Selective aryl radical transfers into N-heteroaromatics from diaryliodonoium salts with trimethoxybenzene auxiliary

Article abstract of DOI:10.3987/COM-16-S(S)90

We have found that a series of trimethoxybenzene-based diaryliodonium(III) salts I (ArI⁺Ar’X^-, where Ar = various aryl groups, Ar’ = 2,4,6-trimethoxyphenyl, X^- = counterion) can exclusively cause Ar-transfers during the base-induced radical couplings with N-heteroaromatic compounds 1 by working the trimethoxybenzene ring (Ar’) as an inert coupling auxiliary. By the treatment with N-heteroaromatics 1 as the solvent, the metal-free arylations utilizing the specific salts I initiated by solid NaOH upon heating selectively produced the corresponding biaryls 2 in good yields without the formation of the trimethoxybenzene (Ar’) coupling product.

Full text of DOI:10.3987/COM-16-S(S)90

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HETEROCYCLES, Vol. 95, No. 2, 2017
HETEROCYCLES, Vol. 95, No. 2, 2017, pp. 1272-1284. © 2017 The Japan Institute of Heterocyclic Chemistry
Received, 1st November, 2016, Accepted, 2nd December, 2016, Published online, 17th February, 2017
DOI: 10.3987/COM-16-S(S)90
SELECTIVE
ARYL
RADICAL
TRANSFERS
INTO
N-HETEROAROMATICS FROM DIARYLIODONOIUM SALTS WITH
TRIMETHOXYBENZENE AUXILIARY
Toshifumi Dohi,a Shohei Ueda,a Akiko Hirai,a Yusuke Kojima,a Koji
Morimoto,a and Yasuyuki Kitab,*
a College of Pharmaceutical Sciences, Ritsumeikan University, 1-1-1 Nojihigashi,
Kusatsu, Shiga, 525-8577, Japan
b
Research Organization of Science and Technology, Ritsumeikan University,
1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577, Japan
*Corresponding author. tel.: +81-77-561-5829; fax: +81-77-561-5829; e-mail:
kita@ph.ritsumei.ac.jp (Y. Kita)ꢀ
Abstract – We have found that a series of trimethoxybenzene-based diaryl-
iodonium(III) salts I (ArI+Ar’X-, where Ar = various aryl groups, Ar’ = 2,4,6-
trimethoxyphenyl, X- = counterion) can exclusively cause Ar-transfers during the
base-induced radical couplings with N-heteroaromatic compounds 1 by working
the trimethoxybenzene ring (Ar’) as an inert coupling auxiliary. By the treatment
with N-heteroaromatics 1 as the solvent, the metal-free arylations utilizing the
specific salts I initiated by solid NaOH upon heating selectively produced the
corresponding biaryls 2 in good yields without the formation of the trimethoxy-
benzene (Ar’) coupling product.
In Celebration of Professor Masakatsu Shibasaki on His 70th Birthday
With respect to the recent green chemistry aspects, the metal-free advances that can avoid the use of
transition metal catalysts and organometallic compounds in the conventional methodologies are strongly
required in the modern coupling reaction developments.1 Hypervalent iodine reagents are now widely
accepted as a safe alternative to toxic heavy-metal oxidants for developing environmentally benign
oxidations and has recently become a significant tool for reproducing the synthetic transformations that
are conventionally performed with rare metal catalysts.2 One of the most significant contributions to this
area is our oxidative cross-coupling strategy for aromatic compounds discovered in recent years. In 2008,

