A new synthetic method for the synthesis of C-substituted [RC(O)CH₂-CB₁₁H₁₁]⁻ carborate anions has been developed. The reaction of [closo-B₁₁H₁₁]²⁻ with terminal alkynes in the presence of a copper catalyst leads to insertion into the boron cluster, and C-substituted [RC(O)CH₂-CB₁₁H₁₁]⁻ carborate anions are formed. These reactions are strongly dependent on the reaction conditions, the solvents, and the alkynes used. The alkynes HCCCO₂Et, HCCCO₂Me, and HCCCONH₂ lead to the formation of [NH₂C(O)CH₂-CB₁₁H₁₁]⁻ as the final product in aqueous ammonia solution. In contrast, the reaction using the alkyne HCCCOMe yields [MeC(O)CH₂-CB₁₁H₁₁]⁻. The products have been fully characterized by multinuclear NMR and IR spectroscopy as well as mass spectrometry. The crystal structures of K[NH₂C(O)CH₂-CB₁₁H₁₁] and [NEt₃CH₂Cl][NH₂C(O)CH₂-CB₁₁H₁₁] have been determined.