomposition of the diazo compound occurred by complex 7
even at refluxing temperature.
With the optimized reaction conditions in hand, the sub-
strate scope of both reactions was examined with complex 7
as the catalyst (the results are summarized in Tables 5 and 6).
Substitution at the benzene ring had a less prominent effect as
the corresponding products 8b–8e and 9b–9e were isolated
in yields of 52–56 and 88–92%, respectively; however, substi-
tution at the C2 position of the pyrrole ring produced a slightly
lower yield of 8 f and 9 f. In contrast, substitution of the NꢀH
bond showed a more prominent effect. The unprotected
indole produced a mixture of CꢀH, NꢀH, and both CꢀH and
NꢀH insertion products, among which the CꢀH insertion prod-
uct 8g was isolated in 38% yield as the major product and by-
products were detected by GC-MS. N-acetyl substitution makes
the pyrrole ring electron deficient, which therefore remained
unreactive toward CꢀH functionalization for both the diazo
compounds. Allyl substitution at NꢀH showed preferential CꢀH
insertion over cyclopropanation (8i and 9h). Both the reac-
tions were very specific for C3 functionalization, and no iso-
meric C2 functionalized products were detected. Attempts to
obtain the C2-functionalized product by blocking the C2 posi-
tion only resulted in recovery of the starting material. The use
of N-methyl-7-azaindole instead of N-methylindole remained
unsuccessful for ethyl diazoacetate (8k), but a moderate yield
Table 3. Optimization of the reaction conditions and catalysts.[a]
Entry
Catalyst
[mol%]
T [8C]
Yield [%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
5
2
1
5
2
1
2
2
2
2
2
2
2
RT
RT
RT
60
60
60
85
60
60
60
60
60
60
56
54
42
58
56
38
60
10
14
16
6
7
7
7
7
1
2
3
4
5
6
45
11
[a] Conditions: Catalyst=1–5 mol%, 1-methyl-1H-indole (1 mmol), slow
addition of ethyl diazoacetate (1.2 mmol) over 30 min, and stirring at the
respective temperatures for 24 h in a nitrogen atmosphere. [b] The reac-
tion was monitored by using GC-MS and the yield was calculated by
using dodecane (1 mmol) as an internal standard.
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(58%) of the corresponding CꢀH functionalized product (9i)
was obtained for methyl phenyldiazoacetate.
Table 5. CꢀH functionalization of indoles with ethyl diazoacetate.[a]
All the results obtained above can be explained by compar-
ing the structures of the dirhodium complexes, the nucleophi-
licity of the diazo compounds, and the formation pathways of
the electrophilic metal carbenoids (Scheme 6). Nakamura and
8a, 54%
8d, 55%
8b, 56%
8e, 52%
8c, 54%
8 f, 48%
8g,38%[b]
8h, 50%
8k, n.r.
8i, 50%
8l, n.r.
8j, n.r.
Scheme 6. Decomposition of the diazo compound and the formation of dir-
hodium carbenoid by complex 7.
[a] Substituted indole (1 mmol), catalyst 7 (0.02 mmol), slow addition of
ethyl diazoacetate (1.2 mmol) over 30 min, and stirring at 608C for 24 h
in a nitrogen atmosphere. Yield of the isolated product is given. [b] CꢀH
insertion product only. [c] n.r.=No reaction.
Berry suggested that the reaction occurs only at one axial site
while the second metal center acts as an electron sink by form-
ing a 3c/4e bonding manifold that involves two rhodium cen-
ters and lone carbene carbon center.[38,3a] Among all the com-
plexes, Rh2(OAc)4, 5, and 7, with at least one accessible axial
site, show much a higher reactivity relative to the other com-
plexes, in which both axial sites are blocked either due to
a stronger coordination of neutral/anionic donor ligands or by
axial preagostic interactions between the {Rh–Rh} unit and the
ortho-methyl hydrogen atoms. The lower reactivity of 2 is at-
tributed to its poor solubility in the chlorinated solvent. In the
case of 5 and 7, both the axial sites in the latter are protected
by the hemilabile amide groups and offer greater accessibility
of the axial sites over 5, which has only one accessible site.
Consequently, 7 shows a much higher reactivity than 5 be-
cause the formation of the metal carbenoid involves an initial
nucleophilic attack of the diazo compound on the Rh center at
the axial site (Scheme 6). Methyl phenyldiazoacetate, with
a greater nucleophilic character, forces an outward rotation of
the amide group, even at room temperature, to de-coordinate
from the axial site, thus allowing the reaction to take place.
However, carrying out the reaction at reflux is necessary for
ethyl diazoacetate. Dimethyl diazomalonate, with the least nu-
cleophilic character, remained unreactive toward the metal-car-
bene formation, even at reflux. Thus, catalyst 7 exhibits better
selectivity of the diazo compounds than Rh2(OAc)4 at room
temperature, although the reactivities are comparable; further-
more, 7 has a higher thermal stability over Rh2(OAc)4. There-
fore, catalyst 7 has the potential to perform a selective CꢀH
functionalization reaction by carefully choosing the diazo
compound and controlling the reaction temperature.
Table 6. CꢀH functionalization of indoles with methyl phenyl-
diazoacetate[a]
9a, 92%
9d, 92%
9b, 90%
9e, 90%
9a, 88%
9 f, 80%
9g, 84%
9h, 86%
9i, 58%
[a] Substituted indole (1 mmol), catalyst 7 (0.01 mmol), slow addition of
methyl phenyldiazoacetate (1.2 mmol) over 30 min, and stirring at room
temperature for 12 h in a nitrogen atmosphere. Yield of the isolated
product is given.
