Palladium(II) Complexes with Anionic N-Heterocyclic Carbene-Borate Ligands as Catalysts for the Amination of Aryl Halides

Article abstract of DOI:10.1021/acs.organomet.6b00217

A series of (allyl)palladium(II) complexes of anionic N-heterocyclic carbenes that contain a weakly coordinating borate moiety (WCA-NHC) were prepared by deprotonation of the carbene IPr with n-butyllithium in the 4-position, which was followed by addition of B(C₆F₅)₃ and reaction of the resulting lithium carbene-borate with [(η³-allyl)Pd(μ-Cl)]₂ (allyl = 2-propenyl, 2-butenyl, 2-methyl-2-propenyl, 1-phenyl-2-propenyl). In polar solvents such as tetrahydrofuran, chloropalladate complexes of the type [Li(THF)₃][(WCA-NHC)PdCl(η³-allyl)] were formed, whereas the reactions in toluene afforded neutral complexes of the type [(WCA-NHC)Pd(η³-allyl)], which exhibit an intramolecular interaction with the N-aryl groups of the carbene ligands. The allyl-palladium complexes were employed as catalysts for the Buchwald-Hartwig amination of various aryl halides with morpholine.

Full text of DOI:10.1021/acs.organomet.6b00217

Article
pubs.acs.org/Organometallics
Palladium(II) Complexes with Anionic N‑Heterocyclic Carbene−
Borate Ligands as Catalysts for the Amination of Aryl Halides
Andrea Winkler, Kai Brandhorst, Matthias Freytag, Peter G. Jones, and Matthias Tamm*
Institut fur Anorganische und Analytische Chemie, Technische Universitat Braunschweig, Hagenring 30, 38106 Braunschweig,
̈
̈
Germany
S
* Supporting Information
ABSTRACT: A series of (allyl)palladium(II) complexes of anionic N-heterocyclic carbenes that
contain a weakly coordinating borate moiety (WCA-NHC) were prepared by deprotonation of
the carbene IPr with n-butyllithium in the 4-position, which was followed by addition of
B(C6F5)3 and reaction of the resulting lithium carbene−borate with [(η3-allyl)Pd(μ-Cl)]2 (allyl =
2-propenyl, 2-butenyl, 2-methyl-2-propenyl, 1-phenyl-2-propenyl). In polar solvents such as
tetrahydrofuran, chloropalladate complexes of the type [Li(THF)3][(WCA-NHC)PdCl(η3-
allyl)] were formed, whereas the reactions in toluene afforded neutral complexes of the type
[(WCA-NHC)Pd(η3-allyl)], which exhibit an intramolecular interaction with the N-aryl groups
of the carbene ligands. The allyl-palladium complexes were employed as catalysts for the
Buchwald−Hartwig amination of various aryl halides with morpholine.
anions (WCA).34 Direct access to the lithium salts of these
INTRODUCTION
■
WCA-NHC ligands was established by deprotonation of
various carbenes such as ItBu, IMes, and IPr (R = tBu, Mes,
Dipp) with n-butyllithium (Robinson’s method)11 and reaction
of the intermediate ditopic anionic carbenes7 with the
appropriate borane electrophile.34,35
N-Heterocyclic carbenes (NHCs) are nowadays ubiquitous and
indispensable as ancillary ligands in homogeneous catalysis,1−5
and quite recently, carbenes incorporating an anionic backbone
have emerged as a new and steadily growing NHC subclass,6−19
with the most prominent examples reported by Guy Lavigne
and co-workers.20−28 Our own contribution to this field was
triggered by the observation that frustrated Lewis pairs
composed of sterically congested imidazolin-2-ylidenes and
fluorinated boranes such as B(C6F5)3 would undergo self-
deactivation with formation of abnormal carbene−borane
Lewis pairs, in which the borane moiety is attached to the 4-
position of the N-heterocycle.29−33 The corresponding anionic
carbenes, formally derived by deprotonation in the 2-position,
were termed WCA-NHC (Figure 1), since the fluorinated
borate moieties can be considered as weakly coordinating
The resulting lithiocarbene species were employed for the
preparation of neutral gold(I) complexes [(WCA-NHC)AuL]
(A, L = PPh3, THT), with the latter, the tetrahydrothiophene
complex, serving as a single-source catalyst for enyne
cycloisomerization in nonpolar solvents.34 Furthermore,
rhodium(I) and iridium(I) complexes were prepared by
reaction of (WCA-NHC)Li species with [(COD)MCl]2
(COD = 1,5-cyclooctadiene), affording neutral complexes of
type B, which exhibit an intramolecular interaction with the N-
aryl groups of the carbene ligands (Figure 1). In all cases,
coordination through the borate-flanking aryl group was
observed, which is substantially stabilized by short intra-
molecular C−F···Cipso contacts involving the metal-bound ipso-
carbon atom. Again, the overall neutral charge of complexes B
allowed the iridium complexes to be used as highly active olefin
hydrogenation catalysts in nonpolar media or in neat alkene
substrate, which is not possible for analogous cationic Crabtree-
type iridium catalysts.35
N-Heterocyclic carbenes have found particularly widespread
use as ancillary ligands in palladium-catalyzed cross-coupling
reactions,36−40 with numerous NHC-Pd systems serving as
(pre)catalysts for the coupling of amines with aryl halides,41−45
i.e., Buchwald−Hartwig amination.46−54 Figure 2 shows a
selection of palladium−carbene complexes. Following publica-
tions on catalytic systems that were composed of imidazolium
Figure 1. Transition metal complexes containing N-heterocyclic
carbene ligands with a weakly coordinating borate moiety (WCA-
NHC).
