Effect of electronic enrichment of NHCs on the catalytic activity of [Pd(NHC)(acac)Cl] in Buchwald-Hartwig coupling

Article abstract of DOI:10.1021/om4010143

A series of [Pd(NHC)(acac)Cl] (NHC = N-heterocyclic carbene) complexes have been synthesized and characterized to investigate the electronic and steric effects of NHC ligands in catalysis. Their reactivity in Buchwald-Hartwig coupling has been explored an

Full text of DOI:10.1021/om4010143

Article
pubs.acs.org/Organometallics
Effect of Electronic Enrichment of NHCs on the Catalytic Activity of
[Pd(NHC)(acac)Cl] in Buchwald−Hartwig Coupling
Gaetan Le Duc, Sebastien Meiries, and Steven P. Nolan*
́
̈
†EaStCHEM School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, U.K.
S
* Supporting Information
ABSTRACT: A series of [Pd(NHC)(acac)Cl] (NHC = N-
heterocyclic carbene) complexes have been synthesized and
characterized to investigate the electronic and steric effects of
NHC ligands in catalysis. Their reactivity in Buchwald−
Hartwig coupling has been explored and compared with that of
their nonmethoxylated congeners. Combining the optimal
steric and electronic properties, [Pd(IHeptOMe)(acac)Cl]
performed excellently in the palladium-catalyzed amination of deactivated aryl chlorides with various anilines.
can increase the efficiency of the catalyst.12 Interestingly, an
INTRODUCTION
■
optimal combination of steric and electronic effects of
substituents appended to the NHC frameworks may improve
the performance of NHC ligands in catalysis.13 In this context,
we report the preparation of the conformationally flexible
[Pd(ITentOMe)(acac)Cl] complexes (Figure 1; 6−8) to
examine together the steric and electronic effects of NHC
ligands. These precatalysts were tested in the Buchwald−
Hartwig arylamination reaction and compared with their
Palladium-catalyzed arylamination has become an important
and widely employed method for the formation of C−N
bonds.1 Of the different substrates that are available,
economically attractive aryl chlorides require special catalyst
design for successful coupling, particularly with deactivated or
hindered anilines. In addition to the phosphine-based ligands
commonly used for this reaction,2 recent reports have shown
that NHC ligands are also efficient catalysts for the Buchwald−
Hartwig coupling.3 Well-defined PdII−NHC precatalysts have
the advantages of being air and moisture stable and of
introducing the palladium and ligand in the optimal ratio
(1:1).4 Easily synthesized in a one-pot procedure from the
NHC·HCl salts, [Pd(NHC)(acac)Cl] complexes are perfect
examples of efficient PdII−NHC precatalysts.5 The catalytic
activity of Pd−NHC complexes is directly related to the unique
properties of the supporting NHC: their strong σ-donor
character facilitates the oxidative addition of aryl halides, and
their steric bulk increases the rate of the reductive elimination.6
Glorius introduced the concept of “flexible steric bulk” to define
adaptable ligands that exist in different conformations: one
small enough to accept substrates in the coordination sphere of
the metal and another one bulky enough to promote
monoligation and reductive elimination.7 As examples, the
bulky IPr* (N,N′-bis(2,6-bis(diphenylmethyl)-4-
́
methylphenyl)imidazol-2-ylidene) reported by Marko and co-
workers8 and further investigated by Nolan9 and the more
flexible IPent (N,N′-bis(2,6-di-3-pentylphenyl)imidazol-2-yli-
dene), reported by Organ,3a,10 performed well in the
Buchwald−Hartwig arylamination reaction. Recently, we
investigated the steric effect of the ITent ligands (“Tent” for
“tentacular”) and we found that IHept (N,N′-bis(2,6-di-4-
heptylphenyl)imidazol-2-ylidene) exhibited excellent catalytic
properties.11 Moreover, it has been reported, by comparison
between [Pd(IPr*)(acac)Cl] (1) and [Pd(IPr*OMe)(acac)Cl]
(2), that modification of the electronic properties of the
aromatic rings, introduced by the presence of a methoxy group,
Figure 1. Pd(NHC)(acac)Cl precatalysts.