HETEROCYCLES, Vol. 95, No. 2, 2017
1273
we first effectively realized the metal-free oxidative C-H cross-coupling between unfunctionalized
aromatic substrates, naphthalenes and alkylbenzenes, using a hypervalent iodine-induced single electron
transfer (SET) oxidation strategy into the naphthalene rings.3 An exciting new metal-free coupling
alternative for the mixed biaryl synthesis of heteroaromatic compounds was then developed in 2009 by
pioneering the novel reactivities of the σ-heteroaryliodonium(III) salts generated in situ by the treatment
of a hypervalent iodine reagent with various heteroaromatic compounds.4 Due to their advantage for
avoiding the use of metal catalysts and metal waste generations, such an oxidative aryl coupling strategy
for making a new C-C bond directly from the C-H groups under metal-free conditions is a promising
greener alternative to the classical methods.5,6
One of the important and well-investigated intermediates for developing new coupling reactions in recent
studies is the diaryliodonium(III) salt, which have two aryl groups bound to a hypervalent iodine(III)
center, having significant applications as highly reactive arylating agent that even allows the metal-free
couplings by the formal SNAr processes.7 In order to achieve such effective couplings, pioneering a new
aromatic ring activation strategy was essential together with the appropriate design of the iodonium salts
for the reactions. In our continuous interest for the metal-free aryl-aryl coupling reactions initiated by
Lewis acids,8 the chemical behaviors and aryl transfer abilities of extensive diaryliodonium salts under
acidic conditions have been revealed in recent years.9 However, controlling the aryl group transfer
reactivities of the diaryliodonium salts under basic conditions has rarely been reported to date, though
there are presently some interesting reports for the metal-free aryl couplings initiated by the base
activators.10 In this paper, we report the new reactivities of the trimethoxybenzene-based diaryliodonium
salts I as a highly selective entry for the base-induced radical couplings with N-heteroaromatics 1, such as
pyrrole, pyridine, and other related compounds (Scheme 1).
new iodonium salts
MeO
X
X
Ar
I
OMe
Ar
N
H
N
H
TfO
MeO
I (Ar = aryl)
X
X
Y
Z
Y
Z
Ar
NaOH, 
N
N
1
2
selective aryl (Ar) transfer
(solvent)
arylated product
X, Y, Z = C or N
Scheme 1. Metal-free aryl coupling of N-heteroaromatics 1 by diaryliodonium(III) salts I
having non-transferring trimethoxybenzene group

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HETEROCYCLES, Vol. 95, No. 2, 2017
In 2012, Zhang, Yu, and coworkers revealed that upon heating of the diaryliodonium salts, pyrroles and
other six-membered N-heteroarenes can cause arylations via some radical mechanism without the use of
any metal catalyst when treating these N-heteroaromatics as a solvent.10a Unfortunately, when using the
diaryliodonium salt having two different aryl groups (Ar1I+Ar2X-, X- = counterion), the coupling events
were uncontrolled regarding transferring of the aryl groups from the iodonium salts, and mixture of the
Ar1 and Ar2 coupling products were produced in the study. For example, the NaOH-promoted coupling
o
reaction of pyrrole 1a as a solvent with (4-anisyl)phenyliodonium triflate at 80 C gave a mixture of the
two arylated products, 2-phenylpyrrole 2aa and 2-(4-anisyl)pyrrole, in 33% and 13% yields, respectively
(Scheme 2, above), in which the less electron-rich phenyl ring seems to be favorable for the aryl transfer.
In our continuous reactivity studies of the diaryliodonium salts attached to electron-rich aryl groups,9 we
have examined various alternative iodonium salts in our library for this transformation, and the salt Ia
having the trimethoxybenzene group11 was found to be optimal for the effective phenylation suppressing
the random aryl transfers of conventional diaryliodonium salts. The result is that instead using the specific
salt Ia as an aryl source for the same treatment of the pyrrole 1a, 2-phenylpyrrole 2aa was smoothly
obtained as a single arylation product in the somewhat higher yield of 65% (Scheme 2, below). Gratefully,
we now report a product as a result of the coupling with the trimethoxybenzene group in the salt Ia was
1
not detected during the reaction (<1% formation by H NMR analysis of the crude reaction mixture).
Scheme 2. Arylation of pyrrole 1a: Aryl transfer selectivities by conventional salt versus new Ia