Received: March 18, 2016
© XXXX American Chemical Society
A
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propenyl) in toluene gave direct access to the zwitterionic
[(WCA-NHC)Pd(η3-allyl)] (1a, R = R′ = H; 1b, R = H, R′ =
Me; 1c, R = Me, R′ = H; 1d, R = Ph, R′ = H) by chloride
substitution (Scheme 1). However, the low solubility of the
Scheme 1. Synthesis of the Palladium(II) [WCA-
NHC)Pd(η3-allyl)] Complexes 1a−d and 3a−d
Figure 2. Representative Pd-NHC precatalysts for the amination of
aryl halides (Dipp = 2,6-diisopropylphenyl).
or imidazolinium salts and the Pd(0) complexes Pd2(dba)3 or
Pd(dba)2 (dba = dibenzylideneacetone), respectively,55,56
dicarbene Pd(0) complexes such as C57,58 were the first
isolable NHC complexes used as catalysts for aryl amina-
tion.59−61 Nevertheless, the majority of catalyst systems contain
stable and readily available Pd(II) complexes such as the
dinuclear complex D.62 Palladium-allyl complexes of type E
represent an intensely studied and prominent class of
precatalyst, since they are usually air- and moisture-stable and
can be conveniently accessed by reaction of [(η3-allyl)Pd(μ-
Cl)]2 complexes with the free carbene ligand.63−69 In addition,
the generation of the active (NHC)Pd(0) catalyst, which
proceeds by reductive elimination of allyl ethers in the presence
of alkoxide bases, is well understood for these systems.
Complexes of the PEPPSI-type F are another class of well-
defined NHC-Pd precatalysts for aryl amination,70−75 in which
the pyridine ligand is responsible for the stabilization of both
the Pd(II) precatalyst and the active NHC-Pd(0) species.76
Precatalysts in which the pyridine ligand is substituted by other
heterocycles have also been reported.77−79 Furthermore, the
acetylacetonate complex G80−83 and the cyclometalated
complexes of type H are recent additions to the ever-growing
family of NHC-Pd amination catalysts, with the latter systems
promoting C−N coupling between aryl sulfides and
amines.84,85
starting materials in toluene prevented the preparation of all
complexes 1 in satisfactory yield. In contrast, if the reaction was
carried out in homogeneous THF solution, the ate-complexes
[Li(THF)n][(WCA-NHC)PdCl(η3-allyl)] 3a−d were conven-
iently isolated in high yield by precipitation with n-pentane.
Drying these precipitates under high vacuum afforded
complexes with varying THF content according to NMR
spectroscopic data and elemental analyses (n = 1, 2), whereas
crystallization from THF/hexane mixtures afforded tris-
(tetrahydrofuran) complexes (n = 3). Subsequent treatment
of these complexes with toluene led to rapid formation of
lithium chloride, and 1a−d were obtained as beige-yellowish
solids after filtration and evaporation. Complex 1a proved to be
air-stable, whereas 1b and 1d are slightly air-sensitive; for 1c,
decomposition was observed even in the solid state, with
formation of a complex mixture of products.
All new palladium compounds 1a−d and 3a−d were
1
characterized by H, 13C, 11B, and 19F NMR spectroscopy (in
Most of the precatalysts in Figure 2 are chloro-palladium(II)
complexes, and we envisaged that the use of anionic WCA-
NHC ligands instead of neutral carbenes would afford
zwitterionic (WCA-NHC)Pd(II) systems in which one chloride
ligand is displaced in favor of an intramolecular arene−
palladium interaction, similar to the situation found for the
Rh(I) and Ir(I) complexes B.35 This assumption proved valid
for a series of Pd-allyl complexes of type 1 (Figure 1), and we
wish to report herein their preparation and structural
characterization, together with their application in catalytic
aryl amination reactions.
