B(C6F5)3-Catalyzed Ring Opening and Isomerization of Unactivated Cyclopropanes

Article abstract of DOI:10.1002/anie.201700864

Catalytic amounts of B(C₆F₅)₃ promote the ring opening and subsequent isomerization of a series of unactivated cyclopropanes to afford terminal olefins in good yields when a hydrosilane and 2,6-dibromopyridine are employed as additives.

Full text of DOI:10.1002/anie.201700864

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201700864
German Edition: DOI: 10.1002/ange.201700864
Boron Catalysis
Hot Paper
B(C6F5)3-Catalyzed Ring Opening and Isomerization of Unactivated
Cyclopropanes
Zi-Yu Zhang+, Zhi-Yun Liu+, Rui-Ting Guo, Yu-Quan Zhao, Xiang Li, and Xiao-Chen Wang*
Abstract: Catalytic amounts of B(C6F5)3 promote the ring
opening and subsequent isomerization of a series of unac-
tivated cyclopropanes to afford terminal olefins in good yields
when a hydrosilane and 2,6-dibromopyridine are employed as
additives.
C
yclopropanes are versatile structural motifs in organic
synthesis. Their high ring strain (ca. 115 kJmolÀ1)[1] is often
À
harnessed to cleave a C C bond for further functionalizations
on the disconnected carbon atoms. Common strategies to
selectively open the cyclopropane ring include 1) the Lewis
acid catalyzed activation of donor–acceptor (D-A) cyclo-
propanes[2–4] and 2) transition-metal-mediated processes that
often proceed by oxidative addition.[5–7] The former approach
requires the presence of electron-donating and -accepting
À
groups to create sufficient electronic bias across the C C
bond (Scheme 1a).[2–4] The latter approach usually utilizes
a chelating group to capture transition metals to facilitate the
process and/or direct transition metals towards the target
[5–7]
À
À
C C bond (Scheme 1b).
However, C C bond cleavage in
Scheme 1. Cyclopropane ring-opening reactions.
simple alkyl- and aryl-substituted cyclopropanes has rarely
been studied because their bonds are much less polarized, and
they do not have a preinstalled functional group to interact
with either Lewis acids or transition metals.[8]
dride migration followed by dissociation of the Lewis acid
provides terminal olefins in good yields in a catalytic process
(Scheme 1d).
We began our investigations with cyclopropane 1a as the
model substrate to test various reaction conditions (Table 1).
Treatment of 1a with 10 mol% B(C6F5)3 in C6D6 at 808C for
24 h gave the ring-opening/isomerization products 2a and 3a
in 15 and 5% yield, respectively (entry 1). Interestingly, we
Recently, a seminal study reported by Stephan and co-
workers revealed that simple aryl cyclopropanes can be
activated with stoichiometric amounts of the frustrated Lewis
pair (FLP) B(C6F5)3/tBu3P, [9] and phenylcyclopropane was
heterolytically cleaved to afford the zwitterionic phosphoni-
um borate as the final product (Scheme 1c). The reaction
possibly occurred by initial Lewis acid activation of the
cyclopropane, prompting cooperative Lewis base attack.[9]
Inspired by this work, we sought to develop a catalytic
process that utilizes the reactivity of B(C6F5)3 towards these
unactivated cyclopropanes. Herein, we report that in the
presence of a hydrosilane and 2,6-dibromopyridine, B(C6F5)3
found that the addition of
a hydrosilane significantly
increased the reaction yield. Et3SiH, Ph3SiH, and (EtO)3SiH
(20 mol%) were all effective, affording olefin 2a as the only
ring-opened product in 66, 70, and 69% yield, respectively
(entries 2–4). We then investigated whether Lewis bases
could further improve the reactivity. In the presence of
t
À
effectively promotes the C C bond cleavage of alkyl-, aryl-,
Ph3SiH, applying Bu3P, which was used by Stephan and co-
and vinyl-substituted cyclopropanes. A subsequent 1,2-hy-
workers to activate aryl cyclopropanes,[9] resulted in full
recovery of the starting material (entry 5). A similar inhib-
itory effect was observed with Ph3P (entry 6). With pyridine
Lewis bases,[10] although 2,6-lutidine and 2,6-di-tert-butylpyr-
idine were not effective (entries 7 and 8), 2,6-dibromopyr-
idine improved the yield to 86% (entry 9; for a detailed
screening of the reaction conditions, see the Supporting
Information). Notably, even in the absence of Ph3SiH, 2,6-
dibromopyridine was found to promote the reaction, albeit in
a lower yield (entry 10). Under the same conditions, com-
monly used Lewis acids, including BF3·OEt2, TiCl4, Sc(OTf)3,
Zn(OTf)2, and Cu(OTf)2, were found to be inactive
(entries 11–15).
[*] Z.-Y. Zhang,[+] Z.-Y. Liu,[+] R.-T. Guo, Y.-Q. Zhao, X. Li,
Prof. Dr. X.-C. Wang
State Key Laboratory and Institute of Elemento-Organic Chemistry
Collaborative Innovation Center of Chemical Science and Engineer-
ing (Tianjin), Nankai University
94 Weijin Road, Tianjin 300071 (China)
E-mail: xcwang@nankai.edu.cn
Homepage: http://www.wangnankai.com/
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700864.
