Analyses of the Structural and Electronic Properties of NHCs with Bicyclic Architectures

Article abstract of DOI:10.1021/acs.organomet.0c00564

Quantification of the steric and electronic properties of ligands is an important task for the development of highly effective chemical transformations when using a metal complex as a catalyst. In this study, we used silver and iridium complexes to perform systematic analyses to quantify the steric and electronic properties of original NHCs, which we refer to as DHASI and BCPSI. In addition to two silver chloride complexes that we previously reported, two novel BCPSIiPrAgCl and BCPSICyAgCl complexes were prepared and used for XRD studies to quantify the steric properties via use of a method referred to as the percent buried volume (%VBur). We then synthesized 6 IrCl(CO)2(HNC) complexes to calculate the TEP values, and the results indicated that the influences of two different core bicyclic motifs were somewhat unclear, while the influences of different N-substituents were readily apparent. Finally, the original NHCs were applied to a copper catalyzed allylic arylation of cinnamyl bromide with PhMgBr. The regioselectivity of the substitutions suggested that the steric environments existing at relatively long distances from the bound copper also exerted a considerable amount of influence over the catalytic properties.

Full text of DOI:10.1021/acs.organomet.0c00564

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Analyses of the Structural and Electronic Properties of NHCs with
Bicyclic Architectures
Shin Ando,* Mio Otsuka, and Tadao Ishizuka*
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ABSTRACT: Quantification of the steric and electronic proper-
ties of ligands is an important task for the development of highly
effective chemical transformations when using a metal complex as a
catalyst. In this study, we used silver and iridium complexes to
perform systematic analyses to quantify the steric and electronic
properties of original NHCs, which we refer to as DHASI and
BCPSI. In addition to two silver chloride complexes that we
previously reported, two novel BCPSIiPrAgCl and BCPSICyAgCl
complexes were prepared and used for XRD studies to quantify the
steric properties via use of a method referred to as the percent
buried volume (%VBur). We then synthesized 6 IrCl(CO)2(HNC)
complexes to calculate the TEP values, and the results indicated
that the influences of two different core bicyclic motifs were somewhat unclear, while the influences of different N-substituents were
readily apparent. Finally, the original NHCs were applied to a copper catalyzed allylic arylation of cinnamyl bromide with PhMgBr.
The regioselectivity of the substitutions suggested that the steric environments existing at relatively long distances from the bound
copper also exerted a considerable amount of influence over the catalytic properties.
sensitive divalent carbons. These somewhat reactive species
can easily be prepared from azolium salt precursors via
deprotonation. The precursor salts are usually very stable
crystalline solids, and straightforward methods to synthesize
these salts have already been developed by many chemists.8
Therefore, a number of NHCs have been synthesized and
investigated to develop useful and effective catalytic processes.
Due to an ease of synthesis and useful geometric features, the
introduction of bulky substituents onto the nitrogen(s) of
imidazolin-2-ylidenes or of imidazolidin-2-ylidenes has become
the most common design (Figure 1a).9 The bulky N-
substituents on these representative core heterocycles
effectively shield the active sites, which has resulted in the
development of highly active NHC-metal catalysts. Because
the structure of NHCs is not in a “cone” form, Nolan and
Cavallo developed the “percent buried volume” (%VBur) as a
parameter that could be used to quantify the steric properties
of many types of ligands such as phosphines and NHCs.10
Cavallo and co-workers additionally developed a steric map to
evaluate the manner in which ligands wrap around bound
INTRODUCTION
■
Steric shielding and electron donation of a ligand are important
properties for making a metal complex if it is to be effective in
catalyzing a specific chemical transformation. Since the fine-
tuning of these factors is known to be a key factor in
developing a highly useful catalytic process, a large number of
ligands have been developed to widen their tuning capacity.1
Sterically hindered ligands aptly protect active, but unstable,
metals with low valence from undesired decompositions, but
too much steric pressure will cause the steric repulsion of
substrates, which could result in ineffective catalysis.
Appropriate steric shielding is also known to accelerate the
catalytic cycle, particularly the reductive elimination step.
Electron donation from ligands activates the bound metals and
the resultant complexes become capable of oxidative addition
to relatively stable bonds such as C−F,2 C−N,3 and even C−
H4 bonds. Quantification of these factors has been attempted,
and the most widely used examples are the Tolman cone
angle5 and the Tolman’s electronic parameter (TEP).6 Tolman
cone angles are beneficial in quantifying the steric influences
that phosphine ligands exert on metal complexes, and the TEP
is widely used to quantify the electron donating ability of
ligands.
Received: August 25, 2020
N-heterocyclic carbenes (NHCs) are ubiquitous ligands in
organometallic chemistry because of their high performance
during the activation of transitional metals and for stabilizing
unstable highly reactive species.7 Although free NHCs show a
relatively high level of stability, they remain air- and moisture-
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RESULTS AND DISCUSSION
Syntheses of BCPSIiPr and BCPSICy Precursors. In this
study, we synthesized BCPSI ligands with isopropyl and
cyclohexyl groups as N-substituents to investigate their
influence compared with the use of DHASIMe,12 DHASIiPr,14
DHASICy,14 and BCPSIMe,13 as reported (Scheme 1a). The
■
Scheme 1. Syntheses of BCPSIiPr and BCPSICy Precursors
syntheses were started with an imination of diketone 713 and
isopropyl amine in the presence of BF3·Et2O, and the diimines
were subsequently reduced with the use of NaBH4 to afford cis-
diamine 9 as a crude oil. The cis-diamine 9 was treated with
formalin to form an imidazolidine ring, followed by oxidation
with NCS to obtain imidazolinium chloride 10 (BCPSIiPr·
HCl) as a BCPSIiPr precursor in a 23% yield over 4 steps. For
the preparation of this BCPSIiPr precursor, one step
cyclization using triethyl orthoformate in the presence of
ammonium chloride resulted in an inferior yield. Another
NHC precursor with cyclohexyl groups was synthesized via a
similar pathway through cis-diamine 11 obtained with the use
of cyclohexylamine. The crude cis-diamine 11 was treated with
triethyl orthoformate in the presence of NH4Cl to afford
imidazolinium chloride 12 (BCPSICy·HCl) as a BCPSICy
precursor in a 37% yield from diketone 7.
Syntheses and Structural Analyses of BCPSIiPrAgCl
and BCPSICyAgCl Complexes. With the NHC precursors in
hand, we synthesized silver complexes via treatment with Ag2O
in CH2Cl2 at 25 °C, which isolated BCPSIiPrAgCl 13 and
BCPSICyAgCl 14 in 62% and 68% yields, respectively
(Scheme 2). These silver complexes were stable under air on
a bench in the solid state and isolated as colorless crystalline
solids. Because we previously analyzed DHASIMeAgCl 412 and
BCPSIMeAgCl 813 complexes using XRD studies, we
Figure 1. (a) The structures of a ubiquitous NHC core. (b)
Imidazolidin-2-ylidenes with a bicyclic architecture on noncarbenic
carbons.
metals, which is helpful in imagining the shape of the reactive
pocket.11
Our group has developed original NHCs that possess
bicyclic architectures on the noncarbenic carbons of
imidazolidin-2-ylidene, which we refer to as DHASI12 or as
BCPSI13 (Figure 1b). These two core examples of NHC
architecture showed good shielding effects when we used them
as ligands to form their silver,12,13 copper,14 and nickel15
complexes. Modification studies on the bicyclo[2.2.1]heptane
motif of BCPSI revealed that an aromatic ring set on an active
site is critical in stabilizing NHC-metal complexes. It should be
emphasized that these stable complexes were formed using
DHASIs equipped with alkyl groups such as methyl, isopropyl,
and cyclohexyl groups or by using BCPSIMe as a ligand.
Generally speaking, metal complexes using imidazolidin-2-
ylidenes are unstable when the N-substituents are small alkyl
groups, whereas DHASIMeAgCl and BCPSIMeAgCl are stable
crystalline solids.16 Herein, we report detailed analyses of
DHASIs and BCPSIs in order to quantify the steric properties
and electron donative abilities. We analyzed their steric
properties via the structural analyses of silver complexes. To
quantify the electron donative abilities of DHASIs and BCPSIs
with Me, i-Pr, and Cy N-substituents, IrCl(CO)2(NHC)
complexes were synthesized and used to calculate the TEP
values via the Ir scale developed by Nolan and co-workers. To
investigate the impact on metal catalysts, we applied ligands to
a copper-catalyzed allylic arylation with PhMgBr.
