Lewis Base-Promoted Ring-Opening 1,3-Dioxygenation of Unactivated Cyclopropanes Using a Hypervalent Iodine Reagent

Article abstract of DOI:10.1002/anie.201713422

A facile and effective system has been developed for the regio- and chemoselective ring-opening/electrophilic functionalization of cyclopropanes through C?C bond activation by [bis(trifluoroacetoxy)iodo]benzene with the aid of the Lewis basic promoter p-toluenesulfonamide. The p-toluenesulfonamide-promoted system works well for a wide range of cyclopropanes, resulting in the formation of 1,3-diol products in good yields and regioselectivity.

Full text of DOI:10.1002/anie.201713422

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Accepted Article
Title: Lewis Base-Promoted Ring-Opening 1,3-Dioxygenation of
Unactivated Cyclopropanes using Hypervalent Iodine Reagent
Authors: Matthew H. Gieuw, Zhihai Ke, and Ying-Yeung Yeung
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201713422
Angew. Chem. 10.1002/ange.201713422
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Lewis Base-Promoted Ring-Opening 1,3-Dioxygenation of
Unactivated Cyclopropanes using Hypervalent Iodine Reagent
Matthew H. Gieuw, Zhihai Ke* and Ying-Yeung Yeung*
Dedicated to Professor Elias J. Corey on the occasion of his 90th birthday
Abstract: A facile and effective system has been developed for the
regio- and chemoselective ring-opening/electrophilic
reactivity of the cyclopropane system (vide infra, Table 1).[4e]
Very recently, two elegant research works demonstrated that
HIR-promoted ring-opening/functionalization of unactivated
cyclopropanes could be achieved by conducting the reactions
using either catalytic amount of iodobenzene and stoichiometric
amount of m-chloroperbenzoic acid (mCPBA) in the presence of
excess amount of Brønsted acid (Scheme 1, eq 1)[8] or iodoxole
reagent in the presence of a strong Lewis acid (Scheme 1, eq
2).[9] Herein, we are pleased to report our recent success in
using sulfonamides as Lewis bases to activate HIRs for the ring-
opening/1,3-dioxygenation of unactivated cyclopropanes. The
transformation is metal-free, the reaction conditions are mild
(neutral and at room temperature), and no strong oxidant is
involved. To the best of our knowledge, this case represents the
first report of Lewis base activation of HIRs for electrophilic
functionalization reactions. While strongly acidic reagents such
as TMSOTf and Ms2NH can activate HIRs for electrophilic
functionalizations of olefins, the use of Lewis bases for the
activation of HIRs offers a complementary platform for relevant
transformations.[10]
functionalization of cyclopropanes via C–C bond activation by
[bis(trifluoroacetoxy)iodo]benzene with the aid of the Lewis basic
promoter p-toluenesulfonamide. Notably, the p-toluenesulfonamide-
promoted system works well for a wide range of cyclopropanes,
resulting in the formation of 1,3-diol products in good yields and
regioselectivity.
Cyclopropane is known to have a high ring strain. The inefficient
orbital overlap in cyclopropane results in three bent carbon-
carbon -bonds that each has a significant degree of -
character.[1] Thus, the reaction chemistry of cyclopropanes
resembles that of olefins in many circumstances.[2] The ring-
opening reactions of cyclopropanes have been an on-going
attractive research area as these reactions afford synthetically
useful 1,3-difunctionalized molecules that are not readily
accessible by other conventional methods. Most of the existing
cyclopropane ring-opening approaches rely on either radical-
initiated processes or the use of activated donor–acceptor
cyclopropanes, which are substrate-specific.[3] In stark contrast,
the ring-opening reactions of unactivated cyclopropanes with
electrophiles remain scarce, which is attributable to the
formidable challenge in controlling the reactivity and
regioselectivity.[4]
Our group has developed several methodologies for the
electrophilic halogenation of olefins using Lewis bases as
catalysts.[5] It is believed that the Lewis base-activated halonium
species could react with olefinic substrate to give the key
intermediate haliranium ion followed by the ring-opening of the
haliranium ion by a nucleophile to give desired halogenated
product.[6] In a continuation of this endeavor, very recently we
applied the Lewis base catalytic protocol to the electrophilic
halocyclization of cyclopropyl substrate and the haliranium ion-
like intermediate is believed to be involved in the reaction to give
the ring-opening/1,3-difunctionalization product.[4b,4c] Hypervalent
In alignment with our research interest on Lewis base-
catalyzed reactions with olefinic and cyclopropyl substrates,[4b,4c]
we hypothesized that a Lewis base promoter could react with
HIR to in situ generate the positively charged species A, which
could be a more active electrophile for the electrophilic ring-
opening/1,3-difunctionalization of unactivated cyclopropanes 1
(Scheme 1, eq 3).[11]
iodine
reagents
(HIRs),
for
example
phenyliodine
bis(trifluoroacetate) (PIFA), are known to readily react with
olefins to give the corresponding 1,2-dioxygenated products and
the reaction mechanism is similar to that of electrophilic
halogenation.[7] However, the electrophilic reaction of
unactivated cyclopropane with PhI(OCOCF3)2 was sluggish in
the absence of external activator, potentially because of the low
[*]
Mr. Matthew H. Gieuw, Dr. Zhihai Ke, Prof. Dr. 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
Scheme 1. Reactions of -character C-C bonds with activated HIR.
