Synthesis, Characterization and Unusual Reactivity of Vinylbenziodoxolones—Novel Hypervalent Iodine Reagents

Article abstract of DOI:10.1002/chem.201603955

A novel type of hypervalent iodine(III) reagents, vinylbenziodoxolones (VBX), has been synthesized in a one-pot reaction from 2-iodobenzoic acid. VBX is bench stable, has been thoroughly characterized and the cyclic structure is supported by X-ray analysis. The reactivity of VBX was investigated in vinylation of nitrocyclohexane, and delivered vinylated products with opposite regioselectivity compared to acyclic vinyl(aryl)iodonium salts. The reagents could become a powerful tool in vinylation reactions under both metal-free and metal-catalyzed conditions.

Full text of DOI:10.1002/chem.201603955

A Journal of
Accepted Article
Title: Synthesis, Characterization and Unusual Reactivity of Vinylbenziodoxolones -
Novel Hypervalent Iodine Reagents
Authors: Elin Stridfeldt; Alexandra Seemann; Marinus J. Bouma; Chandan Dey; An-
ne Ertan; Berit Olofsson
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To be cited as: Chem. Eur. J. 10.1002/chem.201603955
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COMMUNICATION
Synthesis, Characterization and Unusual Reactivity of
Vinylbenziodoxolones – Novel Hypervalent Iodine Reagents
Elin Stridfeldt,[a] Alexandra Seemann,[a] Marinus J. Bouma,[a] Chandan Dey,[a] Anne Ertan[b] and Berit
Olofsson[a]*
Abstract:
A
novel type of hypervalent iodine(III) reagents,
herein we report the first one-pot synthesis, X-ray structure and
preliminary reactivity screen of VBX (Scheme 1b).
vinylbenziodoxolones (VBX), has been synthesized in a one-pot
reaction from 2-iodobenzoic acid. VBX is bench stable, has been
thoroughly characterized and the cyclic structure is supported by X-
ray analysis. The reactivity of VBX was investigated in vinylation of
nitrocyclohexane, and delivered vinylated products with opposite
regioselectivity compared to acyclic vinyl(aryl)iodonium salts. The
reagents could become a powerful tool in vinylation reactions under
both metal-free and metal-catalyzed conditions.
a) Previous Work
Ph
R
S
N3
R
I
O
I
O
R
I
O
NaN3
Ph
SH
20%
O
O
O
57-63%
EBX
R = Me
R = Bu, Hex, Oct, Dec
b) This Work
R
Hypervalent iodine compounds are well-known for their use as
mild and selective oxidants, and have recently proved to be
versatile in a wide variety of other transformations.[1] Iodine(III)
reagents with two carbon ligands are nowadays frequently
employed as group transfer reagents under both metal-
catalyzed and metal-free conditions.[2] One reason for their gain
in popularity is the development of cyclic benziodoxol(on)es,
which tend to be more stable than their acyclic counter-parts.[2b,3]
The use of the benziodoxol(on)e core has enabled the synthesis
of reagents such as trifluoromethylbenziodoxol(on)es (Togni’s
reagents) and azidobenziodoxolones, which are too unstable to
be isolated as acyclic structures.[4] Compounds that are stable
as iodonium salts, such as diaryliodonium salts and
alkynyl(aryl)iodonium salts, have also been obtained as
benziodoxolones.[2b] Ethynylbenziodoxolones (EBX) and cyano-
benziodoxolones have been demonstrated to outperform the
acyclic alkynyl and cyano salts, respectively, in several
applications.[5]
I
I
O
B(OH)2
oxidant, acid
Nu
R
Nu
CO2H
O
VBX
R
Scheme 1. Synthesis and reactivity of VBX.
