Overriding Norrish Type II to Access Cyclopropanols.

The propensity for photoexcited aryl ketones to undergo 1,5-hydrogen atom transfer (HAT) reactions is a fundamental process in organic synthesis and polymer degradation. Since its discovery in 1958, Norrish-Yang photocyclization has become a prominent method to access cyclobutanols under mild photoirradiative conditions. Despite its successful extension to larger ring systems, access to medicinally relevant 3-membered rings remains limited due to the strong kinetic preference for 1,5-HAT over 1,4-HAT. In this work, we demonstrate the first photocyclization of β-boryl aryl ketones to cyclopropanols. Our strategy relies on solvent-controlled selective population of the π,π* triplet state over the n,π* triplet state, with the former preferentially undergoing 1,4-boryl group transfer over 1,5-HAT. This protocol proceeds under mild irradiative conditions with excellent functional group tolerance, providing access to a broad range of highly valuable cyclopropanol scaffolds.