Copper(II) tetrafluoroborate as a novel and highly efficient catalyst for N-tert-butoxycarbonylation of amines under solvent-free conditions at room temperature

Article abstract of DOI:10.1016/j.tetlet.2005.12.044

Commercially available copper(II) tetrafluoroborate hydrate was found to be a highly efficient catalyst for chemoselective N-tert-butoxycarbonylation of amines with di-tert-butyl dicarbonate under solvent-free conditions and at room temperature. Various aromatic amines were protected as their N-tert-butyl carbamates in high yields and in short times. No competitive side reactions such as isocyanate, urea, and N,N-di-t-Boc formation was observed. Chemoselective N-tert-butoxycarbonylation was achieved with substrates bearing OH and SH groups. Chiral α-amino acid esters afforded the corresponding N-t-Boc derivatives in excellent yields.

Full text of DOI:10.1016/j.tetlet.2005.12.044

Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 1087–1091
Copper(II) tetrafluoroborate as a novel and highly efficient
catalyst for N-tert-butoxycarbonylation of amines under
solvent-free conditions at room temperature
Sunay V. Chankeshwara and Asit K. Chakraborti*
Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research (NIPER),
Sector 67, S. A. S. Nagar, Punjab 160 062, India
Received 30 October 2005; revised 2 December 2005; accepted 7 December 2005
Available online 27 December 2005
Dedicated to Professor Goverdhan Mehta
Abstract—Commercially available copper(II) tetrafluoroborate hydrate was found to be a highly efficient catalyst for chemoselective
N-tert-butoxycarbonylation of amines with di-tert-butyl dicarbonate under solvent-free conditions and at room temperature. Var-
ious aromatic amines were protected as their N-tert-butyl carbamates in high yields and in short times. No competitive side reactions
such as isocyanate, urea, and N,N-di-t-Boc formation was observed. Chemoselective N-tert-butoxycarbonylation was achieved with
substrates bearing OH and SH groups. Chiral a-amino acid esters afforded the corresponding N-t-Boc derivatives in excellent yields.
ꢀ 2005 Elsevier Ltd. All rights reserved.
The presence of an amino group in various drug mole-
cules and their key intermediates makes protection/
deprotection1 of the amine functionality a necessity dur-
ing their synthesis. Although an easy process for the
protection of amines is acylation,2 liberation of the
parent amino compound from the acylated derivatives
requires harsh reaction conditions that are often detri-
mental to the purity of the product due to side reactions
under strong alkaline/acidic conditions and at high tem-
peratures.3 Thus, it is desirable to have a protecting
group that may be deprotected under mild conditions.
tert-Butyl carbamates are stable in the presence of a
wide range of nucleophiles and under alkaline condi-
tions and are very labile under mild acidic conditions1a
to liberate the parent amine. The resistance of the N-t-
Boc functionality to nucleophilic attack allows synthetic
manipulation of multifunctional substrates bearing the
protected amine group.4 tert-Butyl carbamate formation
is achieved by treatment of an amine with di-tert-butyl
dicarbonate [(Boc)2O] in the presence of 4-dimethyl-
aminopyridine (DMAP)5 or organic/inorganic bases,6
4-dimethylamino-1-tert-butoxycarbonylpyridinium
chloride7/tetrafluoroborate8 in aqueous NaOH, 2-tert-
butyloxycarbonyloxyimino-2-phenylacetonitrile in the
presence of Et3N in H2O–dioxane,9 tert-butyl 2-pyridyl
carbonate in the presence of Et3N in H2O–DMF,10
and tert-butyl 1-chloroalkyl carbonates in the presence
of K2CO3 in H2O–THF.11 However, these methodolo-
gies have various drawbacks such as long reaction times,
use of solvent, requirement to prepare the tert-butoxy-
carbonylation reagents,8–11 etc. The high toxicity of
DMAP12 and the reagents derived from DMAP restrict
its use. Further, the base catalysed reactions often lead
to the formation of isocyanate,5d,13 urea,5d and N,N-
di-Boc derivatives.5d,14
We speculated that these disadvantages could be
avoided by electrophilic activation of (Boc)2O in the
presence of a Lewis acid (Scheme 1). There are a few
examples of Lewis acid catalysed N-tert-butoxycarbonyl-
ation of amines, including yttria–zirconia in MeCN,15
Zn(ClO4)2Æ6H2O in DCM16 and ZrCl4 in MeCN.17
These require the use of solvent and the preparation
of the catalysts,15 are potentially hazardous18,19 and
require anhydrous conditions.20
Keywords: tert-Butyl carbamates; Amines; Di-tert-butyl dicarbonate;
Copper(II) tetrafluoroborate hydrate; Catalyst; Chemoselective; Sol-
vent free.
