Development of Chiral C2-Symmetric N-Heterocyclic Carbene Rh(I) Catalysts through Control of Their Steric Properties

Article abstract of DOI:10.1021/acs.organomet.8b00823

Chiral square-planar Rh(I) complexes based on new C₂-symmetric NHC ligands have been synthesized selectively in a few steps as single diastereoisomers. These chiral precatalysts were applied to the asymmetric transfer hydrogenation of 1-phenylp

Full text of DOI:10.1021/acs.organomet.8b00823

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Development of Chiral C2‑Symmetric N‑Heterocyclic Carbene Rh(I)
Catalysts through Control of Their Steric Properties
†,‡
†,§
Marc-Antoine Abadie,†,‡ Kirsty MacIntyre,†,‡ Cedric Boulho, Peter Hoggan,†,‡ Frederic Capet,
́
́
́
Francine Agbossou-Niedercorn,†,‡ and Christophe Michon*,†,‡
†
́
Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unite de Catalyse et Chimie du Solide, F-59000
Lille, France
‡ENSCL, UCCS-CCM, (Chimie-C7) CS 90108, 59652 Villeneuve d’Ascq Cedex, France
§ENSCL, UCCS-CS, (Chimie-C7) CS 90108, 59652 Villeneuve d’Ascq Cedex, France
S
* Supporting Information
ABSTRACT: Chiral square-planar Rh(I) complexes based on
new C2-symmetric NHC ligands have been synthesized
selectively in a few steps as single diastereoisomers. These
chiral precatalysts were applied to the asymmetric transfer
hydrogenation of 1-phenylpropanone and to the 1,2-addition
of arylboronic acids to aldehydes. We demonstrated a proper
functionalization of the aromatic rings connected to the
nitrogen atoms of the NHC ligand improved significantly the
asymmetric induction of the chiral Rh(I) NHC catalysts. Bulky substituents allowed a better control of the steric features of the
catalyst quadrants because they behaved as conformational and chirality relays of the NHC chiral backbone.
not have a significant influence on a chemical reaction.
However, the conformation of the aromatic N-substituents can
be controlled provided the rotation around the C−N bond is
restricted by ortho substituents on the aryls (Figure 2).3
Following the pioneering work of Grubbs et al. (Figure 2)
for the first enantioselective ruthenium catalyzed olefin
metathesis in the desymmetrization of achiral trienes,3 chiral
NHC based catalysts have been developed for various organic
reactions using well-defined organometallic species4 or one-pot
procedures.5 In recent reports, steric fine-tuning of such chiral
NHC ligands has been investigated through a modular strategy
and proved to be determining. Indeed, Dorta et al. have
developed chiral NHC ligands substituted by naphthyl
wingtips for effective Pd catalyzed cross-couplings and Ir
catalyzed hydroamination reactions (Figure 2).4h−j,m At the
meantime, Plenio et al. have reported chiral NHC ligands
based on bulky triptycene are useful in the Cu catalyzed
enantioselective borylation of α,β-unsaturated esters (Figure
2).4k Herein, we report on the design and development of new
C2-symmetric NHC ligands based on a chiral diphenylethylene
diamine backbone (Figure 2). The resulting pure diastereo-
meric Rh(I) complexes were applied to the asymmetric
transfer hydrogenation of ketones and to the 1,2-addition of
arylboronic acids to aldehydes. Along our study, we
demonstrated a proper functionalization of the aromatic
rings connected to the nitrogen atoms improved the
asymmetric induction of the enantiomerically pure NHC
INTRODUCTION
■
N-Heterocyclic carbenes (NHCs) have been used as ligands in
transition metal complexes for several decades.1 Numerous
studies on synthesis, coordination, and reactivity of NHC
ligands resulted in significant applications in homogeneous
catalysis for various chemical transformations in racemic and
asymmetric series.2 During the past two decades, chiral
monodentate C2-symmetric N-heterocyclic carbene ligands
were prepared according to three strategies: the use of chiral
N-substituents (Figure 1, A), the synthesis of rigid tricyclic
NHCs (Figure 1, B), and the use of chiral amine backbones
(Figure 1, C).2
NHC ligands of type C draw a parallel with the
stereochemical model of the BINAP ligand. Because the
point chirality is far from the NHC coordination center, it does
Figure 1. General structures of chiral monodentate C2-symmetric N-
heterocyclic carbene (NHC) ligands.
