Enantioselective 6-exo-Bromoaminocyclization of Homoallylic N-Tosylcarbamates Catalyzed by a Novel Monophosphine-Sc(OTf)3 Complex

Article abstract of DOI:10.1021/acs.orglett.5b01779

A highly enantioselective 6-exo-bromoaminocyclization of (E)-homoallylic N-tosylcarbamates catalyzed by a novel monophosphine-Sc(OTf)₃ complex is described, giving a wide variety of optically active oxazinanones with high enantioselectivities.

Full text of DOI:10.1021/acs.orglett.5b01779

Letter
pubs.acs.org/OrgLett
Enantioselective 6-exo-Bromoaminocyclization of Homoallylic
N‑Tosylcarbamates Catalyzed by a Novel Monophosphine-Sc(OTf)3
Complex
Weigang Liu,† Hongjie Pan,† Hua Tian,† and Yian Shi*,†,‡,§
†Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
‡State Key Laboratory of Coordination Chemistry, Center for Multimolecular Organic Chemistry, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093, China
§Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States
S
* Supporting Information
ABSTRACT: A highly enantioselective 6-exo-bromoamino-
cyclization of (E)-homoallylic N-tosylcarbamates catalyzed by
a novel monophosphine-Sc(OTf)3 complex is described, giving
a wide variety of optically active oxazinanones with high
enantioselectivities.
lectrophilic halofunctionalization of olefins, one of the
most fundamental organic reactions, allows the direct
Mechanistic studies showed that the catalyst has stringent
structural requirements for the ligand for the aforementioned
bromocyclization. Two phosphines are essential for the
reaction. To our surprise, optically active 1,3-oxazinan-2-
ones (4) could be obtained with a monophosphine-Sc(OTf)3
complex, but not with a Sc(OTf)3-L1 complex when (E)-
homoallylic N-tosylcarbamates (3) were used as the substrate
(Scheme 2). This observation prompted us to further
investigate this reaction system. Herein, we wish to report
our preliminary studies on this subject.
E
installation of two functional groups onto C−C double bonds
in a stereoselective manner.1 The resulting halides provide
versatile intermediates for further transformations in organic
synthesis. In recent years, asymmetric halogenation of olefins
has received extensive attention,2 which results in the
development of various effective catalytic systems, including
chiral Lewis acid,3,4 chiral phosphoric acid or phosphate,5,6
and chiral base.7,8 Despite that significant progress has been
made in this area, developing new catalytic systems to address
unsolved challenges is highly desirable. During our studies, we
have discovered that a Sc(OTf)3-L1 (Trost ligand9) complex
is an effective catalyst for asymmetric bromocyclization of
(Z)-allyl N-tosylcarbamates (1) (Scheme 1) (Figure 1).10a
Scheme 2
Scheme 1
When (E)-hex-3-enyl tosylcarbamate (3b) was first treated
with N-bromoacetamide and 10 mol % Sc(OTf)3-L1 complex,
a messy mixture was obtained (Table 1, entry 1). It is clear
that this reaction system is not effective for this class of
substrates, which calls for the development of new catalysts.
Various metals and ligands were subsequently examined.
Ironically, high ee’s were obtained with the monophosphine-
Sc(OTf)3 complex, which was previously shown to be
ineffective for the asymmetric bromocyclization of (Z)-allyl
N-tosylcarbamate (Scheme 1).10a For example, 1,3-oxazinan-2-
one 4b was obtained in 18% yield and 93% ee with
monophosphine ligand L2 (Table 1, entry 2). Further studies
showed that the yield and ee were influenced by the amide
Received: June 19, 2015
Figure 1. Selected examples of chiral ligands examined.
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01779
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Studies on the Reaction Conditions
Table 2. Enantioselective Bromoaminocyclization of
Homoallylic N-Tosylcarbamates
a
b
c
entry
Br source
L
t (°C)
yield (%)
ee (%)
1
CH3CONHBr
CH3CONHBr
CH3CONHBr
CH3CONHBr
CH3CONHBr
CH3CONHBr
CH3CONHBr
CH3CONHBr
PhCONHBr
NBP
L1
L2
L3
L4
L5
L6
L7
L8
L5
L5
L5
L5
L5
L5
L5
L5
L5
L5
L5
L5
−
0
messy
18
−
2
0
93
97
92
99
98
97
−
3
0
39
4
0
16
5
0
63
6
0
58
7
0
43
