An alternative to the Sandmeyer approach to aryl iodides

Article abstract of DOI:10.1002/chem.201500151

Iodoarenes are important synthons for a wide range of organic transformations. Here we report a general strategy to prepare singly iodinated electron-rich aromatic compounds through the intermediacy of diaryliodonium salts. This process, which incorporates a phase separation that greatly simplifies product purification, is an attractive replacement for the Sandmeyer approach to iodoarenes that are otherwise difficult to access.

Full text of DOI:10.1002/chem.201500151

DOI: 10.1002/chem.201500151
Communication
&
Synthetic Methods
An Alternative to the Sandmeyer Approach to Aryl Iodides
Bao Hu,[a, b] William H. Miller,[a] Kiel D. Neumann,[a] Ethan J. Linstad,[a] and
Stephen G. DiMagno*[a]
Abstract: Iodoarenes are important synthons for a wide
range of organic transformations. Here we report a general
strategy to prepare singly iodinated electron-rich aromatic
Scheme 1. Tuned electrophilicity in iodoarene synthesis.
compounds through the intermediacy of diaryliodonium
salts. This process, which incorporates a phase separation
Sandmeyer process,[34,35] which involves nitration of an arene,
that greatly simplifies product purification, is an attractive
reduction to the corresponding aniline, diazotization with ni-
replacement for the Sandmeyer approach to iodoarenes
trous acid, and substitution with KI, is the oldest example of
that are otherwise difficult to access.
the use of this approach.[36] Like the nitro group (sp =0.78,
sm =0.71),[33] the aryliodonium substituent is sufficiently elec-
tron-withdrawing (sp =1.37, sm =1.35 for PhI(BF4))[37] to pre-
The significant efforts devoted to iodoarene synthesis speak to
the central roles these iodinated intermediates play in modern
organic chemistry.[1] A large number of reagents have been
used for electrophilic aromatic iodination,[2–6] including N-iodo-
succinimide (NIS)[7–12] and related NÀI compounds,[13–17] iodine
monochloride (ICl),[18–20] bis(pyridine)-iodonium(I) tetrafluorobo-
rate (Py2IBF4),[21,22] dichloroiodates,[23,24] NaI in combination with
various oxidants,[25,26] and silver[12,27–30] or mercury[31,32] salts in
combination with I2. A multitude of reagents and diverse reac-
tion conditions are required to address a central problem in io-
doarene synthesis by electrophilic aromatic substitution (EAS):
the electrophilicity of “I+” reagents must be matched with the
overall “electron-richness” of the arene to obtain selective
iodine substitution. Once on the aromatic ring, the iodine sub-
stituent is not strongly deactivating (Hammett sp =0.18, sm =
0.35, s+ =0.14),[33] so trial and error must be used to find an
iodine source which is sufficiently aggressive, but not so elec-
trophilic that multiple electrophilic aromatic substitutions take
place. This problem is summarized in Scheme 1.
clude further substitution. Moreover, it has been known for
more than a century that diaryliodonium compounds[38–40] can
be converted to aryl iodides efficiently using iodide salts under
mild conditions.[41,42] Here we show that an operationally
simple, two-step approach, which involves formation of a readi-
ly purified diaryliodonium salt intermediate and conversion of
this intermediate to an iodoarene, is an effective and general
process for the selective synthesis of a large variety of monoio-
dinated arenes. Most surprising is that this process occurs with
greater reactivity and selectivity than those employing most
common electrophilic iodination reagents.
The two-step iodination process is outlined in Scheme 2.
The optimization of reaction conditions using aryliodination re-
agents 1-(diacetoxyiodo)-4-nitrobenzene 1a, and dimethyl 5-
One tried and true technique to prevent multiple iodine
substitution is to generate an electron-poor aromatic inter-
mediate which is deactivated toward further EAS, but which
can be converted subsequently to an iodoarene. The classic
[a] Dr. B. Hu, W. H. Miller, Dr. K. D. Neumann, E. J. Linstad,
Prof. Dr. S. G. DiMagno
Department of Chemistry and
Nebraska Center for Materials and Nanoscience
University of Nebraska
Scheme 2. Regioselective iodination of toluene.