Received: November 14, 2018
© XXXX American Chemical Society
A
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S1). The resulting secondary diamines 2a−e were isolated in
good to high yields (e.g., 66−98%) after purification by flash
chromatography. The molecular structures of 2a−e were
consistent with their elemental analyses, mass and NMR
spectra. The corresponding dihydroimidazolium salts 3a−e
were prepared as pure diastereoisomers in 69% to quantitative
yields by reacting amines 2a−e with excess of triethylortho-
formate and tetrafluoroborate ammonium (Scheme 1). Salts
3a−e were characterized by their elemental analyses, mass and
NMR spectra. The identifications of the CH units between the
N atoms are of particular significance in the cations 3a−e as
1
their H and 13C signals are observed, respectively, around δ
8.9 and 158 ppm, which is typical of such dihydroimidazolium
salts. Afterward, Rh(I) complexes 4a−e were prepared in two
steps. At first, pure diastereomeric salts 3a−e were allowed to
react with Ag(I) oxide in acetonitrile at 60 °C for 24 h
(Scheme 1). Second, the resulting Ag(I) complexes reacted
subsequently with [Rh(COD)Cl]2 (COD = 1,5-cycloocta-
diene) without any purification, through transmetalation in
dichloromethane for 64 h (Scheme 1). The resulting Rh(I)
complexes 4a−e were isolated as pure diastereoisomers in
average to high yields (56−90%) after purification by flash
chromatography and recrystallization. They were characterized
by their elemental analyses, mass and NMR spectra. The 13C
NMR resonances of the carbene carbon nuclei were observed
at δ 212−213 ppm with coupling constants J(Rh,C) of 46−47
Hz and confirmed the formation of the carbene complexes.
Because the rotation around the Rh-C axis was restricted by
the steric hindrance, 13C and 1H NMR spectra indicated all the
atoms were inequivalent. Single crystals of Rh(I) complex 4d
were obtained by slow evaporation of a chloroform/n-hexane
mixture. According to an X-ray diffraction analysis, complex 4d
crystallized in an orthorhombic crystal system with a P212121
space group and a Flack parameter of −0.045(12). The
ORTEP of 4d comprised two independent Rh complexes·and
one molecule of n-pentane (Figure 3, Figure S1, Table S1).
Each central Rh atom adopted a square planar coordination
Figure 2. Examples of catalysts based on monodentate C2-symmetric
NHC ligands with chiral amine backbones.
ligand. Because bulky substituents behaved as conformational
and chirality relays of the NHC chiral backbone, the use of
sterically demanding groups allowed a better control of the
steric features of the catalyst quadrants.6
RESULTS AND DISCUSSION
■
The NHC ligand synthesis started by a double Buchwald−
Hartwig amination reaction7 using (S,S)-diphenylethylenedi-
amine (DPEN) and one of the substituted 1-bromo-2-
methoxy-benzene 1a−e previously prepared8 (Schemes 1,
Scheme 1. Synthesis of the Dihydroimidazolium Salts 3a−e and the Rh(I) Complexes 4a−e
B
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further increase of the steric hindrance and led to secondary
alcohol 5 in high yield with an improved 60% ee (entry 7).
Hence, the results demonstrated the use of sterically
demanding substituents allowed a better control of the Rh
catalyst quadrants as they acted as conformational and chirality
relays of the chiral DPEN backbone. Finally, a slight increase of
ee was observed through the use of 10 mol % of tBuOK.
