Carbamic acid 2-trimethylsilylethyl ester as a new ammonia equivalent for palladium-catalyzed amination of aryl halides

Article abstract of DOI:10.1016/j.tetlet.2010.08.100

Carbamic acid 2-trimethylsilylethyl ester (Teoc-NH₂) serves as an ammonia equivalent in the palladium-catalyzed amination of aryl bromides and aryl chlorides. Anilines with sensitive functional groups can be readily prepared using these amine derivatives.

Full text of DOI:10.1016/j.tetlet.2010.08.100

Tetrahedron Letters 51 (2010) 5984–5987
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Tetrahedron Letters
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Carbamic acid 2-trimethylsilylethyl ester as a new ammonia equivalent
for palladium-catalyzed amination of aryl halides
Dibakar Mullick a,b, Prakash Anjanappa a,b, Kumaravel Selvakumar a, , Kandasamy Ruckmani c,
,
⇑
⇑
Manickam Sivakumar d,
⇑
a Synthetic Chemistry, Syngene International Ltd, Biocon Park, Plot 2 & 3, Jigani Link Road, Bommasandra Industial Area, Bangalore 560 099, India
b Bharathidasan Institute of Technology, Bharathidasan University, Tiruchirappalli 620 024, India
c Department of Pharmaceutical Technology, Anna University, Tiruchirappalli 620 024, India
d Department of Chemical & Environmental Engineering, University of Nottingham (Malaysia campus), Semenyih 43500, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 May 2010
Revised 26 July 2010
Accepted 31 August 2010
Available online 6 September 2010
Carbamic acid 2-trimethylsilylethyl ester (Teoc-NH2) serves as an ammonia equivalent in the palladium-
catalyzed amination of aryl bromides and aryl chlorides. Anilines with sensitive functional groups can be
readily prepared using these amine derivatives.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Palladium catalysis
Ammonia surrogates
Cross-coupling
Carbamic acid 2-trimethylsilylethyl ester
Significant progress has been made in the development of tran-
sition metal-catalyzed aminations of aryl halides. Despite the exis-
tence of copper- and nickel-catalyzed versions of this reaction,
palladium remains the metal of choice for this transformation. This
is due principally to the higher efficiency of palladium complexes
in aryl amination, benefitting from developments in related palla-
dium-catalyzed cross-coupling reactions.1–3
As a result, a wide variety of secondary aryl amines can be pre-
pared efficiently using this methodology. Primary aryl amines have
been largely prepared by palladium-catalyzed coupling of aryl
halides with ammonia surrogates. Allylamine was used as an ammo-
nia equivalent and the allyl group from the resulting aryl alkyl amine
was cleaved conveniently using methanesulfonic acid and palla-
dium on carbon.4 Benzophenone imine has frequently been used
as an effective ammonia equivalent. The coupling reactions with this
imine are high yielding and can be performed under extremely mild
reaction conditions. Moreover, the resulting N-aryl imines can be
cleaved using several orthogonal methods that are compatible with
a variety of protecting groups.5 The groups of Hartwig and Buchwald
independently reported the use of inexpensive silyl reagents such as
[LiN(SiMe3)2] and Ph3SiNH2 as ammonia equivalents for palladium-
catalyzed aminations and the resulting aryl silylamines were depro-
tected easily.6 Amidine hydrochloride has recently been used as an
ammonia equivalent for copper-catalyzed couplings with aryl ha-
lides to prepare anilines.7 Hydroxylamine O-benzyl ether as an
ammonia equivalent in the catalytic amination of aryl halides has
been reported.8
Toluene sulfonamides have been used as ammonia surrogates for
palladium-catalyzed aminations.9 Unfortunately, the resulting sul-
fonamide requires strong acidic conditions for its cleavage. Fluori-
nated carbamates have been used as ammonia equivalents for
palladium-catalyzed coupling with aryl halides.10 N,N-Dialkylhydr-
azines were also used as ammonia equivalents for the palladium-
catalyzed coupling of aryl halides, and the resulting aryl hydrazines
were cleaved using palladium on carbon under transfer hydrogena-
tion conditions to give anilines.11 Very recently, ammonia itself has
been used as an amine reagent for the palladium-catalyzed coupling
of aryl halides to afford primary aryl amines.12 Aqueous ammonia
and ammonium chloride were used recently for the synthesis of pri-
mary aryl amines using copper as the catalyst.13
We recently used 2-(trimethylsilyl)ethanesulfonyl amide (SES-
NH2) as an ammonia surrogate and the 2-(trimethylsilyl)etha-
nesulfonyl group was cleaved using CsF in DMF at a relatively high
temperature (100 °C) and requiring a long reaction time.14 In the
search for other suitable and stable ammonia surrogates we found
that carbamic acid 2-trimethylsilylethyl ester (Teoc-NH2) was
an ammonia surrogate for the palladium-catalyzed amination
⇑
Corresponding authors. Tel.: +60 3 8924 8156; fax: +60 3 8924 8017 (M.S.).
E-mail addresses: selva.kumar@syngeneintl.com (K. Selvakumar), ruckmani@
tau.edu.in (K. Ruckmani), Sivakumar.Manickam@nottingham.edu.my (M. Sivaku-
mar).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.08.100