Lincoln, NE 68588-0304 (USA)
(diacetoxyiodo)isophthalate 1b is summarized in Table 1. The
diacetoxyiodoarene compounds 1a and 1b are readily synthe-
sized by oxidation of the corresponding aryl iodides with m-
chloroperoxybenzoic acid (m-CPBA) in acetic acid.[43] Reagents
1a and 1b are unreactive toward typical arenes, like toluene,
at room temperature in CH3CN, but treatment of 1a or 1b
with trimethylsilyltriflate (TMSOTf) in CH3CN increases their
E-mail: sdimagno1@unl.edu
[b] Dr. B. Hu
Institute of Industrial Catalysis
College of Chemical Engineering,
Zhejiang University of Technology
Hangzhou, Zhejiang 310014 (China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500151.
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Table 1. Optimization of conditions for aryliodination/iodination.[a]
Entry
Reagent
X
Ratio[b]
t1 [h]
Yield of 2aa
or 2ab [%][c]
Iodide
source
Equiv.[d]
T [8C]
t2 [h]
Yield of 3a
[%][c]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OMs
TFA
1.0/1.5/1.5
1.0/1.5/1.5
1.0/2.0/4.0
1.0/2.0/4.0
1.0/1.5/3.0
1.0/1.5/3.0
1.0/1.5/3.0
1.0/1.5/3.0
1.0/1.5/3.0
1.0/1.5/3.0
1.0/1.5/1.5
1.0/1.5/1.5
1.0/1.5/3.0
1.0/1.5/3.0
1.0/1.5/3.0
1
8
0.5
1
1
1
1
1
1
1
1
29
1
1.5
1.5
66
98
95
–
–
2.0
–
–
80
–
–
13
–
1
1
4
1
1
1
1
–
–
22
–
–
68
–
TMAI
–
>99
>99
>99
>99
>99
>99
>99
30
TMAI
TMAI
TMAI
TMAI
TMAI
TBAI
NaI
–
2.0
2.0
2.0
4.0
3.0
3.0
3.0
–
120
120
120
80
80
80
80
–
46
71
95
>99
>99
>99
>99
–
9
10
11
12
13
14
15
97
>99
0
–
NaI
–
–
3.0
–
–
120
–
–
>99
–
–
0
–
–
–
–
[a] Reaction conditions: 0.10 mmol scale, 1.5 mL of CD3CN, N2. For details of operations, see Supporting Information. [b] Toluene/1/TMSX. [c] Yields were
determined by 1H NMR spectroscopy. [d] Equivalents of iodide source. TMSOTf=trimethylsilyl trifluoromethanesulfonate; TMAI=tetramethylammonium
iodide; TBAI=tetrabutylammonium iodide; TMSOMs=trimethylsilyl methanesulfonate; TMSTFA=trimethylsilyl trifluoroacetate.
electrophilicity dramatically. This reagent combination gives an
equilibrium distribution of IIII species (ArI(OAc)2, ArI(OAc)(OTf),
and ArI(OTf)2); ligand exchange is indicated by a downfield
luene.[44] For comparison, the room-temperature reactivity and
regioselectivity of several iodination reagents toward toluene
under comparable conditions (0.1m) was examined (Table 2).
A major advantage of this approach is that the ionic diarylio-
donium salt has significantly different solubility than the reac-
tants, permitting an intermediate purification, if desired. For
example, precipitation of p-2ab (X=I) removes the last traces
of the more soluble ortho-substituted diaryliodonium salt inter-
mediate, allowing regiochemically pure 4-iodotoluene (p-3a)
to be prepared easily and in good isolated yield (74%,
Scheme 2).
1
shift in the aromatic signals in the H NMR spectrum upon ad-
dition of TMSOTf, and by generation of TMSOAc. (The relatively
unstable aryliodonium bis(triflate) salts can also be isolated,
and they show similar reactivity (see Supporting Information)).