Vinyl(aryl)iodonium salts are commonly synthesized by coupling
a pre-formed iodine(III) reagent with a vinyl source,[2d] usually
employing (diacetoxyiodo)benzene with vinylboronic acids[10] or
iodosylbenzene with alkenylsilanes.[11] A one-pot procedure from
iodoarenes and terminal alkynes was reported utilizing
FXeOTf,[12] and our group discovered a one-pot synthesis from
iodobenzene, TMS-phenylacetylene and mCPBA.[9h] Benziod-
oxolones are often synthesized in several steps from 2-
iodobenzoic acid, which is oxidized to hydroxybenziodoxolone
and then coupled with a suitable regent.[2b,13] We recently
published one-pot syntheses of both alkynyl salts and
alkynylbenziodoxolones starting from 2-iodobenzoic acid and
either terminal alkynes or alkynylboronic esters.[9b,h]
Vinyl(aryl)iodonium salts have recently been successfully
applied in copper-catalyzed reactions,[6] but the reactivity in
metal-free reactions is rather complex, and the mechanisms
vary with the iodonium salt used.[2d,7] We envisioned better
control of the reactivity in a benziodoxolone structure, as
observed with alkynyl salts vs. EBX. To the best of our
knowledge, vinylbenziodoxolones (VBX) have never been
investigated as reagents. In fact, this compound class has only
been reported as products from addition reactions to
alkynylbenziodoxolones (Scheme 1a).[8] Our group has a long-
term interest in efficient one-pot routes to iodine(III) reagents
from iodoarenes,[9] and investigated this approach to synthesize
the unexplored VBX. The strategy proved to be successful, and
Our envisioned synthesis of VBX was investigated starting
from 2-iodobenzoic acid (1), which was oxidized with mCPBA in
the presence of an acid, and subsequently treated with (E)-
styrylboronic acid (2a, Table 1).[14] To our delight, the reaction
was successful both in the presence of tosic acid and triflic acid,
delivering the acyclic vinyliodonium salts 5a (X= OTs or OTf)
that could be isolated by precipitation in diethyl ether. Salts 5a
were treated with aqueous sodium bicarbonate to yield the
cyclized vinylbenziodoxolone 6a (entries 1-2).[15] The use of
.
BF3 OEt2 only gave minor amounts of product together with
inseparable by-products (entry 3). A 15 min oxidation time (t1)
was sufficient, and longer coupling time (t2) proved indifferent
(entries 4-5).
[a]
[b]
E. Stridfeldt, A. Seemann, Dr. M. J. Bouma, Dr. C. Dey, Prof. B.
Olofsson
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University, SE-106 91 Stockholm, Sweden
E-mail: berit.olofsson@su.se; Website: http://www.organ.su.se/bo/
Dr. A. Ertan
The pinacol ester 3a could be used instead of boronic acid
2a, whereas the silyl reagent 4a only gave traces of product
together with unidentified by-products (entries 6-7). Importantly,
the yield was maintained upon significantly reduced reaction
times t1 and t2 and decreased amount of boronic acid 2a (entries
8-10). Addition of TfOH and 2a at 0 °C slightly increased the
yield of 6a (entry 11).
Department of Materials and Environmental Chemistry, Arrhenius
Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
compared to in 1 (entry 1).[16] Similar shifts of the ipso carbon
have been observed for 1-(phenylethynyl)-1,2-benziodoxol-
3(1H)-one (Ph-EBX) and the acyclic styryl(phenyl)iodonium
tetrafluoroborate 7 (entry 2).[17] The shifts of the vinylic carbons
in 6a are also in agreement with 7 (entry 3)[17b] and the retained
E configuration in 6a is indicated by the coupling constant (J =
15.8 Hz in CDCl3).[14] The cyclic structure of 6a is supported by
the 13C NMR shift of the carbonyl carbon at 166.5 ppm, which is
in agreement with Ph-EBX (entry 4), as well as previously
reported vinylbenziodoxolones.[8a,b] Furthermore, the IR
absorption of the carbonyl in 6a at 1609 cm-1 is in the same
range as known vinyl benziodoxolones[8a,b] and other benziodo-
xolones.[18]
Table 1. Optimization of the synthesis of VBX 6a.[a]
Entry
Acid
t1
2a-4a
(equiv)
t2 (h)
Yield of 6a
Table 2. 13C NMR comparison of 1, 5a, 6a, Ph-EBX and 7 (ppm).
