Cp*CoIII-Catalyzed Branch-Selective Hydroarylation of Alkynes via C–H Activation: Efficient Access to α-gem-Vinylindoles

An efficient, atom-economical, and regioselective insertion of indoles into terminal alkynes has been realized via cobalt(III)-catalyzed C-H activation under mild conditions, leading to efficient synthesis of alpha-gem-vinylindoles. The insertion of the alkynes follows a rare 1,2-selectivity, and silyl alkynes, alkyl alkynes, propargyl alcohols, and protected propargyl amines are all applicable. The mechanism of this hydroarylation system has been studied in detail by a combination of experimental and computational approaches. In the reaction of silyl terminal alkynes, the regioselectivity is dictated by the steric effects of the alkyne substituent, especially in the protonolysis stage. However, for protected propargyl amines, the selectivity results from electronic effects during the insertion step, with protonolysis being insignificant in the determination of selectivity. An internal alkyne also coupled in high efficiency but with low regioselectivity. Comparisons of cobalt, rhodium, and iridium catalysts have also been made in terms of regioselectivity and reactivity, and both are high for cobalt catalysts.