A Green Chemical Approach for Iodination of Pyrimidine Derivatives by Mechanical Grinding under Solvent-Free Conditions

The iodination of pyrimidines is usually carried out by using toxic reagents under acidic conditions, such as with sulfuric acid and nitric acid. To avoid toxic reagents, we developed a simple and eco-friendly approach for the iodination of pyrimidine derivatives under solvent-free conditions using solid iodine and AgNO3 as an electrophilic iodinating reagent. The advantages of this method are the relatively short reaction time (20–30 min), simple set-up procedure, high yields (70–98%), and environmentally friendly reaction conditions. Our novel approach for the iodination of pyrimidines, as well as a variety of their derivatives, will contribute to the development of nucleobase-related drug candidates.


Introduction
Many studies on modified nucleobases have reported that C5-halogeneted (iodo and bromo) pyrimidine is highly active in medicinal usage, and is widely utilized for therapeutic purposes. In particular, iodinated uridine (C5-iodo-2 -deoxyuridine) is widely used as an antiviral drug [1,2]. Iodinated nucleotides are essential precursors for various functional group transformations [3,4]. Therefore, developing a convenient procedure for the iodination of pyrimidines is an important field in synthetic bioorganic chemistry.
In the production of a wide variety of pharmaceutical and bioactive materials, aromatic iodides, including iodinated pyrimidines, play an essential role as intermediates [5,6]. Electrophilic aromatic substitutions are one of the most widely used synthetic methods to create C-I bonds in aromatic iodides [7,8]. In the case of the electron-deficient arenes and heterocycles, they can be iodinated through the combination of molecular iodine (I 2 ) and electrophilic iodinating reagents, such as silver methylsulfonate (AgOMs), as follows (Scheme 1) [9]:

