A highly efficient preparative method of alpha-ylidene-beta-diketones via Au(III)-catalyzed acyl migration of propargylic esters

[(NHC)AuI]-Catalyzed Formation of Conjugated Enones and Enals: An Experimental and Computational Study

The [(NHC)AuI]-catalyzed (NHC=N-heterocyclic carbene) formation of alpha,beta-unsaturated carbonyl compounds (enones and enals) from propargylic acetates is described. The reactions occur at 60 degrees C in 8 h in the presence of an equimolar mixture of [(NHC)AuCl] and AgSbF6 and produce conjugated enones and enals in high yields. Optimization studies revealed that the reaction is sensitive to the solvent, the NHC, and, to a lesser extent, to the silver salt employed, leading to the use of [(ItBu)AuCl]/AgSbF6 in THF as an efficient catalytic system. This transformation proved to have a broad scope, enabling the stereoselective formation of (E)-enones and -enals with great structural diversity. The effect of substitution at the propargylic and acetylenic positions has been investigated, as well as the effect of aryl substitution on the formation of cinnamyl ketones. The presence or absence of water in the reaction mixture was found to be crucial. From the same phenylpropargyl acetates, anhydrous conditions led to the formation of indene compounds via a tandem [3,3] sigmatropic rearrangement/intramolecular hydroarylation process, whereas simply adding water to the reaction mixture produced enone derivatives cleanly. Several mechanistic hypotheses, including the hydrolysis of an allenol ester intermediate and SN2' addition of water, were examined to gain an insight into this transformation. Mechanistic investigations and computational studies support [(NHC)AuOH], produced in situ from [(NHC)AuSbF6] and H2O, instead of cationic [(NHC)AuSbF6] as the catalytically active species. Based on DFT calculations performed at the B3LYP level of theory, a full catalytic cycle featuring an unprecedented transfer of the OH moiety bound to the gold center to the C[triple chemical bond]C bond leading to the formation of a gold-allenolate is proposed.

Catalytic carbophilic activation: catalysis by platinum and gold pi acids

The ability of platinum and gold catalysts to effect powerful atom-economic transformations has led to a marked increase in their utilization. The quite remarkable correlation of their catalytic behavior with the available structural data, coordination chemistry, and organometallic reactivity patterns, including relativistic effects, allows the underlying principles of catalytic carbophilic activation by pi acids to be formulated. The spectrum of reactivity extends beyond their utility as catalytic and benign alternatives to conventional stoichiometric pi acids. The resulting reactivity profile allows this entire field of catalysis to be rationalized, and brings together the apparently disparate electrophilic metal carbene and nonclassical carbocation explanations. The advances in coupling, cycloisomerization, and structural reorganization--from the design of new transformations to the improvement to known reactions--are highlighted in this Review. The application of platinum- and gold-catalyzed transformations in natural product synthesis is also discussed.

Gold-Catalyzed Cyclization of Allene-Substituted Malonate Esters: Synthesis of β,γ-Unsaturated δ-Lactones

An efficient method for the preparation of beta,gamma-unsaturated delta-lactones has been developed. The starting materials for the synthesis of these compounds are allene-substituted malonates which undergo gold-catalyzed cyclization by means of nucleophilic attack of the ester moiety on the allene. It is worth mentioning that this is the first example where an ester group attacks as a nucleophile in a gold-catalyzed transformation of allenes.

Au-Catalyzed Reaction of Propargylic Sulfides and Dithioacetals

Propargylic sulfides and dithioacetals are found to undergo similar transformations as propargylic carboxylates when catalyzed by AuCl or AuCl3, affording indene derivatives through pentannulation of aromatic rings. The reaction presumably involves Au carbene as the reactive intermediate.

Molecular diversity through gold catalysis with alkynes

In this feature article we cover most recent efforts in gold-catalysed transformations, highlighting the wide molecular diversity that can be achieved, in particular with regard to the formation of C-C bonds. Mechanistic interpretations of some cyclisations are based on our own work on the skeletal rearrangement of 1,6-enynes.

Gold-Catalyzed Efficient Formation of Alkenyl Enol Esters/Carbonates from Trimethylsilylmethyl-Substituted Propargyl Esters/Carbonates

A gold-catalyzed efficient method for the preparation of alkenyl enol esters/carbonates is developed. Besides the mild reaction conditions and high catalytic efficiency, the excellent E-selectivity of the nonenolic double bond is remarkable.