C2-symmetric cyclic selenium-catalyzed enantioselective bromoaminocyclization

Article abstract of DOI:10.1021/ja311202e

A catalytic asymmetric bromocyclization of trisubstituted olefinic amides that uses a C₂-symmetric mannitol-derived cyclic selenium catalyst and a stoichiometric amount of N-bromophthalimide is reported. The resulting enantioenriched pyrrolidine products, which contain two stereogenic centers, can undergo rearrangement to yield 2,3-disubstituted piperidines with excellent diastereoselectivity and enantiospecificity.

Full text of DOI:10.1021/ja311202e

Communication
pubs.acs.org/JACS
C2‑Symmetric Cyclic Selenium-Catalyzed Enantioselective
Bromoaminocyclization
Feng Chen, Chong Kiat Tan, and Ying-Yeung Yeung*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
S
* Supporting Information
trisubstituted olefinic amides. To the best of our knowledge,
this represents the first case of a monofunctional Lewis basic
selenium-catalyzed enantioselective bromocyclization process.
We initially examined 2, the selenium analogue of amino−
thiocarbamate catalyst 1 (Figure 1), which was readily obtained
ABSTRACT: A catalytic asymmetric bromocyclization of
trisubstituted olefinic amides that uses a C2-symmetric
mannitol-derived cyclic selenium catalyst and a stoichio-
metric amount of N-bromophthalimide is reported. The
resulting enantioenriched pyrrolidine products, which
contain two stereogenic centers, can undergo rearrange-
ment to yield 2,3-disubstituted piperidines with excellent
diastereoselectivity and enantiospecificity.
lectrophilic halogenation of olefins is a widely used
1
Efundamental organic transformation.
However, the
development of its asymmetric variant, in particular the
catalytic version, remains a challenging task despite the
significant effort devoted to this area.2 On the basis of Brown’s
studies, degenerate halogen exchange of halonium ion with the
olefin causes racemization of the halonium intermediate, which
is believed to be the major obstacle to the development of an
asymmetric halogenation reaction.3
Figure 1. Lewis Basic Sulfur and Selenium Catalysts.
by a one-step coupling between isoselenocyanate and cinchona
alkaloid. When 4-phenyl-4-pentenoic acid was used as the
model substrate, selenocarbamate 2 was found to be a useful
catalyst that offered not only appreciable ee but also a high
reaction rate comparable to that of the thiocarbamate-catalyzed
reaction.12,13 However, it was found that the catalyst was
unstable and decomposed during the reaction. In addition, 2
gradually turns reddish brown if it is not stored under an inert,
anhydrous atmosphere. This could be a result of oxidative
decomposition accompanied by the release of elemental
selenium.14
To obtain a stable Lewis basic selenium catalyst that is
resistant to decomposition, we decided to incorporate the
selenium atom into a dialkyl-substituted system (3). A sugar
alcohol-derived system was chosen in this study since it offers a
well-defined chiral scaffold. Bulky substituents can also be
introduced onto the hydroxyl handles for catalyst optimization.
Mannitol was used in the synthesis, as it is inexpensive and
commercially available. The C2 symmetry of the scaffold also
simplified the catalyst design and modification. Moreover,
different substituents could be introduced to fine-tune the steric
hindrance. The corresponding cyclic selenium catalysts 7 were
readily obtained using a modified literature procedure (Scheme
1).15 Epoxide 4 was treated with different phenols under basic
conditions to yield diols 5. At this stage, the steric side arms
were introduced. Subsequent mesylation followed by nucleo-
philic selenium substitution afforded catalysts 7.