Indeed, it is well established that the transfer hydrogenation of
ketones depends on the nature and amount of the base
cocatalyst (entries 8−10).10c In order to check if catalysts were
decomposing along the reactions, Hg(0) trapping experiments
were performed (entries 3, 6). While decreases of yield and ee
were observed for catalyst 4b, the reaction was almost
quenched using catalyst 4d. At the end of the 5 hours of
reaction, analyses of the catalytic reaction mixtures by dynamic
light scattering (DLS) revealed the presence of particles in the
641 nm range with Hg(0) addition and around 769 nm
without (Figure S2). Though all of these results must be
interpreted with caution,13 they tend to suggest the molecular
catalyst may gradually decompose throughout this catalyzed
transfer hydrogenation to lead to inactive particles which
apparently aggregate during the reaction to give larger particles
(Figure S2).
Figure 3. ORTEP of compound (Sa,Sa,R,R)-4d·(0.5 n-pentane): at
the 50% probability level. One complex, one pentane molecule, and
hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Rh1−Cl1 = 2.3767(15), Rh1−C1 = 1.991(6), Rh1−C7
= 2.226(7), Rh1−C8 = 2.205(7), Rh1−C4 = 2.091(6), Rh1−C11 =
2.3767(15), N1−C1 = 1.344(7), N2−C1 = 1.354(8), C7−C8 =
1.354(10), C4−C11 = 1.384(9), N1−C1−N2 = 106.4(5), N1−C1−
Rh1 = 124.0(5), N2−C1−Rh1 = 129.6(4), C1−Rh1−Cl1 =
90.1(16). CCDC 1848614.9
geometry, being coordinated to the chloride, the 1,5-cyclo-
octadiene, and the carbene ligand without any crystallographic
mirror plane bisecting the molecule in the Cl-Rh-C(NHC)
plane (Figure 3). The measured distances of 1.991(6) Å
(Figure 3) and 2.007(6) Å (Figure S1) between the Rh and
carbon of the carbene proved to be similar to the 2.02−2.07 Å
usually measured for unsaturated10 and saturated11 carbene
ligands.
Afterward, chiral Rh(I) complexes 4a−e were applied as
precatalysts in another reference reaction, the asymmetric 1,2-
addition of phenylboronic acid to 1-naphthaldehyde (Table 2).
The isolated NHC Rh(I) complexes 4a−e were then applied
as precatalysts in two asymmetric organic reactions. At first, we
studied the asymmetric transfer hydrogenation of propiophe-
none to 1-phenylpropanol 5 as such a reaction was often
studied applying chiral NHC Rh(I) complexes as catalysts
(Table 1).10,12 While using 5 mol % of KOtBu as a base in
Table 2. Rh(I) Catalyzed 1,2-Addition of Phenylboronic
Acid to 1-Naphthaldehyde
Table 1. Rh(I) Catalyzed Transfer Hydrogenation of
Propiophenone to 1-Phenylpropanol 5
a
b
entry
precatalyst
base (equiv)
yield (%)
ee (%)
1
2
3
(Ra,Ra,S,S)-4a
(Ra,Ra,S,S)-4b
(Ra,Ra,S,S)-4c
(Ra,Ra,S,S)-4d
(Ra,Ra,S,S)-4e
(Ra,Ra,S,S)-4d
(Ra,Ra,S,S)-4d
(Ra,Ra,S,S)-4d
KF (6)
KF (6)
KF (6)
KF (6)
KF (6)
tBuOK (2)
AgF (2)
KF (2)
54
89
77
82
56
12
0
17
2
21
71
59
a
b
entry
precatalyst
base
yield (%)
ee (%)
c
4
1
2
3
(Ra,Ra,S,S)-4a
(Ra,Ra,S,S)-4b
(Ra,Ra,S,S)-4b
(Ra,Ra,S,S)-4c
(Ra,Ra,S,S)-4d
(Ra,Ra,S,S)-4d
(Ra,Ra,S,S)-4e
(Ra,Ra,S,S)-4d
(Ra,Ra,S,S)-4d
(Ra,Ra,S,S)-4d
tBuOK (5)
tBuOK (5)
tBuOK (5)
tBuOK (5)
tBuOK (5)
tBuOK (5)
tBuOK (5)
tBuOK (10)
tBuONa (10)
KOH (10)
47
80
65
6
99
7
95
99
8
8
13
3
5
6
7
8
c
4
5
6
84
45
49
a
b
Typical experiments are provided. Isolated yields. ee values
measured by HPLC. 71% yield and 71% ee in the presence of one
Hg(0) drop.
