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Sukbok Chang

Publications and source records attributed to Sukbok Chang.

At least 19 recordsLinked to original sources
PubMed2006

Hydrogen-bond-directed highly stereoselective synthesis of Z-enamides via Pd-catalyzed oxidative amidation of conjugated olefins.

An efficient procedure for the preparation of Z-enamides has been developed, involving the reaction of primary amides with conjugated olefins using a Pd/Cu cocatalyst system. It was found that certain additives, such as phosphine oxides and phosphonates, increase the efficiency of the reaction in nonpolar solvents under an oxygen atmosphere, thus producing a variety of Z-enamides in high yields with excellent stereoselectivity under Wacker-type conditions. The oxidative amidation reaction has a broad substrate scope, allowing alkyl, aryl, and vinyl amides to react with olefins conjugated with ester, amide, phosphonate, and ketone groups. The notable preference for the formation of Z-enamides is presumably due to the presence of an intramolecular hydrogen bond between the amido proton and the carbonyl oxygen. The energy difference between two plausible sigma-alkylamidopalladium intermediates, leading to Z- and E-isomeric enamide products, respectively, was calculated to be 4.18 kcal/mol. The beta-hydride elimination step is assumed to be a stereochemistry-determining step in the overall oxidative amidation process, with the energy level for the transition state leading to the Z-enamide being 5.35 kcal/mol lower than that leading to the E-isomer. The efficiency of photoisomerization between Z- and E-enamides was observed to be largely dependent on the substrates' substituents, and certain E-enamides could be obtained in synthetically useful yields by photoirradiation of Z-isomers. Synthetic application of the present method was successfully demonstrated by a direct formal synthesis of cis-CJ-15,801.

Alkenes↗
PubMed2006

Catalytic one-pot synthesis of cyclic amidines by virtue of tandem reactions involving intramolecular hydroamination under mild conditions.

A new synthetic methodology for the generation of cyclic amidines has been developed by the reaction of 1,n-aminoalkynes with electron-deficient azides using a ruthenium catalyst at ambient temperature. The reaction proceeds most likely via a tandem sequence of intramolecular hydroamination of aminoalkynes, cycloaddition of azides with the resulting enamines, and rearrangement of triazoline intermediates. It demonstrates, as the proof-of-principle, that an equilibria cascade sequence can be favorably driven by an irreversible step, thus enabling a facile one-pot synthetic route to deliver molecular complexity under unprecedented mild conditions without relying on the traditional linear approaches.

Journal Article↗
PubMed2006

Halide ions as a highly efficient promoter in the Ru-catalyzed hydroesterification of alkenes and alkynes.

The presence of catalytic amounts of halide salts was found to enhance dramatically the reaction efficiency in the Ru-catalyzed hydroesterification of alkenes and alkynes using a chelating 2-pyridylmethyl formate by lowering the reaction temperature. On the basis of IR and NMR studies, the halide effect on the reaction is mainly attributed to the facile dissociation of the trirutheniumcarbonyl precursor into the presumed active metal species. With this milder condition, the substrate scope has been significantly broadened.

Journal Article↗
PubMed2006

Trimanganese complexes bearing bidentate nitrogen ligands as a highly efficient catalyst precursor in the epoxidation of alkenes.

A series of trinuclear manganese complexes coordinated with neutral bidentate nitrogen ligands, [Mn3L2(OAc)6], were prepared from manganese acetate and the corresponding ligands. Using peracetic acid as the oxidant, the air- and moisture-stable manganese clusters exhibited excellent catalytic activity and selectivity in the epoxidation of olefins under mild conditions. The highest activity was observed with a trinuclear complex containing a 2-pyridylimino ligand, [Mn3(ppei)2(OAc)6] (ppei = 2-pyridinal-1-phenylethylimine). With this system, the substrate scope was extremely wide to include terminal and electron-deficient double bonds of both aliphatic and aromatic alkenes. The high activity was undiminished under the reaction conditions even directly using a mixture of the pyridylimino ligands and manganese acetates, making this process more convenient. It was also observed that analogous trinuclear complexes, such as [Mn3(bipy)2(OAc)6] and [Mn3(phen)2(OAc)6], displayed excellent activities. While radical intermediacy was inferred from the product distribution, kinetic data revealed that the epoxidation is roughly first-order in manganese cluster precursor and oxidant, respectively, and zero-order in olefin. These results led us to propose that the trinuclear complexes [Mn3L2(OAc)6] serve as catalyst precursors that dissociate into monomeric species with the formulation of [MnL2(OAc)2] under the reaction conditions.

