Showing papers in "Journal of Organic Chemistry in 2009"
TL;DR: Improved new methods for the efficient reductive conversion of CO(2) to methanol and/or DME that the authors have developed include bireforming with methane and ways of catalytic or electrochemical conversions.
Abstract: Nature’s photosynthesis uses the sun’s energy with chlorophyll in plants as a catalyst to recycle carbon dioxide and water into new plant life. Only given sufficient geological time can new fossil fuels be formed naturally. In contrast, chemical recycling of carbon dioxide from natural and industrial sources as well as varied human activities or even from the air itself to methanol or dimethyl ether (DME) and their varied products can be achieved via its capture and subsequent reductive hydrogenative conversion. The present Perspective reviews this new approach and our research in the field over the last 15 years. Carbon recycling represents a significant aspect of our proposed Methanol Economy. Any available energy source (alternative energies such as solar, wind, geothermal, and atomic energy) can be used for the production of needed hydrogen and chemical conversion of CO2. Improved new methods for the efficient reductive conversion of CO2 to methanol and/or DME that we have developed include bireformin...
1,236 citations
TL;DR: Conditions for the palladium-catalyzed direct arylation of a wide range of heterocycles with aryl bromides are reported and employ a stoichiometric ratio of both coupling partners, as well as a substoichiometric quantity of pivalic acid, which results in significantly faster reactions.
Abstract: Conditions for the palladium-catalyzed direct arylation of a wide range of heterocycles with aryl bromides are reported. Those conditions employ a stoichiometric ratio of both coupling partners, as well as a substoichiometric quantity of pivalic acid, which results in significantly faster reactions. An evaluation of the influence of the nature of the aryl halide has also been carried out.
516 citations
TL;DR: CuO nanoparticles have been studied for C-N, C-O, and C-S bond formations via cross-coupling reactions of nitrogen, oxygen, and sulfur nucleophiles with aryl halides in the presence of a base such as KOH, Cs(2)CO(3), and K(2]CO( 3) at moderate temperature.
Abstract: CuO nanoparticles have been studied for C−N, C−O, and C−S bond formations via cross-coupling reactions of nitrogen, oxygen, and sulfur nucleophiles with aryl halides. Amides, amines, imidazoles, phenols, alcohols and thiols undergo reactions with aryl iodides in the presence of a base such as KOH, Cs2CO3, and K2CO3 at moderate temperature. The procedure is simple, general, ligand-free, and efficient to afford the cross-coupled products in high yield.
333 citations
TL;DR: It is a heterogeneous process and the copper(II) oxide nanoparticles can be recovered and recycled without loss of activity.
Abstract: The synthesis of substituted benzimidazoles, 2-aminobenzimidazoles, 2-aminobenzothiazoles, and benzoxazoles is described via intramolecular cyclization of o-bromoaryl derivatives using copper(II) oxide nanoparticles in DMSO under air. The procedure is experimentally simple, general, efficient, and free from addition of external chelating ligands. It is a heterogeneous process and the copper(II) oxide nanoparticles can be recovered and recycled without loss of activity.
311 citations
TL;DR: The domination of SPLET over HAT/PCET in case of a flavonoid reacting with electron-deficient radicals such as peroxyls or dpph(*) in polar solvents explains the enhancement of antioxidant activity of 3-hydroxyflavone.
Abstract: The reactions of 10 flavonoids with 2,2-diphenyl-1-picrylhydrazyl radical (dpph(*)) carried out in alcohols always occur significantly faster than in acidified alcohols or in dioxane. These fast kinetics benefit from the contribution of the electron transfer from a flavonoid anion to a radical, a mechanism known as Sequential Proton-Loss Electron-Transfer (SPLET), which adds to the kinetics of single-step Hydrogen Atom Transfer (HAT)/Proton Coupled Electron Transfer (PCET) processes (see Acc. Chem. Res. 2007, 40 , 222.). The domination of SPLET over HAT/PCET in case of a flavonoid reacting with electron-deficient radicals such as peroxyls or dpph(*) in polar solvents explains the enhancement of antioxidant activity of 3-hydroxyflavone. It also elucidates the great acceleration in the reactions of dpph(*) with quercetin, morin, galangin, and 7,8-dihydroxyflavone. The analysis of structure-acidity and structure-activity relationships for 10 flavonoids clearly indicates that hydroxyl group at position 7 is the most acidic site. Thus, in polar solvents this group can participate in radical reaction via SPLET. In nonpolar solvents the most active site in quercetin (a flavonoid antioxidant commonly found in plants) is 3',4'-dihydroxyl moiety and HAT/PCET occurs. However, in ionization-supporting solvents an anion formed at position 7 is responsible for very fast kinetics of quercetin/dpph(*) reaction because both mechanisms participate: HAT (from catechol moiety in ring B) and SPLET (from ionized 7-hydroxyl in ring A). Because of conjugation of rings A, B, and C the final structure of the formed quercetin radical (or quercetin anion radical) is the same for the SPLET and HAT/PCET mechanisms.
