Stereocontrolled Synthesis of Halovinylbenziodoxoles by Hydro- and Iodochlorination of Ethynylbenziodoxoles

Article abstract of DOI:10.1002/chem.201901543

We report herein the synthesis of highly substituted and stereochemically well-defined vinylbenziodoxole (VBX) derivatives through hydrochlorination and iodochlorination of ethynylbenziodoxoles. The hydrochlorination is achieved using pyridine hydrochlori

Full text of DOI:10.1002/chem.201901543

A Journal of
Accepted Article
Title: Stereocontrolled Synthesis of Halovinylbenziodoxoles via Hydro-
and Iodochlorination of Ethynylbenziodoxoles
Authors: Junliang Wu, Xiaozhou Deng, and Naohiko Yoshikai
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To be cited as: Chem. Eur. J. 10.1002/chem.201901543
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Stereocontrolled Synthesis of Halovinylbenziodoxoles via Hydro-
and Iodochlorination of Ethynylbenziodoxoles**
Junliang Wu,* Xiaozhou Deng, and Naohiko Yoshikai*
Abstract: We report herein the synthesis of highly substituted and
Herein, we report on stereoselective hydrochlorination and
iodochlorination reactions of EBX, affording stereochemically
well-defined halogenated VBX compounds (Scheme 1c). The
former reaction was achieved using pyridine hydrochloride in an
exclusive anti fashion, affording a useful building block for
stereoselective synthesis of multisubstituted alkenes. The latter
reaction using iodine monochloride features an unusual syn-
addition pathway, affording an exotic tetrasubstituted VBX
product.
stereochemically well-defined vinylbenziodoxole (VBX) derivatives
through
hydrochlorination
and
iodochlorination
of
ethynylbenziodoxoles. The hydrochlorination is achieved using
pyridine hydrochloride as an HCl source in an anti-fashion under
mild, open-air conditions to afford a 2-chlorinated VBX product,
which serves as a useful building block for the stereoselective
synthesis of trisubstituted alkenes. Meanwhile, iodochlorination with
iodine monochloride proceeds in an unusual syn-pathway,
stereoselectively affording a tetrasubstituted VBX derivative.
Hypervalent iodine compounds bearing benziodoxol(on)e (BX)
backbone have emerged as versatile electrophilic group-transfer
agents in organic synthesis, as the cyclic BX moiety endows the
reagents with well-balanced stability and reactivity suited under
broad range of reaction conditions.[1] Among various BX-based
group transfer reagents, ethynyl–BXs (EBXs) have found
numerous applications in C–alkynyl and heteroatom–alkynyl
bond formations since the pioneering work of Waser.[1a-d,2-4]
Meanwhile, this class of compounds has also found unique
synthetic applications beyond simple alkynyl group transfer,
where the BX group does not act as a mere leaving group but as
an integral part of the product.[5] In this context, EBXs have been
found to serve as precursor to functionalized vinyl–BXs (VBXs),
which would serve as novel alkenyl group transfer agents that
are otherwise difficult to access. We disclosed palladium-
catalyzed 1,1-hydrocarboxylation of various substituted EBXs
via 1,2-iodine(III) shift, affording VBXs bearing enol carboxylate
moiety (Scheme 1a).[6,7] Miyake reported anti-addition of phenols
to aryl–EBXs under visible light irradiation (Scheme 1b), where
the primary VBX product readily transforms into iodovinylether.[8]
More recently, Waser has found simple conditions that allow for
the anti-addition of sulfonamides and phenols to alkyl–EBXs with
retention of the BX moiety in the products.[9,10] As highly
functionalized VBX compounds hold promise as versatile
synthetic building blocks for stereodefined alkenes,[11] further
development of EBX-to-VBX transformations is highly desirable.
Scheme 1. Stereocontrolled synthesis of functionalized VBXs from EBXs.
