A Mild and General One-Pot Synthesis of Densely Functionalized Diaryliodonium Salts

Article abstract of DOI:10.1002/ejoc.201500986

Diaryliodonium salts are powerful and widely used arylating agents in organic chemistry. Here we report a scalable synthesis of densely functionalized diaryliodonium salts from aryl iodides under mild conditions. This two-step, one-pot process has remarkable functional group tolerance, is compatible with commonly employed acid-labile protective group strategies, avoids heavy metal and transition metal reagents, and provides a direct route to stable precursors to PET imaging agents.

Full text of DOI:10.1002/ejoc.201500986

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201500986
A Mild and General One-Pot Synthesis of Densely Functionalized
Diaryliodonium Salts
Linlin Qin,[a][‡] Bao Hu,[a,b][‡] Kiel D. Neumann,[a] Ethan J. Linstad,[a]
Katelyenn McCauley,[a] Jordan Veness,[a] Jayson J. Kempinger,[a] and
Stephen G. DiMagno*[a]
Keywords: Synthetic methods / Iodine / Iodane synthesis / Iodonium salts / Aryl iodides / Oxidation
Diaryliodonium salts are powerful and widely used arylating
agents in organic chemistry. Here we report a scalable syn-
thesis of densely functionalized diaryliodonium salts from
aryl iodides under mild conditions. This two-step, one-pot
process has remarkable functional group tolerance, is com-
patible with commonly employed acid-labile protective
group strategies, avoids heavy metal and transition metal
reagents, and provides a direct route to stable precursors to
PET imaging agents.
These approaches have been reviewed in detail by Merritt
and Olofsson (Scheme 1).[2b] Preparation of the ArIX2 elec-
trophile typically involves an oxidation under acidic condi-
tions.
Introduction
Although diaryliodonium salts (diaryl-λ3-iodanes) were
discovered more than a century ago,[1] their study is under-
going a renaissance in organic chemistry, where they serve
as powerful, and widely applicable, arylation reagents.[2–5]
These compounds arylate a multitude of organic and inor-
ganic nucleophiles to provide functionalized arenes in excel-
lent yields. In particular, the use of diaryliodonium salts as
precursors to radiopharmaceuticals is an area of increas-
ingly active research;[6] Sanford and Scott,[7] Pike,[8]
Coenen,[9] DiMagno,[10] and other groups[11] have all dem-
onstrated successful conversion of diaryliodonium salt pre-
cursors into 18F-labeled compounds for PET imaging.
Regiospecific methods to synthesize unsymmetrical di-
aryliodonium salts generally proceed through ArI(III) inter-
mediates, which are generated either by oxidation of an aryl
iodide or by addition of an I(III) electrophile to an aro-
matic compound. The second step involves coupling the
electrophilic ArIX2 reagent with a second aryl moiety,
either by ligand exchange with an aryl organometallic
nucleophile [Ar-B(OH)2, ArSnR3, ArSiMe3, ArZnCl,
Scheme 1. Synthetic strategies for preparing diaryliodonium
salts.[2b]
Driven by our interest in synthesizing radiofluorinated
ArBF3K], or by electrophilic aromatic substitution of an agents for positron emission tomography (PET) imaging,[10]
electron-rich arene.[2a,12] One-pot oxidation and ligand ex- we sought a general, one-pot method for preparing diverse
change reactions to synthesize diaryliodonium salts directly diaryliodonium salts directly from the protected parent
from arenes and iodoarenes have also been reported.[13] pharmaceuticals. Important parameters that need to be sat-
isfied for the synthesis of radiopharmaceutical precursors
[a] Department of Chemistry & Nebraska Center for Materials and
are (1) functional group tolerance, (2) compatibility with
Nanoscience, University of Nebraska,
commonly employed acid-labile protective group strategies,
(3) the avoidance, at every step of the synthesis, of heavy
metal reagents that could impact long term stability of the
diaryliodonium salts or react with trace 18F-fluoride during
labeling procedures, and (4) scalability. These restrictions
are actually quite onerous, as illustrated in Scheme 2, for
the retrosynthesis of an unsymmetrical diaryliodonium salt
Lincoln, Nebraska 68588-0304, USA
E-mail: sdimagno1@unl.edu
http://chem.unl.edu/dimagno-group
[b] Institute of Industrial Catalysis, College of Chemical
Engineering, Zhejiang University of Technology,
Hangzhou, Zhejiang 310014, P. R. China
[‡] These authors contributed equally to this work.
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500986.
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S. G. DiMagno et al.