THF-d8 for comparability) and by elemental (CHN) analysis
(see the SI for experimental data and NMR spectra). In
comparison with conventional complexes of the type [(IPr)-
PdCl(η3-allyl)] and other NHC-allyl Pd complexes,63−65,86−88
the asymmetry of the WCA-NHC ligand renders the NMR
spectra more complicated, and broad resonances, in particular
for the allyl moieties, generally indicate dynamic behavior in
1
solution. All H NMR spectra exhibit four doublets for the
diastereotopic methyl groups of the two inequivalent Dipp
substituents and a characteristic singlet for the CH hydrogen
atom in the ranges 6.32−6.41 ppm for complexes 1 and 6.13−
6.19 ppm for complexes 3. The 13C NMR signals for the
carbene carbon atoms are found in the range 173.1−177.6 ppm
for 1a−d, which is upfield from the chemical shifts established
for 3a−d (182.6−186.2 ppm). The latter values agree with
those reported for neutral [(IPr)PdCl(η3-allyl)] com-
plexes,63−65,86 and a similar trend to higher field was found
for the carbene signals in cationic complexes of the type
[(IPr)Pd(η3-allyl)(CH3CN)]X (X = BF4, PF6)89 and in
complexes containing weakly coordinating anions, e.g., [(IPr)-
RESULTS AND DISCUSSION
■
Synthesis and Characterization of (WCA-NHC)Pd(II)
Complexes. Similarly to our previously reported procedure,35
the lithium−carbene complex 2 was isolated in a one-pot
reaction by deprotonation of 1,3-bis(diisopropylphenyl)-
imidazolin-2-ylidene (IPr) with n-butyllithium in toluene,11
affording a jelly-like mass, which was treated with a toluene
solution of B(C6F5)3. The toluene solvate 2 was isolated as a
white powder by filtration in almost quantitative yield. The
reaction of 2 with the dimeric complexes [(η3-allyl)Pd(μ-Cl)]2
(allyl = 2-propenyl, 2-butenyl, 2-methyl-2-propenyl, 1-phenyl-2-
PdX(η3-allyl)] (X = OTf, BF4, PF6, AlRF ).87
4
For the latter complexes, it was shown that they are in
equilibrium with their cationic THF solvates [(IPr)Pd(THF)-
B
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(η3-allyl)]+/X− in THF solution,87 suggesting the potential
presence of similar solvates, viz., [(WCA-NHC)Pd(THF)(η3-
allyl)], in THF-d8 solutions of complexes 1. Therefore, the
NMR spectra for the prototypic allyl complex 1a were
additionally recorded in toluene-d8 (see the SI, Figures S14−
S22 for a comparison). The carbene resonance is now found at
168.7 ppm (cf. 177.6 ppm in THF-d8). In toluene-d8, the allyl
moiety gives rise to three well-resolved resonances, a multiplet
at 4.05 ppm for the methine hydrogen atom and two doublets
at 2.53 and 1.87 ppm for the syn- and anti-methylene hydrogen
atoms, respectively. This pattern indicates fast rotation of the
η3-bound C3H5 ligand, which renders the CH2 groups
equivalent. In contrast, coalescence of the two latter signals is
observed in THF-d8 at room temperature, which can be
ascribed to an isomerization via the typical π−σ−π rearrange-
ment occurring in fluxional allyl complexes.90 It is noteworthy
that related complexes, e.g., [(NHC)PdCl(η3-C3H5)] (see SI,
Figure S23),63 generally exhibit a lower degree of fluxionality,
with five distinct allyl multiplets observed at room temper-
ature.87,88
ipso-carbon atom (Pd−C4 = 2.5447(16) Å) and, possibly, one
of the two ortho-carbon atoms (Pd−C5 = 2.6303(17) Å, Pd−
C9 = 3.193(2) Å). These distances correspond to an η2-
coordination mode,91 which leads to a strongly twisted carbene
ligand with disparate Pd−C1−N1 and Pd−C1−N2 angles of
109.06(11)° and 144.98(13)° together with C1−N1−C4 and
C1−N2−C16 angles of 109.11(13)° and 124.09(14)°. The
degree of carbene bending can also be expressed as an
inclination of the Pd−C1 bond by 19.1° to the pseudo-C2 axis
defined by the carbene carbon atom C1 and the centroid of the
imidazole (CtIm) plane. The Pd−C1 bond length of 2.0190(17)
Å is shorter compared to complexes of the type [(IPr)PdCl(η3-