290 citations
TL;DR: A series of salicylaldehyde azine derivatives were found to exhibit interesting aggregation-induced emission enhancement (AIEE) characteristics, and their in situ formation also promises potential applications in fluorescence sensing of hydrazine.
Abstract: A series of salicylaldehyde azine derivatives were found to exhibit interesting aggregation-induced emission enhancement (AIEE) characteristics. In good solvent, all these compounds displayed very weak fluorescence, while strong emission was observed when they were placed in poor solvent. Moreover, the AIEE color of these compounds varied from green to red depending on the substituents on azines. Their in situ formation also promises potential applications in fluorescence sensing of hydrazine.
283 citations
TL;DR: The rhodamine-based sensor 1 exhibits a high selectivity and an excellent sensitivity and is a dual-responsive colorimetric and fluorescent Hg(2+)-specific sensor in aqueous buffer solution.
Abstract: A rhodamine-based sensor 1 was designed and synthesized by incorporation the rhodamine fluorophore and ionophore NS2 with high affinity to Hg2+. Sensor 1 exhibits a high selectivity and an excellent sensitivity and is a dual-responsive colorimetric and fluorescent Hg2+-specific sensor in aqueous buffer solution. In addition, the 1:1 binding mode was proposed based on the 1H NMR and ES(+)MS studies.
282 citations
TL;DR: It is demonstrated, on a test set of 80 conformationally stable molecules of various kinds carrying different functional groups, that, in contrast to what is claimed in the literature, large basis sets are not needed to obtain rather accurate predictions of (1)H NMR chemical shifts by quantum chemical calculations.
Abstract: The purpose of this paper is to convince practitioners of 1H NMR spectroscopy to consider simple quantum chemical calculations as a viable option to aid them in the assignment of their spectra. To this end, it is demonstrated, on a test set of 80 conformationally stable molecules of various kinds carrying different functional groups, that, in contrast to what is claimed in the literature, large basis sets are not needed to obtain rather accurate predictions of 1H NMR chemical shifts by quantum chemical calculations. On the other hand, modeling the solvent by an SCRF-type calculation may improve certain predictions significantly. The best accuracy/cost ratio is provided by GIAO calculations in chloroform as a solvent with the specially parametrized WP04 functional of Cramer et al. using the cc-pVDZ or 6-31G** basis set, closely followed by similar calculations with the ubiquitious B3LYP functional (both predict 1H chemical shifts with an average deviation of ca. 0.12 ppm, if the results are scaled linearly...
251 citations
TL;DR: The results demonstrated that the fluorescence OFF-ON switching of this kind of thiol probe is due to the termination of the ICT effect, which quenches the emission, by a dark S(1) state) by cleavage of the 2,4-dinitrobenzenesulfonyl unit (as acceptor of I CT effect) with thiols, not the re-establishment of the D-pi-A feature of the fluorophore.
Abstract: Green and yellow-emitting 1,6- and 1,8-bis(phenylethynyl) pyrenes (dyes 7, 8, 9, and 10) with different intramolecular charge transfer (ICT) feature were synthesized and the effect of ICT on the photophysical properties of these derivatives were studied by UV−vis absorption spectra, fluorescence emission spectra, and DFT/TDDFT calculations. For the dyes with electron-pushing group (e.g., -dimethylamino, dye 8 and dye 10), structureless and solvent polarity-sensitive fluorescence emission spectra were observed. Conversely, dye with electron-withdrawing group (e.g., −CN, dye 7) shows structured and solvent polarity-independent emission spectra. OFF−ON fluorescent thiol probes 11 and 12 with 2,4-dinitrobenzenesulfonyl protected ethynylpyrene fluorophore were designed based on DFT/TDDFT calculations, which predicts dark state (S1) for these thiol probes (e.g., oscillator strength f = 0.0086 for S1←S0 transition of the probe 11). This dark state is induced by the ICT effect with ethynylated pyrene fluorophore ...
228 citations
TL;DR: This method represents an efficient and facile installation of heterocyclic building blocks onto preexisting organic substructures and obtained in good to excellent yields.
Abstract: A wide variety of bench-stable potassium heteroaryltrifluoroborates were prepared, and general reaction conditions were developed for their cross-coupling to aryl and heteroaryl halides. The cross-coupled products were obtained in good to excellent yields. This method represents an efficient and facile installation of heterocyclic building blocks onto preexisting organic substructures.
217 citations
TL;DR: The selective 2-mono- and 2,6-divinylations of (N-containing heteroaryl)benzenes can be achieved effectively through rhodium-catalyzed oxidative coupling reactions with alkenes through a series of 1,3- divinylbenzene derivatives, some of which exhibit solid-state fluorescence.