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1275
Although the significant utility of this type trimethoxybenzene auxiliary for directing the courses of the
aryl transfers have become recognized in recent few transformations,12 its general applications in the
metal-free couplings are rare and this is the first example revealing its non-transferring ability during the
aryl radical coupling process to furnish the biaryl product.
Strongly encouraged by the new and unique aryl transfer reactivity of the (trimethoxybenzene)iodonium
salt Ia in the arylation, we then examined other representative N-heteroaromatic substrates. For the
imidazole 1b, pyridine 1c, and other six-membered N-heteroaromatics 1d-f, comparable results were
obtained in regard to the selectivity of the phenyl transfer from the salt Ia to the heteroaromatic rings
(Table 1). Since N-heteroaromatic substrates 1b-f were employed as the solvent, the yields of the products
2ba-fa were calculated based on the stoichiometries of the salt Ia used for the reactions in these cases.
The yield of the products changed in some cases in comparison to the pyrrole 1a, and the imidazole 1b
reacted to give the phenylation product 2ba in a slightly decreased yield, that is 44% (entry 1), while the
Table 1. N-Heteroaromatics 1b-f for the selective phenyl-transfer reactions using the salt Iaa
entry
1
substrate
phenylation product (yield)b
1b
1c
2ba (44%)
2
3
2ca (60%, 2-:3-:4- = 32:46:22)
2da (86%)
1d
1e
4
2ea (66%, 2-;4-:5- = 24:60:16)
5c
2fa (67%, 3-:4- = 9:91)
1f
a Reactions were performed using these heteroaromatics 1b-f as a solvent (1 mL) at 110 oC for 10 h in the
presence of the iodonium salt Ia (0.2 mmol) and solid sodium hydroxide (1.5 mmol).
1
b Isolated yield after column chromatography. The regioisomeric ratios were then determined by H
NMR.
c Reaction was progressed at 80 oC.

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pyrazine 1d smoothly produced 2-phenylpyrazine 2da in a good yield (entry 3). The phenylation
selectively occurred at the 2-positions of the imidazole 1b and pyrazine 1d as those previously reported.10
Meanwhile, pyridine 1c and pyridazine 1e were reacted under the same conditions producing the
regiomixture of the phenylated products 2ca and 2ea in a ratio of 2-:3-:4- = 32:46:22 for pyridine 1c
(entry 2) and 2-;4-:5- = 24:60:16 for pyridazine 1e (entry 4). It seems that these results by using the
iodonium salt Ia basically match with the similar results of the phenyl radical generations13 and their
involvement in the radical substitution processes toward the N-heteroaromatic rings. However, this time,
the iodonium salt Ia showed a somehow different outcome of the regio-preferences at the reacting ring
positions for the phenylations when compared to the conventional iodonium salts employed in the early
studies.10 Specifically, the meta-phenylation for pyridine 1c and 4-phenylation for pyrimidine 1e are
favored when using the salt Ia, and the major products were the 3-phenylpyridine 2ca (3-phenyl) and
4-phenylpyrimidine 2ea (4-phenyl). In addition, pyridazine 1f, which is known in aryl radical coupling
processes to give a mixture of 3- and 4-phenylated regioisomers, caused phenylation almost totally at the
4-position and mostly produced 4-phenylpyridazine 2fa in 58% yield as the major product (entry 5).
The unique direction of the trimethoxybenzene group in the aryl transfer is generally for the group of
iodonium salts Ib-f, and no coupling product incorporating the trimethoxybenzene ring was detected in
all the examples shown in Scheme 3. For the coupling with pyrazine 1d, the reaction with the iodonium
salt Ic even proceeded for the introduction of the benzene ring having an electron-donating methoxy
group, and 2-(4-anisyl)pyrazine 2dc was obtained in 40% yield (Eq. 1). Such an electron-rich aryl group
transfer is known to scarcely occur when using the conventional diaryliodonium salts,10 which would
suggest not only the high utility of the iodonium salt Ic over others in this metal-free arylation process,
but also some positive contribution of the trimethoxybenzene group to the reactivity of the salt Ic for
more effectively producing the arylation product 2dc. Also, the benzene ring having an electron-
withdrawing substituent, such as the trifluoromethyl group, could react with the pyrazine 1d to give 2-(4-
trifluoromethylphenyl)pyrazine 2dc as the sole product, albeit in a lower yield compared to the
introduction of the phenyl group by the salt Ia. In addition, the arylation also proceeded using the
iodonium salt Ie having a heteroaromatic group, such as the thiophene ring (Eq. 2). It is interesting to
note that, however, the sterically-congested 2,4-dimethylphenyl and mesityl groups in the salts If and Ig,
which are known to be highly reactive in other couplings with anionic nucleophiles,14 were found not to
cause effective transfer (Eq. 3). This is in good agreement with the facts of the non-transferring ability of
the similarly-substituted trimethoxybenzene ring in the salts I during the base-induced arylating strategy
and of the apparent involvement of a different mechanism from that of other arylating methods for
anionic nucleophiles,14 which thus might give some positive support for the postulated aryl radical
mechanisms in literatures.10

