Electrophilic Bromolactonization of Cyclopropyl Diesters Using Lewis Basic Chalcogenide Catalysts

Article abstract of DOI:10.1002/adsc.201800886

An efficient and regioselective electrophilic bromolactonization of cyclopropylmethyl diesters using triphenylphosphine sulfide (Ph₃PS) or diphenyl selenide (Ph₂Se) as the Lewis basic chalcogenide catalyst has been developed. It was observed that Ph₃PS favored the formation of anti-diastereomer and yielded the multi-functional γ-lactones. Interestingly, the diastereoselectivity was reversed when using Ph₂Se as a catalyst where the syn-product instead of the anti-product was favored. (Figure presented.).

Full text of DOI:10.1002/adsc.201800886

Title: Electrophilic Bromolactonization of Cyclopropyl Diesters Using
Lewis Basic Chalcogenide Catalysts
Authors: Matthew H. Gieuw, Vincent Ming-Yau Leung, Zhihai Ke, and
Ying-Yeung Yeung
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800886
Link to VoR: http://dx.doi.org/10.1002/adsc.201800886

10.1002/adsc.201800886
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Electrophilic Bromolactonization of Cyclopropyl Diesters Using
Lewis Basic Chalcogenide Catalysts
Matthew H. Gieuw, Vincent Ming-Yau Leung, Zhihai Ke*, Ying-Yeung Yeung*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, NT, Hong Kong (China)
[E-mail: chmkz@cuhk.edu.hk; yyyeung@cuhk.edu.hk]
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
of unactivated cyclopropanes using hypervalent
bromolactonization of cyclopropylmethyl diesters using iodine species.[6]
Abstract: An efficient and regioselective electrophilic
triphenylphosphine sulfide (Ph3PS) or diphenyl selenide
(Ph2Se) as the Lewis basic chalcogenide catalyst has been
developed. It was observed that Ph3PS favored the
formation of anti-diastereomer and yielded the multi-
functional -lactones. Interestingly, the diastereoselectivity
was reversed when using Ph2Se as a catalyst where the syn-
product instead of the anti-product was favored.
Based on our experience in Lewis base-catalyzed
halocyclization of olefinic substrates,[7] we envisioned
that
using
different
nucleophilic
partners,
cyclopropane could be ring-opened to form various
classes of compounds through electrophilic
halocyclizations. We have shown that using 1,3-
dibromo-5,5-dimethylhydantoin (DBH) as the
bromine source and triphenylphosphine sulfide as the
Lewis basic chalcogen catalyst, 1,1- and 1,2-
disubstituted cyclopropylmethyl amides could be
cyclized efficiently to give oxazolines and oxazines.[5c]
Besides having amides as the nucleophilic partners,
cyclopropylmethyl carboxylic acids, along with DBH
and triphenylphosphine sulfide as the halogen source
and the catalyst, respectively, could readily undergo
the electrophilic bromolactonization in Markovnikov-
fashion (Scheme 1).[5d] Very recently, we successfully
developed a Lewis base-mediated ring-opening/1,3-
difunctionalization of unactivated cyclopropanes
using phenyliodane bis(trifluoroacetate).[8]
Keywords: Bromolactonization; Cyclopropane;
Diastereoselectivity; Lactones; Lewis base
Cyclopropane is the fundamental core of many
functional molecules and bioactive compounds.[1] It is
featured with three bent -bonds and a high ring-
strain system because of the inefficient orbital
overlapping. The regions with the highest electron
density in cyclopropane are off the C-C connecting
lines. As a result, cyclopropanes have much
resemblance in chemical behavior to olefins due to
the significant -character of their C-C bonds.[2] In the
light of this aspect, significant research efforts have
been devoted to exploiting the reactions using
cyclopropanes in analogue to olefins. Among the
efforts on the development of cyclopropane reactions,
activated cyclopropanes such as donor-acceptor
cyclopropanes have been frequently studied.[3]
Radical initiated reactions are also common in the
cyclopropane ring-opening reactions.[4]
While there are plentiful reports on ring-opening
reactions of activated cyclopropanes, studies on their
unactivated counterparts remain scarce. In 2011, Zyk
and co-workers reported an electrophilic halogenation
of unactivated cyclopropanes, in which they devised a
KICl2
mediated
ring-opening
reaction
of
arylcyclopropanes to yield a mixture of 1,3-dihalide
products.[5a] Recently, Hennecke et al. reported a
range
of
catalyst-free
halocyclization
of
cyclopropanes with tosylamides, hydroxyl, and
carboxyl substituents, yielding the corresponding
Scheme 1. Comparison of Previous and The Present
Studies.
