Gold-catalysed reactions of alcohols: isomerisation, inter- and intramolecular reactions leading to C–C and C–heteroatom bonds

Gold-catalyzed oxidation of arylallenes: Synthesis of quinoxalines and benzimidazoles

A gold-catalyzed oxidation of arylallenes to form α-diketones and aldehydes in good yields is presented. Further directed synthesis of quinoxalines and benzimidazoles, via the condensation of the resulting α-diketones and aldehydes with benzene-1,2-diamine, was achieved in high yields.

When gold can do what iodine cannot do: A critical comparison

Gold catalysis has emerged as one of the most dynamic fields in organic synthesis. Only recently, more and more domino processes, for which gold pre-catalysts were found to be outstandingly effective, were paralleled by employing iodine electrophiles in place of gold compounds. This review highlights how, in certain cases, iodonium activation can match gold-catalyzed reactions to construct identical product scaffolds. Likewise, processes are discussed where mostly identical starting materials are transformed into diverse frameworks depending on whether gold or iodonium activation was used to trigger the reaction.

A comparative study of the Au-catalyzed cyclization of hydroxy-substituted allylic alcohols and ethers

The Au(I)-catalyzed cyclization of hydroxyallylic ethers to form tetrahydropyrans is reported. Employing (acetonitrile)[(o-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate, the cyclization reactions were complete within minutes to hours, depending on the substrate. The reaction progress was monitored by GC, and comparisons between substrates demonstrate that reactions of allylic alcohols are faster than the corresponding ethers. Additionally, it is reported that Reaxa QuadraPure^TM MPA is an efficient scavenging reagent that halts the reaction progress.

High chemoselectivity in the phenol synthesis

Efforts to trap early intermediates of the gold-catalyzed phenol synthesis failed. Neither inter- nor intramolecularly offered vinyl groups, ketones or alcohols were able to intercept the gold carbenoid species. This indicates that the competing steps of the gold-catalyzed phenol synthesis are much faster than the steps of the interception reaction. In the latter the barrier of activation is higher. At the same time this explains the high tolerance of this very efficient and general reaction towards functional groups.

When cyclopropenes meet gold catalysts

Cyclopropenes as substrates entered the field of gold catalysis in 2008 and have proven to be valuable partners in a variety of gold-catalyzed reactions. The different contributions in this growing research area are summarized in this review.

Synthesis of the phenanthrene and cyclohepta[a]naphthalene skeletons via gold(I)-catalyzed intramolecular cyclization of unactivated cyclic 5-(2-arylethyl)-1,3-dienes

The gold(I)-catalyzed hydroarylation of cyclohexa-1,3-dienes bearing an aryl group and a gem-diester in the tether proceeds in a 1,4-addition manner and in a diastereoselective fashion to afford perhydrophenanthrene rings. The reaction proceeded via attack of the aryl group onto the gold-activated cyclic dienes followed by rearomatization and protodeauration to generate perhydrophenanthrenes in good yields. This hydroarylation can be applied to the synthesis of perhydrocyclohepta[a]naphthalenes from aryl-tethered cycloheptadienes and the gold(I) catalyst.

Gold-catalyzed heterocyclizations in alkynyl- and allenyl-β-lactams

New gold-catalyzed methods using the β-lactam scaffold have been recently developed for the synthesis of different sized heterocycles. This overview focuses on heterocyclization reactions of allenic and alkynic β-lactams which rely on the activation of the allene and alkyne component. The mechanism as well as the regio- and stereoselectivity of the cyclizations are also discussed.

Synthesis of Pyrrolo[1,2-a]quinoxalines via gold(I)-mediated cascade reactions

In this study, we developed an efficient tandem process of hydroamination and hydroarylation using a gold catalyst to enable and study the reactions between pyrrole-substituted anilines and alkynes. The gold(I)-catalyzed reactions were achieved in toluene at 80 °C over a reaction time of 1−6 h. These reactions are applicable to a variety of aromatic amino compounds and both the terminal and internal alkynes. Substituted pyrrolo[1,2-a]quinoxalines were obtained in moderate to excellent yields. A presumed mechanism involving intermolecular C−N bond formation and intramolecular nucleophilic reaction via a cationic gold complex has been proposed on the basis of the deuterium labeling studies.

Development of versatile and silver-free protocols for gold(I) catalysis

The use of a versatile N-heterocyclic carbene (NHC) gold(I) hydroxide precatalyst, [Au(OH)(IPr)], (IPr=N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) permits the in situ generation of the [Au(IPr)](+) ion by simple addition of a Brønsted acid. This cationic entity is believed to be the active species in numerous catalytic reactions. (1)H NMR studies in several solvent media of the in situ generation of this [Au(IPr)](+) ion also reveal the formation of a dinuclear gold hydroxide intermediate [{Au(IPr)}(2)(μ-OH)], which is fully characterized and was tested in gold(I) catalysis.

One-pot synthesis of 1,2-dihydropyridines: expanding the diverse reactivity of propargyl vinyl ethers

The catalyzed synthesis of 1,2-dihydropyridines starting from easily accessible propargyl vinyl ethers was realized. The reaction sequence involving a transition metal-catalyzed propargyl-Claisen rearrangement, a condensation step, and a Brønsted acid-catalyzed heterocyclization furnishes the highly substituted heterocycles in moderate to excellent yields. Additionally, a practical one-pot protocol toward 1,2-dihydropyridines and 2H-pyrans starting from propargylic alcohols was developed.

Diversity-oriented approach to pyrrole-imidazole alkaloid frameworks

An exploration of imidazolylpropargyl amides as linchpin synthons for the construction of a diverse array of heterocyclic frameworks, many of which are related to those found in the oroidin derived alkaloids, is described. One such intermediate has been used in a formal total synthesis of cyclooroidin.

Gold-catalyzed cascade cyclization-oxidative alkynylation of allenoates

A gold(I)-catalyzed cascade cyclization-oxidative cross-coupling process has been applied to prepare β-alkynyl-γ-butenolides directly from allenoates and various terminal alkynes. Following an initial gold-catalyzed C-O bond forming allenoate cyclization, a mechanism based on a Au(I)/Au(III) redox cycle has been proposed with Selectfluor acting as the external oxidant.

Gold catalyzed carbocyclization of dienyl acetates to construct mutifunctionalized 3-vinyl cyclohexanol derivatives

A convenient new method was developed to construct six-membered 3-vinylcyclohexanols (and piperidine products) and 6-oxabicyclo[3.2.1]octan-7-one derivatives with high diastereoselectivities from 1,6-dienyl acetates via gold catalysis. The reaction proceeded through the nucleophilic addition of the alkenes onto the allylic cation group via a 6-endo-trig process. The substrate's structure affected the configuration orientation of the allylic cation group in a boatlike transition state, which afforded either the trans-cyclohexanols or cis-piperidine derivatives.