c
c
7
8
9
60
53
Indeed, the enantioselective addition of aryl nucleophiles to
aromatic aldehydes is a well-established route to synthesize
chiral diarylmethanols which are essential building blocks for
the synthesis of various biologically active and pharmacy
relevant compounds.14 Many antihistamines, antiarrhythmic,
and therapeutic agents feature chiral diarylmethanol and diaryl
carbinol derivatives as fundamental structural units. By
comparison to the several chiral phosphine based Rh(I)
complexes which were successfully applied to catalyze this
reaction,15 only few Rh species based on C2-symmetric NHC
have been used.16 Indeed, the selected reaction was reported to
be catalyzed by NHC Rh(I) complex bearing substituents with
planar16a,b or central chirality16c,d on the NHC nitrogen atoms
10
85
21
a
b
Typical experiments are provided. Yields measured by GC. ee
values measured by HPLC. In the presence of one Hg(0) drop.
c
iPrOH at 80 °C, the precatalysts 4a−b substituted,
respectively, by H and Me at the meta position of each 2-
MeO-aromatic unit led to average to good yields of secondary
alcohol 5 along with low enantiomeric excess (ee) (entries
1,2). If complex 4c bearing cyclohexyl substituents led to poor
results (entry 4), the use of complex 4d holding bulky tBu
groups afforded product 5 in a high yield with a 49% ee (entry
5). Complex 4e bearing adamantyl substituents allowed a
C
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Scheme 2. Reaction Scope of the Rh(I) Catalyzed 1,2-Addition of Boronic Acids to Aldehydes (Typical Experiments Are
Provided)
with modest to average ee values. Therefore, after a brief
optimization of the experimental conditions, our study was
started at 80 °C using a tBuOH/MeOH (5:1) solvent mixture
and KF as a base (Scheme S2, Table 2). Rh(I) complexes 4a−
c bearing, respectively, H, Me, and cyclohexyl substituents led
to secondary alcohol 6 in average to good yields but low
enantioselectivities (<21%) (entries 1−3). However, the use of
Rh(I) complex 4d carrying bulky tBu groups afforded 6 in a
high yield (82%) with a 71% ee (entry 4). An increase of the
steric hindrance through the use of complex 4e bearing
adamantyl substituents was here less effective as alcohol 6 was
retrieved in lower yield (56%) and ee (59%) (entry 5). The
use of a reduced amount of KF or even of other bases led to
significant decreases of reactivity and selectivity (entries 6−8).
At that stage, the positive role of a large excess of KF16 remains
unclear as it might act concomitantly as a base and as a ligand
through coordination of the fluoride anion to the Rh(I)
complex. In order to check if any decomposition was occurring
along the catalyzed 1,2-addition of phenylboronic acid to 1-
naphthaldehyde, Hg(0) trapping experiment was performed
using catalyst 4d (entry 4). The reaction yield was reduced by
10%, but no change was noticed for the ee. At the end of the
reactions, analyses of the unfiltered catalytic reaction mixtures
by dynamic light scattering (DLS) revealed the presence of
particles in the 454 nm range with Hg(0) addition and in the
568 nm range without (Figure S3). Because the catalytic
reaction was still running in the presence of Hg(0), the
observed particles were, apparently, not related to any active
Rh species and the molecular catalyst was rather stable
throughout this catalytic process.13
The reaction scope of the enantioselective 1,2-addition of
aryl nucleophiles to aldehydes was then investigated (Scheme
2). Similar to 1-naphthaldehyde, the reaction of heterocyclic
aldehydes with phenylboronic acid led to secondary alcohols
7−9 in low to good yields, ee values being average to good.
