Stereogenic Lock in 1-Naphthylethanamine Complexes for Catalyst and Auxiliary Design: Structural and Reactivity Analysis for Cycloiridated Pseudotetrahedral Complexes

Article abstract of DOI:10.1021/acs.organomet.7b00760

A series of optically active pseudo-tetrahedral five-membered cyclometalated 1-naphthylethanamine iridium(III) complexes were prepared and characterized to analyze the efficacy of the stereogenic conformational lock in both solid and solution phases. The synthesis of the iridacycles was diastereoselective, and the compounds were found to be conformationally rigid. In comparison to its phenyl derivative, the structural lock prevented oxidation of the amine moiety within the five-membered organometallic ring during its synthesis. With up to three stereogenic centers in one of the naphthalene complexes, the stereochemistry of the metallacycle remained stable to both thermal and chemical changes. In terms of catalytic performance, the complexes displayed excellent activity for the asymmetric hydrogen transfer reaction, albeit with modest enantioselectivities.

Full text of DOI:10.1021/acs.organomet.7b00760

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Stereogenic Lock in 1‑Naphthylethanamine Complexes for Catalyst
and Auxiliary Design: Structural and Reactivity Analysis for
Cycloiridated Pseudotetrahedral Complexes
Houguang Jeremy Chen, Ronald Hong Xiang Teo, Yongxin Li, Sumod A. Pullarkat,
and Pak-Hing Leung*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
S
* Supporting Information
ABSTRACT: A series of optically active pseudo-tetrahedral
five-membered cyclometalated 1-naphthylethanamine iridium-
(III) complexes were prepared and characterized to analyze
the efficacy of the stereogenic conformational lock in both
solid and solution phases. The synthesis of the iridacycles was
diastereoselective, and the compounds were found to be
conformationally rigid. In comparison to its phenyl derivative,
the structural lock prevented oxidation of the amine moiety
within the five-membered organometallic ring during its
synthesis. With up to three stereogenic centers in one of the
naphthalene complexes, the stereochemistry of the metalla-
cycle remained stable to both thermal and chemical changes.
In terms of catalytic performance, the complexes displayed excellent activity for the asymmetric hydrogen transfer reaction, albeit
with modest enantioselectivities.
derivative, on the other hand, has its ortho proton too far away
to interact sterically with any neighboring groups, resulting in
the puckered-ring system undergoing a continuous C2/Cs-
interchanging conformational ring flip (Figure 2).
INTRODUCTION
■
Steric interactions play a significant role in the design of chiral
catalysts and auxiliaries for asymmetric catalysis and syntheses.1
To control enantioselectivity in reactions, rational modifica-
tions to the molecular framework are usually incorporated
during its design phase. Specifically, the introduction of a steric
bulk near the active site is a standard approach.2
For multidentate complexes with some form of chiral helicity
on its backbone, a structural lock within the cyclic system could
dramatically amplify their stereoinduction effects. This critical
aspect can be exemplified by the cyclopalladated complexes of
1-phenylethylamine and 1-naphthylethylamine (Figure 1).3
It is well-established that the chiral square planar N,N-
dimethyl-1-naphthylethylamine cyclopalladated complex can
impart conformational rigidity within the five-membered
organometallic ring due to intramolecular steric repulsion
between the methyl substituent at the stereogenic carbon
center and the neighboring naphthalene proton. The phenyl
Figure 2. Possible conformations of cyclopalladated complex (S)-N,N-
dimethyl-1-naphthylethylamine.
Figure 1. Chiral (S)-N,N-dimethyl-1-phenylethylamine and (S)-N,N-
dimethyl-1-naphthylethylamine palladacycles.
Received: October 11, 2017
© XXXX American Chemical Society
A
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This retention of configuration in the naphthalene system
was, therefore, found to be superior since the lack of
interchangeability permits the stereogenic information to be
transferred more efficiently to the substrates from the
cyclopalladated framework via the metal center. The effect of
this critical feature in the design of these palladacyclic analogues
has indeed been evident in numerous asymmetric applications
performed by our group (Figure 3).4
Scheme 1. Cycloiridation of Optically Active Secondary
Amine (S)-1
a
a
Reaction conditions: (i) [IrCp*Cl2]2, NaOAc, CH2Cl2, room
temperature.
The reaction presented two notable findings: (a) the
existence of only one stereoisomer for each of the primary
and secondary amino complexes and (b) the formation of N-
demethylated complex (SC,RIr)-3 where a C−N bond was
cleaved during the transformation. Arguably, the formation of
(SC,RIr)-3 as a minor product is questionable, since it could
have originated from the ortho-metalation of the residual
primary amine present in (S)-1 during its synthesis. To
eliminate this possibility, its racemic variant, prepared via a
protocol that did not involve the primary amine derivative, was
used for cycloiridation (see the Supporting Information for
details). As expected, the reaction once again yielded both
complexes. It is worth mentioning that a similar cleavage of the
C−N bond was observed in one of the byproducts during the
attempted cycloiridation of the tertiary amine (S)-N,N-
dimethyl-1-naphthylamine.9 Such N-demethylation processes
are also prevalent if bulky ligands are utilized for cyclo-
metalation.12
Figure 3. Chiral (S)-1-naphthylethylamine cyclopalladated complex
and its applications.
In addition, structural studies through variation of sub-
stituents within this system were also examined to determine
the boundaries of stereogenic lock.5 With significant knowledge
on the effect of the ligand framework in cyclopalladation
chemistry and consequently asymmetric applications in place,
we now seek to understand and, possibly, extend the concept of
stereogenic lock to metallacycles of other transition-metal
elements. Furthermore, we believe the concept of structural
lock could be applied to other multidentate ligands and five-
membered cyclic organocatalysts with analogous frameworks.
The piano-stool d6-iridium(III)-based cyclometalated com-
pounds have been popular in recent decades.6 These
complexes, although they are of the type ML3X3, exhibit a
pseudo-tetrahedral structure and are active as catalysts for
various reactions.7 Furthermore, the ease of synthesis and
stability of these compounds to air and moisture make such
metallacyclic complexes an obvious choice for investigation
with focus on the aforementioned perspective.
Iridacycle (SC,SN,RIr)-2 was isolated as an orange powder
and, expectedly, is stable in air and moisture. Spectroscopically,
1
the H NMR spectrum displayed one Cp* singlet at 1.67 ppm
at room temperature, indicating the presence of only one
isomer despite the complex consisting of three stereocenters.
