Chiral ionic liquids with a (−)-menthol component as wood preservatives

Abstract

Abstract Research has been conducted on the biotic activity of eight chiral ionic liquids (CILs) with a natural component in the form of (−)-menthol with the configuration (1R,2S,5R) against Basidiomycetes and Fungi imperfecti. Optically active imidazolium salts were tested as follows: several asymmetric 3-alkyl-1-[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium chlorides with various alkyl substituents (from methyl to dodecyl) and one symmetric 1,3-bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium chloride containing two (1R,2S,5R)-(−)-menthol substituents. Mycological tests were carried out with brown-rot and white-rot fungi, as well as with filamentous fungi, i.e. mold fungi and blue stain fungi. The new, optically active compounds demonstrated diverse effectiveness of fungicidal activity, which was dependent on the chemical structure of the functional groups. The natural (−)-menthol component, introduced as a substituent in ionic liquids, turned out to be an optically active compound with activity against wood-destroying fungi. (1R,2S,5R)-(−)-menthol was especially active against the fungi tested. The fungicidal value of imidazolium chloride with two (−)-menthol substituents against Coniophora puteana ranged from 1.8 to 2.9 kg m−3. The adsorption processes on Scots pine wood and resistance to water leaching were examined. The sorption properties of symmetric salts with two optical active (−)-menthol substituents fitted very well with the Freundlich adsorption model. Attenuated total reflection-infrared (ATR-IR) spectra confirmed that methylene groups of the aliphatic substituent, the ring of the heterocyclic imidazole, as well as the ether bridge of the natural (−)-menthol component were fixed in the structure of the impregnated pine wood (Pinus sylvestris L.). The positive results encourage further investigation of the potential application of novel functional CILs (Figure 1) for wood protection.