Hydrochlorination of haloalkynes offers straightforward
approach toward synthetically useful 1,2-chlorohaloalkenes. Zhu
and Xu independently achieved anti-hydrochlorination of
haloalkynes with LiCl/HOAc using palladium and gold catalysts,
respectively,[12] which likely involved anti-chlorometalation of the
C≡C bond as a key step.[13] Ochiai reported a transition metal-
free
anti-hydrochlorination
of
alkynyl(phenyl)iodonium
[*]
Dr. J. Wu, X. Deng, Prof. N. Yoshikai
Division of Chemistry and Biological Chemistry, School of Physical
and Mathematical Sciences
Nanyang Technological University
Singapore 637371, Singapore
E-mail: nyoshikai@ntu.edu.sg
Prof. J. Wu
College of Chemistry and Molecular Engineering, Henan Institutes
of Advanced Technology, Zhengzhou University, Zhengzhou
450001, P. R. China
tetrafluoroborate using LiCl/HOAc or HCl/MeOH, while the
scope of the reaction was demonstrated for only a handful
iodonium salts.[14,15] By contrast to these precedents, the present
anti-hydrochlorination, found during our study on the Pd-
catalyzed 1,1-hydrocarboxylation,[6] was achieved under
extremely simple conditions. Thus, the reaction between Ph-
EBX 1a and pyridine hydrochloride (Py•HCl, 2 equiv) proceeded
smoothly in EtOAc at 50 °C under air, affording the VBX product
2a in quantitative yield exclusively as a cis isomer (Table 1,
entry 1). The compound 2a is stable under ambient conditions
and can be purified by routine chromatography with minimal loss.
The stereochemistry of 2a was unambiguously established by X-
E-mail: wujl@zzu.edu.cn
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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ray crystallographic analysis.[16] The reaction also took place in
ethereal solvents (entries 2 and 3) or chlorinated solvents
(entries 4–6), while substantial decrease in the yield was
observed in some cases. We found that the combination of LiCl
(10 equiv) and HOAc solvent was also effective for the anti-
hydrochlorination, albeit in a modest yield. Note that the use of
pyridine hydrobromide caused decomposition of 1a and failed to
be
extended
to
ethynylbenziodoxolone
and
ethynyl(phenyl)iodonium tosylate, affording the desired products
2u and 2v, respectively, in high yields.
Table 2. anti-Hydrochlorination of EBXs.[a]
CF3
CF3
O
R
I
O
Py•HCl (2 equiv)
CF3
CF3
Cl
I
promote
the
desired
hydrobromination,
while
the
EtOAc, 50 °C, 24 h
hydrobromination product was observed when using LiBr and
HOAc (data not shown).
R
H
1
2
Table 1. Effect of solvents on hydrochlorination.[a]
IIII
Cl
IIII
H
Cl
H
R'
R'
2j (R’ = OMe), 91%
2k (R’ = CF3), 80%
2a (R’ = H), 91%[b]
2b (R’ = Br), 73%
2c (R’ = Cl), 89%
2d (R’ = CHO), 86%
2e (R’ = CO2Et), 91%
2f (R’ = CN), 81%
2g (R’ = Me), 93%
2h (R’ = SiMe3), 61%
2i (R’ = OMe), 31%
X-ray (1a)
Entry
Solvent
EtOAc
1,4-dioxane
DME
Yield [%][b]
IIII
IIII
Cl
Cl
1
99 (97)
88
R'
H
H
2
S
3
87
2o, 94%
2l (R’ = F), 68%
2m (R’ = CF3), 58%
2n (R’ = Me), 42%
4
DCE
91
5
CH2Cl2
CHCl3
68
IIII
IIII
IIII
Cl
Cl
Cl
6
54
R
H
BnO
H
H
7[c]
HOAc
70
2p (R = Me), 95%
2q (R = Bu), 93%
2r, 97%
2s, 69%
[a] The reaction was performed using 0.1 mmol of 1a and 0.2 mmol of Py•HCl
in 1 mL solvent at 50 °C for 24 h. [b] Determined by 19F NMR using 1,4-
bis(trifluoromethyl)benzene as an internal standard. The yield of the isolated
product is shown in the parentheses. [c] The reaction was performed using
LiCl (10 equiv) instead of Py•HCl at room temperature.