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1H NMR (500 MHz, CDCl3): d=8.85 (brs, 1H; NH), 8.57 (d, J=8.6 Hz,
1H; NP), 8.33 (d, J=8.9 Hz, 1H; NP), 7.12 (s, 1H; NP), 4.90 (s, 2H;
Cp), 4.49 (s, 2H; Cp), 4.26 (s, 5H; Cp), 2.72 (s, 3H; Me), 2.66 ppm (s,
3H; Me); 13C NMR (126 MHz, CDCl3): d=170.2 (NHCO), 162.9 (NP),
154.4 (NP), 153.3 (NP), 145.7 (NP), 135.8 (NP), 122.3 (NP), 118.6 (NP),
113.9 (NP), 74.8 (Cp), 71.8 (Cp), 70.2 (Cp), 68.8 (Cp), 25.4 (Me),
18.2 ppm (Me); IR (KBr): n˜ =3474 (br, w, NꢀH), 3228 (br, w), 3084
(sh, w), 1678 (vs; CO), 1601 (vs; NꢀH), 1510 (s), 1454 (m), 1404 (m),
1315 (s), 1285 cmꢀ1 (s); MS (ESI; CH3CN): m/z 386.0951 [M+H]+.
Conclusion
Ferrocene-amide-functionalized 1,8-naphthyridine (NP) based
ligands L1H and L2H have been synthesized to exploit their
hemilabile amide side arm as a weakly coordinating axial
ligand on the RhII–RhII platform. Depending on the ortho sub-
stituent, the lability of the bridging carboxylate moieties, and
hydrogen-bonding interactions between the amide hydrogen
and carboxylate oxygen atoms, a host of structurally diverse
compounds have been isolated that vary in the number,
nature (neutral/anionic), and coordinating modes of the incor-
porated ligands. Among all the complexes reported in this
study, complex 7, with two hemilabile amide side arms at the
axial sites of the {Rh2} core, shows superior activity toward the
CꢀH functionalization of indoles with appropriate diazo com-
pounds. Although the reactivity of 7 is marginally better than
Rh2(OAc)4, it has a higher thermal stability and exhibits better
selectivity toward the diazo compounds at a controlled reac-
tion temperature. Thus, hemilabile ligands appear to serve as
a way to improve thermal stability, control the reaction rate,
and induce selectivity in the dirhodium catalyst.
{[(3-Phenyl-1,8-naphthyridin-2-yl)amino]carbonyl}ferrocene
(L2H): Ligand L2H was synthesized by following a similar procedure
described for the synthesis of L1H by the reaction between ferroce-
necarboxylic acid (2 g, 8.694 mmol) and 2-amino-3-phenyl-1,8-
naphthyridine (2.12 g, 9.563 mmol) to obtain an orange solid
(2.8 g, 74%). Orange block-shaped crystals of X-ray quality were
grown by layering petroleum ether onto a solution of the ligand in
ethyl acetate. 1H NMR (500 MHz, CDCl3): d=9.05 (brs, 1H; NH),
8.39 (brs, 1H; NP), 8.16 (brs, 1H; NP), 8.10 (brs, 1H; NP), 7.58 (brs,
1H; NP), 7.52–7.55 (m, 3H; Ph), 7.45–7.48 (m, 2H; Ph), 4.62 (s, 2H;
Cp), 4.37 (s, 2H; Cp), 4.11 ppm (s, 5H; Cp); 13C NMR (126 MHz,
CDCl3): d=171.2 (NHCO), 168.8 (NP), 162.9 (NP), 154.1 (NP), 147.6
(NP), 139.6 (NP), 138.0 (NP), 136.9 (NP), 130.0 (Ph), 129.1 (Ph), 128.4
(Ph), 121.4 (NP), 71.4 (Cp), 70.0 (Cp), 68.9 ppm (Cp); IR (KBr): n˜ =
3449 (br, w; NꢀH), 3195 (br, w), 3080 (sh, w), 1678 (vs; CO), 1662
(vs; NꢀH), 1608 (m), 1595 (m), 1558 (m), 1522 (m), 1500 (m), 1471
(m), 1457 (m), 1432 (m), 1419 cmꢀ1 (m); MS (ESI; CH3CN):
m/z 434.0956 [M+H]+.
Experimental Section
[Rh2(L1)(m-CH3COO)3] (1): Rh2(CH3COO)4 (40 mg, 0.091 mmol) was
added in one portion to (5,7-dimethyl-1,8-naphthyridin-2-yl)amino-
carbonylferrocene (L1H; 40 mg, 0.104 mmol) dissolved in dichloro-
methane (10 mL). The resulting red solution was stirred at room
temperature for 24 h in a nitrogen atmosphere. The solution was
concentrated to a small volume under reduced pressure, and di-
ethyl ether/petroleum ether (10 mL, 1:2) was added while stirring
to obtain a red precipitate. The precipitate was further washed
with petroleum ether (3ꢂ10 mL) and dried under vacuum to
afford 1 as a red microcrystalline solid (58 mg, 80%). Block-shaped
crystals of X-ray quality were grown by layering petroleum ether
onto a solution of 1 in dichloromethane in a vacuum-sealed glass
tube (o.d.=8 mm). 1H NMR (400 MHz, CD3CN): d=8.02 (d, J=
9.2 Hz, 1H; NP), 7.16 (s, 1H; NP), 7.13 (d, J=9.6 Hz, 1H; NP), 5.14
(s, 2H; Cp), 4.48 (s, 2H; Cp), 4.20 (s, 5H; Cp), 3.84 (s, 3H; MeNP),
2.59 (s, 3H; MeNP), 2.10 (s, 3H; MeOAc), 1.60 ppm (s, 6H; MeOAc);
13C NMR (100 MHz, CD3CN): d=189.9 (COOAc), 185.3 (COOAc), 172.1
(NP), 166.1 (CONCO), 163.8 (NP), 158.6 (NP), 148.0 (NP), 143.0 (NP),
134.0 (NP), 125,7 (NP), 121.9 (NP), 71.0 (Cp), 70.7 (Cp), 69.8 (Cp),
69.3 (Cp), 24.3 (MeNP), 22.9 (MeOAc), 22.7 (MeOAc), 17.6 ppm (MeNP);
IR (KBr): n˜ =3098 (w), 3059 (w), 2988 (w), 2924 (w), 1635 (s, CONCO),
1593 (vs; OAc), 1572 (vs; OAc), 1433 cmꢀ1 (vs); MS (ESI; CH3CN), m/
z: 767.95 [M+H]+; elemental analysis (%) calcd for
C27H27N3O7Fe1Rh2: C 42.27, H 3.55, N 5.48; found: C 42.78, H 3.64, N
5.22.