*
We recently reported that commercially available Cu-
Corresponding author. Tel.: +91 0172 221 4682 686; fax: +91 0172
2214692; e-mail: akchakraborti@niper.ac.in
(BF4)2ÆxH2O is extremely effective for acetylation,2f
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.12.044

1088
S. V. Chankeshwara, A. K. Chakraborti / Tetrahedron Letters 47 (2006) 1087–1091
and benzyl esters of chiral a-amino acids in high yields
X
M
n
and optical purity (entries 31–34) as was evident by
the comparison of the optical rotations.15,26–29 The use
of DMAP leads to racemisation of N-t-Boc protected
amino-acids.30
O
O
NH Ar
2
O
O
O
(Boc) O
X
n
2
M
H
O
O
The advantage of the use of Cu(BF4)2ÆxH2O over the
reported Lewis acid catalysts may be demonstrated by
the following representative examples. Treatment of
aniline with (Boc)2O in the presence of yttria–zirconia
(20% by weight) afforded tert-butyl-N-phenylcarbamate
in 90% yield after 14 h in MeCN.15 A 92% yield could be
obtained after 12 h in DCM on reaction in the presence
of Zn(ClO4)2 (5 mol %).16 In comparison, the use of
Cu(BF4)2ÆxH2O (1 mol %) afforded comparable yields
after 5 min under solvent-free conditions. A more strin-
gent test was N-tert-butoxycarbonylation of 4-nitroani-
line. The ZrCl4 (10 mol %) catalysed reaction carried
out at 80 ꢁC under neat conditions resulted in the forma-
tion of N-t-Boc-4-nitrophenylamine in 3% yield
(GCMS) after 60 min whereas the desired product was
obtained in 50% yield when the reaction was carried
out in the presence of Cu(BF4)2ÆxH2O (1 mol %) under
identical experimental conditions.
MX
n
O
O
O
HN
Ar
O
+
+
Ar
CO
2
OH
O
N
H
Scheme 1. Electrophilic activation of di-tert-butyl dicarbonate during
Lewis acid catalysed tert-butyl carbamate formation from amines.
diacetate formation,21 thia-Michael addition,22 dithio-
lane formation23 and acetal formation.24 Herein
we report an efficient methodology for the protection
of amines as tert-butyl carbamates catalysed by
Cu(BF4)2ÆxH2O under solvent-free conditions and
at room temperature.
Various amines were treated with (Boc)2O in the pres-
ence of Cu(BF4)2ÆxH2O (1 mol %) at ꢀ30–35 ꢁC under
neat conditions. The reactions were monitored by IR
and GCMS. No isocyanate or urea formation was
detected (IR, GCMS). The appearance of a base peak at
m/z 57 along with the molecular ion peak in the GCMS
confirmed the formation of tert-butyl carbamates. No
significant amount of N,N-di-Boc aniline was formed
(GCMS).14 The results are presented in Table 1. The
reactions could also be monitored visually. After addi-
tion of (Boc)2O (1 equiv) to the amine (solid or liquid)
a clear solution was obtained. Effervescence occurred
immediately after the addition of the catalyst to the
reaction mixture, which solidified after the reaction
was complete. Aromatic amines having various substit-
uents such as OMe, Me, Cl, Br, F, OH and SH groups
were converted to their N-t-Boc derivatives efficiently
(entries 1–23). Amino groups attached to aromatic
(entry 29) and non-aromatic (entry 30) heterocycles
afforded quantitative yields. However, for antipyrine
(entry 30), 2 equiv of (Boc)2O was required. In the case
of amino-phenols (entries 13–15) and tyrosine methyl
ester (entry 33), excellent chemoselectivity was observed
and N-t-Boc derivatives were obtained as the sole
products without competitive formation of O-t-Boc
compounds.5d,25 4-Aminothiophenol (entry 16) was
also chemoselectively N-tert-butyloxycarbonylated.
