Structure and Dynamic Behavior of Phosphine Gold(I)-Coordinated Enarnines: Characterization of α-Metalated Iminium Ions

Cationic gold(I) enamine complexes with the (t-Bu)(2)(o-biphenyl)-phosphine ligand were isolated and characterized by NMR spectroscopy and X-ray crystallography. The complexes display highly distorted coordination modes that are consistent with characterization as alpha-metalated iminium ions. The barrier to rotation around the formal enamine C-C double bond has been measured in a geminally disubstituted enamine complex, and it is comparable to the barrier to C C single-bond rotation in electronically restricted alkanes. With additional substitution on the enamine double bond, the complexes remain highly distorted, and the reaction of a mixture of E and Z enamines results in formation of a stereochemically pure gold complex. A survey of binding constants reveals enamines to be significantly stronger donors than any alkenes examined to date, and in the case of a geminally disubstituted enamine, the coordination is stronger even than that of triethylamine. The high stability drives the isomerization of an internal enamine complex generated from an intramolecular hydroamination reaction, to the exocyclic double-bond isomer.