SHORT COMMUNICATION
derived from phenylalanine. Path A dictates an extremely centered on oxidation of the “valuable,” densely-function-
mild oxidation process to satisfy restrictions 1 and 2, while alized aryl ligand.
path B requires generation of an organometallic reagent un-
der weakly basic conditions, and without the assistance of
transition metal catalysis, to satisfy restrictions 1 and 3.
While multistep workarounds for each of these restrictions
may be possible, a direct route involving in situ oxidation
of the suitably protected 4-iodophenylalanine followed by
treatment with an organometallic reagent is clearly pre-
ferred.
Scheme 3. Preparation of ArI(III) reagents with F-TEDA-2BF4
from iodoarenes.
Results and Discussion
Surprisingly, treatment of 4-methoxyiodobenzene 1a
with a mixture of F-TEDA-2BF4 (1.3 equiv.) and trimethyl-
silyl acetate (TMSOAc, 2.6 equiv.) in CD3CN provided
stable 4-methoxyphenyliodonium diacetate 2a in almost
quantitative yield, along with 2 full equivalents of trimethyl-
silyl fluoride (TMSF). In contrast, when the oxidation was
performed under identical conditions, except that the hexa-
fluorophosphate salt of Selectfluor™ (F-TEDA-2PF6) was
employed, an unstable oxidation product formed, which
gradually decomposed. In addition, an excess of TMSOAc
remained at the end of the reaction. The resemblance of the
TEDA-2PF6 oxidation to Shreeve’s “acid-free” F-TEDA-
2BF4/CH3CN oxidation was striking; these data suggested
that the tetrafluoroborate counterion (BF4) played a key
role in stabilizing the oxidation product by serving as a
source of fluoride (Scheme 4).
Scheme 2. Retrosynthesis of an unsymmetrical diaryliodonium salt
derived from phenylalanine.
In 2005, Shreeve[14] and co-workers reported that iodo-
arenes could be oxidized smoothly under neutral conditions
with 2 equiv. of Selectfluor™ (F-TEDA-2BF4, Scheme 3),
but that the ArIF2 products formed proved to be too un-
stable to isolate in good yield. Importantly, they noted that
yields of isolated ArI(III) species could be increased signifi-
cantly in carboxylic acid solvents. Kita[15] and Ishihara[16]
have also reported preparations of ArI(III) reagents with F-
TEDA-2BF4 under acidic conditions. Guided by these
promising results, we reinvestigated the F-TEDA-2BF4
oxidation of aryl iodides with the aim of developing a pro-
of iodoarenes in the presence of F-TEDA-2BF4 and TMSOAc.
tic-acid-free route to stable, yet reactive ArI(III) electro-
Scheme 4. A proposed mechanism for the oxidative diacetoxylation
philes. Our strategy to achieve this goal was simply to re-
Support for the proposed mechanism is found in the ap-
place the proton in acetic or trifluoroacetic acid with a pearance of new signals, derived from multiple fluoroborate
fluorophilic Lewis acid. These efforts led to a direct, one- species, in the 19F NMR spectrum of the reaction mixture,
pot synthesis of unsymmetrical diaryliodonium salts that is and from multiple signals arising from coordinated diazabi-
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Densely Functionalized Diaryliodonium Salts
1
cyclo[2.2.2]octane in the H NMR spectrum. Addition of ium salt synthesis an attractive goal. Potassium (4-meth-
tetrabutylammonium terafluoroborate (TBABF4) to oxid- oxyphenyl)trifluoroborate, AnBF3K, was selected as the nu-
ations performed with F-TEDA-2PF6 proceeded in excel- cleophilic coupling partner because 1) anisole has been
lent yields with generation of the full 2 equiv. of TMSF, shown to be an excellent “dummy” directing group for a
providing support for the critical role of BF4 in facilitating variety of arylation reactions, and 2) aryl transfer to any
formation of 2a. The proposed mechanism is also consistent trifluoroborane generated during the oxidation step would
with the lower fluoride affinity of BF3 (pF = 8.31) com- regenerate the reagent.
pared to PF5 (pF = 9.49),[17] and suggests that the fluoride
Initial NMR-tube scale reactions (dry CD3CN, iodoar-
affinity of cationic ArIF derivatives is on a par with boron enes 1a–1hh) indicated that efficient Selectfluor™ oxidation
trifluoride, and may be reversible. Additional control ex- of diverse iodoarenes in the presence of TMSOAc was ef-
periments in which TMSOAc was combined with F-TEDA- ficient (Ͼ 80% yield) and relatively rapid (2–48 h) at or near
2BF4, with TBABF4, or even TBABF4 in combination with room temperature (See Supporting Information). Upon
4-methoxyphenyliodonium diacetate did not generate any completion of the oxidation, AnBF3K, which resulted in no
TMSF; these results are consistent with a cationic ArIF observable reaction; the reaction mixtures were treated with
species acting as the fluoride transfer agent.
