Rewriting Norrish–Yang Selectivity: Photochemical 1,4-Boryl Transfer to Access Cyclopropanols

The Norrish–Yang photocyclization is a well-established method to access cyclobutanols from simple ketone starting materials. However, access to three-membered ring systems has remained limited due to the strong kinetic preference for 1,5- over 1,4-hydrogen atom transfer. To address this long-standing challenge, we recently developed a novel photocyclization strategy that enables the synthesis of cyclopropanols from β-boryl aryl ketones, in yields ranging from 25% to 96%. This approach relies on the selective population of the π,π* triplet excited state over the n,π* triplet state, enabling preferential 1,4-boryl group transfer over 1,5-HAT. The resulting protocol proceeds under mild irradiative conditions, exhibits broad functional group tolerance, and provides efficient access to a diverse set of cyclopropanol scaffolds. Due to the readily accessible starting materials, along with the simple reaction conditions and set-up, we anticipate that this method will facilitate access to 1-aryl cyclopropanols.