In survey experiments, 1.0 to 2.0 equivalents of 1 were com-
bined with 1.0 to 4.0 equivalents of TMSOTf in CD3CN to gen-
erate the electrophile. This activated aryliodonium species was
treated with one equivalent of toluene and the mixture was al-
lowed to stand at room temperature until all of the toluene
was converted to the diaryliodonium salt 2aa and 2ab
(Table 1). Under the optimized conditions (toluene:1:TMSOTf=
1:1.5:3.0), conversion of arene to the diaryliodonium salt was
complete within one hour. After a second brief survey of reac-
tion conditions, it was found that treatment with 3 equivalents
of NaI and heating to 808C for one hour was sufficient for
complete conversion of the diaryliodonium salt to the corre-
sponding aryl iodides (Table 1). The use of excess iodide serves
two purposes: it drives the ion-exchange for the intermediate
diaryliodonium triflate to the iodide, and it also quenches (by
reduction) any excess ArIX2 reagent remaining upon comple-
tion of the reaction.
Table 2. Comparison of toluene iodination.
Entry Method Conditions
Yield Selectivity
[%][a]
1
2
3
4
A
NIS (1.1 equiv), In(OTf)3 (0.1 equiv),
CH3CN, N2, 238C, 3 days
Ag2SO4 (1.0 equiv), I2 (1.0 equiv),
CH2Cl2, 238C, 1 day
15
55:45
B
<5%
trace
–
–
C
0.83 equiv ICl in CH2Cl2,
238C, N2, overnight
The exceptionally mild conditions under which ArI(OTf)2 is
generated reaffirm that weakly ion-paired aryliodonium cations
are quite potent, yet selective electrophiles: aryliodination of
toluene occurs with quite good regioselectivity. The 90:10 par-
a:ortho diaryliodonium salt product ratio is conserved upon
conversion of these salts to 4-iodotoluene and 2-iodoto-
D[45]
I2, potassium 4-iodylbenzenesulfonate, 98[b] 84:16[b]
H2SO4 (5%), MeCN, 658C
[a] Determined by 1H NMR spectroscopy using an internal standard.
[b] The reported data (see ref. [45]).
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These preliminary reactions,
performed on simple arenes, in-
dicated that this electrophilic io-
dination chemistry was quite
promising for its combination
of reactivity and selectivity,
however, several limitations
were noted. The presence of
free NÀH bonds and phenolic
hydroxyl groups was incompati-
ble with the aryliodination reac-
tion, although simple protective
group strategies provide a work-
around to this problem. Impor-
tantly, the presence of hydro-
gen bonding groups (Scheme 3,
entries 16–18) in the absence of
a reducing functional group is
tolerated.
arenes (1,4-dimethylaminoben-
zene, 1,4-dimethoxybenzene)
Highly
reducing
participated in redox reactions
rather than EAS, indicating that
reduction of the aryliodonium
salt was the principal side reac-
tion that limits the utility of this
method. For iodination of ex-
tremely electron-rich arenes,
less oxidizing aryliodonium elec-
trophiles are preferred (see
below).
The attractiveness of the dia-
ryliodonium salt synthetic ap-
proach is demonstrated by the
synthesis of two compounds
which are quite difficult to pre-
pare by standard methods.
Scheme 3. Synthetic scope of the reagents. Reaction conditions: 0.10 or 0.50 mmol scale, arene/1/TMSOTf/
NaI=1:1.5:3.0:3.0, CD3CN or CH3CN, N2. For details of operations, see Supporting Information. [a] The iodination
required 808C for 2 h. [b] Yields of NMR scale were determined by H NMR spectroscopy using an internal integra-
1
tion standard. [c] Isolated yields. [d] 1.0 mmol scale. [e] 6.0 mmol scale.
