4-[18f]fluorophenylpiperazines by improved hartwig-buchwald n-arylation of 4-[18f]fluoroiodobenzene, formed via hypervalent λ3-Iodane precursors: Application to build-up of the dopamine d4 ligand [18f]fauc 316

Article abstract of DOI:10.3390/molecules20010470

Substituted phenylpiperazines are often neuropharmacologically active compounds and in many cases are essential pharmacophores of neuroligands for different receptors such as D2 -like dopaminergic, serotoninergic and other receptors. Nucleophilic, no-carr

Full text of DOI:10.3390/molecules20010470

Molecules 2015, 20, 470-486; doi:10.3390/molecules20010470
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
4-[18F]Fluorophenylpiperazines by Improved Hartwig-Buchwald
N-Arylation of 4-[18F]fluoroiodobenzene, Formed via Hypervalent
λ3-Iodane Precursors: Application to Build-Up of the Dopamine
D4 Ligand [18F]FAUC 316
Fabian Kügler, Johannes Ermert *, Peter Kaufholz and Heinz H. Coenen
Institute of Neuroscience and Medicine, INM-5: Nuclear Chemistry, Forschungszentrum Jülich,
52425 Jülich, Germany; E-Mails: fabian.kuegler@petnet-solutions.de (F.K.);
p.kaufholz@fz-juelich.de (P.K.); h.h.coenen@fz-juelich.de (H.H.C.)
* Author to whom correspondence should be addressed; E-Mail: j.ermert@fz-juelich.de;
Tel.: +49-2461-61-3110; Fax: +49-2461-61-2119.
Academic Editor: Derek J. McPhee
Received: 24 September 2014 / Accepted: 18 December 2014 / Published: 31 December 2014
Abstract: Substituted phenylpiperazines are often neuropharmacologically active
compounds and in many cases are essential pharmacophores of neuroligands for different
receptors such as D2-like dopaminergic, serotoninergic and other receptors. Nucleophilic,
18
no-carrier-added (n.c.a.) F-labelling of these ligands in an aromatic position is desirable
for studying receptors with in vivo molecular imaging. 1-(4-[18F]Fluorophenyl)piperazine
18
was synthesized in two reaction steps starting by F-labelling of a iodobenzene-iodonium
precursor, followed by Pd-catalyzed N-arylation of the intermediate 4-[18F]fluoro-iodobenzene.
Different palladium catalysts and solvents were tested with particular attention to the polar
solvents dimethylformamide (DMF) and dimethylsulfoxide (DMSO). Weak inorganic bases
like potassium phosphate or cesium carbonate seem to be essential for the arylation step
and lead to conversation rates above 70% in DMF which is comparable to those in
typically used toluene. In DMSO even quantitative conversation was observed. Overall
radiochemical yields of up to 40% and 60% in DMF and DMSO, respectively, were
reached depending on the labelling yield of the first step. The fluorophenylpiperazine
obtained was coupled in a third reaction step with 2-formyl-1H-indole-5-carbonitrile to
yield the highly selective dopamine D4 ligand [18F]FAUC 316.

Molecules 2015, 20
471
Keywords: fluorine-18; Hartwig-Buchwald N-arylation; dopamine D4 radioligand;
iodophenyl-iodonium compounds; iodonium ylides
1. Introduction
Since their first uses in the1960s, cross-coupling reactions using Pd(0) complexes play a major role
in organic syntheses to form carbon-carbon or carbon-heteroatom bonds [1]. This was recently also
recognized by awarding the Nobel Prize to Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki in
2010 [2]. Therefore, it is remarkable that application of this method in radiosyntheses using short-lived
11
18
radionuclides like C (t1/2 = 20.4 min) or F (t1/2 = 109.7 min) is still an exception and only a few
examples of its use are described [3–5]. Cross-coupling reactions allow for a quick synthesis of many
structure motives of biomolecules relevant for non-invasive diagnostic imaging techniques such as
positron emission tomography (PET). Reasons for the rare use of palladium-mediated coupling
methods in radiochemistry with positron-emitters may be the following:
• Whenever possible, multi-step reactions are avoided in radiosyntheses with short-lived nuclides.
• Syntheses with many reactants often lead to problems of reproducibility and optimization under
no-carrier-added (n.c.a.) conditions and complicate a rapid on-line separation of products.
• The typical use of non-polar solvents and dry reaction conditions for the coupling reaction necessitate
an extensive pre-separation of labelled intermediates which is often time and yield consuming.
• Insoluble compounds (mixture of organic and inorganic reactants) complicate the applicability of
the method in remotely controlled synthesis devices.
Otherwise none of these problems are insuperable, and new technical developments, like microfluidic
systems, may help to solve the problem with solubility [6,7]. Since diaryliodonium salts allow to access
n.c.a. [18F]fluorohalobenzenes in high yields and in one reaction step [8–10] the availability of 18F-labelled
electrophilic coupling partners is not any longer a hindrance to apply such coupling methods [5,11–14].
One basic cross-coupling reaction is the Hartwig-Buchwald N-arylation (HBC), which leads to
arylamines, important structure elements of a lot of bioactive compounds. Although alternative
pathways for generating fluoroarylamines have been used including direct fluorination, these proved
18
not to be practical for n.c.a. F-labelled arylamine compounds. Para-alkylamine substituents such as
piperidine or piperazine cause a maximum slow-down of reaction kinetics of SNAr reactions, what is
reflected by very low Hammett σ constants of about −1 of those substituents [15]. Direct labelling of
arylamines with [18F]fluoride was therefore impossible so far. Alternatively, three-step syntheses of n.c.a.
[18F]fluorophenylpiperazine have been developed starting from dinitrobenzene derivatives [16,17].
Thus, the HBC offers the possibility of a two-step procedure to 18F-labelled arylpiperazine or
piperidines. The choice of the solvent is the main challenge is of this reaction. Dimethylformamide
(DMF) which is commonly used for 18F-labelling of iodonium salts failed as solvent for HBC in
previous studies [18,19]. The aim of this work, therefore, was the optimization of HB-coupling
conditions with n.c.a. 18F-labelled iodobenzene to allow for the use of polar media with regard to
facilitate a one-pot synthesis of 1-(4-[18F]fluorophenyl)piperazine. Such reaction conditions avoid both
pre-separation and pre-drying, and may result in a definite increase of efficiency of HBC in radiochemistry.

