Sigmatropic Rearrangements of Hypervalent-Iodine-Tethered Intermediates for the Synthesis of Biaryls

Article abstract of DOI:10.1002/anie.201801132

Metal-free dehydrogenative couplings of aryliodanes with phenols to afford 2-hydroxy-2′-iodobiaryls have been developed. This reaction proceeds through ligand exchange on the hypervalent iodine atom followed by a [3,3] sigmatropic rearrangement and with complete regioselectivity. This coupling, in combination with in situ oxidation by mCPBA, enables the convenient conversion of iodoarenes into desirable biaryls. The obtained biaryls have convertible iodo and hydroxy groups in close proximity, and are thus synthetically useful, as exemplified by the controlled syntheses of π-extended furans and a substituted [5]helicene. DFT calculations clearly revealed that the rearrangement is sigmatropic, with C?C bond formation and I?O bond cleavage proceeding in a concerted manner. Acetic acid, which was found to be the best solvent for this protocol, renders the iodine atom more cationic and thus accelerates the sigmatropic rearrangement.

Full text of DOI:10.1002/anie.201801132

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Synthesis of Biaryls via Hypervalent Iodine-Tethered Sigmatropic
Rearrangement
Authors: Mitsuki Hori, Jing-Dong Guo, Tomoyuki Yanagi, Keisuke Nogi,
Takahiro Sasamori, and Hideki Yorimitsu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201801132
Angew. Chem. 10.1002/ange.201801132
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10.1002/anie.201801132
Angewandte Chemie International Edition
COMMUNICATION
Synthesis of Biaryls via Hypervalent Iodine-Tethered Sigmatropic
Rearrangement
Mitsuki Hori,[a] Jing-Dong Guo,[b] Tomoyuki Yanagi,[a] Keisuke Nogi,[a] Takahiro Sasamori,[c] and Hideki
Yorimitsu*[a]
Dedication
Abstract: Metal-free dehydrogenative coupling of aryliodanes with
Pummerer-type activation. Sulfonium A would undergo charge-
accelerated [3,3] sigmatropic rearrangement[7] to eventually afford
phenols to afford 2-hydroxy-2’-iodobiaryls has been developed. This
reaction proceeds via ligand exchange on the hypervalent iodine atom, 2,2'-difunctionalized biaryls in a regioselective fashion. Although
followed by [3,3] sigmatropic rearrangement, to realize exclusive
regioselectivity. This coupling, in combination with in situ oxidation by
mCPBA, facilitates the convenient conversion of iodoarenes into the
desired biaryls. The obtained biaryls have convertible iodo and
hydroxy groups in close proximity, and are thus synthetically useful,
as exemplified by the controlled syntheses of π-expanded furans and
a substituted [5] helicene. DFT calculations clearly revealed that the
rearrangement is sigmatropic and involves C–C bond formation and
I–O bond cleavage in a concerted manner. Acetic acid, which was
found to be the best solvent for this protocol, would make the iodine
atom more cationic and thus accelerate the sigmatropic
rearrangement.
this would be an attractive method for the metal-free synthesis of
biaryls,[8] the synthetic utility of the resulting sulfanyl-substituted
biaryls is limited because transformations of C(sp2)–S bonds are
less explored[9] as compared to those of carbon–halogen bonds.
A direct solution to the aforementioned problem can be the
use of aryliodanes, instead of aryl sulfoxides, for the synthesis of
more versatile 2’-iodo-2-hydroxybiaryls via intermediate
B
(Scheme 1b). Hypervalent iodine compounds including
aryliodanes have been employed as useful and environmentally
friendly oxidizing reagents, while reduced iodoarene moieties are
generated as “byproducts”.[10] Although various transformations
such as arylation[11] and alkynylation[12] of nucleophiles with
hypervalent iodine reagents have been reported, these reactions
proceed at the C–I bonds of the iodine reagents; thus, the iodo
moiety is not incorporated into the products. On the other hand,
some encouraging precedents involving iodonio-sigmatropic
rearrangement have been developed.[13,14] In 1988, Oh found that
the reactions of aryliodanes with allylsilanes gave the
corresponding ortho-allyl iodoarenes via allylaryliodanes.13a Later,
in an analogous fashion, Ochiai developed ortho- propargylation
of aryliodanes with propargylsilanes.[13b,c] β-Dicarbonyl
