Understanding the highly regioselective cyanothiolation of 1-alkynes catalyzed by palladium phosphine complexes

A computational study with the Becke3LYP DFT functional was carried out on the palladium-catalyzed regioselective cyanothiolation of 1-alkynes with thiocyanates. The full catalytic cycle was computed, starting from the oxidative addition and finishing with the reductive elimination. The two most important issues, namely, the regioselective bond cleavage of thiocyanates and the nature of insertion of 1-alkynes into a Pd - S bond, are discussed. The calculations indicate that the sulfur - cyano (PhS - CN) bond cleavage on Pd(0) is kinetically and thermodynamiclly more favorable than the carbon - sulfur (Ph - SCN) bond cleavage. The kinetic preference of 1,2-insertion over 2,1-insertion in the alkyne insertion step leads to the cyanothiolation products with the SPh group attached to the substituted carbon atom of 1-alkynes.