Atmosphere- and Temperature-Controlled Regioselective Aminobromination of Olefins

Article abstract of DOI:10.1002/adsc.201600821

A complete switch of regioselectivity in the aminobromination of olefins is realized from delicate changes in the reaction temperature from 25 °C to 40 °C and the atmosphere from air to argon, under catalyst-free conditions. The resulting α-bromoamides ca

Full text of DOI:10.1002/adsc.201600821

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DOI: 10.1002/adsc.201600821
Very Important Publication
Atmosphere- and Temperature-Controlled Regioselective
Aminobromination of Olefins
Wesley Zongrong Yu,a Yi An Cheng,a Ming Wah Wong,a,*
and Ying-Yeung Yeunga,b,*
a
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
E-mail: chmwmw@nus.edu.sg
b
Department of Chemistry, The Chinese University of Hong Kong, Shatin, NT, Hong Kong (Peopleꢀs Republic of China)
E-mail: yyyeung@cuhk.edu.hk
Received: July 26, 2016; Revised: November 19, 2016; Published online: && &&, 0000
Supporting information for this article can be found under: http://dx.doi.org/10.1002/adsc.201600821.
Abstract: A complete switch of regioselectivity in
the aminobromination of olefins is realized from
delicate changes in the reaction temperature from
258C to 408C and the atmosphere from air to
argon, under catalyst-free conditions. The resulting
a-bromoamides can be transformed easily to high-
value compounds (e.g., Clobenzorex). Mechanistic
studies revealed that the reaction went through
a radical pathway governed by in-situ generated
bromo(tosyl)amine (TsNHBr).
tion conditions; the reaction was carried out at 408C
in argon using an identical protocol (Scheme 1). It is
remarkable to note that a complete switch in regiose-
lectivity triggered by a small temperature difference
(from 25 to 408C) under catalyst-free conditions is
highly uncommon.[4]
In the literature, catalyst-controlled regioselective
aminobrominations of olefins have been document-
ed.[5] For unsymmetrical olefins (e.g., styrene), al-
though the formation of normal isomer b-bromo-
AHCTUNGTREGaNNUN mide adducts constitutes the vast majority of docu-
Keywords: aminobromination; catalyst-free reac-
tion; halogenation; radical mechanism; regioselec-
tivity
mented protocols, isolation of the opposite isomer a-
bromoamide is, however, less commonly reported.
For the latter, metal catalysts or promoters or uncon-
ventional amides were generally employed to achieve
the desired level of reactivity and selectivity.[6] In this
regard, the intriguing switch in the regioselectivity
under catalyst-free conditions is of particular interest
Haloamides occur widely as fundamental units of in the study of the mechanisms and their synthetic ap-
many natural products and also as versatile intermedi- plications.
ates in numerous organic transformations.[1] Thus,
At the outset of this research, we initiated a system-
concise and efficient ways to perform aminohalogena- atic study using a reaction mixture of styrene (1a),
tion reactions using readily available starting materi-
als are highly sought after by many chemists.[2]
We recently reported a catalyst-free electrophilic
aminobromination of unactivated olefins using the N-
bromosuccinimide (NBS)/sulfonamide protocol. The
reaction was conducted at 258C without the use of an
inert atmosphere to protect the reaction mixture.[3]
When this protocol was applied to terminal olefin
substrates (e.g., styrene), the resultant normal regio-
isomer b-bromoamides, a consequence of Markovni-
kov addition, were furnished in high yields
(Scheme 1). Herein we are pleased to disclose our
more recent results on a complete switch in the regio-
selectivity to afford the opposite regioisomer (a-bro-
moamide from terminal olefins). The switch in regio-
selectivity is solely controlled by the change in reac- lyst-free aminobromination of olefins.
Scheme 1. Complete switch of regioselectivity in the cata-
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Table 1. The effect of temperature and atmosphere on the
aminobromination of 1a.[a]
Entry Temperature Atmosphere Yield
Ratio of
[oC]
[%][b]
2a:3a[c]
1
2
25
40
40
40
40
25
40
25
air
air
62
67
87
85
85
78
57
55
100:0
74:26
0:100
0:100
0:100
44:56
100:0
100:0
Figure 1. Crystal structure of compound 3a.