allyl)], e.g., Pd−Ccarbene = 2.040(11) Å in [(IPr)PdCl(η3-
C3H5)].64 Comparison with the latter system reveals similar
Pd−C distances to the carbon atoms C46−C48 of the 2-
propenyl ligand, i.e., Pd−C46 = 2.089(2) Å, Pd−C47 =
2.137(2) Å, Pd−C48 = 2.203(2) Å in 1a, which indicates a
fairly asymmetrically bound allyl ligand. Consequently, the
C46−C47 bond is somewhat longer than the C47−C48 bond,
cf. 1.355(4) and 1.395(3) Å. The overall structural parameters
of 1b, 1c, and 1d are similar to those discussed above, with Pd−
Ccarbene bond lengths of 2.025(2) Å (1b), 2.0259(16) Å (1c),
and 2.022(3) Å (1d), and the bond lengths and angles within
the Pd−allyl moieties, although these are disordered in 1c and
1d, are in agreement with those established for related 2-
butenyl, 2-methyl-2-propenyl, and 1-phenyl-2-propenyl Pd
complexes.65,86
The complexes 1a−d were further characterized by single-
crystal X-ray diffraction analyses. All complexes crystallize in the
monoclinic space group P21/n; 1a and 1b (which are isotypic)
do not contain additional solvent molecules, whereas 1c and 1d
formed the toluene and dichloromethane solvates 1c·toluene
and 1d·2CH2Cl2, respectively. The molecular structure of 1a is
shown in Figure 3, while those of 1b−d are presented in the SI
The molecular structures of the ate-complexes 3a·THF, 3b·
benzene, and 3c·THF were also established by X-ray diffraction
analysis (see Figure 4 for 3a and Figures S4 and S5 in the SI for
3b and 3c). All structures suffer from disorder within the allyl
moieties, and therefore, the respective bond lengths and angles
have to be treated with caution. The Pd−C1 distances in the
anionic complexes are appreciably longer than those in their
neutral congeners (Table 1), viz., 2.0513(18) Å (3a) vs
2.0190(17) Å (1a), 2.055(2) Å (3b) vs 2.025(2) Å (1b), or
2.051(3) Å (3c) vs 2.0259(16) Å (1c). Similar to complexes of
the type [(IPr)PdCl(η3-allyl)], the Pd atoms in 3a, 3b, and 3c
display distorted square-planar geometries, with the Pd−Callyl
and Pd−Cl bond lengths falling in the expected ranges.65,86 It
should be noted that the WCA-NHC ligand in 3a−c is bound
in the conventional fashion, as indicated by long and equivalent
Pd−Cortho distances (ca. 3.43 Å) and by small inclination angles
of 2.9° (3a), 1.7° (3b), and 3.2° (3c). The lithium counterion
in these complexes is bound to the chlorine atom with Li−Cl =
2.3851(4)/2.4037(6)/2.4064(8) Å in 3a/3b/3c, and the Li
coordination sphere is completed in each case by three THF
ligands to afford slightly distorted tetrahedral geometries.
Theoretical Study of Arene−Palladium Interaction.
The arene−metal interactions observed in the palladium
complexes 1a−d are rare examples in which intramolecular
arene coordination is found for complexes of five-membered
1,3-diaryl NHC ligands, while short Pd···Cipso contacts are
commonly observed for instance in complexes containing
sterically demanding biaryl phosphanes of the Buchwald
type.92−103 To the best of our knowledge, previous structurally
characterized systems are limited to Rh and Ir complexes of
type B (Figure 1), which also bear anionic WCA-NHC
ligands,35 and to the molybdenum and tungsten complexes
[(η5-C5H5)M(IMes)(CO)2][B(C6F5)4], for which η2-coordina-
tion of one of the mesityl substituents was confirmed in the
solid state.104,105 A few other examples are known for six-
membered carbenes,20,106,107 in which the aryl substituents are
Figure 3. ORTEP diagram of 1a with thermal displacement
parameters drawn at 50% probability. The minor disorder component
of one IPr group and all hydrogen atoms are omitted for clarity.
Selected bond lengths [Å] and angles [deg] (see also Table 1): Pd−
C1 2.0190(17), Pd−C4 2.5447(16), Pd−C5 2.6303(17), Pd−C9
3.193(2), Pd−C46 2.089(2), Pd−C47 2.137(2), Pd−C48 2.203(2),
B−C2 1.659(2); N1−C1−N2 105.81(14), C1−N1−C4 109.11(13),
Pd−C1−N1 109.06(11), Pd−C1−N2 144.98(13).
