Seunghoon Shin

Catalytic access to α-oxo gold carbenes by N-O bond oxidants.

Metal carbenes are highly versatile species that mediate various transformations. Recent advances in gold catalysis have allowed catalytic access to α-oxo metal carbenes from the alkyne functionality. Compared with traditional methods that rely upon metal-catalyzed decomposition of diazo precursors, the generation of this synthon occurs in an environmentally more appealing fashion by gold-catalyzed alkyne oxygenation. Hydroxylamine derivatives are typically prepared from hydroxylamine salts that are cheap and can be handled without special precaution. In reactions with an alkyne activated by gold, relatively stable nitrones and related reagents undergo efficient O-atom transfer to form putative α-oxo gold carbenes. The highly reactive nature of these species could be utilized in a variety of cascade transformations. Herein, recent synthetic methods based on this reactivity as well as the currently available mechanistic and structural studies through computational and experimental methods have been discussed. A variety of tandem reactions performed by our laboratory and others have demonstrated the synthetic utility of catalytically generated α-oxo gold carbenes and enabled access to various heterocycles. For example, a reaction between nitrones and alkynes led to azomethine ylides for the [3 + 2] dipolar cycloaddition. Alternatively, α-oxo gold carbenes can be transformed into enolate equivalents through a 1,2-pinacol shift. The addition of hydroxylamine derivatives across triple bonds led to oxoamination, providing α-aminocarbonyl compounds or regioselective Fisher indole-type synthesis. N-O bond cleaving redox chemistry paved the way for intermolecular redox processes, most notably by use of pyridine-N-oxide derivatives with expanding synthetic applications. In closing, other metal-based oxygenations using N-O bond oxidants will be highlighted. One particularly interesting aspect is the process leading to metal vinylidene complexes. Trapping of this intermediate resulted in opposite regioselectivity from gold catalysis in alkyne oxygenation and led to ketene intermediates for use in subsequent cascade transformations.

Geometry-dependent divergence in the gold-catalyzed redox cascade cyclization of o-alkynylaryl ketoximes and nitrones leading to isoindoles

We report geometry-dependent cyclizations of o-alkynylaryl ketoximes and nitrones catalyzed by gold complexes. (E)-Ketoximes undergo N-attack to give isoquinoline-N-oxides. In sharp contrast, (Z)-ketoximes undergo unprecedented O-nucleophilic attack, followed by a redox cascade leading to a novel catalytic entry to isoindoles of diverse scope. The structure of an isoindole was unambiguously supported by X-ray crystallography. We demonstrated the generality of the isoindole synthesis from either (Z)-oximes or nitrones, and presented a mechanistic model of this redox cascade based on the reaction profiles of various substrates.

Gold-Catalyzed Intermolecular Reactions of Propiolic Acids with Alkenes: [4 + 2] Annulation and Enyne Cross Metathesis

A gold-catalyzed intermolecular reaction of propiolic acids with alkenes led to a [4 + 2] annulation or enyne cross metathesis. The [4 + 2] annulation proceeds with net cis-addition with respect to alkenes and provides an expedient route to α,β-unsaturated δ-lactones, for which preliminary asymmetric reactions were also demonstrated. For 1,2-disubstituted alkenes, unprecedented enyne cross metathesis occurred to give 1,3-dienes in a completely stereospecific fashion. DFT calculations and experiments indicated that the cyclobutene derivatives are not viable intermediates and that the steric interactions during concerted σ-bond rearrangements are responsible for the observed unique stereospecificity.

Cross-Coupling of Meyer–Schuster Intermediates under Dual Gold–Photoredox Catalysis

Under dual gold/photoredox catalytic conditions, intermediates from the Meyer-Schuster rearrangement underwent an efficient cross-coupling with arene diazonium salts, leading to α-arylated enones. Diazonium salts assisted the dissociation of the propargyl hydroxyl group by forming alkoxydiazenes in the Meyer-Schuster rearrangement, and the coupling was proposed to proceed through an allenyl methyl ether.

