Imidazolium ring

Table 3.1-5 provides data for a series of ionic liquids in which the only difference is the degree of branching within the allcyl chain at the imidazolium ring 3-position [24, 47]. 

The basis set is 6-31G(d,p), and electron correlation at the MP2 level is included. A similar structure is obtained with the AMI and PM3 semi-empirical methods. Density functional theory at the B3LYP/6-31G(dp,p) level also produced the same structure for this ion-pair. The only observed differences between the semi-empiri-cal and the ab initio structures were slightly shorter hydrogen bonds (PM3 and AMI) between FI, F2, and F5 and the G2-F1 (H18) on the imidazolium ring. 

Kinetic studies indicated that external H interacted with the renal brush border Na /H exchanger (resistant-type) at a single site with apparent pKa 7.3-7.5. Because the imidazolium ring of histidine is the principal group that is titratable in this... 

Recent successes are partly based on the use of aryl or bulky aryl substituents at the nitrogen atoms of the imidazolium rings. As ligands they are strong o-donors (stronger than t-BujP) and hardly rr-acccptors, they are sometimes called singlet carbenes . 

It has been noted that imidazolium ions are not inert. Under mild basic conditions, they are deprotonated to give reaetive nucleophiles. For reactions in basic media, the C2 hydrogen of the imidazolium ring can be replaced with an alkyl group. In one study, the C2 hydrogen was substituted by a methyl group. Ionie liquids based on the C2 methylated imidazolium cation were evaluated for the... 

These supported cycloadducts were then treated with a base (LiOH, NaOH) in a mixture of water and alcohol to give the expected free acid derivatives. However, while the latter compounds were readily recovered, the same was not true for the ionic liquid 4b, which was obtained as a dark brown liquid impure by NMR analysis. Very likely, the basic hydrolysis of the ester function caused the deprotonation of the imidazolium ring leading to a series of undesired side-reactions. Therefore, milder reaction conditions were explored to cleave the Diels-Alder product from the ionic liquid support. Handy and Okello found that the best method was the cyanide-mediated transesterification that gave the corresponding methyl esters 9-11 and allowed recover of 4b in at least 90% yield. It was also demonstrated that the recovered 4b could be used for further supported syntheses. In fact, in two subsequent mns the yields of the final ester compound were similar, indicating that the ionic liquid 4b could be efficiently recycled. 

The solubility of IL is strictly dependent on melting temperature of the solute, which is generally the result of the alkane or phenyl substituents at imidazolium ring. The conclusions can be taken from the solubilihes of benzimidazole and 2-phenylimidazole in water [94] and of every measured imidazoles in organic solvents [83-89]. 

---