Supporting information for this article is given via a link at the end of
the document.
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To test our hypothesis, cyclopropylbenzene (1a) and PIFA
were used as the substrate and the electrophilic reagent,
respectively. The reaction was monitored by 1H NMR using
CDCl3 as the solvent. The background reaction was sluggish
with low conversion (Table 1, entry 1). Although the addition
relatively electron-rich amine systems including benzamide, n-
butylamine and aniline gave sluggish reaction (Table 1, entry
12–14). In particular, aniline completely shut down the reaction
and the starting material was recovered quantitatively. These
results suggest that an amine with suitable Lewis basicity might
be crucial for the high reaction efficiency (vide infra).[14] Finally,
the conversion was further improved by employing 2.1
equivalents of PIFA (Table 1, entry 15).[15] A brief survey on
some solvents revealed the superior performance of CH2Cl2 as
the reaction media.
Table 1. Reaction of cyclopropylbenzene (1a) using various HIRs and
promoters.[a]
With the optimized conditions in hand, the scope of the
dioxygenation reaction was examined (Scheme 2). As a result of
Entry
1
R
Time (d)
Promoter
-
Conversion (%)[b]
CF3
CF3
CF3
CF3
Me
3
28
~60[c]
74[d]
38[e]
0
2
3
TFA
3
3
4-TsNH2
4-TsNMe2
4-TsNH2
4-TsNH2
4-TsNH2
4-TsNH2
2-NsNH2
4-NsNH2
2-TsNH2
PhCONH2
n-BuNH2
PhNH2
4
3
5
5
6
C(CH3)3
CHCl2
CCl3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
5
0
7
10
10
3
0
8
50
27
39
43
12
10
0
9
10
11
12
13
14
15[f]
3
3
3
Scheme 2. One-pot preparation of 1,3-diols 3a-m. [a] Isolated yield of 3 for 2
steps. The reaction time of the first step is indicated. [b] Some starting material
was recovered. [c] 2 mmol scale. [d] Substrate 1l with R = 4-AcO-C6H4 was
used.
3
3
3
4-TsNH2
94
[a] Reaction conditions: cyclopropylbenzene (1a) (0.2 mmol), HIR (1.1 eq),
CDCl3 (0.4 mL) at 22 °C in the absence of light. [b] Based on 1H NMR. [c] 24%
of 1a was converted into 4-iodophenylcyclopropane detected by 1H NMR. [d]
26% of starting material remained. [e] 62% of starting material remained. [f]
2.1 equivalents of PIFA was used. 2-NsNH2 = 2-nitrobenzenesulfonamide, 4-
NsNH2 = 4-nitrobenzenesulfonamide.
the instability of product
2 upon treatment of silica gel
chromatography, 2 was hydrolyzed in an one-pot fashion to its
corresponding diol 3 with the use of K2CO3 and MeOH. In
general, aryl cyclopropanes bearing electron-withdrawing (1b–
1g) and electron-rich (1h–1l) substituents worked well in this
protocol and gave the corresponding products 3b–3l with good
yields and excellent regioselectivity. Particularly, the sterically
hindered 2-methylphenylcyclopropane (1h) could be converted
to the corresponding 1,3-diol (3h) in 91% isolated yield. This
reaction protocol also showed good chemoselectivity, where
electron-deficient carbonyl cyclopropane moiety was left intact
when using 1m as the substrate. In addition, it was found that
the ring-opening and diol formation too place exclusively at the
C–C bond adjacent to the aryl substituent.
of 1 equivalent of trifluoroacetic acid (TFA) greatly accelerated
1
the reaction, H NMR study on the crude sample showed that
24% of 1a was converted into 4-iodophenylcyclopropane as the
side product (Table 1, entry 2).[12] To our delight, the use of
Lewis basic 4-toluenesulfonamide (4-TsNH2) or N,N-dimethyl 4-
toluenesulfonamide (4-TsNMe2) could promote the reaction of
1a and at the same time effectively suppressed the formation of
side products (Table 1, entries 3 and 4), suggesting that the
Lewis basic nitrogen center might play a crucial role in the
reaction. Other less electron-deficient HIRs did not work well
when compared with PIFA (Table 1, entries 5–8 vs 3).[13] Next,
various amines were examined. It was found that the relatively
electron-deficient amides 2-NsNH2 and 4-NsNH2 returned
diminished reactivity (Table 1, entry 9–10). Surprisingly, the
Apart from monosubstituted arylcyclopropane, we also
extended the substrate scope to disubstituted and trisubstituted
cyclopropanes (Scheme 3). 1,2-Disubstituted cyclopropane (1n)
and trisubstituted cyclopropane (1p) were converted into 3n and
3p in good regioselectivity. Trisubstituted cyclopropane (1o) was
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converted into the homoallylic alcohol (3o), presumably obtained
from the elimination of the tertiary alcohol intermediate.