Molecules 2015, 20
472
1-(4-Fluorophenyl)piperazine is itself a neuropharmacologically active substance [20] and thus
interesting as an imaging probe, but it was not 18F-labelled so far. The actual benefit of this molecular
structure for the development of pharmaceuticals lies in the possibility of easy-to-perform coupling by
reductive amination (mild conditions, toleration of moisture and of many functional groups, high yields
and reproducibility) of different aldehyde moieties in order to synthesize various phenylbenzyl-piperazines.
These are important structure motives for different neuroligands, including ligands for D2-like
dopamine receptors, serotonin receptors, sigma receptors [21], adrenergic receptors [22] and calcium
channels [23]. In many cases, an aromatic fluorine substituent increases the affinity and/or selectivity
of a ligand, which is important for its use in radiodiagnostics [24,25].
One very interesting ligand of this group is the highly affine (Ki(D4) = 1 nM) and selective
(Ki(D1-3) = 8600–28,000 nM) dopamine D4 ligand FAUC 316 (1) (see Figure 1) [26]. A direct
radiolabelling of this compound by fluorine-18 or carbon-11 was not carried out so far. However, in
order to achieve an efficient radiolabelling of FAUC analogues, two fluoroethoxy-substituted
derivatives, namely 2-[4-(2-(2-fluoro-ethoxy)phenyl)-piperazin-1-ylmethyl]indole-5-carbonitrile and
2-[4-(4-(2-fluoroethoxy)-phenyl)-piperazin-1-ylmethyl]indole-5-carbonitrile, have been developed which
18
were F-labelled with radiochemical yields of 80% and 47%, respectively [27]. Their respective Ki
values of 2.1 and 9.9 nM for the dopamine D4 receptor subtype are comparable with the Ki value of 1 nM
of FAUC 316. However, the D4 selectivity within the D2 family, best displayed by a 420-fold D4-selectivity
over D2 receptors by 2-[4-(4-(2-fluoroethoxy)-phenyl)piperazin-1-ylmethyl]indole-5-carbonitrile, is
much lower compared to the factor of 19,000 for FAUC 316. Furthermore, a new series of FAUC
derivatives, derived from aminomethyl-substituted pyrazolo[1,5-a]pyridine (FAUC 113 and 213), have
been pharmacologically evaluated and labelled with fluorine-18 [28]. The best derivative of this series,
labelled by 18F-fluoroethylation with 88% RCY, showed a Ki value of 13 nM which again is by a factor of
about 10 lower than that of FAUC 316. The dopamine D4 receptor exhibits a real challenge with regard
to its in vivo determination due to the extremely low concentration (Bmax(brain) ≈ 9–30 fmol/g(tissue))
of this receptor subtype in the mammalian brain [29]. Therefore, a suitable radioligand for PET or an
alternative molecular imaging method does not exist until now [30,31].
F
NC
N
N
N
H
1
Figure 1. FAUC 316, one of the most selective ligands known for the D4 receptor.
Due to the strong and multiple effects of dopamine on the human body it is difficult to differentiate
between the individual influences of the 5 dopamine subtypes since they show a very similar binding
behavior [32,33]. This similarity poses a high demand on the selectivity of ligands for imaging of an
individual subtype of dopaminergic receptors.

Molecules 2015, 20
473
2. Results and Discussion
2.1. Palladium-Mediated N-Arylation
The synthesis of an n.c.a. 4-[18F]fluorohalobenzene as electrophilic coupling compound for HBC
was conducted by using hypervalent λ3-iodane (also called iodonium) species as 18F-labelling
precursor. Although the exact labelling mechanism of the nucleophilic substitution on iodo(III) arene
compounds with fluoride is not known until now [34], it lead to [18F]fluoroaryls also with poorly
activating and even electron withdrawing substituents in high yields. Ideally, it is desirable that all
[18F]fluoroarenes are synthesized by direct labelling of, e.g., a corresponding iodonium compound. In
reality, however, the synthesis of those precursors is limited since it requires harsh oxidative
conditions with a rather poor tolerance of functional groups, and extremely low labelling yields are
often obtained from more complex iodonium precursors. Especially nitrogen-containing aryliodonium
salts are difficult to synthesize by classical routes for diaryliodonium salts [35]. New modifications
were developed to overcome this challenge [36,37], but only very few examples of complex
nitrogen-containing iodonium salts can be found in the literature [38–40]. The fact that all of the
complex iodonium precursors used for a successful 18F-labelling do not contain an amine group, is
only an empirically based information. However, it was verified in our study with the example of
4-(4-benzylpiperazin-1-yl)phenyl(4-methoxyphenyl)iodonium triflate. No 18F-labelling could be
observed using this precursor; that is why a direct one-step synthesis of [18F]FAUC 316 ([18F]1) failed.
Therefore, multi-step reactions are still necessary for 18F-fluorination of these electron rich aryls,
where hypervalent λ3-iodane species, nevertheless, serve as excellent starting material.
Figure 2. Radiosynthesis of 1-[18F]fluoro-4-iodobenzene ([18F]4) by nucleophilic
substitution on iodo(III)arenes under various reaction conditions (n = 3–6).
In this work, 1-[18F]fluoro-4-iodobenzene ([18F]4) was used rather than the bromine-analogue because
higher labelling yields were found and preliminary examinations showed even slightly higher coupling

Molecules 2015, 20
474
18
rates. For comparison, an F-for-N(CH3)3 exchange on 4-iodophenyltrimethylammonium triflate (2)
yielded only 12% ± 3% in DMSO at 160 °C, while the unsymmetrical iodonium salt, 4-iodophenyl-(4′-
methoxyphenyl)-iodonium triflate (3a), yielded [18F]4 at 130 °C in DMF with 40% ± 7% (Figure 2).
Since a convenient synthesis of the symmetrical bis(4-iodophenyl)iodonium salt (3b) was accessible [41],
highest labelling yields of 60% ± 9% could be reached with this iodonium precursor under the same
conditions using the Kryptofix®2.2.2./potassium carbonate system, without any additive or scavenger
which was recommended by Carroll et al. [42]. The solvents DMSO and mixtures of DMSO/DMF
proved less suitable for the 18F-fluorination of the corresponding diaryliodonium salts.
A novel precursor, however, (2,2-dimethyl-5,7-dioxo-1,3-dioxocan-6-yl)(4-iodophenyl)iodonium
ylide (3c) [43], gave still a higher RCY of 70% ± 10%. In contrast to the other precursors the best RCY
was again obtained in dimethylsulfoxide (DMSO) and tetrabutylammonium bicarbonate as anion
activator. Furthermore, a lower reaction temperature of 110 °C could be employed. It is notable, that
even in acetonitrile a RCY up to 38% of [18F]4 was obtained when using the iodonium ylide.
During the preparation of this manuscript, however, Mu et al. published an alternative method for
the synthesis of 1-[18F]fluoro-4-iodobenzene ([18F]4) using triarylsulfonium salts which lead even to
90% RCY of n.c.a. 1-[18F]fluoro-4-iodobenzene [44,45].
N-Arylation was performed using different Buchwald co-ligands of the fourth generation [46], i.e.,
dialkylbiaryl phosphine ligands which were optimized for different substrates. For example, RuPhos is
typically used for coupling of secondary amines in high yields [47]. Therefore, the used catalyst
system was confined on one group.
Typically, for Pd-mediated N-arylation a non-polar solvent like toluene is used together with a strong
base like sodium or potassium tert-butoxide. Normally, weak bases are only used when excellent
tolerance of functional groups is required. With toluene and potassium tert-butoxide high RCYs of about
70% were reached with the ligands DavePhos and RuPhos within 10–15 min reactions time. This is in
agreement with the findings of Wüst et al. [18] who obtained similar yields by coupling an indole
derivative under conditions given in Table 1 under entry 1.
The necessity of a separation and purification of [18F]6 led to a considerable loss of product
especially when starting with the bis(4-iodophenyl)iodonium salt (3b) in DMF as 18F-labelling precursor
which produced a colloidal precipitate upon dilution with water. Furthermore, the bad miscibility of
toluene and water causes problems in cartridge-based separation-processes. For an optimal performance
of the reaction the same solvent should be used for both, labelling and the cross-coupling process, in
order to avoid those problems and to simplify the work-up procedure. DMF is the solvent to consider
in the first place for this purpose since it is typically used for 18F-fluorination of iodonium-precursors.
However, catalyst/base systems which result in best coupling yields within short reaction time in toluene
show no conversion when applied in DMF, even after extension of the reaction time to 1 h (see Table 1).
This is due to a possible side reaction of the HBC during the catalytic circle. While normally a reductive
amination takes place as last step to produce the desired N-aryl compound, a β-hydride-elimination can
lead to the dehalogenated product and a tetrahydropyrazine. As Christensen et al. [48] describe, this path
becomes dominant when deprotonation is slow which normally occurs in a polar solvent like DMF,
DMSO or dimethylacetamide (DMAA) under the employed conditions including NatOBu as base.
Otherwise the successful use of polar solvents [49,50] and even water [51] in HBC is known. In
fact, a lot of catalyst/base systems could be found which lead to high coupling yields in DMF up to 88%