Highly selective ratiometric fluorescent probe for Au3+ and its application to bioimaging

The 4-propargylamino-1,8-naphthalimide based fluorescent probe 1 has been explored as a sensor for selective detection of Au(3+). 4-Amino-1,8-naphthalimides, that possess typical intramolecular charge transfer (ICT) electronic characteristics, have been widely used as versatile platforms for fluorescent probes. The newly designed probe 1 contains a propargylamine moiety at C-4 of the naphthalimide chromophore that reacts with Au(3+) to generate a product that has distinctly different electronic properties from 1. Specifically, the probe undergoes a remarkable change in its absorption spectrum upon addition of Au(3+) that is associated with a distinct color change from yellow to light pink. In addition, a blue shift of ca. 56 nm also takes place in the emission spectra of the probe. Consequently, 1 serves as a reaction-based sensor or so called chemodosimeter for Au(3+). Importantly, surfactants enhance the rate of reaction of 1 with Au(3+), thus, enhancing its use as a real time sensor. Finally, the results of studies probing its application to bioimaging of Au(3+) in live cells show that the probe 1 has a unique ability to sense Au(3+) in cells and, in particular, in lipid droplets within cells.

Stereoselective Palladium-Catalyzed α-Arylation of 3-Aryl-1-Indanones: An Asymmetric Synthesis of (+)-Pauciflorol F

Highly stereoselective, palladium-catalyzed α-arylation reactions of 3-aryl-1-indanones with aryl bromides are described. The use of sodium tert-butoxide as a base in this process is required to elevate the efficiencies and stereoselectivities of these reactions. The new methodology was successfully applied to a highly efficient route for the asymmetric synthesis of (+)-pauciflorol F.

Manipulating interfaces in a hybrid solar cell by in situ photosensitizer polymerization and sequential hydrophilicity/hydrophobicity control for enhanced conversion efficiency

The polyacetylene photosensitizer with quaternary pyridinium salts was layered on CdS nanoparticles films by in situ polymerization of 2-ethynylpyridine and 4-bromobutyric acid. The hydrophilic nature of the polyacetylene is shown to enhance the interfacial contact and electrical coupling between hydrophilic CdS and the polymer. The hydrophilicity of the polymer was modified toward hydrophobicity by anion exchange in order to adequately layer the hydrophobic poly(3-hexylthiophene) by spin coating, power-conversion efficiency 1.18% (AM1.5, I=100mW∕cm2).

Pd nanoparticle-silica nanotubes (Pd@SNTs) as an efficient catalyst for Suzuki–Miyaura coupling and sp2 C–H arylation in water

Silica nanotubes (SNTs) functionalized with Pd-NPs on the inner surface performed as efficient nano-reactors for C–C coupling in water; the nano-confinement offers minimized leaching of Pd and yet efficient mass transfer for Suzuki–Miyaura coupling and C–H arylation of thiazoles in water with very high TON.

Rhodium-catalyzed tandem cyclization–cycloaddition reactions of enynebenzaldehydes: construction of polycyclic ring systems

o-(1,6-Enynyl)benzaldehydes underwent a novel mode of cycloaddition using Rh(I)-precatalyst, via[3+2] cycloaddition of presumed dipolar carbonyl ylide intermediate generated by Rh-catalyst and the utility of this mechanistically intriguing enyne cyclization can be found in a number of polycyclic natural product skeletons.

In situ preparation of CdS nanoparticles imbedded in a polyelectrolyte multilayer for photocurrent generation

CdS nanoparticles (NPs) were prepared and imbedded in polyelectrolyte multilayers (PEMs) in situ and applied to the photocurrent generation. The PEM films of the poly(propylviologen)/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) were prepared using layer-by-layer deposition. The Br− ions of the viologen groups were exchanged with S2− ion by dipping the PEMs films in a Na2S solution. Then, the films were dipped in a solution containing Cd2+ to give CdS NPs of less than 10nm in size. The CdS NP-PEM films form an efficient acceptor-sensitizer dyad system, which showed a good photocurrent of 70nA∕cm2 at 380nm for a 3.5 bilayer sample.