Introduction
Many studies on modified nucleobases have reported that C5-halogeneted (iodo and bromo) pyrimidine is highly active in medicinal usage, and is widely utilized for therapeutic purposes. In particular, iodinated uridine (C5-iodo-2′-deoxyuridine) is widely used as an antiviral drug [1,2]. Iodinated nucleotides are essential precursors for various functional group transformations [3,4]. Therefore, developing a convenient procedure for the iodination of pyrimidines is an important field in synthetic bioorganic chemistry.
In the production of a wide variety of pharmaceutical and bioactive materials, aromatic iodides, including iodinated pyrimidines, play an essential role as intermediates [5,6]. Electrophilic aromatic substitutions are one of the most widely used synthetic methods to create C-I bonds in aromatic iodides [7,8]. In the case of the electron-deficient arenes and heterocycles, they can be iodinated through the combination of molecular iodine (I2) and electrophilic iodinating reagents, such as silver methylsulfonate (AgOMs), as follows (Scheme 1) [9]: The electrophilic iodination of aromatic substrates is quite difficult because iodine has weak reactivity. Thus, this reaction can only occur under harsh reaction conditions such as acidic conditions using nitric acid, acetic acid, or sulfuric acid, or using strong oxidative agents as iodination sources. Environmentally friendly methods for the iodination of pyrimidines have not yet been reported.
We developed solvent-free mechanochemistry methods for the iodination of pyrimidines, namely uracil and cytosine, as well as their derivatives, which have advantages in terms of generating less pollution and lower costs. We developed a procedure for iodination at the C5 position of pyrimidines via a solid-state reaction, performed by grinding all of the reaction mixtures (Figure 1). We performed the reactions using molecular iodine (I 2 ) and various nitrate/nitrite salts (AgNO 3 , NaNO 3 , and NaNO 2 ) as follows (Scheme 2): Ar-H and Ar-I indicate the aromatic molecules and aromatic iodide, respectively. Pyrimidines, which are one type of heterocycles, and their derivatives are also able to be iodinated through similar methods.
The electrophilic iodination of aromatic substrates is quite difficult because iodine has weak reactivity. Thus, this reaction can only occur under harsh reaction conditions such as acidic conditions using nitric acid, acetic acid, or sulfuric acid, or using strong oxidative agents as iodination sources. Environmentally friendly methods for the iodination of pyrimidines have not yet been reported.
We developed solvent-free mechanochemistry methods for the iodination of pyrimidines, namely uracil and cytosine, as well as their derivatives, which have advantages in terms of generating less pollution and lower costs. We developed a procedure for iodination at the C5 position of pyrimidines via a solid-state reaction, performed by grinding all of the reaction mixtures ( Figure 1). We performed the reactions using molecular iodine (I2) and various nitrate/nitrite salts (AgNO3, NaNO3, and NaNO2) as follows (Scheme 2): Scheme 2. Chemical reaction scheme for the synthesis of 5-iodo-pyrimidine derivatives.
In this study, direct iodination at the C5 position of the pyrimidines using nitrate salts and iodine was performed under solvent-free conditions at room temperature (Figure 1). The combination of iodine and nitrate salts was found to be the best reagent for the regioselective iodination reactions shown in Figure 1c. The C5-iodo pyrimidine derivatives were confirmed and identified by 1 H and 13 C NMR and ESI mass spectroscopy. This study suggests an environmentally friendly approach for the iodination of pyrimidine derivatives that will contribute toward the development of nucleobase-related drug candidates.  In this study, direct iodination at the C5 position of the pyrimidines using nitrate salts and iodine was performed under solvent-free conditions at room temperature ( Figure 1). The combination of iodine and nitrate salts was found to be the best reagent for the regioselective iodination reactions shown in Figure 1c. The C5-iodo pyrimidine derivatives were confirmed and identified by 1 H and 13 C NMR and ESI mass spectroscopy. This study suggests an environmentally friendly approach for the iodination of pyrimidine derivatives that will contribute toward the development of nucleobase-related drug candidates.
Ar-H and Ar-I indicate the aromatic molecules and aromatic iodide, respectively. Pyrimidines, which are one type of heterocycles, and their derivatives are also able to be iodinated through similar methods.
The electrophilic iodination of aromatic substrates is quite difficult because iodine has weak reactivity. Thus, this reaction can only occur under harsh reaction conditions such as acidic conditions using nitric acid, acetic acid, or sulfuric acid, or using strong oxidative agents as iodination sources. Environmentally friendly methods for the iodination of pyrimidines have not yet been reported.
We developed solvent-free mechanochemistry methods for the iodination of pyrimidines, namely uracil and cytosine, as well as their derivatives, which have advantages in terms of generating less pollution and lower costs. We developed a procedure for iodination at the C5 position of pyrimidines via a solid-state reaction, performed by grinding all of the reaction mixtures ( Figure 1). We performed the reactions using molecular iodine (I2) and various nitrate/nitrite salts (AgNO3, NaNO3, and NaNO2) as follows (Scheme 2): Scheme 2. Chemical reaction scheme for the synthesis of 5-iodo-pyrimidine derivatives.
In this study, direct iodination at the C5 position of the pyrimidines using nitrate salts and iodine was performed under solvent-free conditions at room temperature (Figure 1). The combination of iodine and nitrate salts was found to be the best reagent for the regioselective iodination reactions shown in Figure 1c. The C5-iodo pyrimidine derivatives were confirmed and identified by 1 H and 13 C NMR and ESI mass spectroscopy. This study suggests an environmentally friendly approach for the iodination of pyrimidine derivatives that will contribute toward the development of nucleobase-related drug candidates.

Optimization of Iodination Reactions of Uracil and Cytosine
A previous study reported that AgNO 3 was found to give better results than other nitrate salts for the simple aromatic substrates [7]. Here, AgNO 3 acted as a Lewis acid and generated nitryl iodide in situ by reaction of silver nitrate with iodine (Scheme 2). Although the grinding reactions were carried out in a solvent-free environment, a few drops of acetonitrile were used for easier grinding. Interestingly, this gentle grinding method was effective for C5 iodination under solid iodine and silver nitrate conditions. Furthermore, the common reaction byproduct was silver iodide (Scheme 2) [7,10].