In the literature, there are a number of reports dealing with
the use of bifunctional catalyst systems in asymmetric
halocyclization processes. Such bifunctionality appears to be
an essential requirement for catalyst design in order to achieve
high reactivity and useful levels of enantioselectivity.4−6
Recently, our research group focused on the development of
an amino−thiocarbamate bifunctional organocatalytic system
and its application to the asymmetric bromocyclization of
olefinic substrates. The Lewis basic sulfur-containing thio-
carbamate is believed to activate the halogen in the electrophilic
cyclization reaction.7
In Denmark’s recent reports, a number of monofunctional
Lewis basic selenium catalysts that showed excellent catalytic
ability in halolactonization reactions were studied.8 In addition,
it has been shown that those monofunctional Lewis basic
selenium catalysts can significantly affect the inherent site
selectivity (through a tight Lewis base−halonium pair
intermediate) in the halolactonization reaction, suggesting
that Lewis basic selenium can potentially be used as a catalyst
in the enantioselective halogenation.8,9 Although there are
several elegant reports on the use of stoichiometric monofunc-
tional chiral Lewis bases as promoters in enantioselective
halogenation processes,4c,10 the application of monofunctional
Lewis basic catalysts in asymmetric halogenation processes
remains unknown.11 Herein we are pleased to report the use of
a sugar alcohol-derived C2-symmetric monofunctional cyclic
selenium species as a Lewis basic catalyst (substoichiometric
amount) in the enantioselective bromoaminocyclization of
The monofunctional selenium catalysts 7 were then
employed in the underexploited asymmetric haloaminocycliza-
Received: November 14, 2012
Published: January 13, 2013
© 2013 American Chemical Society
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Journal of the American Chemical Society
Communication
low enantioselectivity (entries 8 and 9). After this brief study of
1,1-disubstituted olefinic amides, we embarked on the
cyclization of the challenging trisubstituted substrates. To our
delight, the desired product 9d was obtained in 90% yield with
79% ee (entry 10). It is noteworthy that only one diastereomer
was detected exclusively. We also examined substrates 8d and
8e using thiocarbamate 10, a catalyst that gave good yields and
ee in the haloaminocyclization of 1,1-disubstituted olefinic
amide;7c although good ee was obtained, the reaction was
sluggish (entries 11 and 12). This result shows that the
monofunctional cyclic selenium catalysts 7 are more reactive
than the bifunctional amino−thiocarbamate catalyst.12
Scheme 1. Synthesis of Cyclic Selenium Catalysts 7
A systematic screen of different physical parameters was
performed (Table 2). Among different brominating reagents,
tion reaction. The paucity of related reports suggests the high
difficulty of this class of reaction.5,7c,e,h Trisubstituted olefinic
amides 8 (R1 = alkyl) were chosen for study because their
asymmetric cyclization remains a very challenging task. One of
the major obstacles is the sluggishness of the reaction compared
with the O-type halocyclization (also see Table 1). The
resulting pyrrolidines can be transformed to valuable and
unusual substituted piperidine building blocks (vide infra).
A preliminary investigation was performed using catalyst 7a
and 1,1-disubstituted olefinic amide. We were surprised after
some screening to find that the 3-nitrobenzenesulfonamide (3-
nosylamide) was far superior to the corresponding 2- and 4-
nosyl analogues (Table 1, entries 1−3).16 An examination of
Table 2. Optimization of the 7-Catalyzed Bromocyclization
a
Reaction
b
Br
time
yield (%),
ee (%)
entry source
solvent
CH2Cl2
CH2Cl2
catalyst, R
(days)
a
Table 1. Haloaminocyclization of 8 Using 7
t
1
2
3
4
NBS
NBP
7d, Bu
4
4
4
5
90, 79
90, 80
92, 71
90, 89
t
7d, Bu
t
DBH CH2Cl2
7d, Bu
t
NBP
NBP
NBP
NBP
NBP
NBP
CH2Cl2/PhMe
(1:1)
7d, Bu
t
5
6
7
8
9
CH2Cl2/nC6H14
(1:1)
7d, Bu
5
5
5
5
5
87, 81
