An efficient palladium-NHC (NHC=N-heterocyclic carbene) and aryl amination pre-catalyst: [Pd(IPr*)(cinnamyl)Cl]

Article abstract of DOI:10.1002/adsc.201200207

The well-defined [Pd(IPr*)(cinnamyl)Cl] complex is reported as one of the best N-heterocyclic carbene (NHC)-based pre-catalysts for the Buchwald-Hartwig amination reaction. This catalytic system displays high efficiency for the coupling of numerous (heter

Full text of DOI:10.1002/adsc.201200207

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DOI: 10.1002/adsc.201200207
An Efficient Palladium-NHC (NHC=N-Heterocyclic
Carbene)Aryl Amination Pre-Catalyst: [Pd(IPr*)(cinnamyl)Cl]
A
ACHTUNGTRENNUNG
Anthony Chartoire,a Xavier Frogneux,a and Steven P. Nolana,*
a
EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, U.K.
Fax : (+44)-(0)1334-463-808; e-mail: snolan@st-andrews.ac.uk
Received: March 13, 2012; Revised: April 25, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200207.
Abstract: The well-defined [Pd
(IPr*)
E
disadvantages, most notably the use of high catalyst
loadings (typically 2–4 mol%). Moreover, in most
cases, heating is necessary to promote the reaction.
complex is reported as one of the best N-heterocy-
clic carbene (NHC)-based pre-catalysts for the
Buchwald–Hartwig amination reaction. This catalyt-
ic system displays high efficiency for the coupling of
numerous (hetero)aryl chlorides, with a wide range
of amines, at room temperature or at extremely low
catalyst loading (as low as 0.025 mol%).
On the other hand, [PdACTHNUTRGENNU(G NHC)ACHTUNTGREN(NGUN R-allyl)Cl] type com-
plexes[3d,e,5,6] are efficient as easily activated pre-cata-
lysts for amination, at room temperature and low cat-
alyst loading. Recently, Marko and co-workers have
reported on the preparation of IPr* {IPr*=1,3-
bisACTHNUGRTENUNG[2,6-bis(diphenylmethyl)-4-methylphenyl]imidazo-
Keywords: amination; Buchwald–Hartwig cross-
coupling; N-heterocyclic carbene; palladium
2-ylidene}, one of the bulkiest NHCs to date.[7] In our
continuing efforts to develop palladium cinnamyl
complexes for homogeneous catalysis,[3d,e,8] we became
interested in examining how the bulkiness of the IPr*
ligand might affect catalytic performance in the Buch-
wald–Hartwig reaction. We report here the reactivity
The palladium-catalyzed aryl amination reaction has
emerged, in the past decade, as the method of choice
[1]
À
for the creation of C N bonds. To ensure the effi-
ciency of the reaction, the nature of the active palladi-
um species involved in the reaction is of crucial im-
portance. Indeed, in order to circumvent the various
limitations of the cross-coupling type reaction, strong
s-donor ligands are usually employed. To this end,
tertiary bulky phosphines[2] were first introduced, no-
tably by Buchwald and Hartwig as effective catalyst
modifiers in aryl amination. As an alternative to these
sometimes air-sensitive tertiary phosphines, easy to
handle and air-stable N-heterocyclic carbenes (NHCs)
have been found quite beneficial in numerous cross-
coupling reactions as well as in aryl amination.[3] One
of the current main challenges in aryl amination re-
mains the coupling of unactivated aryl chlorides with
primary and secondary amines under mild conditions,
especially at low catalyst loading or/and at room tem-
perature. Among the best catalytic systems described
in the literature are two families of well-defined,
NHC-based palladium complexes. The first makes use
of N-heteroaromatic-stabilized NHC-Pd(II) com-
pounds,[3i,4] including the [Pd-PEPPSI-NHC] precata-
lysts developed by Organ. Despite their high efficien-
Figure 1. Highly efficient NHC-palladium pre-catalysts for
cy, these systems and protocols still suffer from some aryl amination reactions.