Scheme 2. Syntheses of BCPSIAgCl and BCPSICyAgCl
Complexes
B
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attempted XRD studies with these novel silver complexes to
observe the influences of N-substituents. Single crystals of
these complexes were obtained via the slow diffusion of n-
hexane in CH2Cl2 solutions of BCPSIiPrAgCl 13 or
BCPSICyAgCl 14 at ambient temperature, and the ORTEP
diagrams appear in Figure 2a,b. BCPSIiPrAgCl 13 had a
symmetric structure in the solid state, and two methyl groups
on the isopropyl group faced in the direction of the silver
chloride. On the other hand, BCPSICyAgCl 14 was
unsymmetric in the solid state, with two cyclohexyl groups
that had different conformations. However, the most stable
conformation in a solution state will be symmetric, as shown
by the NMR spectra. Despite the fact that silver halide
complexes supported by NHCs possess diverse bonding
motifs,17 all silver chloride complexes with BCPSI ligands
including BCPSIMeAgCl 8 were of a monomeric linear
NHC−Ag−Cl form, which is a relatively rare structure.18
DHASIMe also gave us a DHASIMeAgCl 4 complex in the
same bonding motif, suggesting that our original NHCs with
bicyclic motifs on the noncarbenic carbons tend to form this
linear structure in the solid state. It should be noted that this
tendency is independent of N-substituents in contrast to the
reports of varied structures that depend on N-substituents. We
subsequently attempted to quantify the steric properties of
these novel NHCs by using the SambVca 2.1 program
developed by Cavallo and co-workers (Figure 2c,d).11a
A
calculation was performed using the default parameters
(atomic radii; 1.17, mesh spacing; 0.10 Å), and 2.0 Å was
considered as the distances for the NHC-metal bonds. The
results using a 3.5 Å sphere radius are shown in Figure 2. The
steric map indicated that the steric pressure from the bicyclic
framework was very small in the first sphere around the metal
(northern part of the steric maps). Compared with DHASIMe
and BCPSIMe, the bicyclic architecture seemed to exert fewer
steric effects on BCPSIiPr and BCPSICy. Therefore, the %VBur
for both BCPSIiPr (39.0) and BCPSICy (32.9) was higher
than that for BCPSIMe (29.8), which was mainly the results of
bulkier N-substituents. The difference between BCPSIiPr
(39.0) and BCPSICy (32.9) was caused by the geometries of
the secondary alkyl groups. As mentioned previously, the most
stable structures of these complexes in solution states are
symmetric, as shown in NMR spectra; therefore, the steric
pressure from N-substituents during catalysis cannot be fully
estimated using these calculations for structures in a solid state.
These secondary alkyl N-substituents cause steric repulsion
with the aromatic ring on a bicyclic architecture, and this
repulsion made the dihedral angle between the aromatic ring
and the NHC core N-heterocycles of BCPSIiPr larger than that
of BCPSIMe (angles a in Figure 2e). In a similar manner,
BCPSICy had a larger dihedral angle than that of either
BCPSIMe or BCPSIiPr, which would have weakened the
aromatic ring’s influence on the silver atom. The differences in
dihedral angles were apparent in calculations using a sphere
radii larger than 3.5 Å, which resulted in a steric pressure in the
northern portion of the steric maps of BCPSIMe (Figure S41
in the Supporting Information). Compared with imidazolin-2-
ylidene ligands without a bicyclic structure (IMe, IiPr, and
ICy), higher levels of %VBur for DHASIs and BCPSIs were
observed (Table S5 in the Supporting Information). Notably,
when the longer sphere radii were considered, larger
differences in %VBur were observed. These facts suggested
that the steric state of the outer sphere could be heightened by
the bicyclic architectures on noncarbenic carbons, which would
be useful for widening the capacity when tuning the steric state
of an NHC ligand.
Figure 2. (a) ORTEP diagram of BCPSIiPrAgCl 13. Thermal
ellipsoids are shown at the 50% probability level. Most H atoms are
omitted for clarity. Selected bond lengths (Å) and angles (deg): C−
Ag, 2.086; Ag−Cl, 2.330; N1−C−N2, 109.1; and C−Ag−Cl, 179.0.
(b) ORTEP diagram of BCPSICyAgCl 14. Thermal ellipsoids are
shown at the 50% probability level. Most H atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): C−Ag, 2.084(6);
Ag−Cl, 2.320(3); N1−C−N2, 109.1(5); and C−Ag−Cl, 176.9(2).
(c) Steric map and %VBur of BCPSIiPrAgCl (see Experimental Section
for details). (d) Steric map and %VBur of BCPSICyAgCl (see
Experimental Section for details). (e) Dihedral angles on silver
chloride complexes of BCPSIs.
Syntheses of Ir(COD)Cl(NHC) and IrCl(CO)2(NHC)
Complexes with DHASIs and BCPSIs for Structural
Analyses and Calculations of TEP Values. The Tolman
electronic parameter (TEP) was originally developed using an
IR stretch of carbonyl groups of Ni(CO)3L complexes.6 A
highly electron-donating ligand will weaken a CO triple bond
attached to a metal, which reduces the ν(CO) value. This
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Scheme 3. Syntheses of IrCl(CO)2(NHC) Complexes with DHASIs and BCPSIs
parameter can be used not only for phosphine ligands but also
for NHCs, but the high volatility and toxicity of Ni(CO)4 has
always been a safety drawback for this method. To avoid the
handling of this hazardous material, the Rh scale and Ir scales
are frequently used to quantify the electronic properties.19
Crabtree and co-workers first systematically investigated some
IrCl(CO)2(PR3) complexes to translate the average ν(CO)
value of these complexes to a TEP value based on Ni(CO)3L
complexes.20 Later, Nolan and co-workers used 9 IrCl-
(CO)2(NHC) complexes to extend this Ir system, and the
equation they developed (eq 1; R2 = 0.971) is currently used to
calculate the TEP values of a wide variety of NHC ligands.21
TEP = 0.847[ν (Ir)] + 336.2 cm−1
(1)
av
The syntheses of IrCl(CO)2(NHC) using DHASIs and
BCPSIs are summarized in Scheme 3. We treated NHC
precursors with n-BuLi to generate free carbene in situ, and
subsequently the NHC solution was transferred to a flask
charged with [Ir(COD)Cl]2 to afford Ir(COD)Cl(NHC)
complexes 19−24 as yellow crystalline solids in acceptable
yields. The Ir(COD)Cl(NHC) complexes 19−24 were then
transformed to IrCl(CO)2(NHC) complexes 25−30 via the
bubbling of CO gas into CH2Cl2 solutions at 25 °C, and these
complexes were obtained as pale yellow solids in excellent
yields.
We used Ir(COD)Cl(DHASIiPr) 20 and Ir(COD)Cl-
(BCPSIiPr) 23 for XRD analyses to perform structural
investigations. With both complexes, single crystals for XRD
experiments were grown via the slow diffusion of n-hexane into
CH2Cl2 solutions at ambient temperature. The ORTEP
diagrams are shown in Figure 3. On both complexes, the
geometries of Ir were square planar, and chloride atoms existed
on the other side of the aromatic rings on the bicyclic frames.
The %VBur of both complexes were approximate and smaller
than that of BCPSIiPrAgCl 13 because of the conformations of
isopropyl groups (the parameters for the calculation are same
as that for Figure 2). Similar to the previous analyses using
DHASIMeAgCl 4 and BCPSIMeAgCl 8,13 the aromatic ring of
the DHASIiPr ligand was located further from carbene than
that of the BCPSIiPr ligand, which seems to be a general
difference between these two similar frameworks. This means
that these two similar frameworks have additional sterically
tunable sites, which are aromatic rings located at different
distances from the bound metals. As we show later, these
Figure 3. (a) ORTEP diagram of Ir(COD)Cl(DHASIiPr) 20.
