Metal‐Free [5 + 2] Cyclization of Photogenerated Siloxyketenes and Vinyl Aziridines for the Synthesis of Azepanones

Comprehensive Summary Ketenes constitute a versatile and highly reactive class of intermediates with widespread utility in modern organic synthesis. Among them, siloxyketenes, a class of oxygen‐substituted variants, generated in situ from α‐ketoacylsilanes through a photoinduced [1,3]‐silyl migration, have garnered considerable attention due to their distinctive structural features. However, their synthetic utility has been confined to the construction of four‐ to six‐membered rings. The development of methods that exploit these intermediates for the assembly of medium‐sized rings remains exceedingly limited and is therefore highly desirable. Herein, we developed a metal‐free, visible‐light‐driven [5 + 2] cyclization reaction for the synthesis of azepanone derivatives. Under irradiation of blue LEDs at 20 °C, α‐ketoacylsilanes and vinyl aziridine were used as readily available substrates to realize this cyclization. The reaction features mild reaction conditions, operational simplicity, a broad substrate scope, and it generates diverse azepanones bearing a tertiary alcohol motif in good to excellent yields (up to 96%). The reaction proceeds via a photochemically induced [1,3]‐silyl migration to form reactive siloxyketenes, followed by nucleophilic addition and a ring‐closing process. This work expands the application of α‐ketoacylsilanes as siloxyketene precursors and provides a concise and efficient approach to constructing medium‐sized rings under mild conditions.