Surprisingly, alkyl cyclopropane (1q) was transformed into both
1,3- and 1,4-diol (3q and 3q’).
between the lone pair of the sulfonamide N and the C-I electron-
deficient * orbital of PIFA[18] to give species A1, and this
interaction might disrupt the dimerization of PIFA (Scheme 4).
We also suspect that species A1 might exist in equilibrium with
the charged species A2; the existence of species A2 was
evidenced by the mass spectrometric analysis in both mixtures
Table 2. Reaction of cyclopropanes using different conditions.[a]
Scheme 3. Ring-opening and 1,3-difunctionalization of di-, tri-substituted and
alkyl cyclopropanes using Lewis base-activated HIR. [a] anti:syn of 3n = 3:1.
Further evaluation on the reaction protocol revealed that 4-
TsNMe2 could also be a potent promoter to activate HIR. Some
[a] The reactions were conducted using cyclopropane 1 (0.2 mmol) under
condition (a) or (b). [b] Condition (a): PIFA (2.1 equiv), 4-TsNMe2 (1 equiv) in
CH2Cl2 (0.4 mL) at 22 °C in the absence of light. [c] Condition (b):
iodobenzene (0.2 equiv), mCPBA (1.1 equiv) in TFA/CH2Cl2 (0.4 mL, 1:1 v/v)
at 22 °C in the absence of light. [d] anti:syn = 3:1. [e] All starting material was
consumed but no desired product was detected.
examples are shown in Table
2 in which appreciable
conversions were obtained for the electron-rich (1j, 1r), silyl
ether (1s), and 1,2-disubstituted (1t) substrates. In parallel, we
have compared our protocol [condition (a)] with a recently
reported literature procedure [condition (b)][8] involving the use of
substoichiometric amount of iodobenzene and stoichiometric
amount of co-oxidant m-chloroperbenzoic acid (mCPBA) in an
excess amount of TFA, which can in situ generate PIFA for the
1,3-dioxygenation of a few electron-deficient cyclopropanes.
However, under condition (b) the substrates were consumed but
no desired products were detected (Table 2). These results
highlight the significance in utilizing a Lewis base to promote the
1,3-dioxygenation of cyclopropanes in the absence of strong co-
oxidant and acidic media.
of 4-TsNMe2 and PIFA as well as 4-TsNH2 and PIFA.[19] 19F
NMR study on PIFA was also conducted and two signals were
observed. The major signal at –76.6 ppm and the minor signal at
–78.3 ppm should correspond to the dimer and the monomer of
PIFA, respectively.[20] The ratio of dimeric and monomeric PIFA
reached equilibrium at approximately 72 h.[21] However, the
addition of 4-TsNMe2 to PIFA triggered the consumption of the
dimer as indicated by the diminishment of the signal at –76.6
ppm. Concurrently, the intensity of the signal at –78.3 ppm
increased dramatically in proportion to the decrement of dimeric
PIFA over 72 h.[21] Upon the addition of phenylcyclopropane (1a)
to the mixture, 1a was consumed gradually and the diester
product 2a was formed.[14]
In order to obtain a better understanding on the reaction
1
mechanism, a careful H NMR study was carried out to probe
1
the interaction between PIFA and 4-TsNH2. H NMR study on
pure PIFA alone gave a group of broad signals corresponding to
the aromatic protons. Upon the addition of 4-TsNH2, the signals
of PIFA became sharpened.[16] PIFA is known to exist as a
centrosymmetrical dimer with the secondary intra- and
intermolecular interaction occupies the vacancy of the C-I
antibonding orbital (the electron-deficient * orbital), which could
give rise to broadened proton signals.[17] We speculate that 4-
TsNH2 might coordinate with PIFA through a n→* interaction
Although a more detailed study is needed in order to
elucidate a clear mechanistic picture, a plausible explanation on
the reaction mechanism that is in alignment with the
spectroscopic evidences is shown in Scheme 4. We suspect that
the dimeric PIFA might be inactive towards the electrophilic
functionalization of cyclopropane as a result of the occupied C-I
antibonding orbital of PIFA.[17] Sulfonamide might coordinate
with PIFA and the resulting complex A1 might exist in
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In summary, we devised a reaction protocol to oxidize
cyclopropanes into their respective 1,3-diols from moderate to
excellent yields in a regioselective manner using PIFA as the
oxidant and tosylamides as the promoters. This report highlights
examples of a facile, electrophilic approach to using cyclopropyl
compounds in place of olefins to generate a new class of
structurally diverse 1,3-diol products. This work also opens a
new avenue of utilizing a Lewis base instead of a Brønsted acid
in the activation of hypervalent iodine reagents for electrophilic
functionalization reactions. Effort is now underway to apply the
organocatalysis to other mild and regiocontrollable ring-opening
reactions of cyclopropanes.
