Organocatalytic diastereoselective dibromination of alkenes

Article abstract of DOI:10.1016/j.tetlet.2010.03.043

A highly diastereoselective pyrrolidine-promoted dibromination of alkenes by combination of NBS and succinimide is presented. The pyrrolidine-mediated dibromination of alkenes is higly anti-selective and gives the corresponding products in moderate to high yields and up to >25:1 dr.

Full text of DOI:10.1016/j.tetlet.2010.03.043

Tetrahedron Letters 51 (2010) 2708–2712
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Tetrahedron Letters
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Organocatalytic diastereoselective dibromination of alkenes
Mingzhao Zhu a, Shuangzheng Lin a, Gui-Ling Zhao a,c, , Junliang Sun b,c, Armando Córdova a,c,d,
*
*
a Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden
b Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden
c Berzelii Center EXSELENT on Porous Materials, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden
d Department of Natural Sciences, Engineering and Mathematics, SE-851 70 Sundsvall, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly diastereoselective pyrrolidine-promoted dibromination of alkenes by combination of NBS and
succinimide is presented. The pyrrolidine-mediated dibromination of alkenes is higly anti-selective and
gives the corresponding products in moderate to high yields and up to >25:1 dr.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 13 January 2010
Revised 27 February 2010
Accepted 12 March 2010
Available online 17 March 2010
Dihalo derivatives are important compounds in organic synthesis,
especially in pharmaceutical chemistry.1 In the case of dibromination
of organic substrates, the electrophilic addition of molecular Br2 to
unsaturated carbon–carbon bonds is still the best choice. However,
bromine is hazardous, difficult to handle, and a volatile liquid,2 which
limits its applications. There have been some recent developments
concerning the use of other dibrominating reagents.3,4 For example,
Salazar et al. developed pentylpyridinium tribromide as the source
of Br2 for mono- or dibrominations.3a Recently, Shi and co-workers4a
reported that the combinationof N-bromosuccinimide (NBS) and LiBr
was an efficient method for the dibromination of unsaturated car-
bon–carbon bonds. Moreover, organic amides have been shown to
be potent organocatalysts for the bromoacetoxylation of alkenes
using NBS as the electrophilic bromine source.4c
Thus, we decided to investigate the reaction between a,b-unsat-
urated aldehydes and NBS in the presence of a secondary amine
catalyst and succinimide. Herein, we present a highly diastereose-
lective method for the organocatalytic dibromination of alkenes
using NBS as the source of bromine.
In an initial experiment, we found that pyrrolidine catalyzed the
dibromination of trans-cinnamaldehyde (1a) using pure, freshly
recrystallized NBS (2.2 equiv) as the dibrominating reagent in
CHCl3 (Table 1, entry 1). To our delight, the corresponding product
3a was formed in 17% conversion with excellent anti-diastereose-
lectivity (>25:1 dr). As in our previously reported a-aminosulfeny-
lation reaction, the addition of succinimide (2) improved the
conversion (entry 2). The reaction did not work in MeOH (entry
3) or in THF. A significant improvement in conversion and reaction
time was achieved by increasing the temperature of the reaction
mixture. For example, increasing the temperature to 70 °C led to
the formation of 3a in more than 90% conversion with >25:1 dr
(anti/syn) within 2 h (entry 6). However, a small amount of elimi-
nation product 4a was also formed.
Moreover, decreasing the amount of succinimide (2) to
0.2 equiv did not affect the conversion (95%) and product 3a was
obtained with >25:1 dr (anti/syn) (entry 8). Replacing pyrrolidine
with Et3N as the organic mediator significantly decreased the con-
version (entry 9) and only a trace amount of 3a was formed when
no organic base was added (entry 10). We also compared our
method to that reported by Shi and co-workers.4 Thus, the addition
of LiBr to the reaction mixture was investigated (entries 11 and
12). We found that no reaction occurred when pyrrolidine was
not added (entry 11) and 80% conversion was observed in the pres-
ence of pyrrolidine and LiBr. However, the diastereoselectivity of
the reaction decreased to a moderate 4:1.