one equivalent of (diacetoxyiodo)arenes are insufficiently
A brief survey of the oxidation conditions (Table 1) electrophilic to engage this aryl nucleophile. Formation of
showed that a variety of electron-rich and electron-poor ar- the diaryliodonium salt commenced immediately after
enes could be oxidized smoothly to (diacetoxyiodo)arenes, the addition of a CD3CN solution of either trimethylsilyl
2, at or near room temperature; the electron-poor arenes trifluoroacetate (TMSTFA) or trimethylsilyl triflate
required slightly more aggressive conditions to complete the (TMSOTf), and was complete within 10 min at room tem-
oxidation. Reactions performed in CD3CN and monitored perature. {As we have shown previously, a ligand exchange
by 1H NMR spectroscopy indicated that the oxidation pro- reaction between iodine and silicon [ArI(OAc)2 + TMSX
ceeded in excellent yield (Ͼ90%) in all cases; some losses ↔ ArI(OAc)(X) + TMSOAc] creates a Lewis acid buffer
were associated with the subsequent isolation of the system that can be used to tune the electrophilicity of the
ArI(OAc)2 products. Importantly, the straightforward prep- I(III) center over a significant range.}[18] Iodine arylation
aration of 2g (Scheme 5) shows that the oxidation occurs with AnBF3K proceeded in excellent yield (Ͼ 95%) in all
under effectively neutral conditions; BOC protecting groups cases.
are not cleaved during this process.
To demonstrate the practicality of the method, we pre-
pared a relatively large number of unsymmetrical diaryl-
iodonium triflates and hexafluorophosphates (Scheme 6).
These compounds, which were selected largely for their po-
tential use as radiosynthesis precursors, featured acid-sensi-
tive 3(g, h, i, n, t, u, v, aa–hh) and base-sensitive 3(t–y,
aa–ee) functionality. The larger scale reactions followed the
procedure described above for the NMR tube reactions,
except that an aqueous work-up, ion-exchange, and
recrystallization of the products were performed (see Sup-
porting Information). Significant losses were incurred dur-
ing the isolation and recrystallization of some of the prod-
ucts, but even quite densely functionalized compounds
could be prepared successfully on a multigram scale.
Although most of the reactions reported in Scheme 6
were performed using glove box techniques, the use of rig-
orously anhydrous conditions and highly purified reagents
and solvents are not required. For example, 3t was synthe-
sized on the benchtop under nitrogen using as received rea-
gents, and the yields were comparable to those obtained
under rigorously dry conditions. In fact, the method is ro-
bust enough to be transferred to pilot plant facilities; as
an example, a cGMP synthesis of a 100 gram lot of 3v, a
diaryliodonium salt precursor to 18F–6-fluoro-l-
DOPA[10a,10b] was recently completed[19] by a commercial
concern. The reason this system tolerates the presence of
adventitious moisture is that the oxidation generates its own
acetate buffer if water is present during the course of the
reaction. If extremely acid-sensitive functional groups are
Table 1. Preparation of (diacetoxyiodo)arenes 2 from iodoarenes
1.[a]
Entry
ArI
R
T [°C]
t [h]
Prod. Yield [%][b]
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
4-OMe
2-OMe
3-CF3
3-CN
3,5-CO2Me
4-NO2
23
23
23
23
50
50
3
3
8
8
22
33
2a
2b
2c
2d
2e
2f
70
65
80
70
93
85
[a] Reaction conditions: 0.50 mmol scale, 6.0 mL of CH3CN, N2.
For details of operations, see Supporting Information. [b] Isolated
yields.
Scheme 5. Synthesis of (diacetoxyiodo)arene 2g from iodoarene 1g.
The losses associated with purification and isolation of present in the molecule, additional leeway for the generation
(diacetoxyiodo)arenes made a direct, one-pot diaryliodon- of Brønsted acids from water can be gained by adding N-
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S. G. DiMagno et al.
SHORT COMMUNICATION
Scheme 6. Diaryliodonium salts prepared using the reported method.
chloromethyldiazabicyclo[2.2.2]octane
(CH2Cl-DABCO) at the beginning of the reaction. This
compound, which is also generated in situ during the initial
tetrafluoroborate
Conclusions
The scalable, heavy-metal-free, one-pot, two step synthe-
oxidation reaction, is a mild and non-reducing tertiary sis of diaryliodonium salts described here has a remarkable
amine base that can scavenge small amounts of protic acid functional group tolerance, and is compatible with com-
side products, if desired.
