Efficient Catalytic Enantioselective Hydroxyamination of α-Aryl-α-Cyanoacetates with 2-Nitrosopyridines

Challenges and recent advancements in the synthesis of α,α-disubstituted α-amino acids

α,α-Disubstituted α-amino acids (α-AAs) have improved properties compared to other types of amino acids. They serve as modifiers of peptide conformation and as precursors of bioactive compounds. Therefore, it has been a long-standing goal to construct this highly valuable scaffold efficiently in organic synthesis and drug discovery. However, access to α,α-disubstituted α-AAs is highly challenging and largely unexplored due to their steric constraints. To overcome these, remarkable advances have been made in the last decades. Emerging strategies such as synergistic enantioselective catalysis, visible-light-mediated photocatalysis, metal-free methodologies and CO2 fixation offer new avenues to access the challenging synthesis of α,α-disubstituted α-AAs and continuously bring additional contributions to this field. This review article aims to provide an overview of the recent advancements since 2015 and discuss existing challenges for the synthesis of α,α-disubstituted α-AAs and their derivatives.

Photoinduced, Copper-Catalyzed Enantioconvergent Alkylations of Anilines by Racemic Tertiary Electrophiles: Synthesis and Mechanism

Transition-metal catalysis of substitution reactions of alkyl electrophiles by nitrogen nucleophiles is beginning to emerge as a powerful strategy for synthesizing higher-order amines, as well as controlling their stereochemistry. Herein, we report that a readily accessible chiral copper catalyst (commercially available components) can achieve the photoinduced, enantioconvergent coupling of a variety of racemic tertiary alkyl electrophiles with aniline nucleophiles to generate a new C-N bond with good ee at the fully substituted stereocenter of the product; whereas this photoinduced, copper-catalyzed coupling proceeds at -78 °C, in the absence of light and catalyst, virtually no C-N bond formation is observed even upon heating to 80 °C. The mechanism of this new catalytic enantioconvergent substitution process has been interrogated with the aid of a wide array of tools, including the independent synthesis of proposed intermediates and reactivity studies, spectroscopic investigations featuring photophysical and EPR data, and DFT calculations. These studies led to the identification of three copper-based intermediates in the proposed catalytic cycle, including a chiral three-coordinate formally copper(II)-anilido (DFT analysis points to its formulation as a copper(I)-anilidyl radical) complex that serves as a persistent radical that couples with a tertiary organic radical to generate the desired C-N bond with good enantioselectivity.

Cyanation of glycine derivatives

We report a catalytic oxidative C-H cyanation of glycine derivatives using a simple copper(i) catalyst with NFSI as an oxidant via a radical process to furnish α-cyano glycine derivatives, which are useful intermediates for organic synthesis. CuCl acted as both a one-electron reductant and a transition-metal catalyst in this transformation. NFSI served as a one-electron oxidant and generated a N-centered radical as a H-abstractor. The reaction displayed broad substrate scope and mild reaction conditions.