2-Aminonicotinaldehyde,[15i] 2-amino-3-phenyl-1,8-naphthyridine,[39]
substituted indoles, dimethyl diazomalonate, and methyl phenyl-
diazoacetate were prepared by following reported proce-
dures.[29,34–36] Ethyl diazoacetate was purchased from Sigma Aldrich.
Dirhodium precursors, such as [Rh2(CH3COO)4(CH3CN)2],[40]
[Rh2(CF3COO)4(Et2O)2],[40] and [Rh2(CH3COO)2(CH3CN)6](BF4)2 were
[41]
prepared as reported earlier.
Syntheses
{[(5,7-Dimethyl-1,8-naphthyridin-2-yl)amino]carbonyl}ferrocene
(L1H): Oxalyl chloride (1.1 mL, 13 mmol) was added dropwise to
a suspension of ferrocenecarboxylic acid (2 g, 8.694 mmol) in di-
chloromethane (50 mL) at 08C. A few drops of DMF were added to
the reaction mixture, and the reaction began immediately. The
mixture was slowly brought to room temperature and stirred for
12 h. All the solvent and excess oxalyl chloride were removed
under reduced pressure to afford ferrocenoyl chloride as a dark-red
solid in almost quantitative yield. This crude product was directly
used in the next step without further purification.
2-Amino-5,7-dimethyl-1,8-naphthyridine (1.66 g, 9.563 mmol) was
dissolved in THF (100 mL) and triethylamine (3 mL, 21.6 mmol) was
added to the solution. The solution was chilled to 08C and a sus-
pension of ferrocenoyl chloride (prepared in the previous step) in
THF was added portionwise over 30 min. The resulting mixture
was slowly brought to room temperature while a white precipitate
of triethylamine hydrochloride deposited on the walls of the flask.
Stirring was continued for 12 h at room temperature and then
heated to reflux for an additional 12 h. After completion of the re-
action, the solvent was removed by means of rotary evaporation.
The residue was taken in dichloromethane and extracted with a sa-
turated aqueous solution of sodium bicarbonate. The organic
phase was dried over anhydrous Na2SO4 and evaporated to obtain
a dark-brown solid. The residue was purified by column chroma-
tography on silica gel with ethyl acetate/petroleum ether (1:1) as
the eluent to obtain the product as an orange solid (3 g, 90%).
[Rh2(L2)(m-CH3COO)3] (2): Complex 2 was synthesized by following
the similar procedure described for the synthesis of complex 1 by
reaction of Rh2(CH3COO)4 (40 mg, 0.091 mmol) and L2H (45 mg,
0.104 mmol) to obtain 57 mg (74%) of the product. IR (KBr): n˜ =
3094 (w), 3060 (w), 2980 (w), 2924 (w), 1630 (s; CONCO), 1588 (vs;
OAc), 1574 (vs; OAc), 1416 cmꢀ1 (s); MS (ESI; CH3CN): m/z 815.9095
[M+H]+; elemental analysis (%) calcd for C31H27N3O7Fe1Rh2: C
45.67, H 3.34, N 5.15; found: C 44.80, H 3.62, N 5.32.
[Rh2(L1H)2(CF3COO)4] (3): Rh2(CF3COO)4 (50 mg, 0.076 mmol) was
dissolved in dichloromethane (10 mL) and L1H (60 mg, 0.156 mmol)
was added in one portion. Immediately after the addition, the
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color of the solution became deep red. The solution was further
stirred for 24 h at room temperature in a nitrogen atmosphere and
then concentrated to a small volume under reduced pressure. Pe-
troleum ether (10 mL) was added to the residue while stirring to
obtain a red precipitate, which was further washed with petroleum
ether (3ꢂ10 mL) and dried under vacuum to afford 3 as a deep-
red solid (82 mg, 76%). Block-shaped crystals of X-ray quality were
grown by layering petroleum ether onto a solution of 3 in di-
chloromethane inside a vacuum-sealed glass tube (o.d.=8 mm).
1H NMR (500 MHz, CD2Cl2): d=10.17 (s, 2H; NH), 7.80 (d, J=9.2 Hz,
2H; NP), 7.74 (d, J=9.5 Hz, 2H; NP),6.84 (s, 2H; NP), 4.97 (s, 4H;
Cp), 4.61 (s, 4H; Cp), 4.30 (s, 10H; Cp), 2.48 (s, 6H; MeNP), 2.11 ppm
(s, 6H; MeNP); 13C NMR (126 MHz, CD2Cl2): d=176.5 (OTfAc), 173.6
(OTfAc), 170.2 (NHCO), 167.0 (NP), 164.0 (NP), 156.0 (NP), 150.3
(NP), 147.2 (NP), 132.4 (NP), 125.3 (NP), 116.8 (CF3), 114.1 (CF3), 73.2
(Cp), 73.0 (Cp), 72.3 (Cp), 70.7 (Cp), 69.7 (Cp), 68.9 (Cp), 21.4 (MeNP),
17.6 ppm (MeNP); 19F NMR (470 MHz, CD2Cl2): d=ꢀ73.8, ꢀ74.3,
ꢀ74.6, ꢀ74.7 ppm; IR (KBr): n˜ =3204 (w; NꢀH), 3106 (w), 2963 (w),
1678 (s, CO), 1672 (s; CO), 1645 (s; NꢀH), 1597 (s; OTfAc), 1580 (s;
OTfAc), 1514 (s; OTfAc), 1425 (s), 1195 cmꢀ1 (vs; CF3); MS (ESI;
CH3CN): m/z 1314.92 [MꢀCF3COO]+; elemental analysis (%) calcd
for C50H38N6O10F12Fe2Rh2: C 42.04, H 2.68, N 5.88; found: C 42.45, H
2.91, N 5.20.
