Synthesis and catalytic activity of fluorous chiral primary amine-thioureas†
Three enantiopure fluorous thioureas featuring a free –NH2 group were synthesized by direct addition of aromatic isothiocyanates bearing a single n-C8F17 substituent in the ortho, meta and para position, respectively, to enantiopure (1R,2R)-1,2-diaminocyclohexane. The catalytic behavior of these bifunctional molecules was assessed in representative Michael-type reactions. The three fluorous thioureas performed similarly in all the reactions tested, thus showing that the position of the fluorous ponytail does not have a major influence on the catalytic behavior of this class of compounds. In particular, excellent enantioselectivities (up to 99% ee) and yields (up to 98%) were obtained for the addition of aliphatic aldehydes to maleimides to give α-substituted succinimides. Recyclability of these primary amine-based thioureas was found to be limited by the concurrent formation of imine-derivatives during the catalytic process, leading to a structural modification of the organic catalyst.