3
argon[d]
argon[d]
nitrogen[d]
argon[d]
oxygen
oxygen
4[e]
5
Table 2. Scope of the aminobromination of 1a.[a]
6
7
8
[a]
Reactions were carried out with 1a (0.18 mmol), TsNH2
(0.18 mmol), and NBS (0.36 mmol) in CH2Cl2 (3 mL) in
the absence of light.
[b]
[c]
Combined isolated yield of 2a and 3a.
The ratio was determined by 1H NMR analysis on the
isolated product mixture.
[d]
[e]
See Figures S12 and S13 in the Supporting Information
for atmosphere specification.
The reaction was conducted in the presence of light from
a household fluorescent bulb.
TsNH2 and NBS in CH2Cl2 under different reaction
temperatures and atmospheres to investigate the in-
fluence of these conditions on the regioselectivity. Re-
actions performed in air at 258C gave only normal
isomer b-bromoamide 2a (Table 1, entry 1). On the
other hand, an appreciable amount of the opposite
isomer a-bromoamide 3a was detected at 408C
(entry 2). Interestingly, replacing air with argon or ni-
trogen at 408C resulted in the exclusive formation of
3a (entries 3 and 5). Light appears to have a negligible
effect on the reaction (entry 4). A key indication that
an inert atmosphere was necessary for the regioselec-
tivity arises from the observation that a mixture of 2a
and 3a with a ratio of 44:56 was obtained when the
reaction was performed in argon, even when the tem-
perature was lowered to 258C (entry 6). Indeed, only
2a was isolated at both 258C and 408C when the reac-
tions were conducted under pure oxygen atmosphere
(entries 7 and 8). These results indicate the intimate
relationship between the reaction temperature and at-
mosphere for the complete switch of regioselectivity
of this aminobromination reaction. The structure of
3a was confirmed by an X-ray crystallographic study
(Figure 1).[7]
[a]
Reactions were carried out with olefin 1 (0.18 mmol),
TsNH2 (0.18 mmol) and NBS (0.36 mmol) in CH2Cl2
(3 mL) at 408C under an argon atmosphere in the ab-
sence of light.
[b]
[c]
[d]
[e]
1
The ratio of 2:3 was determined by H NMR analysis on
the product mixture (2d:3d=0.5:1; 2e:3e=1:1.3).
The syn:anti ratio of 3 (syn-3e:anti-3e=0.3:1; syn-3f:anti-
3f=0.2:1).
Yield reported in parenthesis is obtained with (E)-2-
bromo-2-butene as substrate.
Yield reported when (Z)-2-bromo-2-butene was used as
substrate.
The scope of this reaction is indicated by the 19 ex- a variety of aliphatic linear chain olefins (1k–1r), fur-
amples listed in Table 2. In general, the reaction pro- nishing the a-bromoamide products 3 in high yields
ceeded smoothly with styrenic olefins containing elec- and excellent regioselectivities. In contrast, a lower
tron-deficient substituents (1b and 1c), substituted ar- 2:3 ratio was observed in the reaction of electron-rich
omatic rings (1h–1j), 1,1-disubstituted olefin (1g) and olefins (1d and 1e).[8] The structures of 3c, 3n and 3r
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lecular reaction is entropically more favourable than
an intermolecular reaction.[10] This result may further
suggest that two different mechanistic pathways (vide
infra) are involved.
To obtain a better understanding of the tempera-
ture effect on the reaction, a variable temperature
1H NMR study on a mixture of TsNH2 and NBS in
CDCl3 was performed. At 258C, two new sets of
peaks appeared, which corresponded to the formation
of the TsNHBr and TsNBr2 species (Figure 2, spec-
trum A). However, when the temperature was raised
to 408C, the signal of TsNBr2 completely disappeared
while TsNHBr remained (Figure 2, spectrum B). Fur-
thermore, when styrene (1a) was added to the mixture
containing TsNHBr at 408C, the opposite regioisomer
3a was detected (see Figure S3 in the Supporting In-
formation).
