Premium
Synthesis and electrochemical evaluation of 2‐substituted imidazolium salts
Author(s) -
Hogan David T.,
Sutherland Todd C.
Publication year - 2018
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.3784
Subject(s) - chemistry , substituent , trifluoromethanesulfonate , bromide , electrochemistry , polar effect , density functional theory , ionic liquid , medicinal chemistry , computational chemistry , combinatorial chemistry , organic chemistry , catalysis , electrode
Abstract Herein, we report the synthesis, electrochemical, and computational evaluation of six 2‐substituted imidazolium bromides and six 2‐substituted imidazolium triflates. All final compounds were obtained in 2 or fewer synthetic steps from inexpensive starting materials and display a single, irreversible electrochemical reduction. The reduction potentials span a range greater than 1 V depending on the electron withdrawing power of the 2‐substituent. Imidazolium bromides such as Bn 2 (H)ImBr reduce with E 1/2 = −2.70 V vs Fc/Fc + , whereas the electron‐withdrawing Br‐containing analog Bn 2 (Br)ImBr reduces at only −1.58 V vs Fc/Fc + . The reduction potential of imidazolium bromides obeys a linear free energy relationship to σ m Hammett constants, whereas imidazolium triflates correlate better with the σ p Hammett constants. These results indicate that the stabilizing effect of the 2‐substituent is anion‐sensitive, changing from induction to resonance upon exchanging bromide for triflate. Predicted electron affinities from density functional theory–optimized structures of imidazolium cations and reduced species more closely match experimental data for the triflates, suggesting that a triflate anion does not electronically perturb the imidazolium core as much as a bromide. Taken together, these data highlight the dual modularity of imidazolium salts by changing both 2‐substituent and anion.