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.
通过将分别在正位、偏位和对位具有单个 n-
C8F17 取代基的芳香族异
硫氰酸酯直接加到对映体 (1R,2R)-
1,2-二氨基环己烷上,合成了三种具有游离 -NH2 基团的对映体荧光
硫脲。在具有代表性的迈克尔型反应中,对这些双功能分子的催化行为进行了评估。这三种有荧光的
硫脲类化合物在所有测试反应中的表现相似,从而表明荧光马尾的位置对这类化合物的催化行为没有重大影响。特别是在脂肪醛与马来
酰亚胺的加成反应中,获得了极好的对映选择性(ee高达 99%)和收率(高达 98%),从而得到了 α-取代的琥珀
酰亚胺。研究发现,这些
伯胺基
硫脲的可回收性受到催化过程中同时形成的
亚胺衍
生物的限制,从而导致有机催化剂结构的改变。