pyrrolidines,
tetrahydrofurans
and
lactones,
respectively.[5b] An emerging research direction
involves the oxidative ring-opening/functionalization
1
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10.1002/adsc.201800886
Advanced Synthesis & Catalysis
To further exploit the field of Lewis base- Interestingly, it was realized that water played an
mediated functionalization of cyclopropyl compounds, important role in the efficiency of the reaction; the
we were in search of different nucleophilic partners. reaction time was dramatically reduced from 7 days to
Herein, we are pleased to report an efficient and 12 hours upon the addition of 1 equivalent of H2O
highly
regioselective
bromocyclization
to give
of (Table 1, entry 4 vs 5). It was also shown that water
multi- alone could not promote the halocyclization (Table 1,
cyclopropylmethyl
diesters
functionalized lactones catalyzed by chalcogenide entry 6). A brief survey on a series of solvents
catalysts (Scheme 1). It is also interesting to realize revealed the superior performance of CH2Cl2 in
that the diastereoselectivity could be dictated by the offering
sulfide or selenide catalyst. diastereoselectivity (See Supporting Information
The study began with the cyclization of Table S1 for details). Surprisingly, the
satisfactory
reaction
yield
and
cyclopropylmethyl 1,3-diester 1a using N- diastereoselectivity could be dictated by the option of
bromosuccinimide (NBS) as the halogen source and chalcogenide. It was observed that replacing Ph3PS
triphenylphosphine sulfide as the chalcogen catalyst with Ph3PSe as the catalyst in the bromocyclization of
under different conditions (Table 1).
1a gave 3a as the major diastereomeric product
(Table 1, entry 7). The diastereoselectivity in favor of
of 3a was further increased when using diphenyl
selenide as the catalyst (Table 1, entry 8). The
relatively more electron-deficient catalyst (4-F3C-
C6H4)2Se returned an even higher dr but with much
lower reaction rate (Table 1, entry 9). On the other
hand, the methoxy-substituted catalyst (4-MeO-
C6H4)2Se gave excellent yield but with the significant
diminishment of the dr (Table 1, entry 10).
Table
1.
Electrophilic
Bromolactonization
Cyclopropylmethyl Diester 1a.[a]
entry catalyst
H2O
time
(h)
yield
dr
(%) (2a:3a)[b]
(equiv)
1
2[c]
3[c]
4
5
6
-
-
0
0
0
0
1
1
1
1
60
18
24
168
12
16
36
60
0
0
-
-
Ph3PS
Ph3PS
Ph3PS
-
Ph3PSe
Ph2Se
(4-F3C-
C6H4)2Se
(4-MeO-
C6H4)2Se
complex mixture
77
92
0
86
92
2.5:1
2.2:1
-
1:1.3
1:2.6
7
8
9
1
1
72
48
32
99
1:4.5
1:1.7
10
[a]
Reactions were carried out with 1a (0.05 M), NBS (2
o
equiv), and catalyst (10 mol %) in CH2Cl2 (1 mL) at 25 C
[b]
in the absence of light. The yields are isolated yields.
Determined by 1H NMR.
Reactions were conducted
[c]
under household 18 W fluorescent lamp.
Bromolactonization of 1a was sluggish in the
absence of Ph3PS in dark (Table 1, entry 1). Unlike
the halocyclization of cyclopropylmethyl carboxylic
acids that could be triggered by household
fluorescence lamp (18 W),[5d] bromocyclization of 1a
remained sluggish in the presence of light (entry 2). In
the presence of both light and a catalytic amount of
Ph3PS, a mixture of poly-brominated products was
obtained (entry 3). In sharp contrast, halocyclization
of 1a underwent smoothly to give the desired cyclized
products 2a and 3a in 77% isolated yield and 2.5:1 dr
when the reaction was conducted with 10 mol % of
Ph3PS in the absence of light (entry 4). The structure
of 2a was confirmed by an X-ray crystallographic
analysis on a single crystal sample (Figure 1).[9]
Figure 1. X-Ray Crystallographic Structure of 2a.