O
I
O
OTs
Cl
Cl
I
IIII
Cl
IIII
Ph
H
Ph
H
H
2t, 83%[c]
2u, 95%
2v, 93%
Table 2 summarizes the scope of the anti-hydrochlorination
of EBXs. For EBXs derived from
a
variety of
[a] The reaction was performed on a 0.1 mmol scale under the conditions in
Table 1, entry 1. The symbol IIII in the product structure refers to the
benziodoxole moiety. [b] The reaction was performed on a 5 mmol scale. [c] 4
equiv of Py•HCl was used.
(hetero)arylacetylenes, the reaction proceeded smoothly to
afford the desired VBX products 2a–2o in moderate to excellent
yields. A gram-scale reaction of 1a (2.35 g) afforded 2a in
excellent yield (2.30 g, 91%). We observed significant effect of
the substituent on the aryl group on the efficiency of the
hydrochlorination. Thus, electron-withdrawing groups, such as
halogen, aldehyde, ester, and cyano groups, at the para-position
typically led to high yields (see 2b–2f, 73–91%), while EBX
bearing an electron-donating para-methoxy group underwent
substantial decomposition, affording the desired product 2i only
in 31% yield. Aryl–EBXs bearing ortho substituents afforded the
desired products 2l–2n in apparently lower yields (42–68%),
most likely due to the steric effect. 2-Thienyl–EBX afforded the
desired product 2o in excellent yield regardless of the electron-
rich nature of the thienyl group. EBXs derived from
alkylacetylenes and enyne were also amenable to
hydrochlorination, producing the products 2p–2s in good to
excellent yields. Interestingly, a bis-EBX substrate underwent
exclusive hydrochlorination on only one of the alkyne moieties
even in the presence of excess Py•HCl (4 equiv) to afford 2t, for
unknown reasons. Notably, the present hydrochlorination could
Iodochlorination of alkynes with iodine monochloride or
equivalent reagents comprised of electrophilic iodine source and
chloride anion has been explored as an approach to 1,2-
iodochloroalkenes.[17] The reaction typically occurs with anti
selectivity,[18] which can be easily rationalized by a mechanism
involving formation of a cyclic iodonium species and subsequent
anti-addition of a chloride anion. Iodochlorination of haloalkynes
was also achieved using the Barluenga reagent (I(Py)2•BF4) and
LiCl, featuring anti selectivity.[19] Given this background, we were
surprised to find that the reaction of 1a with ICl in CH2Cl2
completed quickly within 30 min under open air to afford the syn-
iodochlorination product 3a in high yield, whose stereochemistry
was unambiguously confirmed by X-ray crystallographic analysis
(Table 3).[16]
A
variety of aryl–EBXs also underwent
iodochlorination to afford the products 3b–3l as single
stereoisomers in moderate to good yields. The reaction was
particularly high-yielding with aryl–EBXs bearing aldehyde and
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cyano groups (3e and 3f), while it became somewhat sluggish
with those bearing fluorine (3d) and methoxy (3g and 3h)
substituents. Unfortunately, alkyl–EBXs decomposed under the
reaction conditions and failed to give the desired products.
Table 3. syn-Iodochlorination of EBXs.[a]
Cl
I
I
R
I
O
ICl (2 equiv)
CF3
CF3
R
CH2Cl2, rt, 30 min
CF3
CF3
O
1
3
Cl
I
Cl
I
IIII
IIII
R'
R'
3h (R’ = OMe), 62%
3i (R’ = F), 80%
X-ray (3a)
3a (R’ = H), 90%
3b (R’ = Br), 72%
3c (R’ = Cl), 72%
3d (R’ = F), 61%
3e (R’ = CHO), 86%
3f (R’ = CN), 90%
3g (R’ = OMe), 60%
Cl
I
Cl
I
R'
IIII
IIII
S
3l, 79%
3j (R’ = F), 79%
3k (R’ = Me), 73%
[a] The reaction was performed on a 0.1 mmol scale. The symbol IIII in the
product structure refers to the benziodoxole moiety.
