π Activation of Alkynes in Homogeneous and Heterogeneous Gold Catalysis

The
activation of alkynes toward nucleophilic attack upon coordination
to gold-based catalysts (neutral and positively charged gold clusters
and gold complexes commonly used in homogeneous catalysis) is investigated
to elucidate the role of the σ donation and π back-donation
components of the Au–C bond (where we consider ethyne as prototype
substrate). Charge displacement (CD) analysis is used to obtain a
well-defined measure of σ donation and π back-donation
and to find out how the corresponding charge flows affect the electron
density at the electrophilic carbon undergoing the nucleophilic attack.
This information is used to rationalize the activity of a series of
catalysts in the nucleophilic attack step of a model hydroamination
reaction. For the first time, the components of the Dewar–Chatt–Duncanson
model, donation and back-donation, are put in quantitative correlation
with the kinetic parameters of a chemical reaction.