4,4′-Di-tert-butyl-2,2′-bipyridinium Trifluoromethanesulfonate

Abstract

4,4′-Di-tert-butyl-2,2′-bipyridinium trifluoromethanesulfonate was synthesized by stirring 4,4′-Di-tert-butyl-2,2′-bipyridine with scandium(III) trifluoromethanesulfonate in acetonitrile, followed by precipitation with diethyl ether. The structure of the new compound was characterized by FT-IR, ¹H, ¹³C{¹H} and ¹⁹F{¹H} NMR spectroscopy and CHN elemental analysis. This is a safe and simple method to obtain mono-protonated bipyridinium trifluoromethanesulfonate without the direct use of trifluoromethanesulfonic acid.

1. Introduction

Protonated pyridinium salts are widely used in synthetic organic reactions (e.g., oxidation reactions [1], Brønsted acid-catalyzed reactions [2,3,4], and photochemical reactions [5]), host-guest chemistry [6,7], and proton-coupled electron transfer processes [8]. Although 2,2′-bipyridines are one of the most commonly used ligands for metals, their protonated forms, the mono- or di-protonated 2,2′-bipyridinium salts [9], are also used as an oxidizing agent [10,11], fluorinating agent [12], and ligand precursor for a palladium catalyst [13]. Protonated pyridinium salts are typically synthesized from the reaction of the corresponding pyridines with Brønsted acids. However, most of these acids (e.g., hydrochloric acid, tetrafluoroboric acid, and trifluoromethanesulfonic acid (TfOH)) are corrosive, toxic, and difficult to handle. Therefore, it is desirable to develop an alternative method to avoid the direct use of such acids.

During the course of our study on 4,4′-Di-tert-butyl-2,2′-bipyridine-based early transition metal complexes [14,15,16,17,18,19,20,21], we developed interest in synthesizing a new scandium(III) complex using a scandium(III) salt and 4,4′-Di-tert-butyl-2,2′-bipyridine (4,4′-^tBubpy; 1). Among the scandium(III) salts, scandium(III) trifluoromethanesulfonate (Sc(OTf)₃) is an efficient and stable Lewis acid catalyst that maintains its catalytic activity even in the presence of water [22]. In addition, Sc(OTf)₃ has recently been used in combination with chiral nitrogen-donor ligands to achieve various enantioselective transformations [23]. Hence, we examined the complexation of 4,4′-^tBubpy (1) with Sc(OTf)₃. Unexpectedly, however, we found that a mono-protonated bipyridinium trifluoromethanesulfonate 2 was obtained instead of a scandium bipyridine complex under the reaction conditions described later (Section 2). We herein report the synthesis of the new mono-protonated bipyridinium trifluoromethanesulfonate 2 without the direct use of TfOH.

2. Results and Discussion

Reaction of 1 with Sc(OTf)₃ in acetonitrile at ambient temperature afforded a pale pink solution. Subsequent addition of undried diethyl ether to this solution resulted in the formation of a white precipitate. Characterization of the compound revealed that the expected scandium bipyridine complex was not formed; instead, mono-protonated bipyridinium trifluoromethanesulfonate 2 was formed (Scheme 1).

The ¹H NMR spectrum of 2 in CDCl₃ shows three signals in the aromatic region at δ 8.88 (d, J = 5.6 Hz, 2H), 8.50 (d, J = 1.6 Hz, 2H), and 7.72 (dd, J = 5.6, 1.6 Hz, 2H), and these signals are shifted downfield with respect to those of 1 (Figure 1). This suggests the existence of a protonated nitrogen atom in 2 [13] (¹H, ¹³C{¹H} and ¹⁹F{¹H} NMR spectra of 2 are included in the Supplementary Materials). We also obtained long plate-like crystals by slow diffusion of diethyl ether into the acetonitrile solution of 2. Although the obtained crystals were not of good quality, preliminary X-ray structure analysis of the crystals has confirmed the molecular structure of 2.

Although Sc(OTf)₃ is often used as a stable Lewis acid even in water [22], this salt reversibly converts to TfOH through the hydrolysis of Sc³⁺ [24,25]. In the present reaction, we speculate that Sc(OTf)₃ reacts with the water in the reaction solvent to reversibly generate TfOH, followed by protonation of 1 to afford 2. Bipyridinium salt 2 has low solubility in an acetonitrile–diethyl ether mixture and readily precipitates in the mixed solvent, shifting the reaction equilibrium to the product 2 side. Although it has already been known that the protonation of a pyridine derivative occurs in the presence of Sc(OTf)₃ and water in organic solvent [24], to our knowledge the preparation of protonated pyridinium trifluoromethanesulfonates based on this mechanism has not been explored. The protocol presented herein will provide safe and simple access not only to mono-protonated bipyridinium trifluoromethanesulfonates but also to a variety of protonated pyridinium trifluoromethanesulfonates without the direct use of TfOH.

3. Materials and Methods

3.1. General

All the reagents and solvents were purchased from chemical companies and used without further purification. ¹H NMR spectra were recorded on a JEOL JNM-ECS400 (400 MHz) FT NMR system or JEOL JMN-ECX400 (400 MHz) FT NMR system in CDCl₃ with Me₄Si as an internal standard. ¹³C{¹H} NMR spectrum was recorded on a JEOL JNM-ECS400 (100 MHz) FT NMR system in CDCl₃. ¹⁹F{¹H} NMR spectrum was recorded on a Bruker AVANCE NEO 400 spectrometer (376 MHz). The IR spectrum was recorded on a JASCO FT/IR-410 spectrometer.

3.2. Synthesis of 4,4′-Di-tert-butyl-2,2′-bipyridinium Trifluoromethanesulfonate (2)

Scandium(III) trifluoromethanesulfonate (247.5 mg, 0.5 mmol) and 4,4′-Di-tert-butyl-2,2′-bipyridine (1; 406.2 mg, 1.5 mmol) were stirred in acetonitrile (2.5 mL) at ambient temperature for 2 h. Diethyl ether was added to the resulting pale pink solution, and a white precipitate was obtained. The precipitate was collected and washed with diethyl ether to give 2 (270.8 mg, 43% yield based on 1) as a white powder. Mp 216.5–217.0 °C; ¹H NMR (CDCl₃, 400 MHz) δ 8.88 (d, J = 5.6 Hz, 2H), 8.50 (d, J = 1.6 Hz, 2H), 7.72 (dd, J = 5.6, 1.6 Hz, 2H), 1.47 (s, 18H) [Note: The proton (H⁺) signal of compound 2 could not be observed in the range of −2.5 to 20.5 ppm. This might be due to the rapid proton exchange of N–H.]; ¹³C{¹H} NMR (CDCl₃, 100 MHz) δ 168.0, 148.1, 146.2, 123.8, 120.6, 120.4 (q, J = 318.8 Hz), 36.2, 30.2; ¹⁹F NMR (CDCl₃, 376 MHz) δ −78.2; IR (KBr, cm⁻¹) 3104, 2974, 1631, 1597, 1485, 1442, 1284, 1266, 1227, 1206, 1153, 1035, 848; Anal. Calcd for C₁₉H₂₅F₃N₂O₃S: C, 54.53; H, 6.02; N, 6.69. Found: C, 54.61; H, 5.89; N, 6.57.

4. Conclusions

The new mono-protonated bipyridinium trifluoromethanesulfonate 2 has been synthesized from 4,4′-^tBubpy using Sc(OTf)₃ without the direct use of TfOH. We consider that the present protocol will provide easy and safe access to a variety of protonated pyridinium trifluoromethanesulfonates.