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1277
Scheme 3. Reactions of pyrazine 1d with a set of diaryliodonium(III) salts Ib-g:
The transferring aryl groups
One of the advantages in using the diaryliodonium salts I bearing the trimethoxybenzene group lies in
their easy preparative operations,11,12 and the specific aryl group-transfer abilities of the (trimethoxy-
benzene)iodonium salts I can now allow the generation of the salts and their use in the base-induced
couplings to occur in a one-pot manner. For example, 2-phenylpyrazine 2da was also obtained starting

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from the iodosobenzene (PhIO) by treatment with equimolar amounts of triflic acid (TfOH) and the
trimethoxybenzene in dichloromethane11 followed by thermal decomposition of the formed iodonium salt
Ia for the aryl coupling after replacement of the solvent by pyrazine 1d and then the addition of NaOH
(Scheme 4). The smooth formation of the product 2da corresponds to the high-yield formation of the
iodonium salt Ia in its preparative step.
Scheme 4. One-Pot diaryliodonium salt formation and aryl coupling
In summary, we have newly found in this study a series of promising diaryliodoum salts I having the
trimethoxybenzene group (ArI+Ar’X-, where Ar = various aryl groups, Ar’ = 2,4,6-trimethoxyphenyl, X-
= counterion) that are promising for strongly directing the aryl transfer abilities of the salts during the
base-induced radical arylations toward the N-heteroaromatics 1. As a result of the exclusive aryl transfer
control for the Ar groups over the introduced trimethoxybenzene during the aryl radical processes, the
new iodonium salts I can expect to give the selective productions of the biaryls 2 and avoid
contamination of the undesired arylation products as conventional problems by the competitive aryl
transfers from the iodonium salts having two different aryl ligands. With such excellent and unique
arylation characteristics now revealed, further applications of the specific iodonium salts I in more
extensive metal-free coupling processes are currently being investigated in our laboratory, the results of
which will be reported in due course.
EXPERIMENTAL
Melting point (mp) is uncorrected. All 1H- and 13C-NMR spectra of the products were measured in CDCl3
13
o
1
by spectrometers operating at 400 MHz (100 MHz for C NMR) at 25 C. Chemical shifts of H-NMR
were recorded in parts per million (ppm, G) relative to tetramethylsilane (G = 0.00 ppm) as an internal
standard. Data are reported as follows: chemical shift in ppm (G), multiplicity (s = singlet, d = doublet, t =
triplet, q = quartet, brs = broad singlet, m = multiplet), coupling constant (J) in Hz, and integration.

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13
Chemical shifts of C-NMR were reported in ppm with the solvent as reference peak (CDCl3: G = 77.0
ppm). Absorptions of infrared spectra (IR) are reported in reciprocal centimeters (cm-1) for representative
peaks. Flash column chromatography was performed with Merck Silica Gel 60 (230-400 mesh) eluting
with hexane and ethyl acetate for isolation of the products. Analytical thin-layer chromatography (TLC)
was carried out on Merck Silica Gel F254 plates (0.25 mm). The spots and bands were detected by UV
light of irradiation (254, 365 nm) and/or by staining with 5% phosphomolybdic acid followed by heating.
Materials
The diaryliodonium(III) salts I having the trimethoxybenzene group were prepared by using our reported
condensation procedure of iodosoarenes,11 which were obtained by literature oxidation procedures15 from
corresponding commercial iodoarene compounds. The N-heteroaromatic compounds 1 used in this study
for the coupling reactions are commercially available and used as they stand. All other chemicals for
performing the experiments and chromatography were obtained from commercial suppliers and used as
received without further purification.
Typical experimental procedure for arylations: The reaction of pyrazine 1d with iodonium salt Ia
The metal-free arylations of N-heteroaromatic compounds 1 using (trimethoxybenzene)iodonium salts I
were performed according to the literature procedure.10a In the case of pyrazine 1d, the iodonium salt Ia
(104 mg, 0.2 mmol) and sodium hydroxide (ca. 60 mg, 7.5 equiv) were added to pyrazine solution (1 mL)
in reflux tube. After stirring at 110 oC for 10 h, the remaining pyrazine was distilled by rotary evaporator
under reduced pressure. The resulting oily residue including the pheylation product 2da was diluted with
EtOAc (20 mL), and washed with brine (15 mL) and water (15 mL), and then the organic layer was dried
over anhydrous sodium sulfate. After removal of the solvent, the crude product was subjected to column
chromatography on silica-gel (hexane/EtOAc = 5/1) to give a pure 2-phenylpyrazine 2da (26.8 mg, 0.17
mmol) in 86% yield as a colorless crystals.
Characterization of the arylation products 2
The physical and spectral data of all the products 2 well matched those previously reported.
2-Phenylpyrrole (2aa)10a
Colorless solid; IR (KBr): 3083, 3043, 1567, 1480, 1430, 1341, 1262 cm-1; 1H-NMR (400 MHz, CDCl3):
6.27-6.30 (m, 1H), 6.51 (brs, 1H), 6.83-6.85 (m, 1H), 7.19 (t, J = 7.3 Hz, 1H), 7.34 (t, J = 8.2 Hz, 2H),
7.47 (dd, J = 9.2, 1.0 Hz, 2H) ppm; 13C-NMR (100 MHz, CDCl3): 105.9, 110.1, 118.8, 123.8, 126.2,
128.9, 132.4, 132.8 ppm.

