Modular Access to Diverse Amphoteric α-Boryl Aldehydes or Borylated Heterocycles via Photoredox Catalysis

The stable α-boryl aldehydes are important amphoteric synthetic intermediates that act as linchpins in organic synthesis. However, current approaches to these important building blocks typically rely on oxidative processes, which might lead to issues such as lack of product diversity and functional group compatibility. Here, we report the design and utilization of 2-(BMIDA) acrylaldehyde as a suitable coupling partner toward photoredox-catalyzed intermolecular radical addition reaction for the rapid synthesis of functionalized α-boryl aldehydes. With a commercially available photocatalyst and readily accessible alkyltrifluoroborates as radical precursors, a wide range of α-boryl aldehydes were obtained in up to 90% yield. Preliminary mechanistic studies suggested that the reaction might proceed through intermolecular radical addition and radical-polar crossover processes. The synthetic potential of the method was further demonstrated by scale-up reactions, several synthetic transformations, and derivatization of bioactive molecules. Moreover, by simply switching the radical precursors to α-amino acids, diverse 4-boryl 2,3-dihydropyrroles could be accessed via the intermolecular radical addition/cyclization sequence. Furthermore, the MIDA boronate-bearing six- or seven-membered heterocycles could also be readily obtained when the corresponding β- or γ-amino acid was used. This strategy provides a facile, robust, and diversity-oriented synthetic approach to functionalized α-boryl aldehydes or borylated heterocycles.