In the research field of organocatalysis, amine-catalyzed dom-
ino, cascade and tandem reactions have been developed.5 In this
context, we recently found that chiral pyrrolidines catalyze enan-
tioselective aminosulfenylation of
a,b-unsaturated aldehydes
using N-(benzylthio)succinimides as both a nucleophilic and an
electrophilic reactant (Eq. 1).6 The reaction proceeds very well in
the presence of a secondary amine catalyst together with a cata-
lytic amount of succinimide providing the corresponding amino-
sulfenylation products in high yields and enantioselectivities.
O
O
O
O
N
H
O
N
*
N SBn
*
+
R
H
R
H
succinimide, CHCl3
SBn
O
1-(benzylthio)pyrrolidine-2,5-dione
ð1Þ
Encouraged by these results, we decided to investigate the
scope and limitations of the dibromination of
aldehydes 1a–f using 2.2 equiv of NBS, 0.2 equiv of succinimide
a,b-unsaturated
* Corresponding authors. Tel.: +46 8 162479; fax: +46 8 154908 (A.C.).
E-mail addresses: zhaogl@organ.su.se (G.-L. Zhao), acordova@organ.su.se,
(2), and 0.2 equiv of pyrrolidine (Table 2).
armando.cordova@miun.se (A. Córdova).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.043

M. Zhu et al. / Tetrahedron Letters 51 (2010) 2708–2712
2709
Table 1
Screening of the reaction conditions for the dibromination of trans-cinnamaldehyde (1a)a
O
NH
Br
O
O
O
2
NBS,
O
H
H
+
H
pyrrolidine
(20 mol%)
Br
4a
Br
3a
1a
Entry
NBS (equiv)
2 (equiv)
Concd (M)
Solvent
Time (h)
Temp (°C)
Conv. of 3ab (%)
dr of 3ab (anti/syn)
3a:4ab
1
2
3
4
5
6
7
8
2.2
2.2
2.2
2
0
0.07
0.07
0.07
0.5
0.5
0.5
5
0.5
0.5
0.5
0.1
0.1
CHCl3
CHCl3
MeOH
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
THF
15
15
15
24
24
2
4
2
1
1
rt
rt
rt
rt
rt
70
rt
60
60
60
rt
17
57
0
>25:1
>25:1
—
>25:1
>25:1
>25:1
>25:1
>25:1
>25:1
—
>25:1
>25:1
—
70:30
82:18
83:17
85:15
79:21
>25:1
—
1.5
1.5
0.5
0.5
0.5
0.5
0.2
0.5
0.5
1.5
1.5
58
71
>90
74
95
29
<5
0
4
2.2
2.2
2.2
2.2
2.2
2
9c
10d
11e
12f
15
15
—
4:1
—
>25:1
2
THF
rt
80
a
Experimental conditions: To a stirred solution of trans-cinnamaldehyde (0.5 mmol) in 1–7 mL of solvent were added successively NBS (recrystallized from boiling water),
succinimide, and pyrrolidine. The reaction vial was sealed and the solution was stirred under the conditions displayed in the Table.
b
Determined by 1H NMR analyses of the crude reaction mixture.
20 mol % of Et3N was used instead of pyrrolidine.
Reaction run without pyrrolidine.
2 equiv of LiBr added and no pyrrolidine.
c
d
e
f
2 equiv of LiBr was added.