Alkenes↗
PubMed2006

A facile access to N-sulfonylimidates and their synthetic utility for the transformation to amidines and amides.

[reaction: see text] It is shown that N-sulfonylimidates can be efficiently prepared by a three-component coupling of terminal alkynes, sulfonyl azides, and alcohols with use of a copper catalyst and an amine base. The reaction is characterized by mild conditions, high selectivity, and tolerance with various functional groups. Facile transformation of imidates to amidines was also achieved by sodium cyanide. Additionally, a protocol for the extremely efficient Pd-catalyzed [3,3]-sigmatropic rearrangement of allylic sulfonimidates to N-allylic sulfonamides has been developed.

Journal Article↗
PubMed2005

Copper-catalyzed hydrative amide synthesis with terminal alkyne, sulfonyl azide, and water.

It is shown for the first time that N-sulfonyl amides can be efficiently prepared by an unconventional approach of the hydrative reaction between terminal alkynes, sulfonyl azides, and water in the presence of copper catalyst and amine base under very mild conditions. The present route is quite general, and a wide range of alkynes and sulfonyl azides are readily coupled catalytically with water to furnish amides in high yields. A variety of labile functional groups are tolerated under the conditions, and the reaction is regioselective in that only terminal alkynes react while double or internal triple bonds are intact. The reaction can be readily scaled up and is also adaptable to a solid-phase procedure with high efficiency.

Journal Article↗
PubMed2005

WO3 nanoparticles on MCM-48 as a highly selective and versatile heterogeneous catalyst for the oxidation of olefins, sulfides, and cyclic ketones.

[reaction: see text] It is shown that nanosized WO(3) particles supported on MCM-48 work as a highly efficient and selective heterogeneous catalyst for the oxidation of olefins, sulfides, and cyclic ketones using hydrogen peroxide or peracetic acid. The catalytic activity of the supported tungstate was dependent on the nature of the supporting materials and particle size. The catalyst system employs environmentally benign oxidants in halide-free solvents, and it does not require phase-transfer agents and pH control.

Journal Article↗
PubMed2005

Highly efficient one-pot synthesis of N-sulfonylamidines by Cu-catalyzed three-component coupling of sulfonyl azide, alkyne, and amine.

A highly efficient, mild, practical, and catalytic multicomponent reaction for the synthesis of N-sulfonylamidines has been developed. This reaction has an extremely wide scope with regard to all three coupling components of alkyne, sulfonyl azide, and amine. Two plausible mechanistic pathways involving ketenimine or triazole intermediate are tentatively presented for the copper-catalyzed three-component coupling reactions.

Alkynes↗
PubMed2004

Substituent effects on the Z/E-selectivity in cross-metathesis of conjugated enynes.

Cross-metathesis of a range of conjugated enynes with alkenes turns out to proceed with preferential formation of Z-isomers over E-isomers up to >25:1. Careful studies including substrate modification and control experiments revealed that the reaction proceeds under kinetic rather than thermodynamic control. Driving forces for this substrate-dependent Z-selectivity are attributed to the steric hindrance between substituents on the reacting enynes and NHC ligand of the ruthenium catalyst in the putative metallacyclobutane, as well as chelation effects of suitably positioned functional groups to Ru, which is strongly supported by ab initio calculations.

Journal Article↗
PubMed2004

Notable coordination effects of 2-pyridinesulfonamides leading to efficient aziridination and selective aziridine ring opening.