Scheme 2. Synthetic utility and reaction scale-up. Reactions
conditions: (a) Ph3SnH, AIBN, toluene, reflux; (b) acetone/
H2O, reflux, 12 h; (c) 2-chlorobenzyl bromide, K2CO3,
MeCN, 258C, 24 h; (d) Mg, MeOH, 258C, 24 h.
were confirmed by X-ray crystallographic studies (see
Figures S14–S18 in the Supporting Information).[7]
The utility of this reaction is illustrated by trans-
forming
3
into high-value building blocks
(Scheme 2A). For example, the aliphatic amide and
a-hydroxyamide can be easily synthesized by treating
3a with Ph3SnH/AIBN in refluxing toluene and in re-
fluxing acetone/water, respectively. In addition, the
amphetamine drug, Clobenzorex (Asenlixꢂ),[9] could
be synthesized from 3e in a three-step sequence. The
reaction can be readily scaled up without loss of re-
gioselectivity (Scheme 2B), thereby demonstrating its
practicality.
The chemoselective nature of this reaction is illus-
trated by subjecting olefinic amide 1i to the bromina-
tion reactions (Scheme 3). In the absence of TsNH2,
1i readily underwent electrophilic bromocyclization to
Figure 2. Variable temperature 1H NMR spectra of a mixture
of TsNH2 and NBS in CDCl3 at 258C and 408C.
1
As TsNBr2 was not observed at 408C by H NMR
yield 3i’ and 3i’’. Interestingly, intermolecular bromo- and TsNHBr was consumed in the presence of sty-
ACHTUNGTRENNUNGamide adduct 3i was obtained exclusively in the pres- rene (1a) to give 3a, we speculate that TsNHBr could
ence of 1 equivalent of TsNH2. Typically, an intramo- potentially function as the brominating species.[11] To
rationalize the opposite regioselectivity, two possible
mechanistic pathways emerged as potential candi-
dates: the involvement of (i) an aziridinium ion[12] or
(ii) free radical species.
To get a better understanding on the mechanistic
picture, controlled experiments were performed. It
was found that the aminobromination reaction was in-
hibited when the radical scavenger 2,2,6,6-tetrameth-
yl-1-piperidinyloxy (TEMPO) was added (Scheme 4).
In addition, the isolation of a TEMPO-incorporated
product 4 that has the same regioselectivity as 3a
might indicate the involvement of the benzylic radical
species B in the formation of 3a.[13] In the other set of
experiments, no reaction was observed in the absence
Scheme 3. Bromination of 1i in the presence or absence of
TsNH2.
of NBS, suggesting that the formation of 4 might not
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Scheme 4. Probing the reaction mechanism using a radical
scavenger.
Scheme 6. Plausible mechanistic pathways for the atmos-
phere- and temperature-dependent aminobromination of
olefins.
be triggered by TEMPO. The atmosphere-depend-
ence of the reaction also points to the involvement of
radicals in the bromination process that is responsible
for the complete switch of regioselectivity (Table 1,
entries 3 and 7).
A by the TsNHꢀ anion in a Markovnikov fashion
In the case of aminobromination of 1e, a mixture of then furnishes the normal regioisomer 2a. When the
syn and anti diastereomers 3e was obtained in a dr of reaction atmosphere is changed to argon, a switch in
0.3:1 (Table 2). This diastereomeric preference could the mechanistic pathway appears to occur and the
be rationalized using a benzylic radical species based radical pathway is predominant; TsNHBr now might
on the allylic-[1,3]-strain model (Scheme 5).[14] We add to substrate 1 to form a benzylic radical species
speculate that the steric effect arising from the methyl B, which could then be trapped by the Br radical to
group might play a dominant role in determining the furnish the opposite regioisomer 3a. Separately, we
preferred conformation that leads to the increased se- had considered two other possible pathways to form
lectivity for anti-3e over syn-3e.
3a: (i) rearrangement of 2a to 3a, and (ii) the involve-
ment of a (1,2-dibromoethyl)benzene intermediate
followed by a nucleophilic substitution by TsNH2 (see
Figures S4 and S5 in the Supporting Information).
