Catalytic, Z-Selective, Semi-Hydrogenation of Alkynes with a Zinc–Anilide Complex

The reversible activation of dihydrogen with a molecular zinc anilide complex is reported. The mechanism of this reaction has been probed through stoichiometric experiments and DFT calculations. The combined evidence suggests that H2 activation occurs by addition across the Zn–N bond via a four-membered transition state in which the Zn and N atoms play a dual role of Lewis acid and Lewis base. The zinc hydride complex that results from H2 addition, has been shown to be remarkably effective for the hydrozincation of C=C bonds at modest temperatures. The scope of hy-drozincation includes alkynes, alkenes, and a 1,3-butadiyne. For alkynes, the hydrozincation step is stereospecific leading exclusively to the syn-isomer. Competition experiments show that the hydrozincation of alkynes is faster than the equivalent alkene substrates. These new discoveries have been used to develop an unprecedented catalytic sys-tem for the semi-hydrogenation of alkynes. The catalytic scope includes both aryl and alkyl substituted internal al-kynes and proceeds with high alkene : alkane (96 : 4) and Z : E ratios (>91 : 9). This work offers a first example of selective hydrogenation catalysis using zinc complexes.