Received: October 16, 2013
© XXXX American Chemical Society
A
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[Pd(ITent)(acac)Cl] analogues (3−5) to evaluate the influence
The coupling of 4-chloroanisole with 4-fluoroaniline was
chosen as the test reaction for our initial optimization. After
an initial base/solvent screening, the use of KOtAm in toluene
at 80 °C was found to provide the best conditions, leading to
complete conversion of the starting materials with 0.25 mol %
of 6−8 in 2 h. As the base is also involved in the activation of
the catalyst, its choice is crucial. A plausible mechanism involves
a chloride/alkoxide anion exchange followed by a rearrange-
ment of the acac moiety and then a reductive elimination step
to generate the active Pd species.3k Moreover, as in the case of
3−5,11 the use of more polar solvents appeared to be
detrimental to the reaction (Table 1).
of the methoxy group.
RESULTS AND DISCUSSION
■
By reacting the corresponding ITentOMe·HCl14 salts with
Pd(acac)2 in 1,4-dioxane at reflux, as described in the
literature,5 an additional family of palladium precatalysts,
[Pd(ITentOMe)(acac)Cl], can be easily prepared via a synthetic
route that does not require the isolation of the free NHC. The
newly prepared [Pd(IPentOMe)(acac)Cl] (6; IPentOMe = N,N′-
bis(2,6-di-3-pentyl-4-methoxyphenyl)imidazol-2-ylidene), [Pd-
(IHeptOMe)(acac)Cl] (7; IHeptOMe = N,N′-bis(2,6-di-4-heptyl-
4-methoxyphenyl)imidazol-2-ylidene), and [Pd(INonOMe)-
(acac)Cl] (8; INonOMe = N,N′-bis(2,6-di-4-nonyl-4-
methoxyphenyl)imidazol-2-ylidene) were obtained as air- and
moisture-stable yellow solids in high purity and good yields:
respectively 82%, 87%, and 78% (Scheme 1).
a
Table 1. Optimization for Amination with 6−8
b
Scheme 1. Preparation of [Pd(ITentOMe)Cl] (6−8)
entry
cat.
solvent
toluene
base
KOtBu
conversion, %
1
6
6
6
6
6
6
6
6
6
6
7
8
91
100
85
2
toluene
KOtAm
LiHMDS
KOtBu
KOtAm
LiHMDS
KOtBu
KOtAm
LiHMDS
KOtAm
KOtAm
KOtAm
3
toluene
4
1,4-dioxane
1,4-dioxane
1,4-dioxane
48
5
35
6
18
c
7
DME
40
8
DME
37
9
DME
21
10
11
12
DMF
23
toluene
toluene
100
100
The new [Pd(ITentOMe)(acac)Cl] complexes 6−8 were
a
Reagents and conditions: ArX (0.5 mmol), Ar’NH2 (0.55 mmol),
base (0.55 mmol), solvent (1.0 mL), 6−8 (0.25 mol %) . Conversion
to coupling product based on starting aryl chloride by GC, average of
1
b
characterized. Interestingly, H NMR spectroscopy revealed
that the gap between the singlets of the acac methyl groups
depends on the length of the R alkyl groups of the NHC ligand:
c
three runs. DME = dimethoxyethane.
1
the H spectrum of 6 exhibited two singlets at 1.61 and 1.78
1
ppm for the methyl groups of the acac ligand. The H NMR
To compare the catalytic properties of 3−515 and 6−8, the
reaction was performed at lower catalyst loadings. Couplings of
4-chloroanisole with 4-fluoroaniline and 3-trifluoromethylani-
line were chosen as benchmark reactions. By employing 0.05
mol % of precatalyst at 80 °C, 70% conversion was obtained
with 6, 98% with 7, and 86% with 8 (Table 2, entry 1). At 110
°C, the coupling of 4-chloroanisole and 3-trifluoromethylaniline
provided the desired products in 53%, 85%, and 76% GC
conversions using 6−8, respectively (Table 2, entry 2). It
appeared that [Pd(IHeptOMe)(acac)Cl] (7) is the most efficient
catalyst in every case. For both benchmark reactions, we
previously observed the same trend with the [Pd(ITent)-
(acac)Cl] precatalysts (Table 2): 4 was more efficient than 3
and slightly more efficient than 5.11 The results obtained with
[Pd(ITentOMe)(acac)Cl] are in agreement with our previous
results and confirmed the influential role of the length of the R-
alkyl chains. This effect was optimal with the IHeptOMe ligand.
