Overriding the alkynophilicity of gold: catalytic pathways from higher energy Au(I)-substrate complexes and reactant deactivation via unproductive complexation in the gold(I)-catalyzed propargyl Claisen rearrangement.

Computational and experimental analysis of unusual substituent effects in the Au-catalyzed propargyl Claisen rearrangement revealed new features important for the future development of Au(I) catalysis. Despite the higher stability of Au-alkyne complexes, they do not always correspond to the catalytically active compounds. Instead, the product emanates from the higher energy Au(I)-oxygen complex reacting via a low barrier cation-accelerated oxonia Claisen pathway. Additionally, both intra and intermolecular competition from other Lewis bases present in the system, for the Au(I) catalyst, can lead to unproductive stabilization of the substrate/catalyst complex, explaining hitherto unresolved substituent effects.

Gold(I)-catalyzed Claisen rearrangement of allenyl vinyl ethers: missing transition states revealed through evolution of aromaticity, Au(I) as an oxophilic Lewis acid, and lower energy barriers from a high energy complex.

Curtin-Hammett analysis of four alternative mechanisms of the gold(I)-catalyzed [3,3] sigmatropic rearrangement of allenyl vinyl ethers by density functional theory calculations reveals that the lowest energy pathway (cation-accelerated oxonia Claisen rearrangement) originates from the second most stable of the four Au(I)-substrate complexes in which gold(I) coordinates to the lone pair of oxygen. This pathway proceeds via a dissociative transition state where the C-O bond cleavage precedes C1-C6 bond formation. The alternative Au(I) coordination at the vinyl π-system produces a more stable but less reactive complex. The two least stable modes of coordination at the allenyl π-system display reactivity that is intermediate between that of the Au(I)-oxygen and the Au(I)-vinyl ether complexes. The unusual electronic features of the four potential energy surfaces (PESs) associated with the four possible mechanisms were probed with intrinsic reaction coordinate calculations in conjunction with nucleus independent chemical shift (NICS(0)) evaluation of aromaticity of the transient structures. The development of aromatic character along the "6-endo" reaction path is modulated via Au-complexation to the extent where both the cyclic intermediate and the associated fragmentation transition state do not correspond to stationary points at the reaction potential energy surface. This analysis explains why the calculated PES for cyclization promoted by coordination of gold(I) to allenyl moiety lacks a discernible intermediate despite proceeding via a highly asynchronous transition state with characteristics of a stepwise "cyclization-mediated" process. Although reaction barriers can be strongly modified by aryl substituents of varying electronic demand, direct comparison of experimental and computational substituent effects is complicated by formation of Au-complexes with the Lewis-basic sites of the substrates.

Gold(I)-catalyzed Claisen rearrangement of allenyl vinyl ethers; synthesis of substituted 1,3-dienes.

Synthesis of substituted 1,3-dienes was achieved via gold(I)-catalyzed Claisen rearrangement of allenyl vinyl ethers. The N-heterocyclic carbene gold chloride catalyst (IPrAuCl) was superior in terms of activity and selectivity and afforded the 3,3-product in excellent yields. A proposed cation-π inter-action played a significant role in affecting the reaction rate.

Solvent controlled mechanistic dichotomy in a Au(III)-catalyzed, heterocyclization triggered, Nazarov reaction.

Tandem Au(III)-catalyzed heterocyclization/Nazarov cyclizations leading to substituted carbocycle fused furans are described. An interesting dichotomy of reaction pathways as a function of solvent, confirmed by the isolation and trapping of reaction intermediates, provided a basis for computational studies that supported the experimental findings.