Cationic Gold Catalysis Research Articles

Investigating chemical diversity: o-propargylphenols as key compounds in the divergent synthesis of 2-substituted benzofurans and chromenes.

In this study, we explored and optimized a MW-enhanced divergent approach for the synthesis of 2-substituted benzofurans and chromenes, starting from seventeen substituted o-propargylphenols characterized by a monoaryl substitution on the propargylic sp3 carbon. Firstly, we developed a robust platform for the preparation of a library of o-propargylphenols. Under basic conditions, o-propargylphenols reacted regioselectively to yield benzofurans in yields ranging from 43% to 100%. Conversely, under cationic gold catalysis, we were able to obtain the corresponding 4H-chromenes, albeit in more variable yields (from 25% to 93%) and slightly lower regioselectively. We also proposed plausible mechanisms to explain the divergent outcomes observed. Our findings underscore the potential of diversity-oriented synthesis in the investigation of molecular complexity. Our neglected o-propargylphenols have proven to be versatile and strategic starting materials for accessing oxygen-containing heterocyclic scaffolds through intramolecular cyclization reactions.

Diversification of 4-bromo-1H-indole-3-carbaldehyde-derived Ugi adducts: Access to the azepino[3,4,5-cd]indoles and spiroindolines

4-Bromo-1H-indole-3-carbaldehyde has been explored as a key building block for a post-Ugi assembly of azepino[3,4,5-cd]indoles and spiroindolines. First, the corresponding Ugi adducts were successfully employed in palladium-catalyzed reductive Heck cyclization producing a range of azepinoindoles. Then, 4-bromo-1H-indole-3-carbaldehyde-derived Ugi adducts were subjected to cationic gold catalysis affording tetracyclic spiroindolines. Furthermore, the aryl bromide moiety retained in the resulting spiroindolines was found to be amendable to further functionalization via Suzuki and Sonogashira couplings.

AuI -Catalyzed Haloalkynylation of Alkenes.

The formal insertion of alkenes into aromatic chloro- and bromoalkynes takes place under cationic gold catalysis. This haloalkynylation reaction can be performed with cyclic, gem-disubstituted and monosubstituted alkenes, using BINAP, triazolo[4,3-b]isoquinolin-3-ylidene ligands or SPhos, respectively. The products were isolated in moderate to excellent yields and with complete diastereo- and regioselectivity; the halogen atom bonding the more substituted carbon of the alkene. Preliminary experiments showed that the enantioselective haloalkynylation of cyclopentene can be performed with (S)-BINAP to afford the insertion products with moderate to good enantioselectivities.

Improving Homogeneous Cationic Gold Catalysis through a Mechanism-Based Approach.

Homogeneous gold catalysis is regarded as a landmark addition to the field of organic synthesis. It is the most effective way to activate alkynes for the addition of a diverse host of nucleophiles. However, the literature reveals that a relatively high catalyst loading is needed in many gold-catalyzed applications (1-10 mol %), which is impractical in large-scale synthesis or multistep synthesis because of the high price and recyclization difficulty of the gold. A more thorough understanding of the factors that operate on homogeneous gold catalysis can provide better guidelines for the future design of more efficient gold-catalyzed reactions. In this Account, we will summarize our group's extensive investigation of factors impacting cationic gold catalysis, namely, the effects of ligands, counterions, additives, and catalyst decay and deactivation, using a mechanism-based approach with the aim of improving the efficiency of homogeneous gold catalysis. Through NMR-assisted kinetic studies, we investigated the above factors. Our systematic ligand effect investigation provided a clearer understanding of how ligands influence each of the three stages in the gold catalytic cycle. On the basis of this study, we synthesized a novel phosphine ligand and achieved parts per million-level gold catalysis by manipulating the electron density of the substituents and the steric strain around phosphorus. Our investigation of counterion effects led to the design of a gold affinity index and hydrogen-bonding basicity index for counterions, which can forecast the reactivity of counterions in cationic gold catalysis. We studied the adverse silver effects in cationic gold catalyst activation and proposed a more efficient practical guide. Our additive effect investigation revealed that additives that are good hydrogen-bond acceptors increase the efficiency of gold-catalyzed reactions in those occurrences where protodeauration is the rate-determining step. The first detailed experimental analysis of gold catalyst decay and the influence of each component in the reaction system (substrate, counterion, solvent) on the decay process was also conducted. We found that high-gold-affinity impurities (halides, bases) in solvents, starting materials, filtration, or drying agents decrease the reactivity of a gold catalyst but that a suitable acid activator can reactivate the gold catalyst and enable the reaction to proceed smoothly at competitively low gold catalyst loadings. The effects of acid additives were also systematically investigated using typical reactions. We are convinced that better mechanistic understandings will offer clearer guidelines for the search for more efficient gold-catalyzed reactions.