It appears that in the case of 8 the additional bulk is too far
from the coordination sphere of the metal and does not
influence the catalytic properties of the complex. We also
observed that the activity of each [Pd(ITentOMe)(acac)Cl]
complex was superior to that of its [Pd(ITent)(acac)Cl]
analogues, as observed on a comparison between 1 and 2.12 As
the gain of activity is general of all the ITentOMe series, these
results prove the positive effect of the methoxy group. As the
methoxy substituent resides away from the coordination sphere
of the metal, the stronger σ-donor properties of the ITentOMe
spectrum of 7 revealed that these singlets were shifted upfield,
the first one slightly (1.76 ppm) and the second one more
significantly (1.46 ppm). This suggests that, in solution, the
magnetic and chemical environment around the metal is
directly affected by the length of the R alkyl groups appended
to the NHC ligand. Complex 8 exhibited approximately the
same gap as 7 (1.48 and 1.76 ppm). This result could be
explained by the similar bulk of 7 and 8 in the coordination
sphere of the metal. Consequently, the alkyl group of 8 could
be partially located further away from the metal center. We
have previously observed similar results with the [Pd(ITent)-
(acac)Cl] series. The gap between the singlets of the acac
methyl groups increased with the length of the R alkyl chain
(until R = Et): [Pd(IPr)(acac)Cl] (1.82 and 1.84 ppm), 3 (1.53
and 1.73 ppm), 4 (1.40 and 1.77 ppm). In addition, 5 exhibited
singlets similar (1.40, 1.75) to those of 4.11 It appeared that
these observations are general for the ITent and ITentOMe
series and were not fortuitous. Such spectroscopic information
may help to explain the effect of the NHC bulk on the catalysts
in solution and may be correlated with their activity in
homogeneous catalysis. Having prepared our new precatalysts,
we then investigated their catalytic properties in Buchwald−
Hartwtig arylamination. Recent reports identified electron-rich
aryl halides and electron-deficient anilines as highly disfavored
coupling partners.9b For this reason, these challenging
substrates were examined with our new acac-bearing catalysts.
B
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Table 2. Comparison of [Pd(ITent)(acac)Cl] (3−5) and
[Pd(ITentOMe)(acac)Cl] (6−8) Precatalysts in
Table 3. Scope of the Buchwald−Hartwig Arylamination
a
with [Pd(IHeptOMe)(acac)Cl] (7)
a
Arylamination
a
Reagents and conditions: ArX (0.5 mmol), Ar’NH2 (0.55 mmol),
b
KOtAm (0.55 mmol), toluene (1.0 mL). 3−8 0.05 mol %, 80 °C, 3
c
d
h;. 3−8 0.1 mol %, 110 °C, 6 h. Conversion to coupling product
based on starting aryl chloride by GC, average of three runs.
ligand in comparison with ITent ligand could explain the
difference in the catalytic activity observed. This extra σ
donation could offer greater stabilization of the Pd0−NHC
complex.3a,16 [Pd(IHeptOMe)(acac)Cl] (7) combined the
optimal length of the alkyl chain and the presence of the
methoxy group and gave the best results for C−N coupling.
[Pd(IHeptOMe)(acac)Cl] (7) was selected as the precatalyst
to explore the scope of the arylamination reaction. The scope of
the reaction was explored with the previously optimized
conditions, using 0.05−0.2 mol % of 7 in toluene at 80 °C
or at reflux in the presence of KOtAm. The system displayed
excellent catalytic activity for the coupling of various substrates.