Table 2
Dibromination of
a
,b-unsaturated aldehydes 1a–1fa
O
NH
2
O
Br
O
Br
O
NBS (2.2 equiv), (20 mol%)
NaBH4, MeOH
0 oC
R
H
R
OH
R
H
CHCl3, 60 oC, 2 h
pyrrolidine,
(20 mol%)
Br
Br
1
5
3
Entry
1d
R
Product
Yieldb (%)
58
drc (anti/syn)
1a
1b
1c
5a
>25:1
2e
5b
30
>25:1
Br
3f
5c
35
>25:1
4
5
6
n-Bu
Me
Et
1d
1e
1f
5d
5e
5f
62
55
53
>25:1
>25:1
>25:1
a
Experimental conditions: To a stirred solution of aldehyde (0.5 mmol) in CHCl3 (1 mL) were added successively NBS (1.1 mmol, recrystallized from boiling water),
succinimide (0.1 mmol), and pyrrolidine (0.1 mmol). The reaction vial was sealed and the solution was heated at 60 °C for 2 h.
b
Isolated yield of the corresponding alcohol 5 after in situ reduction of product 3.
c
Anti/syn ratio determined by 1H NMR analysis.
d
8% of reduced elimination product 4a was isolated.
11% of reduced elimination product 4b was isolated.
16% of reduced elimination product 4c was isolated.
e
f
The dibromination of
a
,b-unsaturated aldehydes 1a–f pro-
tuent, a small amount of the corresponding elimination product 4
was also formed (entries 1–3). In the case of enals 1 with aliphatic
substituents, only the corresponding dibrominated alcohols 5d–f
were formed (entries 4–6).
With these results in hand, we next investigated whether we
could catalyze an enantioselective version of this reaction using
chiral pyrrolidine derivatives as catalysts for the dibromination
ceeded with excellent diastereoselectivity and the corresponding
alcohols 5a–f were obtained in moderate to good yields after
in situ reduction with NaBH4.7 For example, the dibromination of
1a and subsequent reduction gave the corresponding product 5a
in 58% yield with >25:1 dr (anti/syn) (entry 1). In the case of the
dibromination of a,b-unsaturated aldehydes 1 with an aryl substi-

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M. Zhu et al. / Tetrahedron Letters 51 (2010) 2708–2712
Table 3
Dibromination of simple alkenes 1i–na
R3
R1
R2
R3
Br
R1
R2
NBS (2.2 eq.), 2 (20 mol%)
H
Br
3
CHCl3, 60 oC
pyrrolidine
(20 mol%)
1
Entry
1
Substrate
Product
Time (h)
1.5
Yieldb (%)
62
drc (anti/syn)
Br
Ph
1j
—
Br
3j
Ph
Br
2d
3
4
2
47
—
Br
1i
1k
3i
Br
Br
Ph
Ph
90e
9:1
3k
3l
Ph
Ph
Br
Ph
1l
Ph
4
3
53
>10:1
Br
Br
Br
Br
1m
Ph
Ph
5
6
3
6
75f
72
—
Ph
3m
3m'
3m:3m'= 5:6
Br
>10:1
1n
3n
Br
a
Experimental conditions: To a stirred solution of alkene 1 (0.5 mmol) in CHCl3 (1 mL) were successively added NBS (1.1 mmol, recrystallized from boiling water),
succinimide (0.1 mmol), and pyrrolidine (0.1 mmol). The reaction vial was sealed and the solution was heated at 60 °C for the time displayed in the Table.
b
Isolated yield of pure anti-isomer 3.
C
Anti/syn ratio determined by 1H NMR analysis.
d
5 equiv of NBS was used.
e
Product 3k can decompose on silica gel, hence the reaction was purified by extraction with pentane.
Combined yield of compounds 3m and 3m0.
f
of cinnamaldehyde (1a) or crotonaldehyde (1e). However, the cor-
responding products were formed in similar yields to those ob-
served when pyrrolidine was used and with only <5% ee.
Dibromination of cinnamaldehyde (1a) in the presence of (S)-2-
[diphenyl(trimethylsilyloxy)methyl]pyrrolidine (20 mol %) gave
the corresponding product 5a, after in situ reduction, in 45% yield
with >25:1 dr and almost 0% ee. Based on these results, we suggest
that the pyrrolidine derivatives were not able to control the stere-
oselectivity by changing the iminium activation of the enals. We
also found that the dibromination process was highly regioselec-
tive. As shown in the dibromintaion of citral (1g) (trans/cis = 1:1),
only selective bromination of the electron-rich double bond was
Figure 1. ORTEP picture of 2,3-dibromo-3-phenylpropan-1-ol 5a.