Molecules 2015, 20
475
within 15 min which is an acceptable reaction time for multi-step 18F-chemistry. The pivotal point is the
use of a weak base like K3PO4 or Cs2CO3. Here small but clearly higher yields are obtained with
K3PO4 in DMF. In DMSO even higher yields were observed with a reverse trend concerning the use of
K3PO4 and Cs2CO3 (Table 1, entry 10–19). The system Pd2(dba)3, RuPhos and Cs2CO3 in DMSO lead
to a nearly complete conversion within 15 min at 100 °C. In all solvents, a double coupling of
n.c.a. 4-[18F]fluoroiodobenzene with piperazine was not observed, which is probably due to the low
concentration of the n.c.a. radiotracer [18F]4 present in the reaction mixture. Another possible side
reaction, the coupling of 4-[18F]fluorophenylpiperazine with excessive precursor, which was not
separated from the reaction mixture, was also not found.
Table 1. HBC of 1-[18F]fluoro-4-iodobenzene ([18F]4) and piperazine. Total RCY is
related to coupling yields based on [18F]4.
Radiochemical Conversion
Entry
Catalyst System
Base
Solvent
Yield/Reaction Time
70%/15 min
73%/15 min
25%/30 min
60%/5 min
1
2
3
Pd2(dba)3/DavePhos
Pd2(dba)3/RuPhos
[CinnamylPdCl]2/BrettPhos NaOtBu
NaOtBu
NaOtBu
Toluene
Toluene
Toluene
4
Pd(OAc)2/RuPhos
NaOtBu
Toluene
74%/10 min
40%/30 min
0%/60 min
0%/60 min
0%/60 min
5
6
7
8
9
Pd2(dba)3/DavePhos
Pd2(dba)3/DavePhos
Pd(OAc)2/RuPhos
[P(tBu)3PdBr]2
NaOtBu
NaOtBu
NaOtBu
K3PO4
1,4-dioxane
DMF
DMF
DMF
DMF
[P(tBu)3PdBr]2
K2CO3
0%/60 min
50%/6 min
10
Pd2(dba)3/RuPhos
K3PO4
DMF
88%/15 min
31%/15 min
67%/15 min
51%/15 min
28%/15 min
11%/15 min
60%/15 min
~95%/15 min
82%/15 min
77%/15 min
11
12
13
14
15
16
17
18
19
Pd[P(tBu)3]2
K3PO4
K3PO4
Cs2CO3
K3PO4
Cs2CO3
K3PO4
Cs2CO3
K3PO4
K3PO4
DMF
DMF
DMF
DMF
DMF
DMSO
DMSO
DMSO
DMSO
Pd2(dba)3/JohnPhos
Pd2(dba)3/JohnPhos
Pd2(dba)3/XPhos
Pd2(dba)3/XPhos
Pd2(dba)3/RuPhos
Pd2(dba)3/RuPhos
Pd2(dba)3/JohnPhos
Pd2(dba)3/XPhos
The necessity of a weaker base is surprising since a deprotonation process is estimated to be inferior
under this condition. Otherwise, the strength of the base does not seem to have a considerable influence
18
on the coupling reaction. The anion activator Kryptofix®2.2.2./K2CO3 used for F-fluorination is still
present in the DMF solution, which is strongly and basically due to the chelated potassium. Here,
deiodination is still comparatively low, but the fact that in general highest yields are obtained in

Molecules 2015, 20
476
DMSO can be a result of the weaker basic conditions during labelling, when tetrabutyl-ammonium
bicarbonate was used as anion activator as was done with precursor 3c. The results do not allow for a
definite conclusion on the mechanism during this step of the HBC. However, the reason for slow
deprotonation in polar media may be a steric hindrance of base due to coordination with the solvent.
Carbonate and phosphate which can also coordinate to palladium may have better kinetics during this
step due to their multidentate character.
2.2. Synthesis and Evaluation of the D4 Agonist [18F]FAUC 316
The synthesis of [18F]FAUC 316 was performed by a reductive amination of 1-(4-[18F]fluoro-
phenyl)piperazine ([18F]6) with 2-formyl-1H-indole-5-carbonitrile (7) (see Figure 3). A pre-purification
of 1-(4-[18F]fluorophenyl)piperazine ([18F]6) is essential to use it for application in a radiopharmaceutical
synthesis following the HBC. The liquid-liquid extraction of [18F]6 with hydrochloric acid (2 mol/L)
allowed a selective separation of the reaction mixture containing amines as well as water soluble
substances. The progress of extraction could easily be traced following the distribution of radioactivity
between the two phases. After addition of sodium hydroxide until pH 9, organic compounds were fixed
on a Sep-Pak C18 cartridge and eluted with DMSO as solvent of the following reaction. The liquid-liquid
extraction was almost quantitative in contrast to a direct solid phase extraction on the weak cation
exchanger Strata X-CW which is a faster process, but only provides a product yield of 20% ± 4%.
100
80
60
40
18F]FAUC 316 ([18F]1)
18F]9
20
[
[
0
4
6
8
10
12
14
16
18
20
22
time [min]
Figure 3. Time dependence of the reductive amination of the indole-2-carbaldehydes 7 and
8 with 1-(4-fluorophenyl)piperazine yielding [18F]FAUC 316 ([18F]1) and the corresponding
ligand [18F]9 without a nitrile substituent for comparison. Reaction conditions: NaBH3CN,
AcOH, DMSO, 110 °C.

