A Mild and Chemoselective Photoredox-Catalyzed Reduction of Aromatic Ketones

A mild, chemoselective reduction of aromatic ketones was discovered and investigated. The combination of photoredox and Lewis acid catalysis with an organic hydrogen source reduced aromatic ketones in good to high yield. Optimization found 2-phenylbenzothiazoline to be a sufficiently strong source of hydrogen in combination with an iridium photosensitizer and lanthanum triflate. Effective photomediated reduction of some substrates was also observed in the absence of photocatalyst and Lewis acid or with photocatalyst only. While yields were typically higher under catalytic conditions, some acid-sensitive substrates were more effectively reduced in the absence of Lewis acid. The reaction was generally high yielding, and chemoselecte, while tolerant of complex and functionally rich molecules.