However, with 2-aminothiophenol (entry 17), 2-hydroxy-
benzothiazole was isolated as the sole product (IR,
NMR and MS). The presence of electron-withdrawing
groups such as CN, COMe and NO2 reduced the nucleo-
philicity of the nitrogen atom of the amino group
significantly and these substrates required longer times
and elevated temperatures to afford moderate yields
(entries 24–28).
We have described herein Cu(BF4)2ÆxH2O as a highly
efficient catalyst for the formation of tert-butyl carba-
mates from amines at room temperature. The advanta-
ges include (i) the use of a cheap, easy to handle and
commercially available catalyst, (ii) solvent-free and
room temperature reaction conditions, (iii) short reac-
tion times and (iv) high yields. With the enforcement
of tight legislation for environment protection in chem-
ical transformations, the solvent-free reaction condi-
tions employed in the present methodology constitutes
a greener protocol for the desired transformation.31
Typical procedure for N-tert-butyloxycarbonylation of
amines. To a magnetically stirred mixture of aniline
(0.235 g, 2.5 mmol, 1 equiv) and (Boc)2O (0.545 g,
2.5 mmol,
1 equiv),
Cu(BF4)2ÆxH2O
(6.0 mg,
0.025 mmol, 1 mol %) was added and the mixture was
stirred at 30–35 ꢁC until completion of the reaction
(5 min, TLC, IR, GCMS). The mixture was diluted with
EtOAc (25 mL), washed with water (2 · 10 mL), dried
(Na2SO4) and concentrated under reduced pressure to
afford tert-butyl-N-phenylcarbamate (white solid,
0.485 g, 98%, entry 1, Table 1), mp 132 ꢁC. IR (KBr):
3314, 2985, 1689, 1598, 1531, 1245, 1150 cmÀ1 1H
.
NMR (CDCl3, 300 MHz) d = 1.51 (s, 9H), 6.55 (br s,
1H), 6.99–7.04 (m, 1H), 7.24–7.36 (m, 4H). 13C NMR
(CDCl3, 75 MHz) d = 28.32, 80.45, 118.52, 122.98,
128.93, 138.32, 152.7. MS (EI): m/z 193 (M+). This data
is identical with those of an authentic sample.16
The remaining reactions were carried out following this
general procedure. In most cases, the products obtained
after workup were of sufficient purity (spectral data) and
did not require any further purification. Wherever
required (<90% conversion), purification was achieved
by triturating with EtOAc–hexane or by passing
through a column of silica-gel and eluting with
hexane–EtOAc. On each occasion, the spectral data
The mildness of the present methodology was exempli-
fied by the formation of N-t-Boc derivatives of methyl

S. V. Chankeshwara, A. K. Chakraborti / Tetrahedron Letters 47 (2006) 1087–1091
1089
Table 1. Cu(BF4)2ÆxH2O-catalysed tert-butyl carbamate formation from amines under solvent-free conditionsa
Entry
Amine
Time (min)
Yield (%)b,c
NH2
R1
R2
R4
R3
1
2
3
4
5
6
7
8
R1 = R2 = R3 = R4 = H
5
5
98
92
100
90
100
100
100
100
100
98
98
96
86
90
100
96
100d
100
90
100
100
100
80
R1 = R2 = R4 = H; R3 = OMe
R1 = R3 = R4 = H; R2 = OMe