Molecules 2015, 20
477
Table 2. Sequence of the two-step radiosynthesis of n.c.a. 2-((4-(4-[18F]fluorophenyl)-
piperazin-1-yl)methyl)-1H-indole-5-carbonitrile ([18F]FAUC 316).
Time (min) RCY (%) Compounds and Conditions
Structures
0
5
Drying process
100
Addition of iodonium
ylide 3c
15
30
70
60
Heating at 110 °C
Purification step
Addition of piperazine
and HBC reagents
47
37
Heating at 100 °C
Purification step
Addition of 2-formyl-1H-indole-
5-carbonitrile and NaBH3CN
Stirring
45
80
28
10
Pre-purification
HPLC-separation
Formulation
The synthesis of the cyanoindole precursor 7 for coupling was performed according to methods
from the literature [27]. The standard compound FAUC 316 was prepared by reductive amination of
commercially available 1-(4-fluorophenyl)piperazine (2) with the aldehyde 7 and acidic sodium
cyanoborohydride in methanol obtaining a RCY of about 50%. Compound 7 was also used for
radioactive coupling of the 18F-labelled amine ([18F]6) obtaining radiochemical yields of 85% ± 5% of
[18F]1 after 15 min using DMSO as solvent. In order to study whether the strong electron withdrawing
effect of the cyano group of compound 7 has an influence on the reductive amination reaction, the

Molecules 2015, 20
478
coupling was also studied with the unsubstituted indole-2-carbaldehyde 8. However, no significant
difference in yield was observed in this case (see Figure 3). Obviously, the cyano substituent does not
influence this reaction. The overall RCY of [18F]1 was about 10% and 18% within 80 and 120 min,
respectively, depending if a separation of the aryliodide intermediate [18F]4 was necessary. Maximum
molar activities of the product reached 90 GBq/µmol and the radiochemical purity was determined by
HPLC to be >98%. In Table 2, the whole reaction sequence under optimized conditions is summarized.
For evaluation of the pharmacological properties of the highly affine D4 radioligand [18F]FAUC 316
for later in vivo application its accumulation behavior on rat brain slices was examined by in vitro
autoradiography. Thereby, the binding of the radioligand was competed with spiperone and the
inactive standard compound FAUC 316 1. Spiperone, an unselective but high affine dopaminergic
ligand, was chosen because no applicable gold standard for D4 exists. The extent of competition is
lower than 10% for spiperone and even less than 5% for 1. Non-specific binding refers to the binding
of a ligand to other sites in the tissue, e.g., other receptors, enzymes or membrane transporters, or to
distribution of the ligand into the lipid cell membrane [52]. Therefore, in spite of the excellent affinity
and selectivity reported for the inactive ligand, the preliminary in vitro rat brain studies with
[18F]FAUC 316, exhibiting a comparatively high molar activity, the high non-specific binding of both
radiofluorinated tracers in rat brain does not encourage to recommend them as in vivo PET imaging
agents for dopamine D4 receptors in humans.
3. Experimental Section
3.1. General
All reagents and anhydrous solvents were purchased from Aldrich (Steinheim, Germany) or Fluka
(Buchs, Switzerland) and used without further purification. 2-(Hydroxylmethyl)-1H-indole-5-
carbonitrile was available from Aurora Fine Chemicals (Graz, Austria). The preparation of
4-iodophenyltrimethylammonium triflate (2) [53], 4-iodophenyl-(4′-methoxyphenyl)-iodonium triflate
(3a), bis(4-iodophenyl)iodonium triflate (3b) [41], (2,2-dimethyl-5,7-dioxo-1,3-dioxocan-6-yl)(4-
iodophenyl)iodonium ylide (3c) [43], 2-formyl-1H-indole-5-carbonitrile [27] (7) and 2-((4-(4-
fluorophenyl)piperazine-1-yl)methyl)-1H-indole [54] (9) were synthesized by published methods and
were confirmed by NMR-spectroscopy and mass spectrometry.
Sep-Pak C-18 plus-cartridges were purchased from Waters (Eschborn, Germany), Li-Chrolut EN
cartridges and glass columns (65 × 10 mm) from Merck (Darmstadt, Germany). Thin layer
chromatography (TLC) was done on precoated plates of silica gel 60F254 (Merck) or Alumina N. The
compounds were detected by UV at 254 nm. HPLC was performed on the following system from
Dionex (Idstein, Germany): an Ultimate 3000 LPG-3400A HPLC pump, an Ultimate 3000 VWD-3100
UV/VIS-detector (272 nm), a UCI-50 chromatography interface, an injection valve P/N 8215.
Reversed-phase HPLC was carried out using a Gemini 5 µm C18 110 Å column from Phenomenex
(Aschaffenburg, Germany), with a dimension of 250 mm × 4.6 mm (flow 1 mL/min) for analytical
19
separations and 250 mm × 10 mm (flow 5 mL/min) for semi-preparative applications. 1H, 13C and F
NMR spectra were recorded on a Bruker DPX Avance 200 spectrometer with samples dissolved in
CDCl3 or DMSO-d6. All shifts are given in δ ppm using the signals of the corresponding solvent as a

Molecules 2015, 20
479
reference. Mass spectra were obtained with a Finnigan Automass Multi mass spectrometer with an
electron beam energy of 70 eV. High resolution electron spray mass spectra were recorded on a LTQ
FT Ultra (Thermo Fischer). Melting points are uncorrected and were determined on a Mettler FP-61
apparatus in open capillaries.
3.2. Chemistry
2-((4-(4-Fluorophenyl)piperazine-1-yl)methyl)-1H-indole-5-carbonitrile, FAUC 316 (1) (alternative to
literature method [26]). 2-Formyl-1H-indole-5-carbonitrile (7) (200 mg, 1.18 mmol),
1-(4-fluorophenyl)piperazine (318 mg, 1.76 mmol), sodium cyanoborohydride (1 g, 4.72 mmol) and
acetic acid (280 µL, 4.72 mmol) were dissolved in 12 mL of methanol and heated at 60 °C for 20 h.
After cooling to room temperature the solution was extracted with ethyl acetate, washed with brine and
dried over sodium sulfate. Upon vacuum evaporation the product was purified by flash chromatography
(n-hexane/ethyl acetate 2:1) to obtain 190 mg (0.57 mmol, 49%) of 1. TLC (dichloromethane/methanol
95:5): Rf = 0.73. 1H NMR (200.13 MHz, DMSO-d6) δ 2.58 (t, br, J = 4.16 Hz, J = 5.1 Hz, 4 H), 3.12
(t, br, J = 5.22 Hz, J = 4.12 Hz, 4 H), 3.73 (s, 2 H), 6.50 (s, 1 H), 6.95–7.05 (m, 4 H), 7.40 (dd,
13
J = 8.42, J = 1.56, 1 H), 7.51 (d, J = 8.46, 1 H), 8.01 (s, 1 H), 11.67 (s, 1 H) ppm. C NMR (50.32
MHz, DMSO-d6) δ 49.42, 53.09, 55.25, 60.22, 101.25, 102.10, 112.73, 115.48, 115.91, 117.49,
117.64, 121.32, 123.96, 125.50, 128.14, 138.64, 139.38, 148.37, 170.78 ppm. 19F NMR (188.28 MHz,
DMSO-d6) δ −125.61 ppm. FT-MS (ESI): 335.11 m/z (100) [M + H]+.
3.3. Radiochemistry
3.3.1. Reactive n.c.a [18F]Fluoride
N.c.a. [18F]fluoride was produced via the 18O(p,n)18F nuclear reaction by bombardment of
isotopically enriched [18O]water in a Ti-target [55] with 17 MeV protons at the JSW cyclotron BC
1710 (INM-5, Forschungszentrum Jülich). The produced [18F]fluoride was isolated from the irradiated
water through electrochemical adsorption on a Sigradur-Anode (HTW Hochtemperatur-Werkstoffe
GmbH, Germany) and subsequent desorption into 500 µL of pentadistilled water after recovery of
the 18O-enriched water [56]. An aliquot of the [18F]fluoride solution (10–50 μL, 75–375 MBq) was
filled into a 5-mL conical vial (Reactivial) containing 1 mL of acetonitrile and either 10 mg of
Kryptofix®2.2.2 and 13 μL of an aqueous 1 M potassium carbonate solution or 80 µL of a 0.64 M
tetrabutylammonium bicarbonate solution [57]. The solvent was evaporated under a stream of argon at
80 °C and 600 mbar. This azeotropic drying was repeated twice using each time 1 mL of dry
acetonitrile, followed by evaporation at 8–15 mbar for 5 min.
3.3.2. General Preparation of 1-[18F]Fluoro-4-iodobenzene ([18F]4)
Method A: A solution of bis(4-iodophenyl)iodonium triflate [41] (20 mg, 30 μmol) dissolved in
0.5 mL of anhydrous DMF was added to the vial containing the dried [18F]fluoride/Kryptofix®2.2.2
complex at 130 °C. Monitoring of the reaction progress was performed by radio HPLC of about
30–50 μL aliquots taken at regular time intervals (Gemini 5 μ RP18 A110, 250 × 4.6 mm, 1 mL/min,
isocratic 75:25:0.5 v/v/v CH3CN/H2O/TEA) in order to determine the optimal reaction time.

