The stereochemical impact of chiral ionic liquids on [2+2] photochemical cycloadditions and aldol reactions

Abstract

Eight chiral ionic liquids have been prepared for use as solvents to test whether they can induce enantioselectivity in both photochemical [2+2] cycloadditions and aldol reactions using a variety of starting materials. The chiral ionic liquids include 1-butyl-3-methylimidazolium Llactate (chiral anion), 1-butyl-3-methylimidazolium camphorsulfonate (R and S chiral anions), 2- amino-N,N,N-trimethyl-1-butanol-bis(trifluoromethanesulfon)imidate (R and S chiral cations), 1- butyl-3-menthoxymethylimidazolium bis(trifluoromethanesulfonyl)amide (chiral cation), 1- [(3R)-3,7-dimethyloct-6-enyl]-3-methyl-1H-imidazolium bromide (chiral cation), and 1-butyl-3- [(3R)-3,7-dimethyl-6-enyl]-1H-imidazolium bromide (chiral cation). The stability of each chiral ionic liquid was tested to ensure they were well suited for both reaction types. To date, the eight chiral ionic liquids have been used as solvents to impart enantioselectivity in 28 photoadditions and 22 aldol reactions. Specific rotations of the purified products have been obtained. Enhanced optical activity was seen when switching from the achiral environment that utilized classical organic solvents to the chiral environment that utilized chiral ionic liquids as the solvent. Enantiomeric excesses could not be calculated. All products have been characterized by gas chromatography (GC), infrared (IR) spectrometry, mass spectrometry (MS), and most of them by 1H NMR spectroscopy.