193.4 (COOAc), 192.1 (COOAc), 174.6 (NHCO), 172.8 (NP), 163.6 (NP),
157.7 (NP), 153.7 (NP), 152.1 (NP), 140.6 (NP), 127.2 (NP), 122,5 (NP),
78.6 (Cp), 75.3 (Cp), 72.1 (Cp), 70.6 (Cp), 29.5 (MeNP), 24.3 (MeOAc),
22.7 (MeOAc), 18.7 (MeNP), 4.5 (MeCH3CN); IR (KBr): n˜ =3240 (w; NꢀH),
3093 (w), 3000 (w), 2938 (w), 2334 (w; CH3CN), 2306 (w; CH3CN),
1638 (s, CO), 1592 (m; NꢀH), 1558 (s; OAc), 1422 (vs), 1063 cmꢀ1
(vs; BF4); MS (ESI; CH3CN): m/z 877.98 [MꢀBF4]+; elemental analysis
(%) calcd for C29H31N5O5B2F8Fe1Rh2: C 36.10, H 3.24, N 7.26; found:
C 35.78, H 2.98, N 7.42.
[Rh2(L2H)2(m-CH3COO)2(CH3CN)4][(BF4)2]
(6):
[Rh2(CH3COO)2-
(CH3CN)6]
(BF4)2 (40 mg, 0.054 mmol) was dissolved in dichloromethane
(10 mL) and L2H (48 mg, 0.111 mmol) was added in one portion.
The solution became red immediately and was stirred at room
temperature for 30 min. The solution was taken into four glass
tubes (o.d.=8 mm ), layered with petroleum ether, and sealed
under nitrogen. Red block-shaped crystals of 6 were formed after
3—4 days, which were collected and combined for further analysis
(36 mg, 44%). 1H NMR (500 MHz, CDCl3): d=9.06 (brs, 2H; NH),
8.62 (brs, 2H; NP), 8.38 (brs, 2H; NP), 8.26 (brs, 2H; NP), 7.96 (brs,
2H; NP), 7.79 (brs, 4H; Ph), 7.71 (brs, 6H; Ph), 4.52–4.75 (4 s, 8H;
Cp), 4.27 (s, 5H; Cp), 4.14 (s, 5H; Cp), 2.25 (s, 3H; MeOAc), 2.16 (s,
3H; MeOAc), 1.97 ppm (brs, 12H; CH3CN); IR (KBr): n˜ =3384 (m; Nꢀ
H), 3352 (m; NꢀH), 3061 (w), 2980 (w), 2925 (w), 2363 (w; CH3CN),
2330 (w; CH3CN), 1676 (s; CO), 1656 (s; NꢀH), 1567 (s; OAc), 1505
(m), 1452 (vs), 1413 (vs), 1072 cmꢀ1 (vs; BF4); MS (ESI; CH3CN): m/
z 1277.21 [MꢀBF4ꢀCH3CN)4]+; elemental analysis (%) calcd for
C62H56N10O6B2F8Fe2Rh2: C 48.72, H 3.69, N 9.17; found: C 47.78, H
3.98, N 9.42.
[Rh2(L2)(L2H)(CF3COO)3] (4): Complex 4 was synthesized by follow-
ing the similar procedure described for the synthesis of complex 3
through a reaction of Rh2(CF3COO)4 (50 mg, 0.076 mmol) and L2H
(68 mg, 0.157 mmol) to obtain 78 mg (73%) of product. Red block-
shaped crystals of X-ray quality were grown by layering petroleum
ether onto a solution of 4 in dichloromethane in a vacuum-sealed
glass tube (o.d.=8 mm). 1H NMR (500 MHz, CD2Cl2): d=9.20 (brs,
1H; NH), 7.23–8.46 (br, 18H; NP, Ph), 5.25 (s, 2H; Cp), 5.14 (s, 2H;
Cp), 4.74 (s, 2H; Cp), 4.63 (s, 2H; Cp), 4.33 (s, 5H; Cp), 4.27 ppm (s,
5H; Cp); 13C NMR (126 MHz, CD2Cl2): d=175.5 (OTfAc), 173.9
(OTfAc), 171.9 (NHCO), 170.4 (NP), 169.5 (NP), 164.5 (NCO), 163.4
(NP), 156.0 (NP), 154.0 (NP), 149.2 (NP), 147.1 (NP), 141.3 (NP), 140.2
(NP), 134.7 (NP), 131.0 (Ph), 130.1 (Ph), 129.9 (Ph), 129.5, (Ph), 128.8
(NP), 128.0 (NP), 123.0 (NP), 122.4 (NP), 113.3 (CF3), 111.0 (CF3), 73.9
(Cp), 73.7 (Cp), 71.4 (Cp), 70.8 ppm (Cp); 19F NMR (470 MHz,
CD3CN): d=ꢀ74.7, ꢀ74.8, ꢀ74.9, ꢀ75.0, ꢀ75.2, ꢀ75.3, ꢀ75.6,
ꢀ76.2 ppm; IR (KBr): n˜ =3352 (w; NꢀH), 3086 (w), 2927 (w), 1675
(s; NHCO), 1642 (s; NCO), 1598 (s; NꢀH), 1574 (s; OTfAc), 1520 (s;
OTfAc), 1477 (s), 1422 (vs), 1198 (vs; CF3), 1155 cmꢀ1 (vs; CF3); MS
(ESI; CH3CN): m/z 1296.94 [MꢀCF3COO]+; elemental analysis (%)
calcd for C56H37N6O8F9Fe2Rh2: C 47.69, H 2.64, N 5.96; found: C
46.78, H 2.98, N 5.42.