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Table 1: Optimization of the ring-opening and isomerization reaction of
Table 2: Scope of 1,1-disubstituted cyclopropanes.[a]
cyclopropane 1a.[a]
Entry Lewis acid Additive (mol%)
2a [%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
B(C6F5)3
B(C6F5)3
B(C6F5)3
B(C6F5)3
B(C6F5)3
B(C6F5)3
B(C6F5)3
B(C6F5)3
B(C6F5)3
B(C6F5)3
BF3·OEt2
TiCl4
–
15 (5)[c]
66
70
Et3SiH (20)
Ph3SiH (20)
(EtO)3SiH (20)
Ph3SiH (20)/tBu3P (10)
Ph3SiH (20)/Ph3P (10)
Ph3SiH (20)/2,6-lutidine (10)
Ph3SiH (20)/2,6-di-tert-butylpyridine (10) 52
Ph3SiH (20)/2,6-dibromopyridine (10)
2,6-dibromopyridine (10)
Ph3SiH (20)/2,6-dibromopyridine (10)
Ph3SiH (20)/2,6-dibromopyridine (10)
Ph3SiH (20)/2,6-dibromopyridine (10)
Ph3SiH (20)/2,6-dibromopyridine (10)
Ph3SiH (20)/2,6-dibromopyridine (10)
69
n.d.
n.d.
18
86
60
n.d.
n.d.
n.d.
n.d.
n.d.
Sc(OTf)3
Zn(OTf)2
Cu(OTf)2
[a] Unless otherwise specified, all reactions were performed in 0.5 mL
C6D6 with 0.1 mmol 1a under N2 atmosphere. [b] Determined by NMR
analysis with CH2Br2 as the internal standard. [c] Yield of 3a; 3a was not
formed in all other entries. n.d.=not detected.
With optimized reaction conditions in hand, we inves-
tigated the generality of this reaction with a series of
cyclopropanes (Table 2). The reaction worked well with
1-methyl-1-(p-methoxyphenyl)cyclopropane, providing the
desired product in 81% yield (entry 2), whereas electron-
deficient 1-methyl-1-(p-bromophenyl)cyclopropane only
underwent moderate conversion into the product, which
was isolated in 50% yield (entry 3). The reaction was found to
be particularly efficient for spiro cyclopropanes with an
indane or tetralin scaffold, and high conversions were
achieved even with substrates bearing electron-withdrawing
substituents such as F, Cl, or Br (entries 4–8). The significantly
decreased yield of isolated product 2d compared to the NMR
yield was due to product evaporation during workup because
of its low boiling point. The enhanced reactivity of these
substrates was attributed to the ring strain of the spiro-bicyclic
backbone, which facilitates the cleavage of the cyclopropane
ring. However, for the spiro cyclopropane linked to a benzo-
cycloheptene ring (1i), the reactivity dropped significantly,
presumably owing to the decreased ring strain (entry 9). We
also subjected 1,1-dialkylcyclopropanes to these reaction
conditions. The olefins 2j, 2k, and 2l were obtained in good
yields after the ring-opening/isomerization reaction
(entries 10–12). For 1,1-diarylcyclopropanes, however, only
cyclopropane 1m, which bears two electron-rich aryl rings,
worked well, delivering olefin 2m in 94% yield (entry 13).
Notably, other olefin isomers were either not formed or
formed only in trace amounts (< 5%, GC-MS) during our
studies with these 1,1-disubstituted cyclopropanes. Unfortu-
nately, monoalkyl-, monoaryl-, and 1,2-disubstituted cyclo-
propanes were unreactive under the current reaction con-
ditions (see the Supporting Information for details).
[a] Unless otherwise specified, all reactions were performed in 0.5 mL
solvent (C6D6 for entry 1 and [D8]toluene for all other entries) with
1 (0.1 mmol) under N2 atmosphere. Products 2a–2i and 2l are all
racemic. [b] Yields of isolated products are given. Yields determined by
NMR analysis with an internal standard are given in parentheses.
[c] (EtO)3SiH (0.2 equiv) was used instead of Ph3SiH to avoid deme-
thylation/silylation of the methoxy group. [d] With B(C6F5)3 (20 mol%)
and 2,6-dibromopyridine (20 mol%). PMP=para-methoxyphenyl,
TBS=tert-butyldimethylsilyl.