Thermal ellipsoids are shown at the 50% probability level. H atoms
are omitted for clarity. A COD ligand appears as a wireframe model.
Selected bond lengths (Å) and angles (deg): C−Ir, 2.025(3); Ir−Cl,
2.375(1); N1−C−N2, 109.6(3); and C−Ir−Cl, 89.3(1). (b) ORTEP
diagram of Ir(COD)Cl(BCPSIiPr) 23. Thermal ellipsoids are shown
at the 50% probability level. H atoms are omitted for clarity. A COD
ligand appears as a wireframe model. Selected bond lengths (Å) and
angles (deg): C−Ir, 2.017; Ir−Cl, 2.327; N1−C−N2, 109.1; and C−
Ir−Cl, 92.1. (c) Steric map and %VBur of Ir(COD)Cl(DHASIiPr) (see
Experimental Section for details). (d) Steric map and %VBur of
Ir(COD)Cl(BCPSIiPr) (see Experimental Section for details).
differences had an unexpectedly strong influence on the
catalytic activities of these copper complexes.
D
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With the use of IrCl(CO)2(NHC) complexes 25−30, the IR
spectra were measured to calculate the TEP values of these
NHC ligands (Table 1). Huynh previously reported extensive
and the results are summarized in Table 2. We used silver
chloride complexes as an NHC transfer reagent to form a
Table 2. Application to a Copper Catalyzed Allylic Arylation
of Cinnamyl Bromide 31 with PhMgBr
Table 1. TEP Values of DHASIs, BCPSIs, and Other
Common NHCs
entry
112
2
3
413
5
ligand
combined yield (%)
SN2:SN2′
DHASIMe
DHASIiPr
DHASICy
BCPSIMe
BCPSIiPr
BCPSICy
IMe
98
90
90
99
95
85
98
20:80
40:60
23:77
12:88
7:93
entry
ligand
TEP value (cm−1
)
1
2
3
4
5
6
DHASIMe
DHASIiPr
DHASICy
BCPSIMe
BCPSIiPr
BCPSICy
2056
2053
2051
2055
2053
2052
6
7
4:96
49:51
(NHC)CuCl complex in situ, which was directly used as a
catalyst. It was somewhat surprising that the use of DHASIiPr
and DHASICy resulted in a selectivity for SN2′-substituted
products that was inferior to that of DHASIMe. In contrast to
the results with DHASIs, BCPSIiPr and BCPSICy generated
the SN2′ product with selectivity that was higher than that
generated by BCPSIMe. Since the TEP values were the same,
the drastic differences between DHASIiPr and BCPSIiPr are
thought to arise from the steric differences on the bicyclic
structures. Although the steric properties were similar based on
their %VBur and steric maps in the first sphere around the
metal, these results obviously suggested that the outer steric
environments of the 3.5 Å sphere (see Figures S38 and S42
and Table S5 in the Supporting Information) strongly affected
the regioselectivity. At the same time, higher SN2′-selectivity
with DHASIMe and with BCPSIMe than with DHASICy
cannot be explained by using %VBur values alone even if we
considered the values in large spheres. One possible reason for
these phenomena might be the distances between the bound
copper and the aromatic ring, because we had previously
observed the exceptional influence of the phenyl group.13 The
steric maps of DHASIMe and of BCPSIMe in a 5.0 Å sphere
clearly showed the steric pressure of phenyl groups in the
northern portions (Figures S36 and S41), which are closer to
the carbenes than the phenyl group of DHASICy (Figure S40).
To obtain the SN2′ product in higher selectivity with this
particular substitution reaction, the closer aromatic ring on the
BCPSI core and the bulky N-substituents were effective, which
suggested the necessity to consider steric tuning on the range
from the metal center in a sphere wider than 3.5 Å sphere.
studies about the relationship between TEP values and
structures of NHCs, in which the limitations for the
quantification of electron donative abilities based on TEP
values is suggested.19 However, the TEP value remains the
most aptly studied parameter, which should help explain the
relative electronic properties of NHCs, particularly NHCs with
a unique core structure. In this study, we aimed to investigate
the influences of bicyclic architectures, particularly the
influences of aromatic rings on the different distances from
the bound metals. We used CH2Cl2 solutions of IrCl-
(CO)2(NHC) complexes 25−30 because the solvents had a
considerable amount of influence on the TEP values.19 The
results are summarized in Table 1. Unfortunately, we found no
significant changes in the TEP values between NHCs with and
without bicyclic architectures, although the fluctuations based
on the N-substituents seemed influenced by the core
structures.22 However, because of the limited resolution of
IR spectroscopy, the results of only these observations are not
sufficiently convincing to support the theory that these slightly
different degrees of the fluctuations based on N-alkyl groups
were the result of the influences of aromatic rings.19 In
addition to other parameters for quantification of electronic
properties, further studies with a series of NHCs such as a
BCPSI core equipped with varied substituents on the aromatic
ring might be helpful to explain the influence of π-electron of
aromatic rings.
The Influences that NHCs with a Bicyclic Architecture
Exert on Regioselectivity during Copper-Catalyzed
Allylic Arylation with PhMgBr. We finally applied these 6
NHC ligands to a copper-catalyzed allylic arylation of cinnamyl
bromide with PhMgBr to evaluate the influence of the
structure on catalytic activity. Mechanistically, a bulky and
electron-deficient ligand is thought to be preferable because it
heightens SN2′ selectivity.23 Our group previously found that
the DHASIMeCuCl complex, which forms in situ, generates
SN2′-substituted products as its major yield (SN2:SN2′ =
20:80), whereas IMeCuCl gave us a 1:1 mixture of SN2 and
SN2′ substituted products. BCPSIMeCuCl, which also forms in
situ, generated SN2′ substituted products in a ratio (SN2:SN2′ =
12:88) that was higher than that of DHASIMeCuCl. In
addition to these previous results, we attempted the reaction
with DHASIiPr, DHASICy, BCPSIiPr, and BCPSICy ligands,
CONCLUSION
■
In this study, we synthesized two novel NHC precursors,
BCPSIiPr·HCl 10 and BCPSICy·HCl 12, and used systematic
investigations to quantify the electronic and steric properties of
our previously developed NHCs possessing bicyclic frame-
works. The steric properties of BCPSI ligands with N-isopropyl
and N-cyclohexyl groups were analyzed using silver chloride
complexes. The secondary alkyl groups on nitrogen caused
steric repulsion to an aromatic ring, which resulted in dihedral
angles between the aromatic ring and the NHC core that were
larger than those of BCPSIMeAgCl. Analyses using the
SambVca 2.1 program indicated that the steric influences of
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with sat. NH4Cl aq. (5.0 mL × 1), dried over Na2SO4, filtered, and
concentrated under reduced pressure to obtain a crude diimine. The
crude residue was dissolved in MeOH (2.0 mL), and the solution was
cooled in an ice bath. NaBH4 (46.9 mg, 1.2 mmol) was added to the
solution at 0 °C, and the mixture was stirred at 25 °C for 1 h. The
reaction was then quenched with sat. NH4Cl aq. (2.0 mL), and the
mixture was extracted with EtOAc (4.0 mL × 3). The combined
organic layers were dried over Na2SO4, filtered, and concentrated
under reduced pressure to afford a crude cis-diamine (110 mg) as a
colorless powder. The crude diamine was dissolved in CH2Cl2/EtOH
(7/3, 3.0 mL), and 30% formalin was added to the solution at 25 °C.
After stirring at the same temperature for 2 h, The reaction solution
was then concentrated under reduced pressure, and the crude mixture
was diluted with EtOAc (3.0 mL) and H2O (3.0 mL). The layers were
separated, and the aqueous layer was extracted with EtOAc (4.0 mL ×
3). The combined organic layers were washed with brine (5.0 mL ×
1), dried over Na2SO4, filtered, and concentrated under reduced
pressure. The residue was dissolved in 30% EtOAc in n-hexane, and
the solution was passed through a short pad of silica gel, which was
then eluted with the 30% EtOAc in n-hexane. The filtrate was
concentrated under reduced pressure to afford a crude imidazolidine
(62.5 mg) as a colorless oil. The crude imidazolidine was dissolved in
Et2O (1.5 mL), and NCS (61.7 mg, 0.46 mmol) was added at 25 °C.
