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The mechanism of gold(III) [Au(III)]-catalyzed isomerization of alkyl-substituted allenes to conjugated dienes in the presence of a nitroso compound (additive) was studied quantum mechanically using hybrid density functional PBE0 with 6-31G** basis set for lighter atoms and (aug)-ccpVDZ basis set and LANL2 electron core potential for Au atom. Several pathways, involving the nitroso compound in a free or bound state to the gold-allene (GA) complex, were investigated. Calculated results reveal that the unbound nitroso compound acts as a better proton transferring agent in the isomerization process and utilizes its own nitrogen atom to carry the proton. While comparing the efficiency of other basic reagents to carry out the process, it appeared that the moderate basicity of the nitroso compound plays a crucial role to reduce the activation barrier of the reaction pathway. A similar pathway was also investigated using a gold(I) [Au(I)] catalyst and found to be less favorable than the process catalyzed by a Au(III) catalyst. All these facts agree well with the experimental reports for the reaction.

Gold-catalyzed isomerization of propargylic ester to a diketone derivative is a fascinating example for the generation of the C-C bond in organoaurate chemistry as it is one of the few reactions that exploit the nucleophilicity of organoaurates to a migrating acyl group. The proposed mechanistic pathway, involving the formation of a four-membered intermediate, has never been substantiated by any theoretical or experimental evidence. Detailed theoretical calculation suggests that the formation of an alkylideneoxoniumcyclobutene intermediate is highly unlikely. Instead, an acyl migration, assisted by the chlorine ligand in the square planar geometry of metal complex offers an alternative mechanism that can justify the reasonable activation barrier and the associated stereochemical feature involved in the reaction. The initial mandatory steps of the catalytic process such as allene formation (af) and rotamerization of allene-bound gold complex (ra) are found to be quite facile. However, the final step, acyl migration (am), that takes place through the formation of an intermediate with C-Cl bond, acts as the rate-determining step of the reaction. The mechanism also justifies the lack of sufficient activity of Au(I) salt to catalyze the isomerization process.

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The mechanistic pathways of metal-catalyzed pentannulation and hexannulation of aromatic enediyne were studied quantum mechanically with Pt and Au salts. In agreement with the experimental facts, our result shows that the pentannulation favors over the hexannulation under Pt-catalyzed conditions and the reverse possibility favors when the Pt salt is replaced with an Au one. The Pt-catalyzed reaction involves a long-range acyl migration that follows the cyclization step. Our study reveals that such migration takes place under the assistance of a ligand of the metal atom. Moreover, the variation of aromaticity (probed by the change of the nucleus-independent chemical shift (0) value) in the cyclization steps shows that both processes maintain the development of the aromatic character of the generated intermediate during the progress of the reaction.