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Table 2. Scope of the amination reaction at room tempera-
Table 1. Optimization of the reaction conditions at room
ture.[a]
temperature.[a]
Entry
Product
Time [h]
Yield [%][b]
1
2
3
3
97
91
Entry
Solvent
Base
Conversion [%][b]
1
2
3
4
5
6
7
8
toluene
toluene
toluene
toluene
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
DME
DME
DME
DME
DME
KO-t-Bu
NaO-t-Bu
KO-t-Am
LiHMDS
KO-t-Bu
NaO-t-Bu
KO-t-Am
LiHMDS
KO-t-Bu
NaO-t-Bu
KO-t-Am
KO-t-Am
LiHMDS
46
0
40
37
57
0
57
38
3
4
22
3
82
89
9
43
0
89
10
11
12[c]
13
5
6
3
79
>99 (91)[d]
6
6
90
94
[a]
Reagents and conditions: 4-chloroanisole (1 equiv.), mor-
pholine (1.1 equiv.),
1
(0.5 mol%), solvent, base
7
46
(1.1 equiv.), 258C, 20 h.
Conversion to coupling product based on 4-chloroanisole
determined by GC.
Catalyst loading of 1 mol%, reaction time of 3 h.
Isolated yield after chromatography on silica gel, average
of two runs.
[b]
[a]
Reagents and conditions: (Het)ArCl (1 equiv.), amine
(1.1 equiv), 1 (1 mol%), KO-t-Am (1.1 equiv.), DME,
258C, time.
Isolated yield after chromatography on silica gel, average
of two runs.
[c]
[d]
[b]
of the new [Pd
amination reactions at room temperature and at very (Table 2, entries 2 and 3) and heteroaryl (Table 2,
low catalyst loadings (Figure 1). entry 6) chlorides with cyclic dialkylamines, affording
(cinnamyl)Cl] pre-catalyst 1[9] in
ACHTUNGTRENNUNG(IPr*)ACHTUGNTRENNUGN
The reactivity of pre-catalyst 1 was initially studied the expected compounds in excellent yields (79–97%)
at room temperature. To optimize the reaction condi- and in short reaction times (3–6 h). In the case of 2-
tions, the reactivity of the challenging unactivated 4- chloroanisole, due to a combination of electronic de-
chloroanisole with morpholine was explored activation and steric hindrance, the reaction requires
(Table 1). To this end, eight different bases and three 22 h to efficiently yield the desired coupled product
solvents were tested. We noticed that a strong base is (Table 2, entry 3) in 82% yield. If the more challeng-
required to promote the reaction (KO-t-Am, KO-t-Bu ing N-methylaniline is used, the reaction rate is signif-
or LiHMDS), weaker bases (KOH, NaOH, Cs2CO3 icantly decreased (Table 2, entry 7), now requiring
and K3PO4) led to lower or no conversion.[10] Surpris- 46 h. This result is insightful as it illustrates the high
ingly, NaO-t-Bu provides no conversion showing that stability of the active palladium(0) species under
the cation of the alkoxide base in these reactions these reaction conditions at room temperature. It is
plays a significant role. Finally, the optimal conditions noteworthy that the efficiency of the transformation
for room temperature reactivity make use of potassi- is highly dependent on the nature of the amine.
um tert-amylate (KO-t-Am) as base in 1,2-dimethoxy- Indeed, if a primary amine is used under these condi-
ethane (DME) with 1 mol% pre-catalyst 1 (Table 1, tions (aniline or 2,6-dimethylaniline), no conversion
entry 12, conversion >99%) and lead to complete of the substrate is observed and the starting materials
conversion in only 3 h.