After being darkened with aluminum foil, the reaction mixture was
stirred at the same temperature for 23 h. The colorless precipitate was
filtered and purified by a column chromatography (5% MeOH/
CH2Cl2) on silica gel (20 mL) to afford BCPSIiPr·HCl 10 (45.3 mg,
0.14 mmol, 23% from diketone 7) as a colorless powder. mp 99−102
the aromatic rings on BCPSIiPr and on BCPSICy were smaller
than that on BCPSIMe particularly when the sphere radii are
larger than 3.5 Å, and that the secondary N-alkyl groups
showed increased %VBur values. To quantify the electronic
properties of DHASIs and BCPSIs, the 6 IrCl(CO)2(NHC)
complexes were prepared and subsequently analyzed using IR
spectra. The IR stretch of carbonyl showed that the TEP values
of DHASIs and of BCPSIs changed depending on the N-alkyl
groups, whereas the impact of bicyclic motifs was not so
obvious. Further extensive studies are required to explain the
high stabilities of metal complexes supported by DHASIs and
BCPSIs. Finally, to examine the influence that our NHCs
could exert on a metal catalyst, we applied 6 NHCs equipped
with bicyclic frameworks to a copper-catalyzed allylic arylation
of cinnamyl bromide with PhMgBr. The regioselectivity of this
transformation was changed depending on the N-substituents
as well as on the bicyclic core structure. The results obviously
indicated that the outer steric environments of the 3.5 Å
sphere of the bound metal affected the properties of the
catalysts. Further investigations to develop catalytic trans-
formations using the unique steric and electronic properties of
DHASIs and of BCPSIs are under way in our laboratory.
EXPERIMENTAL SECTION
■
General Procedure and Chemicals. Caution! Since CO gas is
highly toxic and flammable, handle with extreme care. All reactions were
carried out under an argon atmosphere with freshly distilled solvents
under anhydrous conditions, unless otherwise noted. Anhydrous
CH2Cl2, DMSO, and THF were purchased and used without further
distillation. Other reagents were used without further purification.
Yields refer to chromatographically and spectroscopically (1H NMR)
1
°C; H NMR (600 MHz, CDCl3, 300 K) δ 9.09 (s, 1H, NCHN),
7.26−7.21 (m, 4H, ArH), 4.93 (dd, J = 1.8, 1.8 Hz, 2H, CH), 3.86
(sept, J = 6.6 Hz, 2H, NCH(CH3)2), 3.82 (s, 2H, CH), 2.09 (ddd, J =
10.2, 1.8, 1.8 Hz, 1H, CHAHB), 1.84 (ddd, J = 10.2, 1.8, 1.8 Hz, 1H,
CHAHB), 1.39 (d, J = 6.6 Hz, 6H, NCH(CH3)2), 1.36 (d, J = 10.2 Hz,
6H, NCH(CH3)2) ppm; 13C{1H} NMR (151 MHz, CDCl3, 302 K) δ
154.7 (NCN), 141.9 (Ar), 127.6 (Ar), 123.8(Ar), 66.0 (CH), 50.8
(CH(CH3)2), 48.6 (CH), 47.3 (CH2), 21.3 (CH3), 20.9 (CH3) ppm;
HRMS (ESI+, TOF) m/z calcd. for C18H25N2+ [M − Cl]+: 269.2012,
found 209.2028.
homogeneous materials unless otherwise noted. H and 13C NMR
1
spectra were recorded at 600 and 151 MHz, respectively. Chemical
shifts are reported in δ ppm and reference either an internal
tetramethylsilane or solvent peaks. The following abbreviations are
used to indicate the multiplicities: s = singlet; d = doublet; t = triplet;
q = quartet; qt = quintet; sex = sextet; sept = septet dd = doublet of
doublets; ddd = doublet of doublet of doublets; dddd = doublet of
doublet of doublet of doublets; dt = doublet of triplets; app =
apparent; m = multiplet; br = broad. Melting points were uncorrected.
High-resolution mass spectra were recorded on a high resolution
quadrupole time-of-flight instrument operating in an electrospray
ionization (ESI) mode or on a double-focusing magnetic-sector mass
analyzer operating in a fast atomic bombardment (FAB) mode.
Buried volume calculations were performed using the SambVca 2.1
web application available online at https://www.molnac.unisa.it/
OMtools/sambvca2.1/index.html. The crystal structures were up-
loaded in .xyz format. The xz plane was defined by the imidazolidine
ring system, the z-axis was oriented along the xz projection of NHC-
metal bond. All Ag, Ir, Cl, COD ligands and hydrogen atoms were
excluded from the analysis. The following parameters were used for
the calculations: NHC−metal bond distances, 2.0 Å; Bondi radii
scaled by 1.17; and mesh spacing for numerical integration, 0.10. The
results within a 3.5 Å sphere appear in Figures 2 and 3, and the results
from larger spheres are shown in the Supporting Information. NHC
precursors, DHASIMe·HCl,12 DHASIiPr·HBF4,14 DHASICy·HCl,14
and BCPSIMe·HCl,13 were prepared according the procedure we
previously reported.
BCPSICy·HCl 12. A flask was charged with (1R,4S)-1,2,3,4-
tetrahydro-1,4-methanonaphthalene-2,3-dione13 7 (177 mg, 1.03
mmol) and a magnetic stir bar, and the flask was evacuated and
backfilled with argon (this process was repeated three times).
Cyclohexylamine (0.57 mL, 5.15 mmol) was added to the flask at
25 °C, followed by BF3·Et2O (0.13 mL, 1.06 mmol). After stirring at
the same temperature for 15 h, the reaction solution was diluted with
EtOAc (20 mL), and the resulting solution was washed with sat.
NH4Cl aq. (20 mL × 1), dried over Na2SO4, filtered, and
concentrated under reduced pressure to obtain a crude diimine.
The crude residue was dissolved in MeOH (3.4 mL), and the solution
was cooled in an ice bath. NaBH4 (77.9 mg, 2.06 mmol) was added to
the solution at 0 °C, and the mixture was stirred at 25 °C for 1 h. The
reaction was then quenched with sat. NH4Cl aq. (2.0 mL), and the
mixture was extracted with EtOAc (4.0 mL × 3). The combined
organic layers were dried over Na2SO4, filtered, and concentrated
under reduced pressure to afford a crude cis-diamine as a colorless
powder. The crude diamine was dissolved in toluene, and (EtO)3CH
(0.5 mL) and NH4Cl (66.1 mg, 1.24 mmol) were added to the
resultant solution. The reaction solution was heated at 100 °C for 16
h, and then concentrated under reduced pressure. The crude residue
was dissolved in CH2Cl2, and insoluble solid was filtered-off. n-
Hexane was added to the filtrate to precipitate the imidazolinium
chloride. The colorless solid was filtered, and dried under reduced
pressure to afford BCPSICy·HCl 12 (67.6 mg, 0.38 mmol, 37% from
Syntheses of NHC Precursors. BCPSIiPr·HCl 10. A flask was
charged with (1R,4S)-1,2,3,4-tetrahydro-1,4-methanonaphthalene-
2,3-dione13 7 (107 mg, 0.62 mmol) and a magnetic stir bar, and
the flask was evacuated and backfilled with argon (this process was
repeated three times). Toluene (2.0 mL) was added to dissolve the
diketone 7. Isopropylamine (0.27 mL, 3.1 mmol) was then added to
the flask at 25 °C, followed by BF3·Et2O (0.07 mL, 0.62 mmol). After
stirring at the same temperature for 15 h, the reaction solution was
diluted with EtOAc (5.0 mL), and the resulting solution was washed
1
diketone 7) as a colorless powder. mp 125−128 °C; H NMR (600
MHz, CDCl3, 300 K) δ 8.86 (s, 1H, NCHN), 7.25−7.20 (m, 4H,
ArH), 4.99 (dd, J = 1.8, 1.8 Hz, 2H, CH), 3.82 (s, 2H, CH), 3.42
(dddd, J = 12.0, 12.0, 3.6, 3.6 Hz, 2H, Cy), 2.12 (app d, J = 12.0 Hz,
1H, CHAHB), 2.06 (ddd, J = 9.6, 1.8, 1.8 Hz, 1H, CHAHB), 1.90−1.83
(m, 4H, Cy), 1.73 (dddd, J = 12.0, 12.0, 12.0, 3.6 Hz, 2H, Cy), 1.68−
1.63 (m, 4H, Cy), 1.52 (dddd, J = 12.0, 12.0, 12.0, 3.6 Hz, 2H, Cy),
F
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1.34−1.30 (m, 4H, Cy), 1.24 (ddddd, J = 12.0, 12.0, 12.0, 3.6, 3.6 Hz,
2H, Cy) ppm; 13C{1H} NMR (151 MHz, CDCl3, 300 K) δ 155.0
(NCN), 141.7 (Ar), 127.5 (Ar), 123.6 (Ar), 65.6 (CH), 57.5 (Cy),
48.5 (CH), 47.1 (CH2), 31.7 (Cy), 31.1 (Cy), 25.3 (Cy), 25.0 (Cy),
24.9 (Cy) ppm; HRMS (ESI+, TOF) m/z calcd. for C24H33N2+ [M −
Cl]+: 349.2638, found 349.2643.
