Higher-Order Cyclopropenimine Superbases: Direct Neutral Brønsted Base Catalyzed Michael Reactions with α‑Aryl Esters

The
synthesis and characterization of six new classes of higher-order
superbases, including five that incorporate cyclopropenimine functionality,
has been achieved. We propose a nomenclature that designates these
as the CG2, GC2, PC3, PC1, C3, and GP2 classes of superbases. The pKBH+ values were measured to be between 29.0
and 35.6 in acetonitrile. Linear correlations of ten superbase basicities
vs that of their substituents demonstrated the insulating effect of
the cyclopropenimine core. The molecular structures of several of
these materials were obtained by single-crystal X-ray analysis, revealing
interesting aspects of conformational bias and noncovalent organization.
The types of superbasic cores and substituents were each reliably
shown to affect selectivity for deprotonation over alkylation. Higher-order
cyclopropenimine and guanidine superbase stability to hydrolysis was
found to correlate to basicity. Finally, a GC2 base was
found to catalyze conjugate additions of α-aryl ester pronucleophiles,
representing the first report of a neutral Brønsted base to catalyze
such reactions.

成功合成并表征了六类新型高阶超碱，其中五类包含环丙烯亚胺官能团。本研究提出了一种命名法，将这些超碱命名为CG2、GC2、PC3、PC1、C3和GP2系列。在乙腈中测定的pKBH+值介于29.0至35.6之间。十个超碱碱度与其取代基之间的线性相关性揭示了环丙烯亚胺核心的隔离效应。通过单晶X射线分析获得了这些材料中若干分子的结构，揭示了构象偏倚和非共价组织的有趣方面。已可靠地证明，超碱核心和取代基的类型分别影响对去质子化反应相对于烷基化反应的选择性。高阶环丙烯亚胺和胍基超碱对水解的稳定性被发现与碱度相关。最终，发现一种GC2碱能够催化α-芳基酯亲核前体的共轭加成反应，这是首次报道一种中性的布朗斯台德碱能够催化此类反应。