Camphorpyrazolium-based chiral functional ionic liquids

The solvent-less quaternization of N-phenyl-camphorpyrazole 4 with BrC4H9, IC2H4OCH3, and IC2H4OC2H4OC4H9 afforded the corresponding chiral pyrazolium halides 5a, 6a, and 7a in excellent yields. The anions were modified either by trihalide formation with Br2 and I2, or by salt metathesis with LiNTf2 and NaCo(CO)4. All pyrazolium salts bearing the di-ether side chain 7a-d were liquids at room temperature, while the X-ray crystal structure of the bis(trifluoromethylsulfonyl)amide salt of the corresponding mono-ether analogue 6c (mp 97 °C) revealed intermolecular H-bonding interactions. Furthermore, an improved protocol for the well-known but notoriously low-yielding synthesis of (+)-hydroxymethylenecamphor 3 is disclosed.