Typical Procedure for the Syntheses of NHC−AgCl
Complexes. A flask was charged with NHC precursor (imidazoli-
nium chloride, 1.0 equiv), Ag2O (2.0 equiv), molecular sieves 4 Å
(100 w/w%), and a magnetic stir bar. The resultant flask was
evacuated and backfilled with argon (repeated three times), and
CH2Cl2 ([precursor] = 0.1 M) was added to the flask. The reaction
mixture was darkened with aluminum foil and stirred for 24 h at 25
°C. The mixture was then filtered through a short pad of Celite, which
was washed with CH2Cl2. The combined filtrate and washings were
concentrated under reduced pressure to reduce the total volume of
the solution to ca. 1 mL. n-Hexane was added to the resulting solution
to precipitate the NHC−AgCl complex, which was filtered and dried
under reduced pressure to afford the title complexes.
dried under reduced pressure to afford the [Ir (COD)Cl(NHC)]
complex.
[Ir(COD)Cl(DHASIMe)] 19. Yield: 9.2 mg (0.27 mmol, 15% from
1
1.79 mmol of DHASIMe·HCl); mp 234 °C (decomp) ; H NMR
(600 MHz, CDCl3, 300 K) δ 7.31 (dd, J = 5.4, 3.6 Hz, 2H, ArH), 7.29
(dd, J = 5.4, 3.6 Hz, 2H, ArH), 7.18 (dd, J = 5.4, 3.6 Hz, 2H, ArH),
7.16 (dd, J = 5.4, 3.6 Hz, 2H, ArH), 4.59 (dd, J = 1.2, 1.2 Hz, 2H,
CH), 4.38 (app dd, J = 3.0, 3.0 Hz, 2H, COD), 4.20 (dd, J = 1.2, 1.2
Hz, 2H, CH), 3.36 (s, 6H, NCH3), 2.09−2.00 (m, 2H, COD), 2.00−
1.92 (m, 2H, COD), 1.90 (app dd, J = 3.0, 3.0 Hz, 2H, COD), 1.64−
1.52 (m, 2H, COD), 1.42 (tt, J = 7.2, 7.2 Hz, 2H, COD) ppm;
13C{1H} NMR (151 MHz, CDCl3, 300 K) δ 208.5 (Ccarbene), 139.8
(Ar), 138.5 (Ar), 127.2 (Ar), 126.9 (Ar), 125.7 (Ar), 125.1 (Ar), 84.9
(COD), 68.5 (CH), 52.6 (COD), 46.1 (CH), 34.6 (CH3), 33.3
(COD), 29.3 (COD) ppm; HRMS (ESI+, TOF) m/z calcd. for
C27H30N2ClIrNa+ [M + Na]+: 633.1619, found 633.1591.
[Ir(COD)Cl(DHASIiPr)] 20. Yield: 13.8 mg (21 μmol, 26% from 0.08
1
mmol of DHASIiPr·HBF4); mp 230 °C (decomp.); H NMR (600
MHz, CDCl3, 300 K) δ 7.29 (dd, J = 5.4, 3.6 Hz, 2H, ArH), 7.27 (dd,
J = 5.4, 3.6 Hz, 2H, ArH), 7.16 (dd, J = 5.4, 3.6 Hz, 2H, ArH), 7.14
(dd, J = 5.4, 3.6 Hz, 2H, ArH), 5.38 (sept, J = 6.6 Hz, 2H,
NCH(CH3)2), 4.58 (s, 2H, CH), 4.40 (dd, J = 1.2, 1.2 Hz, 2H, CH),
4.33 (app dd, J = 3.0, 3.0 Hz, 2H, COD), 2.10 (app dd, J = 3.0, 3.0 Hz,
2H, COD), 2.09−2.00 (m, 2H, COD), 2.00−1.92 (m, 2H, COD),
1.62−1. 54 (m, 2H, COD), 1.54 (d, J = 6.6 Hz, 6H, NCH(CH3)2),
1.43 (tt, J = 7.2, 7.2 Hz, 2H, COD), 1.31 (d, J = 6.6 Hz, 6H,
NCH(CH3)2) ppm; 13C{1H} NMR (151 MHz, CDCl3, 300 K) δ
212.7 (Ccarbene), 140.7 (Ar), 139.0 (Ar), 127.1 (Ar), 126.6 (Ar), 126.3
(Ar), 125.0 (Ar), 83.9 (COD), 64.0 (CH), 53.1 (CH), 52.6 (COD),
49.1 (CH), 33.4 (COD), 29.3 (COD), 23.4 (CH3), 20.4 (CH3) ppm;
HRMS (ESI+, TOF) m/z calcd. for C31H38N2ClIrNa+ [M + Na]+:
689.2245, found 689.2219.
BCPSIiPrAgCl 13. Yield: 64 mg (0.15 mmol, 70% from 0.22 mmol
1
of BCISIiPr·HCl); mp 227−229 °C; H NMR (600 MHz, CDCl3,
300 K) δ 7.18 (dd, J = 5.4, 3.0 Hz, 2H, ArH), 7.14 (dd, J = 5.4, 3.0
Hz, 2H, ArH), 4.66 (dd, J = 1.8, 1.8 Hz, 2H, CH), 3.92 (sept, J = 6.6
Hz, 2H, NCH(CH3)2), 3.64 (s, 2H, CH), 2.00 (app dd, J = 9.6, 1.2
Hz, 1H, CHAHB), 1.74 (ddd, J = 9.6, 1.2, 1.2 Hz, 1H, CHAHB), 1.32
(d, J = 6.6 Hz, 6H, NCH(CH3)2), 1.26 (d, J = 10.2 Hz, 6H,
NCH(CH3)2) ppm; 13C{1H} NMR (151 MHz, CDCl3, 300 K) δ
142.8 (Ar), 126.8 (Ar), 124.0 (Ar), 66.3 (CH), 52.6 (CH(CH3)2),
49.5 (CH), 48.5 (CH2), 22.8 (CH3), 22.5 (CH3) ppm (the carbenic
carbon was not observed); HRMS (ESI+, TOF) m/z calcd. for
C36H48N4Ag+ [L2Ag]+: 643.2924, found 643.2913. Although the MS
spectrum showed that m/z L2Ag+ was the parent peak, the structure
was a L−Ag−Cl form in a solid state.