(%)[b]
1
TsOH.H2O
TfOH
17 h
17 h
17 h
2 h
2a (2.0)
2a (2.0)
2a (2.0)
2a (2.0)
2a (2.0)
3a (2.0)
4a (2.0)
2a (2.0)
2a (1.4)
2a (1.1)
2a (1.4)
2
36
69
<10
73
71
59
<5
69
69
60
73
Entry
Solvent
peak
1
5a[a]
6a
Ph-EBX
7
2
2
1
2
CD3OD
CDCl3
C-I
C-I
94.1
94.9
114.9
-
115.5
115.1
-
-
.
116.2[17a]
110.2[17b]
3
BF3 OEt2
2
151.6[17b]
95.8[17b]
4
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
2
154.5
99.7
3
4
CDCl3
CDCl3
C=C
C=O
-
-
-
-
5
2 h
17
17
17
1
171.1
166.5
166.7[17a]
-
6
2 h
[a] Not soluble in CDCl3.
7
2 h
8
15 min
15 min
15 min
15 min
The benziodoxolone structure of VBX 6a was unambiguously
assigned by X-ray crystallography.[19] The crystals were
prepared by layering a solution of 6a in DMSO with diethyl ether
and submitted to single crystal X-ray analysis (Figure 1). The
molecular conformation has a distorted T-shape with an O9-I1-
C10 angle of 165.8°, which is similar to the reported arylbenz-
iodoxolones and alkynylbenziodoxolones.[17a,20] The bond angle
O9-I1-C6 is 72.9° and the C6-I1-C10 angle of 93.7° are also
similar to previous reports, as are the bond lengths between I1-
C6 of 2.121 (Å) and I1-C10 of 2.100 (Å). The I1-O9 bond
distance of 2.51 Å is significantly larger than in alkynylbenz-
iodoxolones, but in similar length as arylbenziodoxolones.[17a,20]
This bond length trend is in agreement with the larger trans
influence exerted by vinyl and aryl groups compared to alkynyl
and trifluoromethyl groups.[3,21]
9
1
10
11[c]
1
1
[a] Reaction conditions: 1 (0.4 mmol), mCPBA and acid in CH2Cl2 (2 mL) were
stirred at RT for time t1. 2a-4a was added and the mixture was stirred at RT for
time t2. The solvent was removed and 5a was precipitated in Et2O. 5a was
then cyclized in CH2Cl2 (2 mL) and saturated NaHCO3 (2 mL) and isolated by
extraction with CH2Cl2. [b] Isolated yields. [c] TfOH and 2a added at 0 °C.
We were pleased to see that the synthesis of VBX 6a could be
performed in a one-pot manner without isolation of 5a (Scheme
2). Significantly, the reaction was easily scaled up to 5 mmol
with maintained yield.
Scheme 2. One-pot synthesis of VBX 6a.
The structure of (E)-styrylbenziodoxolone 6a was compared to
other similar compounds by several analytical methods,
including 13C NMR (Table 2). The iodine(III) oxidation state of 5a
and 6a is in accordance with the 13C NMR shift of the ipso
carbon, which moved considerably downfield in 5a and 6a
Figure 1. X-ray structure of 6a with thermal ellipsoids displayed with 50%
probability. Selected bond lengths: I1-O9 2.510 Å, I1-C6 2.121 Å, I1-C10
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2.100 Å. Selected bond angles: O9-I1-C6 72.9 °, O9-I1-C10 165.8 °, C6-I1-
C10 93.7 °.
With the structure of VBX 6a firmly established, other boronic
acids 2 were utilized in the one-pot sequence to investigate the
scope of the methodology (Scheme 3).[14] Aromatic boronic acids
with different para-substituents were first investigated, and their
electronic properties were found to influence the reaction
substantially. Methyl-substituted reagent VBX 6b was formed in
good yield, whereas the more electron-donating methoxy-
substituted boronic acid proved too reactive under the reaction
conditions, and gave an impure, dark colored product. This
could be avoided by employing tosic acid instead of triflic acid,
delivering VBX 6c in good yield. The corresponding acyclic
reagent 4-methoxystyryl(phenyl)iodonium tetrafluoroborate was
reported to decompose rapidly at room temperature, illustrating
the generally observed stabilizing effect of the benziodoxolone
core.[22] Synthesis of the electron-deficient trifluoromethyl VBX
6d could be effected by heating to 50 °C and prolonged reaction
time. Unfortunately, aliphatic vinyl boronic acids were less
suitable for the developed one-pot conditions, and reactions with
cyclohexylboronic acid provided VBX 6e in moderate yield.