are recovered unchanged.[11]
The scope of the amination reaction at room tem-
Comparing with literature results, complex 1 be-
perature was next studied using these optimized con- longs to a limited family of NHC-based pre-catalysts
ditions. The results are summarized in Table 2. The allowing amination at room temperature. The catalyst
system displays good efficiency for the coupling of loading used is very competitive with previously pub-
non-activated (Table 2, entries 1, 4 and 5), deactivated lished results {better than [Pd-PEPPSI-IPr] (2–
2
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An Efficient Palladium-NHC (NHC=N-Heterocyclic Carbene)Aryl Amination Pre-Catalyst
Table 3. Optimization of the reaction conditions at low-cata-
a short reaction time of 2 h and the desired coupled
products are isolated in reasonable to excellent yields
(58–97%). In the case of the coupling between 4-
chlorotoluene and morpholine, the catalyst loading
can even be decreased to as low as 0.025 mol% with-
out any loss of efficiency (Table 4, entry 2). The cata-
lytic system reveals very high efficacy for the prepara-
tion of sterically hindered products (Table 4, entries 5
and 10, 94–95%). To our delight, the sequence is also
extremely efficient for the challenging couplings of
unactivated chlorides with N-methylaniline in only
2 h (Table 4, entries 8 and 17), without increasing the
amount of palladium. Additionally, the sequence tol-
erates chiral centers (entry 14) and the use of N-meth-
ylpiperazine[13] (entry 18). In the case of the dibutyl-
lyst loading.[a]
Entry
Solvent
Base
Conversion [%][b]
1
2
3
4
DME
DME
KO-t-Bu
KO-t-Am
KO-t-Bu
KO-t-Am
KO-t-Bu
KO-t-Am
KO-t-Am
8
7
50
63
63
78
1,4-dioxane
1,4-dioxane
toluene
toluene
toluene
5[12]
6
7[c]
>99 (91)[d]
AHCTUNGTREGaNNUN mine the system shows its limits and a catalyst load-
ing of 0.1 mol% is necessary to obtain a reasonable
isolated yield (entry 4, 58%). This latter result is nev-
[a]
Reagents and conditions: 4-chloroanisole (1 equiv.), mor-
pholine (1.1 equiv.), (0.025 mol%), solvent, base
(1.1 equiv.), reflux, 20 h.
Conversion to coupling product based on 4-chloroanisole
determined by GC.
Catalyst loading of 0.05 mol%, reaction time of 2 h.
Isolated yield after chromatography on silica gel, average
of two runs.
1
[b]
Table 4. Scope of the amination reaction at low catalyst
[c]
[d]
loading.[a]
Entry
Product
Time [h]
Yield [%][b]
ACTHNUTRGNEUNG
4 mol%),[3i] similar to [Pd(SIPr)CpCl] (1 mol%),[3j]
1
2
2
92
95
but not as low as when using [Pd
ACHUTGTNRENN(GU SIPr)ACHUTNGTREN(NUGN cinnamyl)Cl]
2[c]
(0.1 mol%)[3d]}.
Because palladium is an expensive metal, the use of
low catalyst loadings is crucial when preparing large
amounts of products. However, a decrease of the cat-
alyst loading generally requires a subsequent increase
of the reaction temperature. A rapid optimization of
the reaction conditions using 0.025 mol% of 1 was
then investigated at the reflux temperatures of DME,
1,4-dioxane and toluene (Table 3). The best result was
obtained combining KO-t-Am as the base in toluene
(Table 3, entry 6, 78%). In that case and in compari-
son with the reactivity of the system at room tempera-
ture, the efficiency of the reaction is driven by the
highest boiling point solvent (toluene) rather than the
most stabilizing one (DME). Finally, if 0.05 mol% of
1 is used, the reaction reaches completion after only
2 h and the expected coupled product is isolated in an
excellent yield (Table 3, entry 7, 91%).
The scope of the amination reaction using
0.05 mol% of 1 was explored. The results are summar-
ized in Table 4. The system displays very good effi-
ciency for the coupling of non-activated (Table 4, en-
tries 1–6 and 10–15), deactivated (Table 4, entries 7–9
and 17–18) and heteroaryl (Table 4, entries 16–18)
chlorides with a wide range of amines including pri-
mary alkyl- and arylamines, cyclic and acyclic dialkyl-
amines, N-methylaniline and N-methylpiperazine. In
most of the cases the reaction is complete after
3
2
92
58
4[d]
18
5
2
95
6
7
2
2
76
91
8
2
8
2
93
86
94
9
10
11
2
94
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Experimental Section
Table 4. (Continued)
Entry
Product
Time [h]
2
Yield [%][b]
97
General Procedure for the Amination Reaction at
Room Temperature
12
In a glovebox, in a vial equipped with a stirring bar and
sealed with a screw cap fitted with a septum was added KO-
t-Am (139 mg, 1.1 mmol). Outside the glovebox, were added
the aryl chloride (1.0 mmol), the amine (1.1 mmol) and fi-
13
8
8
2
83
92
83
nally a solution of the [PdACTHNUTRGNENG(U IPr*)ACHTUNGTERN(NUGN cinnamyl)Cl] pre-catalyst
1 (11.7 mg, 1 mol%) in 1 mL of anhydrous DME. The reac-
tion mixture was then stirred (800 rpm) at room tempera-
ture for 3–46 h. The solution was then filtered through
Celiteꢁ and eluted with DCM. The filtrate was evaporated
under vacuum. The crude product was finally purified by
flash chromatography on silica gel. The reported yields are
the average of two runs.