Good yields are obtained with electron-poor anilines and
electron-rich aryl chlorides, which are reported to be
challenging coupling partners (Table 3, entries 1−4 and 8−
10).10 The system appeared unaffected by the presence of
substituents in the ortho position of the aryl chlorides:
couplings of 2-chloroanisole and 4-chloroanisole with 4-
fluoroaniline gave very similar results (Table 3, entries 1 and
3). Similar results are observed for the coupling of 2-
chloroanisole or 4-chloroanisole with 3-trifluoromethylaniline
(Table 3, entries 8 and 9). Moreover, very good yields were
obtained with sterically hindered substrates (Table 3, entries 4,
6, and 11). The increased conformational flexibility of IHeptOMe
may allow it to better accommodate sterically hindered
substrates in the coordination sphere of the metal center.7,17
In all cases, the product of biarylation is never observed,
attesting that this catalytic system is selective for the
monoarylation of ArNH2. Finally, various anilines were
successfully coupled with deactivated 1,3-dimethoxychloroben-
zene (Table 3, entries 5, 6, and 9) and, for the first time, with
very deactivated 1,3,5-trimethoxychlorobenzene at low catalyst
loading (Table 3, entry 11), attesting to the high reactivity of 7.
We also investigated the efficiency of our catalyst with more
nucleophilic amines. Nonactivated aryl chlorides were success-
fully coupled with N-methylaniline at low catalytic loading (as
low as 50 ppm of 7), and remarkable catalyst productivity
(turnover number up to 18000) was observed (Table 4, entries
2 and 3). These results are comparable with results obtained
with the most efficient Pd/phosphine systems for similar
substrates.18
a
Reagents and conditions: ArX (0.5 mmol), Ar’NH2 (0.55 mmol),
KOtAm (0.55 mmol), 7 (x mol %), toluene (1.0 mL), 80 °C, 3 h.
b
Isolated yields after chromatography on silica gel, average of two runs.
c
110 °C, 6 h.
CONCLUSION
■
The new [Pd(ITentOMe)(acac)Cl] complexes were obtained as
well-defined air- and moisture-stable precatalysts in good yields.
C
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Article
was evaporated and the crude product was dissolved in pentane. The
solution was filtered on a pad of silica covered with Celite, and the pad
was eluted with pentane. After evaporation of the solvent and drying
under high vacuum, the desired complex was obtained as a yellow
powder (159 mg, 87%). 1H NMR (300 MHz, CDCl3): δ 6.96 (s, 2H),
6.70 (s, 4H), 4.99 (s, 1H), 3.84 (s, 6H), 2.76 (m, 4H), 2.10 (m, 4H),
1.76 (s, 3H), 1.46 (s, 3H), 1.68−1.04 (m, 28H), 0.79 (t, J = 7.1 Hz,
24H). 13C NMR (75 MHz, CDCl3): δ 186.0, 183.6, 159.5, 154.5,
146.0, 130.0, 125.1, 110.1, 99.6, 52.3, 39.7, 39.6, 38.5, 26.5, 25.7, 21.3,
20.8, 14.7. Anal. Calcd for C50H79ClN2O4Pd: C, 65.70; H, 8.71; Cl, N,
3.06. Found: C, 65.64; H, 8.80; N, 3.15.
Table 4. Arylamination with Low Catalytic Loadings
Procedure for the Synthesis of [Pd(INonOMe)(acac)Cl]. In a
Schlenk flask equipped with a magnetic stirring bar were added
INonOMe·HCl (221 mg, 0.27 mmol) and Pd(acac)2 (62 mg, 0.2 mmol)
in dry 1,4-dioxane (5 mL) under an atmosphere of nitrogen. The
reaction mixture was refluxed for 40 h. After this time, dioxane was
evaporated and the crude product was dissolved in pentane. The
solution was filtered on a pad of silica covered with Celite, and the pad
was eluted with pentane. After evaporation of the solvent and drying
under high vacuum, the desired complex was obtained as a yellow
powder (160 mg, 78%). 1H NMR (300 MHz, CDCl3): δ 6.96 (s, 2H),
6.72 (s, 4H), 5.00 (s,1H), 3.86 (s, 6H), 2.76 (m, 4H), 2.13 (m, 4H),
1.76 (s, 3H), 1.48 (s, 3H), 1.68−1.10 (m, 44H), 0.83 (m, 24H). 13C
NMR (75 MHz, CDCl3): δ 185.9, 183.4, 159.5, 154.4, 146.0, 130.0,
124.9, 110.0, 99.7, 55.3, 39.7, 37.0, 36.0, 30.4, 29.8, 26.6, 25.8, 23.5,
23.4, 14.2, 14.0. Anal. Calcd for C58H95ClN2O4Pd: C, 67.88; H, 9.33;
N, 2.73. Found: C, 67.72; H, 9.46; N, 2.88.