R1 = OMe; R2 = R3 = R4 = H
R1 = R4 = H; R2 = R3 = OMe
R1 = R3 = OMe; R2 = R4 = H
R1 = R2 = R4 = H; R3 = Me
R1 = R3 = R4 = H; R2 = Me
R1 = Me; R2 = R3 = R4 = H
R1 = R4 = Me; R2 = R3 = H
R1 = R3 = Me; R2 = R4 = H
R1 = R3 = R4 = Me; R2 = H
R1 = R2 = R4 = H; R3 = OH
R1 = R3 = R4 = H; R2 = OH
R1 = OH; R2 = R3 = R4 = H
R1 = R2 = R4 = H; R3 = SH
R1 = SH; R2 = R3 = R4 = H
R1 = R2 = R4 = H; R3 = F
R1 = F; R2 = R3 = R4 = H
R1 = R2 = R4 = H; R3 = Br
R1 = Me; R2 = R4 = H; R3 = Br
R1 = R2 = R4 = H; R3 = Cl
R1 = R3 = R4 = H; R2 = Cl
R1 = R2 = R4 = H; R3 = CN
R1 = R2 = R4 = H; R3 = COMe
R1 = R2 = R4 = H; R3 = NO2
R1 = R3 = R4 = H; R2 = NO2
R1 = NO2; R2 = R3 = R4 = H
10
30
30
45
10
15
30
30
30
30
60
60
60
30
60
10
15
30
30
30
60
6 h
24 h
60
60
60
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
50
60
50e
64e
33e
29
30
100
N
NH2
NH2
Me
Me
N
100f
O
30
N
60
CO2Me
NH2
31
10
95
CO2Me
NH2
32
33
10
10
99
90
CO2Me
NH2
HO
CO2Bn
NH2
34
10
96
BnO
a The amine (2.5 mmol) was treated with (Boc)2O (1 equiv; except for entry 30) in the presence of Cu(BF4)2ÆxH2O (1 mol %) at room temperature
(ꢀ30–35 ꢁC; except for entries 26–28) under neat conditions.
b Isolated yield of the corresponding N-t-Boc derivative.
c The products were characterised by IR, NMR and MS.
d 2-Hydroxybenzothiazole was isolated as the sole product (IR, NMR and MS).
e The reaction was carried out at 80 ꢁC.
f The reaction was carried out with 2 equiv of (Boc)2O.

1090
S. V. Chankeshwara, A. K. Chakraborti / Tetrahedron Letters 47 (2006) 1087–1091
(IR, 1H NMR and MS) of known compounds were
found to be identical with those reported in the litera-
ture. Physical data for previously unknown compounds
are provided below.
References and notes
1. (a) Greene, T. W.; Wuts, P. G. M. Protecting Group in
Organic Synthesis, 3rd ed.; John Wiley and Sons: New
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Chem. Commun. 2003, 44, 1896–1897; (c) Chakraborti, A.
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(d) Chakraborti, A. K.; Sharma, L.; Gulhane, R.; Shivani
Tetrahedron 2003, 59, 7661–7668; (e) Chakraborti, A. K.;
Gulhane, R.; Shivani Synlett 2003, 1805–1808; (f) Chakra-
borti, A. K.; Gulhane, R.; Shivani Synthesis 2004, 111–
115; (g) Chakraborti, A. K.; Gulhane, R. Synlett 2004,
627–630.
(2,4-Dimethylphenyl)carbamic acid tert-butyl ester
(Table 1, entry 11): Mp: 84–85 ꢁC IR (KBr): 3273,
3255, 2977, 2927, 1699, 1513, 1374, 1361, 1171 cmÀ1
.
1H NMR (CDCl3, 300 MHz) d = 1.50 (s, 9H), 2.17 (s,
3H), 2.24 (s, 3H), 6.22 (br s, 1H), 6.95 (d, 2H,
J = 11.4 Hz), 7.56 (d, 1H, J = 6.9 Hz). MS (EI): m/z
221 (M+). Anal. Calcd (%) for C, 70.56; H, 8.65; N,
6.33. Found C, 70.62; H, 8.67; N, 6.35. (4-Fluorophe-
nyl)carbamic acid tert-butyl ester (Table 1, entry 18):
Mp: 124–125 ꢁC IR (KBr): 3300, 2976, 1694, 1529,
1407, 1306, 1248, 1153 cmÀ1 1H NMR (300 MHz,
.