Predicting Counterion Effects Using a Gold Affinity Index and a Hydrogen Bonding Basicity Index

We have developed a gold affinity index and hydrogen bonding basicity index for counterions and have used these indexes to forecast their reactivity in cationic gold catalysis.

Acidic Co-Catalysts in Cationic Gold Catalysis.

A systematic study on the effects of Lewis or Brønsted acid co-catalysts in gold-catalyzed reactions was undertaken using representative reactions (O-, N-, and C-nucleophilic additions to alkynes). Through these reactions, it was demonstrated that an acidic co-catalyst can increase the catalyst turnover significantly, enabling practical reaction rates at competitive catalyst loadings (<1 mol %). Further investigation is currently underway to improve the understanding of the subtle principles underlying these experimental observations.

Revisiting the Influence of Silver in Cationic Gold Catalysis: A Practical Guide.

An excess amount of silver salt to generate cationic gold from a gold catalyst precursor such as L-Au-Cl almost always has adverse effects on the reactivity of the cationic gold catalyst. A preformed L-Au(+)X(-) complex, generated by sonication followed by centrifugation, increases the reactivity in a gold catalyzed reaction. The adverse silver effect might be caused by the interaction of silver salts with gold intermediates.

Enantioselective cyclizations of silyloxyenynes catalyzed by cationic metal phosphine complexes.

The discovery of complementary methods for enantioselective transition metal-catalyzed cyclization with silyloxyenynes has been accomplished using chiral phosphine ligands. Under palladium catalysis, 1,6-silyloxyenynes bearing a terminal alkyne led to the desired five-membered ring with high enantioselectivities (up to 91% ee). As for reactions under cationic gold catalysis, 1,6- and 1,5-silyloxyenynes bearing an internal alkyne furnished the chiral cyclopentane derivatives with excellent enantiomeric excess (up to 94% ee). Modification of the substrate by incorporating an α,β-unsaturation led to the discovery of a tandem cyclization. Remarkably, using silyloxy-1,3-dien-7-ynes under gold catalysis conditions provided the bicyclic derivatives with excellent diastereo- and enantioselectivities (up to >20:1 dr and 99% ee).

ChemInform Abstract: Synthetic Evolutions in the Nucleophilic Addition to Alkynes

AbstractReview: 65 refs.

ChemInform Abstract: Fluorine‐Enabled Cationic Gold Catalysis: Functionalized Hydration of Alkynes.

AbstractA one‐pot tandem addition/oxidative coupling/fluorination sequence performed with the alkyne, an arylboronic acid, water, and Selectfluor in the presence of a gold(I) catalyst affords α‐aryl‐α‐fluoroketones.

Synthetic evolutions in the nucleophilic addition to alkynes

This short account describes our efforts to transform the simple nucleophilic addition of alkynes into a more efficient, selective and environmentally benign synthetic tool. We have circumvented the lack of regioselectivity in the gold-catalyzed triple bond addition of water through neighboring group participation and in the process we developed a ‘functionalized hydration’ (multiple bond formation and hydration in a one-pot process) using fluorine-engendered cationic gold catalysis. In addition, we have conducted the synthesis of O-heterocycles through a gold-catalyzed tandem addition/cycloisomerization sequence, the synthesis of N-heterocycles through a copper-catalyzed cyclization-triggered addition of alkynes, and a green synthesis of thioethers ‘on water’ without catalyst or initiator. These nucleophilic synthetic evolutions, catapulted by a simple addition to an alkyne, will surely contribute to provide a wider synthetic access to sophisticated biological targets.

Fluorine‐Enabled Cationic Gold Catalysis: Functionalized Hydration of Alkynes

A one-pot tandem addition/oxidative coupling/fluorination sequence performed with the alkyne, an arylboronic acid, water, and Selectfluor in the presence of a gold(I) catalyst affords α-aryl-α-fluoroketones.

Fluorine-Enabled Cationic Gold Catalysis: Functionalized Hydration of Alkynes

Fluor macht's möglich: Eine kationische Fluor-Gold-Spezies, die durch Fluorierung eines niedervalenten Goldkomplexes hergestellt wurde, vermittelt die „funktionalisierende Hydratation“ von Alkinen. Eine Dreifachbindung wird hierbei genutzt, um in einem Eintopfprozess unter milden Bedingungen eine Carbonylgruppe und zwei neue Bindungen am α-Kohlenstoffatom einzuführen (siehe Schema). Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.