Stereochemical inversion at the nitrogen and iridium central
chirality was also found to be nonexistent at other temperatures
(discussed in greater detail below). As mentioned earlier, this
was surprising since numerous Cp*-based cycloiridated
complexes were found to provide diastereomers upon cyclo-
metalation.6d,13
We herein report the synthesis of several cyclometalated 1-
naphthylalkylamine iridium(III) complexes. A comparative
study involving the structural aspects and their effectiveness
in catalytic asymmetric hydrogen transfer reactions will also be
discussed.
To characterize and determine the existence of the
stereogenic lock in the complex, crystals of iridacycle 2, in
the form of orthorhombic orange blocks, were obtained
through recrystallization from a mixture of hexane and
dichloromethane (Figure 4).
RESULTS AND DISCUSSION
■
Synthesis, Characterization, and Stereogenic Lock. It
is widely established that cyclometalation is driven by both
electronic and steric factors at the donor moiety.8 As such, our
initial attempts at cycloiridation involved the tertiary amine (S)-
N,N-dimethyl-1-naphthylethylamine as the ligand, identical
with that used in our model palladacycle. Disappointingly, the
amine failed to cyclometalate and the reaction afforded
numerous side products.9
It is worth mentioning that a primary amine derived
iridacycle has been prepared by Ikariya and co-workers
previously for their study on asymmetric hydrogen transfer
reactions.10 As such, we believe that the cyclometalation
protocol could be sensitive to steric factors, although it can also
be a driving force via the Thorpe−Ingold effect.11 To
understand how steric hindrance affects cycloiridation of such
a framework, we turned to its secondary amine derivative (S)-1.
Gratifyingly, the procedure proceeded efficiently, affording the
two cyclic organometallic complexes (SC,SN,RIr)-2 and (SC,RIr)-
3, with the former being the major product (Scheme 1).
The complex conforms to a pseudotetrahedral structure at
the iridium center, with stereochemistry at carbon, nitrogen,
and iridium determined to be S, S, and R, respectively,
according to Cahn−Ingold−Prelog sequence rules.14 Along the
plane of the naphthylene ring, the complex resembles an
expected octahedral geometry, typical of pseudotetrahedral
complexes, with its bulky Cp* moiety occupying much of the
coordination sphere, as evident from the bond angles between
the carbon, nitrogen, chlorine, and iridium entities. In addition,
the structure features a five-membered organometallic ring
adopting a static λ conformation with the stereogenic methyl
substituent locked in place at the axial position (Figure 5).
The determination of the stereogenic lock in the solution
1
phase was performed by means of 2D H−1H NOESY NMR
spectroscopy (Figure 6). Cross peaks relating to Ha−Hc
interactions revealed that the five-membered puckered ring
B
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Article
interchanging conformational ring flip within the molecule.
Overall, this information provided evidence to support the
static λ configuration, as observed in the puckered-ring system
(Figure 5).
The orange primary amine-based ortho-iridated complex 3
could be isolated as a byproduct from the reaction, although the
fraction was only evident to us on chromatography at a 1 mmol
scale. Despite its low yield (2%) in the reaction, it should be
noted that the iridacycle could be generated in higher yields
(90%) through direct cyclometalation of the primary amine
under similar experimental conditions.
Like the secondary amine analogue, iridacycle 3 was found to
afford only one diastereomer based on the Cp* singlet (1.74
ppm) within the 1H NMR spectrum. The spectrum also
revealed a set of broad nonequivalent NH2 signals at 3.70 and
4.78 ppm, typical of chelating complexes. Single-crystal X-ray
crystallographic studies determined that the recrystallized
ortho-metalated complex conforms to a pseudotetrahedral
structure with chirality at carbon and iridium to be S and R,
respectively (Figure 7). The molecule also presented a static λ
conformation about the organometallic ring in its solid state.
Figure 4. Molecular structure of cycloiridated complex (SC,SN,RIr)-2
with thermal ellipsoids shown at 50% probability. Hydrogen atoms
except H(C11) and H(N1) are omitted for clarity. Selected bond
lengths and angles: N−Ir (2.151 Å), C−Ir (2.040 Å), Cl−Ir (2.425 Å),
N−Ir−C (76.7°), N−Ir−Cl (82.5°), C−Ir−Cl (86.6°).
Figure 5. Preferred λ conformation of cycloiridated complex
(SC,SN,RIr)-2.
Figure 7. Molecular structure of cycloiridated complex (SC,RIr)-3 with
thermal ellipsoids shown at 50% probability. Hydrogen atoms except
H(C11) are omitted for clarity. Selected bond lengths and angles: N−
Ir (2.122 Å), C−Ir (2.060 Å), Cl−Ir (2.431 Å), N−Ir−C (76.7°), N−
Ir−Cl (82.5°), C−Ir−Cl (86.8°).
In the solution phase, (SC,RIr)-3 presented similar key
interactions, although fewer cross peaks were seen due to the
increased spatial capacity, in comparison to (SC,SN,RIr)-2 (see
1
the 2D H-1H NOESY NMR spectrum of (SC,RIr)-3 in the
Supporting Information). In particular, the continual presence
of the C-chiral methyl group adjacent aromatic proton
interaction fixes the organometallic ring system to a single
conformation.
While the apparent progression to the investigation was to
include the phenyl derivatives in the study, the desired primary
amine and secondary amine analogues were unattainable under
similar conditions or at elevated temperatures. The primary
amine (R)-1-phenylethanamine gave only coordination product
(R)-4 and unidentifiable compounds at room temperature and
60 °C, respectively (Scheme 2). In the case of the secondary
amine (R)-N-methyl-1-phenylethan-1-amine ((R)-5), the race-
mic imino-iridacycle 6 can be obtained as the major product. It
should also be mentioned that an additional optically active
orange oil-like substance, in significant amounts, could also be
obtained. However, further purification and characterization of
the fraction proved to be challenging.
Figure 6. 2D 1H−1H NOESY NMR spectrum of cycloiridated
complex (SC,SN,RIr)-2.
has adopted a single configuration, affirming that a structural
lock exists between the methyl substituent at the carbon
stereogenic center and its neighboring naphthalene proton. In
addition, the spectrum exhibited spatial interactions between
the C-chiral methyl substituent, as well as the N-chiral methyl
group, with the large cyclopentadienyl methyl moiety, as shown
by cross peaks Hc−HCpMe and Hd−HCpMe respectively.