Molecules 2015, 20
480
After 10 min reaction time 100 mg of Celite 503 suspended in 20 mL of water was added to the
reaction mixture and the total solid was removed by a LiChrolut cartridge (Merck) with a 10 µm PTFE
strainer. After washing with 1 mL of water the almost clear solution was passed through a Sep-Pak
C18 cartridge, which was previously conditioned with 8 mL of ethanol and 8 mL of water. After
washing the cartridge with 5 mL of water and drying for 5 min by a stream of argon the product was
subsequently eluted through an unconditioned Alumina N cartridge with 2 mL of anhydrous toluene
into a second reaction vial.
Method B: A solution of (2,2-dimethyl-5,7-dioxo-1,3-dioxocan-6-yl)-(4-iodophenyl)iodonium
ylide [43] (20 mg, 40 µmol) dissolved in 0.5 mL of anhydrous DMSO was added to the vial containing
the dried [18F]fluoride/tetrabutylammonium bicarbonate complex at 110 °C. Monitoring of the reaction
progress was done by radio HPLC of about 30–50 µL aliquots taken at regular time intervals (Gemini
5 μ RP18 A110, 250 × 4.6 mm, 1 mL/min, isocratic 75:25:0.5 v/v/v CH3CN/H2O/TEA) in order to
determine the optimal reaction time. The work-up procedure is identical to that of method A.
3.3.3. General Procedure of N-Arylation via Hartwig-Buchwald Coupling
The palladium catalyst Pd2(dba)3, palladium(π-cinnamyl) chloride dimer, or Pd(OAc)2 (5.5 µmol),
11 µmol of the biaryl phosphine ligand 2-dicyclohexylphosphino-2′-(N,N-dimethylamino)biphenyl
(DavePhos), 2-dicyclohexyl-phosphino-2′,6′-diisopropoxybiphenyl (RuPhos), or 2-(dicyclohexyl-
phosphino)-3,6-dimethoxy-2′,4′,6′-triisopropyl-1,1′-biphenyl (BrettPhos), 100 µmol of the base
(NaOtBu or K2CO3 or K3PO4 or Cs2CO3) and 50 µmol of the amine compound were placed in a
reaction vial equipped with silicon septum and a magnetic stirrer. The vial was evacuated for 30 min at
10 mbar and set under an atmosphere of dry argon Thereafter, [18F]4 was directly added by passing the
precursor dissolved in toluene, 1,4-dioxane or THF through an AluminaN® drying cartridge into the
reaction vial and the mixture was heated at 100 °C.
When DMF or DMSO was used for N-arylation, [18F]4 was directly transferred into the second vial
containing all reactants under an atmosphere of argon. Reaction progress was monitored by
radio-HPLC (Gemini 5 µm RP18 A110, 250 × 4.6 mm, 1 mL/min, isocratic 75:25:0.5 v/v/v
CH3CN/H2O/TEA) of aliquots of about 30–50 µL diluted tenfold with the elution solvent and typically
containing about 37 kBq of activity.
3.3.4. Pre-Purification of 4-[18F]Fluorophenylpiperazine ([18F]6)
Method A: Liquid-liquid extraction of 4-[18F]fluorophenylpiperazine ([18F]6). The reaction mixture
was extracted twice with 2 mL of hydrochloric acid (2 M) and the aqueous phases were treated with
4 mL of a 2 M sodium hydroxide solution. The solution was afterwards passed through a Sep Pak C18
cartridge, conditioned with 8 mL of ethanol and 8 mL of water, and dried for 2 min in a stream of
argon. The product was then eluted with 1–2 mL of anhydrous DMSO or acetonitrile.
Method B: Solid-liquid extraction of 4-[18F]fluorophenylpiperazine ([18F]6). The reaction mixture
was diluted with 5 mL of methanol and passed through a Strata-X-CW weak cation exchanger
cartridge, conditioned with 10 mL of methanol and 10 mL of water. After the cartridge was washed
with 5 mL of an ammonium acetate buffer (pH 6.3) and 5 mL of methanol, the product was eluted with
1–2 mL of a mixture of acetonitrile and methanol 80:20 (v/v) or DMSO containing 2% of formic acid.