[Ir(COD)Cl(DHASICy)] 21. Yield: 24.0 mg (35 μmol, 32% from 0.11
1
mmol of DHASICy·HCl); mp 247 °C (decomp.); H NMR (600
BCPSICyAgCl 14. Yield: 30 mg (0.06 mmol, 68% from 0.09 mmol
MHz, CDCl3, 300 K) δ 7.29 (dd, J = 5.4, 3.6 Hz, 2H, ArH), 7.27 (dd,
J = 5.4, 3.6 Hz, 2H, ArH), 7.15 (dd, J = 5.4, 3.6 Hz, 2H, ArH), 7.13
(dd, J = 5.4, 3.6 Hz, 2H, ArH), 4.98 (dddd, J = 12.0, 12.0, 1.2, 3.0, 3.0
Hz, 2H, Cy), 4.59 (s, 2H, CH), 4.40−4.35 (m, 4H, CH, COD), 2.12
(br d, J = 12.0 Hz, 2H, Cy), 2.07 (app dd, J = 3.0, 3.0 Hz, 2H, COD),
2.00−1.79 (m, 12H, Cy, COD), 1.75 (br d, J = 12.0 Hz, 2H, Cy),
1.63−1. 57 (m, 2H, COD), 1.53−1.38 (m, 8H, COD, Cy), 1.27−1.18
(m, 2H, Cy) ppm; 13C{1H} NMR (151 MHz, CDCl3, 300 K) δ 212.7
(Ccarbene), 140.8 (Ar), 139.1 (Ar), 127.0 (Ar), 126.6 (Ar), 126.2 (Ar),
125.0 (Ar), 83.4 (COD), 64.8 (CH), 61.2 (Cy), 52.7 (COD), 49.1
(CH), 34.7 (Cy), 33.6 (COD), 31.2 (Cy), 29.5 (COD), 27.0 (Cy),
26.1 (Cy), 26.0 (Cy) ppm; HRMS (FAB+) m/z calcd. for
C37H46N2ClIrNa+ [M]+: 746.2979, found 746.2994.
1
of BCPSICy·HCl); mp 171−173 °C; H NMR (600 MHz, CDCl3,
300 K) δ 7.16 (dd, J = 5.4, 3.0 Hz, 2H, ArH), 7.13 (dd, J = 5.4, 3.0
Hz, 2H, ArH), 4.66 (dd, J = 1.8, 1.8 Hz, 2H, CH), 3.64 (s, 2H, CH),
3.45 (dddd, J = 12.0, 12.0, 3.6, 3.6 Hz, 2H, Cy), 2.02 (app d, J = 12.0
Hz, 1H, CHAHB), 1.97 (app dd, J = 9.6, 0.6 Hz, 1H, CHAHB), 1.89−
1.79 (m, 4H, Cy), 1.72−1.66 (m, 4H, Cy), 1.66−1.59 (m, 2H, Cy),
1.51 (dddd, J = 12.0, 12.0, 12.0, 3.6 Hz, 2H, Cy), 1.38−1.24 (m, 6H,
Cy), 1.14 (ddddd, J = 12.0, 12.0, 12.0, 3.6, 3.6 Hz, 2H, Cy) ppm;
13C{1H} NMR (151 MHz, CDCl3, 300 K) δ 142.8 (Ar), 126.6 (Ar),
123.9 (Ar), 66.6 (CH), 60.7 (Cy), 49.5 (CH), 48.0 (CH2), 33.5 (Cy),
33.1 (Cy), 25.70 (Cy), 25.67 (Cy), 25.3 (Cy) ppm (the carbenic
carbon was not observed); HRMS (ESI+, TOF) m/z calcd. for
C48H64N4Ag + [L2Ag]+: 803.4176, found 803.4153. Although the MS
spectrum showed that m/z L2Ag+ was the parent peak, the structure
was a L−Ag−Cl form in a solid state.
[Ir(COD)Cl(BCPSIMe)] 22. Yield: 40.1 mg (80 μmol,40% from 0.20
1
mmol of BCPSIMe·HCl); mp 234 °C (decomp.); H NMR (600
MHz, CDCl3, 300 K) δ 7.21 (dd, J = 5.4, 3.0 Hz, 2H, ArH), 7.10 (dd,
J = 5.4, 3.0 Hz, 2H, ArH), 4.45 (dd, J = 1.8, 1.8 Hz, 2H, CH), 4.32
(app dd, J = 3.0, 3.0 Hz, 2H, COD), 3.60 (dd, J = 1.8, 1.8 Hz, 2H,
CH), 3.23 (s, 6H, NCH3), 2.04−1.96 (m, 2H, COD), 1.95−1.88 (m,
5H, CHAHB, COD), 1.66 (ddd, J = 9.6, 1.2, 1.2 Hz, 1H, CHAHB),
1.59−1.52 (m, 2H, COD), 1.41−1.33 (m, 2H, COD) ppm; 13C{1H}
NMR (151 MHz, CDCl3, 300 K) δ 206.5 (Ccarbene), 142.9 (Ar), 126.7
(Ar), 122.8 (Ar), 84.1 (COD), 69.9 (CH), 52.1 (COD), 47.8 (CH),
47.4 (CH2), 35.0 (CH3), 33.1 (COD), 29.1 (COD) ppm; HRMS
(FAB+) m/z calcd. for C22H28N2ClIr+ [M]+: 548.1570, found
548.1581.
Typical Procedure for the Syntheses of [Ir (COD)Cl(NHC)]
Complexes. A flask was charged with a ligand precursor
(imidazolinium chloride of imidazolinium tetrafluoroborate, 1.0
equiv of Ir metal) and a magnetic stir bar, and the flask was
evacuated and backfilled with argon (repeated three times). THF
([precursor] = ca. 0.2 M) was added to the flask, and the resultant
mixture was cooled in an ice bath. A commercial solution of n-BuLi
(1.54 M in n-hexane, 2.0 equiv of NHC precursor) was added to the
mixture, and the reaction solution was stirred 15 min at 25 °C. The
solution containing in situ formed NHC ligand was then transferred
to another flask which was charged with [Ir(COD)Cl]2 (1.0 equiv)
and THF ([Ir] = 0.5 M), and the resultant solution was stirred for 4.5
h at 25 °C. The mixture was then filtered through a short pad of
Celite, and the filter cake was washed with CH2Cl2. The combined
filtrate and washings were concentrated under reduced pressure, and
the residue was dissolved in CH2Cl2, and the solution was filtered
through a short pad of silica gel, eluted with CH2Cl2. The eluted
solution was concentrated under reduced pressure to ca. 1.0 mL. n-
Hexane was added to the residual solution to precipitated the title
complex as a bright yellow crystalline solid. The solid was filtered and
[Ir(COD)Cl(BCPSIiPr)] 23. Yield: 14.5 mg (23 μmol, 23% from 0.10
mmol of BCISIiPr·HCl); mp >250 °C; 1H NMR (600 MHz, CDCl3,
300 K) δ 7.19 (dd, J = 5.4, 3.0 Hz, 2H, ArH), 7.10 (dd, J = 5.4, 3.0
Hz, 2H, (ArH), 5.19 (sept, J = 6.6 Hz, 2H, NCH(CH3)2), 4.62 (dd, J
= 1.8, 1.8 Hz, 2H, CH), 4.29 (app dd, J = 3.0, 3.0 Hz, 2H, COD), 3.55
(dd, J = 1.8, 1.8 Hz, 2H, CH), 2.06−1.97 (m, 2H, COD), 1.96−1.88
(m, 5H, CHAHB, COD), 1.72 (ddd, J = 9.6, 1.2, 1.2 Hz, 1H, CHAHB),
1.59−1.52 (m, 2H, COD), 1.43−1.35 (m, 2H, COD), 1.39 (d, J = 6.6
Hz, 6H, NCH(CH3)2), 1.29 (d, J = 6.6 Hz, 6H, NCH(CH3)2) ppm;
13C{1H} NMR (151 MHz, CDCl3, 300 K) δ 210.3 (Ccarbene), 143.8
G
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(Ar), 126.0 (Ar), 124.4 (Ar), 83.5 (COD), 64.3 (CH), 52.8 (CH),
51.9 (COD), 50.0 (CH), 48.8 (CH2), 33.4 (COD), 29.3 (COD), 22.6
(CH3), 20.8 (CH3) ppm; HRMS (ESI+, TOF) m/z calcd. for
C26H36N2ClIrNa+ [M + Na]+: 627.2088, found 627.2059.
= 1.8, 1.8 Hz, 2H, CH), 3.12 (s, 6H, NCH3), 2.03 (ddd, J = 9.6, 1.2,
1.2 Hz, 1H, CHAHB), 1.70 (ddd, J = 9.6, 1.2, 1.2 Hz, 1H, CHAHB)
ppm; 13C{1H} NMR (151 MHz, CDCl3, 300 K) δ 198.3 (Ccarbene),
181.8 (CO), 166.8 (CO), 142.0 (Ar), 127.7 (Ar), 122.9 (Ar),
70.0 (CH), 48.1 (CH), 46.8 (CH2), 35.5 (CH3) ppm; HRMS (ESI+,
TOF) m/z calcd. for C16H16N2O2ClIrNa+ [M + Na]+: 519.0422,
found 519.0397.