Furthermore, the lack of NOE interactions of Hb and He with
the bulky Cp* ring system detailed the absence of
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Scheme 2. Attempted Direct Cycloiridation of Optically
Active 1-Phenylethylamine Derivatives
Scheme 3. Cycloiridation of N-Methyl-1-
naphthalenemethylamine (7)
a
a
a
Reaction conditions: (i) [IrCp*Cl2]2, NaOAc, 1,2-dichloroethane, 60
°C.
cycloiridation of the imine derivative, since the oxidation of the
naphthalene-based amine moiety was not observed during the
process.
a
Reaction conditions: (i) [IrCp*Cl2]2, NaOAc, CH2Cl2, room
Table 1. Selected Bond Lengths (Å) and Bond Angles (deg)
of Structurally Analogous Cycloiridated Complexes
temperature.
Oxidation of the amine moiety during the process of
cyclometalation is not unheard of and has been observed by
Pfeffer and co-workers during their cycloiridation procedure.15
The stepwise oxidation−cyclometalation process presents one
of the methods for obtaining these imino-metallacycles. Stirling
has reported the racemization of enantiopure amines by an
iridium(III) Cp* dihalogen complex through an imino
intermediate, although it tends to proceed much more slowly
in chloro complexes than in its bromo counterparts.16 The
resultant imine could then undergo ortho-metalation to afford
the cycloiridated imine species. Alternatively, a cyclometala-
tion−oxidation protocol could take place. Feringa and de Vries
have documented the self-oxidation of a cationic (κ2-C,N)-N-
methylbenzylamine cycloiridated system to its imino variant in
solution.13b
It is important to note that Ikariya and Kayaki have recently
synthesized various chiral cycloiridated benzylic amino
complexes.6b As such, we questioned the difference in
reactivities between the amines of Ikariya and Kayaki with
those of ours. It is known that the cyclopalladated equivalents
of the former amines were conformationally rigid through
stereogenic lock in both solid and solution phases.5b,17 Noting
that our structurally locked iridacycles (SC,SN,RIr)-2 and
(SC,RIr)-3 can be isolated in their optically active forms, we
believed that the locking mechanism must have hindered the
oxidative process. To provide the cycloiridated imine complex,
the coordinated or cycloiridated amine would need to adopt a
planar intermediary structure. However, due to the immense
steric crowding about the metal center, the C-chiral methyl
substituent-adjacent naphthalene proton interaction may have
prevented the oxidation of the amine. We proceeded to verify
the concept utilizing N-methyl-1-naphthalenemethylamine (7),
a naphthyl-CH2-amine for which its cyclopalladated species is
not known to be conformationally rigid, for cycloiridation
(Scheme 3). Consistent with our hypothesis, the reaction
afforded the aldimino complex rac-8 in good yield.
bond length (Å)
N−Ir
CAr−Ir
Cl−Ir
(SC,SN,RIr)-2
(SC,RIr)-3
rac-9
2.151
2.122
2.137
2.040
2.060
2.425
2.431
2.418
2.032
bond angle (deg)
N−Ir−CAr
N−Ir−Cl
CAr−Ir−Cl
(SC,SN,RIr)-2
(SC,RIr)-3
rac-9
76.7
76.7
76.4
82.5
82.5
83.5
86.6
86.8
88.5
Assessment of the optically active secondary amine iridacycle
(SC,SN,RIr)-2 showed a slightly longer N−Ir bond in
comparison to the primary amino analogue (SC,RIr)-3. Despite
the difference in electronic densities, it seems that steric
crowding within the complex limits the σ donation of the lone
electron pair from the electronically richer secondary amine
ligand to the electron-deficient iridium. The data also revealed a
significantly larger dihedral angle between Cstereogenic center−N
and the aromatic ring axes for (SC,SN,RIr)-2 in comparison to
the primary amine counterpart (28.43° vs 19.15°). This was
despite smaller deviation to the N−Ir and CAr−Ir bond lengths,
and N−Ir−CAr and CAr−Ir−Cl bond angles.
With reference to the cycloiridated imine complex rac-9, the
N−Ir bond length was found to lie between the corresponding
bond distances in (SC,SN,RIr)-2 and (SC,RIr)-3. This further
instills the idea of steric crowding within (SC,SN,RIr)-2, since the
imino moiety is known to be less basic than the amine
functionality. Spatially, it is obvious that the imine ligand
framework presents the least steric crowding within the
complex. This was supported by the small dihedral angle of
13.92° and the larger bond angles about the iridium center.
Finally, on comparison of the cycloiridated primary amine
complex (SC,RIr)-3 with its structurally analogous ruthenium
Structural Analysis of the Pseudotetrahedral Iridium-
(III) Complexes. To gain a further understanding of the steric
and electronic properties of the synthesized iridium(III)
complexes, we prepared the analogous imine iridacycle for
comparison in the solid state. It is important to reiterate that
the imino complex has to be synthesized from the direct
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counterparts,18 the iridacycle is consistent in terms of
conformation. The structurally-locked ruthenacycles also
exhibited a static λ conformation with the stereogenic carbon
center at the axial position in both the solid and solution phase.
Unsurprisingly, when the naphthalene moiety was traded for a
phenyl substituent, the 1,3-diaxial η6-aromatic−methyl inter-
action within the ruthenacycle pushed for a more stable δ
conformation in the solid phase.19 Consequently, a rapid
interchanging conformational ring flip could occur within the
puckered ring system on solvation.
The analysis, thus, presents plausible explanations to the
diastereospecificity of the ortho-iridated complex (SC,SN,RIr)-2.
With the orientation of its C-chiral methyl group already
defined by the amine, the ligand framework would have to
conform itself to its most sterically favorable configuration
within the three-legged piano-stool Cp*-iridium(III) complex
upon cyclometalation. Such diastereoselectivity has also been
observed in similar sterically demanding complexes with planar
chirality within its framework.6c,20
The VT NMR studies revealed no visible epimerization at
both low and elevated temperatures. The spectroscopic handle
remained as a singlet during both cooling and heating
processes. Furthermore, the multiplities of the other resonance
signals generally remained unchanged with only slight deviation
in chemical shifts.