[Rh2(L2H)2(m-CH3COO)2][(BF4)2] (7): Complex 7 was synthesized by
following the similar procedure described for the synthesis of com-
plex
5 by reaction of [Rh2(CH3COO)2(CH3CN)6](BF4)2 (40 mg,
0.054 mmol) and L2H (48 mg, 0.111 mmol) to obtain 52 mg (71%)
of the product. Red block-shaped crystals of X-ray quality were
grown by layering diethyl ether onto an solution of 7 in acetoni-
trile in
a
vacuum-sealed glass tube (o.d.=8 mm). 1H NMR
(400 MHz, CD3CN): d=9.35 (s, 1H; NH), 9.34 (s, 1H; NH), 8.95 (s,
2H; NP), 8.39 (d, J=8.2 Hz, 2H; NP), 8.35 (s, 2H; NP), 7.82–7.89 (m,
6H; Ph), 7.71 (d, J=7.8 Hz, 4H; Ph), 7.60 (dd, J=7.8 Hz, 5.5 Hz, 2H;
NP), 5.04 (s, 2H; Cp), 4.87 (s, 2H; Cp), 4.76 (s, 2H; Cp), 4.39 (s, 10H;
Cp), 4.07 (s, 2H; Cp), 1.96 ppm (s, 6H; MeOAc); 13C NMR (100 MHz,
CD3CN): d=191.6 (COOAc), 173.8 (NHCO), 167.8 (NP), 158.9 (NP),
153.1 (NP), 148.5 (NP), 143.5 (NP), 142.8 (NP), 138.9 (NP), 134.4 (NP),
131.7 (Ph), 131.4 (Ph), 130.7 (Ph), 75.7 (Cp), 72.4 (Cp), 71.3 (Cp),
66.2 (Cp), 24.0 ppm (MeOAc); IR (KBr): n˜ =3363 (m; NꢀH), 3090 (w),
2926 (w), 1653 (vs; CO), 1597 (s; NꢀH), 1549 (m), 1518 (s; OAc),
1405 (s), 1083 cmꢀ1 (vs; BF4); MS (ESI; CH3CN): m/z 1277.21
[MꢀBF4]+; elemental analysis (%) calcd for C54H44N6O6B2F8Fe2Rh2: C
47.55, H 3.25, N 6.16; found: C 45.98, H 2.96, N 6.22.
[Rh2(L1H)(m-CH3COO)2(CH3CN)2][(BF4)2] (5): [Rh2(CH3COO)2(CH3CN)6]
(BF4)2 (50 mg, 0.067 mmol) was added in one portion to a solution
of L1H (28 mg, 0.074 mmol) in 1,2-dichloroethane (10 mL). The
color of the solution became red immediately after this addition.
The resulting red solution was heated to reflux for 24 h in a nitro-
gen atmosphere and cooled to room temperature. The solution
was concentrated to a small volume under reduced pressure and
diethyl ether (10 mL) was added while stirring to obtain a red pre-
cipitate. The precipitate was further washed with diethyl ether (3ꢂ
10 mL) and dried under vacuum to afford 5 as a red microcrystal-
line solid (53 mg, 82%). Block-shaped crystals of X-ray quality were
grown by layering petroleum ether onto a solution of 5 in di-
General procedure for the catalytic CꢀH functionalization of
indoles
Complex 7 (0.02 mmol, 27 mg) and substituted indole (1 mmol)
were dissolved in dry 1,2-dichloroethane (3 mL) in a flame-dried
schlenk round-bottomed flask in a nitrogen atmosphere. A solution
of ethyl diazoacetate (1.2 mmol, 126 mL) in 1,2-dichloroethane
(2 mL) was added dropwise to the resulting solution over 30 min
at room temperature. The temperature of the reaction mixture was
raised to 608C, and the reaction was stirred for 24 h. After comple-
tion of the reaction, the solvent was evaporated under reduced
pressure and the crude product was purified by flash chromatogra-
phy on silica gel with ethyl acetate/petroleum ether (2:98) as the
1
chloromethane a vacuum-sealed glass tube (o.d.=8 mm). H NMR
(500 MHz, CD3CN): d=9.54 (s, 1H; NH), 8.63 (d, J=8.9 Hz, 1H; NP),
7.76 (d, J=8.9 Hz, 1H; NP),7.62 (s, 1H; NP), 5.35 (s, 1H; Cp), 5.32 (s,
1H; Cp), 4.87 (s, 1H; Cp), 4.86 (s, 1H; Cp), 4.43 (s, 5H; Cp), 3.48 (s,
3H; MeNP), 2.74 (s, 3H; MeNP), 2.42 (s, 3H; MeOAc), 2.13 (s, 6H;
CH3CN), 1.80 ppm (s, 3H; MeOAc); 13C NMR (126 MHz, CD3CN): d=
Chem. Eur. J. 2014, 20, 16537 – 16549
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16547
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Sanau’, S. Stiriba, M. A. Ubeda, Organometallics 1997, 16, 880–886;
c) M. P. Doyle, J. Org. Chem. 2006, 71, 9253–9260; d) M. P. Doyle, Q.
Zhou, C. E. Raab, G. H. P. Roos, S. H. Simonsen, V. Lynch, Inorg. Chem.
1996, 35, 6064–6073; e) M. P. Doyle, J. P. Morgan, J. C. Fettinger, P. Y. Za-
valij, J. T. Colyer, D. J. Timmons, M. D. Carducci, J. Org. Chem. 2005, 70,
5291–5301.
eluent. (For methyl phenyldiazoacetate, complex
7 (14 mg,
0.01 mmol) was taken and the reaction mixture was stirred at
room temperature for 12 h. The crude product was purified by
flash chromatography on silica gel with ethyl acetate/petroleum
ether (5:95) as the eluent.
[7] a) F. A. Cotton, E. V. Dikarev, M. A. Petrukhina, Angew. Chem. 2000, 112,
2452–2454; Angew. Chem. Int. Ed. 2000, 39, 2362–2364; b) F. A. Cotton,
E. V. Dikarev, M. A. Petrukhina, S. Stiriba, Organometallics 2000, 19,
1402–1405; c) F. A. Cotton, E. V. Dikarev, M. A. Petrukhina, R. E. Taylor, J.
Am. Chem. Soc. 2001, 123, 5831–5832; d) F. A. Cotton, E. V. Dikarev,
M. A. Petrukhina, J. Am. Chem. Soc. 2001, 123, 11655–11663; e) M. A.
Petrukhina, K. W. Andreini, J. Mack, L. T. Scott, Angew. Chem. 2003, 115,
3497–3501; Angew. Chem. Int. Ed. 2003, 42, 3375–3379.
