Accessing N-Unprotected Unnatural α-Amino Acids by Half-Sandwich Chiral-at-Metal Aldehyde Catalysis: Scope and Mechanistic Study

Chiral aldehyde catalysis has emerged as an efficient and step-economical protocol for constructing unnatural α-amino acids. Although binaphthyl or pyridoxal aldehyde catalysts featuring axial or carbon-centered chirality were widely utilized, there is an eager request for alternative catalyst scaffold featuring high reactivity and precise selectivity control. Herein, we report the recently discovered half-sandwich chiral-at-metal aldehyde is a versatile asymmetric catalyst with remarkably broad scope. Direct asymmetric α-C−H functionalization of N-unprotected glycine esters with four types of electrophiles (51 examples, all > 91% ee) has been successfully realized for accessing structurally diverse unnatural α-amino acids. Gram-scale experiment and the catalyst recovery underscored the potential practicability of this reaction.