90, 90
90, 88
85, 84
90, 92
c
d
c
c
t
CH2Cl2/PhMe
(1:1)
7d, Bu
t
CH2Cl2/PhMe
(1:1)
7d, Bu
i
CH2Cl2/PhMe
(1:1)
7e, Pr
b
c
entry amide, R, R1
catalyst, Ar
time (days) yield (%), ee (%)
CH2Cl2/PhMe
(1:1)
7f, CMe2Et
1
2
3
4
5
6
7
8
9
8a, 4-Ns, H
8b, 3-Ns, H
8c, 2-Ns, H
8b, 3-Ns, H
8b, 3-Ns, H
8b, 3-Ns, H
8b, 3-Ns, H
8b, 3-Ns, H
8b, 3-Ns, H
8d, 3-Ns, Me
8d, 3-Ns, Me
8e, 4-Ns, Me
7a, Ph
0.5
0.5
0.5
0.5
0.5
0.5
2
71, 11
67, 41
82, 1
a
7a, Ph
Reactions were carried out with 8d (0.03 mmol), 7 (0.006 mmol),
and the Br source (0.033 mmol) in the solvent (0.03 M) in the
7a, Ph
b
c
7b, 4-MePh
7c, 3,5-Me2Ph
7d, 4-tBuPh
7d, 4-tBuPh
7d, 4-tBuPh
7d, 4-tBuPh
7d, 4-tBuPh
10
62, 41
80, 37
77, 49
88, 71
NR
absence of light. Isolated yields. The reaction was conducted at a
d
concentration of 0.02 M. The reaction was conducted at a
concentration of 0.015 M.
d
e
f
0.5
0.5
4
N-bromophthalimide (NBP) offered the best enantioselectivity
(entry 2). In the solvent screen, a CH2Cl2/toluene solvent
blend (1:1, 0.02 M) was found to be optimal, giving the desired
pyrrolidine 9d with 90% ee (entry 6). Further fine-tuning of the
catalyst bulkiness showed that the 1,1-dimethylpropyl-sub-
stituted catalyst 7f gave 9d with up to 92% ee (entry 9).
With the optimized conditions in hand, we further expanded
the substrate scope (Table 3). Substrates with various R1
substituents returned good yields and ees (entries 1−7). In
addition, the reaction was readily scalable without diminishing
the enantioselectivity (entry 3). Substrates 8k and 8l containing
multiple double bonds also furnished the desired pyrrolidine
products (entries 8 and 9), in sharp contrast to the results for
other simple catalytic systems. With simple Lewis bases such as
1,4-diazabicyclo[2.2.2]octane and triphenylphosphine sulfide,
no reaction was observed at low temperature. At room
temperature, the reaction was sluggish and accompanied by
14, 15
90, 79
21, 65
28, 78
10
11
12
4
10
4
a
Reactions were carried out with amide 8 (0.03 mmol), catalyst 7
(0.006 mmol), and NBS (0.033 mmol) in CH2Cl2 (1.0 mL) in the
b
c
d
absence of light. Ns = nosyl. Isolated yields. 1:1 CH2Cl2/toluene
was used as the solvent. NCS was used as the halogen source. NIS
was used as the halogen source.
e
f
different Ar substituents in 7 showed that a 4-tert-butylphenoxy
unit gave 49% ee (entries 4−6). Changing the solvent from
CH2Cl2 to 1:1 CH2Cl2/toluene gave the desired product with
71% ee (entry 7). Changing the halogen source from N-
bromosuccinimide (NBS) to N-chlorosuccinimide (NCS) or
N-iodosuccinimide (NIS) resulted in sluggish reactions with
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Journal of the American Chemical Society
Communication
a
eq 2). These motifs resemble many interesting bioactive
compounds.20
Table 3. Bromoaminocyclization of 8 Using 7f
We speculate that the mechanism of this cyclic-selenium-
catalyzed bromocyclization reaction may involve the following
sequence (Scheme 3): (1) Coordination of the Lewis basic
b
entry
amide
R1
R2
yield (%), ee (%)
Scheme 3. Proposed Mechanism of the Lewis Basic
Selenium-Catalyzed Bromoaminocyclization
1
2
8e
8f
Et
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
93, 91
90, 90
82, 91
91, 90
91, 83
70, 60
92, 83
66, 84
91, 90
15, 79
62, 59
85, 83
61, 79
43, 29
92, 95
89, 51
90, 75
11, 2
nPr
nPr
nBu
iBu
Bn
cd
,
3
4
5
6
7
8
9
8f
8g
8h
8i
d
d
8j
EtPh
allyl
homoallyl
Ph
8k
8l
10
11
12
13
14
15
16
17
18
8m
8n
8o
8p
8q
8r
8s
8t
selenium 7 to NBP would form activated electrophilic
brominating species A. (2) Subsequent interaction between A
and the olefinic substrate 8 would then give a tight selenium-
coordinated bromonium intermediate species, B. We suspect
that the tight pair may help mitigate the halogen degeneration
and racemization (without the use of a bifunctional pocket that
could encapsulate the halonium intermediate), which may
explain the high enantioselectivity. (3) Se−Br−olefin inter-
mediate8c B would then undergo SN2 attack by the sulfonamide
to give the desired product 9 with regeneration of catalyst 7.