Scheme 4. Ochiai’s vinylation of nitrocyclohexane.[23]
While C-functionalized nitroalkanes are important scaffolds for
further transformations,[24] the reports on C-vinylation of
nitroalkanes are limited and include the use of stoichiometric
amounts of lead reagent,[25] copper-catalyzed carbohalogenation
of alkynes[26] or Michael addition to electron-deficient
acetylenes.[27] We were hence interested to see whether the
VBX reagents would result in chemoselective vinylation and thus
provide a useful synthetic route to C-functionalized nitroalkanes.
Vinylbenziodoxolone 6a and the corresponding stryryl-
(phenyl)iodonium salt 7 were submitted to Ochiai’s conditions,[23]
as detailed in Table 3. Reactions with the vinyl(phenyl)iodonium
salt 7 delivered a product mixture with vinylated product 8 as the
major product, together with minor amounts of the terminal
alkene 9 and the phenylated product 10 (entry 1). To our
surprise, the major product using VBX 6a was instead the
terminal alkene 9 (entry 2). Furthermore, the reaction proceeded
with complete chemoselectivity, i.e. no arylated product 10 was
1
detected by H NMR or GC-MS. The reaction conditions were
briefly screened,[14] to conclude that increased amount of VBX 6a
(entry 3) or prolonged reaction time (entry 4) did not improve the
conversion to 9. On the other hand, conditions similar to those
used in our previously reported arylation of nitrocyclohexane[28]
further increased the product ratio 9:8 and delivered 9 in a
satisfactory yield of 57% (entry 5). Addition of 2 equivalents of
the radical scavenger 1,1-diphenylethylene (DPE) did not affect
the yield, indication that radicals are not involved in product or
byproduct formation.[29]
Table 3. Reactivity difference between VBX 6a and acyclic salts 7.[a]
Scheme 3. Scope of one-pot synthesis of VBX 6. Isolated yields on 0.4 mmol
scale, see SI for details. [a] TsOH instead of TfOH. [b] At 50 °C for 3 h.
All reactions were performed without precautions to avoid air or
moisture. The VBX products are bench stable and can be stored
at room temperature in the dark for at least one month without
signs of decomposition. To further support the benziodoxolone
structures of products 6b-e the intermediate acyclic salts 5b-e
were isolated and analyzed for comparison.[14] All acyclic salts
were prepared in similar yields as the corresponding
benziodoxolones, indicating a quantitative cyclization step.
The reactivity of this novel compound class was
subsequently investigated and compared to known reactivity of
acyclic vinyliodonium salts. Ochiai and co-workers reported the
vinylation of nitrocyclohexane with cyclohexenyl(aryl)iodonium
tetrafluoroborates, which delivered a mixture of vinylated and
arylated nitrocyclohexanes (Scheme 4).[23]
Entry
Solvent
Reagent
(equiv)
t (h)
Yield (%)[b]
8
9
10
1
THF
THF
THF
THF
DME
DME
7 (1.1)
18
18
18
72
72
72
55
12
<5
2
6a (1.1)
6a (2.0)
6a (1.1)
6a (2.0)
6a (2.0)
9
36
37
35
57
57
n.d.
n.d.
n.d.
n.d.
n.d.
3
10
8
4
5[c]
6[c][d]
4
4
[a] Reaction conditions: Nitrocyclohexane (0.24 mmol) and tBuOK (1.1 equiv)
were stirred at RT for 1 h before addition of 6a or 7. [b] 1H NMR yield
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calculated using 1,3,5-trimethoxybenzene as internal standard, n.d. = not
detected. [c] See SI for details. [d] 2 equiv. DPE added.
The change in product distribution indicates a mechanistic
difference between reactions with VBX 6a and acyclic salt 7.
