Regioselective C(sp3)–H borylation via a diarylboryl anion surrogate in sp2–sp3 diboranes(5)

Abstract

Transition-metal-free C–H functionalization is a long-standing goal in synthetic chemistry. While a few main-group species have shown promise in C(sp²)–H activation through insertion or nucleophilic aromatic substitution (S_NAr) pathways, C(sp³)–H functionalization remains underdeveloped due to the intrinsic inertness of saturated carbon centers and the insufficient reactivity of reported main-group species. Herein, we report regioselective C(sp³)–H borylation in sp²–sp³ diboranes(5), mediated by a highly reactive B(o-tolyl)₂⁻ surrogate. It selectively inserts into α-C(sp³)–H bonds of alkyl groups to generate a family of anionic 1,1-diborylalkyl species. The resulting anions are readily converted to neutral gem-diborylalkanes by using TMSOTf. In substrates lacking α-C(sp³)–H bonds, β-C(sp³)–H activation becomes operative, producing organic salts and olefins. Competitive experiments reveal a clear preference for α- over β-C(sp³)–H activation. DFT calculations support a concerted insertion pathway and explain the preference for α- over β- and ortho-C–H activation. This study showcases the potential of reactive diarylboryl anion surrogates as powerful reagents for achieving regioselective C(sp³)–H borylation and expands the synthetic landscape of main-group-element-mediated C–H activation.