2
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10.1002/adsc.201800886
Advanced Synthesis & Catalysis
A systematic halogen source screening was also Table 3. Ph3PS Catalyzed Bromolactonization of 1.[a]
carried out and it was shown that NBS is still superior
to other bromine sources such as DBH, N-
bromoacetamide (NBA), N-bromophthalimide (NBP),
2,4,4,6-tetrabromocyclohexa-2,5-dienone
(TBCO)
(Table 2, entries 1-5). The reaction could also proceed
using NIS with satisfactory yield and
diastereoselectivity (Table 2, entry 6).
Table 2. Electrophilic Lactonization of Cyclopropylmethyl
Diester 1a with Different Halogen Sources.[a]
entry
halogen
source (X)
time
(h)
yield
(%)
dr
(2a:3a)[b]
1
2
3
NBS (Br)
DBH (Br)
NBA (Br)
NBP (Br)
TBCO (Br)
NIS (I)
12
24
24
24
24
96
92
82
79
78
79
69
2.2:1
2.0:1
1.5:1
2.1:1
2.2:1
2.0:1
4
5
6[c]
[a]
Reactions were carried out with 1a (0.05 M), halogen
source (2 equiv), water (1 equiv), and Ph3PS (10 mol %) in
o
CH2Cl2 (1 mL) at 25 C in the absence of light. The yields
1
are isolated yields. [b] Determined by H NMR. [c] NIS was
used instead of NBS. The dr indicated is the ratio of the
iodinated product 4:5.
With the optimized conditions in hand, a variety of
cyclopropylmethyl diesters 1 were subjected to the
investigation (Table 3). In general, good reaction
yields and diastereoselectivity in favor of the anti-
products
2
were achieved. For 1c, aromatic
bromination was also taken place at the para-position
to the methoxy group. For substrates with R1 as
electron-deficient substituents such as 1e (3,4-Cl2-
C6H3), 1f (4-Cl-C6H4) and 1g (4-Br-C6H4), good
yields and satisfactory diastereoselectivity were
obtained. Excellent diastereoselectivity was observed
with 1i (4-NO2-C6H4) although the reaction rate was
relatively slow. Substrates with alkyl substituent at R1
(1j) or R2 (1k) were investigated as well. The
corresponding cyclized products were obtained in
satisfactory yields but the dr was significantly
diminished. We suspect that the change in substituent
might interrupt the diastero-determining step (vide
infra, Scheme 2). Changing the alkyl malonate from
methyl to ethyl (1l) or tert-butyl (1m) was also found
to have a negative impact on the diastereoselectivity.
For the case of tert-butyl ester substrate 1m, the
reaction rate was found to be insensitive to the
amount of water. Apart from 1,1-disubstituted
cyclopropylmethyl diesters, this protocol also worked
well for trans-1,2-disubstituted cyclopropylmethyl
[a]
Reactions were carried out with 1 (0.05 M), NBS (2
equiv), H2O (1 equiv), Ph3PS (10 mol %) in CH2Cl2 (1 mL)
o
at 25 C in the absence of light. The yields are isolated
yields. [b] 38% of the starting material recovered. [c] 10% of
[d]
starting material recovered.
60% of starting material
[e]
recovered.
Without 1 equiv. of H2O, 30% of starting
material recovered. [f] 15% of starting material recovered.
3
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10.1002/adsc.201800886
Advanced Synthesis & Catalysis
diester 1n, where the major diastereomer 2n was the active halogenating species A.[12] The -
isolated in good yield and its relative configuration charactered -bond in cyclopropyl substrate 1 might
was confirmed by COSY and NOESY experiments.[10] then interact with the electrophilic brominating
We have also investigated some entries using species A to form the putative bromiranium-like
diphenyl selenide as the chalcogenide catalyst. In intermediate B. We suspect that the diastereo-
contrast to the reactions catalyzed by Ph3PS, the determining step C1 with Ph3PS as the catalyst might
formation of lactones 3 (syn-diastereomer) were follow a steric control mechanism in which the bulky
favored instead of lactones 2 (anti-diastereomer) substituent R2 might sit at the pseudo-equatorial
when Ph2Se was used as the catalyst. In addition, the position. This model could also explain the poor dr in
diastereoselectivity was reversed and the syn-products the reaction with 1k [the steric effect of the
3 were favored regardless of the electronic properties substituent (R2 = Me) might be less significant] and
of the substrates (Table 4).