[8] a) T. Oohara, H. Nambu, M. Anada, K. Takeda, S. Hashimoto, Adv. Synth.
Catal. 2012, 354, 2331–2338; b) A. Biffis, M. Braga, M. Basato, Adv.
Synth. Catal. 2004, 346, 451–458; c) Y. Lou, T. P. Remarchuk, E. J. Corey,
J. Am. Chem. Soc. 2005, 127, 14223–14230; d) K. Guthikonda, J. D. Bois,
J. Am. Chem. Soc. 2002, 124, 13672–13673; e) A. Padwa, D. J. Austin,
A. T. Price, M. A. Semones, M. P. Doyle, M. N. Protopopova, W. R. Win-
chester, A. Trans, J. Am. Chem. Soc. 1993, 115, 8669–8680; f) M. P. Doyle,
M. S. Shanklin, Organometallics 1994, 13, 1081–1088.
Acknowledgements
This work was financially supported by the Department of Sci-
ence and Technology (DST), India. J.K.B. thanks DST for the
Swarnajayanti fellowship; M.S. thanks CSIR, India for a fellow-
ship; P.D. thanks CSIR, India for a SPM fellowship; and T.G.
thanks UGC India for a fellowship.
Keywords: catalysis · CꢀH activation · hemilability · hydrogen
bonds · naphthyridine · rhodium
[1] a) M. P. Doyle in Modern Rhodium-Catalyzed Organic Reactions (Eds.: P. A.
Evans), Wiley-VCH, Weinheim, 2005, pp. 341–355; b) D. J. Timmons,
M. P. Doyle in Multiple Bonds Between Metal Atoms (Eds.: F. A. Cotton,
C. A. Murillo, R. A. Walton), Springer, Heidelberg, 2005, pp. 591–632;
c) M. P. Doyle, M. A. McKervey, T. Ye, Modern Catalytic Methods for Or-
ganic Synthesis with Diazo Compounds: From Cyclopropanes to Ylides,
Wiley, Hoboken, 1998; d) H. M. L. Davies, A. R. Dick in Top. Curr. Chem.,
Vol. 292 (Eds.: J. Yu, Z. Shi), Springer, Heidelberg, 2010, pp. 303–345;
e) K. P. Kornecki, J. F. Berry, D. C. Powers, T. Ritter in Prog. Inorg. Chem.,
Vol. 58 (Ed: K. D. Karlin), Wiley, Hoboken, 2014, pp. 225–302; f) M. P.
Doyle in Top. Organomet. Chem., Vol. 13 (Eds.: K. H. Dçtz), Springer, Hei-
delberg, 2004, pp. 203–222 g) K. P. Kornecki, J. F. Briones, V. Boyarskikh,
F. Fullilove, J. Autschbach, K. E. Schrote, K. M. Lancaster, H. M. L. Davies,
J. F. Berry, Science 2013, 342, 351–354; h) H. M. L. Davies, J. R. Manning,
Nature 2008, 451, 417–424.
[2] a) J. Hansen, H. M. L. Davies, Coord. Chem. Rev. 2008, 252, 545–555;
b) E. V. Dikarev, D. K. Kumar, A. S. Filatov, A. Anan, Y. Xie, T. Asefa, M. A.
Petrukhina, ChemCatChem 2010, 2, 1461–1466; c) D. K. Kumar, A. S. Fi-
latov, M. Napier, J. Sun, E. V. Dikarev, M. A. Petrukhina, Inorg. Chem.
2012, 51, 4855–4861.
[3] a) J. F. Berry, Dalton Trans. 2012, 41, 700–713; b) A. F. Trindade, J. A. S.
Coelho, C. A. M. Afonso, L. F. Veiros, P. M. P. Gois, ACS Catal. 2012, 2,
370–383; c) P. Hirva, P. Lahuerta, J. Pe’rez-Prieto, Theor. Chem. Acc.
2005, 113, 63–68; d) J. D. Aguirre, D. A. Lutterman, A. M. Angeles-Boza,
K. R. Dunbar, C. Turro, Inorg. Chem. 2007, 46, 7494–7502; e) Y. Lian, K. I.
Hardcastle, H. M. L. Davies, Angew. Chem. 2011, 123, 9542–9545;
Angew. Chem. Int. Ed. 2011, 50, 9370–9373.
[4] a) P. M. P. Gois, A. F. Trindade, L. F. Veiros, V. Andrꢃ, M. T. Duarte, C. A. M.
Afonso, S. Caddick, F. G. N. Cloke, Angew. Chem. 2007, 119, 5852–5855;
Angew. Chem. Int. Ed. 2007, 46, 5750–5753; b) A. F. Trindade, P. M. P.
Gois, L. F. Veiros, V. Andre’, M. T. Duarte, C. A. M. Afonso, S. Caddick,
F. G. N. Cloke, J. Org. Chem. 2008, 73, 4076–4086; c) J. A. S. Coelho, A. F.
Trindade, R. Wanke, B. G. M. Rocha, L. F. Veiros, P. M. P. Gois, A. J. L. Pom-
beiro, C. A. M. Afonso, Eur. J. Org. Chem. 2013, 1471–1478; d) K. P. Kor-
necki, J. F. Berry, Chem. Commun. 2012, 48, 12097–12099; e) A. J.
Catino, J. M. Nichols, R. E. Forslund, M. P. Doyle, Org. Lett. 2005, 7,
2787–2790; f) A. J. Catino, J. M. Nichols, H. Choi, S. Gottipamula, M. P.
Doyle, Org. Lett. 2005, 7, 5167–5170; g) E. C. McLaughlin, M. P. Doyle, J.
Org. Chem. 2008, 73, 4317–4319; h) A. J. Catino, R. E. Forslund, M. P.
Doyle, J. Am. Chem. Soc. 2004, 126, 13622–13623.
[5] a) Z. Li, H. T. Chifotides, K. R. Dunbar, Chem. Sci. 2013, 4, 4470–4485;
b) M. E. Moragues, J. Esteban, J. V. Ros-Lis, R. Martꢄnez-MꢅÇez, M. D.
