Isolable silyl- and germyl-thioketones dearomatize their own benzene rings at room temperature

Although dearomatization of benzene converts flat molecules into three-dimensional (3D) frameworks, the stability of the benzene ring makes this transformation difficult. Main-group compounds that achieve it are typically highly reactive low-valent species or contain weak multiple bonds and usually require elaborate design. Thioketones engage an adjacent aryl ring only with strongly electron-poor partners or fused arenes, and the products rearomatize. Whether a simple, isolable main-group functional group can dearomatize a non-fused benzene ring under mild conditions has remained unclear so far. Here, we introduce a substituent-based strategy that switches on this reactivity, i.e., thioketones bearing a σ-donating silyl or germyl group dearomatize their own benzene rings at room temperature without the need for metals, light, or redox reagents. Whereas conventional thioketones are inert toward alkenes under identical conditions, these compounds, including the first structurally characterized silyl-thioketone and the first isolable germyl-thioketones, undergo cascade cycloadditions with alkenes and alkynes to give air-stable 3D frameworks. Rather than strengthening frontier-orbital interactions, the group-14 substituent polarizes the C═S bond and renders bond formation highly asynchronous, thus markedly reducing the distortion required to disrupt the aromatic ring. These results establish main-group substitution as a design strategy for unlocking latent dearomative reactivity under mild conditions.