Well-defined allylnickel chloride/N-heterocyclic carbene [(NHC)Ni (allyl)Cl] complexes as highly active precatalysts for C-N and C-S cross-coupling reactions

Article abstract of DOI:10.1002/adsc.201000223

The room temperature Buchwald-Hartwig amination of heteroaromatic chlorides has been achieved using the sterically bulky allylnickel chloride/ N-heterocyclic carbene [(IPr)Ni (allyl)Cl]complex as a well-defined precatalyst. Arylation of aromatic thiols, affording high yields of products under low catalyst loadings, has also been promoted with the same complex.

Full text of DOI:10.1002/adsc.201000223

FULL PAPERS
DOI: 10.1002/adsc.201000223
Well-Defined Allylnickel Chloride/N-Heterocyclic Carbene
[
(NHC)Ni
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(allyl)Cl] Complexes as Highly Active Precatalysts for
À
À
C N and C S Cross-Coupling Reactions
a
b
a,
Marꢀa Josꢁ Iglesias, Auxiliadora Prieto, and M. Carmen Nicasio *
a
Laboratorio de Catꢂlisis Homogꢁnea, Departamento de Quꢀmica y Ciencia de los Materiales, Unidad Asociada al CSIC,
Campus de El Carmen s/n, Universidad de Huelva, 21007 Huelva, Spain
Fax : (+34)-959-219-942; phone: (+34)-959-219-948; e-mail: mcnica@dqcm.uhu.es
Departamento de Ingenierꢀa Quꢀmica, Quꢀmica Fꢀsica y Quꢀmica Orgꢂnica, Campus del Carmen s/n, Universidad de
b
Huelva, 21007 Huelva, Spain
Received: March 22, 2010; Revised: June 4, 2010; Published online: August 16, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000223.
Abstract: The room temperature Buchwald–Hartwig low catalyst loadings, has also been promoted with
amination of heteroaromatic chlorides has been the same complex.
achieved using the sterically bulky allylnickel chlo-
ride/N-heterocyclic carbene [(IPr)Ni ACHUTNGRNEGUN( allyl)Cl] com-
plex as a well-defined precatalyst. Arylation of aro- Keywords: N-arylamination; CÀS cross-coupling; N-
matic thiols, affording high yields of products under heterocyclic carbenes; nickel
Introduction
spite of the fact that Ni-based catalysts are attractive,
not only due to their lower cost when compared with
Transition metal-mediated cross-coupling reactions palladium, but also because of the higher reactivity of
[
6]
are one of the most powerful tools for the formation Ni(0) species toward aryl chlorides. A possible ex-
of CÀC and CÀheteroatom bonds, palladium usually planation for the lack of nickel-based catalysts could
[
1]
being the metal of choice for these transformations.
be related either to problems in reducing stable Ni(II)
[7,8]
In most cases, versatile and highly efficient CÀN precursors
cross-coupling transformations are based in the use species, or to the handling of air-unstable Ni(0).
of palladium and phosphane ligands. In the last During the last years, significant efforts have been
decade, the substitution of tertiary phosphanes by made to overcome those drawbacks. Thus, Knochel
their N-heterocyclic carbene (NHC) ligand surrogates et al. have described the use of polymethylhydrosil-
has led to more active Pd-based catalysts for cross- oxane as the reducing agent in the nickel-catalyzed
coupling processes. Over the past few years, Nolan amination of aryl chlorides, employing Ni(acac) /
to catalytically active zerovalent nickel
[
2]
[9]
[
10]
AHCTUNGTRENNUNG
[
3]
ACHTUNGTRENNUNG
2
and co-workers have developed several catalytic sys- 3,5,6,8-tetrabromophenanthroline as the catalytic
tems for cross-coupling processes involving the use of system with excellent results. On the other hand, air-
well-defined NHC-containing palladium(II) precata- stable, well-defined Ni(II) precatalysts – such as trans-
[
4]
[11]
lysts. In this context, [(NHC)Pd
plexes have been reported to perform the Buchwald– [NiX ACHTUNGERTNNNUG( PPh ) AHCTUNTGNERUNG( NHC)] species – have been examined
2 3
A
H
U
G
R
N
N
(p-allyl)Cl] com- [(PPh ) Ni
A
H
U
G
R
N
U
G
and mixed PPh /NHC, trans-
3
2
3
[
12]
Hartwig amination of aryl chlorides with sterically in CÀN coupling reactions. These complexes are
hindered substrates in high yields even at room tem- easily reduced to the active Ni(0) species (the former
[5]
perature. These Pd(II) precatalysts were easily acti- in the presence of NHC ligands) without the aid of
vated in the presence of NaO-t-Bu, the base added to external reductants. In addition, a family of phos-
deprotonate the N-nucleophile, leading to NHC-Pd(0) phane-free Ni(II)-NHC complexes of composition
[
5a,b]
[13]
active species which initiates the catalytic cycle.
[CpNi AHCTUNGRETNNUN(G NHC)Cl] have been reported to exhibit only
In contrast with the knowledge based on palladium moderate catalytic activity in both aryl halide dehalo-
catalysts, nickel-catalyzed CÀheteroatom bond-form- genation and aryl amination reactions. Such a low ac-
ing reactions have been less studied. And this is in
Adv. Synth. Catal. 2010, 352, 1949 – 1954
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1949

Marꢀa Josꢁ Iglesias et al.
FULL PAPERS
tivity could be the result of the lack of an easy reduc- Table 1. Optimization studies for the Ni-catalyzed N-aryla-
[a]
tion of morpholine.
tion of Ni(II) precursors to Ni(0) species.
On the basis of Nolanꢄs work on the use of well-de-
fined (NHC)Pd(II) precatalysts in cross-coupling re-
actions, we decided to investigate whether the
[
b]
[
(NHC)Ni
activated to (NHC)Ni(0) active species under suitable
reaction conditions. The complexes [(NHC)Ni(p-al-
but they
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(p-allyl)Cl] analogues could also be easily Entry
Catalyst
Base
Solvent
Yield [%]
1
2
3
4
5
6
7
8
9
1
1
2
3
2
2
2
2
2
2
2
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
KO-t-Bu
LiO-t-Bu
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
THF
THF
THF
THF
5
AHCTUNGTRENNUNG
98
71
75
57
19
54
0 [
[
14]
lyl)Cl] have been previously synthesized
have not been yet used as catalysts in cross-coupling
processes.
THF
Herein, we describe the use of these NHC-Ni(II)
Toluene
DMF
i-PrOH
THF
complexes as precatalysts in CÀN and CÀS bond-
forming reactions. We have found that the complex
c]
98[
[
(IPr)Ni ACHTUNGTRENNUNG( p-allyl)Cl] can be easily activated even at
d]
0
THF
98
room temperature allowing the room temperature
nickel-catalyzed Buchwald–Hartwig coupling of aryl
and heteroaryl chlorides. To the best of our knowl-
edge, this is the first example based on this metal that
operates at room temperature. Noteworthy, we have
also expanded this system to CÀS cross-coupling reac-
[a]
Reaction conditions: morpholine (1.5 mmol); 4-chloroto-
luene (1 mmol); base (1.2 mmol); nickel complex (5
mol%); solvent (1 mL); T=608C; t=24 h.
Isolated yields (average of two runs).
t=2 h.
[
[
[
b]
c]
d]
T=room temperature (258C); t=24 h.
tions.
Results and Discussion
displayed the best catalytic behaviour, affording the
desired product in quantitative yield after 24 h
(entry 2). No reaction was observed in the absence of
the metal complex. Optimization of the reaction con-
ditions was therefore performed with 2 as the precata-
Catalytic Activity of [(NHC)Ni
in the Buchwald–Hartwig Reaction
ACHTUNGTRENNUNG( allyl)Cl] Complexes
The complexes [(IMes)Ni
(allyl)Cl] (2) and [(SIPr)Ni
were readily prepared in high yields according to the Thus, as inferred from entries 4 and 5, the nature of
ACHTUNGTRENNUNG
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ACHTUNGTRENNUNG
[
14]
literature procedures.
In a first series of experi- the cation of the base seems to affect the activation of
[15]
ments, complexes 1–3 were tested as precatalysts in the precatalyst.
Also, solvents such as toluene,
the Buchwald–Hartwig amination reaction. Following DMF or i-PrOH were detrimental for the yield of the
a protocol similar to that reported by Fort and co- coupling product (entries 6–8). The reaction time was
[7b]
workers, the reaction of morpholine and 1-chloro- then optimized by carrying out the amination reaction
4
1
-methylbenzene was carried out in the presence of with 2 in only 2 h at 608C (entry 9). Alternatively, the
.2 equiv. of the base (NaO-t-Bu) using 5 mol% of the same experiment was carried out at room tempera-
Ni(II) precatalyst 1–3 at 608C in THF. The results are ture. To our delight, the N-arylated product was ob-
shown in Table 1. Complex 2, bearing the IPr ligand, tained in quantitative yields at room temperature in
24 h (entry 10). As far as we are aware, this consti-
tutes the first example of a nickel-catalyzed N-aryla-
tion of morpholine under such mild conditions.
After these promising results, a series of different
aryl chlorides were reacted with morpholine using the
optimised conditions: complex 2 (5 mol%) as the cat-
alyst, NaO-t-Bu (1.2 equiv.) as the base and THF as
the solvent (Table 2). Electron-neutral, electron-rich
and electron-poor aryl chlorides proved to be suitable
for this reaction affording N-arylated products in
quantitative yields within 2–4 h at 608C (entries 1–5),
with the sole exception of p-MeOC H Cl which re-
6
4
quired extended heating to furnish the coupling prod-
uct in 72% yield (entry 3). The use of ortho-chloroto-
Scheme 1. ACHTUNGTRENNUNG[ (NHC)Ni ACTHUNGTRENNNUG
work.
tion about the steric hindrance that negatively influ-
1950
asc.wiley-vch.de
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 1949 – 1954

Well-Defined Allylnickel Chloride/N-Heterocyclic Carbene [(NHC)Ni ACHUTNGRENNUG( allyl)Cl] Complexes
Table 2. Buchwald–Hartwig cross-coupling reactions of aryl in aryl amination reactions (1058C, 7–20 h). Com-
[
a]
chlorides.
pared to these precedents, our system provides better
conversions under milder conditions. Such features
could be explained in terms of the presence of the
allyl moiety in the precursor which would help in the
reduction of Ni(II) to Ni(0) species, probably in a
similar way as it has been proposed for Pd(II) ana-
[b]
Entry
Chloride
Time [h]
2
Yield [%]
99[
[
5a,b]
logues.
1
2
3
4
c]
24
98
Given the catalytic capabilites of complex 2 for the
amination of 2-chlorobenzoxazole and 2-chlorobenzo-
thiazole, we wondered about the extension of this
system toward the use of heteroaromatic compounds,
and therefore a series of N-containing heteroaromatic
chlorides as coupling partners was examined next. We
were pleased to find that the reaction of 2-chloropyri-
dine with morpholine reached completion in only 1 h
at room temperature using 2.5 mol% of the Ni(II)
catalyst (Table 3, entry 1), in the first example of this
reaction catalyzed with nickel at room-temperature.
Under the same conditions, the reaction of several
cyclic secondary amines with chloropyridine or 2-
chloroquinoline occurred within 3 h in quantitative
yields (Table 3, entries 1–5). The more sterically de-
manding dibutylamine required longer reaction times
to give the coupling product although in lower yield
2
2
98[
c]
4
98
24
72
2
96[
c]
24
97
4
2
96[
5
6
c]
4
98
24
24
61
70
7
8
9
24
24
72
70
Table 3. Buchwald–Hartwig reactions of heteroaromatic
[a]
chlorides at room temperature.
[
a]
Reaction conditions: morpholine (1.5 mmol); aryl chlo-
ride (1 mmol); NaO-t-Bu (1.2 mmol), 2 (5 mol%); THF
(
1 mL); T=608C.
[
[
b]
c]
Isolated yields (average of two runs).
T=room temperature (258C).
[b]
Entry Amine
1
Chloride
Time [h] Yield [%]
1
98
95
99
99
99
62
71
76
enced the conversions (entries 6 and 7). It is worth-
while to point out that the conversions obtained after
2
3
4
5
6
7
1
2
–4 h at 608C could also be achieved upon carrying
out the same experiments at room temperature for
4 h. Finally, the amination of 2-chlorobenzothiazole
2
1
and 2-chlorobenzoxazole, less commonly employed
substrates in these coupling processes, were examined.
Although reaction rates were slower, precatalyst 2
was also effective providing the products in good
yields (entries 8 and 9).
1
3
The results obtained with our system are, at least,
comparable to those reported by Yang and co-work-
8
[11a]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ers
using [Ni ACHTUNGTRENNUNG( PPh ) ACHTUNETGRNNNU(G phenyl)Br]/IPr·HCl as the cat-
3 2
alyst, although in our case the use of phosphane is
avoided. A related system has been described by Mat-
24
8
[
12]
subara and co-workers, based in the complex [NiCl2
(PPh )(IPr)] as the catalyst for the amination of aryl
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
8
3
halides. However, only aryl bromides could be cou-
pled with less basic amines such as anilines at higher
temperatures and longer reaction times (1008C, 24 h).
[a]
Reaction conditions: amine (1.5 mmol); heteroaromatic
chloride (1 mmol); NaO-t-Bu (1.2 mmol), 2 (2.5 mol%);
THF (1 mL).
[b]
Similar[ conditions were described for Nolanꢄs
13]
system
employing [CpNi
A
H
U
G
E
N
N
(NHC)Cl] as precatalysts
Isolated yields (average of two runs).
Adv. Synth. Catal. 2010, 352, 1949 – 1954
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1951

Marꢀa Josꢁ Iglesias et al.
FULL PAPERS
(
62% isolated yield, entry 6). Moreover, morpholine with NaO-t-Bu as the base afforded the diaryl sulfide
was also coupled with the less electron-deficient 3- in high yields, minimizing the undesired phenyl disul-
chloropyridine at room temperature after 24 h fide, C H ÀSÀSÀC H (Table 4, entry 2). The effect of
6
5
6
5
(
entry 7). Finally, the reaction of 2-chloropyridine the halide was investigated, 4-chloro- and 4-iodoto-
with aniline was slower than those of secondary alkyl- luene being employed to obtain, with the latter, the
amines but afforded only the monoarylation product coupling product in 95% yield (entries 3 and 4). Fur-
in 76% yield. It is interesting to note that compared ther experiments were preformed to optimize the cat-
with 2, Pd(II) analogues [(NHC)Pd ACHUTNGRENNUG( R-allyl)Cl] re- alyst loading, leading to a value of 2 mol% of precata-
quired the presence of a modified allyl moiety in their lyst 2 as the most appropriate. Interestingly, Zhang
molecule for the room temperature activation of and co-workers have reported for the experiment de-
[
5e,f]
these precatalysts in aryl amination reactions.
scribed in entry 2 a low conversion (14% GC yield),
although generating in situ the (IPr)-Ni(0) precatalyst
with an NHC/Ni ratio of 1. This is in contrast with
our results, based in the use of isolated, well-defined
Ni-complexes, a feature that seems to be crucial in
Catalytic Activity of [(IPr)Ni
Forming Reactions
A
H
U
G
R
N
U
G
The formation of C
tance since aryl sulfides are intermediates in the syn-
thesis of valuable compounds with biological, pharma-
A
H
U
G
R
N
U
G
[a]
Table 5. Coupling of aryl halides with thiols.
were required. Very recently, Zhang and co-workers
have reported the first (NHC)-based nickel(0) cata-
[
18]
lyst for CÀS coupling reactions.
This in situ pre-
1
[b]
Entry
R
H
H
Aryl halide
Cat. load
Yield [%]
pared catalyst system (NHC/Ni=2) has allowed the
coupling of thiophenol with aryl bromides and iodides
in high yields with 1–4 mol% of Ni catalyst. These re-
sults prompted us to evaluate the performance of
1
2
1
2
76
82
86
97
1
2
[
(IPr)Ni
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(allyl)Cl] as the catalyst in CÀS bond-forming
reactions.
3
H
1
95
Thiophenol and 4-bromotoluene were selected as
representative substrates to screen the experimental
conditions (Table 4). Initially, the best conditions pre-
viously found for the N-arylation of morpholine were
tested but no conversion was observed. Optimization
studies revealed that 5 mol% of 2 in DMF at 1008C
4
5
6
7
H
H
H
H
2
3
5
5
86
90
78
86
Table 4. Optimization studies for the Ni-catalyzed arylation
[a]
of thiophenol.
8
9
H
H
5
2
95
83
[b]
Entry
X
Cat. load
Solvent
Yield [%]
1
2
3
4
6
7
8
Br
Br
Cl
I
I
I
5
5
5
5
1
2
0
DME
DMF
DMF
DMF
DMF
DMF
DMF
62
90
30
95
89
95
0
10
11
12
OMe
Me
Cl
2
2
2
85
99
96
I
[
a]
[a]
Reaction conditions: thiophenol (1.1 mmol); aryl halide
1 mmol); NaO-t-Bu (1.2 mmol); solvent (1 mL); T=
008C; t=24 h.
Isolated yields (average of two runs).
Reaction conditions: thiol (1.1 mmol); aryl halide
(1 mmol); NaO-t-Bu (1.2 mmol); DMF (1 mL); T=
1008C.
(
1
[
b]
[b]
Isolated yields (average of two runs).
1952
asc.wiley-vch.de
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 1949 – 1954

