Rhodium-Catalyzed Ring Expansion Reactions for the Concise Construction of Densely Functionalized Oxathionines and Oxathiocines

Ring expansion reactions have been proven to be efficient tools for building medium-sized rings with broad applications in synthetic and medicinal chemistry. However, the strategy involving pericyclic or pseudopericyclic pathways via sulfur ylide-initiated [1,4]-sigmatropic rearrangement remains an under-exploited area. Herein, we disclose an interesting ring expansion of thiochromenes and aromatic thiophenes via rhodium catalysis, which enables the straightforward assembly of polyaromatic oxathionines and oxathiocines with diverse functionalities. The mechanistic investigation via DFT calculations revealed this ring expansion transformation to be a [1,4]-sigmatropic rearrangement via a metal-free sulfur-based ylide.