B. Das et al. / Tetrahedron Letters 47 (2006) 7551–7556
7555
H
O
O
Cat.
O+
R NH2
(Boc) O
2
R NH2
R NH2
H
O
O
O
O+
H
O
O
O
CO2
+
+
O
O
O
OH
O
NHR
NHR
Scheme 3.
groups. Silica gel is less expensive than several organic
polymers and readily available and it possesses the
capacity to anchor an organic chain easily. The experi-
mental procedure is simple and the structures of all
the products were determined from their spectroscopic
Lutz, C.; Lutz, V.; Knochel, P. Tetrahedron 1998, 54,
385–6402.
6
4
. (a) Porta, F.; Cenini, S.; Pizzotti, M.; Crotti, C. Gazz.
Chim. Ital. 1985, 115, 275–277; (b) Pandey, P. K.; Dagade,
S. P.; Upadhyay, R. K.; Dongare, M. K.; Kumar, P.
ARKIVOK 2000, vii, 28–33; (c) Bartoli, G.; Bosco, M.;
Locatelli, M.; Marcantoni, E.; Massaccesi, M.; Melchi-
orre, P.; Sambri, L. Synlett 2004, 1794–1798; (d) Heydari,
A.; Hosseini, S. E. Adv. Synth. Catal. 2005, 347, 1929–
1932.
1
13
(
IR, H, C NMR, and MS) and elemental analysis
9
data. The catalyst could be consecutively recycled three
times without the loss of its activity. It can be mentioned
here that the acidic ion-exchange resin, Amberlyst-15
was used earlier for the deprotection of Boc-amines.10
We attempted to utilize the catalyst for the Boc protec-
tion of amines but the yields were not satisfactory.
5. (a) Das, B.; Ramu, R.; Reddy, M. R.; Mahender, G.
Synthesis 2005, 250–254; (b) Das, B.; Venkateswarlu, K.;
Mahender, G.; Mahender, I. Tetrahedron Lett. 2005, 46,
3
041–3044; (c) Das, B.; Ramu, R.; Ravikanth, B.; Reddy,
K. R. Tetrahedron Lett. 2006, 47, 779–782; (d) Das, B.;
Thirupathi, P.; Mahender, I.; Reddy, V. S.; Rao, Y. K. J.
Mol. Catal. A: Chem. 2006, 247, 233–239.
. Hajela, S. P.; Johnson, A. R.; Xu, J.; Sunderland, C. J.;
Cohen, S. M.; Caulder, D. L.; Raymond, K. N. Inorg.
Chem. 2001, 40, 3208–3216.
7. (a) Shylesh, A.; Sharma, S.; Mirajkar, S. P.; Sing, A. P. J.
Mol. Catal. A: Chem. 2004, 212, 219–228; (b) Karimi, B.;
Khalkhali, M. J. Mol. Catal. A: Chem. 2005, 232, 113–117.
. (a) Corma, A. Chem. Rev. 1995, 95, 559–614; (b) Corma,
A.; Garc ´ı a, H. Catal. Today 1997, 38, 257–308.
The mechanism of the conversion is possibly similar to
that which operates for the Lewis-acid induced Boc pro-
tection of amines. The sulfonic-acid-functionalized sil-
ica catalyzes the reaction by the electrophilic activation
1
1
6
of (Boc) O. A plausible mechanism is shown in Scheme
2
3
.
In conclusion, we have described how sulfonic-acid-
functionalized silica is a remarkably efficient heteroge-
neous catalyst used here for the first time for the mono
8
9
. General experimental procedure: To a mixture of an
Boc protection of a wide range of amines using (Boc) O.
2
amine (1.1 mmol) and (Boc)
5 mL) sulfonic-acid-functionalized silica [prepared
from silica (10 g) and 3-marcaptopropyltrimethoxysilane
5 mmol)] (20 mg) were added. The mixture was stirred at
2 2 2
O (1.0 mmol) in CH Cl
Aromatic amines containing electron-withdrawing
groups also afforded the desired derivatives in good
yields. Chiral substrates were resistant to racemization
and labile functionalities such as esters were compatible
in the conversion. The protocol is a highly chemoselec-
tive offering potential in different applications. The
method also has several other advantages such as simple
experimental procedures, mild reaction conditions,
excellent yields of mono Boc protected amines and reus-
ability of the catalyst.
7
b
(
(
room temperature and the reaction was monitored by
TLC. After completion the mixture was filtered. The
catalyst was washed with CHCl3 (2 · 5 mL), EtOH
(2 · 5 mL) and diethyl ether (2 · 5 mL) and subsequently
dried for recyclization purpose. The filtrate was concen-
trated and the residue was subjected to column chroma-
tography over silica gel using hexane–EtOAc as eluent to
obtain a pure Boc protected amine.
The recovered catalyst was utilized consecutively three
times for the Boc protection of amine group of 1-
phenylethylamine (Table 1, entry b) following the above
procedure for 5 min in each case to afford the correspond-
ing N-Boc derivative with the yields of 97%, 96%, and
Acknowledgements
The authors thank the CSIR and the UGC, New Delhi,
for financial assistance.
9
4%.
1
13
The spectral (IR, H and C NMR and MS) and
elemental analysis data of some representative products
are given below.
References and notes
À1
1
Compound 2f: IR (neat): 3414, 1690, 1364, 1306 cm ; H
1
. Greene, T. W.; Wuts, P. G. M. In Protective Group in
Organic Synthesis, 2nd ed.; Wiley: New York, 1999; pp
NMR (200 MHz, CDCl ): d 4.91 (1H, br s), 3.56 (1H, br
s), 3.03 (1H, dd, J = 12.0, 4.0 Hz), 2.88–2.74 (2H, m),
3
5
03–550, and references cited therein.
2.72–2.60 (2H, m), 2.52 (1H, dd, J = 12.0, 4.0 Hz), 1.89–
1.62 (5H, m), 1.59–1.41 (11H, m); C NMR (50 MHz,
1
3
2
. Pope, B. M.; Yamamoto, X.; Tarbell, D. S. Org. Synth.
Coll. 1988, VI, 418.
. (a) Bailey, S. W.; Chandrasekaran, R. Y.; Ayling, J. E. J.
Org. Chem. 1992, 57, 4470–4477; (b) Kn o¨ lker, H. J.;
Braxmeier, T. Tetrahedron Lett. 1996, 37, 5861–5864; (c)
CDCl ): d 155.4, 79.2, 51.7, 46.8, 46.0, 31.1, 28.4, 23.9;
3
+
3
FABMS: m/z 229 [M+H] ; Anal. Calcd for C H N O :
1
2
24
2
2
C, 63.15; H, 10.52; N, 12.28. Found: C, 62.98; H, 10.45; N,
12.20.