Enantioselective Synthesis of Chiral Amides by a Phosphoric Acid Catalyzed Asymmetric Wolff Rearrangement

The enantioselective addition of potent nucleophiles to ketenes poses challenges due to competing background reactions and poor stereocontrol. Herein, we present a method for enantioselective phosphoric acid catalyzed amination of ketenes generated from α-aryl-α-diazoketones. Upon exposure to visible light, the diazoketones undergo Wolff rearrangement to generate ketenes. The phosphoric acid not only accelerates ketene capture by amines to form a single configuration of aminoenol intermediates but also promotes an enantioselective proton-transfer reaction of the intermediates to yield the products. Mechanistic studies elucidated the reaction pathway and explained how the catalyst expedited the transformation and controlled the enantioselectivity.

A Spiro Phosphamide Catalyzed Enantioselective Proton Transfer of Ylides in a Free Carbene Insertion into N-H Bonds

Free carbene readily causes multiple side reactions due to its high energy, thus its asymmetric transformation is very difficult. We present here our findings of high-pK_a Brønsted acid catalysts that enable free carbene insertion into N-H bonds of amines to prepare chiral α-amino acid derivatives with high enantioselectivity. Under irradiation with visible light, diazo compounds produce high-energy free carbenes that are captured by amines to form free ylide intermediates, and then the newly designed high-pK_a Brønsted acids, chiral spiro phosphamides, promote the proton transfer of ylides to afford the products. Computational and kinetic studies uncover the principle for the rational design of proton-transfer catalysts and explain how the catalysts accelerate this transformation and provide stereocontrol.

Synthesis of Axially Chiral N-Arylindoles via Atroposelective Cyclization of Ynamides Catalyzed by Chiral Brønsted Acids

In recent years, asymmetric catalysis of ynamides has attracted much attention, but these reactions mostly constructed central chirality, except for a few examples on the synthesis of axially chiral compounds which exclusively relied on noble-metal catalysis. Herein, a facile access to axially chiral N-heterocycles enabled by chiral Brønsted acid-catalyzed 5-endo-dig cyclization of ynamides is disclosed, which represents the first metal-free protocol for the construction of axially chiral compounds from ynamides. This method allows the practical and atom-economical synthesis of valuable N-arylindoles in excellent yields with generally excellent enantioselectivities. Moreover, organocatalysts and ligands based on such axially chiral N-arylindole skeletons are demonstrated to be applicable to asymmetric catalysis.