Unlike the fixed stereogenic center at carbon, isomerization
at the nitrogen center is probable within the complex due to
nitrogen inversion upon decoordination. Furthermore, due to
the acidic nature of the proton on the amine moiety, it could
isomerize if a sufficiently strong base is present. As such, we
aimed to determine if isomerization would take place at the
nitrogen atom when a deprotonation−protonation protocol
was effected. Iridacycle (SC,SN,RIr)-2 was subjected to
deprotonation with potassium tert-butoxide in d2-dichloro-
methane at room temperature under an inert atmosphere
(Scheme 4). Upon complete conversion to the amido
Scheme 4. Experiment on the Stability of Stereochemistry at
a
the Nitrogen Center
Stability of Stereochemistry within Cycloiridated
Complex (SC,SN,RIr)-2. With three stereogenic centers that
can be generated diastereoselectively, the ortho-iridated
complex (SC,SN,RIr)-2 could present a variety of potential
applications ranging from asymmetric catalysis and syntheses to
therapeutic biological agents.7,21
As such, it is imperative to investigate the stability of the
stereogenic centers upon chemical transformation.1a
a
Reaction conditions: (i) KOtBu, CD2Cl2; (ii) HCl(aq), room
As with various pseudotetrahedral cycloiridated complexes,
epimerization of the cycloiridated compound is not uncom-
mon,13 albeit less likely due to the fixed C-chiral center in
complex (SC,SN,RIr)-2. Nonetheless, we proceeded with NMR
studies to analyze the possible epimerization of the iridacycle in
solution.
temperature.
intermediate (S)-10, the dark red mixture was hydrolyzed
with aqueous hydrochloric acid to return iridacycle (SC,SN,RIr)-
2. Spectroscopic analysis revealed no isomerization of the
hydrolyzed complex, offering a spectrum similar to that of pure
(SC,SN,RIr)-2. Analysis of the compound’s optical activity also
returned a similar specific optical rotation ([α]D(21 °C, c 0.1) =
+106.8°), indicating that the complex remained structurally
unaltered.
In another experiment, the stability of the chiral center at
iridium was explored. It is postulated that an exchange of ligand
at the metal center must undergo a dissociative mechanism,
since the 18-electron species is coordinatively saturated. The
replacement of a chloride ligand with a phosphine moiety
would also provide a spectroscopic handle for the determi-
nation of epimerization by 31P{1H} NMR. The ligated chloride
in cycloiridated complex rac-2 was first abstracted with silver
hexafluorophosphate in acetonitrile at room temperature
(Scheme 5). Triphenylphosphine was subsequently added to
the pale yellow mixture, with its crude 31P{1H} NMR spectrum
revealing a singlet at 16.0 ppm, indicative of only one
At room temperature, the stereochemistry of the complex is
stable, as indicated by its 1H NMR spectrum. However, it is not
known if the compound could isomerize when it is subjected to
heat. The complex was dissolved in 1,2-dichloroethane (bp 83
°C) and placed under reflux conditions over a period of 5 days.
The resulting red-brown solution was subsequently evaporated
1
to dryness prior to analysis via H NMR spectroscopy. When
the methyl substituents on the Cp* ligand were used as a
spectroscopic handle for the determination of isomeric ratio,
the resultant solution revealed only one Cp* resonance signal
at 1.67 ppm, offering no indication of isomerization. To further
investigate possible stereochemical inversion within the
molecule during the heating process, variable-temperature
(VT) NMR studies were performed in d-chloroform (bp 61
°C; mp −64 °C) at high and low temperatures (Figure 8).
−
coordinated triphenylphosphine complex, and a PF6 septet
resonance signal at −144.3 ppm. The phosphino complex rac-
Scheme 5. Experiment on the Stability of Stereochemistry at
the Iridium Center during Ligand Exchange
Figure 8. Variable temperature (VT) 1H NMR studies on cyclo-
iridated complex (SC,SN,RIr)-2 from −60 to 60 °C. The chemical shift
range from 4.7 to 8.0 ppm has been magnified for clarity.
E
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11 was subsequently recrystallized, and its molecular structure
and relative stereochemistry were confirmed by single-crystal X-
ray crystallography (see the molecular structure of rac-11 in the
Supporting Information).
Table 2. Asymmetric Hydrogen Transfer Reaction of
Acetophenone by Chiral Cycloiridated Complexes
a
Effects of Structure on Catalysis: Application in
Asymmetric Hydrogen Transfer Reaction. With the
knowledge that the stereogenic lock within the naphthalene-
based cycloiridated complexes are active under a broad range of
temperatures and reaction conditions, we proceeded to
examine its effectiveness in the transmission of stereogenic
information to the substrate in asymmetric applications.
Considering that many pseudotetrahedral metallacycle cata-
lyzed reactions involved asymmetric hydrogen transfer
reactions,22 a model catalytic reduction was employed for the
study.
The transfer hydrogenation protocol was carried out with
acetophenone as the substrate and 2-propanol as the reducing
agent. The optically active cycloiridated precursors comprised
of complexes (SC,SN,RIr)-2 and (SC,RIr)-3, which then can be
readily deprotonated with potassium tert-butoxide to provide
the active amido species. The initial run of the catalytic reaction
was performed at −15 °C, and its activity profile is shown in
Figure 9.
b
bc
,
entry
1
temp (°C)
time (h)
0.25
conversn (%)
ee (%)
20
20
99
75
96
97
+52
+58
+59
+69
d
e
2
96
3
4
−15
−30
1
1.5
a
Reactions were carried out with acetophenone (0.5 mmol), 1,4-
dimethoxybenzene (0.5 mmol), (SC,SN,RIr)-2 (0.01 mmol), and
KOtBu (0.05 mmol) in iPrOH at the desired temperature for the
stated duration. Conversion and ee were determined by HPLC
notation was determined by optical
activity studies. Reaction was conducted without KOtBu. Reactions
b
c
(Diacel Chiralpak IF column).
d
e
were halted after 96 h reaction time.
(52%; entry 1). Finally, at lower temperature (−30 °C), the
reaction remained efficient with a slight increase in
enantioselectivity to 69%, in comparison to its counterpart at
−15 °C (59%).
CONCLUSION
■
A series of pseudotetrahedral cycloiridated 1-naphthylethyl-
amine complexes were synthesized, characterized, and studied
for their structure and stereoselectivity in the catalytic
asymmetric hydrogen transfer reaction. The investigation
revealed the relevance of a stereogenic lockdefined by the
fixation of configuration within a small ring system through
steric interaction within the molecular frameworkin both
structural aspects, in terms of diastereoselectivity and stereo-
stability, and enantioselectivity in asymmetric catalysis.
Furthermore, we reviewed the practicality for structural lock
to avoid side reactions within the complex which may render
the enantiopure compound less beneficial for asymmetric
applications.
Figure 9. Conversion (blue) and enantiomeric excess (orange)
profiles of the asymmetric hydrogen transfer reaction of acetophenone
by chiral cycloiridated complexes at −15 °C.