(Figures S1−S3). In all cases, the composition [(WCA-
NHC)Pd(η3-allyl)] is confirmed, and pertinent structural data
are assembled in Table 1. Since the binding of the WCA-NHC
ligand in complexes 1a−d is quite similar, we will restrict the
following discussion to the molecular structure of 1a. In
agreement with the situation found for the Rh and Ir complexes
B (Figure 1), the Dipp substituent adjacent to the borate
moiety displays an arene−metal interaction that involves the
C
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a
Table 1. Selected Bond Lengths [Å] and Angles [deg] for Palladium NHC Complexes
1a
1b
1c
1d
3a
3b
3c
4
M−Ccarbene
M−Cipso
M−Cortho
M−C′ortho
M−L
2.0190(17)
2.5447(16)
2.6303(17)
3.193(2)
2.025(2)
2.474(2)
2.671(2)
3.043(2)
2.0259(16)
2.5771(16)
2.721(2)
2.022(3)
2.542(3)
2.655(4)
3.165(3)
2.0513(18)
3.433(2)
3.540(2)
4.322(2)
2.055(2)
3.419(2)
3.554(2)
4.259(2)
2.4037(6)
2.051(3)
3.423(3)
3.521(3)
4.272(3)
2.4064(8)
2.074(2)
3.466(2)
3.652(2)
4.307(2)
2.1732(17)
130.75(14)
124.74(14)
4.6
3.222(2)
b
b
b
c
2.3851(4)
M−C1−N1
M−C1−N2
109.06(11)
144.98(13)
19.1
106.60(15)
147.55(17)
21.5
108.71(11)
145.17(12)
19.5
108.7(2)
144.8(2)
18.9
130.40(12)
124.92(12)
2.9
130.12(14)
125.81(14)
1.7
131.64(18)
124.51(18)
3.2
a
incl.
Pd−C(1)
Pd−C(2)
Pd−C(3)
CF···Cipso
2.089(2)
2.137(2)
2.203(2)
2.652(2)
2.091(2)
2.160(2)
2.193(2)
2.670(3)
2.085(4)
2.159(2)
2.251(2)
2.738(2)
2.111(6)
2.179(8)
2.346(6)
2.731(4)
2.087(10)
2.159(3)
2.232(7)
2.092(2)
2.171(2)
2.195(2)
2.861(2)
2.140(9)
2.142(4)
2.208(4)
2.723(3)
2.121(2)
2.161(2)
2.202(2)
2.796(2)
2.848(2)
a
b
c
Inclination defined by 180° − (M−C1−CtIm) (CtIm = centroid of the imidazole ring). L = Cl. L = NC4H8O.
atom and an ortho-fluorine atom of one C6F5 group are indeed
present in the solid-state structures of 1a−d, and their range
from 2.652(2) to 2.738(2) Å is similar to those found for
complexes B with ArF = C6F5 (Figure 1).
To assess the strength of these intramolecular interactions,
we initially carried out a series of DFT calculations on the
prototypic allyl complex 1a. Since noncovalent and dispersion
interactions can be expected to play a crucial role in these
species, a thorough evaluation of several commonly used
density functionals was considered to be mandatory. Accord-
ingly, we employed the standard density functional
B3LYP,110,111 including the two variants B3LYP-GD3112 and
B3LYP-GD3BJ,113 which include empirical dispersion correc-
tions, and the B97D,114 B97D3,113 and M06L115 functionals
developed by Grimme and Truhlar, respectively. For all atoms
except Pd the 6-311G(d,p) basis set of Pople was used, whereas
Pd was described by the well-tempered all-electron basis set
WTBS of Huzinaga.116,117 We studied the ability of the
different density functionals to reproduce the experimentally
observed structure of 1a by running geometry optimizations in
the gas phase starting from the X-ray crystal structure and
compared the resulting structural parameters. As further
outlined in the Supporting Information (Table S7), the
M06L density functional reproduced the experimentally
determined structure best, and therefore, all further computa-
tions were performed with this density functional.
Figure 4. ORTEP diagram of 3a in 3a·C4H8O with thermal
displacement parameters drawn at 50% probability. The minor
disorder components of coordinated THF, a disordered THF of
solvation, and all hydrogen atoms are omitted for clarity. Selected
bond lengths [Å] and angles [deg] (see also Table 1): Pd−C1
2.0513(18), Pd−Cl 2.3851(4), Pd−C4 3.433(2), Pd−C5 4.322(2),
Pd−C9 3.540(2), Pd−C46 2.087(10), Pd−C47 2.159(3), Pd−C48
2.232(7), B−C2 1.652(3); N1−C1−N2 104.62(15), C1−N1−C4
119.67(14), Pd−C1−N1 130.40(12), Pd−C1−N2 124.92(12).