Iodination Reaction Using Ag 2 SO 4 , NaNO 3 , and NaNO 2
In order to clarify the salt effect on the solvent-free iodination of pyrimidine derivatives, we also performed the same reaction with Ag 2 SO 4 , NaNO 3 , and NaNO 2 as the Lewis acid to generate nitryl iodide.
In the case of uracil, the reaction yield to 5-iodo-uracil was 73% when Ag 2 SO 4 was used (Table 3). However, the reaction with NaNO 3 and NaNO 2 led to reaction yields of only 33% and 38%, respectively (Table 3). A similar result was observed for the iodination of uridine (Table 3). Surprisingly, when Ag 2 SO 4 , NaNO 3 , and NaNO 2 were used, only <50% of dU and mU could be converted to 5I-dU and 5I-mU, respectively (Table 3).
In the case of cytidine derivatives, all iodination reactions with Ag 2 SO 4 , NaNO 3 , and NaNO 2 showed reaction yields lower than 35%, except the iodination of mU with Ag 2 SO 4 ( Table 3). The iodination of mU with Ag 2 SO 4 had 80% yield ( Table 3).
The synthetic strategy for the electrophilic iodination of pyrimidine was an electrophilic reaction of the reactive iodine species produced by nitrate salt. Table 3 summarizes the direct iodination and the oxidative iodination required for I + species, which were generated by an electrophilic iodinating reagent (Ag 2 SO 4 ), in comparison to the other metal catalysts (NaNO 3 and NaNO 2 ) [12][13][14]. The reactivity was different depending on each pyrimidine substrate. Based on the obtained results, we can conclude that the order of the reactivity of the nitrate salts is AgNO 3 >> Ag 2 SO 4 >> NaNO 3 >> NaNO 2 .
It has been reported that electrophilic reactions can be catalyzed by metal-based salts [15,16]. Silver-containing salts can more efficiently catalyze this reaction [17]. Our results revealed that AgNO 3 is a better catalyst for the direct iodination reaction compared to Ag 2 SO 4 , even though both contain the Ag + ion. In the presence of oxygen, NaNO 3 and NaNO 2 can also promote this reaction, although their efficiencies were slightly lower than the Ag + -based catalysts. In addition, NaNO 2 showed higher efficiency than NaNO 3 , because it can be converted to nitrogen oxide [12,18].

General Information for Experiments
All the reagents and solvents were purchased from local commercial suppliers. The elemental iodine and nitrate salts were purchased from Dungsun and used without further purification. The 1 H NMR spectra were recorded on a Bruker 300-MHz instrument with DMSO as the solvent and TMS as the internal standard (TMS, δ = 0.00 ppm). The 13 C NMR spectra were recorded on a Bruker 500 MHz instrument with DMSO as the solvent. All the products were identified by comparison of their spectral and physical data with those of the known sample. The progress of the reaction and the purity of the products were checked by thin-layer chromatography (TLC) on silica gel 60 F254-coated TLC plates (Merck KGaA, Darmstadt, Germany) and visualized by short-UV light at 254 nm. The coupling constants (J) were reported in Hz. Peak multiplicity was indicated as follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; and dd, doublet of the doublet. Mass spectra were obtained by an LCQ Fleet ion trap mass spectrometer using positive-ion (ESI+) and negative-ion (ESI-) mode electrospray ionizations (ThermoScientific; Qual Browser software, version 2.0.7, Thermo Fischer Scientific, Waltham, MA, USA). An amount of 20 µL of the sample was diluted with HPLC grade methanol solution for analysis. HPLC separation with UV was performed on a 1290 infinity UHPLC system (Agilent technology, Waldbroen,