MXenes as Heterogeneous Thermal Catalysts: Regioselective Anti-Markovnikov Hydroamination of Terminal Alkynes with 10 h Turnover Frequencies.

Due to their conductive properties and optoelectronic tunability, MXenes have revolutionized the area of electrocatalysis and active materials in supercapacitors. In comparison, there are only a few reports on MXenes as thermal catalysts for general organic reactions. Herein, the unprecedented catalytic activity of TiC MXene for the hydroamination of alkynes is reported, overcoming the limitations of poor activity, lack of selectivity, and stability, which are generally encountered in the solid catalysts known so far. In the case of TiC, hydroamination exhibits almost complete selectivity for the anti-Markovnikov isomer, for both aliphatic amines and less-reactive aromatic amines. TiC also efficiently catalyzes intramolecular hydroamination, leading to the formation of indol heterocycles. The catalytic hydroamination of C-C multiple bonds is a reaction with complete atom efficiency that may form C-N bonds from convenient reagents. The maximum number of hydroamination sites on the TiC nanosheets is quantified by thermoprogrammed NH desorption. The measured TOF values are on the order of 10 h, with the highest TOF value being 350 h for 1-hexyne hydroamination by butylamine. Therefore, TiC is among the few heterogeneous hydroamination catalysts studied, with its activity per site being comparable to the best hydroamination catalysts reported so far. Density functional theory calculations on the models indicate the cooperation of neighboring Ti atoms in the mechanism. Considering the compositional and structural versatility of MXenes, the present findings open the door for further application of MXenes in other general organic reactions.