Norrish-Yang-type cyclopropanation via functional group migration with photosensitizer at ppb loading

The classic Norrish-Yang cyclization of ketones and their alkene counterparts is a well-known photochemical rearrangement strategy utilized for building cyclobutanes through hydrogen atom transfer. Herein, a noncanonical Norrish-Yang-type rearrangement involving the migration of functional groups, such as cyano and aryl, is established through energy transfer catalysis, facilitating facile assembly of three-membered cyclopropanes. With a focus on reactions translocating a cyano group, this protocol showcases a broad substrate scope (58 entries), operates under mild reaction conditions, and requires an extremely low catalyst loading (100 ppb, 12 h, 97% yield). Impressively, a total turnover number (TTN) of 1.15 × 10⁷ was recorded. Mechanistic experiments uncovered a unique fluorescence enhancement phenomenon of photocatalysts, characterized by a linear Stern-Volmer plot with an unusual negative slope. The present Norrish-Yang-type cyclization significantly expands the synthetic repertoire of photochemical rearrangement for preparing distinct ring frameworks.