Stereoselective Synthesis of Chiral β-Fluoro α-Amino Acids via Pd(II)-Catalyzed Fluorination of Unactivated Methylene C(sp3) H Bonds: Scope and Mechanistic Studies

The synthesis of fluorinated complex molecules via direct C(sp(3))-H fluorination is attractive yet remains challenging. Here we describe the Pd(II)-catalyzed fluorination of unactivated methylene C(sp(3))-H bonds by an inner-sphere mechanism. This method allows the site- and diastereoselective fluorination of beta-methylene C(sp(3))-H bonds of alpha-amino acid derivatives. A range of substrates containing both aliphatic and benzylic C(sp(3))-H bonds were compatible with this protocol, leading to an array of beta-fluorinated alpha-amino acids. Stoichiometric fluorination of an isolated palladacycle intermediate takes place rapidly under very mild reaction conditions (room temperature, 5-10 min). Data from preliminary mechanistic studies are consistent with direct C-F reductive elimination from a high-valent intermediate.