Consequently, we have attempted to calculate the electronic
energy profile for the interconversion between the syn-isomer
of 1a (IN1, Figure 5, left) and its respective anti-isomer (IN2,
Figure 5, right). As expected, the experimentally observed syn-
isomer is stabilized by 3.54 kcal/mol compared to its anti-
isomer, albeit to a smaller extent than found for the Rh and Ir
complexes B (vide supra). Attempts to locate stationary points
for this interconversion afforded a transition state (TS) that
connects IN1 with the intermediate IN1a, in which the
palladium−arene interaction is replaced by a short C−H···Pd
contact of 2.016 Å with a Dipp methyl group (Figure 5). It is
interesting to note that an interaction with this methyl group is
already present in IN1 with a Pd−H distance of 2.502 Å, which
falls in the range of typical anagostic interactions commonly
observed in square-planar d8 metal complexes.118−122 In
contrast, the very short Pd−H distance in IN1a (2.016 Å) is
comparable to those reported for T-shaped Pd(II) complexes,
in which the fourth coordination site is occupied by a CH
hydrogen atom.123,124 For instance, a Pd−H distance of 2.048
Å was found in the solid state for the carbene−allyl complex
in closer proximity to the metal atom for geometric reasons.
Despite this limited structural evidence, it was recently
established that, in addition to the conventional picture of
metal−NHC bonding,108 interaction can occur through π-face
donation from the aromatic N-substituents, predominantly
through the ipso-carbon atoms. It was also concluded that this
interaction can have an impact in catalysis and offers a handle
for the rational tuning of the electronic properties of (Pd)
catalysts.109
Our previous theoretical calculations for the Rh and Ir
complexes B revealed that the experimentally observed syn-
isomer, in which the metal atom is bound to the borate-flanking
aryl group, is energetically favored by ca. 6−9 kcal/mol over the
hypothetical anti-isomer, which exhibits interaction with the
other aryl substituent. Furthermore, in both the experimental
and the calculated structures, the presence of short C−F···Cipso
contacts with C−F distances well below the sum of the van der
Waals radii provided an explanation for the discrimination of
the two arene sites.35 Similar contacts involving the ipso-carbon
D
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Figure 5. Part of the calculated electronic energy profile for intramolecular arene coordination in complex 1a (IN1) and its transformation to the
hypothetical anti-isomer (IN2). All values and structures correspond to local minima or transition states optimized at the M06L/6-311G(d,p) level
(for details see the SI), and energies are reported with respect to the lowest energy conformer IN1. C6F5 groups of the borate moiety and
unimportant hydrogen atoms have been omitted for clarity.
[(CAAC)Pd(η3-C3H5)]BF4 (CAAC = cyclic alkyl amino
Table 2. Computed Energy Differences between the
carbene).125 According to our DFT calculations, the barrier
Respective syn- and anti-Isomers, with Variations in the
Borate Unit Only
a
height for the interconversion of IN1 to IN1a amounts to 1.53
kcal/mol, which indicates that the Pd−arene interaction is quite
weak. Nevertheless, since the T-shaped intermediate IN1a is
destabilized by 0.85 kcal/mol relative to IN1, arene
coordination is favored over CH coordination by this amount.
The pathway from IN1 to IN1a can be described as a rotation
of the carbene about the Pd−C bond or equivalently as the
rotation of the allyl group about the same bond.
borate group
ΔE(anti−syn) [kcal/mol]
B(C6F5)3
3.54
2.36
2.43
3.29
2.33
B(C6F5)2(C6F4H)
B(C6F3H2)3
B(C6H5)3
BH3
a
All reported values correspond to energy differences between fully
optimized structures at the M06L/6-311G(d,p) level (for details see
the SI).
Since all attempts to find a transition state that leads from
IN1a anywhere closer on the pathway to IN2 have failed, in
principle, two different diastereomers of IN2 need to be
considered, which differ in the orientation of the methine
hydrogen atom of the allyl group. One possible isomer, IN2, is
depicted in Figure 5, which lies 3.54 kcal/mol higher in energy
than IN1, whereas the energy of the second isomer (see SI,
IN2_b), where the methine hydrogen atom is oriented toward
the opposite site of the allyl group, is 3.14 kcal/mol relative to
IN1. Although it remains unclear how the actual rearrangement
of the allyl group would proceed, both isomers are close in
energy, and we will limit the discussion to the diastereomer
IN2, where the allyl orientation is equivalent to the one in IN1.
Regardless of the relative allyl orientation, the syn-isomer is
3.14−3.54 kcal/mol more stable than the anti-isomer, and since
both isomers differ only in the relative position of the borate
unit with respect to the Dipp substituents, it seems reasonable
that this unit must be held responsible for the preferential
coordination on one side. As already noted, in both the
experimental and the calculated structures, the presence of
short C−F···Cipso contacts is observed, which may contribute to
the preferred coordination on the borate-flanking site. In order
to test this hypothesis, we have carried out a series of DFT
calculations on the energy difference between the respective
syn- and anti-isomers, where we have systematically varied the
substituents on the borate unit (Table 2).
the original system. In the next step, we replaced all six ortho-
fluorine atoms in the borate unit by hydrogen atoms. The
energy difference between both isomers remains similar at 2.43
kcal/mol. The situation is less clear-cut if we replace all 15
fluorine atoms by hydrogen atoms, since now the energy
difference between both isomers is 3.29 kcal/mol, which is
almost the same difference as in the original system. We have to
stress that we currently have no explanation for this behavior,
but π-stacking effects might also have to be considered. In order
to exclude any π-stacking or steric effects, we drastically
simplified the borate unit to the BH3 unit. Even then, the
energy difference between the syn- and anti-isomer remained at
2.33 kcal/mol.
