Structure and Dynamic Behavior of Phosphine Gold(I)-Coordinated Enamines: 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 α-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.