In terms of activity, both complexes performed outstandingly
with the hydrogen transfer protocol approaching near
completion (ca. 90% conversion) after 30 min, although the
reactivities decrease exponentially with time after the initial
burst at the beginning. Both compounds displayed similar
reactivity with their conversion of acetophenone within 4% of
each other at any point of time. With regard to its
enantioselectivity, the catalysts only gave modest enantiomeric
excess (ee) at best, with the primary amino-iridacycle achieving
the highest measured ee after 15 min at 65%, although the
reaction was only moderately complete (78%). Expectedly, the
enantioselectivity declined with time due to the continuous
oxidation−reduction processes within the reaction vessel,
reaching complete racemization overnight for both complexes.
We proceeded to vary the reaction conditions with complex
(SC,SN,RIr)-2 as the catalyst. At room temperature, the transfer
hydrogenation protocol progressed extremely efficiently, with
the reaction approaching complete conversion within 15 min,
albeit with a slight reduction in enantioselectivity. In the
absence of base, the reaction was found to proceed, although its
reactivity was greatly hindered, being incomplete even after 4
days of reaction time at room temperature. Nonetheless, the
entry gave an ee similar (Table 2, 58%; entry 2) to that of the
amido complex mediated reaction at the same temperature
Through this work, we aimed to establish the concepts of
stereogenic lock which could be applied to the rational design
of chiral catalysts and templates for asymmetric applications.
We have also demonstrated the relevance of conformational
rigidity in the outcome of asymmetric applications.
EXPERIMENTAL SECTION
■
General Considerations. Reactions involving air- or moisture-
sensitive compounds were carried out by means of conventional
Schlenk techniques under a positive pressure of nitrogen gas. Unless
stated otherwise, chemicals and solvents were used as received from
commercial vendors without further purification. When necessary,
anhydrous solvents were freshly distilled and dried according to
standard procedures prior to use. Nuclear magnetic resonance (NMR)
spectra were recorded on a Bruker Avance III 400 (BBFO 400)
1
spectrometer at 400 MHz for H NMR, 100 MHz for 13C NMR, and
161 MHz for 31P{1H} NMR. Chemical shifts (δ) are quoted in ppm
and referenced to chemical shifts of residual solvent peaks. High-
resolution mass spectrometry via electrospray ionization (ESI) was
performed on the Waters Q-Tof Premier spectrometer. Elemental
analysis was performed on the EuroVector Euro EA elemental analyzer
at Nanyang Technological University Division of Chemistry and
Biological Chemistry Central Facilities Laboratory. Optical rotation
studies were measured on the Jasco P-1030 polarimeter in a 0.1 dm
F
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Article
polarimetry cell at the specified temperature using the D line of
sodium (589 nm) as the source of light.
HCl (1 M, 1.5 mL) was added to the dark red solution, and this
mixture was stirred for 1 h. The organic phase of the biphasic solution
was collected, and the aqueous partition was further extracted with
dichloromethane. The organic extracts were then combined, dried over
anhydrous MgSO4 ,and evaporated to dryness, affording an orange
powder.
General Procedure for the Cycloiridation of 1-Arylalkyl-
amines. [Cp*IrCl2]2 (40 mg, 0.05 mmol) and NaOAc (10 mg, 0.12
mmol) were added to a stirred solution of 1-arylalkylamine (0.10
mmol) in dichloromethane (3 mL). The reaction mixture was stirred
at room temperature until full conversion or over 3 days, whichever
was shorter. The crude reaction mixture was then evaporated to
dryness and purified by column chromatography on silica gel with
hexane/ethyl acetate as the eluent.
(SC)-(1,2,3,4,5-Pentamethylcyclopentadienyl){(κ2-C,N)-1-[1-(N-
methylamido)ethyl]naphthyl}iridium(III) complex ((SC)-10): 1H
3
NMR (400 MHz, CD2Cl2) δ 1.45 (d, JHH = 6.4 Hz, ArCH(CH3),
3
1.94 (s, 15H, Cp(CH3)), 2.80 (q, JHH = 6.4 Hz, ArCH(CH3)), 3.48
(d, 4JHH = 0.6 Hz, 3H, N(CH3)), 7.19−8.19 (m, 6H, ArH); 13C NMR
(100 MHz, CD2Cl2) δ 10.00, 19.96, 54.57, 82.01, 87.57, 122.65,
124.12, 124.78, 124.89, 127.46, 128.25, 130.89, 135.10, 155.37, 160.26.
Procedure for the Determination of Stereostability at
Iridium. AgPF6 (7.6 mg, 0.03 mmol) was added to a stirred
acetonitrile solution (3 mL) of complex (SC,SN,RIr)-2 (11 mg, 0.02
mmol) in the dark. The reaction mixture was stirred at room
temperature for 1 h. PPh3 (5.2 mg, 0.02 mmol) was subsequently
added to the reaction mixture, and this mixture was stirred for another
2 h. The crude mixture was filtered over Celite, evaporated to dryness,
and recrystallized from a solvent mixture of hexane and ethyl acetate.
rac-(1,2,3,4,5-pentamethylcyclopentadienyl){(κ2-C,N)-1-[1-(N-
methylamino)ethyl]naphthyl}(triphenylphosphine)iridium(III) Hexa-
(SC,SN,RIr)-(1,2,3,4,5-Pentamethylcyclopentadienyl){(κ2-C,N)-1-[1-
(N-methylamino)ethyl]naphthyl}iridium(III) chloride ((SC,SN,RIr)-2):
1
yield, 53 mg (97%); [α]D(22 °C, c 0.5) = +102.1°; H NMR (400
3
MHz, CD2Cl2) δ 1.24 (d, JHH = 6.5 Hz, 3H, ArCH(CH3)), 1.67 (s,
3
15H, Cp(CH3)), 3.02 (d, JHH = 6.4 Hz, 3H, NH(CH3)), 4.69 (dq,
3JHH = 4.2 and 6.4 Hz, 1H, ArCH(CH3)), 5.02 (br s, 1H, NH(CH3)),
7.20−7.74 (m, 6H, ArH); 13C NMR (100 MHz, CD2Cl2) δ 9.36,
17.39, 39.57, 66.89, 87.35, 122.66, 123.41, 125.41, 126.05, 127.92,
128.38, 131.07, 136.07, 144.65, 153.10; HRMS (ESI) m/z [negative
mode] calcd for C23H29ClIrN 547.1618, found 547.1619. Anal. Calcd
for C23H29ClIrN: C, 50.49; H, 5.34; N, 2.56. Found: C, 50.02; H, 5.56;
N, 2.31.
