Mechanism of Gold(I)-Catalyzed
Rearrangements of Acetylenic
Amine-<i>N</i>-Oxides: Computational Investigations Lead
to a New Mechanism Confirmed by Experiment
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Abstract
Quantum mechanical studies of the mechanism of gold-catalyzed
rearrangements
of acetylenic amine-<i>N</i>-oxides to piperidinones or
azepanones have revealed a new mechanism involving a concerted heteroretroene
reaction, formally a 1,5 hydrogen shift from the <i>N</i>-alkyl groups to the vinyl position of a gold-coordinated methyleneisoxazolidinium
or methyleneoxazinanium. Density functional calculations (B3LYP, B3LYP-D3)
on the heteroretroene mechanism reproduce experimental regioselectivities
and provide an explanation as to why the hydrogen is transferred from
the smaller amine substituent. In support of the proposed mechanism,
new experimental investigations show that the hydrogen shift is concerted
and that gold carbenes are not involved as reaction intermediates