This iodination process may be used with a variety of aryl IIII
reagents. We chose reagents 1a and 1b for their disparate re-
tention factors on normal phase silica gel TLC plates, since
flash chromatography was used to isolate the iodoarene prod-
ucts. However, aryl IIII reagents that form aryl iodides having
differential boiling points, phase behavior, or solubility could
also be employed to streamline final purification of sensitive
aryl iodides.
Methyl 2-(3-iodo-4-isobutylphenyl)propanoate 3j has not been
isolated previously, although (3-bromo-4-isobutylphenyl)propa-
noate has been obtained using a Sandmeyer approach.[46] Ibu-
profen methyl ester (Scheme 4) is difficult to iodinate regiose-
lectively with conventional electrophilic reagents, and the re-
sulting mixtures of the 3-iodo and 2-iodo derivatives 2j pres-
ent a separations challenge (see Supporting Information).
Treatment with 1b under standard conditions yields a 90:10
mixture of the two regioisomeric diaryliodonium salts 2j,
which, upon crystallization of the mixture provided one pure
regioisomer 3-iodo 2j in good yield. The conversion of the re-
giochemically pure diaryliodonium salt to the iodoarene was
followed by silica gel chromatography (EtOAc/hexane, 1:40) to
remove dimethyl 5-iodoisophthalate and provide methyl 2-(3-
iodo-4-isobutylphenyl)propanoate 3j in good overall yield
(65%). It bears emphasis that this procedure is operationally
simpler, and uses milder reagents and conditions than those
required to obtain the same compound using approaches that
employ a diazonium salt intermediate.
To examine the scope of this process, a number of arenes
were subjected to aryliodination/iodination (Scheme 3). For
most arenes, treatment with 1.5 equivalents of 1 and 3 equiva-
lents of TMSOTf resulted in >99% conversion to the diarylio-
donium intermediate within 1 h at room temperature. As ex-
pected, in no case was a multiple addition product observed.
The resultant diaryliodonium salts were converted rapidly (<
1 h) to the corresponding aryl iodides by treatment with
3 equivalents of NaI in acetonitrile at 808C. In cases where the
intermediate diaryliodonium iodide (R=5-(dimethyl isophthal-
yl)) was sparingly soluble in acetonitrile, more aggressive heat-
ing was used.
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but of larger magnitude than
those observed in the direct io-
dination of BPAD, indicating
that the increased selectivity of
the aryliodination reaction most
likely results from
through-bond electronic effect
rather than secondary,
a direct,
a
through-space, intramolecular
p–p interaction involving the
electron-rich and electron-poor
rings in 2k. Given the expected
attenuation of such an electron-
ic effect through the saturated
bridge, the selectivity for the
singly substituted product is
surprising and impressive.
The aryliodoination approach
to the preparation of aryl io-
dides retains many of the ad-
vantages of the Sandmeyer re-
action, but it is operationally
simpler. In cases where re-
gioisomers are obtained, forma-
tion of polar, diaryliodonium
salt intermediates that are read-
ily purified simplifies the isola-
Scheme 4. Regioselective iodination of ibuprofen methyl ester (top) and BPAD (bottom).
Perhaps the most impressive example of selective monoiodi-
nation using the aryliodonation approach is seen with bisphe-
nol A dimethyl ether (BPAD, Scheme 4). As is expected from
the symmetrical structure of BPAD, typical electrophilic iodinat-
ing reagents such as NIS/In(OTf)3, ICl, or I2/AgSO4 gave insepa-
rable mixtures of BPAD and singly and multiply iodinated de-
rivatives, even when a significant excess of BPAD (3 equiv) was
used (see Supporting Information). In contrast, when a three-
fold excess of BPAD was treated with the mild and selective
ArIIII reagent 1-(diacetoxyiodo)-4-methoxybenzene 1c, and tri-
methylsilyl trifluoroacetate (TMSTFA), a single diaryliodonium
salt product 2k was obtained. The diaryliodonium salt inter-
mediate 2k was readily separated from excess BPAD by pre-
cipitation of the salt from ethyl acetate with methyl tert-butyl
ether (MTBE). The isolated diaryliodonium salt 2k was convert-
ed smoothly to the iodinated derivative in good yield. It bears
emphasis that a single electrophilic aromatic substitution prod-
uct is not expected from a statistical analysis that assumes that
each ring of BPAD is equally susceptible to EAS. For example,
treatment of BPAD with NIS gave complex mixtures of prod-
ucts when the reaction was run with the same stoichiometry.
