Gold-Catalyzed Cyclizations of 1,6-Diynes

Gold(I)-Catalyzed Cycloisomerization of 3-Alkoxyl-1,6-diynes: A Facile Access to Bicyclo[2.2.1]hept-5-en-2-ones

A novel gold‐catalyzed cycloisomerization of 1,6‐diynes was achieved, providing an atom‐economic approach to a diverse set of bicyclo[2.2.1]hept‐5‐en‐2‐ones in moderate to good yields. With unsymmetrical starting materials with two different internal alkynyl substituents, to some extent, the regioselectivity could be controlled by both electronic and steric factors. This unprecedented reactivity pattern may inspire new and unconventional strategies for the preparation of bridged ring systems.

Alkynes as Synthetic Equivalents of Ketones and Aldehydes: A Hidden Entry into Carbonyl Chemistry

The high energy packed in alkyne functional group makes alkyne reactions highly thermodynamically favorable and generally irreversible. Furthermore, the presence of two orthogonal π-bonds that can be manipulated separately enables flexible synthetic cascades stemming from alkynes. Behind these "obvious" traits, there are other more subtle, often concealed aspects of this functional group's appeal. This review is focused on yet another interesting but underappreciated alkyne feature: the fact that the CC alkyne unit has the same oxidation state as the -CH2C(O)- unit of a typical carbonyl compound. Thus, "classic carbonyl chemistry" can be accessed through alkynes, and new transformations can be engineered by unmasking the hidden carbonyl nature of alkynes. The goal of this review is to illustrate the advantages of using alkynes as an entry point to carbonyl reactions while highlighting reports from the literature where, sometimes without full appreciation, the concept of using alkynes as a hidden entry into carbonyl chemistry has been applied.

Au(I)-Catalyzed Dimerization of Two Alkyne Units-Interplay between Butadienyl and Cyclopropenylmethyl Cation: Model Studies and Trapping Experiments

In recent years, Au(I)-catalyzed reactions proved to be a valuable tool for the synthesis of substituted cycles by cycloaromatization and cycloisomerization starting from alkynes. Despite the myriad of Au(I)-catalyzed reactions of alkynes, the mono Au(I)-catalyzed pendant to the radical dimerization of nonconjugated alkyne units has not been investigated by quantum chemical calculations. Herein, by means of quantum chemical calculations, we describe the mono Au(I)-catalyzed dimerization of two alkyne units as well as the transannular ring closure reaction of a nonconjugated diyne. We found that depending on the system and the method used either the corresponding cyclopropenylmethyl cation or the butadienyl cation represents the stable intermediate. This circumstance could be explained by different stabilizing effects. Moreover, the calculation reveals a dramatic (>10¹²-fold) acceleration of the Au(I)-catalyzed reaction compared to that of the noncatalyzed radical variant. Trapping experiments with a substituted 1,6-cyclodecadiyne using benzene as a solvent at room temperature as well as studies with deuterated solvents confirm the calculations. In this context, we also demonstrate that trapping of the cationic intermediate with benzene does not proceed via a Friedel-Crafts-type reaction.

On the Gold-Catalyzed Generation of Vinyl Cations from 1,5-Diynes

Conjugated 1,5-diynes bearing two aromatic units at the alkyne termini were converted in the presence of a gold catalyst. Under mild conditions, aryl-substituted dibenzopentalenes were generated. Calculations predict that aurated vinyl cations are key intermediates of the reaction. A bidirectional approach provided selective access to the angular annulated product in high yield, which was explained by calculations.

Gold-catalysed reactions of diynes

In this critical review the reactivity patterns observed with different types of diyne substrates in gold catalysis are discussed. Apart from the many examples from homogeneous catalysis, the few examples from heterogeneous gold catalysis are also included. With a proper arrangement of the two alkynes unique and exciting reactivity patterns like 1,3-carbonyl transpositions, carbene transfer reactions, cascade annulations, macrocyclisations or the formation of gold vinylidene intermediates are observed. These reactions are of interest for organic synthesis, for pharmaceutical and medicinal chemistry and for material science.

Non-covalent interactions in coinage metal complexes of 1,2,4-triazole-based N-heterocyclic carbenes

Seven coinage metal(I) complexes bearing two different triazole-based N-heterocyclic carbene (NHC) ligands, [1-tert-butyl-4-{2-[(N,N-dimethylamino)methyl]phenyl}-3-phenyl-1H-1,2,4-triazol-4-ium-5-ide and 1-tert-butyl-4-(4-methylphenyl)-3-phenyl-1H-1,2,4-triazol-4-ium-5-ide], were synthesized and fully characterized in solution by NMR spectroscopy as well as in the solid state by X-ray diffraction techniques. Furthermore, the XRD analysis showed that the bidentate coordination of the amino group substituted NHC ligand, previously observed for rhodium and palladium complexes, does not take place in the solid state structure of Au(I) complexes with various halide ligands. Nevertheless, the formation of sets of two head-to-tail oriented monomers aggregated via a weak metallophilic contact was revealed for both NHC ligands as well as for all three coinage metals with different halides. These experimental data correlate quite well with the previously published theoretical study on related complexes.

Recent advances in the gold-catalyzed additions to C-C multiple bonds

C-O, C-N and C-C bonds are the most widespread types of bonds in nature, and are the cornerstone of most organic compounds, ranging from pharmaceuticals and agrochemicals to advanced materials and polymers. Cationic gold acts as a soft and carbophilic Lewis acid and is considered one of the most powerful activators of C-C multiple bonds. Consequently, gold-catalysis plays an important role in the development of new strategies to form these bonds in more convenient ways. In this review, we highlight recent advances in the gold-catalyzed chemistry of addition of X-H (X = O, N, C) bonds to C-C multiple bonds, tandem reactions, and asymmetric additions. This review covers gold-catalyzed organic reactions published from 2008 to the present.

Construction of cyclic enones via gold-catalyzed oxygen transfer reactions

During the last decade, gold-catalyzed reactions have become a tour de force in organic synthesis. Recently, the gold-, Brønsted acid- or Lewis acid-catalyzed oxygen transfer from carbonyl to carbon–carbon triple bond, the so-called alkyne–carbonyl metathesis, has attracted much attention because this atom economical transformation generates α,β-unsaturated carbonyl derivatives which are of great interest in synthetic organic chemistry. This mini-review focuses on the most recent achievements on gold-catalyzed oxygen transfer reactions of tethered alkynones, diynes or alkynyl epoxides to cyclic enones. The corresponding mechanisms for the transformations are also discussed.

Syntheses of α-pyrones using gold-catalyzed coupling reactions

Sequential alkyne activation of terminal alkynes and propiolic acids by gold(I) catalysts yields compounds having α-pyrone skeletons. Novel cascade reactions involving propiolic acids are reported that give rise to α-pyrones with different substitution patterns.