[reaction: see text] We have developed, on the basis of a chelation-strategy, an efficient copper-catalyzed aziridination protocol with the use of 5-methyl-2-pyridinesulfonamide and PhI(OAc)(2). The reaction proceeds smoothly under mild conditions to give aziridines in moderate to good yields in the absence of external ligands or bases. The coordination-assisted approach offers the additional benefits that efficient deprotection of the N-substituent and selective aziridine ring-opening are effectively achieved.

Journal Article↗
PubMed2004

Cooperative multi-catalyst systems for one-pot organic transformations.

One-pot co-catalyst systems are covered in this tutorial review. It is divided into three sections according to the reaction types: i) one catalyst performs a desired reaction as the second catalyst restores the first catalytic species back into its original state for the next catalytic cycles, ii) two catalysts carry out sequential organic transformations, in which the first step is carried out by one catalyst to afford certain intermediates being to be subjected to the second catalyst for the next step, and iii) cooperative catalytic actions on both substrates by suitable catalysts proceed in a substrate-selective manner followed by the subsequent coupling of the two activated adducts providing the desired products.

Journal Article↗
PubMed2004

Ruthenium-catalyzed Heck-type olefination and Suzuki coupling reactions: studies on the nature of catalytic species.

Ruthenium-catalyzed Heck olefination and Suzuki cross coupling reactions have been developed. When starting with a ruthenium complex [RuCl(2)(p-cymene)](2) as a homogeneous catalyst precursor, induction periods were observed and ruthenium colloids of zero oxidation state were generated under catalytic conditions. Isolated ruthenium colloids carried out the olefination, implying that active catalytic species are ruthenium nanoclusters. To support this hypothesis, ruthenium nanoparticles stabilized with dodecylamine were independently prepared via a hydride reduction procedure, and their catalytic activity was subsequently examined. Olefination of iodobenzene with ethyl acrylate was efficiently catalyzed by the ruthenium nanoparticles under the same conditions, which could be also reused for the next runs. In poisoning experiments, the conversion of the olefination was completely inhibited in the presence of mercury, thus supporting our assumption on the nature of catalytic species. No residual ruthenium was detected from the filtrate at the end of the reaction. On the basis of the postulation, a heterogeneous catalyst system of ruthenium supported on alumina was consequently developed for the Heck olefination and Suzuki cross coupling reactions for the first time. It turned out that substrate scope and selectivity were significantly improved with the external ligand-free catalyst even under milder reaction conditions when compared to results with the homogeneous precatalyst. It was also observed that the immobilized ruthenium catalyst was recovered and reused up to several runs with consistent efficiency. Especially in the Suzuki couplings, the reactions could be efficiently carried out with as low as 1 mol % of the supported catalyst over a wide range of substrates and were scaled up to a few grams without any practical problems, giving coupled products with high purity by a simple workup procedure.

Journal Article↗
PubMed2003

Conjugated enynes as a new type of substrates for olefin metathesis.

[reaction: see text] It has been demonstrated for the first time that conjugated enynes can be employed as a facile substrate in olefin metathesis with the use of a bispyridine-substituted ruthenium benzylidene catalyst. Cross-metathesis of the enynes with alkenes turns out to proceed with preferential formation of (Z)-isomers over (E)-isomers up to >25:1 in moderate to good yields. The intramolecular version of conjugated enynes affords novel butadienyl cycloalkenes, which are a highly useful synthetic building blocks, in acceptable yields.

Journal Article↗
PubMed2003

Ru-catalyzed hydroamidation of alkenes and cooperative aminocarboxylation procedure with chelating formamide.

[reaction: see text] A strategy of chelation-assisted activation of formamide was employed to achieve hydroamidation of alkenes to generate one-carbon-elongated amides in moderate to good selectivity and yields. Also reported is the two-metal-catalyzed cooperative aminocarboxylation of aryl iodides, in which Ru is presumed to catalyze decarbonylation of formamide to release carbon monoxide and amine for the subsequent Pd-catalyzed aminocarboxylation routes, thus enabling the net transformation to be performed in the absence of external CO pressure.

Journal Article↗