Visible-Light-Enabled Paterno-Buchi Reaction via Triplet Energy Transfer for the Synthesis of Oxetanes

One of the most efficient ways to synthesize oxetanes is the light-enabled [2 + 2] cycloaddition reaction of carbonyls and alkenes, referred to as the PaternO-Biichi reaction. The reaction conditions for this transformation typically require the use of high energy UV light to excite the carbonyl, limiting the applications, safety, and scalability. We herein report the development of a visible-light-mediated Paterno-Biichi reaction protocol that relies on triplet energy transfer from an iridium-based photocatalyst to the carbonyl substrates. This mode of activation is demonstrated for a variety of aryl glyoxylates and negates the need for both visible-light-absorbing carbonyl starting materials and UV light to enable access to a variety of functionalized oxetanes in up to 99% yield.