D. Mullick et al. / Tetrahedron Letters 51 (2010) 5984–5987
5985
Table 2
O
Teoc-NH2 as an ammonia surrogate in the palladium-catalyzed coupling of aryl and
O
SiMe3
heterocyclic chloridesa
X
HN
O
Me3Si
O
NH2
Entry
1
Substrate
Product
Yieldb (%)
93
R
R
Pd(OAc)2, Xantphos,
Cs2CO3, 1,4-dioxane, 100 oC
Cl
NHTeoc
NHTeoc
NHTeoc
MeOC
OHC
MeOC
OHC
X = Cl, Br
Cl
Scheme 1. Synthesis of Teoc-protected anilines.
2
3
4
5
6
7
8
9
92
97
93
89
91
83
88
90
Cl
EtO2C
EtO2C
Table 1
Cl
NHTeoc
Teoc-NH2 as an ammonia surrogate in the palladium-catalyzed coupling of aryl and
heterocyclic bromidesa
NC
NO2
Cl
NC
NO2
Entry
1
Substrate
Product
Yieldb (%)
97
NHTeoc
Br
NHTeoc
MeOC
MeOC
O2N
NO2
Cl
MeOC
MeOC
O2N
NO2
MeO2C
EtO2C
MeO2C
EtO2C
NHTeoc
Br
NHTeoc
NHTeoc
2
3
98
93
CF3
CF3
Cl
NHTeoc
Br
CN
Cl
CN
OHC
Br
OHC
NHTeoc
NHTeoc
CHO
4
96
MeO
O2N
NO2
Cl
MeO
O2N
NO2
CHO
NHTeoc
Br
NHTeoc
5
6
7
93
92
90
MeO
MeO
NC
NC
NC
NC
Br
Br
NHTeoc
NHTeoc
10
41c
Cl
NH2
OMe
OMe
F
F
Cl
NH2
43c
79
NC
NC
11
12
13
MeO
MeO
Br
NHTeoc
N
Cl
N
NHTeoc
8
9
95
90
71
NC
NC
Br
NHTeoc
Cl
NHTeoc
NHTeoc
N
N
72
NC
NC
Br
NHTeoc
Cl
10
MeO
MeO
14
15
90
88
N
N
O2N
N
O2N
N
MeO
Br
MeO
NHTeoc
Cl
NHTeoc
11
12
13
14
97
98
95
87
MeO2C
O2N
MeO2C
O2N
N
N
Br
NHTeoc
a
Reaction conditions: aryl chloride (1 mmol), Teoc-NH2 (1.2 mmol), Pd(OAc)2
(0.03 mmol), Xantphos (0.06 mmol), Cs2CO3 (2.0 mmol) 1,4-dioxane (7 ml), Schlenk
tube, 120 °C, 15 h.
OMe
Br
OMe
NHTeoc
b
Isolated yield.
c
Incomplete reaction; the Teoc group cleaved and the product was isolated as
F3C
F3C
the free amine.
Br
NHTeoc
N
N
(Scheme 1). The Teoc-protecting group can be removed under mild
conditions using a fluoride source. The fluoride ion attacks the sil-
icon atom which leads to b-elimination and this releases the
deprotected amine and volatile products such as fluorotrimethylsi-
lane and ethylene. Teoc-NH2 can be prepared from the reaction of
phenyl chloroformate and trimethylsilyl ethanol in the presence of
pyridine and the resulting carbonate can be transformed directly
into the Teoc carbamate (Teoc-NH2) using ammonia.15 Teoc-NH2
can also be prepared from trimethylsilyl ethanol by the reaction
with CDI.16
CHO
CHO
15
93
F3C
O2N
Br
Br
F3C
O2N
NHTeoc
NHTeoc
16
17
75
86
N
N
N
N
Br
NHTeoc
a
Reaction conditions: aryl bromide (1 mmol), Teoc-NH2 (1.2 mmol), Pd(OAc)2
(0.03 mmol), Xantphos (0.06 mmol), Cs2CO3 (2.0 mmol) 1,4-dioxane (7 ml), Schlenk
In the present investigation, we studied the potential of Teoc-
NH2 as a reagent for the synthesis of primary arylamines from
methyl 4-bromobenzoate using a palladium source such as
tube, 100 °C, 15 h.
b
Isolated yield.