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The scope of the reaction was successfully extended to
vinyl-substituted cyclopropanes, which produced non-conju-
gated 1,4-dienes after the ring-opening/isomerization reaction
(Table 3). The reaction worked well with a number of styryl
cyclopropanes bearing different substituents (4a–4g), afford-
ing the desired 1,4-dienes in moderate to good yields
Table 3: Scope of vinyl cyclopropanes.[a]
Scheme 2. Mechanistic studies.
ular Friedel–Crafts reaction. Then, [D2]-labeled cyclopropane
8 was subjected to the standard reaction conditions. Two
isotopomers (compounds 9 and 10) were formed depending
À
on which C C bond was cleaved. The formation of olefin 10
À
clearly shows that a 1,2-hydride migration occurs after C C
bond cleavage. To investigate the role of the hydrosilane
additive, one equivalent of Et3SiD was used in the reaction of
1a, but no deuterium incorporation was detected in 2a. We
also monitored the reaction of 1a by NMR spectroscopy, but
failed to observe any silicon-containing intermediates. These
experimental results indicate that the hydrosilane additive,
albeit crucial for the reactivity, is not directly reacting with the
cyclopropane. Whereas hydrosilanes have been shown to act
as water scavengers during catalysis with B(C6F5)3,[11] the
absence of Ph3SiOSiPh3 in the reaction mixture ruled out this
possibility. At the end of the reaction, Ph3SiH and 2,6-
dibromopyridine were mostly recovered. According to stud-
ies performed by the groups of Piers,[12] Oestreich,[13] and
Sakata,[14] hydrosilanes can coordinate to perfluoroaryl bor-
À
anes through the interaction of the Si H bond with the boron
atom to form an acid–base adduct. Moreover, this adduct
formation was recently confirmed by single-crystal X-ray
analysis by Piers, Tuononen, and co-workers.[15] Therefore, we
speculate that the hydrosilane might function as a borane-
specific Lewis base in our reaction, facilitating the dissocia-
tion of B(C6F5)3 from the reactive intermediate by compet-
itive coordination and thereby improving catalyst turnover.
A possible reaction mechanism is depicted in Scheme 3.
Cyclopropane ring opening with B(C6F5)3 gives zwitterionic
borate I, which subsequently undergoes 1,2-hydride migration
to afford intermediate II. Afterwards, dissociation of B(C6F5)3
from II assisted by hydrosilane coordination gives the olefin
product. During this process, the stabilization of I through
a reversible FLP-type interaction[9] (I to III) might account
for the yield enhancement upon addition of 2,6-dibromopyr-
idine.
[a] Unless otherwise specified, all reactions were performed in 0.5 mL
solvent ([D8]toluene for entries 1, 2, 5, 6, 7, 9, and 10 and toluene for all
other entries) with 4 (0.1 mmol) under N2 atmosphere. [b] Yields of
isolated products are given. Yields determined by NMR analysis with an
internal standard are given in parentheses. [c] With B(C6F5)3 (10 mol%)
and 2,6-dibromopyridine (10 mol%).
(entries 1–7). Again, the significantly decreased yields of
isolated products compared to the NMR yields were due to
product evaporation during workup. The reaction also
proceeded with vinyl cyclopropanes bearing aliphatic sub-
stituents (4h–4l), providing the dienes in yields of 62 to 80%
(entries 8–12).
To study the mechanism, we performed several experi-
ments (Scheme 2). When cyclopropane 6 was reacted with
catalytic amounts of B(C6F5)3 in the presence of 2,6-dibro-
mopyridine, 9,10-dihydroacridine 7 was obtained in 70%
yield, suggesting the generation of a B(C6F5)3-ligated carbo-
cation intermediate that was trapped through an intramolec-
In summary, we have developed a catalytic cyclopropane
ring-opening reaction that provides olefins by B(C6F5)3-
À
mediated C C bond cleavage. This reaction is compatible
with a broad range of unactivated cyclopropanes that are
difficult to activate by other Lewis acids or transition metals.
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Scheme 3. Possible reaction mechanism.
We anticipate that our findings will enable the development
of other catalytic processes initiated by Lewis acid activation
of unactivated cyclopropanes. Further studies exploring such
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laboratory.
Acknowledgements
Financial support was provided by the National Natural
Science Foundation of China (21602114), the 1000-Talent
Youth Program, and Nankai University. We thank Prof.
Michael P. Doyle at UTSA for helpful discussions.
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Conflict of interest
The authors declare no conflict of interest.
À
Keywords: boron · carbocations · C C activation ·
isomerization · strained molecules
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À
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[11] We found that treatment of H2O (1 equiv) with Ph3SiH (2 equiv)
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Manuscript received: January 24, 2017
Final Article published: && &&, &&&&
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Communications
Boron Catalysis
Z.-Y. Zhang, Z.-Y. Liu, R.-T. Guo,
Y.-Q. Zhao, X. Li,
X.-C. Wang*
&&&& — &&&&
À
B(C6F5)3-Catalyzed Ring Opening and
Isomerization of Unactivated
Cyclopropanes
Opening inert cyclopropanes: B(C6F5)3-
catalyzed ring-opening and isomerization
reactions of aryl-, alkyl-, and vinyl-substi-
tuted cyclopropanes are described. These
reactions generate terminal olefins after
a sequential process of C C bond cleav-
age, 1,2-hydride migration, and B(C6F5)3
dissociation. The addition of 2,6-dibro-
mopyridine and Ph3SiH is crucial for the
reactivity.
6
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