1. Introduction

Since their initial discovery, the halogen azides (IN₃, ClN₃, BrN₃, and FN₃) have been considered challenging compounds requiring skill to control/manipulate, due to their risky chemical reactivity, volatility and potential toxicity [1]. Halogen azides are highly explosive and their stability is believed to ascend in the following order: FN₃ < IN₃ < BrN₃ < ClN₃ [2]. The 1, 2-bromoazidation of alkenes was first reported by Hassner and Boerwinkle [3] by the addition of bromine azide generated from bromine and sodium azide. Subsequently, Hassner and co-workers [4-8] made a major contribution to understanding the mechanism of halogen azide addition to olefins and its regiospecificity through two different pathways, namely a radical or an ionic process. Also, BrN₃ was synthesized by many in situ systems starting from [Br₂ gas/NaN₃] which was modified by Hassner to [Br₂-HCl-NaN₃] [3-5], hydrazoic acid [NXS-HN₃, X=I, Br] [9] and [Br₂/NaN₃ in base] [10]. However, most of these methods have significant hazards related to explosivity and toxicity. The convenience and versatility of in situ techniques were initiated by Krief in 1974 [NBS-NaN₃] [11-16], [NBS-TMSN3] [17] and [metal triflates-NBS-TMSN₃] [18-20].Lastly, several sources of Br⁺ such as TsNBr₂ [21] and PhNMe₃Br₃ [22] have been used with trimethylsilylazide. Many of the described protocols exhibit lack of selectivity, low-yield and require toxic or exotic reagents. So, the [NBS-TCSN₃] reaction system is anticipated as a dependable option for bromoazidation. Haloazidation represents an important transformation in organic synthesis in which vicinal haloazide compounds are versatile precursors of vinyl azides [23], amines [24], heterocycles [25] and particularly aziridines [26]. Therefore, the haloazidation of olefinic double bonds still remains an important and challenging task for organic chemists.

2. Experimental 2.1. Typical procedure for the synthesis of azidobromoarylidine-malononitrile

In a dry, two-neck, round bottom flask equipped with a rubber septum, a magnetic stirring bar and a condenser and surrounded by a light excluding jacket, a mixture of TCS (10 mmol), and sodium azide (10 mmol) was stirred for 10 min, then NBS (10 mmol) and benzalmalononitrile (10 mmol) in CH₃CN (15 mL) were added. This mixture was allowed to stir with the exclusion of moisture at r.t. for the specified time. The mixture was then poured into icecold H₂O (~100 mL) and extracted with diethyl ether (2 × 25 mL), then the extract was dried over anhydrous Na₂SO₄. The solvent was removed under reduced pressure and the obtained product was collected and identified after drying.

Data for representative examples are shown:

2-Azido-1-bromo-1, 1-dicyano-2-(4-bromophenyl)ethane (d): Mp 93-95 ℃; IR (KBr, cm^-1): ν 2934, 2899 (CH), 2224 (CN), 2122 (N₃). 1H NMR (300 MHz, CDCl₃): δ 7.77-7.73 (d, 2H, J=7.8 Hz, ArH), 7.52-7.44 (d, 2H, J=7.8 Hz, ArH), 5.10 (s, 1H, CH); ¹³C NMR (75 MHz, CDCl₃): d 132.92, 132.50, 131.71, 129.99 (Ar-C), 110.03 (2CN), 69.86 (CH), 29.88 (C); MS (EI 70 eV) m/z: 353 (M+); Anal. Calcd. for C₁₀H₅Br₂N₅ (352.89): C, 33.83; H, 1.42; Br, 45.02; N, 19.73. Found: C, 33.80; H, 1.36; Br, 45.00; N, 19.65.

2, 4-Diazido-1, 3-dibromo-1, 1-dicyano-4-phenylbutane (g): Mp. 135 ℃; IR (KBr, cm^-1): ν 2934 (CH), 2250 (CN), 2118 (N₃); ¹H NMR (300 MHz, CDCl₃): δ 7.55-7.35 (m, 5H, ArH), 5.10-5.15 (d, 2H, J=18 Hz), 5.08-5.05 (d, 2H, J=18 Hz), 4.80-4.65 (dd, 1H, J=18 Hz); ¹³C NMR (75 MHz, CDCl₃): δ 136, 129, 128.52, 127 (Ar-C), 109.50, 108.50 (2CN), 69 (C-4), 62 (C-2), 52 (C-3), 28.50 (C-1); MS (EI 70 eV) m/z: 422 (M⁺); Anal. Calcd. for C12H8Br2N8 (421.92): C, 33.99; H, 1.90; Br, 37.69; N, 26.42. Found: C, 33.93; H, 1.85; Br, 37.63; N, 26.35.

3. Results and discussion 3.1. Chemistry

During last decade, several research groups used TCS as a catalyst in synthetic organic chemistry [27]. Also, TCS and silzic play a significant role in our research group activities [28, 29]. In conjunction with our interests, we present, in this work, the utilization of bromine azide to access 1, 2-azidobromides as an anti-Markovinkov addition product of the olefinic double bond in benzalmalononitriles. The reaction occurs by mixing TCS/NaN₃ at a molar ratio 1:1 for 10 min, then adding 1 mol of NBS and 1 mol of benzalmalononitrile in acetonitrile as a solvent and stirring for the appropriate time at r.t. (Scheme 1).