[IrCl(CO)2(BCPSIiPr)] 29. Yield: 10.3 mg (18.6 μmol, 96% from
19.4 μmol of [Ir(COD)Cl(BCPSIiPr)] 23); mp 168−171 °C; IR
(CH2Cl2) ν(CO) = 2065, 1987 cm−1; 1H NMR (600 MHz,
CDCl3, 300 K) δ 7.24 (dd, J = 5.4, 3.0 Hz, 2H, ArH), 7.14 (dd, J =
5.4, 3.0 Hz, 2H, ArH), 4.81 (sept, J = 6.6 Hz, 2H, NCH(CH3)2), 4.70
(dd, J = 1.8, 1.8 Hz, 2H, CH), 3.65 (dd, J = 1.8, 1.8 Hz, 2H, CH),
2.02 (ddd, J = 9.6, 1.2, 1.2 Hz, 1H, CHAHB), 1.77 (ddd, J = 9.6, 1.2,
1.2 Hz, 1H, CHAHB), 1.35 (d, J = 6.6 Hz, 6H, NCH(CH3)2), 1.30 (d,
J = 6.6 Hz, 6H, NCH(CH3)2) ppm; 13C{1H} NMR (151 MHz,
CDCl3, 300 K) δ 198.9 (Ccarbene), 181.9 (CO), 166.9 (CO),
143.0 (Ar), 126.5 (Ar), 124.5 (Ar), 65.0 (CH), 52.5 (CH), 49.9
(CH), 48.9 (CH2), 22.5 (CH3), 20.5 (CH3) ppm; HRMS (ESI+,
TOF) m/z calcd. for C20H24N2O2ClIrNa+ [M + Na]+: 575.1048,
found 575.1022.
[IrCl(CO)2(BCPSICy)] 30. Yield: 8.8 mg (14.0 μmol, 93% from 15.0
μmol of [Ir(COD)Cl(BCPSICy)] 24); mp 181−184 °C; IR
(CH2Cl2) ν(CO) = 2063, 1985 cm−1; 1H NMR (600 MHz,
CDCl3, 300 K) δ 7.21 (dd, J = 5.4, 3.0 Hz, 2H, ArH), 7.12 (dd, J =
5.4, 3.0 Hz, 2H, ArH), 4.68 (dd, J = 1.8, 1.8 Hz, 2H, CH), 4.36 (dddd,
J = 12.0, 12.0, 3.6, 3.6 Hz, 2H, Cy), 3.63 (dd, J = 1.8, 1.8 Hz, 2H,
CH), 2.02−1.95 (m, 3H, CHAHB, Cy), 1.93−1.84 (m, 4H, Cy), 1.82−
1.75 (m, 2H, Cy), 1.74−1.65 (m, 3H, CHAHB, Cy), 1.61−1.49 (m,
4H, Cy), 1.43−1.29 (m, 2H, Cy), 1.12 (ddddd, J = 12.0, 12.0, 12.0,
3.6, 3.6 Hz, 2H) ppm; 13C{1H} NMR (151 MHz, CDCl3, 300 K) δ
199.0 (Ccarbene), 182.1 (CO), 167.0 (CO), 143.1 (Ar), 126.5
(Ar), 124.5 (Ar), 65.7 (CH), 60.5 (Cy), 50.0 (CH), 48.6 (CH2), 33.2
(Cy), 31.2 (Cy), 26.0 (Cy), 25.6 (Cy), 25.5 (Cy) ppm; HRMS (ESI+,
TOF) m/z calcd. for C26H32N2O2ClIrNa+ [M + Na]+: 655.1674,
found 655.1673.
[Ir(COD)Cl(BCPSICy)] 24. Yield: 23.3 mg (33 μmol, 33% from 0.10
mmol of BCPSICy·HCl); mp 242 °C (decomp.); 1H NMR (600
MHz, CDCl3, 300 K) δ 7.17 (dd, J = 5.4, 3.0 Hz, 2H, ArH), 7.08 (dd,
J = 5.4, 3.0 Hz, 2H, ArH), 4.60 (dd, J = 1.8, 1.8 Hz, 2H, CH), 4.78
(dddd, J = 12.0, 12.0, 3.6, 3.6 Hz, 2H, CH), 4.32 (app dd, J = 3.0, 3.0
Hz, 2H, COD), 3.53 (dd, J = 1.8, 1.8 Hz, 2H, CH), 2.07−1.96 (m,
6H, Cy, COD), 1.95−1.85 (m, 7H, CHAHB, Cy, COD), 1.83−1.76 (m,
2H, Cy), 1.73−1.66 (m, 3H, CHAHB, Cy), 1.65−1.55 (m, 4H, Cy,
COD), 1.50−1.31 (m, 8H, Cy, COD), 1.13 (dddd, J = 12.0, 12.0, 3.6,
3.6 Hz, 2H, Cy) ppm; 13C{1H} NMR (151 MHz, CDCl3, 300 K) δ
210.2 (Ccarbene), 143.9 (Ar), 125.9 (Ar), 124.5 (Ar), 83.1 (COD), 65.2
(CH), 60.2 (Cy), 52.4 (COD), 50.0, 48.6 (CH2), 33.8, 33.6, 31.1,
29.5, 26.7, 25.92, 25.85; LRMS (ESI+, TOF) m/z calcd. for
C32H44N2ClIrNa+ [M + Na]+: 703, found 703. Anal. Calcd for
C32H44N2ClIr (684.3850): C, 56.16; H, 6.48; N, 4.09. Found: C,
56.03, H, 6.83; N, 4.11.
Typical Procedure for the Syntheses of [IrCl(CO)2(NHC)]
Complexes. CO gas was bubbled through a solution of [(NHC)-
Ir(COD)Cl] in CH2Cl2 (0.01 M) for 10 min. Ar gas was then
bubbled for 5 min, and the resultant solution was concentrated under
reduced pressure. The residue was washed with n-hexane and the
insoluble solid was filtered, washed with a minimum amount of n-
hexane, and dried under reduced pressure to afford the title complex
as a pale yellow powder.
[IrCl(CO)2(DHASIMe)] 25. Yield: 9.0 mg (16.2 μmol, 94% from 17.2
μmol of [Ir(COD)Cl(DHASIMe)] 19); mp 190 °C (decomp) ; IR
(CH2Cl2) ν(CO) = 2070, 1989 cm−1; 1H NMR (600 MHz,
CDCl3, 300 K) δ 7.35 (dd, J = 5.4, 3.0 Hz, 4H, ArH), 7.22 (dd, J =
5.4, 3.6 Hz, 2H, ArH), 7.21 (dd, J = 5.4, 3.6 Hz, 2H, ArH), 4.67 (s,
2H, CH), 4.29 (dd, J = 1.2, 1.2 Hz, 2H, CH), 3.25 (s, 6H, CH3) ppm;
13C{1H} NMR (151 MHz, CDCl3, 300 K) δ 199.9 (Ccarbene), 181.8
(CO), 166.6 (CO), 139.2 (Ar), 137.7 (Ar), 127.6 (Ar), 127.5
(Ar), 125.5 (Ar), 125.2 (Ar), 68.7 (CH), 45.8 (CH), 34.9 (CH3)
ppm; HRMS (ESI+, TOF) m/z calcd. for C21H18N2O2ClIrNa+ [M +
Na]+: 581.0578, found 581.0560.