compounds also underwent iodonio-sigmatropic rearrangement
with aryliodanes to afford the α-arylated products via
aryl(vinyloxy)iodanes.[14]
Biaryls are privileged structures in various areas of organic
chemistry and in industrial applications. Among numerous
methods for the synthesis of biaryls,[1] transition-metal-catalyzed
cross-coupling of aryl halides with organometallic reagents is the
most reliable. In the last decade, catalytic dehydrogenative C–
H/C–H coupling of two aromatic compounds emerged as another
promising strategy from the viewpoints of atom and step economy
and extensively studied.[2] However, in this method, high catalyst
loadings are often required to achieve high efficiency. In addition,
contaminations of products by heavy metals would be a fatal
problem in research on bioactive agents as well as organic
electronic devices.[3] Transition-metal-free dehydrogenative C–
H/C–H cross-coupling has been actively investigated as an ideal
strategy to overcome these drawbacks.[4]
Previous work
Me
O
Ar1
Ar1
SMe
OH
S
Me
S
(CF3CO)2O
– CF3CO2H
Ar1
(a)
– H
O
Recently we developed metal-free C–H/C–H cross-coupling
of aryl sulfoxides with phenols in the presence of trifluoroacetic
anhydride (Scheme 1a).[5,6] The key intermediate of this reaction
is transiently S–O-linked sulfonium A, generated through
Ar2
Ar2
OH
Ar2
A
This work
OAc
Ar1
I
Ar1
I
OAc
I
OH
O
Ar1
(b)
– AcOH
– AcOH
Ar2
OAc
Ar2
B
[a]
M. Hori, T. Yanagi, Dr. K. Nogi, Prof. Dr. H. Yorimitsu
Department of Chemistry, Graduate School of Science,
Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: yori@kuchem.kyoto-u.ac.jp
Scheme 1. Dehydrogenative biaryl synthesis tethered by heteroatom in high
oxidation state
As a model reaction to confirm our hypothesis, we selected
dehydrogenative coupling of 2-naphthol (1a) with 2-
(diacetoxyiodo)naphthalene (2a). In our first trial, simple mixing of
1a with 2a in CH2Cl2 provided the desired coupling product 3aa in
38% NMR yield, along with many unidentified byproducts (Table
S1, entry 2). After extensive screening of solvents, acetic acid
(AcOH) was found to be optimal , and 3aa was obtained in 70%
NMR yield (Table S1, entry 10). It is worth noting that the reaction
in the presence of TEMPO or in the dark had a negligible effect
on the reaction efficiency (Table S1, entries 11 and 12), which
[b]
[c]
Dr. J.–D. Guo
Institute for Chemical Research,
Kyoto University, Gokasho, Uji, Kyoto 611-0011 (Japan)
Prof. Dr. T. Sasamori
Graduate School of Natural Sciences,
Nagoya City University, Yamanohata 1, Mizuho-cho, Mizuho-ku,
Nagoya, Aichi 467-8501(Japan)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201801132
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COMMUNICATION
R
eliminated the possibility of intermediacy of radical species via
single-electron oxidation.[4,15]
I
1.5 equiv mCPBA
2.0 equiv 1a or 1f
OH
I
Ar
AcOH, 40 °C, 3 h
AcOH, 25 °C, 2 h
Under the optimal reaction conditions, 3aa was isolated in
64% yield (Scheme 2). 6-Bromo-2-hydroxynaphthalene (1b)
reacted smoothly to yield 3ba. The reaction of 2-hydroxy-7-
methoxynaphthalene (1c) furnished the corresponding binaphthyl
3ca in moderate yield. However, its 6-methoxy isomer afforded a
complicated mixture probably because cooperative electron
donation from the 6-methoxy and 2-hydroxy groups through
conjugation prioritized undesired oxidation of the more electron-
rich naphthalene core.[16] Electron-withdrawing groups such as
formyl and cyano groups were tolerated under the optimized
conditions and binaphthyls 3da–ga were obtained in moderate to
good yields.
4
Ar
3aa–ai
OH
OH
I
OH
I
OH
I
I
MeO
I
R
R
3aa: 60%[a]
3ab: 43%[b]
OMe
3ae (R = H):
3ac (R = H): 49%[a] 3af (R = OMe): 0%
3ad (R = Br): 39% 3ag (R = NO2): 0%
7%[a]
CO2Me
OH
CO2Me
OH
OH
I
I
+
+
OH
I
R
MeO
I
AcO
TIPSO
I
OH
I
R
+
OAc
OMe
OTIPS
3ai
AcOH
25 °C, 2 h
OH
3fh
3fh'
3ai'
60% (5.0:1)
57% (4.7:1)
1a–g
2a
1.2 equiv
Scheme 3. Scope of iodoarenes. [a] NMR yield. [b] 2 equiv of mCPBA.
tBu
3aa–ga
tBu
tBu
Br
OH
NC
OH
OH
I
OH MeO
OH
OH
1h
AcO
(a)
+
O
+
3aa: 64%
3ba: 50%
3ca: 50%
CO2Me
3da: 55%
I
OHC
CONHPh
OAc
3ha: 21%
path a
5ha: 20%
2a
OH
OH
OH
simple two-fold
aromatization
– HI
3ea: 65%
3fa: 67%
3ga: 56%
Scheme 2. Scope of naphthols.
tBu
tBu
tBu
We then attempted to explore the scope of aryliodanes.