8
0
messy
12
9
0
98
10
11
12
13
14
15
0
messy
−
24
−
−
93
TBCO
0
DBDMH
0
NBS
0
messy
39
−
CH3CONHBr
CH3CONHBr
CH3CONHBr
CH3CONHBr
CH3CONHBr
CH3CONHBr
CH3CONHBr
CH3CONHBr
CH3CONHBr
rt
98
>99
>99
>99
99
99
99
−
−15
−30
−50
−15
−15
−15
−15
−15
66
d
16
60
d
17
32
e
18
44
f
19
50
g
20
53
21
−
messy
h
22
L5
−
a
The reactions were carried out with substrate 3b (0.10 mmol), Br
source (0.12 mmol), and Sc(OTf)3-L (1:1) (0.010 mmol) in CH2Cl2
b
(1.0 mL) for 24 h unless otherwise stated. Isolated yield.
c
d
e
Determined by chiral HPLC analysis. For 96 h. With 3 Å MS
f
g
h
(0.025 g). With 4 Å MS (0.025 g). With 5 Å MS (0.025 g). Without
Sc(OTf)3.
substituents (Table 1, entries 2−8) with ligand L5 being the
best, giving 4b in 63% yield and 99% ee in CH2Cl2 at 0 °C.
Among the bromine sources examined (Table 1, entries 5, 9−
13), N-Bromoacetamide was found to be the most effective
for the reaction. The reaction temperature was further
examined (Table 1, entries 5, 14−17), and the optimal
results were obtained at −15 °C. Lower yields were obtained
when the reaction was carried out in the presence of
molecular sieves (Table 1, entries 18−20). Control experi-
ments showed that both Sc(OTf)3 and the ligand were crucial
for the reaction. Little reaction was observed with Sc(OTf)3
alone (Table 1, entry 21), and a messy mixture was formed
with ligand L5 in the absence of Sc(OTf)3 (Table 1, entry
22).
As shown in Table 2, the bromocyclization with the
Sc(OTf)3-L5 complex can be extended to a variety of (E)-
homoallylic N-tosylcarbamates (Table 2, entries 1−9) (the X-
ray structure of 4b is shown in Figure 2). While the yield was
moderate, the enantioselectivity was remarkably high (97−
99% ee). Various functional groups such as OBn, Cl, N3,
alkene, and alkyne can be present in the side chains (Table 2,
entries 5−9). The starting material largely remained when
(E)-4-phenylbut-3-enyl tosylcarbamate was used as the
substrate (R = Ph). For (E)-hexa-3,5-dienyl tosylcarbamate
(R = vinyl), a messy mixture was obtained. Lower ee’s were
obtained for (Z)- and terminal substrates (Table 2, entries 10
and 11).
a
The reactions were carried out with substrate 3 (0.50 mmol),
Sc(OTf)3-L5 (1:1) (0.050 mmol), and CH3CONHBr (0.60 mmol) in
b
CH2Cl2 (5.0 mL) at −15 °C for 48 h unless otherwise stated. The
reactions were carried out with DBDMH at 0 °C. The absolute
configurations of 4b and 4f were assigned based on their X-ray
structures. For entries 1, 3−5, and 7−9, the absolute configurations
were tentatively proposed by analogy. Isolated yield. Determined by
chiral HPLC analysis.
c
d
e
The reaction can be carried out on a relatively large scale.
As exemplified by 3b (Scheme 3), 2.11 g of oxazinanone 4b
B
DOI: 10.1021/acs.orglett.5b01779
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
could bring opportunities to develop new reaction processes.
Further efforts will be devoted to understanding the reaction
mechanism, developing new catalytic systems, and expanding
the substrate scope.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterizations, X-ray structures,
data for determination of enantiomeric excess, and NMR
spectra. The Supporting Information is available free of charge
on the ACS Publications website at DOI: 10.1021/
acs.orglett.5b01779.
Figure 2. X-ray structure of 4b.
Scheme 3. Bromoaminocyclization on a Gram Scale
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: Yian.Shi@colostate.edu.
Notes
was obtained in 56% yield with >99% ee. 1,3-Oxazinan-2-one
4b can serve as a versatile chiral building block for further
transformations as illustrated in Scheme 4. Treating 4b with
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Basic Research
Program of China (973 program, 2011CB808600), the
National Natural Science Foundation of China (21172221),
and the Chinese Academy of Sciences for the financial
support.
Scheme 4. Synthetic Transformations of Bromide 4b
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exo-bromoaminocyclization of (E)-homoallylic N-tosylcarba-
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