Synthesis and Electrochemical Properties of 2,5-Disubstituted 1,4-Bis(4,5-diphenyl-1 H -imidazol-2-yl)benzene Derivative
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hesis and Electrochemical Properties of 2,5-Disubstituted 1,4-Bis(4,5-diphenyl-1H-imidazol-2-yl)benzene Derivatives D. B. Chugunova,*, E. V. Okinaa, A. S. Timoninaa, L. A. Klimaevaa, and Yu. M. Selivanovaa a
Ogarev Mordovian National Research State University, Saransk, 430005 Russia *e-mail: [email protected] Received February 7, 2020; revised April 17, 2020; accepted April 21, 2020
Abstract—2,5-Disubstituted 1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzenes have been synthesized, and their electrochemical properties have been studied by cyclic voltammetry, in particular the effect of substituents on the redox characteristics has been examined. The reversible electrochemical redox transformation quinone– biradical of the title compounds occurs at positive potentials. Keywords: biradicals, monoradicals, cyclic voltammetry, 1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzene
DOI: 10.1134/S1070428020070167 metry. It was found that steric strain created by two bromine atoms in the central benzene ring is responsible for unusual redox properties of these compounds, in particular for the shift of their reduction potentials to the positive region. The reduction potentials of 2,5-disubstituted 1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzenes are unprecedented among substituted cyclohexa-1,4-dienes.
Due to their high stability toward atmospheric oxygen, triarylimidazolyl radicals are fairly convenient and widely used models for studying structure–reactivity relationships in the series of stable radicals [1–3]. However, there are almost no published data on biradicals derived from these compounds. We previously synthesized 1,4-bis(imidazolyl)benzene systems whose oxidation gave the corresponding quinoid structures, and the latter showed paramagnetic susceptibility on heating as a result of rupture of quinoid bonds. It was presumed that the presence of a substituent in the ortho position of the central benzene ring with respect to the imidazolyl substituent should hamper formation of quinoid structures and that biradicals derived therefrom should possess unusual redox properties. Herein, we report the synthesis of 2-mono- and 2,5-dibromo-1,4-bis(4,5-diphenyl-1H-imidazolyl-2)benzenes as the simplest bisimidazoles. Their electrochemical properties were studied by cyclic voltam-
2,5-Disubstituted 1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzenes 2a–2c were synthesized as outlined in Scheme 1. Initial 2-bromo- and 2,5-dibromoterephthalic acids necessary for the synthesis of dialdehydes 1b and 1c were prepared according to the procedures described in [4, 5]. Dialdehydes 1a–1c were reacted with 2 equiv of benzil and 4 equiv of ammonium acetate to obtain bis-imidazoles 2a–2c, and the latter were oxidized to quinones with potassium hexacyanoferrate(III) in the two-phase system benzene– water in the presence of KOH (Scheme 2). Quinones 3a–3c separated from the reaction mixtures as solids partially soluble in benzene. According to the TLC data
Scheme 1. CHO O
R1 + 2 R
2
Ph + 4 NH OAc 4
Ph
(1) AcOH, Δ (2) Aq. NH3
R1
H N
Ph
N
+ 4 AcOH +
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