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HETEROCYCLES, Vol. 95, No. 2, 2017
2-Phenylimidazole (2ba)16a
1
Colorless solid; IR (KBr): 3053, 2904, 1564, 1504, 1488, 1415, 1108, 942 cm-1; H-NMR (400 MHz,
13
CDCl3): 7.10 (m, 2H), 7.29-7.36 (m, 3H), 7.80-7.82 (m, 2H) ppm; C-NMR (100 MHz, CDCl3): 123.2,
125.5, 128.5, 128.8, 130.3, 147.2 ppm.
Phenylpyridines (2ca, mixture of regioisomers)10
The phenylation products were obtained as a mixture of regioisomers with a ratio of 2-:3-:4- = 32:46:22,
1
determined by H NMR analysis after chromatography (hexane: EtOAc). The major product, 3-phenyl-
pyridine (2ca), was isolated for charactarization (see below).
3-Phenylpyridine (2ca)10b
Yellow oil; 1H-NMR (400 MHz, CDCl3):ꢀ2.18 (t, J = 7.8 Hz, 2H), 2.56 (t, J = 7.8 Hz, 2H), 3.80 (s, 3H),
13
6.39 (d, J = 10.2 Hz, 2H), 6.84 (d, J = 10.2 Hz, 2H) ppm; C-NMR (100 MHz, CDCl3): 123.6, 127.2,
128.1, 129.1, 134.4, 136.6, 137.8, 148.4, 148.5 ppm.
2-Phenylpyrazine (2da)10
1
Colorless solid; H-NMR (400 MHz, CDCl3): 7.54-7.45 (m 3H), 8.00-8.03 (m, 2H), 8.50-8.51 (m, 1H),
13
8.63-8.64 (m, 1H), 9.04 (d, J = 1.96 Hz, 1H) ppm; C-NMR (100 MHz, CDCl3): 127.0, 129.1, 129.9,
136.4, 142.3, 142.9, 144.2, 152.9 ppm.
Phenylpyrimidines (2ea, mixture of regioisomers)10b
The phenylation products were obtained as a mixture of regioisomers with a ratio of 2-:3-:4- = 24:60:16,
1
determined by H NMR analysis after chromatography (hexane: EtOAc). The major product, 4-phenyl-
pyrimidine (2ea), was isolated for charactarization (see below).
4-Phenylpyrimidine (2ea)16b
Colorless solid; 1H-NMR (400 MHz, CDCl3): 7.71 (dd, J = 6.1, 1.5 Hz, 1H), 8.06-8.09 (m, 2H), 8.75 (d, J
= 5.36 Hz, 1H), 9.25 (d J = 0.96 Hz, 1H) ppm; 13C-NMR (100 MHz, CDCl3): 117.0, 127.1, 129.1, 131.1,
136.5, 157.5, 159.1, 163.9 ppm.
Phenylpyridazines (2fa, mixture of regioisomers)13a
The phenylation products were obtained as a mixture of regioisomers with a ratio of 3-:4- = 9:91,
1
determined by H NMR analysis after chromatography (hexane: EtOAc). The major product, 4-phenyl-
pyridazine (2fa), was isolated for charactarization (see below).

