Transition Metal-Free Borata-Alkene/Alkyne Metathesis Enables Access to Cyclopentenes.

Metathesis reactions are fundamental transformations in organic synthesis, enabling efficient reorganization of unsaturated bonds and underpinning broad applications in pharmaceuticals, functional materials, and polymer science. To date, the conceptual and methodological development of metathesis transformations has been focused predominantly on carbon-carbon unsaturated bonds, and strategies that combine unsaturated bond reorganization with the direct incorporation of heteroatoms remain rare. Additionally, the established metathesis methodologies have been developed typically within the framework of transition metal catalysis, and transition metal-free alternatives are scarcely explored. Here, we demonstrate that borata-alkenes can serve as non-metal alkylidene surrogates to enable a transition metal-free hetero-enyne metathesis to access cyclopentene derivatives. This process accommodates diverse electrophiles and can be integrated into cascade sequences, allowing three- and four-component couplings from readily accessible 1,1,1-triborylalkanes. The resulting chemodivergent products include cyclopentenyl boronic esters, geminal borylsilylalkanes, conjugated dienes, and, in a sequential four-component variant, highly substituted ketones and enol ethers. Mechanistic studies and density functional theory (DFT) calculations support a borata-alkene-mediated metathesis pathway, establishing a transition metal-free approach for cyclopentene synthesis via carbon-carbon bond reorganization.