Enantiospecific Homo-Boron-Wittig Reaction: Direct Conversion of Chiral Epoxides to Cyclopropanes

Deoxygenative functional-group interconversions─such as Wittig olefination─are foundational transformations that convert abundant oxygenated feedstocks into higher-value carbon frameworks by leveraging strong element-oxygen affinity. Extending this logic from carbonyl olefination to epoxide deoxygenation, namely, the Homo-Wittig reaction, offers a direct entry to cyclopropanes, yet general, enantiospecific solutions have remained limited in scope and substitution patterns. Here we report a ZnBr₂-enabled, enantiospecific "Homo-Boron-Wittig" cyclopropanation that converts chiral epoxides to cyclopropylboronic esters using boron-stabilized α-carbanions derived from geminal diboryl alkanes. The reaction tolerates diverse functional groups and accommodates monosubstituted, 1,2-disubstituted, 1,1-disubstituted, and trisubstituted epoxides, delivering densely substituted cyclopropanes─including tertiary boronic esters and tri-, tetra-, and pentasubstituted products─with excellent enantiospecificity. Mechanistic experiments support boron-to-zinc transmetalation under the reaction conditions, and DFT calculations indicate a concerted intramolecular substitution pathway that enforces stereochemical inversion and rationalizes the observed diastereoselectivity. Subsequent diastereoconvergent radical cross-coupling improves diastereoselectivity while preserving enantiopurity, highlighting the value of these products as versatile cyclopropane linchpins.