A Synthetic Method to Hydrogen Radicals.

Atomic hydrogen (H•), the union of an electron and a proton, is the smallest radical of the universe. Over the past century, advanced techniques such as electric discharge and mercury UV sensitization enabled a fundamental understanding of the reactivity of H• but fell short of providing synthetic methods to exploit its intriguing properties. Here we report a light-mediated method to access hydrogen radicals under synthetically useful conditions. This strategy enabled the mild, metal-free reduction of highly functionalized alkenes and halides. Mechanistic, EPR, and DFT studies suggest hydrogen radical generation via a Rydberg radical intermediate, with potential applications across the chemical and biological sciences.