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Synlett 2006(1): 0155-0156  
DOI: 10.1055/s-2005-922794
SPOTLIGHT
© Georg Thieme Verlag Stuttgart · New York

N-Alkylimidazolium Tetrafluoroborate

Shafi A. Siddiqui*
Organic Chemistry: Technology Division, National Chemical ­Laboratory, Dr. Homi Bhabha Road, Pune - 411 008, MS, India
e-Mail: shafi@dalton.ncl.res.in;
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Publikationsdatum:
16. Dezember 2005 (online)

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Introduction

In recent years, studies of low-waste routes and reusable reaction media for enhanced selectivity and energy minimization have occupied the interests of synthetic organic chemists the world over. [1] In this context, the use of room-temperature ionic liquids (ILs) as ‘green’ solvents in ­organic synthetic processes has gained considerable ­importance due to their solvating ability, negligible vapor pressure, and easy recyclability. They have the potential to be highly polar yet noncoordinating. In addition to the above-mentioned salient features of ILs as reaction media, they have also been shown to promote and catalyze ­organic transformations of commercial importance under ambient conditions, without the need for any added ­catalyst or ligand.

N-Butylimidazolium tetrafluoroborate ([Hbim]BF4) was first reported by Srinivasan et al. [2] They also synthesized several newer ionic liquids, with varying anions, belonging to this class. These ILs have been fully characterized in terms of 1H and 13 C NMR spectra, thermal stability (TGA/DTA), polarity (using Reichardt’s dye), viscosity and density.

ILs have shown enhanced reactivity and selectivity when employed as reaction media in the absence of any added catalyst due to their inherent Brønsted acidity as compared to the conventional molecular solvents and reagents. Their Brønsted acidic character has been attributed to the acidity of reagents as indicated by signi­ficant downfield shift exhibited by this proton in 1H NMR spectrum, e.g. the NH proton of [Hbim]BF4 shows a NH proton shift of 14.59 ppm. In literature, there are reports on the use of the ionic liquid N-methylimiazolium ­tetrafluoroborate [Hmim]BF4 as reaction medium-cum-promoter. N-Alkylimidazolium tetrafluoroborates [2] can be easily prepared from imdazole by alkylation at room temperature via n-alkylimidazoles followed by quaternization to form the crude ionic liquid 1. This was further purified by evaporating water under pressure followed by filtration through silica gel.