t-Butyl N,N-dibromocarbamate (BBC) - New reagent for aminobromination of terminal alkenes

Article abstract of DOI:10.1016/S0040-4039(01)00662-1

t-Butyl N,N-dibromocarbamate, easily obtained by bromination of t-butyl carbamate in aqueous potassium carbonate, reacts smoothly with a variety of terminal alkenes to afford the corresponding β-bromo-N-Boc-amines upon reduction with aqueous sodium sulphi

Full text of DOI:10.1016/S0040-4039(01)00662-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 4539–4540
t-Butyl N,N-dibromocarbamate (BBC)—new reagent for
aminobromination of terminal alkenes
Anna Klepacz and Andrzej Zwierzak*
:
Institute of Organic Chemistry, Technical University (Politechnika), Zeromskiego 116, 90-924 Lodz 40, Poland
Received 1 March 2001; accepted 19 April 2001
Abstract—t-Butyl N,N-dibromocarbamate, easily obtained by bromination of t-butyl carbamate in aqueous potassium carbonate,
reacts smoothly with a variety of terminal alkenes to afford the corresponding b-bromo-N-Boc-amines upon reduction with
aqueous sodium sulphite. Immediate deprotection of N-Boc-amines with gaseous HCl yields b-bromoamine hydrochlorides in
good yields. © 2001 Elsevier Science Ltd. All rights reserved.
As part of our studies aimed at developing a new,
phosphorus free reagent for free-radical aminobromina-
tion of alkenes, we performed the synthesis of t-butyl
N,N-dibromocarbamate (BBC) 2 using t-butyl carba-
mate 1 as a starting material. The bromination of crude
1 using elemental bromine was carried out in aqueous
potassium carbonate at room temperature to give 2 in
excellent yield (Scheme 1).5
Direct functionalization of a carbonꢀcarbon double
bond by free-radical or ionic addition of various pseu-
dohalogens has been the subject of numerous and
intensive studies. In a series of our earlier papers1–4 we
have shown that diethyl N,N-dichlorophosphoramidate
(DCPA)1,2 and its N,N-dibromoanalogue (DBPA)3,4
behave as typical pseudohalogens towards alkenes. The
latter compound in particular appeared to be a very
promising and versatile reagent capable of reacting with
alkenes either via a radical-chain mechanism3 or by an
ionic addition pathway,4 depending upon the reaction
conditions. The DBPA–alkene adducts, upon reduction
with sodium bisulphite, could subsequently be dephos-
phorylated to the corresponding b-bromoamine
hydrochlorides, useful precursors of aziridines.
Scheme 1.
Scheme 2.
Keywords: t-butyl carbamate; free-radical addition; oxazolidinones; Boc-amine deprotection.
* Corresponding author. Fax: (48-42)636-55-30.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00662-1

4540
A. Klepacz, A. Zwierzak / Tetrahedron Letters 42 (2001) 4539–4540
Scheme 3.
Table 1. Preparation of b-bromoamine hydrochloridesa
Entry
R
R1
Yield (%)
Mpb (°C)
Lit.3 mp (°C)
1
2
3
4
5
6
7
H
Ph
Me
Et
Bu
Bn
Pr
Neo-C5H11
83
78
80
54.5
56
165–167
157–159
149–150
183–185
180–182
175–177
110–112
158–159
151–153
133–135
Me
Me
H
H
H
174–176
c
c
c
59
55
H
a All new compounds were fully characterized by MS, IR, and 1H NMR spectra.
b Crystallized from ethanol–ether. All compounds decomposed on melting.
c Not reported previously.
Crude BBC 2 was found to react smoothly and regiose-
lectively with terminal alkenes in an anti-
reported previously.3 The use of 2 in other similar
synthetic applications is currently under investigation.
3
Markovnikov fashion (which is evident from NMR
spectroscopy) yielding the corresponding N-bromoad-
ducts 4 which could subsequently be reduced by means
of 12% aqueous sodium sulphite at 5–10°C to give
b-bromo-N-Boc-amines 5 (Scheme 2).
References
1. Zwierzak, A.; Koziara, A. Tetrahedron 1970, 26, 3527–3537.
2. Koziara, A.; Zwierzak, A. Tetrahedron 1976, 32, 1649–1654.
3. Zawadzki, S.; Zwierzak, A. Tetrahedron 1981, 37, 2675–
2681.
4. Osowska-Pacewicka, K.; Zwierzak, A. Tetrahedron 1985,
41, 4117–4725.
Careful examination of the NMR spectrum of the
crude BBC–styrene adduct revealed the presence of ca.
7% of 5-phenyl-oxazolidin-2-one 8 contaminating the
expected b-bromo-N-Boc-amine 7. The latter was
slowly transformed into 8 on standing in solution even
at room temperature. The presence of oxazolidinone 8
in the BBC–styrene adduct can be explained as a result
of cyclization of 7 via an intramolecular SN2 displace-
ment. The analogous conversion of b-mesyloxy-N-Boc-
amines into oxazolidinones has been reported recently
(Scheme 3).7
5. Bromine (35.16 g, 0.22 mol) was added dropwise with
stirring for 40 min to a solution of crude t-butyl carbamate
16 (prepared in CH2Cl2, mp 90–93°C, yield ca. 100%, purity
ꢀ90%; 12.9 g, 0.11 mol) and K2CO3 (15.2 g, 0.11 mol) in
water (200 ml) at ambient temperature. The resulting
mixture was stirred for 2 h, CH2Cl2 (100 ml) was then added,
the organic layer separated, and the aqueous phase was
extracted with CH2Cl2 (3×30 ml). Combined extracts were
washed with water (30 ml), dried, and evaporated to give
24.6 g (90%) of 2 as an orange solid. Crude 2 was
contaminated with ca. 9% of t-butyl N-bromocarbamate.
Analytically pure sample 2 (prepared from pure 1, mp
107–108°C and washed with cold pentane) had mp 93–95°C;
1H NMR (250 MHz, CDCl3): l (ppm) 1.50 (s, 9H); 13C
NMR (63 MHz, CDCl3): l (ppm) 27.3, 86.2, 156.2; IR
(KBr): 2992, 1696, 1368, 1280, 1264, 1248, 1144, 872, 744
cm−1. MS (CI): 274 (M+1, 41%), 276 (M+3, 96%), 278 (M+5,
40%).
The formation of variable amounts of oxazolidinones
was also observed on attempted addition of BBC 2 to
other alkenes. In order to minimize this undesired side
reaction, the b-bromo-N-Boc-amines 5 were not iso-
lated but immediately deprotected just after addition by
means of gaseous HCl in CH2Cl2 to give b-bromoamine
hydrochlorides 6 (Table 1). All BBC additions to termi-
nal alkenes exhibit characteristic features which are
indicative of spontaneously initiated free-radical chain
reactions.
6. Loev, B.; Kormendy, M. F. J. Org. Chem. 1963, 28,
3421–3426.
7. Benedetti, F.; Norbedo, S. Tetrahedron Lett. 2000, 41,
10071–10074.
In summary, we have shown that terminal alkenes can
easily be converted into b-bromoamine hydrochlorides
6 utilizing the new aminobrominating reagent 2. The
yields of 6 are higher and their purity is better than
.