Experimental and Theoretical Study of the Effectiveness and Stability of Gold(I) Catalysts Used in the Synthesis of Cyclic Acetals

Different [AuL]+ fragments
(L = tertiary phosphines,
ylides, or NHC carbene) have been tested under mild conditions as
suitable catalysts for the transformation of terminal or internal
alkynes into the corresponding cyclic acetals upon reaction with ethylene
glycol. We have obtained a moderate to negligible activity when using
tertiary phosphines or nonstabilized ylides as ligands. However, a
very high catalytic activity is reached when the IPr N-heterocyclic
carbene ligand is used. We have analyzed the key stages in this type
of gold-catalyzed reaction, namely, (i) electronic activation (alkynophilicity);
(ii) protodeauration; and (iii) decomposition of the gold catalyst.
The first two stages have been analyzed through DFT computation of
the minimum-energy reaction pathways employing different catalysts.
An explanation of the catalysts’ stability has been proposed
through the analysis of in situ time-resolved nuclear
magnetic resonance spectra of the catalysts.