5986
D. Mullick et al. / Tetrahedron Letters 51 (2010) 5984–5987
Table 3
selectively from products containing acid-sensitive groups such
as tert-butyl ester, enol ether, tert-butyl carbamate, aldol, and
imine. (Table 3).
In summary, we have reported the synthesis of several aryl car-
bamic acid 2-trimethylsilylethyl esters from aryl bromides and aryl
chlorides using Teoc-NH2. The Teoc group can be cleaved selec-
tively without affecting other functional groups and establishes
Teoc-NH2 as a useful ammonia surrogate for aryl amination reac-
tions. Further studies on N-substituted Teoc-NH2 are in progress.
Teoc-NH2 as an ammonia surrogate in the palladium-catalyzed coupling of aryl
bromides containing sensitive functional groups and selective Teoc group cleavagea
Entry Substrate
Product
Yieldb (%)
96 (98)
ButO2C
Br
Br
ButO2C
NHTeoc
NHTeoc
1
2
96 (94)
80 (93)
ButO2C
ButO2C
BocHN
BocHN
Br
NHTeoc
NHTeoc
Acknowledgments
3
O
O
O
The authors, P. Anjappa, D. Mullick, and K. Selvakumar thank
Goutam Das, COO, Syngene international Ltd for granting permis-
sion to carry out this work at Syngene. M. Sivakumar and K. Ruck-
mani thank Bharathidasan and Anna Universities for support.
Br
4
5
92 (90)
88 (95)
O
O
Br
O
NHTeoc
NHTeoc
NHTeoc
Supplementary data
Ph
N
Br
Ph
N
6
7
79 (96)
80 (96)
Ph
Ph
Supplementary data (experimental and analytical data for new
compounds) associated with this article can be found, in the online
version, at doi:10.1016/j.tetlet.2010.08.100.
Br
BocHN
Br
BocHN
TeocHN
CO2But
NHBoc
CO2But
8
(52)c
References and notes
NHBoc
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a
Reaction conditions: aryl bromide (1 mmol), Teoc-NH2 (1.2 mmol), Pd(OAc)2
(0.03 mmol), Xantphos (0.06 mmol), Cs2CO3 (2.0 mmol) 1,4-dioxane (7 ml), Schlenk
tube, 100 °C, 15 h.
b
Isolated yield of coupled product. Yield of free amine after Teoc deprotection is
given in the brackets.
c
Incomplete reaction; the Teoc group of the crude product was cleaved and the
product was isolated as the free amine.
Pd(OAc)2 or Pd2(dba)3 with rac-BINAP in toluene or 1,4-dioxane as
solvent. The maximum yield obtained using this system was only
60%.
Although the yield was moderate, the product formation from
Teoc-NH2 was convincing and thus we screened different catalysts.
Changing the ligand from BINAP to Xantphos resulted in an in-
crease in the yield. Also, by changing the solvent, base, and reaction
temperature, we were able to obtain a 97% isolated yield of product
from methyl 4-bromobenzoate using
a Pd(OAc)2–Xantphos–
Cs2CO3 catalyst system in 1,4-dioxane at 100 °C for 15 h. Aryl bro-
mides with various functional groups such as cyano, ester, nitro,
aldehyde, and methoxy reacted under the above-optimized condi-
tions without undergoing any other side reactions (Table 1).17
Having been successful with aryl bromides, we next extended
this reaction to aryl chlorides. Aryl chloride bond activation is an
industrially important field of research due to the lower cost of aryl
chlorides compared to aryl bromides and aryl iodides.18 We used
the same reaction conditions for the coupling of Teoc-NH2 with
aryl chlorides, but the temperature was increased from 100 °C to
120 °C. The reaction worked well with aryl chlorides possessing
different substituents such as cyano, ester, keto, nitro, and alde-
hyde, and the yields obtained were more than 80% in most cases.
Surprisingly, this reaction only worked moderately well with aryl
chlorides with electron-donating groups (Table 2, entries 10 and
11), whereas heterocyclic aryl chlorides reacted well with Teoc-
NH2 (Table 2).
The aminated aryl trimethylsilylethyl ester products were very
stable and could be used for further transformations without
affecting the Teoc group. The Teoc group was cleaved selectively
using CsF in DMF without affecting other functional groups.19 We
have carried out Teoc deprotection with several products having
acid-sensitive groups, for example, the Teoc group was cleaved

D. Mullick et al. / Tetrahedron Letters 51 (2010) 5984–5987
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(6.7 mg, 0.03 mmol), Xantphos (34.7 mg, 0.06 mmol), Teoc-NH2 (193 mg,
1.2 mmol), and Cs2CO3 (650 mg, 2.0 mmol). The Schlenk tube was evacuated
and back-filled with argon. 3-Bromobenzoic acid tert-butyl ester (257 mg,
1.0 mmol) and 1,4-dioxane (7 ml) were added and the Schlenk tube was sealed
with a Teflon screw cap and placed in a preheated oil bath at 100 °C for aryl
bromides and 120 °C for aryl chlorides for 15 h. After cooling to room
temperature, H2O (10 ml) was added and the reaction mixture extracted
with EtOAc (20 ml). The combined organic layer was washed with brine
(10 ml), dried over Na2SO4, filtered, and concentrated in vacuo. The product
was purified by flash chromatography. Yield: 324 mg, 96%.
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