Marcos, M. Martꢄnez, J. Soto, F. Sancenꢆn, J. Am. Chem. Soc. 2011, 133,
15762–15772; c) J. Esteban, J. V. Ros-Lis, R. Martꢄnez-MꢅÇez, M. D.
Marcos, M. Moragues, J. Soto, F. Sancenꢆn, Angew. Chem. 2010, 122,
5054–5057; Angew. Chem. Int. Ed. 2010, 49, 4934–4937; d) M. C. Pir-
rung, A. T. Morehead Jr., J. Am. Chem. Soc. 1994, 116, 8991–9000.
[6] a) J. Lloret, J. J. Carbo’, C. Bo, A. Lledꢆs, J. Pꢃrez-Prieto, Organometallics
2008, 27, 2873–2876; b) F. Estevan, P. Lahuerta, J. Pe’rez-Prieto, M.
[9] a) J. L. Roizen, M. E. Harvey, J. D. Bois, Acc. Chem. Res. 2012, 45, 911–
922; b) A. DeAngelis, O. Dmitrenko, G. P. A. Yap, J. M. Fox, J. Am. Chem.
Soc. 2009, 131, 7230–7231; c) D. T. Boruta, O. Dmitrenko, G. P. A. Yap,
J. M. Fox, Chem. Sci. 2012, 3, 1589–1593; d) A. DeAngelis, D. T. Boruta,
J. Lubin, J. N. Plampin, G. P. A. Yap, J. M. Fox, Chem. Commun. 2010, 46,
4541–4543.
[10] a) T. Ren, C. Lin, E. J. Valente, J. D. Zubkowski, Inorg. Chim. Acta 2000,
297, 283–290; b) J. L. Bear, C. L. Yao, R. S. Lifsey, J. D. Korp, K. M. Kadish,
Inorg. Chem. 1991, 30, 336–340; c) H. T. Chifotides, K. V. Catalan, K. R.
Dunbar, Inorg. Chem. 2003, 42, 8739–8747; d) D. A. Lutterman, N. N.
Degtyareva, D. H. Johnston, J. C. Gallucci, J. L. Eglin, C. Turro, Inorg.
Chem. 2005, 44, 5388–5396; e) M. W. Cooke, M. P. Santoni, G. S. Hanan,
A. Proust, B. Hasenknopf, Dalton Trans. 2009, 3671–3673; f) D. Char-
trand, G. S. Hanan, Chem. Commun. 2008, 727–729.
[11] a) N. G. Connelly, T. Einig, G. G. Herbosa, P. M. Hopkins, C. Mealli, A. G.
Orpen, G. M. Rosair, F. Viguri, J. Chem. Soc. Dalton Trans. 1994, 2025–
2039; b) C. J. Adams, R. A. Baber, N. G. Connelly, P. Harding, O. D. Hay-
ward, M. Kandiah, A. G. Orpen, Dalton Trans. 2007, 1325–1333; c) M. B.
Hursthouse, M. A. Mazid, T. Clark, S. D. Robinson, Polyhedron 1993, 12,
563–565.
[12] a) F. Estevan, P. Lahuerta, J. Lloret, M. Sanaffl, M. A. Ubeda, J. Vila, Chem.
Commun. 2004, 2408–2409; b) F. Estevan, P. Lahuerta, J. Lloret, J.
Pe’rez-Prieto, H. Werner, Organometallics 2004, 23, 1369–1372; c) J.
Lloret, F. Estevan, P. Lahuerta, P. Hirva, J. Pe’rez-Prieto, M. Sanaffl, Chem.
Eur. J. 2009, 15, 7706–7716; d) F. Estevan, J. Lloret, M. Sanaffl, M. A.
Úbeda, Organometallics 2006, 25, 4977–4984; e) J. Lloret, M. Stern, F.
Estevan, M. Sanaffl, M. A. Úbeda, Organometallics 2008, 27, 850–856;
f) F. A. Cotton, C. A. Murillo, S. E. Stiriba, X. P. Wang, R. M. Yu, Inorg.
Chem. 2005, 44, 8223–8233.
[13] a) M. El-Deftar, F. G. Adly, M. G. Gardiner, A. Ghanem, Current Organic
Chemistry 2012, 16, 1808–1836; b) M. P. Doyle, D. C. Forbes, Chem. Rev.
1998, 98, 911–935; c) R. Hrdina, L. Guꢃnꢃe, D. Moraleda, J. Lacour, Or-
ganometallics 2013, 32, 473–479; d) C. Qin, V. Boyarskikh, J. H. Hansen,
K. I. Hardcastle, D. G. Musaev, H. M. L. Davies, J. Am. Chem. Soc. 2011,
133, 19198–19204; e) R. P. Reddy, G. H. Lee, H. M. L. Davies, Org. Lett.
2006, 8, 3437–3440.
[14] H. M. L. Davies, R. E. J. Beckwith, Chem. Rev. 2003, 103, 2861–2903.
[15] a) W. R. Tikkanen, E. Binamira-Soriaga, W. C. Kaska, P. C. Ford, Inorg.
Chem. 1983, 22, 1147–1148; b) W. R. Tikkanen, E. Binamira-Soriaga, W. C.
Kaska, P. C. Ford, Inorg. Chem. 1984, 23, 141–146; c) A. T. Baker, W. R.
Tikkanen, W. C. Kaska, P. C. Ford, Inorg. Chem. 1984, 23, 3254–3256;
d) J. P. Collin, A. Jouaiti, J. P. Sauvage, W. C. Kaska, M. A. McLoughlin,
N. L. Keder, W. T. A. Harrison, G. D. Stucky, Inorg. Chem. 1990, 29, 2238–
2241; e) A. Sinha, S. M. W. Rahaman, M. Sarkar, B. Saha, P. Daw, J. K. Bera,
Inorg. Chem. 2009, 48, 11114–11122; f) S. K. Patra, N. Sadhukhan, J. K.
Bera, Inorg. Chem. 2006, 45, 4007–4015; g) M. Majumdar, S. K. Patra, M.