Though the cyclohexanecarboxaldehyde reacted in a similar
way to offer alcohol 10, the reaction of linear 3-phenylpropanal
afforded 11 in lower yield and ee. A similar trend was observed
for the reaction of alkyl or halide substituted benzaldehydes
with phenylboronic acid as alcohols 12−15 were obtained in
good yields but much lower enantioselectivities. Finally, the
reaction of 1-naphthaldehyde with fluoro- or methoxy-
substituted phenylboronic acid resulted in alcohols 16−17
with fair yields but low ee values. On the whole, hindered
substrates seemed to allow better asymmetric induction,
probably due to more efficient ligand−substrate interactions.
Hence, the enantioselectivity of such Rh(I) catalyzed 1,2-
addition of aryl nucleophiles to aldehydes proved to be highly
dependent on the reagent molecular structures.
CONCLUSIONS
■
Herein, we have reported the straightforward synthesis of
enantiomerically pure Rh(I) complexes based on new C2-
symmetric NHC ligands bearing a chiral diphenylethylene
diamine backbone. Applications to the asymmetric transfer
hydrogenation of ketones and the 1,2-addition of arylboronic
acids to aldehydes were subsequently investigated. Though the
enantioselectivity of the developed Rh(I) NHC catalysts
proved to depend on the substrate structure, we demonstrated
a proper functionalization of the aromatic rings connected to
D
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product was carried out through flash chromatography on silica gel
with a solid deposit and using petroleum ether and acetone solvent
mixtures with a 20% elution gradient. Due to the hygroscopic nature
of the resulting imidazolium salt, the latter was repeatedly dissolved in
dry toluene and then evaporated under vacuum. Such an azeotrope
treatment needed to be repeated 3 times in order to obtain a dry
product which required to be stored under nitrogen.
the carbene nitrogen atoms improved drastically the
asymmetric induction of the implied Rh(I) NHC catalysts.
Indeed, the presence of sterically demanding substituents
allowed a better control of the steric features of the catalyst
quadrants because these bulky groups behaved as conforma-
tional and chirality relays of the NHC chiral backbone. Further
applications of this concept will be reported in due course.
General Procedure for the Synthesis of Rh(I) Complexes.
Ag2O (2 equiv) was introduced to a Schlenk tube containing the dry
imidazolium compound and a magnetic stirring bar. After a drying
under vacuum of 30 min, CH3CN (7 mL, degassed using the freeze−
pump−thaw method) was transferred to the Schlenk under a nitrogen
flow, and the resulting mixture was stirred at 60 °C for 24 h in the
darkness. Afterward, [Rh(COD)Cl]2 (0.5 equiv) was dissolved in
CH2Cl2 (3 mL, degassed using the freeze−pump−thaw method) in a
second Schlenk tube and transferred to the reaction mixture
previously cooled at room temperature. The reaction was then
pursued in the darkness under stirring and a nitrogen flow for 64 h. At
the end, the reaction mixture was filtered over Celite with an ethyl
acetate wash. The resulting yellow solution was concentrated under
vacuum and purified by flash chromatography on neutral alumina with
a liquid deposit and using petroleum ether and acetone solvent
mixtures with a 15% elution gradient. After the corresponding
fractions were gathered and evaporated under vacuum, the resulting
yellow solid was recrystallized twice from dichloromethane and
pentane.
General Procedure for the Rh(I) Catalyzed Transfer Hydro-
genation of Ketones. Prior to the reaction, dry isopropanol was
degassed under a nitrogen flow for 2 h. Catalyst (1 mol %) and
potassium tert-butoxide (10 mol %) were introduced in a Schlenk
tube containing a magnetic stirring bar and dried under vacuum for 10
min. Under nitrogen, distilled propiophenone (0.2 mmol, 0.030 mL)
and dry isopropanol (2 mL) were transferred to the Schlenk tube via
syringe. The reaction mixture was then heated to 80 °C under stirring
for 5 h. After this time, a 0.1 mL aliquot of the reaction mixture was
taken for GC analysis to ensure the reaction had gone to completion.
At the end, the isopropanol was evaporated under vacuum and the
product was purified by flash chromatography on silica gel with a
(9:1) petroleum ether and ethyl acetate solvent mixture. Typical
experiments are provided in Table 1.
EXPERIMENTAL SECTION
■
General Methods and Instrumentation. All solvents were
dried using standard methods and stored over molecular sieves (4 Å).