Nonetheless, these possibilities have been ruled out
by subjecting either 2a or (1,2-dibromoethyl)benzene
to the optimized reaction conditions (both in the pres-
ence and absence of NBS), which did not give any re-
action in both cases.
We have also studied the reaction using other
amides and the preliminary results are shown in
Table 3. It is particularly noteworthy that analogous
to the reaction with TsNH2, TsNHMe also furnished
the isomer b-bromoamide 6 exclusively in 78% yield
Scheme 5. Allylic [1,3]-strain model for 1e.
1
(entry 2). A H NMR study on a mixture of TsNHMe
At this stage, the results from the abovementioned
mechanistic studies would likely suggest a radical
pathway involving benzylic radical B for the current
reaction protocol. While we cannot completely rule
out the possibility of the involvement of an aziridini-
um ion intermediate, several pieces of experimental
evidence disfavor such a mechanistic proposal for this
type of aminobromination (see Figure S6 in the Sup-
porting Information). Based on the information in
our hand, we attempted to construct a mechanistic
proposal for the regioselective aminobromination re-
action with substrate 1, TsNH2 and NBS (Scheme 6).
At 408C, mixing TsNH2 and NBS gives TsNHBr
which could then react through two different path-
ways. Under an oxygen atmosphere, the radical path-
way could be inhibited[15] and TsNHBr might add to
substrate 1 via an ionic pathway to form bromiranium
intermediate A.[3] Subsequent nucleophilic attack on
Table 3. Aminobromination of 1a with amides.[a]
Entry R1
R2
H
Yield [%] Ratio of 5:6[b]
1
2
3
4
5
p-MeC6H4SO2
p-MeC6H4SO2
EtOCO
p-NO2C6H4CO
(PhO)2PO
87
0:100
0:100
0:100
50:50
75:25
Me 78
H
H
H
28
28
35
[a]
Reactions were carried out with 1a (0.18 mmol), RNH2
(0.18 mmol) and NBS (0.36 mmol) in CH2Cl2 (3 mL) at
408C under an argon atmosphere in the absence of light.
[b]
1
The ratio of 5:6 was determined by H NMR analysis on
the product mixture
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References
and NBS in CDCl3 revealed that TsNBrMe could be
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sence of oxygen) that enable the complete switch of
regioselectivity in the aminobromination of unactivat-
ed and unsymmetrical olefins. The practicality and
utility of this protocol were demonstrated with the
gram scale preparation of the a-bromoamide, and
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Experimental Section
General Procedure for the Regioselective
Aminobromination of Olefins 1
An oven-dried 10-mL Schlenk flask was purged with argon
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gas and placed in an oil bath pre-heated to 408C. Tosyl-
ACHTUNGTRENNUNGamide (30.4 mg, 0.18 mmol, 1.0 equiv.) was added in CH2Cl2
(3 mL) and in the absence of light were added olefin
1 (20.5 mL, 0.18 mmol, 1.0 equiv.) and N-bromosuccinimide
(62.9 mg, 0.36 mmol, 2.0 equiv.) at 408C. (Note: during the
addition of solid reagents, maintain the argon atmosphere
by increasing the flow rate of argon gas). Upon completion,
0.26 g of silica gel were added and the mixture concentrated
under vacuum. The residue was purified by flash column
chromatography (hexane:EtOAc 9:1!7:1 eluent) to yield
a-bromoamide 3.
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Acknowledgements
We thank the financial support from National University of
Singapore Tier 1 Funding (grant no. 143-000-605-112) and
The Chinese University of Hong Kong Direct Grant (no.
4053157). We are also grateful for scholarships to W. Z. Y.
(NUS Research Scholarship) and Y. A. C. (NGS Scholar-
ship). Special thanks to Prof. S. E. Denmark for his valuable
advice and encouragement.
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might be present at low concentration beyond the
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1H NMR detection limit. However, our experimenta-
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Atmosphere- and Temperature-Controlled Regioselective
Aminobromination of Olefins
7
Adv. Synth. Catal. 2017, 359, 1 – 7
Wesley Zongrong Yu, Yi An Cheng, Ming Wah Wong,*
Ying-Yeung Yeung*
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