Molecules 2015, 20
481
3.3.5. General Preparation of [18F]1 and [18F]9 by Reductive Amination
Sodium cyanoborohydride (4 mg, 64 µmol) in 50 µL of DMSO and 1H-indole-2-carbaldehyde (8)
(5.8 mg, 40 µmol) or 2-formyl-1H-indole-5-carbonitrile (7) (6.8 mg, 40 µmol) in DMSO (50 µL) and
acetic acid (40 µL) were added to the reaction vial containing [18F]6 in DMSO. The solution was
stirred for about 15 min. For determination of reaction progress the aliquots were analyzed by
radio-HPLC using different systems: (A) Phenomenex Luna 5 µm C18(2) 100 Å 250 × 4 mm,
1 mL/min, isocratic 60:40:0.03 v/v/v CH3CN/H2O/TEA pH 9, (B) Phenomenex Gemini 5 µm C18
100 Å 250 × 4.6 mm, 1 mL/min, isocratic 60:40:0.03 v/v/v CH3CN/H2O/TEA pH 9, (C) Phenomenex
Luna 5 µm PFP(2) 100 Å 250 × 4.6 mm, 1 mL/min, isocratic 50:50:0.01 v/v/v CH3CN/H2O/TEA
pH 7.8. After addition of 20 mL of water the solution was passed through a Sep Pak C18 cartridge
followed by washing with 5 mL water and drying with air. Then the cartridge was eluted with 1 mL of
acetonitrile and the eluate injected on a semi-preparative HPLC system (Phenomenex Luna 5 µm
PFP(2) 100 Å 250 × 10 mm, 4 mL/min, isocratic 50:50:0.01 v/v/v CH3CN/H2O/TEA pH 7.8). The
collected fraction was diluted with 15 mL of water, passed through a SepPak C18 cartridge, which was
then washed with 5 mL of water and dried in a stream of argon. The product was eluted from the
cartridge with 4 mL of diethylether, then the solvent evaporated in vacuo (800 up to 330 mbar) and the
product re-dissolved in 100–300 µL of ethanol. The molar activity was determined by HPLC using a
UV mass calibration curve as described earlier in detail [58].
3.3.6. In Vitro Evaluation of [18F]FAUC 316 ([18F]1)
After anesthetizing and decapitation of rats (4–6 month old female Wistar rats with 230–250 g body
weight), whole brains were rapidly removed and immediately frozen at −80 °C until use. Brain
sections were prepared in a cryostat microtome (CM 3050; Leica; section thickness 20 µm) at −20 °C,
thaw-mounted onto silica-coated object slides, dried on silica gel overnight at 4 °C and stored at
−80 °C until use. Institutional approval of animal studies was obtained from local government
authorities (KFA: 9.93.2.10.35.07.244).
Incubation conditions for in vitro autoradiography of all tested substances were similar to those
previously described by Zhang et al. [59]. All incubations were performed at 22 °C in Tris-HCl buffer
(50 mmol/L, pH 7.4). After pre-incubation in buffer for 10 min rat brain slices were incubated in
10 nmol/L of [18F]FAUC 316 for 30 min either with 10 µmol/L of competitor (spiperone or standard 1)
or with the same amount of DMSO, respectively. They were washed twice for 5 min in ice-cold
Tris-HCl buffer, rapidly rinsed with ice-cold distilled water, and placed under a stream of dry air for
rapid drying. Object slides were exposed to γ-sensitive phosphor-imaging plates for 15–30 min and
then laser scanned by a phosphor imager BAS 5000 (Fuji). The analysis of receptor autoradiography
was processed according to standard image analysis software (AIDA 2.31; Raytest Isotopenmeßgeräte,
Germany). Non-specific binding was defined as the residual activity in the presence of cold standard.
Thus, specific binding was calculated as the difference between total and non-specific binding.

Molecules 2015, 20
482
4. Conclusions
Hartwig-Buchwald coupling is traditionally performed in toluene together with a strong base like
sodium or potassium tert-butoxide. This Pd-mediated reaction was chosen in order to synthesize
the intermediate n.c.a. 1-(4-[18F]fluorophenyl)piperazine ([18F]6) in a one-pot synthesis starting
from piperazine and 1-[18F]fluoro-4-iodobenzene ([18F]4). Iodonium salts enabled the synthesis of
1-[18F]fluoro-4-iodobenzene in one step using the polar solvents DMF and DMSO with RCY up to
70%. The use of toluene and NaOtBu enables the conversion of [18F]4 into [18F]6 with a RCY of 74%
in the presence of Pd(OAc)2/RuPhos. However, this necessitated the separation of [18F]6 by solid
phase extraction in order to transfer the intermediate into toluene as non-polar solvent which led to a
considerable loss of product.
When using DMF or DMSO as solvent for the coupling, this separation step could be omitted as
18
those solvents are typically also used for the F-labelling reaction. Thus, biarylphosphonium ligands
together with the multidental inorganic bases K3PO4 or Cs2CO3 were best suited for the arylation step
and led to conversion rates of more than 70% in DMF, which is comparable to those in toluene.
Even in DMSO, quantitative conversation was also observed. Overall radiochemical yields of
1-(4-[18F]fluorophenyl)piperazine of up to 40% and 60% were reached in DMF and DMSO,
respectively, depending on the labelling yield of the first step. An additional reaction step led to a quick
radiosynthesis of the D4 agonist [18F]FAUC 316 in good radiochemical yields and high molar activity.
The radioligand showed, however, a high non-specific binding in first in vitro autoradiographic studies.
Acknowledgments
The authors wish to thank Wiebke Sihver for the in vitro evaluation studies, Marcus H. Holschbach
and Dirk Bier for NMR and mass measurements of organic compounds, respectively, and for excellent
help and discussion with regard to synthetic and pharmacological aspects.
Author Contributions
F.K., J.E. and H.H.C. designed the research; F.K. and P.K. performed the experimental work; F.K.,
J.E. and H.H.C. wrote the manuscript. All authors discussed, edited and approved the final version.
Conflicts of Interest
The authors declare no conflict of interest.
References
1. Heck, R.F. Arylation, methylation, and carboxyalkylation of olefins by group VIII metal
derivatives. J. Am. Chem. Soc. 1968, 90, 5518–5526.
2. Wu, X.F.; Anbarasan, P.; Neumann, H.; Beller, M. From Noble Metal to Nobel Prize:
Palladium-Catalyzed Coupling Reactions as Key Methods in Organic Synthesis. Angew. Chem.
Int. Ed. 2010, 49, 9047–9050.