However, preparation of aryliodanes was often troublesome
because of their instability and difficulty in purification. We thus
attempted to develop a protocol involving oxidation of iodoarenes
and subsequent dehydrogenative coupling of naphthols with the
aryliodanes generated in situ. Gratifyingly, oxidation of 2-
iodonaphthalene (4a) with dry m-chloroperbenzoic acid (mCPBA),
followed by treatment of 2-naphthol (1a), afforded 3aa in 60%
overall yield (Scheme 3). Using this convenient procedure, we
could investigate the scope of iodoarenes 4. Even when a larger
amount of mCPBA was used, 2,6-diiodonaphthalene (4b)
underwent selective monoarylation to yield binaphthyl 3ab as the
sole product.[17] The reaction was not limited to iodonaphthalene
derivatives; 3,5-dimethoxyiodobenzene (4c) and 2-bromo-3,5-
dimethoxyiodobenzene (4d) also underwent the dehydrogenative
coupling to afford 3ac[18] and 3ad, respectively. On the other hand,
the reaction with iodobenzene (4e) gave the product 3ae in only
7% yield. When 4-methoxyiodobenzene (4f) was employed, the
desired product 3af was not obtained. In this case, ipso
substitution at the C–I bond of 4f with 2-naphthol (1a) proceeded,
resulting in C–C bond formation with loss of the iodo moiety to
afford 1-(4-methoxyphenyl)-2-naphthol in 59% yield.[19] The
electron-deficient 4-nitroiodobenzene (4g) did not undergo this
reaction and was almost completely recovered. 3-Iodoanisole
(4h) reacted with naphthol 1f regioselectively to provide a mixture
of 3fh and 3fh’ in favor of less crowded 3fh. Good selectivity was
observed in the reaction of triisopropylsilyloxy-substituted 1i.
O
O
O
H
H
path b
I
I
I
OAc
OAc
8
6
7
OH
I
OH
I
1a
+
O
+
(b)
conditions in
Scheme 3
4j
3aj: 36%
5aj: 20%
Scheme 4. Cases where two aromatization processes compete.
Not only naphthols (as shown in Scheme 2) but also phenol
derivatives participated in the C–C bond forming process.[20]
When 4-tert-butylphenol (1h) was reacted with 2a, a mixture of
the expected biaryl 3ha and benzonaphthofuran 5ha was
obtained (Scheme 4a).[21] The formation of the mixture provides
important information, that is, difference in the rates of
rearomatization into a phenolic ring and an iodonaphthalene ring.
After the [3,3] sigmatropic rearrangement of intermediate 6,
doubly dearomatized intermediate 7 can go through one of the
two pathways: simple double rearomatization to afford biaryl 3ha
(path a) or preferential rearomatization into the more aromatic
benzene ring over that into the less aromatic naphthalene,
inducing intramolecular nucleophilic attack to yield 8 (path b).
Subsequent elimination of HI would furnish benzonaphthofuran
5ha. A similar trend was observed in the reaction of 9-
iodophenanthrene (4j) with 1a, wherein aromatizations into the
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10.1002/anie.201801132
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NC
more aromatic naphthalene and less aromatic phenanthrene
competed (Scheme 4b). These observations are supportive of our
proposed mechanistic scheme involving iodonio-sigmatropic
rearrangement.
OTf
1) Tf2O, pyridine
tBu
3da
2) 4-tBuC6H4B(OH)2
K2CO3
cat. Pd(PPh3)2Cl2
9: 87%
(from 3da)
We carried out DFT calculations to investigate the reaction
mechanism further. The coupling of 1a with 2a was chosen as a
computational model reaction (Figure 1). As expected in Scheme
1, we could find a reaction pathway involving the formation of a
transient I–O bond and subsequent [3,3] sigmatropic
rearrangement. As the first step, the ligand exchange on the
iodine atom was calculated to occur from complex INT1 with
concomitant intramolecular deprotonation of the naphthol by the
leaving acetoxy group (TS1). The calculated activation barrier to
the ligand exchange is 13.3 kcal/mol, reflecting smooth
deprotonation via a six-membered cyclic transition state. The
subsequent rearrangement from INT2 to INT3[22] was calculated
to be sigmatropic: C1–C2 bond formation and I–O5 bond
NC
tBu
hν
benzene
10: 33%
Scheme 6. Application to the synthesis of dorsally benzo-fused [5] helicene.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Numbers
JP16H01149 and JP25107002 as well as JST ACT-C Grant
Number JPMJCR12ZE, Japan.
