Continuing mechanistic study of gold catalyzed alkene hydroamination

Homogeneous gold catalysis has emerged as a powerful method for activating multiple bonds toward nucleophilic addition and for involvement in selective skeletal rearrangements. The functionalized products of these reactions are important building blocks in syntheses of pharmaceuticals and materials. The mechanisms of these processes are not fully understood and this continues to present a challenge to optimization. Reactions involving allenes and alkynes are far more widespread than those of alkenes and have been the focus of the majority of mechanistic studies. In a pioneering report by Toste, treatment of amino alkenes with the non-traditional trigold oxonium complex (Ph3P)Au3OBF4 in the presence of triethylamine resulted in smooth aminoauration to generate beta-N substituted alkyl gold complexes. An important observation during this study was the inability of said complexes to undergo protodemetalation. However, the study was limited to the use of nitrogen nucleophiles and complexes with triarylphosphine ligands. We have examined ligand effects, substrate effects, and base effects, each from a structural and kinetic perspective and have elaborated on the reactivity of these important alkyl gold intermediates. A most significant observation is that catalysis occurs faster than can be accounted for by alkyl gold protodeauration, and so alkene gold activation modes that do not involve protodeauration of discrete alkyl gold intermediates must be considered. To probe alternative activation modes, we have expanded our mechanism study to include classic gold catalysts (monomeric LAu+) with various ligands and under different reaction conditions, and the results of these studies are reported herein.