The dominant role of the bromonium intermediate B would
explain the excellent regioselectivity (Markonikov type) and
stereoselectivity (anti relationship of Br and Ns).
Me
4-ClPh
Me
3-ClPh
Me
4-FPh
Me
4-CF3Ph
2-MePh
3-MePh
4-MePh
4-MeOPh
Me
Me
Me
8u
Me
a
Reactions were carried out with 8 (0.03 mmol), 7f (0.006 mmol),
and NBP (0.033 mmol) in 1:1 CH2Cl2/PhMe (1.6 mL) in the absence
of light. Isolated yields. The reaction was conducted on a 0.3 mmol
b
c
d
scale. The reaction time was 6 days.
In summary, we have developed the first Lewis basic
selenium-catalyzed asymmetric bromocyclization of trisubsti-
tuted olefinic amides. The enantioenriched pyrrolidine
products, which contain two stereogenic centers, can undergo
rearrangement to yield the corresponding 2,3-disubstituted
piperidines with excellent diastereoselectivity and enantiospe-
cificity. Further studies of the mechanism and applications of
this class of catalysts to other asymmetric halogenation
processes are underway.
the formation of multiple side products.17 This suggests that
the cyclic selenium catalyst 7 offers not only good control of
the enantioselectivity but also high positional selectivity. For
substrate 8m, for which R1 = R2 = Ph, the exo-cyclized product
was obtained exclusively with 79% ee despite the low reaction
rate (entry 10).18 Some substrates with different R2 substituents
were also examined and returned good to excellent ee’s (entries
11−13 and 15−17). Relatively strong electron-rich and
electron-poor aryl R2 groups, however, gave disappointing
ee’s (entries 14 and 18). Notably, the reactions were highly
diastereoselective, as products 9 were obtained exclusively with
dr >99%. The absolute configurations of 9 were established by
an X-ray crystallographic study of 9d.13
The cyclized products 9 could be further manipulated to
form various interesting building blocks. For instance, treat-
ment of 9d with silver trifluoroacetate in 3:1 acetone/water at
60 °C gave 2,3-disubstituted piperidine 11 (Scheme 2, eq 1).19
This rearrangement was found to be highly diastereoselective
and enantiospecific.18 Subsequent dehydration of 11 furnished
unsaturated piperidine 12. On the other hand, hydroboration of
the olefin in 9l gave multifunctional pyrrolidine 13 (Scheme 2,
ASSOCIATED CONTENT
* Supporting Information
Experimental details, CIF files, and spectroscopic and analytical
data for new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
chmyyy@nus.edu.sg
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Scheme 2. Syntheses of 11−13
We thank the National University of Singapore (Grant 143-
000-509-112) for financial support. We acknowledge the
receipt of a NUS Research Scholarship (to F.C.) and a
President’s Graduate Fellowship (to C.K.T.) and offer special
thanks to Prof. Scott E. Denmark (University of Illinois) for
valuable discussions.
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(9) For studies using Lewis basic selenium as the catalyst in
halolactonization, see: (a) Mellegaard, S. R; Tunge, J. A. J. Org. Chem.
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