General Procedure for Buchwald−Hartwig Cross Coupling
of Aryl Halides with Anilines. In a glovebox, a glass vial equipped
with a stirring bar was charged with KOtAm (0.55 mmol) and sealed
with a screw cap fitted with a septum. The vial was then loaded with
the neat aniline (0.55 mmol) and the aryl halide (0.50 mmol) outside
the glovebox. Finally, a premade solution of the precatalyst in
anhydrous solvent (prepared in the glovebox) was injected at room
temperature under argon and the reaction mixture was stirred and
heated until completion, as indicated by GC. Then, water was added to
the reaction mixture, the organic layer was extracted with Et2O and
dried over magnesium sulfate, and the solvent was evaporated in
vacuo. The product was purified by flash chromatography on silica gel.
a
Reagents and conditions: ArCl (0.5 mmol), Ar’NH2 (0.55 mmol),
b
KOtAm (0.55 mmol), 7 (x ppm), toluene (1.0 mL), 110 °C. Isolated
yields after chromatography on silica gel, average of two runs.
They proved to be superior to their [Pd(ITent)(acac)Cl]
analogues. The study confirmed that the “flexible steric bulk”
concept is essential in securing high catalytic activity with Pd−
NHC complexes and this effect is greatest with [Pd(IHeptOMe)-
(acac)Cl]. Combining optimal steric and electronic properties,
[Pd(IHeptOMe)(acac)Cl] exhibited excellent catalytic activity in
Buchwald−Hartwig coupling reactions with various substrates,
even the most deactivated. Moreover, excellent productivity
(TON > 18000) was also observed with nucleophilic anilines.
This study will help to identify which characteristics are crucial
for the design of future N-heterocyclic carbene ligands in
palladium catalysis.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text and figures giving detailed procedures for the amination
reactions and NMR spectra for complexes 6−8 and cross-
coupling products. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.P.N.: snolan@st-andrews.ac.uk.
Notes
EXPERIMENTAL SECTION
■
■
Procedure for the Synthesis of [Pd(IPentOMe)(acac)Cl]. In a
Schlenk flask equipped with a magnetic stirring bar were added
IPentOMe·HCl (223 mg, 0.37 mmol) and Pd(acac)2 (85 mg, 0.28
mmol) in dry 1,4-dioxane (6 mL) under an atmosphere of nitrogen.
The reaction mixture was refluxed for 40 h. After this time, dioxane
was evaporated and the crude product was dissolved in pentane. The
solution was filtered on a pad of silica covered with Celite, and the pad
was eluted with pentane. After evaporation of the solvent and drying
under high vacuum, the desired complex was obtained as a yellow
powder (184 mg, 82%). 1H NMR (300 MHz, CDCl3): δ 7.01 (s, 2H),
6.73 (s, 4H), 5.04 (s, 1H), 3.86 (s, 6H), 2.66 (m, 4H), 2.11 (m, 4H),
1.78 (s, 3H), 1.61 (s, 3H), 1.75−1.40 (m, 12H), 0.97 (t, J = 7.3 Hz,
12H), 0.75 (t, J = 7.4 Hz, 12H). 13C NMR (75 MHz, CDCl3): δ 186.1,
183.6, 159.5, 154.8, 145.5, 130.2, 125.2, 110.4, 99.7, 55.2, 41.5, 28.5,
27.4, 27.1, 26.7, 26.0, 12.4, 11.5. Anal. Calcd for C42H63ClN2O4Pd: C,
62.91; H, 7.92; N, 3.49. Found: C, 62.84; H, 8.03; N, 3.53.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the EC for funding through the
seventh framework project SYNFLOW. Umicore AG is
thanked for its generous gifts of materials. S.P.N. is a Royal
Society Wolfson Research Merit Award holder.
REFERENCES
■
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D
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dx.doi.org/10.1021/om4010143 | Organometallics XXXX, XXX, XXX−XXX