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Hye-Young Jang

Publications and source records attributed to Hye-Young Jang.

14 recordsLinked to original sources
PubMed2005

Enantioselective reductive cyclization of 1,6-enynes via rhodium-catalyzed asymmetric hydrogenation: C-C bond formation precedes hydrogen activation.

Asymmetric hydrogenation of 1,6-enynes using chirally modified cationic rhodium precatalysts enables enantioselective reductive cyclization to afford alkylidene-substituted carbocycles and heterocycles in a completely atom economical fashion. Good to excellent yields and exceptional levels of asymmetric induction are observed across a structurally diverse set of substrates. Mechanistic studies involving hydrogen-deuterium crossover experiments, along with the observance of nonconjugated cycloisomerization products 14c and 15c, suggest rhodium(III) metallocyclopentene formation occurs in advance of hydrogen activation. This oxidative coupling-hydrogenolytic cleavage motif should play a key role in the design of related hydrogen-mediated couplings.

Journal Article↗
PubMed2004

Self-assembled metallocycles with two interactive binding domains.

Five metallocycles 1 a-e have been self-assembled from S-shaped bispyridyl ligands 2 a-e and a palladium complex, [Pd(dppp)(OTf)(2)] (dppp=1,3-bis(diphenylphosphanyl)propane), and have been characterized by elemental analysis and various spectroscopic methods including (1)H NMR spectroscopy and electrospray ionization (ESI) mass spectrometry. These metallocycles all are monocyclic compounds, but can fold to generate two binding domains bearing hydrogen-bonding sites based on pyridine-2,6-dicarboxamide units. The binding properties of the metallocycles with N,N,N',N'-tetramethylterephthalamide (G) have been probed by means of ESI mass spectrometry and (1)H NMR spectroscopy. The results both in the gas phase and in solution are consistent with the fact that the metallocycles accommodate two molecules of the guest G. Thus, the ESI mass spectra clearly show fragments corresponding to the 1:2 complexes in all cases. (1)H NMR studies on 1 a and G support the formation of a 1:2 complex in solution; the titration curves are nicely fitted to a 1:2 binding isotherm, but not to a 1:1 binding isotherm. In addition, a Job plot also suggests a 1:2 binding mode between 1 a and G, showing maximum complexation at approximately 0.33 mol fraction of the metallocycle 1 a in CDCl(3). The binding constants K(1) and K(2) are calculated to be 1600 and 1400 M(-1) (+/-10 %), respectively, at 25 degrees C in CDCl(3), indicative of positively cooperative binding. This positive cooperativity was confirmed by the Hill equation, affording a Hill coefficient of n = 1.6. Owing to insufficient solubility in CDCl(3), for comparison purposes the binding properties of the metallocycles 1 b-e were investigated in a more polar medium, 3 % CD(3)CN/CDCl(3). (1)H NMR titrations revealed that the metallocycles all bind two molecules of the guest G with Hill coefficients ranging from 1.4 to 1.8. This positive cooperativity may be attributed to a structural reorganization of the second binding cavity when the first guest binds to either one of the subcavities present in the metallocycles.

Hydrogen Bonding↗
PubMed2004

Rhodium-catalyzed reductive cyclization of 1,6-diynes and 1,6-enynes mediated by hydrogen: catalytic C-C bond formation via capture of hydrogenation intermediates.

Catalytic hydrogenation of carbon-, nitrogen- and oxygen-tethered 1,6-diynes 1a-9a and 1,6-enynes 10a-18a using cationic Rh(I) precatalysts at ambient temperature and pressure enables reductive carbocyclization to afford 1,2-dialkylidene cyclopentanes 1b-9b and monoalkylidene cyclopentanes 10b-18b, respectively, in good to excellent yields and as single alkene stereoisomers. Notably, the 1,3-diene and alkene containing cyclization products 1b-9b and 10b-18b are not subject to over-reduction under the conditions of catalytic hydrogenation in which they are formed. Reductive cyclization 1,6-diyne 1a and 1,6-enyne 10a performed under an atmosphere of D(2) provides the carbocyclization products deuterio-1b and deuterio-10b, respectively, which incorporate two deuterium atoms. The collective data are consistent with a catalytic mechanism involving heterolytic activation of elemental hydrogen (H(2) + Rh(+)X(-) --> Rh-H + HX) followed by Rh(I)-mediated oxidative cyclization of the 1,6-diyne or 1,6-enyne substrates to afford (hydrido)Rh(III)-based metallocyclopentadiene and metallocyclopentene intermediates, respectively. These transformations represent the first examples of metal-catalyzed reductive carbocyclization mediated by hydrogen.

