Nickel Catalysis Enables Hetero [2+2+1] Cycloaddition between Yne‐Isothiocyanates and Isonitriles with Low Catalyst Loading

Nickel(II) can be used to catalyze the hetero [2+2+1] cycloaddition of 2‐alkynylaryl isothiocyanates and isonitriles in 2‐methyltetrahydrofuran (2‐MeTHF) to give a wide array of thieno[2,3‐ b ]indoles in excellent yields. The reaction is featured by employing as little as 0.3 mol% nickel(II) acetylacetonate [Ni(acac) 2 ] under air conditions in the absence of any additives (additional reducing agents and external ligands). This is the first successful example to apply nickel(II) directly in hetero [2+2+1] cycloadditions.