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6385–6402.
CDCl3): d = 1.50 (s, 9H), 6.52 (br s, 1H), 6.94–6.99
(m, 2H), 7.28–7.32 (m, 2H). MS (EI): m/z 211 (M+).
Anal. Calcd (%) for C, 62.55; H, 6.68; F, 8.99; N,
6.63; O, 15.15. Found C, 62.45; H, 6.68; N, 6.65. (3-
Chlorophenyl)carbamic acid tert-butyl ester (Table 1,
entry 23): Mp: 69–70 ꢁC IR (KBr): 3368, 3306, 2987,
5. (a) Grehn, L.; Ragnarsson, U. Angew. Chem., Int. Ed.
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2938, 1695, 1592, 1519, 1400, 1268, 1237, 1178 cmÀ1
.
1H NMR (300 MHz, CDCl3): d = 1.51 (s, 9H), 6.53
(br s, 1H), 7.0 (d, 1H, J = 6.75 Hz), 7.13–7.21 (m, 2H),
7.52 (s, 1H). 13C NMR (CDCl3, 75 MHz) d = 28.29,
81.02, 116.37, 118.48, 123.02, 129.91, 134.72, 139.54,
152.43. MS (EI): m/z 227 (M+). Anal. Calcd (%) for
C, 58.03; H, 6.20; Cl, 15.57; N, 6.15. Found C, 57.84;
H, 6.18; Cl, 15.59; N, 6.18. (4-Cyanophenyl)carbamic
acid tert-butyl ester (Table 1, entry 24): Mp: 113–
114 ꢁC IR (KBr): 3369, 2995, 2947, 2227, 1694, 1613,
´
7. Guibe-Jampel, E.; Wakselman, M. J. Chem. Soc. Chem.
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1586, 1504, 1409, 1283, 1239, 1151, 1058, 834 cmÀ1. H
´
NMR (300 MHz, CDCl3): d = 1.52 (s, 9H), 6.74 (br s,
1H), 7.48 (d, 2H, J = 8.70 Hz), 7.57 (d, 2H,
J = 8.76 Hz). 13C NMR (CDCl3, 75 MHz) d = 28.21,
81.68, 105.77, 118.08, 119.02, 133.28, 142.58, 154.26.
MS (EI): m/z 218 (M+). Anal. Calcd (%) for C, 66.04;
H, 6.47; N, 12.84. Found C, 66.24; H, 6.43; N, 12.89.
(4-Acetylphenyl)carbamic acid tert-butyl ester (Table 1,
entry 25): Mp: 137–138 ꢁC IR (KBr): 3246, 2975, 1719,
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1663, 1593, 1535, 1320, 1238, 1153 cmÀ1 1H NMR
.
(300 MHz, CDCl3): d = 1.51 (s, 9H), 2.56 (s, 3H), 7.11
(br s, 1H), 7.48 (d, 2H, J = 8.61 Hz), 7.90 (d, 2H,
J = 8.03 Hz). 13C NMR (CDCl3, 75 MHz) d = 26.28,
28.22, 81.15, 117.43, 129.78, 131.69, 143.10, 152.26,
197.01. MS (EI): m/z 235 (M+). Anal. Calcd (%) for
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N, 6.05. (1,5-Dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-
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1626, 1556, 1526, 1363, 1252, 1180, 1058 cmÀ1 1H
.
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3H), 3.03 (s, 3H), 5.95 (br s, 1H), 7.28–7.47 (m, 5H).
MS (EI): 303 m/z (M+). Anal. Calcd (%) for C, 63.35;
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Supplementary data
19. The preparation of yttria–zirconia involves use of sulfuric
acid at 500 ꢁC.
20. ZrCl4 is highly moisture sensitive, decomposes on storing
and liberates corrosive HCl fumes.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.tetlet.2005.12.044.

S. V. Chankeshwara, A. K. Chakraborti / Tetrahedron Letters 47 (2006) 1087–1091
1091
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