Department of Organic Chemistry, University of Geneva, Geneva, Switzerland.
Angew Chem Int Ed Engl. 2020 Apr 6;59(15):6273-6277. doi: 10.1002/anie.202000579. Epub 2020 Feb 20.
The concept of anion-π catalysis focuses on the stabilization of anionic transition states on aromatic π surfaces. Recently, we demonstrated the occurrence of epoxide-opening ether cyclizations on aromatic π surfaces. Although the reaction proceeded through unconventional mechanisms, the obtained products are the same as those from conventional Brønsted acid catalysis, and in agreement with the Baldwin selectivity rules. Different mechanisms, however, should ultimately lead to new products, a promise anion-π catalysis has been reluctant to live up to. Herein, we report non-trivial reactions that work with anion-π catalysis, but not with Brønsted acids, under comparable conditions. Namely, we show that the anion-π templated autocatalysis and epoxide opening with alcoholate-π interactions can provide access to unconventional ring chemistry. For smaller rings, anion-π catalysis affords anti-Baldwin oxolanes, 2-oxabicyclo[3.3.0]octanes, and the expansion of Baldwin oxetanes by methyl migration. For larger rings, anion-π templated autocatalysis is thought to alleviate the entropic penalty of folding to enable disfavored anti-Baldwin cyclizations into oxepanes and oxocanes.
阴离子-π 催化的概念侧重于在芳香 π 表面上稳定阴离子过渡态。最近，我们证明了环氧化合物开环醚环化反应在芳香 π 表面上发生。尽管反应通过非常规机制进行，但得到的产物与传统的 Brønsted 酸催化相同，并且与 Baldwin 选择性规则一致。然而，不同的机制最终应该会产生新的产物，这是阴离子-π 催化一直难以实现的承诺。在此，我们报告了在可比条件下，阴离子-π 催化但不与 Brønsted 酸发生的非平凡反应。也就是说，我们表明，阴离子-π 模板自催化和与醇盐-π 相互作用的环氧化合物开环可以提供非常规的环化学反应途径。对于较小的环，阴离子-π 催化提供反 Baldwin 噁唑烷、2-氧杂双环[3.3.0]辛烷和通过甲基迁移扩展的 Baldwin 环氧乙烷。对于较大的环，阴离子-π 模板自催化被认为可以缓解折叠的熵罚，从而使不利的反 Baldwin 环化反应能够进入噁烷和氧杂环烷。