All sensitive salts were weighed in a glovebox. All reactions were
carried out under a dry nitrogen atmosphere using standard Schlenk
techniques and were repeated two to four times. Analytical thin layer
chromatography (TLC) was performed on Merck precoated 0.20 mm
silica gel Alugram Sil 60 G/UV254 plates. Flash chromatography was
1
carried out with Macherey silica gel (Kielselgel 60). H (300 MHz),
13C{1H} (75 MHz) NMR spectra were acquired on Bruker Avance
spectrometers. Chemical shifts (δ) are reported downfield of Me4Si in
ppm, and coupling constants are expressed in Hz. Chloroform-d was
purchased from Eurisotop and freshly filtered through basic alumina
prior to use. 1,3,5-Trimethoxybenzene and 1,2,4,5-tetrachlorobenzene
1
were used as internal standards when needed. H NMR shifts are
given relative to the residual solvent resonance of CDCl3 (at 7.26
ppm), and 13C{1H} NMR shifts are given relative to the residual
solvent peak of CDCl3 (at 77.16 ppm). HPLC analyses were
performed on a Hitachi LaChromElite equipment with a Peltier oven
and a DAD detector. Optical rotations were measured on a
PerkinElmer 343 using a 10 cm cell. Gas chromatography analyses
were done on a GC Shimadzu 2010+ with FID detectors using
Supelco SPB-5 column (30 m, 0.25 mm, 0.25 μm) and with nitrogen
as gas carrier. GC-MS analyses were performed on a Shimadzu
QP2010+ (EI mode) using a Supelco column SLBTM-5 ms (30m,
0.25 mm, 0.25 μm). HRMS-ESI analyses were performed at CUMA-
Pharm. Dept.-University Lille Nord de-France. Elemental analyses
were performed on an Elementar Vario Micro Cube apparatus at
UCCS, University Lille Nord de France. X-ray diffraction analyses
were performed at UCCS, University Lille Nord de France, on a
Bruker APEX DUO. DLS measurements were performed on a
Malvern Zetasizer Nano Series at 25 °C after decantation of the crude
reaction solutions (20 min) and without any filtration. Aldehyde
substrates were used as received. Purchased boronic acids were
purified by flash chromatography on silica gel prior to use.
General Procedure for the Synthesis of Diamine Precursors.
Rac-BINAP (10 mol %) was introduced in a large Schlenk tube along
with a magnetic stirring bar. In a glovebox, Pd2(dba)3 (5 mol %) and
NaOtBu (2.8 equiv) were subsequently added. Under a nitrogen flow,
toluene (4 mL) was transferred to the Schlenk and the resulting
mixture was stirred at room temperature for 30 min. In a separate
Schlenk flask, (S,S)-DPEN (x mmol from glovebox) was dissolved in
4 mL of toluene and transferred to the reaction mixture via cannula.
The temperature was set at 30 °C, and the reaction was stirred for a
further 60 min. Afterward, the substituted 2-bromoanisole (2*x
mmol) was dissolved in 4 mL of toluene in a third Schlenk tube, and
the resulting solution was transferred to the reaction medium via
cannula. The reaction was then stirred for 24 h at 115 °C under a
nitrogen flow. At the end, the resulting mixture was cooled and the
remaining solvents were evaporated under vacuum. The residue was
then dissolved in ethyl acetate and filtered over Celite. Purification
was performed by flash chromatography on silica gel with a solid
deposit and using petroleum ether and ethyl acetate solvent mixtures
with a 5% elution gradient.
General Procedure for the Rh(I) Catalyzed Additions of
Boronic Acids to Aldehydes. Catalyst 7d (3 mol %), purified
boronic acid (2 equiv, 0.3 mmol), and potassium fluoride (6 equiv,
0.9 mmol, 0.052 g) were introduced in a Schlenk tube containing a
magnetic stirring bar and dried under vacuum for 10 min. Under
nitrogen, distilled t-butanol (2.5 mL) and methanol (0.5 mL), as well
as the aldehyde (0.15 mmol), were transferred to the Schlenk tube via
syringe. The reaction mixture was then left under stirring at 80 °C for
4 h. At the end, the reaction was cooled and subsequently quenched
with brine. The aqueous solution was extracted thrice with ethyl
acetate (3 × 10 mL), and the organic phases were dried over
anhydrous magnesium sulfate and concentrated under vacuum.
