Photochemical and Thermal Concerted 1,3-Sigmatropic Rearrangements of a π-Extended Methylenecyclopropane

The reversible interconversion between methylenecyclopropane (MCP) and trimethylenemethane (TMM) has been extensively studied, yet precise control over their spin states remains challenging. Here, we designed and synthesized a propeller-shaped π-extended MCP derivative and its corresponding TMM derivative with anthroxyl moieties. The MCP derivative was isolated as a stable compound, and upon photoirradiation, it generated the triplet-state TMM derivative, which reverted to the MCP derivative without decomposition. In contrast, thermal excitation induced a unique carbon-carbon bond fluxionality within the MCP core via a concerted 1,3-sigmatropic rearrangement rather than a stepwise diradical pathway. These findings demonstrate that external stimuli such as light and heat enable spin-state control and dynamic bond fluxionality, providing new insights into organic stimuli-responsive materials.