| Olefin amination using aminium radical cations is a classical method for C--N bond formation; however, catalytic variants that utilize simple secondary amine precursors remain largely undeveloped. We developed a new visible-light photoredox protocol for the intramolecular anti-Markovnikov hydroamination of aryl olefins that proceeds through catalytically generated aminium radical intermediates. A range of structurally diverse N-aryl heterocycles can be prepared in good to excellent yields under conditions significantly milder than those required by conventional aminium-based protocols.;During the development of our photocatalytic anti-Markovnikov hydroamination system, we became interested in using photon upconversion, a method to transform multiple photons of low energy wavelengths of light into a single photon of high energy, in synthetic chemistry. We established the feasibility of the proposal by applying the technique to upconvert from low energy wavelengths to blue light to promote our previously described hydroamination reaction. We later became interested in photon upconversion's ability to access high energy singlet excited states to promote classically UV photochemistry. We demonstrate that coumarins can collect upconverted energy to furnish high-energy singlet excited state intermediates that can participate in subsequent [2+2] cycloadditions. Steady-state luminescence experiments along with time-resolved measurements provide support for the proposed mechanism. These findings establish a proof of concept for the applicability of upconversion systems to mediate a classical UV photochemical transformation using low-energy visible light inputs. |