The addition of an ArIIII species to one ring of BPAD deacti-
vates the second ring toward electrophilic aromatic substitu-
tion, even though the two rings are separated by a quaternary
tion of a single regioisomer. The solubility characteristics of the
diaryliodonium salt intermediates also make it possible to
obtain good yields of iodinated arenes even when the reaction
is carried out at low conversion. The reactivity of the aryliodi-
nation reagents can be tuned to match a variety of electron-
rich arenes, and the polarity of the aryliodoination reagents
(1a, 1b, or 1c) can be varied to simplify product isolation by
silica gel chromatography. This method allows one to prepare
iodoarenes that normally cannot be prepared regioselectively
by direct electrophilic aromatic substitution, and it also pro-
vides a means to isolate iodoarenes that would be extremely
challenging to separate from their unsubstituted parent com-
pounds. Finally, these reagents are distinguished by a unique
combination of reactivity and selectivity, as they are simultane-
ously more aggressive, regioselective, and chemoselective than
standard reagents for electrophilic aromatic iodination.
Experimental Section
Typical procedure for the synthesis of 4-iodotoluene: Under an
atmosphere of dry N2, TMSOTf (1.50 mmol, 333 mg, 3.0 equiv) was
added slowly to a stirred solution of dimethyl 5-(diacetoxyiodo)i-
sophthalate 1b (0.75 mmol, 329 mg, 1.5 equiv) in dry CH3CN
(1.0 mL).
A solution of distilled toluene (0.50 mmol, 46 mg,
1.0 equiv) in dry CH3CN (1.0 mL) was added dropwise and the mix-
ture was allowed to stand at 238C for 1.0 h. The completion of the
reaction was monitored by TLC. Approximately 10 min after the ad-
dition of solid sodium iodide (1.50 mmol, 225 mg, 3.0 equiv) with
stirring, a yellow solid precipitated from the solution. The precipi-
tated solid was collected by filtration, washed with acetonitrile and
dried in vacuo to provide diaryliodonium iodide as a yellow solid
1
sp3-carbon atom. An analysis of the H NMR spectrum indicates
that the aromatic signals arising from the unsubstituted ring in
the diaryliodonium salt intermediate 2k move upfield, and the
methyl resonance arising from the methoxy group of the un-
substituted ring moves downfield upon formation of 2k from
BPAD. These chemical shift changes are in the same direction,
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1
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Acknowledgements
Research reported in this publication was supported by Insti-
tute of Biomedical Imaging and Bioengineering of the National
Institutes of Health under award number R01 EB015536. The
content is solely the responsibility of the authors and does not
necessarily represent the official views of the National Insti-
tutes of Health. Bao Hu also thanks the National Natural Sci-
ence Foundation of China (No. 21202148) for support.
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Keywords: aryl iodides · diaryliodonium salt · electrophilic
substitution · regioselectivity · Sandmeyer reaction
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Received: January 13, 2015
Published online on && &&, 0000
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
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Synthetic Methods
B. Hu, W. H. Miller, K. D. Neumann,
E. J. Linstad, S. G. DiMagno*
&& – &&
Reactive and selective aryliodination
reagents: An operationally simple ap-
proach to the preparation of aryl io-
dides is described. Electrophilic aromatic
substitution to form polar, and highly
deactivated diaryliodonium salt inter-
mediates is followed by conversion to
the corresponding aryl iodides with NaI.
This two-step process, which incorpo-
rates a phase separation that greatly
simplifies product purification, is an ef-
fective replacement for the Sandmeyer
reaction.
An Alternative to the Sandmeyer
Approach to Aryl Iodides
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&
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!