Although BrN₃ generated in situ is soluble in organic solvents [2], many solvents (CH₂Cl₂, CH₃NO₂, CH₂Cl₂ + CH₃NO₂, n-hexane and acetonitrile) were examined and it was determined that acetonitrile has higher solubility as a starting material, less reaction time, high reaction yield and easy work up and also that BrN₃ is sensitivity to light and so the reaction is run with a jacketed flask to exclude light. The suggested mechanism for the formation of bromine azide is explained in Scheme 2 [30].

The proposed reaction mechanism between azidobromides with benzalmalononitrile is explained by two pathways [31]: Pathway A shows the BrN₃ fragmentation by a radical or an ionic substituent as explained by Hassener and coworker. In the presented reaction, the radical fragmentation was excluded because the reaction did not give mixed products and the reaction is done in dark jacket; Pathway B shows the anti-addition step of BrN₃ to benzalmalononitrile (Scheme 3).

In order to support the validity and regioselectivity of the reaction reagent, the data in Table 1 summarizes a preliminary survey of the vicinal azidobromination of benzalmalononitrile. All of the substrates (entries 1-6) underwent anti-Markovnikov addition type reaction where the electron withdrawing power of the two CN groups shift the electron density to the a-position and from this the radical mechanism becomes excluded. Also, the reaction was done with cinnamalmalononitrile (entry 7) using two equivalents of reagents under the same conditions and after 45 min, the 2, 4-diazido-1, 3-dibromo-1, 1-dicyano-4-phenyl (g) was obtained in excellent yield proving the efficacy and regioselectivity of this reagent.

The structures of the resulting vicinal azidobrominations were elucidated by spectroscopic methods. The IR spectrum showed peaks at ν=2250-2215, 2126-2118 cm^-1 corresponding to CN and N₃, respectively. The 1H NMR spectrum of the synthesized products revealed a doublet signal for (1H) in CH at δ 5.07-5.85 and at δ 8.75-7.29 refers to ArCH ppm. From the results presented in Table 1, the reaction times were short (10-30 min) and the yields were high (77%-90%) for all products, especially in the presence of strong electron withdrawing group like nitro group. In the case of fluorine substitution, the mesomeric effect of the 2p orbital of the aromatic ring decreases the withdrawing power electronegativity of fluorine atom rather than 3p orbital of chlorine and 4p orbital of bromine, and also the steric hindrance of Cl and NO₂ groups in the ortho position of the aromatic ring play the main role in increasing the reaction time in the case of compound f.

3.2. Molecular docking methodology

Thiamine pyrophosphate (TPP) riboswitches regulate essential genes in bacteria by changing conformation upon binding intracellular TPP. Riboswitches are promising antimicrobial targets because of their highly specific small molecule recognition prevalent in bacteria, and the association with genes essential for pathogen survival or virulence. And it was noticed that most ligands used against protein targets only recently has this approach been applied to RNA, leading to the discovery of several small molecules that bind the Escherichia coli thiM TPP riboswitch. Thus, even though further substantial development is necessary, fragments have the potential to be elaborated in ligands specific for TPP riboswitches. The standard docking protocol using MOE 2015.10 software was used [32] to study the interaction between the prepared ligand molecules (a-g) and TPP riboswitches. From the validation based on comparing the calculated binding energy scores (kcal/mol) of the prepared ligands (Table 1) and energy score (-6.43 kcal/mol) of the reference ligand [33], there are three ligands (c, e and g) which give less energy scores related to the strong interaction with TPP riboswitch binding sites by forming eight hydrogen bonds.

The selection of vicinal bromoazide scaffold analogs (a-g) to build a pharmacophore for potential Aurora kinase A inhibitors was based on their high potency. The superposition of model example e was generated a pharmacophore (Fig. 1).

The active site analysis of the TPP riboswitches RNA was performed from MOE database similar nucleotide residues receptor as C57, U20, C73, G19, G42, G72, A43, G40, U39 and C58. The interaction between ligand compound e and TPP pocket residue was done by eight hydrogen bonds; the Br atom attached C73 by distance 3.76 Å and energy -1.2 kcal/mol, the N atom in azide attached with C57, G19, G19 by distance (3.51, 3.08 and 3.50 Å, respectively) and energy (-2.4, -5.7 and -2.0 kcal/mol, respectively), the nitrogen atom in cyano group attached U20 and G19 by distance (3.38 and 2.94 Å, respectively) and energy (-1.2 and -2.1 kcal/mol, respectively) and the oxygen atom in nitro groups attached G40 by distance 3.02 Å and energy -2.4 kcal/mol. Also, the interaction between TPP and reference ligand was explained in 3D-structure by forming four hydrogen bonds; nitrogen in azide group with U39 and C73 (3.29 and 4.05 Å respectively), nitrogen in pyrimidine ring with G19 and G40 (3.30 and 3.22 Å respectively) (Fig. 2).

4. Conclusion

Vicinal bromoazide was synthesized from arylidene malononitrile. Thr reaction was started by the generation of BrN3 in situ by the interaction between sodium azide, N-bromosuccinimide and TCS in acetonitrile as a solvent at ambient temperature. This catalytic process represents a highly regioselective method for the synthesis of 1, 2-bromoazides. The reaction products were obtained in excellent yield and characterized by IR, MS, 1H NMR, ¹³C NMR, and elemental analysis. The molecular docking for the synthesized molecules was performed to prove that the synthesized molecules gave strong interaction with TPP riboswitches compared with 5-(azidomethyl)-2-methylpyrimidin-4-amine as the reference molecule.