O
HBr
+
NH
N
Br
(possibly derived
from decomposition of NBS
2
O
1i
2
and or pyrrolidine)
Br
O
Br
N
Br
N
H
3i
O
Scheme 1. Possible pyrrolidine activation.

M. Zhu et al. / Tetrahedron Letters 51 (2010) 2708–2712
2711
achieved (Eq. 2a). This was also the case for the reaction with enal
1h where only the terminal olefin was dibrominated (Eq. 2b).
Moreover, the dibromination of a mixture of olefinic substrates
1a and 1i gave only the product 3i derived from alkene 1i (Eq. 3).
as the Br source. The corresponding dibromo products were iso-
lated in moderate to high yields and with up to >25:1 dr (anti/
syn). It is noteworthy that the use of NBS together with an organic
catalyst makes it possible to avoid the use of molecular Br2, which
Br
NBS, 2,
pyrrolidine
CHO
CHO
3 h, CHCl3
ð2aÞ
Br
1g
3g
1g:3g = 5:7
Br
NBS, 2,
pyrrolidine
CHO
CHO
ð2bÞ
3 h, CHCl3
Br
1h
3h
1h:3h = 1:4
O
O
Br
Br
H
NBS, 2,
pyrrolidine, CHCl3
H
+
1a
+
Br
Br
ð3Þ
3i
3a
1i
Before reaction: 1a: 1i = 1: 2
After 1 h, 1a: 1i: 3a: 3i = 1: 1.8: 0: 1.4
After 2 h, 1a: 1i: 3a: 3i = 1: 1.8: 0: 1.5
With this information in hand, we realized that the scope of this
organocatalytic reaction could be broadened to other types of al-
kenes. Thus, we investigated the dibromination of other alkenes
1 using our optimized reaction conditions (Table 3).
makes the reaction safer and more environmentally friendly. Fur-
ther elaboration of this transformation and a study of its mechanis-
tic aspects are ongoing in our laboratory.
Our metal-free system had a broad substrate scope and both
acyclic and cyclic alkenes gave the corresponding dibromination
products 3i–3n in moderate to high yields.8 For example, the
dibrominations of terminal alkenes 1j and 1i gave the corre-
sponding dibromo compounds 3j and 3i in 62% and 47% yields,
respectively (entries 1 and 2). The dibromination of 1,2-disubsti-
tuted alkenes 1k and 1l gave the corresponding products 3k and
3l in 90% and 53% yield and with high diastereoselectivity,
respectively (entries 3 and 4). In the case of alkene 1m, the
monobrominated product 3m0 was also formed together with
3m in a 6:5 ratio (entry 5). Moreover, the dibromination of cyl-
cohexene 1n gave the corresponding anti-product 3n in 72%
yield and >10:1 dr (entry 6).
Acknowledgements
We gratefully acknowledge the Swedish National Research
Council and Swedish Governmental Agency for Innovation Systems
(VINNOVA) for financial support. The Berzelii Center EXSELENT is
financially supported by VR and VINNOVA.
References and notes
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The relative stereochemistry of compounds 3 was anti as estab-
lished by X-ray analysis of alcohol 5a9 (Fig. 1). The mechanism of this
dibromination process is still under investigation. However, molec-
ular Br2 may be the brominating agent4a,10 and can be formed under
the present reaction conditions. Moreover, pyrrolidine can activate
bromination reagents11 such as NBS by making them more electro-
philic. Therefore, another possibility is that pyrrolidine activates
NBS via the catalytic cycle shown in Scheme 1. As a comparison,
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acetoxylation of alkenes.4c
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Ramachary, D. B.; Chowdari, N. S.; Barbas, C. F., III Angew. Chem., Int. Ed. 2003,
In summary, we have reported a convenient and highly diaste-
reoselective organocatalytic method for the dibromination of al-
kenes using pyrrolidine and its derivatives as catalysts and NBS

2712
M. Zhu et al. / Tetrahedron Letters 51 (2010) 2708–2712
42, 4233; (h) Marigo, M.; Franzén, J.; Poulsen, T. B.; Zhuang, W.; Jørgensen, K. A.