Molecules 2015, 20
483
11
3. Allard, M.; Fouquet, E.; James, D.; Szlosek-Pinaud, M. State of art in C labelled radiotracers
synthesis. Curr. Med. Chem. 2008, 15, 235–277.
18
4. Ermert, J. F-Labelled Intermediates for Radiosynthesis by Modular Build-Up Reactions: Newer
Developments. BioMed Res. Int. 2014, 2014, doi:10.1155/2014/812973.
5. Way, J.; Bouvet, V.; Wuest, F. Synthesis of 4-[18F]fluorohalobenzenes and Palladium-mediated
18
Cross-coupling Reactions for the Synthesis of F-labeled Radiotracers. Curr. Org. Chem. 2013,
17, 2138–2152.
6. Elizarov, A.M. Microreactors for radiopharmaceutical synthesis. Lab Chip 2009, 9, 1326–1333.
7. Lu, S.Y.; Pike, V.W. Micro-reactors for PET tracer labeling. In PET Chemistry: The Driving
Force in Molecular Imaging; Schubiger, P.A., Lehmann, L., Friebe, M., Eds.; Springer-Verlag:
Berlin-Heidelberg, Germany, 2007; Volume 62, pp. 271–287.
8. Ermert, J.; Hocke, C.; Ludwig, T.; Gail, R.; Coenen, H. Comparison of pathways to the versatile
synthon of no-carrier-added 1-bromo-4-[18F]fluorobenzene. J. Label. Compd. Radiopharm. 2004,
47, 429–441.
18
9. Ross, T.L.; Ermert, J.; Hocke, C.; Coenen, H.H. Nucleophilic F-fluorination of heteroaromatic
iodonium salts with no-carrier-added [18F]fluoride. J. Am. Chem. Soc. 2007, 129, 8018–8025.
10. Shah, A.; Pike, V.W.; Widdowson, D.A. The synthesis of [18F]fluoroarenes from the reaction of
cyclotron-produced [18F]fluoride ion with diaryliodonium salts. J. Chem. Soc. Perkin Trans. 1
1998, 2043–2046, doi:10.1039/A802349B.
11. Allain-Barbier, L.; Lasne, M.C.; Perrio-Huard, C.; Moreau, B.; Barré, L. Synthesis of
4-[18F]fluorophenyl-alkenes
and
-arenes
via
palladium-catalyzed
coupling
of
4-[18F]fluoroiodobenzene with vinyl and aryl tin reagents. Acta Chem. Scand. 1998, 52, 480–489.
12. Wüst, F.R.; Kniess, T. No-carrier added synthesis of 18F-labelled nucleosides using Stille
cross-coupling reactions with 4-[18F]fluoroiodobenzene. J. Label. Compd. Radiopharm. 2004, 47,
57–468.
18
13. Steiniger, B.; Wuest, F.R. Synthesis of F-labelled biphenyls via SUZUKI cross-coupling with
4-[18F]fluoroiodobenzene. J. Label. Compd. Radiopharm. 2006, 49, 817–827.
14. Marrière, E.; Rouden, J.; Tadino, V.; Lasne, M.C. Synthesis of analogues of (-)-cytisine for in vivo
studies of nicotinic receptors using positron emission tomography. Org. Lett. 2000, 2, 1121–1124.
15. Hansch, C.; Leo, A.J. Substituent Constants for Correlation Analysis in Chemistry and Biology;
Wiley-lnterscience: New York, NY, USA, 1979.
16. Collins, M.; Lasne, M.C.; Barré, L. Rapid synthesis of N,N′-disubstituted piperazines. Application
to the preparation of no carrier added 1-(4-[18F]fluorophenyl)piperazine and of an [18F]-selective
ligand of serotoninergic receptors (5HT2 antagonist). J. Chem. Soc. Perkin Trans. 1 1992, 23,
3185–3188.
17. Mishani, E.; Cristel, M.E.; Dence, C.S.; McCarthy, T.J.; Welch, M.J. Application of a novel
phenylpiperazine formation reaction to the radiosynthesis of a model fluorine-18-labeled
radiopharmaceutical (18FTFMPP). Nucl. Med. Biol. 1997, 24, 269–273.
18. Wüst, F.R.; Kniess, T. N-Arylation of indoles with 4-[18F]fluoroiodobenzene: Synthesis of
18F-labelled σ2 receptor ligands for positron emission tomography (PET). J. Label. Compd.
Radiopharm. 2005, 48, 31–43.

Molecules 2015, 20
484
19. Betts, H.M.; Robins, E.G. 2-Bromo-6-[18F]fluoropyridine: Two-step fluorine-18 radiolabelling via
transition metal-mediated chemistry. J. Label. Compd. Radiopharm. 2014, 57, 215–218.
20. Scherman, D.; Hamon, M.; Gozlan, H.; Henry, J.P.; Lesage, A.; Masson, M.; Rumigny, J.F.
Molecular pharmacology of niaprazine. Prog. Neuropsychopharmacol. Biol. Psychiatry 1988, 12,
989–1001.
21. Glennon, R.A.; Yousif, M.Y.; Ismaiel, A.M.; el-Ashmawy, M.B.; Herndon, J.L.; Fischer, J.B.;
Server, A.C.; Howie, K.J.B. Novel 1-phenylpiperazine and 4-phenylpiperidine derivatives as
high-affinity.sigma. ligands. J. Med. Chem. 1991, 34, 3360–3365.
22. Wu, W.N.; McKown, L.A.; Kuo, G.H. Hepatic metabolism of two α-1A-adrenergic receptor
antagonists, phthalimide-phenylpiperazine analogs (RWJ-69205 and RWJ-69471), in the rat, dog
and human. Eur. J. Drug Metab. Pharmacokinet. 2006, 31, 271–276.
23. Mockett, B.G.; Guévremont, D.; Wutte, M.; Hulme, S.R.; Williams, J.M.; Abraham, W.C.
Calcium/Calmodulin-dependent protein kinase II mediates group I metabotropic glutamate
receptor-dependent protein synthesis and long-term depression in rat hippocampus. J. Neurosci.
2011, 31, 7380–7391.
24. Ismail, F.M.D. Important fluorinated drugs in experimental and clinical use. J. Fluor. Chem.
2002, 118, 27–33.
25. Gakh, A.A.; Burnett, M.N. Extreme modulation properties of aromatic fluorine. J. Fluor. Chem.
2011, 132, 88–93.
26. Hübner, H.; Kraxner, J.; Gmeiner, P. Cyanoindole Derivatives as Highly Selective Dopamine D4
Receptor Partial Agonists: Solid-Phase Synthesis, Binding Assays, and Functional Experiments.
J. Med. Chem. 2000, 43, 4563–4569.
27. Tietze, R.; Hocke, C.; Löber, S.; Hübner, H.; Kuwert, T.; Gmeiner, P.; Prante, O. Syntheses and
radiofluorination of two derivatives of 5-cyano-indole as selective ligands for the dopamine
subtype-4 receptor. J. Label. Compd. Radiopharm. 2006, 49, 55–70.
28. Prante, O.; Tietze, R.; Hocke, C.; Löber, S.; Hübner, H.; Kuwert, T.; Gmeiner, P. Synthesis,
Radiofluorination, and In Vitro Evaluation of Pyrazolo[1,5-a]pyridine-Based Dopamine D4
Receptor Ligands: Discovery of an Inverse Agonist Radioligand for PET. J. Med. Chem. 2008,
51, 1800–1810.
29. Primus, R.J.; Thurkauf, A.; Xu, J.; Yevich, E.; McInerney, S.; Shaw, K.; Tallman, J.F.;
Gallager, D.W., II. Localization and characterization of dopamine D4 binding sites in rat and
human brain by use of the novel, D4 receptor-selective ligand [3H]NGD 94-1. J. Pharmacol.
Exp. Ther. 1997, 282, 1020–1027.
30. Banerjee, A.; Prante, O. Subtype-selective dopamine receptor radioligands for PET imaging:
Current status and recent developments. Curr. Med. Chem. 2012, 19, 3957–3966.
31. Prante, O.; Maschauer, S.; Banerjee, A. Radioligands for the dopamine receptor subtypes.
J. Label. Compd. Radiopharm. 2013, 56, 130–148.
32. Ye, N.; Neumeyer, J.L.; Baldessarini, R.J.; Zhen, X.; Zhang, A. Update 1 of: Recent Progress
in Development of Dopamine Receptor Subtype-Selective Agents: Potential Therapeutics for
Neurological and Psychiatric Disorders. Chem. Rev. 2013, 113, PR123–PR178.
33. Löber, S.; Hübner, H.; Tschammer, N.; Gmeiner, P. Recent advances in the search for D3- and
D4-selective drugs: probes, models and candidates. Trends Pharmacol. Sci. 2011, 32, 148–157.

