posted on 2023-03-27, 19:04authored byGreg J. Baker, Andrew J. P. White, Ian J. Casely, Damian Grainger, Mark R. Crimmin
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 density functional theory (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 CC bonds at modest temperatures.
The scope of hydrozincation 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 a catalytic system for the
semi-hydrogenation of alkynes. The catalytic scope includes both aryl-
and alkyl-substituted internal alkynes and proceeds with high alkene:
alkane, Z:E ratios, and modest functional
group tolerance. This work offers a first example of selective hydrogenation
catalysis using zinc complexes.