HETEROCYCLES, Vol. 95, No. 2, 2017
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4-Phenylpyridazine (2fa)16c
1
Brown solid; H-NMR (400 MHz, CDCl3): 7.50-7.57 (m, 3H), 7.66-7.70 (m, 3H), 9.23 (d, J = 5.36 Hz,
13
1H), 9.47 (d, J = 1.00 Hz, 1H) ppm; C-NMR (100 MHz, CDCl3): 123.3, 127.1, 129.5, 130.2, 134.5,
138.5, 150.0, 151.4 ppm.
2-(4-Methylphenyl)pyrazine (2db)16b
Colorless solid; 1H-NMR (400 MHz, CDCl3): 7.31 (d, J = 7.8 Hz, 2H), 7.89-7.91 (m, 2H), 8.46 (d, J = 2.4
13
Hz, 1H), 8.60-8.61 (m, 1H), 8.99 (d, J = 1.4 Hz, 1H) ppm; C-NMR (100 MHz, CDCl3): 21.4, 126.9,
129.8, 133.6, 140.2, 142.1, 142.6, 144.1, 152.9 ppm.
2-(4-Methoxyphenyl)pyrazine (2dc)16d
Colorless solid; 1H-NMR (400 MHz, CDCl3): 7.00-7.04 (m, 2H), 7.95-7.99 (m, 2H), 8.42 (d, J = 2.9 Hz,
1H), 8.57-8.58 (m, 1H), 8.96 (d, J = 1.4 Hz, 1H) ppm; 13C-NMR (100 MHz, CDCl3): 55.4, 114.5, 128.3,
128.9, 141.6, 142.1, 142.0, 152.5, 161.2 ppm.
2-(4-Trifluoromethylphenyl)pyrazine (2dd)16e
Colorless solid; 1H-NMR (400 MHz, CDCl3): 7.77 (d, J = 7.8 Hz, 2H), 8.14 (d, J = 8.0 Hz, 2H), 8.58 (d, J
13
= 1.4 Hz, 1H), 8.68 (m, 1H), 9.08 (m, 1H) ppm; C-NMR (100 MHz, CDCl3): 126.0 (q, J = 3.8 Hz),
127.2 , 131.7 (q, J = 32.6 Hz), 139.6, 142.3, 143.8, 144.4, 151.2 ppm.
2-(3-Thienyl)pyrazine (2de)16f
Colorless solid; 1H-NMR (400 MHz, CDCl3): 7.45 (dd, J = 4.9, 2.9 Hz, 2H), 7.67-7.69 (m, 1H), 7.99-8.00
13
(m, 1H), 8.43 (br, 1H), 8.56 (br, 1H), 8.92 (br, 1H) ppm; C-NMR (100 MHz, CDCl3): 124.8, 125.8,
127.0, 139.0, 141.9, 142.5, 144.2, 149.1 ppm.
2-(2,4-Dimethylphenyl)pyrazine (2df)16g
Yellow oil; 1H-NMR (400 MHz, CDCl3): 2.31 (s, 6H), 7.06 (d, 1H, J = 7.8 Hz), 7.07 (s, 1H), 7.27 (d, 1H,
J = 8.2 Hz), 8.51 (d, 1H, J = 2.4 Hz), 8.67 (dd, 1H, J = 2.4, 1.4 Hz), 8.71 (d, 1H, J = 1.4 Hz) ppm;
13C-NMR (400 MHz, CDCl3): 20.2, 21.2, 126.9, 129.8, 131.9, 133.8, 136.1, 139.2, 142.0, 143.8, 145.0,
155.7 ppm.
ACKNOWLEDGEMENTS
This work was partially supported by a Grant-in-Aid for Scientific Research (A) and Encouragement of
Young Scientists (A) from JSPS. T.D. is thankful to a Grant-in-Aid for Scientific Research (C) from JSPS,

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Challenging Exploratory Research (JSPS), and a research fund from the Asahi Glass Foundation.
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