1m (the bulky tert-butyl ester group sitting at the
Since the diastereoselectivity could be dictated by the pseudo-axial position might be disfavored). The
chalcogenide catalysts, we speculate that the Lewis bases subsequent intramolecular nucleophilic attack on the
might play a crucial role in controlling the diastereo- bromiranium-like ion in C1 by the carboxylate group
determining steps. NMR experiment on a mixture of 1,3- in substrate
1
could then be executed in
diester, NBS, and Ph2Se was performed and a significant Markovnikov-fashion to furnish the cyclized species
down-field shift of the Se signal was detected, attributed to D1. We believe that demethylation of the methyl
the coordination of the ester’s oxygen to the Se center of group at the oxonium ion of species D1 by the
the Ph2SeBr selenonium species.[11] On the other hand, succinimide counter-anion might not be the
there was no observable change in the signal from the 31P dominated pathway in giving the cyclized product 2
NMR experiment on a mixture of 1,3-diester, NBS, and because N-methylsuccinimide was not detected in the
Ph3PS.
reaction. Since the reaction rate was relatively low
when water was not added (Table 1, entry 4 vs 5) but
water itself could not promote the reaction (Table 1,
entry 7), a possible dominated reaction pathway from
species D1 to lactone 2 might go through intermediate
E1 as a result of the water attack at the carbonyl
carbon of the oxonium moiety.[13] Since water was
found to be not necessary for the cyclization of tert-
butyl ester substrate 1m (Table 3), we suspect the
tert-butyl group might be eliminated in the form of
isobutene through the E1 or E2 mechanism. In the
case with Ph2Se as the catalyst, the ester moiety might
interact with the species A (LB = Ph2Se) as evidenced
by the 77Se NMR experiment. The Se-O interaction
might direct the bromination through transition state
C2 and the higher dr dictated by the more electron-
deficient catalyst (4-F3C-C6H4)2Se (Table 1, entry 9)
could be attributed to the stronger Se-O interaction.
Subsequently, product 3 could be accomplished
through the cyclization-demethylation sequence
C2→D2→E2. A more detailed study is required in
order to elucidate the sole origin of the
diastereoselectivity.
Table 4. Ph2Se Catalyzed Bromolactonization of 1.[a]
In summary, we have devised an efficient catalytic
protocol to synthesize multi-functionalized lactones 2
and 3 by the electrophilic bromolactonization of
cyclopropylmethyl diesters 1. In most cases, both 2
(anti-diastereomer) and 3 (syn-diastereomer) could be
preferentially synthesized using Ph3PS or Ph2Se as the
catalyst, respectively. The synthetic application and
more sophisticated mechanistic studies are underway.
[a]
Reactions were carried out with 1 (0.05 M), NBS (2
equiv), H2O (1 equiv), Ph2Se (10 mol %) in CH2Cl2 (1 mL)
o
at 25 C in the absence of light. The yields are isolated
yields. [b] 35% of the starting material was recovered.
Although a detailed mechanism has yet to be
determined, a plausible mechanistic picture is
depicted in Scheme 2 based on the acquired data and
our experience in the chalcogenide-catalyzed
halocyclization reactions.[7] Since the chalcogenides
exhibited appreciable catalytic property in the
reactions, we speculate that the Lewis basic
chalcogenides could activate the Br in NBS to give
4
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10.1002/adsc.201800886
Advanced Synthesis & Catalysis
Acknowledgements
We thank the financial support from RGC General Research Fund
of HKSAR (CUHK 14304417) and The Chinese University of
Hong Kong Direct Grant (4053268). Equipment was partially
supported by the Faculty Strategic Fund for Research from the
Faculty of Science of the Chinese University of Hong Kong,
China.
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was added into the reaction mixture. The reaction was then
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with DI water (2 mL) and extracted with CH2Cl2 (3 × 5 ml).
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Advanced Synthesis & Catalysis
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6
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10.1002/adsc.201800886
Advanced Synthesis & Catalysis
COMMUNICATION
Electrophilic Bromolactonization of Cyclopropyl
Diesters Using Lewis Basic Chalcogenide Catalysts
Adv. Synth. Catal. 2018, Volume, Page – Page
Matthew H. Gieuw, Vincent Ming-Yau Leung,
Zhihai Ke*, Ying-Yeung Yeung*
7
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