(SC,RIr)-(1,2,3,4,5-Pentamethylcyclopentadienyl){(κ2-C,N)-1-[1-
aminoethyl]naphthyl}iridium(III) chloride ((SC,RIr)-3): yield, 11 mg
(2%) (from N-demethylation of secondary amine at 1 mmol scale), 48
mg (90%) (from direct cycloiridation of primary amine); [α]D(22 °C,
1
fluorophosphate (rac-11): yield, 9.7 mg (53%) (recrystallized); H
NMR (400 MHz, CDCl3) δ 1.14 (d, 3JHH = 6.6 Hz, 3H, ArCH(CH3)),
1.65 (d, 15H, Cp(CH3)), 2.94 (d, 3JHH = 6.3 Hz, 3H, NH(CH3)), 3.92
(m, 1H, ArCH(CH3)), 4.38 (br s, 1H, NH(CH3)), 7.11−7.74 (m,
21H, ArH); 13C NMR (100 MHz, CDCl3) δ 9.48, 16.83, 41.15, 68.86,
96.15 (d, Cp(CH3)), 123.47, 123.99, 126.32, 126.54, 128.27, 128.49,
1
3
c 0.5) = +110.3°; H NMR (400 MHz, CD2Cl2) 1.32 (d, JHH = 6.6
Hz, 3H, ArCH(CH3)), 1.74 (s, 15H, Cp(CH3)), 3.70 (br s, 1H, NH),
4.78 (br s, 1H, NH), 5.06 (m, 1H, ArCH(CH3)), 7.21−7.74 (m, 6H,
ArH); 13C NMR (100 MHz, CD2Cl2) δ 10.77, 23.95, 60.92, 88.44,
124.08, 124.93, 126.82, 127.39, 129.61, 129.82, 132.59, 137.72, 147.15,
155.07; HRMS (ESI) m/z [negative mode] calcd for C22H27ClIrN
533.1461, found 533.1456. Anal. Calcd for C22H27ClIrN: C, 49.56; H,
5.10; N, 2.63. Found: C, 49.46; H, 5.29; N, 2.79.
1
128.92 (d, JCP = 10.2 Hz), 131.20, 132.01, 133.93 (br s), 134.44 (d,
4JCP = 2.4 Hz), 136.79, 136.91, 146.69; 31P{1H} NMR (161 MHz,
−
CDCl3) δ −144.32 (sep, PF6 ), 16.03 (s, PPh3); HRMS (ESI) m/z
[negative mode] calcd for C41H44F6IrNP2 919.2483, found 919.2482.
Anal. Calcd for C41H44F6IrNP2: C, 53.59; H, 4.83; N, 1.52. Found: C,
53.39; H, 4.78; N, 1.29.
(1,2,3,4,5-Pentamethylcyclopentadienyl){(κ2-C,N)-1-[1-(N-
methylimino)ethyl]phenyl}iridium(III) chloride (rac-6): yield, 21 mg
(42%); 1H NMR (400 MHz, CD2Cl2) δ 1.67 (s, 15H, Cp(CH3)), 2.50
(s, 3H, ArC(CH3)), 3.84 (s, 3H, N(CH3)), 6.95−7.76 (m, 4H, ArH);
13C NMR (100 MHz, CD2Cl2) δ 8.99, 14.65, 45.09, 88.67, 121.07,
127.34, 130.77, 135.10, 148.18, 167.89, 180.04; HRMS (ESI) m/z
[negative mode] calcd for C19H25ClIrN 495.1305, found 495.1292.
Anal. Calcd for C19H25ClIrN: C, 46.10; H, 5.09; N, 2.83. Found: C,
45.62; H, 5.38; N, 2.83.
General Procedure for Catalytic Asymmetric Hydrogen
Transfer Reactions. Acetophenone (58.3 μL, 60 mg, 0.5 mmol),
1,4-dimethoxybenzene (69 mg, 0.5 mmol), and KOtBu (2.8 mg, 0.025
mmol) were added sequentially to a stirred solution of cycloiridated
complex (0.01 mmol) in dry 2-propanol (10 mL) at the desired
temperature. The reaction mixture was stirred at the temperature for
the indicated time. The pale brown mixture was then diluted with
water (10 mL), extracted with ethyl acetate (10 mL × 3), washed with
brine (20 mL), dried over anhydrous MgSO4, filtered, and evaporated
to dryness. The crude mixture was then purified by column
chromatography on silica gel with hexane/ethyl acetate as the eluent
to afford a colorless oil.
(1,2,3,4,5-Pentamethylcyclopentadienyl){(κ2-C,N)-1-[(N-
methylimino)methyl]naphthyl}iridium(III) chloride (rac-8): yield, 32
mg (61%) (heating to 60 °C in 1,2-dichloroethane required); 1H
NMR (400 MHz, CD2Cl2) δ 1.76 (s, 15H, Cp(CH3)), 3.99 (s, 3H,
N(CH3)), 7.32−8.10 (m, 6H, ArH), 9.07 (s, 1H, HC(N)); 13C
NMR (100 MHz, CD2Cl2) δ 9.05, 49.71, 89.57, 121.79, 123.35,
126.87, 128.82, 129.95, 130.55, 132.81, 134.33, 139.61, 171.64, 174.53;
HRMS (ESI) m/z [negative mode] calcd for C22H25ClIrN 531.1305,
found 531.1297. Anal. Calcd for C22H25ClIrN: C, 49.75; H, 4.74; N,
2.64. Found: C, 49.59; H, 5.21; N, 2.91.
1
3
1-Phenylethanol: H NMR (400 MHz, CDCl3) δ 1.49 (d, JHH
=
6.4 Hz, 3H, PhCH(CH3)), 1.88 (d, 3JHH = 3.4 Hz, 1H, OH), 4.89 (dq,
3JHH = 3.3 Hz, JHH = 6.4 Hz, 1H, PhCH(CH3)), 7.25−7.38 (m, 5H,
3
ArH); 13C NMR (100 MHz, CDCl3) δ 25.12, 70.39, 125.35, 127.45,
128.48, 145.78; HRMS (ESI) m/z [negative mode] calcd for C8H10O
112.0732, found 122.0726. Anal. Calcd for C8H10O: C, 78.65; H, 8.25.
Found: C, 78.69; H, 8.44.
