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Piperidine‐appended imidazolium ionic liquids as task‐specific catalysts: computational study, synthesis, and multinuclear NMR
Author(s) -
Laali Kenneth K.,
Jamalian Arezu,
Borosky Gabriela L.
Publication year - 2016
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.3541
Subject(s) - piperidine , chemistry , carbene , moiety , ionic liquid , methylene , proton nmr , computational chemistry , solvent , density functional theory , alkylation , medicinal chemistry , catalysis , stereochemistry , organic chemistry
Imidazolium ionic liquids (IMILs) with a piperidine moiety appended via variable length methylene spacers (with n = 1–4) were studied computationally to assess their potential to act as internal base for N ‐heterocyclic carbene (NHC) generation. Proton transfer energies computed by B3LYP/6‐311+G(2d,p) were least endothermic for the basic‐IL with n = 3, whose optimized structure showed the shortest C 2 ‐H‐‐‐‐N(piperidine) distance. Inclusion of counter anion (Cl or NTf 2 ) caused dramatic conformational changes to enable close contact between the acidic C 2 ‐H and the anions. To examine the prospect for internal C 2 ‐H‐‐‐‐N coordination, multinuclear NMR data ( 1 H, 15 N, and 13 C) were computed by gauge independent atomic orbitals–density functional theory (GIAO‐DFT) in the gas phase and in several solvents by the PCM method for comparison with the experimental NMR data for the basic ILs (with n = 2–4) synthesized in the laboratory. These studies indicate that interactions with solvent and counter ion are dominant forces that could disrupt internal C 2 ‐H‐‐‐‐N coordination/proton transfer, making carbene generation from these basic‐ILs unlikely without an added external base. Therefore, the piperidine‐appended IMILs appear suitable for application as dual solvent/base in organic/organometallic transformations that require the use of mild base, without the necessity to alkylate at C‐2 to prevent N ‐heterocyclic carbene formation. Copyright © 2016 John Wiley & Sons, Ltd.