Acknowledgements
We thank the financial support from RGC General Research
Fund of HKSAR (CUHK 14304417). Equipment was partially
supported by the Faculty Strategic Fund for Research from the
Faculty of Science of the Chinese University of Hong Kong. We
also thank Prof. Thomas Wirth (Cardiff University) for his
valuable advice.
Scheme 4. A plausible reaction mechanism.
Keywords: Cyclopropanes • Diols • Hypervalent iodine •
Organocatalysis • Oxygenation • Sulfonamide
equilibrium with the more electrophilic HIR species A2.[22] The -
character -bond of cyclopropane 1 might then interact with
species A2 to give the iranium-like species B1. Substitution of
B1 by the trifluoroacetate anion in a Markovnikov fashion could
give C (Scheme 4, path a) and subsequent degradation of
species C could give the product 2. Alternatively, intermediate C
could be formed from the carbocation species B2 (Scheme 4,
path b), which could be generated through the ring-opening of
species B1. At this stage, we believe that both paths (a) and (b)
could lead to the desired 1,3-difunctionalized product 2 while
path (a) could possibly be the predominant one because anti-
products were preferentially formed with substrate 1n and 1t. In
another set of NMR study, it was found that other stronger Lewis
bases could also form similar Lewis base-PIFA adducts. For
instance, aniline could readily react with PIFA to give the
corresponding aniline-PIFA adduct. However, the adduct was
found to be intact upon the addition of 1a and the ring-opening
reaction was sluggish.[14] It appears that a stronger Lewis base
(e.g. aniline) could coordinate tightly with PIFA, but the resulting
adduct seems to be too stable for subsequent reaction as
indicated by its inertness towards the ring-opening of
cyclopropane substrate. On the other hand, tosylamide with
proper Lewis basicity appears to be matching with HIR to form
the amide-PIFA adduct A that has suitable reactivity; this could
explain why amines that have stronger Lewis basicity could not
efficiently promote the reaction (Table 1, entries 12–14). We had
suspected that sulfonamide might react with PIFA to in situ
generate TFA (e.g. collapse of A2) which could promote the
reaction. However, this possibility was ruled out by two findings:
(1) in the 19F NMR study on a mixture of tosylamide (TsNH2 or
TsNMe2) and PIFA, no TFA was detected; (2) TsNMe2, which
has no acidic proton and cannot trigger the formation of TFA,
could still readily promote the formation of diol 3 (Table 2).
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[13] The 1,3-bis(trichloroacetic)ester product (Table 1, entry 8) readily
decomposed during the purification process. The structure was
confirmed by HRMS (ESI) analysis (Calc’d for C13H10Cl6O4 [M+Na]+:
464.85737; found 464.85728).
[14] The details are shown in the Supporting Information Figure S1 and S2.
[15] >95% conversion could still be achieved with 1 equivalent of PIFA after
9 days.
[16] The details are shown in the Supporting Information Figure S3.
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[19] The details are shown in the Supporting Information Figure S4 and S5.
[20] The details are shown in the Supporting Information Figure S6.
[21] The details are shown in the Supporting Information Figure S7.
[22] It was reported that TsNH2 could accelerate electrophilic cyclization of
olefinic urea using HIR, although the role of TsNH2 was not studied. For
reference, see: 10a.
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10.1002/anie.201713422
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Lewis base works. A facile and
effective system has been developed
for the regio- and chemoselective
ring-opening/electrophilic
Matthew H. Gieuw, Zhihai Ke,*
Ying-Yeung Yeung*
Page No. – Page No.
functionalization of cyclopropanes
via C–C bond activation by
[bis(trifluoroacetoxy)iodo]benzene
with the aid of the Lewis base
Lewis Base-Promoted Ring-
Opening 1,3-Dioxygenation of
Unactivated
Cyclopropanes
using
Hypervalent Iodine
promoter
p-toluenesulfonamide.
Reagent
Notably, the p-toluenesulfonamide-
promoted system works well for a
wide range of cyclopropanes,
resulting in the formation of 1,3-diol
products in good yields and
regioselectivity.
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