Scheme 2 illustrates proposed reaction pathways for the
present hydrochlorination and iodochlorination reactions. The
hydrochlorination may proceed through β-addition of a chloride
ion to EBX and subsequent protonation of the resulting vinyl
anion by pyridinium proton (Scheme 2a).[9,14,15] Preliminary DFT
calculations on a model system (B3LYP/SDD for I, 6-31+G(d) for
other atoms) allowed us to locate a transition state of chloride
addition (TS1), which adopts a substantially bent structure and
leads to a vinyl anion CP1 (see the Supporting Information for
more detail). CP1 has cis arrangement of the Cl and BX groups,
and its protonation would afford the anti-hydrochlorination
product. By contrast to the hydrochlorination, it would be
reasonable to assume that the iodochlorination is initiated by
electrophilic activation of EBX by I+ (Scheme 2b). DFT
Scheme 2. Proposed reaction pathways of anti-hydrochlorination and syn-
iodochlorination of EBX, and DFT-optimized structures.
The present hydrochlorination products would serve as
versatile building blocks for the stereoselective synthesis of
multisubstituted alkenes, as illustrated in Scheme 3. The product
2a was readily converted to enyne 4 in a quantitative yield by
Sonogashira coupling. A triarylethene 6 was stereoselectively
prepared by a sequence of Stille and Kumada couplings in good
overall yield. Meanwhile, to date, our attempts on using the
iodochlorination products in the cross-coupling chemistry have
been largely unsuccessful.[20] Nonetheless, further exploration is
ongoing on their potential use as building blocks for
tetrasubstituted alkenes and also as functionalized vinylidene-
type synthons.[19a]
optimization of
a
complex between Ph-EBX and I+
spontaneously converged to CP2, with essentially complete
formation of the new C–I bond (2.08 Å compared to 2.13 Å in the
product) and substantial elongation of the C–IIII bond (2.51 Å
compared to 2.12 Å in the starting material). This barrierless
structural reorganization would be due to the intrinsic weakness
of the C–IIII bond. Thus, CP2 may be better regarded as alkynyl
iodide activated by BX cation rather than EBX activated by I
cation. As such, a chloride anion would approach from the
opposite side of the BX group to avoid steric hindrance, thus
ensuring the syn-iodochlorination stereochemistry.
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In summary, we have reported anti-hydrochlorination and
syn-iodochlorination of EBXs for the synthesis of highly
functionalized VBXs. Both the reactions are achieved using
extremely simple reagents under mild, open-air conditions with
high stereoselectivity. The 2-chloro VBXs synthesized by the
hydrochlorination serve as versatile building blocks for the
stereoselective synthesis of trisubstituted alkenes. The
[13] For Pd-catalyzed hydrochlorination of alkynes, see also: a) J. Derosa, A.
L. Cantu, M. N. Boulous, M. L. O'Duill, J. L. Turnbull, Z. Liu, D. M. De
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a
unique syn-selective addition
pathway, and the utility of the unusual trihalogenated alkene
products is currently under investigation.
[14] M. Ochiai, K. Uemura, K. Oshima, Y. Masaki, M. Kunishima, S. Tani,
Tetrahedron Lett. 1991, 32, 4753-4756.
[15] For
other
examples
of
hydrofunctionalization
of
Acknowledgements
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This work was supported by the Ministry of Education
(Singapore) and Nanyang Technological University (MOE2016-
T2-2-043). We thank Dr. Yongxin Li (Nanyang Technological
University) for his assistance with the X-ray crystallographic
analysis.
Keywords: hypervalent iodine compounds • alkyne •
hydrohalogenation • dihalogenation • cross-coupling
[16] CCDC 1900632 (2a) and 1900633 (3a) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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alkynylation product via cleavage of both the C–I bonds in a modest
yield (45%). Further studies on chemoselective and iterative coupling
are currently underway.
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Entry for the Table of Contents (Please choose one layout)
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Junliang Wu,* Xiaozhou Deng, and
Naohiko Yoshikai*
Page No. – Page No.
Stereocontrolled Synthesis of
Halovinylbenziodoxoles via Hydro-
and Iodochlorination of
We report herein the synthesis of highly substituted and stereochemically well-
defined vinylbenziodoxole (VBX) derivatives through hydrochlorination and
iodochlorination of ethynylbenziodoxole (EBX). The hydrochlorination is achieved
using pyridine hydrochloride as an HCl source in an anti-fashion under mild
conditions, while the iodochlorination with iodine monochloride proceeds in an
unusual syn-pathway.
Ethynylbenziodoxoles
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