(1,2,3,4,5-Pentamethylcyclopentadienyl){(κ2-C,N)-1-[1-(N-
methylimino)ethyl]naphthyl}iridium(III) chloride (rac-9): yield, 22
mg (41%) (cis/trans mixture of N-methyl-1-(naphthalen-1-yl)ethan-1-
imine was used); 1H NMR (400 MHz, CDCl3) δ 1.73 (s, 15H,
Cp(CH3)), 2.98 (s, 3H, C(NMe) (CH3)Ar)), 3.98 (s, 3H, NCH3),
7.32−8.22 (m, 6H, ArH); 13C NMR (100 MHz, CDCl3) δ 9.16, 20.65,
45.68, 89.34, 122.11, 122.46, 125.92, 129.57, 130.95, 131.17, 132.80,
134.49, 140.83, 174.92, 180.39; HRMS (ESI) m/z [negative mode]
calcd for C23H27ClIrN 545.1461, found 545.1451. Anal. Calcd for
C23H27ClIrN: C, 50.68; H, 4.99; N, 2.57. Found: C, 50.50; H, 5.07; N,
2.73.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00760.
Preparative methods for the ligands, spectral data of
synthesized compounds, and X-ray crystallographic data
of compounds (SC,SN,RIr)-2, (SC,RIr)-3, rac-9, and rac-11
(PDF)
Procedure for the Determination of Stereostability at
Nitrogen. KOtBu (3.4 mg, 0.03 mmol) was added to a stirred
mixture of complex (SC,SN,RIr)-2 (11 mg, 0.02 mmol) in d2-
dichloromethane (1.5 mL). The reaction mixture was stirred at
room temperature for 24 h. The crude reaction mixture was then
Accession Codes
CCDC 1577410−1577411, 1578300, and 1578840 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
1
filtered and characterized by H NMR spectroscopy. Dilute aqueous
G
DOI: 10.1021/acs.organomet.7b00760
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Giardiello, M.; Hilton, S. T.; Russell, D. R. Dalton Trans. 2003, 4132−
4138.
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.
(7) (a) Jiang, X.; Tang, W.; Xue, D.; Xiao, J.; Wang, C. ACS Catal.
2017, 7, 1831−1835. (b) Wang, C.; Xiao, J. Chem. Commun. 2017, 53,
3399−3411. (c) Michon, C.; MacIntyre, K.; Corre, Y.; Agbossou-
Niedercorn, F. ChemCatChem 2016, 8, 1755−1762. (d) Matsunami,
A.; Kuwata, S.; Kayaki, Y. ACS Catal. 2016, 6, 5181−5185. (e) Corre,
Y.; Iali, W.; Hamdaoui, M.; Trivelli, X.; Djukic, J. P.; Agbossou-
Niedercorn, F.; Michon, C. Catal. Sci. Technol. 2015, 5, 1452−1458.
(f) Haak, R. M.; Berthiol, F.; Jerphagnon, T.; Gayet, A. J. A.;
Tarabiono, C.; Postema, C. P.; Ritleng, V.; Pfeffer, M.; Janssen, D. B.;
Minnaard, A. J.; Feringa, B. L.; de Vries, J. G. J. Am. Chem. Soc. 2008,
130, 13508−13509.
(8) (a) Albrecht, M. Chem. Rev. 2010, 110, 576−623. (b) Vicente, J.;
Saura-Llamas, I. Comments Inorg. Chem. 2007, 28, 39. (c) Dunina, V.
V.; Zalevskaya, O. A.; Potapov, V. M. Russ. Chem. Rev. 1988, 57, 250−
269. (d) Bruce, M. I. Angew. Chem., Int. Ed. Engl. 1977, 16, 73−86.
(9) Chen, H. J.; Teo, R. H. X.; Wong, J.; Li, Y.; Pullarkat, S. A.;
Leung, P.-H. Hindrance to Cyclometalation − A Study on N-
Demethylation Triggered from ortho-Metalation of Sterically-
Hindered Ligands. Manuscript in preparation.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for P.-H.L.: pakhing@ntu.edu.sg.
ORCID
Houguang Jeremy Chen: 0000-0001-5384-2576
Pak-Hing Leung: 0000-0003-3588-1664
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Ministry of Education
Academic Research Fund (AcRF-RG108/15). We are also
grateful to Nanyang Technological University for Ph.D.
scholarships to H.J.C. and R.H.X.T.
(10) Arita, S.; Koike, T.; Kayaki, Y.; Ikariya, T. Organometallics 2008,
27, 2795−2802.
(11) (a) Jung, M. E.; Piizzi, G. Chem. Rev. 2005, 105, 1735−1766.
(b) Beesley, R. M.; Ingold, C. K.; Thorpe, J. F. J. Chem. Soc., Trans.
1915, 107, 1080−1106.
REFERENCES
■
(1) (a) Malcolmson, S. J.; Meek, S. J.; Sattely, E. S.; Schrock, R. R.;
Hoveyda, A. H. Nature 2008, 456, 933−937. (b) Jiang, C.; Li, Y.; Tian,
Q.; You, T. J. Chem. Inf. Comput. Sci. 2003, 43, 1876−1881.
(2) (a) Yu, J.; Long, J.; Yang, Y.; Wu, W.; Xue, P.; Chung, L. W.;
Dong, X.-Q.; Zhang, X. Org. Lett. 2017, 19, 690−693. (b) Luo, S.;
Zhang, X.; Zheng, Y.; Harms, K.; Zhang, L.; Meggers, E. J. Org. Chem.
2017, 82, 8995−9005. (c) Sigman, M. S.; Miller, J. J. J. Org. Chem.
2009, 74, 7633−7643.
(12) (a) Yap, J. S. L.; Ding, Y.; Yang, X.-Y.; Wong, J.; Li, Y.; Pullarkat,
S. A.; Leung, P.-H. Eur. J. Inorg. Chem. 2014, 2014, 5046−5052.
(b) See Leng Yap, J.; Chen, H. J.; Li, Y.; Pullarkat, S. A.; Leung, P.-H.
Organometallics 2014, 33, 930−940.
(13) (a) Arthurs, R. A.; Horton, P. N.; Coles, S. J.; Richards, C. J. Eur.
J. Inorg. Chem. 2017, 2017, 229−232. (b) Jerphagnon, T.; Gayet, A. J.
̌ ́
A.; Berthiol, F.; Ritleng, V.; Mrsic, N.; Meetsma, A.; Pfeffer, M.;
Minnaard, A. J.; Feringa, B. L.; de Vries, J. G. Chem. - Eur. J. 2009, 15,
12780−12790.
(3) (a) Wild, S. B. Coord. Chem. Rev. 1997, 166, 291−311.
(b) Alcock, N. W.; Hulmes, D. I.; Brown, J. M. J. Chem. Soc., Chem.
Commun. 1995, 395−397. (c) Valk, J.-M.; Claridge, T. D. W.; Brown,
J. M.; Hibbs, D.; Hursthouse, M. B. Tetrahedron: Asymmetry 1995, 6,
2597−2610. (d) Alcock, N. W.; Brown, J. M.; Hulmes, D. I.