Kannan, K. R. Dunbar, J. K. Bera, Inorg. Chem. 2008, 47, 2212–2222;
h) C. S. Campos-Fernꢅndez, X. Ouyang, K. R. Dunbar, Inorg. Chem. 2000,
Chem. Eur. J. 2014, 20, 16537 – 16549
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39, 2432–2433; i) C. S. Campos-Fernꢅndez, L. M. Thomson, J. R. Galꢅn-
Mascarꢆs, X. Ouyang, K. R. Dunbar, Inorg. Chem. 2002, 41, 1523–1533.
[16] a) V. P. Litvinov, Adv. Heterocycl. Chem. 2006, 91, 189–300; b) C. He, S. J.
Lippard, Tetrahedron 2000, 56, 8245–8252; c) J. Kuzelka, S. Mukhopad-
hyay, B. Spingler, S. J. Lippard, Inorg. Chem. 2003, 42, 6447–6457; d) C.
He, S. J. Lippard, Inorg. Chem. 2001, 40, 1414–1420; e) T. C. Davenport,
T. D. Tilley, Angew. Chem. 2011, 123, 12413–12416; Angew. Chem. Int.
Ed. 2011, 50, 12205–12208.
[17] R. K. Das, B. Saha, S. M. W. Rahaman, J. K. Bera, Chem. Eur. J. 2010, 16,
14459–14468.
[18] N. Sadhukhan, A. Sinha, R. K. Das, J. K. Bera, J. Organomet. Chem. 2010,
695, 67–73.
[19] a) A. Sinha, M. Majumdar, M. Sarkar, T. Ghatak, J. K. Bera, Organometallics
2013, 32, 340–349; b) M. Brookhart, M. L. H. Green, G. Parkin, Proc. Nat.
Acad. Sci. USA 2007, 104, 6908–6914.
[20] H. Kitamura, T. Ozawa, K. Jitsukawa, H. Masuda, Y. Aoyama, H. Einaga,
Inorg. Chem. 2000, 39, 3294–3300.
Oxford, 1999; c) A. J. Kochanowska-Karamyan, M. T. Hamann, Chem. Rev.
2010, 110, 4489–4497.
[27] H. M. L. Davies, S. J. Hedley, Chem. Soc. Rev. 2007, 36, 1109–1119.
[28] E. Wenkert, M. E. Alonso, H. E. Gottlieb, E. L. Sanchez, R. Pellicciari, P. Co-
golli, J. Org. Chem. 1977, 42, 3945–3949.
[29] J. M. Fraile, K. L. Jeune, J. A. Mayoral, N. Ravasiob, F. Zaccheria, Org.
Biomol. Chem. 2013, 11, 4327–4332.
[30] S. J. Hedley, D. L. Ventura, P. M. Dominiak, C. L. Nygren, H. M. L. Davies, J.
Org. Chem. 2006, 71, 5349–5356.
[31] S. Muthusamy, C. Gunanathan, S. Arulananda Babu, E. Suresh, P. Dasti-
dar, Chem. Commun. 2002, 824–825.
[32] A. DeAngelis, V. W. Shurtleff, O. Dmitrenko, J. M. Fox, J. Am. Chem. Soc.
2011, 133, 1650–1653.
[33] Y. Lian, H. M. L. Davies, Org. Lett. 2012, 14, 1934–1937.
[34] M. B. Johansen, M. A. Kerr, Org. Lett. 2010, 12, 4956–4959.
[35] W. Chan, S. Yeung, Z. Zhou, A. S. C. Chan, W. Yu, Org. Lett. 2010, 12,
604–607.
[21] J. D. Carr, S. J. Coles, W. W. Hassan, M. B. Hursthouse, K. M. Abdul Malik,
J. H. R. Tucker, J. Chem. Soc. Dalton Trans. 1999, 57–62.
[22] F. A. Cotton, E. A. Hillard, C. A. Murillo, J. Am. Chem. Soc. 2002, 124,
5658–5660.
[23] F. A. Cotton, K. R. Dunbar, M. G. Verbruggen, J. Am. Chem. Soc. 1987,
109, 5498–5506.
[24] C. A. Crawford, J. H. Matonic, J. C. Huffman, K. Folting, K. R. Dunbar, G.
Christou, Inorg. Chem. 1997, 36, 2361–2371.
[25] a) M. P. Doyle, R. Duffy, M. Ratnikov, L. Zhou, Chem. Rev. 2010, 110, 704–
724; b) H. M. L. Davies, D. Morton, Chem. Soc. Rev. 2011, 40, 1857–1869;
c) C. N. Slattery, A. Ford, A. R. Maguire, Tetrahedron 2010, 66, 6681–
6705.
[36] Y. Cai, S. Zhu, G. Wang, Q. Zhou, Adv. Synth. Catal. 2011, 353, 2939–
2944.
[37] R. Gibe, M. A. Kerr, J. Org. Chem. 2002, 67, 6247–6249.
[38] E. Nakamura, N. Yoshikai, M. Yamanaka, J. Am. Chem. Soc. 2002, 124,
7181–7192.
[39] M. Sarkar, J. K. Bera, Proc. Natl. Acad. Sci. India, Sect. A 2014, 84, 227–
234.
[40] G. A. Rempel, P. Legzdins, H. Smith, G. Wilkinson in Inorganic Syntheses,
Vol. 13 (Ed.: F. A. Cotton), McGraw-Hill, New York, 1972, pp. 90–91.
[41] G. Pimblett, C. D. Garner, W. Clegg, J. Chem. Soc. Dalton Trans. 1986,
1257–1263.
[26] a) P. M. Dewick, Medicinal Natural Products: A Biosynthetic Approach,
Wiley, Hoboken, 2009; b) D. H. R. Barton, K. Nakanishi, O. MethCohn,
J. W. Kelly, Comprehensive Natural Products Chemistry, Pergamon Press,
Received: April 3, 2014
Published online on October 21, 2014
Chem. Eur. J. 2014, 20, 16537 – 16549
www.chemeurj.org
16549
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim