Palladium-catalyzed amination of aryl sulfides with anilines

Article abstract of DOI:10.1002/anie.201404355

A combination of a palladium-NHC catalyst and potassium hexamethyldisilazide enables the amination of aryl sulfides with anilines to afford a wide variety of diarylamines. The reaction conditions are versatile enough for the reaction of even bulky ortho-substituted aryl sulfides. This amination can be applied to the modular synthesis of N-aryl carbazoles from the corresponding ortho-bromothioanisoles. As aryl sulfoxides undergo extended Pummerer reactions to afford ortho-substituted aryl sulfides, the Pummerer products are thus useful substrates for the amination to culminate in efficient syntheses of a 2-anilinobenzothiophene and an indole as proof-of-principle of the utility of the extended Pummerer reaction/amination cascade. Switching S to N: Transformation of inert C-S bonds of aryl sulfides into C-N bonds is achieved using a palladium complex with an N-heterocyclic carbene ligand. Thanks to the efficient conditions for amination of bulky sulfides, the synthesis of carbazole products from 2-biphenylylamine is achieved through oxidative C-N bond formation. When combined with extended Pummerer reactions, intriguing nitrogen-containing molecules are prepared.

Full text of DOI:10.1002/anie.201404355

Angewandte
Chemie
DOI: 10.1002/anie.201404355
Synthetic Methods
Palladium-Catalyzed Amination of Aryl Sulfides with Anilines**
Tomohiro Sugahara, Kei Murakami,* Hideki Yorimitsu,* and Atsuhiro Osuka
Abstract: A combination of a palladium–NHC catalyst and
potassium hexamethyldisilazide enables the amination of aryl
sulfides with anilines to afford a wide variety of diarylamines.
The reaction conditions are versatile enough for the reaction of
even bulky ortho-substituted aryl sulfides. This amination can
be applied to the modular synthesis of N-aryl carbazoles from
the corresponding ortho-bromothioanisoles. As aryl sulfoxides
undergo extended Pummerer reactions to afford ortho-sub-
stituted aryl sulfides, the Pummerer products are thus useful
substrates for the amination to culminate in efficient syntheses
of a 2-anilinobenzothiophene and an indole as proof-of-
principle of the utility of the extended Pummerer reaction/
amination cascade.
reactions of aryl sulfides.[16] We envisioned that such elec-
tron-donating and bulky ligands would also aid the catalytic
amination of aryl sulfides. Herein, we report amination
reactions of aryl sulfides with aryl amines using a Pd–NHC
catalyst.
The reaction conditions were first optimized for the
amination reaction (Table 1; for details of the catalysts used,
see Figure S1 in the Supporting Information). Treatment of
Table 1: Optimization of the amination reaction conditions.
T
he transition-metal-catalyzed amination of aryl halides[1,2]
has attracted considerable attention from material and
medicinal chemists because a significant number of functional
molecules[3] and drugs[4] contain aryl amine structures.
Entry
Catalyst
Base
Yield [%][a]
1
2
3
4
5
6
7
8
Pd-PEPPSI-IPr
Pd-PEPPSI-IPr
Pd-PEPPSI-IPr
Pd-PEPPSI-IPr
Pd-PEPPSI-IPr
Pd-PEPPSI-IPr
None
K2CO3
KOH
KOtBu
0
0
7
À
Although aminations of inert C O bonds of phenol deriva-
KHMDS/Toluene
NaHMDS/THF
LiHMDS/THF
KHMDS/Toluene
KHMDS/Toluene
KHMDS/Toluene
KHMDS/Toluene
KHMDS/Toluene
KHMDS/Toluene
KHMDS/Toluene
KHMDS/Toluene
KHMDS/Toluene
KHMDS/Toluene
KHMDS/Toluene
76[b]
38
5
tives, such as mesylate,[5] tosylates,[6] carbamates,[7] sulfama-
tes,[7a,8] pivalate esters,[9] phosphate,[10] and methyl ethers,[11]
0
À
are established, it is still difficult to convert C S bonds of aryl
sulfides into C N bonds.
a difficult transmetalation step, which is caused by the strong
interaction between a cationic transition-metal center and an
[12,13]
SingaCycle-A1
SingaCycle-A3
84
88[b]
72
50
84
91[b,c]
0
5
25
0
À
The difficulty originates from
9
10
11
12
13
14
15
16
17
[IPrPdCl(p-allyl)]
[Pd2(dba)3]/IPr·HCl
[{(p-allyl)PdCl}2]/IPr·HCl
SingaCycle-A3
Pd-PEPPSI-IMes
Pd-PEPPSI-IPent
Pd-PEPPSI-SIPr
NiBr2·diglyme/IPr·HCl
À
anionic thiolate moiety. Although many conversions of C S
bonds into C C bonds are known,[14,15] no catalytic conversion
À
À
À
of C S into C N bonds has been reported.
Our recent studies showed that palladium and nickel
catalysts bearing an N-heterocyclic carbene (NHC) ligand are
À
sufficiently active for C C bond-forming cross-coupling
[a] Product yield calculated from 1H NMR experiments. Diphenylme-
thane was used as an internal standard. [b] Yield of isolated product.
[c] p-Toluidine (1.2 equiv), SingaCycle-A3 (2.5 mol%), KHMDS/toluene
(2.5 equiv), dioxane (0.5m). Highlighted line indicates optimum reaction
conditions.
[*] T. Sugahara, Prof. Dr. K. Murakami, Prof. Dr. H. Yorimitsu,
Prof. Dr. A. Osuka
Department of Chemistry
Graduate School of Science, Kyoto University
Kitashirakawa, Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: yori@kuchem.kyoto-u.ac.jp
thioanisole with p-toluidine (1.5 equiv) in the presence of the
catalyst Pd-PEPPSI-IPr[1c,d,h] (5 mol%; PEPPSI = pyridine-
enhanced pre-catalyst preparation, stabilization, and initia-
tion; IPr= 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)
and K2CO3 (3 equiv) in dioxane at 1008C for 12 h gave none
of the expected diaryl amine 1a (entry 1). We speculated that
Prof. Dr. K. Murakami
The Hakubi Center for Advanced Research
Kyoto University (Japan)
E-mail: kmura@kuchem.kyoto-u.ac.jp
Prof. Dr. H. Yorimitsu
ACT-C, JST (Japan)
[**] This work was supported by Grants-in-Aid from MEXT (Nos.:
24106721 “Reaction Integration” and 25107002 “Science of Atomic
Layers”) and from JSPS (Nos.: 24685007 (Young Scientists (A)),
23655037, and 26620081 (Exploratory Research)). K.M. acknowl-
edges JSPS Postdoctoral Fellowship for Young Scientists. Mr.
Vincent Guꢀrin and Dr. Alexandre Baralle are acknowledged for
preparation of starting materials.
this disappointing result is caused by
a
sluggish
transmetalation step.[17] The same reaction was tested using
stronger bases to accelerate the transmetalation step
(entries 2–6). Although KOH and KOtBu were not effective,
employment of KHMDS (potassium hexamethyldisilazide)
provided 1a in 76% yield (entries 2–4). The choice of
countercation was important: analogues NaHMDS and
LiHMDS afforded 1a in only 38% and 5% yields, respec-
Supporting Information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404355.
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.
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Communications
Table 2: Reagent scope of the reaction.
tively (entries 5,6). The reaction did not proceed in the
absence of a palladium catalyst, thereby eliminating a pos-
sible SNAr (nucleophilic aromatic substitution) pathway
(entry 7).[12] Palladium precursors also affected the efficiency
(entries 8–13). Palladium compound SingaCycle-A3[18]
showed higher catalytic activity than Pd-PEPPSI-IPr to
obtain 1a in 88% yield (entry 9). Further optimization
revealed that the highest yield (91%) for the reaction was
obtained in the presence of SingaCycle-A3 (2.5 mol%), p-
toluidine (1.2 equiv), and KHMDS (2.5 equiv) under concen-
trated conditions (0.5m; entry 13). The choice of the Pd–IPr
catalyst combination was crucial. Employment of the less
bulky IMes ligand (IMes = 1,3-bis(2,4,6-trimethylphenyl)imi-
dazol-2-ylidene) or the more bulky IPent ligand (IPent = 1,3-
bis(2,6-di-3-pentylphenyl)imidazol-2-ylidene) showed lower
catalytic activity (entries 14,15). The reaction with the
SIPr ligand, which is an analogue of the IPr ligand
with a saturated imidazolium core, also resulted in a lower
product yield (entry 16). A combination of a nickel salt
(NiBr2·diglyme) and the IPr ligand was not effective
(entry 17).
The scope of the amination reaction was then investigated
(Table 2). We first studied the effect of alkylsulfanyl leaving
groups. The odorless 1-dodecylsulfanyl group[19] and bulky
tert-butylsulfanyl and phenylsulfanyl groups participated in
the reaction (entries 1–4). A wide scope of anilines was also
successfully employed in this reaction. Bulky o-toluidine
reacted smoothly to give 1c in high yield (entry 6). Unsub-
stituted aniline (entry 7) as well as anilines having an
electron-donating group (entries 8,11) or an electron-
withdrawing group (entries 9,10) were successfully used in
the amination reaction. By protection of the NH2 substituent
using benzyl groups, the reagent survives under the basic
reaction conditions (entry 11).[20] Amination with 2-pyridyl-
amine was also successful but required a longer reaction time
(entry 12). A variety of aryl sulfides were also tested in the
reaction (entries 13–25). Aryl sulfides having various sub-
stituents, such as o-methyl (entry 15), electron-withdrawing
fluoro and trifluoromethyl (entries 16,18), electron-donating
methoxy (entry 19), THP-protected hydroxy[20] (entry 20,
THP = tetrahydropyran), and acetal-protected formyl[20]
(entry 21), underwent the amination. Unfortunately, 4-
trifluoromethylaniline derivative 2e could not successfully
isolated from the reaction as a result of its instability under
basic conditions (entry 17). The reaction conditions were also
applicable to bulky polyaromatic sulfides, such as 1-naphthyl,
9-phenanthryl, and 9-anthracenyl sulfides (entries 22–24). 2-
Biphenylylamine 2m was obtained in good yield (73%) and
was easily transformed into the corresponding carbazole (see
below, Scheme 1).
Entry Ar1
R
R2
1 or 2 Yield [%][a]
1
Ph
Me
4-Me
1a
1a
1a
1a
1b
1c
1d
1e
1 f
91
(73)
(65)
(99)
92[b]
75
85
90
88
2
Ph
C12H25 4-Me
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
tBu
Ph
4-Me
4-Me
3-Me
2-Me
H
4-MeO
4-F
3-CF3
4-Bn2N
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
1g
1h
65[b]
74[b]
75[c,d]
72[c]
80[c]
64
Ph
Ph
(2-Pyridyl) 1i
4-MeC6H4
3-MeC6H4
2-MeC6H4
4-FC6H4
4-CF3C6H4
3-CF3C6H4
4-MeOC6H4
4-THPOC6H4
4-Me
4-Me
4-Me
4-Me
2a
2b
2c
2d
2e
2 f
65[c]
0
C12H25 4-Me
C12H25 4-Me
Me
Me
83
4-Me
4-Me
2g
2h
73[e]
60[b,d]
21
Me
4-Me
2i
76[b]
22
Me
4-Me
2j
68[b]
23
24
25
C12H25 4-Me
2k
2l
80[f]
81[b]
73[b]
C12H25 4-Me
Me
4-Me
2m
[a] Yield of isolated product. Product yield calculated from 1H NMR
experiments are in parentheses. [b] SingaCycle-A3 (10 mol%). [c] Sin-
gaCycle-A3 (5 mol%). [d] 18 h. [e] SingaCycle-A3 (2.5 mol% ꢁ3).
[f] SingaCycle-A3 (7.5 mol%).
Since the amination provided potassium diarylamides,
such as 3, before aqueous work-up, the one-pot synthesis of
triarylamine 4 was achieved by the subsequent reaction of 3
with 1-bromonaphthalene [Eq. (1)].[21]
Taking advantage of the efficient amination of bulky aryl
sulfides (for example Table 2, entry 25), we undertook the
modular synthesis of carbazoles. Compounds 6 (6a–6e;
Scheme 1) were obtained through the palladium-catalyzed
Suzuki reaction of 2-bromoaryl methyl sulfide (5a) or its
methyl derivative 5b, which are readily available through
bromination of aryl methyl sulfide. Amination of the com-
pound 6 derivatives provided the corresponding amines 7.
The resulting diarylamines 7 can be employed as the key
precursors of substituted carbazoles 8 through direct oxida-
tive C N bond formations.[22,23] Treatment of 7 in the presence
À
2
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Scheme 2. Extended Pummerer/amination reaction sequences. [a] Tf2O
(1.4 equiv), K2CO3 (3 equiv), toluene, 258C, 1 h, then ethanolamine
(5 equiv), 258C, 3 h. [b] p-Toluidine (1.2 equiv), SingaCycle-A3
(10 mol%), KHMDS/toluene (2.5 equiv), dioxane (0.5m), 1008C, 18 h.
[c] Tf2O (1.5 equiv), MeCN, 258C, 0.5 h, then 2,6-lutidine (2.5 equiv),
808C, 20 h. [d] p-Toluidine (4-MeC6H4NH2; 1.3 equiv), SingaCycle-A3
(10 mol%), KHMDS/toluene (2.6 equiv), dioxane (0.5m), 1008C, 12 h.
Another example includes the method of Eberhart and
Procter for the ortho-propargylation of aryl sulfoxides, which
afforded methyl-2-propargylphenyl sulfide (12).[26b] When
compound 12 was used under the amination conditions,
a tandem amination/cyclization reaction occurred to yield the
corresponding indole 13 in 80% yield.[27]
In summary, we have developed new amination of aryl
sulfides by using a state-of-the-art palladium–IPr catalyst. The
wide scope of the reaction means that various aryl sulfides
and anilines, including bulky ortho-substituted substrates and
substrates with base-proof protection, can be employed. A
new route to the modular synthesis of N-arylcarbazoles can be
achieved through amination of 2-biphenylyl sulfides followed
Scheme 1. Top: Modular synthesis of N-arylcarbazole derivatives 8
from bromoaryl methyl sulfide reagents 5. Bottom: Structures and
overall cumulative yields of final N-arylcarbazole products 8a–8e.
Yields for individual reaction steps to form intermediates 6a–6e and
7a–7e and final products 8a–8e given in parentheses. [a] 2 h.
[b] 1508C.
of a catalytic amount of [Pd(OAc)2] with [Cu(OAc)2] as an
oxidant in DMF for 3 h at 1408C furnished carbazole
compounds 8 in high yields. A wide variety of carbazoles,
including a ladder-type molecule 8e, were obtained through
this three-step strategy in 25–40% overall yields from 5.
Interestingly, the ring-closing reaction of 7d proceeded
regioselectively at the ortho position of the methoxy group
to give carbazole 8d.
We have regarded sulfur as a halogen surrogate of higher
functionality because of the relatively recent advancements in
extended Pummerer reactions,[16,24–26] where sulfoxide
moieties are converted into the corresponding sulfides with
À
by intramolecular C N bond formation under oxidative
conditions. We have also demonstrated that extended Pum-
merer reaction/amination sequences are highly efficient for
the rapid synthesis of complex molecules. Our work will be an
important milestone to disseminate carbon–sulfur-based
organic synthetic methods that complement carbon–halo-
gen-based syntheses. The development of more general
aminations of aryl sulfides under mild conditions is ongoing
in our laboratory.
Received: April 15, 2014
Revised: June 2, 2014
Published online: && &&, &&&&
À
intriguing C C bond formation. As a limited variety of
transformations of the resulting alkylsulfanyl moieties are
available,[14] we have been striving after new methods for
Keywords: amination · aryl sulfides ·
À
transformations of C S bonds to expand the potential of the
.
N-heterocyclic carbene ligands · palladium · Pummerer reaction
Pummerer products. We envisioned that combining extended
Pummerer reactions with our catalytic amination would
provide a new method to synthesize nitrogen-containing
aromatic compounds. For example, we employed our benzo-
thiophene synthesis from the readily available compound,
ketene dithioacetal monoxide 9[25b] (Scheme 2). The Pum-
merer cyclization of 9 with trifluoromethanesulfonic anhy-
dride (Tf2O) followed by demethylation with ethanolamine
afforded 2-methylsulfanyl-3-phenylbenzothiophene (10) in
90% yield. The resulting methylsulfanyl group of 10 was
replaced with p-toluidine using the amination method.
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M. L. Turner (The University of Manchester), WO 2012/
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[12] For examples of SNAr reactions of electron-deficient aryl
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see: a) H. Yorimitsu, J. Synth. Org. Chem. Jpn. 2013, 71, 341;
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2185; c) T. Kobatake, S. Yoshida, H. Yorimitsu, K. Oshima,
Angew. Chem. 2010, 122, 2390; Angew. Chem. Int. Ed. 2010, 49,
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Angewandte
Chemie
2340; d) K. Murakami, J. Imoto, H. Matsubara, S. Yoshida, H.
Yorimitsu, K. Oshima, Chem. Eur. J. 2013, 19, 5625; e) S.
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f) T. Kobatake, D. Fujino, S. Yoshida, H. Yorimitsu, K. Oshima,
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K. Oshima, Org. Lett. 2007, 9, 5573.
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Ed. 2013, 52, 4008; c) K. S. Feldman, M. D. Fodor, J. Org. Chem.
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[26] For recent examples on extended Pummerer reactions from
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Maulide, J. Am. Chem. Soc. 2013, 135, 7312; b) A. J. Eberhart,
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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Angewandte
Communications
Communications
Synthetic Methods
Switching S to N: Transformation of inert
À
À
C S bonds of aryl sulfides into C N
bonds is achieved using a palladium
complex with an N-heterocyclic carbene
ligand. Thanks to the efficient conditions
for amination of bulky sulfides, thesyn-
thesis of carbazole products from 2-
biphenylylamine is achieved throughoxi-
T. Sugahara, K. Murakami,*
H. Yorimitsu,* A. Osuka
&&&& — &&&&
Palladium-Catalyzed Amination of Aryl
Sulfides with Anilines
À
dative C N bond formation. When com-
bined with extended Pummerer reactions,
intriguing nitrogen-containing molecules
are prepared.
6
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!