5.31 (d, J = 11.0 Hz, 1H), 4.73 (ddd, J = 11.1 Hz, J = 4.3 Hz, J = 2.6 Hz, 1H), 4.36
(ddd, J = 12.7 Hz, J = 7.8 Hz, J = 4.3 Hz, 1H), 4.28 (ddd, J = 12.7 Hz, J = 6.4 Hz,
J = 2.6 Hz, 1H), 2.26–2.30 (m, 1H). 13C NMR (CDCl3, 100 MHz) d 139.8, 128.9,
128.7, 127.8, 65.8, 59.2, 52.3. HRMS (ESI): calcd for [M+Na] (C9H10Br2ONa)
requires m/z 314.8991, found 314.8988. The chiral amine (S)-2-[diphenyl-
(trimethylsilyloxy)methyl]pyrrolidine (20 mol %) gave the corresponding
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(iso-hexane/i-PrOH = 95:5, k = 210 nm) 0.5 ml/min, tR1 = 40.0 min, tR2
=
45.1 min.
8. Typical experimental procedure: To a stirred solution of alkene 1 (0.5 mmol) in
CHCl3 (1 mL) were successively added NBS (1.1 mmol, recrystallized from
boiling water), succinimide (0.1 mmol) and pyrrolidine (0.1 mmol). The
reaction vial was sealed and the mixture was stirred at 60 °C for the time
indicated in Table 3. After the reaction was complete, the corresponding
dibromo product 3 was separated by flash chromatography on silica gel. The
spectral data were in accordance with the literature: compound 3j: (a) Dewkar,
G. K.; Narina, S. V.; Sudalai, A. Org. Lett. 2003, 5, 4501–4504. Compound 3i: (b)
Lexa, D.; Saveant, J. M.; Schaefer, H. J.; Binh, S. K.; Vering, B.; Wang, D.-L. J. Am.
Chem. Soc. 1990, 112, 6162–6177. Compounds 3k, 3l and 3m: (c) Butcher, T. S.;
Zhou, F.; Detty, M. R. J. Org. Chem. 1998, 63, 169–176; (d) Ye, C.-F.; Shreeve, J. M.
J. Org. Chem. 2004, 69, 8561–8563. Compound 3m’: (e) Donohoe, T. J.; Fishlock,
L. P.; Procopiou, P. A. Org. Lett. 2008, 10, 285–288.
6. Zhao, G.-L.; Rios, R.; Vesely, J.; Eriksson, L.; Córdova, A. Angew. Chem., Int. Ed.
2008, 47, 8468–8472.
7. Typical experimental procedure: To a stirred solution of aldehyde 1 (0.5 mmol) in
CHCl3 (1 mL) were successively added NBS (1.1 mmol, recrystallized from
boiling water), succinimide (0.1 mmol) and pyrrolidine (0.1 mmol). The reaction
vial was sealed and the mixture was stirred at 60 °C for 2 h. Next, the solution was
cooled to 0 °C and methanol (1 mL) was added followed by addition of NaBH4
(1.5 mmol) in portions. The reaction was quenched by addition of satd aq NH4Cl
solution (2 mL) and extracted with EtOAc (2 ꢀ 50 mL). The combined organic
layers were dried over anhydrous Na2SO4, filtered and concentrated. The residue
was purified by flash silica gel chromatography to give the corresponding
dibromo alcohol 5. Data for 5a: 1H NMR (CDCl3, 400 MHz) d 7.34–7.43 (m, 5H),
9. CCDC 745659 contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
10. Finkelstein, M.; Hart, S. A.; Moore, W. M.; Ross, S. D.; Eberson, L. J. Org. Chem.
1986, 51, 3548. and references cited therein.
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