This series of calculations shows that the short C−F···Cipso
contact between the ortho-fluorine atom and the Dipp
substituent contributes to a relative stabilization of the syn-
isomer IN1 over the respective anti-isomer IN2 of the order of
about 1.1 kcal/mol (Table 2). This interpretation is further
underlined by the existence of a bond-critical point between the
ortho-fluorine and the ipso-carbon atoms (for details see the SI).
Still, this interaction cannot be held responsible for the overall
stabilization of 3.54 kcal/mol of IN1 over IN2, and we propose
that the remaining stabilization stems from an inductive effect
caused by the borate unit.
In a first step, we exchanged the ortho-fluorine atom that
shows the short C−F···Cipso contact by a hydrogen atom in IN1
and IN2 and computed the energy difference after performing
an unconstrained geometry optimization. This energy differ-
ence amounts to 2.36 kcal/mol, which is 1.18 kcal less than in
Catalytic Amination of Aryl Halides. We envisaged that
the reversibility of the metal−arene interaction in complexes
1a−d might give access to reactive and catalytically active 14-
E
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Article
electron palladium species,125 and Buchwald−Hartwig cross-
coupling of 4-bromotoluene and morpholine was chosen as a
test reaction (Figure 6). An extensive screening of the reaction
DME), full conversion and high isolated yields were found in
the case of 4-bromotoluene after 4 h at 60 °C with NaOtAm or
NaOtBu serving as the base (entries 1 and 2). As expected, the
reaction proceeded much faster at 80 °C, with full or almost full
conversion reached within ca. 30−45 min (entries 3 and 4).
With 4-chlorotoluene, the corresponding N-arylamine was also
isolated in high yield, albeit at the expense of a higher catalyst
loading (2 mol % at 60 °C, entry 5) or a higher reaction
temperature (80 °C, entry 6). The electronically activated
substrate 1-bromo-4-(trifluoromethyl)benzene was completely
converted within less than 30 min at 60 °C (entry 7), whereas
deactivated 1-bromo-4-methoxybenzene required longer reac-
tion times (8 h at 60 °C, entry 8) or a higher reaction
temperature of 80 °C to furnish the coupling product in
excellent yield within 1 h (entry 9). Finally, amination of the
sterically hindered mesityl bromide reached completion within
4 h at 80 °C (entry 10). Aniline was also successfully employed
for the amination of 4-bromotoluene and 1-bromo-4-
(trifluormethyl)benzene, and the corresponding secondary
amines were selectively formed in high yield at 60 and 80 °C
(Table 5, entries 1−4). However, prolonged reaction times
were required compared to morpholine. In contrast, Suzuki−
Miyaura coupling between 4-bromotoluene and phenylboronic
acid resulted in only moderate conversion of 21% and 66% after
8 h at 60 or 80 °C, respectively (entries 5 and 6). Further
optimization by variation of the catalyst/base/solvent combi-
nation was not carried out for this reaction.
It should be noted that the catalytic performance of 1a−d
cannot compete with the best allyl NHC catalyst systems
reported to date,63,66,67,80,73,74 but they show similar perform-
ance to e.g. several of the earlier PEPPSI systems.70−72 Our
results indicate, however, that arene−palladium interactions as
present in the precatalysts 1a−d might also play an important
and previously unrecognized role in Pd-catalyzed aryl
amination mediated by other NHC palladium systems.109
To demonstrate that arene−palladium bonding in the
precatalysts 1a−d is likely to be reversibly cleaved during a
catalytic cycle, the methallyl complex 1b was treated with
morpholine in THF at room temperature, resulting in
instantaneous decolorization of the light yellow solution
(Scheme 2). The same color change was also observed during
the setup of the catalytic reaction mixtures, suggesting
coordination of morpholine to the Pd atom.126 Addition of
hexane afforded a white precipitate in quantitative yield. In
addition to the carbene and allyl ligands, NMR spectroscopy
indicated the presence of a coordinated morpholine molecule
(Figures S48−S51) and therefore the formation of the amine
complex 4, which was confirmed by X-ray diffraction analysis.