Styryliodonium salt 7 has been used in metal-free reactions to
deliver internal alkenes by thermal solvolysis with acetic
acid.[2d,30] The formation of terminal alkenes has been reported in
halogenations of 7, and was assumed to proceed via addition of
HBr to in situ formed phenylacetylene.[31] This type of
mechanism seems unlikely in reactions with nitrocyclohexane,
and we are currently investigating the mechanism of the reaction.
[3]
[4]
[5]
[6]
In conclusion,
a straightforward one-pot synthesis of
vinylbenziodoxolones (VBX) has been developed. The cyclic
structure is supported by X-ray analysis of the styrylbenz-
iodoxolone 6a. A preliminary study into the reactivity of this
novel class of vinylation reagents showed that reaction between
VBX and nitrocyclohexane result in regioselective formation of
terminal alkene 9, which is opposite to the regioselectivity
observed with acyclic vinyliodonium salt 7. The unusual product
distribution indicates a different mechanistic pathway, which is
currently under investigation in our lab. The related ethynyl-
benziodoxolones (EBX) have recently proven superior to acyclic
alkynyliodonium salts in several applications, and the VBX
reagents are currently being explored as powerful reagents in
organic synthesis.
[7]
[8]
[9]
Experimental Section
[10]
[11]
[12]
Synthesis of Vinylbenziodoxolones 6
2-Iodobenzoic acid 1 (99.0 mg, 0.40 mmol, 1 equiv) was added to a
microwave vial. CH2Cl2 (2 mL) was added, followed by mCPBA (0.44
mmol, 1.1 equiv). The mixture was cooled to 0 °C and TfOH (0.40-0.60
mmol, 1-1.5 equiv) was added. The solution was stirred at RT for 15 min,
and then cooled down to 0 °C. The boronic acid 2 (0.56-0.80 mmol, 1.4-
2.0 equiv) was added in one portion and rinsed down with CH2Cl2 (0.5
mL) and the mixture was left to stir at RT or 50 °C for 1-3 h. Saturated
NaHCO3 (2 mL) was then added and the mixture was stirred vigorously
at RT for 1 h. The reaction mixture was diluted with CH2Cl2 and H2O and
the layers were separated. The aqueous phase was extracted with
CH2Cl2 and the combined organic phases were washed with water and
brine. The solution was dried over Na2SO4, filtered and concentrated.
After precipitation with Et2O the formed solid was filtered off and washed
with Et2O to obtain vinylbenziodoxolone 6 as a white solid.
[13]
[14]
[15]
For details, see the Supporting Information.
The cyclization to 6a proved highly efficient despite that
vinyl(aryl)iodonium salts are known to be easily deprotonated even
by weak bases, see: M. Ochiai, K. Uemura, Y. Masaki, J. Am.
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[16]
[17]
[18]
[19]
CCDC 1481493 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre.
[20]
[21]
M. S. Yusubov, R. Y. Yusubova, V. N. Nemykin, V. V. Zhdankin, J.
Org. Chem. 2013, 78, 3767-3773.
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Iris R. M. Tébéka is kindly acknowledged for early work on
synthesis of acyclic vinyliodonium salts. The Swedish Research
Council (2011-3608, 2015-04404) and Carl Trygger Foundation
(CTS 11:343, 14:359) are acknowledged for financial support.
[22]
[23]
[24]
[25]
Keywords: benziodoxolones • hypervalent iodine • structure
analysis • synthetic methods • vinylation
[26]
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Chemistry - A European Journal
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Chemistry - A European Journal
10.1002/chem.201603955
COMMUNICATION
Entry for the Table of Contents
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Elin Stridfeldt, Alexandra Seemann,
Marinus J. Bouma, Chandan Dey, Anne
Ertan and Berit Olofsson*
Page No. – Page No.
Synthesis, Characterization and
Unusual Reactivity of Vinylbenz-
iodoxolones – Novel Hypervalent
Iodine Reagents
Novel vinylation reagent: Vinylbenziodoxolones (VBX) are stable hypervalent
iodine(III) reagents that are efficiently synthesized in a one-pot reaction. The cyclic
structure is supported by X-ray analysis. The reactivity of VBX was investigated in
C-vinylation of nitrocyclohexane, and resulted in opposite regioselectivity compared
to vinyl(aryl)iodonium salts.
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