14[e]
15
General Procedure for the Amination Reaction at Low
Catalyst Loading
In a glovebox, in a vial equipped with a stirring bar and
sealed with a screw cap fitted with a septum was added KO-
t-Am (139 mg, 1.1 mmol) in toluene (0.85 mL). Outside the
glovebox, were added the aryl chloride (1.0 mmol), the
16
17
2
2
2
68[f]
84
amine (1.1 mmol) and finally a solution of the [Pd
ACHTUNGTRENNUNG(IPr*)-
AHCTUNGTREG(NNNU cinnamyl)Cl] pre-catalyst 1 (150 mL, 0.05 mol%, prepared
18
88
from 11.7 mg of the pre-catalyst in 3 mL of toluene). The re-
action mixture was then stirred (800 rpm) while refluxing
the toluene during 2–18 h. The solution was then cooled, fil-
tered through Celiteꢁ and eluted with DCM. The filtrate
was evaporated under vacuum. The crude product was final-
ly purified by flash chromatography on silica gel. The re-
ported yields are the average of two runs.
More details about optimization reactions at room tem-
perature as well as the NMR spectra and the characteriza-
tion data for all the compounds are available in the Support-
ing Information.
[a]
Reagents and conditions: (Het)ArCl (1 equiv.), amine
(1.1 equiv.), 1 (0.05 mol%), KO-t-Am (1.1 equiv.), tolu-
ene, reflux, time.
Isolated yield after chromatography on silica gel, average
of two runs.
[b]
[c]
[d]
[e]
Catalyst loading of 0.025 mol%.
Catalyst loading of 0.1 mol%.
The chirality of the final product has been confirmed by
chiral HPLC.
An unidentified by-product was formed.
[f]
Acknowledgements
ertheless very competitive with literature results since
high catalyst loadings are generally reported for such
a hard cross-coupling.[2f,14]
To the best of our knowledge, this catalyst loading
of 0.05 mol% is the second best described in the liter-
ature for the amination of chlorides using NHC-based
palladium precatalysts.[15]
We gratefully acknowledge the EC for funding through the
seventh framework programme SYNFLOW and the EaSt-
CHEM School of Chemistry. Arnaud Boreux is thanked for
his experimental contribution on this project. Umicore AG is
thanked for its generous gifts of materials. SPN is a Royal
Society Wolfson Research Merit Award holder.
In summary, we have shown the [Pd
ACHTUNGTRNE(NUNG IPr*)-
ACHTUNGTRENNUNG(cinnamyl)Cl] complex to be one of the best NHC-
based pre-catalysts for Buchwald–Hartwig cross-cou-
pling. The system exhibits excellent catalytic activity
at room temperature, and also at extremely low cata-
lyst loading. In these conditions, the couplings of nu-
merous (hetero)aryl chlorides with a wide range of
amines have been achieved, yielding the correspond-
ing aryl amines in excellent yields (58–97%).
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[15] [PdACHTNUTRGENNUG(SIPr)ACHTUNGTREN(NGUN cinnamyl)Cl] is still efficient at 0.01 mol%
for the coupling of deactivated chlorides and
0.001 mol% for the coupling of the 2-chloropyridine.
Adv. Synth. Catal. 0000, 000, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
5
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These are not the final page numbers!

COMMUNICATIONS
6 An Efficient Palladium-NHC (NHC=N-Heterocyclic
Carbene)Aryl Amination Pre-Catalyst:
[PdACHTUNGTRENNUNG(IPr*)ACHTUNGTRENNUNG(cinnamyl)Cl]
Adv. Synth. Catal. 2012, 354, 1 – 6
Anthony Chartoire, Xavier Frogneux, Steven P. Nolan*
6
asc.wiley-vch.de
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 0000, 000, 0 – 0
ÝÝ
These are not the final page numbers!