Well-Defined Allylnickel Chloride/N-Heterocyclic Carbene [(NHC)Ni ACHUTNGRENNUG( allyl)Cl] Complexes
this catalytic system. Finally, in the absence of nickel lected with a Varian GC-3900 chromatograph with a flame
ionization detector.
catalyst no coupling product was observed.
With the optimal conditions in hand (1.2 equiv. of
General Catalytic Procedure for the Buchwald–
Hartwig Reaction
NaO-t-Bu, DMF as solvent, 1008C), an array of aryl
halides and thiols were examined, the results being
shown in Table 5. The protocol proved suitable to pre-
pare a series of aryl sulfides in 76–99% yields with 1–
The catalyst [(IPr)Ni
ACHTUNGTRENUNNG( allyl)Cl] 2 (0.05 mmol), sodium tert-
butoxide (1.2 mmol) and tetrahydrofuran (THF) (1 mL)
5
mol% of nickel catalyst. In general, yields did not were added in turn to an ampoule equipped with a magnetic
vary significantly by changing the electronic nature of stir bar. The amine (1.5 mmol) and the aromatic chloride
the substituent both in the aryl iodide and in the thio- (1 mmol) were added under a nitrogen atmosphere. The re-
action was stirred at the given temperature (608C or room
phenol (entries 1–5 and 10–12). In good accord with
temperature) for the given time (1–24 h). When the reaction
the already mentioned preference for iodoaryls, full
had reached completion (monitored by GC) or no further
selectivity was observed when using 1-chloro-4-iodo-
conversion was observed, the mixture was allowed to cool to
benzene with thiophenol, affording the corresponding
room temperature, diluted with ethyl acetate (10 mL) and
4-chlorophenyl phenyl thioether as the only product
filtered through celite. The clean solution was evaporated to
dryness, and the residue was purified by flash chromatogra-
phy on silica gel.
(
entry 4). Also, conversions were not affected by the
presence of substituents in the ortho-position of the
aryl halide (entry 9). As expected, aryl bromides were
less reactive requiring a higher catalyst loading
General Catalytic Procedure for the Arylation of
Thiols with Aryl Halides
(
5 mol%) in order to achieve good yields of aryl sul-
fides (entries 6–8).
The catalyst [(IPr)Ni ACHUTNGRNENUG( allyl)Cl] 2 (0.01–0.05 mmol), sodium
tert-butoxide (1.2 mmol) and DMF (1 mL) were added to an
ampoule equipped with a magnetic stir bar. The thiol
Conclusions
(
1.1 mmol) and the aryl halide (1 mmol) were added under
a nitrogen atmosphere. The reaction was stirred at 1008C
for 24 h in an oil bath. The reaction mixture was allowed to
In summary, the complexes [(NHC)Ni ACHUTNGRNENGU( allyl)Cl] have
been employed for the first time as precatalysts in CÀ cool to room temperature, diluted with ethyl acetate
N and CÀS bond forming reactions. [(IPr)Ni
ACHTUNGTRENNUNG
(allyl)Cl] (10 mL) and filtered through celite. The clean solution was
displays the higher activity in the Buchwald–Hartwig evaporated to dryness and the residue was purified by flash
chromatography on silica gel with petroleum ether to afford
amination of aryl chlorides. This precatalyst is easily
activated at room temperature allowing the coupling
the desired product.
of heteroaromatic chlorides with secondary cyclic and
acyclic and primary amines in good to excellent
yields. These are the first examples of room-tempera-
ture nickel-catalyzed coupling of amines and aryl hal-
Acknowledgements
ides. Furthermore, the singly ligated [(IPr)Ni ACHUTNRGNENUG( allyl)Cl]
We thank Junta de Andalucꢀa (Proyecto P07-FQM-02745)
and MEC (Proyecto CTQ2008-00042BQU) and Consolider
Ingenio 2010 (Grant No. CSD2006-0003) for financial sup-
port. M.J.I. thanks Junta de Andalucꢀa for a research fellow-
performs efficiently the arylations of aromatic thiols
with loadings of 1–5 mol% affording high yields of
coupling products.
Further studies are currently underway to expand ship. We wish to thank Prof. Pedro J. Pꢁrez (Univ Huelva)
the reaction scope and to investigate the activation of for helpful comments and discussions.
Ni(II) precatalyst to the postulated active Ni(0) spe-
cies.
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Experimental Section
General Methods
[
All reactions and manipulations were carried out under an
atmosphere of nitrogen with standard Schlenk techniques.
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4
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Adv. Synth. Catal. 2010, 352, 1949 – 1954
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1953

Marꢀa Josꢁ Iglesias et al.
FULL PAPERS
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