[IrCl(CO)2(DHASIiPr)] 26. Yield: 8.6 mg (14.0 μmol, 93% from 15.1
μmol of [Ir(COD)Cl(DHASIiPr)] 20); mp 215−218 °C; IR
(CH2Cl2) ν(CO) = 2066, 1986 cm−1; 1H NMR (600 MHz,
CDCl3, 300 K) δ 7.33 (dd, J = 5.4, 3.6 Hz, 2H, ArH), 7.32 (dd, J =
5.4, 3.6 Hz, 2H, ArH), 7.19 (dd, J = 5.4, 3.6 Hz, 2H, ArH), 7.18 (dd, J
= 5.4, 3.6 Hz, 2H, ArH), 4.92 (sept, J = 6.6 Hz, 2H, NCH(CH3)2),
4.63 (s, 2H, CH), 4.49 (dd, J = 1.2, 1.2 Hz, 2H, CH), 1.46 (d, J = 6.6
Hz, 6H, NCH(CH3)2), 1.37 (d, J = 6.6 Hz, 6H, NCH(CH3)2) ppm;
13C{1H} NMR (151 MHz, CDCl3, 300 K) δ 200.7 (Ccarbene), 181.7
(CO), 167.1 (CO), 140.3 (Ar), 138.1 (Ar), 127.2 (Ar), 126.6
(Ar), 126.5 (Ar), 124.9 (Ar), 64.7 (CH), 52.6 (NCH(CH3)2), 48.4
(CH), 23.4 (CH3), 19.8 (CH3) ppm; HRMS (ESI+, TOF) m/z calcd.
for C25H26N2O2ClIrNa+ [M + Na]+: 637.1204, found 637.1178.
[IrCl(CO)2(DHASICy)] 27. Yield: 9.1 mg (13.1 μmol, 97% from 13.5
μmol of [Ir(COD)Cl(DHASIiPr)] 21); mp 232 °C (decomp.); IR
(CH2Cl2) ν(CO) = 2063, 1983 cm−1; 1H NMR (600 MHz,
CDCl3, 300 K) δ 7.33 (dd, J = 5.4, 3.0 Hz, 2H, ArH), 7.32 (dd, J =
5.4, 3.0 Hz, 2H,. ArH), 7.18 (dd, J = 5.4, 3.0 Hz, 2H, ArH), 7.17 (dd,
J = 5.4, 3.0 Hz, 2H, ArH), 4.66 (s, 2H, CH), 4.50−4.43 (m, 4H, CH,
Cy), 1.90−1.65 (m, 12H, Cy), 1.48−1.38 (m, 4H, Cy), 1.29−1.22 (m,
2H, Cy) ppm; 13C{1H} NMR (151 MHz, CDCl3, 300 K) δ 200.8
(Ccarbene), 181.9 (CO), 167.2 (CO), 140.3 (Ar), 138.2 (Ar),
127.1 (Ar), 126.51 (Ar), 126.49 (Ar) 124.9 (Ar), 65.1 (CH), 60.9
(NCy), 48.4 (CH), 34.1 (Cy), 30.9 (Cy), 26.1 (Cy), 25.8 (Cy), 25.7
(Cy) ppm; HRMS (ESI+, TOF) m/z calcd. for C31H34N2O2ClIrNa+
[M + Na]+: 717.1830, found 717.1821.
Typical Procedure for an Allylic Arylation. CH2Cl2 (0.50 mL)
was added to a flask charged with CuCl (2.5 mg, 25 μmol), silver-
carbene complex (25 μmol), and a magnetic stir bar at 25 °C. The
resulting suspension was stirred at the same temperature for 10 min,
and a solution of cinnamyl bromide (99 mg, 0.50 mmol) in CH2Cl2
(0.50 mL) was added to the resulting mixture, followed by cooling in
a dry ice/EtOH bath. A solution of PhMgBr (0.29 mL of 3.0 M
solution in Et2O diluted with 0.25 mL of CH2Cl2) was added to the
reaction mixture by a syringe pump over 15 min. Once the addition
was complete, the resulting mixture was stirred for another 1 h at the
same temperature. The reaction mixture was then diluted with Et2O
(2.0 mL) and quenched with 2 M HCl aq. (2.0 mL) at −78 °C. The
resulting mixture was allowed to warm to 25 °C and extracted with
Et2O (3 × 4 mL). The combined organic extracts were dried over
Na2SO4, filtered, and concentrated under reduced pressure. The crude
residue was dissolved in CH2Cl2, and anthracene (89 mg, 0.5 mmol)
was added to the solution as an external standard to calculate the yield
1
based on the crude H NMR. After the removal of the solvent, the
1
ratios of the products were calculated based on the crude H NMR.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00564.
Copies of 1H and 13C NMR spectra of all new
compounds and crystallographic data of complexes 13,
14, 20, and 23 (PDF)
[IrCl(CO)2(BCPSIMe)] 28. Yield: 8.5 mg (17.1 μmol, 94% from 18.2
μmol of [Ir(COD)Cl(BCPSIMe)] 22); mp 170 °C (decomp.); IR
(CH2Cl2) ν(CO) = 2068, 1987 cm−1; 1H NMR (600 MHz,
CDCl3, 300 K) δ 7.28 (dd, J = 5.4, 3.0 Hz, 2H, ArH), 7.18 (dd, J =
5.4, 3.0 Hz, 2H, ArH), 4.52 (dd, J = 1.8, 1.8 Hz, 2H, CH), 3.72 (dd, J
Accession Codes
CCDC 2025127−2025130 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
H
https://dx.doi.org/10.1021/acs.organomet.0c00564
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Organometallics
pubs.acs.org/Organometallics
Article
Ed. N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient
Synthetic Tools; The Royal Society of Chemistry: Cambridge, U.K.,
2011.
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.
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Cesar, V. Synthetic Routes to N-Heterocyclic Carbene Precursors.
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AUTHOR INFORMATION
Corresponding Authors
■
(9) Izquierdo, F.; Manzini, S.; Nolan, S. P. The Use of the Sterically
Demanding IPr* and Related Ligands in Catalysis. Chem. Commun.
2014, 50, 14926−14937.
Shin Ando − Faculty of Life Sciences, Kumamoto University,
Chuo-ku, Kumamoto 862-0973, Japan; orcid.org/0000-
0002-1087-5391; Email: ando@gpo.kumamoto-u.ac.jp
Tadao Ishizuka − Faculty of Life Sciences, Kumamoto
University, Chuo-ku, Kumamoto 862-0973, Japan;
Email: tstone@gpo.kumamoto-u.ac.jp
(10) (a) Clavier, H.; Nolan, S. P. Percent Buried Volume for
Phosphine and N-Heterocyclic Carbene Ligands: Steric Properties in
Organometallic Chemistry. Chem. Commun. 2010, 46, 841−861.
(b) Hillier, A. C.; Sommer, W. J.; Yong, B. S.; Petersen, J. L.; Cavallo,
L.; Nolan, S. P. A Combined Experimental and Theoretical Study
Examining the Binding of N-Heterocyclic Carbenes (NHC) to the
Cp*RuCl (Cp* = η5-C5Me5) Moiety: Insight into Stereoelectronic
Differences between Unsaturated and Saturated NHC Ligands.
Organometallics 2003, 22, 4322−4326. (c) Poater, A.; Cosenza, B.;
Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo, L. SambVca:
A Web Application for the Calculation of the Buried Volume of N-
Heterocyclic Carbene Ligands. Eur. J. Inorg. Chem. 2009, 2009,
1759−1766.
Author
Mio Otsuka − Faculty of Life Sciences, Kumamoto University,
Chuo-ku, Kumamoto 862-0973, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00564
Notes
(11) (a) Falivene, L.; Cao, Z.; Petta, A.; Serra, L.; Poater, A.; Oliva,
R.; Scarano, V.; Cavallo, L. Towards the Online Computer-Aided
Design of Catalytic Pockets. Nat. Chem. 2019, 11, 872−879.
(b) Falivene, L.; Credendino, R.; Poater, A.; Petta, A.; Serra, L.;
Oliva, R.; Scarano, V.; Cavallo, L. SambVca 2. A Web Tool for
Analyzing Catalytic Pockets with Topographic Steric Maps. Organo-
metallics 2016, 35, 2286−2293.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was financially supported in part by JSPS
KAKENHI Grant Numbers 18K06583 and 19K06998. We are
grateful to Dr. Hiroyasu Sato at the Rigaku Corporation for
assistance with the crystallographic analysis.
(12) Ando, S.; Matsunaga, H.; Ishizuka, T. Syntheses of Effectively-
Shielded N-Heterocyclic Carbene Ligands. Tetrahedron 2013, 69,
1687−1693.
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