RESUMEN
Deracemization is an attractive strategy for asymmetric synthesis, but intrinsic energetic challenges have limited its development. Here, we report a deracemization method in which amine derivatives undergo spontaneous optical enrichment upon exposure to visible light in the presence of three distinct molecular catalysts. Initiated by an excited-state iridium chromophore, this reaction proceeds through a sequence of favorable electron, proton, and hydrogen-atom transfer steps that serve to break and reform a stereogenic C-H bond. The enantioselectivity in these reactions is jointly determined by two independent stereoselective steps that occur in sequence within the catalytic cycle, giving rise to a composite selectivity that is higher than that of either step individually. These reactions represent a distinct approach to creating out-of-equilibrium product distributions between substrate enantiomers using excited-state redox events.
Asunto(s)
Electrones , Luz , Urea/química , Carbono , Catálisis , Hidrógeno/química , Oxidación-Reducción , Protones , EstereoisomerismoRESUMEN
An enantioselective vinylcyclopropane ring-opening/cycloaddition cascade is described. The active thiyl radical catalysts are generated in situ via UV light-promoted homolysis of cystine-based dimers. Amide-functionalization of the peptide at the 4-proline position is essential for effective asymmetric induction. Stereochemical communication is dependent on steric interactions with this substituent that are enforced by H-bonding to the peptide backbone.
Asunto(s)
Péptidos/química , Catálisis , Reacción de Cicloadición , Disulfuros , Estructura Molecular , EstereoisomerismoRESUMEN
Conditions for the C-CN activation and intramolecular cyanoesterification of alkynes to provide butenolides in good to excellent yields are presented. Pd catalysts, high temperatures/short reaction times (microwave irradiation), and Lewis basic solvents minimized competitive decarbonylation. Less sterically encumbered, electron-rich alkynes underwent cyanoesterification with greater ease compared to sterically encumbered, electron-deficient alkynes. The results led to the hypothesis that migratory insertion of the alkyne, rather than C-CN activation, might be the product-determining step.