Stereodivergent Synthesis of (Z)- and (E)-Thioenamides

Significance

In contrast to the analogous en­amides, there was no general mild procedure to prepare thioenamides 3. A ruthenium catalyst mediates a highly regioselective anti-Markovnikov addition of thioamides 1 to terminal alkynes. The mildness of the reaction is illustrated by the wide variety of functional groups tolerated (5-10). The method is remarkable in view of many potential selectivity issues: (1) thioamides 1 are ambident nucleophiles, but only N-addition occurs; (2) exclusive terminal addition is observed; and (3) the stereochemistry of the alkene is controlled by the ligands. The reaction shows that thioamides are competent substrates despite sulfur-containing reagents being known to poison metal catalysts.