Molecules 2015, 20
485
34. Grushin, V.V.; Demkina, I.I.; Tolstaya, T.P. Unified mechanistic analysis of polar reactions of
diaryliodonium salts. J. Chem. Soc. Perkin Trans. 2 1992, 505–511, doi:10.1039/P29920000505.
35. Stang, P.J.; Olenyuk, B.; Chen, K. Preparation of nitrogen-containing bis(heteroaryl)iodonium
salts. Synthesis 1995, 937–938, doi:10.1055/s-1995-4023.
36. Bielawski, M.; Malmgren, J.; Pardo, L.M.; Wikmark, Y.; Olofsson, B. One-Pot Synthesis and
Applications of N-Heteroaryl Iodonium Salts. ChemistryOpen 2014, 3, 19–22.
37. Chun, J.H.; Pike, V.W. Selective syntheses of no-carrier-added 2- and 3-[18F]fluorohalopyridines
through the radiofluorination of halopyridinyl(4[prime or minute]-methoxyphenyl)iodonium tosylates.
Chem. Commun. 2012, 48, 9921–9923.
38. Jang, K.S.; Jung, Y.W.; Gu, G.; Koeppe, R.A.; Sherman, P.S.; Quesada, C.A.; Raffel, D.M.
4-[18F]Fluoro-m-hydroxyphenethylguanidine: A Radiopharmaceutical for Quantifying Regional
Cardiac Sympathetic Nerve Density with Positron Emission Tomography. J. Med. Chem. 2013,
56, 7312–7323.
39. Moon, B.S.; Kil, H.S.; Park, J.H.; Kim, J.S.; Park, J.; Chi, D.Y.; Lee, B.C.; Kim, S.E. Facile
aromatic radiofluorination of [18F]flumazenil from diaryliodonium salts with evaluation of their
stability and selectivity. Org. Biomol. Chem. 2011, 9, 8346–8355.
40. Lee, B.C.; Lee, K.C.; Lee, H.; Mach, R.H.; Katzenellenbogen, J.A. Strategies for the Labeling of
Halogen-Substituted Peroxisome Proliferator-Activated Receptor γ Ligands: Potential Positron
Emission Tomography and Single Photon Emission Computed Tomography Imaging Agents.
Bioconjug. Chem. 2007, 18, 514–523.
41. Cardinale, J.; Ermert, J.; Coenen, H.H. Convenient preparation of (4-iodophenyl)aryliodonium salts.
Tetrahedron 2012, 68, 4112–4116.
42. Carroll, M.A.; Nairne, J.; Smith, G.; Widdowson, D.A. Radical scavengers: A practical solution
to the reproducibility issue in the fluoridation of diaryliodonium salts. J. Fluor. Chem. 2007, 128,
127–132.
43. Cardinale, J.; Ermert, J. Simplified synthesis of aryliodonium ylides by a one-pot procedure.
Tetrahedron Lett. 2013, 54, 2067–2069.
44. Mu, L.; Fischer, C.R.; Holland, J.P.; Becaud, J.; Schubiger, P.A.; Schibli, R.; Ametamey, S.M.;
18
Graham, K.; Stellfeld, T.; Dinkelborg, L.M.; et al. F-radiolabeling of aromatic compounds using
triarylsulfonium salts. Eur. J. Org. Chem. 2012, 2012, 889–892.
45. Way, J.D.; Wuest, F. Automated radiosynthesis of no-carrier-added 4-[18F]fluoroiodobenzene:
A versatile building block in 18F radiochemistry. J. Label. Compd. Radiopharm. 2014, 57, 104–109.
46. Surry, D.S.; Buchwald, S.L. Biaryl phosphane ligands in palladium-catalyzed amination.
Angew. Chem. Int. Ed. 2008, 47, 6338–6361.
47. Surry, D.S.; Buchwald, S.L. Dialkylbiaryl phosphines in Pd-catalyzed amination: A user’s guide.
Chem. Sci. 2011, 2, 27–50.
48. Christensen, H.; Kiil, S.; Dam-Johansen, K.; Nielsen, O.; Sommer, M.B. Effect of solvents on the
product distribution and reaction rate of a Buchwald-Hartwig amination reaction. Org. Process
Res. Dev. 2006, 10, 762–769.
49. Ohshita, K.; Ishiyama, H.; Oyanagi, K.; Nakata, H.; Kobayashi, J. Synthesis of hybrid molecules
of caffeine and eudistomin D and its effects on adenosine receptors. Bioorg. Med. Chem. 2007,
15, 3235–3240.

Molecules 2015, 20
486
50. Tasler, S.; Mies, J.; Lang, M. Applicability aspects of transition metal-catalyzed aromatic
amination protocols in medicinal chemistry. Adv. Synth. Catal. 2007, 349, 2286–2300.
51. Leadbeater, N.E. Fast, easy, clean chemistry by using water as a solvent and microwave heating:
The Suzuki coupling as an illustration. Chem. Commun. 2005, 2881–2902, doi:10.1039/b500952a.
52. Foreman, J.C.; Johansen, T. Textbook of Receptor Pharmacology, 2nd ed.; CRC Press:
Boca Raton, FL, USA, 2003.
53. Gail, R.; Coenen, H.H. A one step preparation of the n.c.a. fluorine-18 labelled synthons:
4-Fluorobromobenzene and 4-fluoroiodobenzene. Appl. Radiat. Isot. 1994, 45, 105–111.
54. Ambrogio, I.; Cacchi, S.; Fabrizi, G. Palladium-Catalyzed Synthesis of 2-(Aminomethyl)indoles
from Ethyl 3-(o-Trifluoroacetamidophenyl)-1-propargyl Carbonate. Org. Lett. 2006, 8, 2083–2086.
55. Qaim, S.M.; Stöcklin, G. Production of Some Medically Important Short-Lived Neutron-Deficient
Radioisotopes of Halogens. Radiochim. Acta 1983, 34, 25–40.
56. Hamacher, K.; Hirschfelder, T.; Coenen, H.H. Electrochemical cell for separation of
[18F]fluoride from irradiated 18O-water and subsequent no carrier added nucleophilic fluorination.
Appl. Radiat. Isot. 2002, 56, 519–523.
57. Coenen, H.H.; Klatte, B.; Knöchel, A. Preparation of n.c.a. [17-18F]-fluoroheptadecanoic acid in
high yields via aminopolyether supported, nucleophilic fluorination. J. Label. Compd. Radiopharm.
1986, 23, 455–466.
58. Ermert, J.; Stüsgen, S.; Lang, M.; Roden, W.; Coenen, H.H. High molar activity of [11C]TCH346
via [11C]methyl triflate using the “wet” [11C]CO2reduction method. Appl. Radiat. Isot. 2008, 66,
619–624.
59. Zhang, K.; Weiss, N.T.; Tarazi, F.I.; Kula, N.S.; Baldessarini, R.J. Effects of alkylating agents on
dopamine D3 receptors in rat brain: Selective protection by dopamine. Brain Res. 1999, 847, 32–37.
Sample Availability: All compounds used in the study are commercially available.
© 2014 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/4.0/).