Tetrahedron: Asymmetry 1993, 4, 743−756.
(4) Selected publications on asymmetric Claisen rearrangement
reactions and carbon−carbon (C−C) bond formations: (a) Leung, P.-
H.; Ng, K.-H.; Li, Y.; White, A. J. P.; Williams, D. J. Chem. Commun.
1999, 2435−2436. (b) Wong, J.; Gan, K.; Chen, H. J.; Pullarkat, S. A.
Adv. Synth. Catal. 2014, 356, 3391−3400. Reviews on asymmetric [4
+ 2]-cycloaddition and hydrophosphination reactions: (c) Chew, R. J.;
Leung, P.-H. Chem. Rec. 2016, 16, 141−158. (d) Leung, P.-H. Acc.
Chem. Res. 2004, 37, 169−177.
(14) Sloan, T. E. Stereochemical Nomenclature and Notation in
Inorganic Chemistry. In Topics in Stereochemistry. John Wiley & Sons,
Inc 2007, 12, 1−36 and references therein.
(15) Barloy, L.; Issenhuth, J.-T.; Weaver, M. G.; Pannetier, N.; Sirlin,
C.; Pfeffer, M. Organometallics 2011, 30, 1168−1174.
(16) Stirling, M. J.; Mwansa, J. M.; Sweeney, G.; Blacker, A. J.; Page,
M. I. Org. Biomol. Chem. 2016, 14, 7092−7098.
(17) Dunina, V. V.; Kazakova, M. Y.; Grishin, Y. K.; Malyshev, O. R.;
Kazakova, E. I. Russ. Chem. Bull. 1997, 46, 1321−1330.
(18) Sortais, J.-B.; Pannetier, N.; Holuigue, A.; Barloy, L.; Sirlin, C.;
Pfeffer, M.; Kyritsakas, N. Organometallics 2007, 26, 1856−1867.
(19) Attar, S.; Nelson, J. H.; Fischer, J.; de Cian, A.; Sutter, J.-P.;
Pfeffer, M. Organometallics 1995, 14, 4559−4569.
(5) Selected publications relating to stereogenic lock in cyclo-
palladated complexes: (a) Yap, J. S. L.; Li, B. B.; Wong, J.; Li, Y.;
Pullarkat, S. A.; Leung, P.-H. Dalton Trans. 2014, 43, 5777−5784.
(b) Dunina, V. V. Curr. Org. Chem. 2011, 15, 3415−3440. (c) Ding,
Y.; Zhang, Y.; Li, Y.; Pullarkat, S. A.; Andrews, P.; Leung, P.-H. Eur. J.
Inorg. Chem. 2010, 2010, 4427−4437. (d) Ding, Y.; Chiang, M.;
Pullarkat, S. A.; Li, Y.; Leung, P.-H. Organometallics 2009, 28, 4358−
4370. (e) Ding, Y.; Li, Y.; Zhang, Y.; Pullarkat, S. A.; Leung, P.-H. Eur.
J. Inorg. Chem. 2008, 2008, 1880−1891. (f) Ng, J. K. P.; Chen, S.; Tan,
G. K.; Leung, P.-H. Tetrahedron: Asymmetry 2007, 18, 1163−1169.
(6) (a) Navarro, M.; Smith, C. A.; Albrecht, M. Inorg. Chem. 2017,
56, 11688−11701. (b) Sato, Y.; Kayaki, Y.; Ikariya, T. Chem. - Asian J.
2016, 11, 2924−2931. (c) Arthurs, R. A.; Ismail, M.; Prior, C. C.;
Oganesyan, V. S.; Horton, P. N.; Coles, S. J.; Richards, C. J. Chem. -
Eur. J. 2016, 22, 3065−3072. (d) Sabater, S.; Baya, M.; Mata, J. A.
(20) Scheeren, C.; Maasarani, F.; Hijazi, A.; Djukic, J.-P.; Pfeffer, M.;
Zaric,
3345.
(21) (a) Zimbron, J. M.; Passador, K.; Gatin-Fraudet, B.; Bachelet,
́
S. D.; Le Goff, X.-F.; Ricard, L. Organometallics 2007, 26, 3336−
C.-M.; Plazuk, D.; Chamoreau, L.-M.; Botuha, C.; Thorimbert, S.;
̇
Salmain, M. Organometallics 2017, 36, 3435−3442. (b) Mukhopadhyay,
S.; Gupta, R. K.; Paitandi, R. P.; Rana, N. K.; Sharma, G.; Koch, B.;
Rana, L. K.; Hundal, M. S.; Pandey, D. S. Organometallics 2015, 34,
́
4491−4506. (c) Liu, Z.; Romero-Canelon, I.; Qamar, B.; Hearn, J. M.;
Habtemariam, A.; Barry, N. P. E.; Pizarro, A. M.; Clarkson, G. J.;
Sadler, P. J. Angew. Chem., Int. Ed. 2014, 53, 3941−3964.
(22) (a) Pannetier, N.; Sortais, J.-B.; Issenhuth, J.-T.; Barloy, L.;
Sirlin, C.; Holuigue, A.; Lefort, L.; Panella, L.; de Vries, J. G.; Pfeffer,
M. Adv. Synth. Catal. 2011, 353, 2844−2852. (b) Jerphagnon, T.;
Haak, R.; Berthiol, F.; Gayet, A. J. A.; Ritleng, V.; Holuigue, A.;
Pannetier, N.; Pfeffer, M.; Voelklin, A.; Lefort, L.; Verzijl, G.;
Tarabiono, C.; Janssen, D. B.; Minnaard, A. J.; Feringa, B. L.; de Vries,
J. G. Top. Catal. 2010, 53, 1002−1008.
́
Organometallics 2014, 33, 6830−6839. (e) Feghali, E.; Barloy, L.;
Issenhuth, J.-T.; Karmazin-Brelot, L.; Bailly, C.; Pfeffer, M. Organo-
metallics 2013, 32, 6186−6194. (f) Kashiwame, Y.; Kuwata, S.; Ikariya,
T. Organometallics 2012, 31, 8444−8455. (g) Djukic, J.-P.; Iali, W.;
Pfeffer, M.; Le Goff, X.-F. Chem. - Eur. J. 2012, 18, 6063−6078.
(h) Davies, D. L.; Al-Duaij, O.; Fawcett, J.; Singh, K. Organometallics
2010, 29, 1413−1420. (i) Davies, D. L.; Al-Duaij, O.; Fawcett, J.;
H
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