Tertiary and Quaternary Carbon Formation via Gallium-Catalyzed Nucleophilic Addition of Organoboronates to Cyclopropanes

GaCl₃ and (IPr)­GaCl₃/AgSbF₆ formed γ-tertiary and γ-quaternary carbons via homoconjugate addition of organoboron nucleophiles to diester- and ketone-functionalized cyclopropanes. Electron donor group cyclopropane substituents were not needed, allowing electron-deficient aryl, alkenyl, alkyl, and hydrogen-substituted cyclopropanes to be used. The catalytic conditions were compatible with alkenyl, alkynyl, and aryl nucleophiles, including ortho-substituted aromatics, to synthesize highly hindered quaternary carbons. Alkynyl nucleophiles formed substituted cyclopentenes. A control experiment supports an intermediate carbocation in quaternary carbon center formation.