Purification of the product was performed via preparative thin layer
chromatography on glass silica plates. Typical experiments are
provided in Scheme 2.
(1S,2S)-N1,N2-Bis(5-(tert-butyl)-2-methoxyphenyl)-1,2-diphenyl-
ethane-1,2-diamine (2d). To synthesize compound 2d, reagents
were used as follows:
(S,S)-DPEN (0.376 g, 1.8 mmol), ( )BINAP (0.110 g, 10 mol %),
Pd2(dba)3 (0.081 g, 5 mol %), NaOtBu (0.476 g, 5.0 mmol), and 4-
tBu-2-bromoanisole 1d (0.862 g, 3.6 mmol). The product was
obtained as a white solid (0.714 g, 75% yield), Rf = 0.6, petroleum
ether and ethyl acetate solvent mixture (95:5).
1H NMR (300 MHz, CDCl3): δ 0.98 (s, 18 H, tBu), 3.74 (s, 6 H,
OMe), 4.51 (s, 2 H, CH), 5.22 (bs, 2 H, NH), 6.30 (s, 2 HAr) 6.51
(dd, J = 8.3, J = 2.2, 2 HAr), 6.59 (d, J = 8.3, 2 HAr), 7.04 (m, 10 HAr).
13C{1H} NMR (75 MHz, CDCl3): δ 31.5 (3 CH3, Me), 34.2 (C),
General Procedure for the Synthesis of Imidazolium Salts.
The diamine precursor (1 equiv) and NH4BF4 (2 equiv) were
introduced in a Schlenk tube containing a magnetic stirring bar. After
a drying under vacuum of 30 min, CH(OEt)3 (3 mL) was transferred
to the Schlenk under a nitrogen flow, and the remaining mixture was
stirred overnight at 120 °C. After 12 h, the reaction was stopped and
the remaining solvent evaporated under vacuum. Purification of the
55.7 (CH3, OMe), 64.6 (CH), 109.1 (CH), 110.1 (CH), 113.3 (CH),
127.3 (CH), 127.8 (2 CH), 128.2 (2 CH), 136.7 (C), 140.9 (C),
143.9 (C), 145.4 (C). HRMS (ESI+): m/z calcd for C36H45N2O2
E
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[MH+] 537.34756, found 537.34604. Elemental analysis: calcd (%)
for (C36H44N2O2): C, 80.56; H, 8.26; N, 5.22; found C, 80.77; H,
8.32; N, 5.08. [α]2D0 = −103 (CH2Cl2, 9 mM).
(Ra,Ra,4S,5S)-1,3-Bis(5-(tert-butyl)-2-methoxyphenyl)-4,5-di-
phenyl-4,5-dihydro-1H-imidazol-3-ium Tetrafluoroborate (3d). To
synthesize compound 3d, diamine 2d (0.714 g, 1.3 mmol) was
combined with NH4BF4 (0.279 g, 2.7 mmol) in 3 mL of CH(OEt)3.
The product was obtained as a beige solid (0.738 g, 87% yield). Rf =
0.6, petroleum ether and acetone solvent mixture (2:8).
AUTHOR INFORMATION
Corresponding Author
*E-mail: christophe.michon@univ-lille.fr.
ORCID
Christophe Michon: 0000-0002-0138-2923
Notes
The authors declare no competing financial interest.