cleavage occur in
a
concerted manner via TS2. The
rearrangement computationally proceeds over a rather high
activation barrier of 20.4 kcal/mol. Considering that the reaction
proceeded very smoothly at room temperature, we conducted
calculations for the rearrangement by adding another explicit
molecule of acetic acid (AcOH) to simulate solvation by hydrogen
bonding. Notably, the activation energy for the sigmatropic
rearrangement from AcOH-coordinating INT2’ via AcOH-
coordinating TS2’ was calculated to be significantly lower (10.5
kcal/mol).[23] At the transition state TS2’, the acetoxy ligand on the
iodine interacts with the proton of the externally added AcOH to
render the iodine center more electron-deficient. As Maulide[7] and
Shafir[14c,d,24] proposed that a cationic charge at the heteroatom
center would accelerate sigmatropic rearrangement, we presume
that acetic acid would solvate the acetato ligand on iodine to
accelerate the sigmatropic rearrangement.
Keywords: biaryl • hypervalent iodine compound • sigmatropic
rearrangement • aromaticity • DFT calculations
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The synthetic utility of 2-hydroxy-2’-iodobiaryls 3 was next
verified. Owing to the high reactivity of their C–I bonds, biaryls 3
underwent intramolecular SNAr cyclization under basic conditions
to afford dinaphthofurans 5aa, 5ca, and 5ai in excellent yields
(Scheme 5). As another derivatization, we attempted the
controlled synthesis of a [5] helicene derivative with a specific
substitution pattern by taking advantage of the different
reactivities of the C–I and C–O bonds (Scheme 6). Triflation of
3da and subsequent iodo-selective Suzuki-Miyaura coupling
afforded 9 in 87% yield. Photo-induced ring closure[25] of 9
furnished dorsally benzo-fused [5] helicene 10.
Ar1
Ar1
[5]
[6]
OH
I
KOH or Cs2CO3
O
DMSO, 110-120 °C, 12 h
Ar2
Ar2
3
5
[7]
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MeO
O
O
O
5aa: 91%
5ca: 79%
5aj: 82%
Scheme 5. Application to the synthesis of π-expanded furans.
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careful computational analyses (See the relevant section and Figures
S2–S4 in Supporting Information for details).
[23] An extremely high activation barrier (37.9 kcal/mol) was calculated for
the possible C–C bond formation between the 3-position of 1a and the 1-
position of 2a (See Supporting Information for details).
[24] A. Shafir, Tetrahedron Lett. 2016, 57, 2673–2682.
[14] a) J. Zhu, A. R. Germain, J. A. Porco, Jr. Angew. Chem. Int. Ed. 2004,
43, 1239–1243; Angew. Chem. 2004, 116, 1259–1263; b) Z. Jia, E.
Gálves, R. M. Sebastián, R. Pleixats, Á. Álvarez-Larena, E. Martin, A.
[25] Y. Kurata , S. Ots uk a, N. Fuk ui, K. Nogi , H. Yori mits u, A. Os uk a, Org. Lett.
2017, 19, 1274–1277.
Me
‡
Me
∆G (kcal/mol)
O
O
O
O
O
H
Me
H
O
I
Me
O
O
O5
I
O
O5
O
‡
Me
C3
C2
O
O
C1
C2
O
Me
C1
I
H
O
O
O5
O
O
Me
I
20.6
TS2
Me
O
Me
O
Me
I
O
O
H
O
14.3
O
O
H
INT3
O5
O
O5
11.9
11.9
TS1
TS2’
8.2
‡
INT3’
AcO
Me
O
H
O
Me
AcO
O
1.4
H
H
0.2
INT2
Me
O
O
Me
0.0
O
–1.4
I
H
O
O
O
INT2’
O5
I
O5
O
1a
+
2a
INT1
C3
One additional AcOH to coordinate
(involved for modeling solvation)
C1
C2
C1
C2
Figure 1. DFT reaction profile calculated at the level of B3LYP-D3(BJ)/Def2-TZVP (for I)/6-311+G(d,p) (for C, H, O) with PCM (AcOH). One innocent AcOH that
should be included in the pathway from 1a+2a to INT3 to balance the total energies with those in the pathway including solvation is omitted for clarity.
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10.1002/anie.201801132
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
M. Hori, J.-D. Guo, T. Yanagi, K. Nogi, T.
Sasamori, H. Yorimitsu*
Page No. – Page No.
Synthesis of Biaryls via Hypervalent
Iodine-Tethered Sigmatropic
Rearrangement
Dehydrogenative coupling of aryliodanes with phenols affords 2-hydroxy-2’-
iodobiaryls of synthetic utility, proceeding via ligand exchange on the hypervalent
iodine atom followed by [3,3] sigmatropic rearrangement. This coupling, in
combination with in situ oxidation by mCPBA, facilitates the convenient conversion
of iodoarenes into the desired biaryls. DFT calculations clearly justify the reaction
mechanism and reveals acetic acid solvent would accelerate the sigmatropic
rearrangement by hydrogen bonding.
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