The molecular structure is shown in Figure 7. Overall, the
structural parameters are very similar to those found in the ate-
complex 3b, in which morpholine is replaced by a chloro ligand
(Table 1). The Pd−C1 bond is slightly elongated, cf. 2.055(2)
Å in 3b and 2.074(2) Å in 4, and the anionic carbene ligand is
bound in the conventional fashion, with an inclination of 4.6°
ruling out any significant Pd−arene interaction. Finally, it
should be noted that heating of THF-d8 or toluene-d8 solutions
of complex 4 at 60 °C was carried out to induce and follow the
deprotonation of the coordinated morpholine ligand. However,
no conclusive NMR spectra could be recorded and gradual
decomposition with formation of a black precipiate (elemental
palladium) was observed.
Figure 6. Catalytic performance of 1a−d and 3a in the aryl amination
reaction of 4-bromotoluene with morpholine.
conditions by variation of temperature, catalyst loading, solvent,
and base was carried out for the precatalyst 1a, which showed
optimum performance in 1,2-dimethoxyethane (DME) solution
at 60 °C with sodium tert-butoxide as base (see the SI, Tables
S2−S5 for details). Under these reaction conditions, the
complexes 1a and 1d showed the best performance, reaching
almost full conversion within 2−4 h (Figure 6 and Table 3,
Table 3. Amination of 4-Bromotoluene with Morpholine
entry
catalyst
time [h]
GC yield [%]
1
2
3
4
5
1a
1b
1c
1d
3a
4
8
8
4
4
100
28.9
52.3
99.9
61.3
entries 1 and 4). The conversion−time diagram in Figure 6
reveals an induction period for both complexes, and initiation
was found to be even slower for 1b and 1c. The active catalysts
formed from the latter complexes also suffered from faster
deactivation, which reached conversions of only 28.9% and
52.3% after 8 h (entries 2 and 3). For the ate-complex 3a, the
reaction is fastest at the beginning, but similar deactivation and
incomplete conversion (61.3%, entry 5) was also observed for
this catalyst system.
The amination of 4-chlorotoluene was also achieved in a
satisfactory manner with the precatalysts 1a and 1d, with 1a
exhibiting the better combination of activity and stability
(Figure S53). In addition, the robust nature of 1a allowed the
catalytic studies to be performed on the benchtop without
special requirements to exclude air and moisture. Therefore,
further screening with morpholine and selected aryl halide
substrates were carried out with 1a (Table 4). Under the
reaction conditions described above (1 mol % catalyst loading,
F
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Organometallics
Article
a
Table 4. Aryl Amination of Aryl Halides with Morpholine
a
1 mol % catalyst, 0.75 mmol of base, 0.5 mmol of aryl halide, 0.6 mmol of morpholine, 5 mL of DME. The conversion was monitored by GC with
b
n-decane as internal standard. The yields were determined after isolation of the pure products by quenching and filtration. 2 mol % catalyst.
a
Table 5. Aryl Amination: Additional Substrate Screening
a
Catalyst 1a (1 mol %), 0.75 mmol of NaOtBu, 0.5 mmol of aryl halide, 0.6 mmol of amine/boronic acid, 5 mL of DME. The conversion was
monitored by GC with n-decane as internal standard. Isolated yields were determined after isolation of the pure products by quenching, filtration,
and purification by column chromatography.
cal calculations reveal that coordination through the borate-
flanking arene site is favored, which can be ascribed, at least
partially, to the presence of short intramolecular C−F···Cipso
contacts. In addition, inductive effects attributable to the borate
moiety have to be considered. The [(WCA-NHC)Pd(allyl)]
complexes studied herein are suitable precatalysts for aryl
amination reactions, indicating that such arene−palladium
interactions may generally contribute to the stabilization of
coordinatively unsaturated species and should therefore have a
considerable impact on the reaction mechanism of Pd-catalyzed
cross-couplings. Consequently, a deeper understanding of these
interactions offers a handle for the rational tuning of the
electronic properties of (Pd) catalysts by variation of the
(anionic) backbone substituent.
Scheme 2. Addition of Morpholine to 1b
CONCLUSION
■
It has been confirmed that N-heterocyclic carbenes of the 1,3-
diarylimidazolin-2-ylidene type that bear a weakly coordinating
borate moiety (WCA-NHC) afford complexes with intra-
molecular arene−metal interactions. Experimental and theoreti-
G
DOI: 10.1021/acs.organomet.6b00217
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00217.
■
Namyslo, J. C.; Nieger, M.; Schmidt, A. Chem. Commun. 2014, 50,
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Supplementary data containing full experimental details
and X-ray diffraction data (PDF)
Crystallographic data (CIF)
Structure files (MOL)
Structure files (PDB)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: m.tamm@tu-bs.de.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
́
(25) Cesar, V.; Misal Castro, L. C.; Dombray, T.; Sortais, J.-B.;
■
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
In memory of Guy Lavigne. The work was supported by the
Deutsche Forschungsgemeinschaft (DFG) through grant TA
189/9-1.
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