monly employed acid-labile protective group strategies. Un-
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Densely Functionalized Diaryliodonium Salts
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 Institutes of Health
(NIH). B. H. also thanks the National Natural Science Foundation
of China (NSFC) (grant number 21202148) for support. Disclos-
ures: The chemistry reported in this article is the subject of patent
applications (PCT/US 2013/066830, WO 2014/066772) which have
been licensed to Ground Fluor Pharmaceuticals, Inc., Lincoln, NE
(GFP). SGD and KDN are shareholders in GFP.
symmetrical diaryliodonium salts possessing quite reactive
functionalities, including succinimidyl esters and male-
imides typically used for bioconjugation, may be synthe-
sized with relative ease from their parent aryl iodides. This
improved capability should streamline the development of
diaryliodonium salts from complex, pharmaceutically rel-
evant aromatic compounds and expand the scope of poten-
tial arylation reagents available for organic synthesis. More-
over, given the increasing importance of diaryliodonium
salts as precursors to 18F-fluorinated radiopharmaceuticals,
this simple methodology provides a direct route to stable
precursors to PET imaging agents.
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Experimental Section
Typical procedure for the benchtop, one-pot synthesis of a diaryl-
iodonium salt 3t: Under an atmosphere of dry nitrogen, methyl
(S)-2-[bis(tert-butoxycarbonyl)amino]-3-(4-iodophenyl)propanoate
(3.0 mmol, 1.52 g, 1.0 equiv.), SelectfluorTM (3.9 mmol, 1.39 g,
1.3 equiv.) and 15 mL of dry CH3CN were introduced to an oven-
dried Schlenk flask. A solution of TMSOAc (7.8 mmol, 1.03 g,
2.6 equiv.) in dry CH3CN (5 mL) was added by syringe dropwise
with stirring. This colorless mixture was stirred at room tempera-
ture for 24 h; a 1H NMR spectrum obtained from a withdrawn
aliquot confirmed that the oxidation had reached Ͼ90% at this
time. Solid potassium (4-methoxyphenyl)trifluoroborate (0.64 g,
3.0 mmol, 1.0 equiv.) was added directly to the flask against a flow
of nitrogen. Once the added solid was dissolved and a homogen-
eous mixture was obtained (3 min), a solution of TMSOTf (0.67 g,
2.7 mmol, 0.9 equiv.) in 10.0 mL of dry CH3CN was added drop-
wise by syringe and the yellowish mixture was allowed to stir at
room temperature for 10 min. The solvents were removed under
reduced pressure and 100 mL of 0.1 m acetate buffer (pH = 5) was
added. The mixture was extracted with CH2Cl2 (3ϫ 50 mL). The
combined organic layers were added to a separatory funnel and
washed with water (50 mL), and the wash water was extracted fur-
ther (2ϫ 50 mL) with CH2Cl2. The combined organic extracts were
dried with sodium sulfate, filtered, and the solvent was removed by
rotary evaporation. The residue was placed under dynamic vacuum
to obtain a yellow-tinged foam. This solid was dissolved in 10.0 mL
ethyl acetate and added dropwise to a mixture of MTBE and hex-
ane (1:4) to precipitate the diaryliodonium triflate product. The
obtained solid was dissolved in 1 mL acetonitrile/water (9:1 by vol-
ume) solution and passed down an Amberlite IRA-400 ion ex-
change column (triflate counterion). After removal of the solvents
under reduced pressure, the purified iodonium triflate product
(1.89 g, 83%) of was obtained as a colorless solid. 1H NMR
(CD3CN, 400 MHz): δ = 8.02 (d, J = 9.1 Hz, 2 H), 7.96 (d, J =
8.4 Hz, 2 H), 7.33 (d, J = 8.4 Hz, 2 H), 7.05 (d, J = 9.1 Hz, 2 H),
5.16 (dd, J1 = 10.9, J2 = 4.8 Hz, 1 H), 3.84 (s 3 H), 3.70 (s, 3 H),
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3.43 (dd, J1 = 14.0, J2 = 4.8 Hz, 1 H), 3.21 (dd, J1 = 14.0, J2
=
10.9 Hz, 1 H), 1.27 (s, 18 H) ppm. 13C NMR (CD3CN, 176 MHz):
δ = 170.2, 163.4, 151.6, 143.7, 137.8, 134.9, 133.6, 122.0, 120.2,
118.2, 111.6, 101.5, 83.1, 58.6, 55.8, 52.1, 35.3, 27.0 ppm. 19F NMR
(CD3CN, 376 MHz): δ = –79.3 (s, 3 F) ppm. HRMS: (ESI) calcd.
for C27H35INO7 [M – OTf]+ 612.1458, found 612.1453.
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Acknowledgments
Research reported in this publication was supported by Institute of
Biomedical Imaging and Bioengineering of the National Institutes
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SHORT COMMUNICATION
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