■
1H NMR (300 MHz, CDCl3): δ 1.14 (s, 18 H, tBu), 3.96 (s, 6 H,
OMe), 5.76 (s, 2 H, CH), 6.87 (m, 2 HAr), 7.24 (m, 2 HAr), 7.27 (m,
2 HAr), 7.40 (m, 10 HAr), 8.93 (s, 1 H, imid). 13C{1H} NMR (75
MHz, CDCl3): δ 31.1 (3 CH3, Me), 34.2 (C), 56.4 (CH3, OMe), 75.5
(CH), 111.8 (CH), 122.4 (C), 124.0 (CH), 127.3 (CH), 128.0 (2
CH), 129.6 (2 CH), 130.1 (CH), 135.2 (C), 144.7 (C), 151.0 (C),
158.0 (CH, imid). HRMS (ESI+): m/z calcd for C37H43N2O2 [M+]
547.33191, found 547.32965. Elemental analysis: calcd (%) for
(C37H43BF4N2O2 + 1 H2O) C, 68.10; H, 6.95; N, 4.29; found C,
67.84; H, 7.02; N, 4.09. [α]2D0 = −304 (CH2Cl2, 1 mM).
(Ra,Ra)-Chloro-(1,5)-cyclooctadiene-((4S,5S)-1,3-bis(5-tertio-
butyl-2-methoxy-phenyl)-4−5-diphenylimidazolidin-2-ylidene)-
rhodium (4d). To synthesize complex 4d, Ag2O (0.269 g, 1.2 mmol)
and [Rh(COD)Cl]2 (0.143 g, 0.3 mmol) reacted with imidazolium
salt 3d (0.369 g, 0.6 mmol). The complex was obtained as a yellow
solid (0.329 g, 72% yield). Rf = 0.7, petroleum ether and acetone
solvent mixture (85:15). Recrystallization by using CH2Cl2 and n-
hexane, followed by drying under vacuum (0.329 g, 72% yield).
1H NMR (300 MHz, CDCl3): δ 1.15 (s, 9 H, tBu), 1.28 (s, 9 H,
tBu), 1.35 (m, 3 H, CH2, COD), 1.58 (m, 3 H, CH2, COD), 1.91 (m,
2 H, CH2, COD), 3.13 (m, 1 H, CH, COD), 3.49 (m, 1 H, CH,
COD), 3.80 (s, 3 H, OMe), 3.90 (s, 3 H, OMe), 4.52 (m, 1 H, CH,
COD), 4.64 (m, 1 H, CH, COD), 4.92 (d, J = 6.0, 1 H, CH), 5.50 (d,
J = 6.0, 1 H, CH), 6.82 (dd, J = 9.0, 4 HAr), 7.00 (d, J = 3.0, 2 HAr),
7.30 (m, 9 HAr), 7.57 (d, J = 8.5, 2 HAr), 8.51 (d, J = 1.3, 1 HAr).
13C{1H} NMR (75 MHz, CDCl3): δ 27.3 (CH2), 29.6 (CH2), 31.2
(CH2), 31.4 (3 CH3, CMe), 31.6 (3 CH3, CMe), 33.3 (CH2), 33.9
ACKNOWLEDGMENTS
■
The University of Lille is acknowledged for a Ph.D. fellowship
(M.-A.A). The University of Edinburgh, ENSCL, and E.U. are
thanked for an Erasmus+ fellowship (K.M.). The University of
St-Andrews and ENSCL are acknowledged for a training
fellowship (P.H.). The CNRS, the Chevreul Institute (FR
̀
́
2638), the Ministere de l’Enseignement Superieur et de la
́
Recherche, the Region Nord − Pas de Calais, and the FEDER
are acknowledged for supporting and partially funding this
̈
work. Mr. A. Plumyoen is thanked for preliminary experiments.
́
Mrs. C. Meliet (UCCS) is thanked for elemental analyses. Mrs.
C. Delabre (UCCS) is thanked for HPLC analyses. Dr. M.
Kouach, Mrs. N. Duhal, Mrs. C. Lenglart, and Ms. A. Descat
(Univ. Lille, CUMA) are thanked for HRMS analyses. Mr. M.
Ortega Vaz and Dr. C. Pierlot are thanked for their help with
DLS measurements. This paper is dedicated to Emeritus Prof.
Robert J. Angelici (Iowa State University), one of the mentors
of Dr. Christophe Michon.
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DOI: 10.1021/acs.organomet.8b00823
Organometallics XXXX, XXX, XXX−XXX