Journal Article↗
PubMed2004

Hydrogen-mediated C-C bond formation: catalytic regio- and stereoselective reductive condensation of alpha-keto aldehydes and 1,3-enynes.

Hydrogenation of 1,3-enynes in the presence of alpha-keto aldehydes using cationic Rh(I) catalysts enables regio- and stereoselective reductive coupling to the acetylenic terminus of the enyne to afford (E)-2-hydroxy-3,5-dien-1-one products. Reductive condensation of 1-phenyl but-3-en-1-yne 1a with phenyl glyoxal 2a performed under an atmosphere of D(2) provides the product of mono-deuteration, (E)-2-hydroxy-3-deuterio-3,5-dien-1-one deuterio-3a, in 85% yield. Competition experiments involving catalytic hydrogenation of phenyl glyoxal in the presence of equimolar quantities of 1,4-diphenylbutadiene and 1,4-diphenylbut-3-en-1-yne 10a, as well as 1,4-diphenylbut-3-en-1-yne 10a and 1,4-diphenylbutadiyne, are chemoselective for coupling to the more highly unsaturated partner, suggesting a preequilibrium involving precoordination and exchange of the pi-unsaturated pronucleophiles with the catalyst prior to C-C bond formation, as well as a preference for coordination of the most pi-acidic reacting partner, as explained by the Dewar-Chatt-Duncanson model for alkyne coordination.

Aldehydes↗
PubMed2004

Catalytic C-C bond formation via capture of hydrogenation intermediates.

Although catalytic hydrogenation has been practiced for over a century, use of hydrogen as a terminal reductant in catalytic C-C bond formation has been restricted to processes involving migratory insertion of carbon monoxide, e.g., alkene hydroformylation and related Fischer-Tropsch-type reactions. In an effort to develop hydrogenation as a new method for catalytic cross-coupling, a catalytic system enabling capture of hydrogenation intermediates was recently developed in our lab. These results support the feasibility of developing a broad new family of hydrogen-mediated C-C bond formations.

Journal Article↗
PubMed2003

First catalytic reductive coupling of 1,3-diynes to carbonyl partners: a new regio- and enantioselective C-C bond forming hydrogenation.

Upon exposure of diynes and glyoxals to cationic Rh(I) catalysts under 1 atm of hydrogen gas, regioselective condensation occurs to afford highly unsaturated enyne products without over-reduction. In the presence of chiral phosphine ligands, reductive coupling products are obtained in high enantiomeric excess at ambient temperature and pressure. The present studies are among the first examples of the electrophilic trapping of organometallic intermediates obtained transiently under the conditions of catalytic hydrogenation.

Alkenes↗
PubMed2003

Quantitative comparison of kinetic stabilities of metallomacrocycle-based rotaxanes.

Four mononuclear metallomacrocycles with identical cavities but different transition metals (Os(VI), Pd(II), Pt(II), and Re(I)) were prepared. With these metallomacrocycles, the corresponding rotaxanes 2-Os, 2-Pd, 2-Pt, and 2-Re were self-assembled by hydrogen-bonding interactions. The kinetic stabilities of the rotaxanes were determined quantitatively and compared with each other by (1)H NMR spectroscopic techniques, including two-dimensional exchange spectroscopy (2D-EXSY) experiments. The activation free energies (DeltaG( not equal )) for the exchange between the rotaxanes 2-Os, 2-Pd and 2-Pt and their free components were determined to be 15.5, 16.0, and 16.4 kcal mol(-1), respectively. These magnitudes imply that the rotaxanes 2-Os, 2-Pd and 2-Pt are kinetically labile at room temperature and exist only as equilibrium mixtures with free components in solution. In contrast, the rotaxane 2-Re is kinetically stable enough to be isolated in pure form by silica gel chromatography under ordinary laboratory conditions. However, at higher temperatures (>60 degrees C) 2-Re was slowly disassembled into its components until the equilibrium was established. The rate constants were measured at three different temperatures, and the Eyring plot yielded the activation enthalpy DeltaH(not equal)=35 kcal mol(-1) and the activation entropy DeltaS(not equal)=27 eu for the disassembly of the rotaxane 2-Re in Cl(2)CDCDCl(2). These thermodynamic parameters gave the activation free energy DeltaG(not equal)(off)=27.1 kcal mol(-1) at 25 degrees C. Consequently, 2-Re is one example of a novel metallomacrocycle-based rotaxane that contains a coordination bond with enough strength to allow both for isolation in pure form around room temperature and for self-assembly at higher temperatures.

Journal Article↗
PubMed2002

Reductive generation of enolates from enones using elemental hydrogen: catalytic C-C bond formation under hydrogenative conditions.

Exposure of enones to elemental hydrogen in the presence of a Rh(I) catalyst enables reductive enolate generation, as evidenced by electrophilic trapping of the enolate by appendant and exogenous aldehyde partners. The significance of these findings resides in the ability to regioselectivity generate and transform transition metal enolates under catalytic conditions that circumvent formation of stoichiometric byproducts.

Alcohols↗
PubMed2002

Diastereoselective cycloreductions and cycloadditions catalyzed by Co(dpm)(2)-silane (dpm = 2,2,6,6-tetramethylheptane-3,5-dionate): mechanism and partitioning of hydrometallative versus anion radical pathways.

In the presence of phenylsilane and 5 mol % cobalt(II) bis(2,2,6,6-tetramethylheptane-3,5-dionate), aryl-substituted monoenone monoaldehydes and bis(enones) undergo reductive cyclization to afford syn-aldol and anti-Michael products, respectively. For both aldol and Michael cycloreductions, five- and six-membered ring formation occurs in good yield with high levels of diastereoselectivity. Cycloreduction of monoenone monoaldehyde 1a in the presence of d(3)-phenylsilane reveals incorporation of a single deuterium at the enone beta-position as an equimolar mixture of epimers, inferring rapid isomerization of the kinetically formed cobalt enolate prior to cyclization. The deuterated product was characterized by single-crystal neutron diffraction analysis. For bis(enone) substrates, modulation of the silane source enables partitioning of the competitive Michael cycloreduction and [2 + 2] cycloaddition manifolds. A study of para-substituted acetophenone-derived bis(enones) reveals that substrate electronic features also direct partitioning of cycloreduction and cycloaddition manifolds. Further mechanistic insight is obtained through examination of the effects of enone geometry on product stereochemistry and electrochemical studies involving cathodic reduction of bis(enone) substrates. The collective experiments reveal competitive enone reduction pathways. Enone hydrometalation produces metallo-enolates en route to aldol and Michael cycloreduction products, that is, products derived from coupling at the alpha-position of the enone. Electron-transfer-mediated enone reduction produces metallo-oxy-pi-allyls en route to [2 + 2] cycloadducts and, under Ni catalysis, homoaldol cycloreduction products, that is, products derived from coupling at the beta-position of the enone. The convergent outcome of the metal-catalyzed and electrochemically induced transformations suggests the proposed oxy-pi-allyl intermediates embody character consistent with the mesomeric metal-complexed anion radicals.

Anions↗
PubMed2002

Organocatalytic Michael cycloisomerization of bis(enones): the intramolecular Rauhut-Currier reaction.

The utilization of enones as latent enolates enables regioselective enolate formation from chemically robust presursors. In this communication, we report a catalytic Michael cycloisomerization of bis(enones) under Morita-Baylis-Hillman conditions. Upon exposure to 10 mol % tributylphosphine, bis(enone) substrates afford both five- and six-membered ring products. Notably, unsymmetrical bis(enones) possessing sufficient steric or electronic bias yield single isomeric products.

Alkenes↗
PubMed2002

Anion radical chain cycloaddition of tethered enones: intramolecular cyclobutanation and Diels-Alder cycloaddition.

[reaction: see text] The anion radicals of certain bis(enones), generated by cathodic reduction, are observed to participate in intramolecular cyclobutanation, yielding bicyclo[3.2.0]heptane derivatives through an anion radical chain mechanism. Evidence for stepwise cycloaddition involving distonic anion radical intermediates is presented. In addition to the novel anion radical cyclobutanations, an unprecedented intramolecular anion radical Diels-Alder product is observed. Parallel trends in substrate scope vis-à-vis the Co-catalyzed bis(enone) cyclobutanation are discussed.

Journal Article↗