Abstract: The selective 2-mono- and 2,6-divinylations of (N-containing heteroaryl)benzenes can be achieved effectively through rhodium-catalyzed oxidative coupling reactions with alkenes. The installation of two different vinyl groups is also possible by a simple one-pot manner. Thus, a series of 1,3-divinylbenzene derivatives, some of which exhibit solid-state fluorescence, is readily prepared.
TL;DR: A new parameter, CP3, is introduced, based on comparing differences in calculated shift with differences in experimental shift, which is significantly more successful at making correct structure assignments with high confidence than are the currently used parameters of the mean absolute error and the correlation coefficient.
Abstract: GIAO NMR chemical shifts have been calculated for a set of 28 pairs of diastereoisomers in order to test the ability of NMR shift calculation to distinguish between diastereomeric structures. We compare the performance of several different parameters for quantifying the agreement between calculated and experimental shifts from the point of view of assigning structures and introduce a new parameter, CP3, based on comparing differences in calculated shift with differences in experimental shift, which is significantly more successful at making correct structure assignments with high confidence than are the currently used parameters of the mean absolute error and the correlation coefficient. Using our new parameter in conjunction with Bayes’ theorem, stereostructure assignments can be made with quantifiable confidence using shifts obtained in single point calculations on molecular mechanics geometries without computationally expensive ab initio geometry optimization.
TL;DR: A series of 1,3-disubstituted-2-imidazolium carboxylates, an adduct of CO(2) and N-heterocyclic carbenes, were synthesized and characterized using single crystal X-ray, thermogravimetric, IR, and NMR analysis.
Abstract: A series of 1,3-disubstituted-2-imidazolium carboxylates, an adduct of CO2 and N-heterocyclic carbenes, were synthesized and characterized using single crystal X-ray, thermogravimetric, IR, and NMR analysis. The TGA analysis of the NHC-CO2’s shows that as steric bulk on the N-substituent increases, the ability of the NHC-CO2 to decarboxylate increases. The comparison of NHC-CO2’s with and without methyls at the 4,5-position indicate that extra electron density in the imidazolium ring enhances the stability of an NHC-CO2 thereby making it less prone to decarboxylation. Single crystal X-ray analysis shows that the torsional angle of the carboxylate group and the C−CO2 bond length with respect to the imidazolium ring is dependent on the steric bulk of the N-substituent. Rotamers in the unit cell of a single crystal of ItBuPrCO2 (2f) indicate that the C−CO2 bond length increases as the N-substituents rotate toward the carboxylate moiety, which suggests that rotation of the N-substituents through the plane of ...
TL;DR: The described catalytic system should provide an efficient alternative to mercury-based methodologies and be useful in synthetic programs and a plausible mechanism for the hydration of alkynes through ketal intermediates is proposed on the basis of kinetic studies.
Abstract: Hydration of a wide range of alkynes to the corresponding ketones has been afforded in high yields at room temperature by using gold(I)-phosphine complexes as catalyst, with no acidic cocatalysts required. Suitable substrates covering alkyl and aryl terminal alkynes, enynes, internal alkynes, and propargylic alcohols, including enantiopure forms, are cleanly transformed to the corresponding ketones in nearly quantitative yields. Acid-labile groups present in the substrates are preserved. The catalytic activity strongly depends on both the nature of the phosphine coordinated to the gold (I) center and the softness of the counteranion, the complex AuSPhosNTf(2) showing the better activity. A plausible mechanism for the hydration of alkynes through ketal intermediates is proposed on the basis of kinetic studies. The described catalytic system should provide an efficient alternative to mercury-based methodologies and be useful in synthetic programs.
TL;DR: The straightforward and efficient synthesis of alpha-pyrone and butenolide derivatives has been achieved by the rhodium-catalyzed oxidative coupling reactions of substituted acrylic acids with alkynes and alkenes, respectively.
Abstract: The straightforward and efficient synthesis of α-pyrone and butenolide derivatives has been achieved by the rhodium-catalyzed oxidative coupling reactions of substituted acrylic acids with alkynes and alkenes, respectively. Some α-pyrones obtained exhibit solid-state fluorescence.
TL;DR: It is found that methanol and benzene are excellent reference standards for computing NMR shifts of sp(3)- and sp-sp(2)-hybridized carbon atoms, respectively, and this multi-standard approach (MSTD) performs better than TMS in terms of accuracy and precision and also displays much lower dependence on the level of theory employed.
Abstract: The influence of the reference standard employed in the calculation of 13C NMR chemical shifts was investigated over a large variety of known organic compounds, using different quantum chemistry methods and basis sets. After detailed analysis of the collected data, we found that methanol and benzene are excellent reference standards for computing NMR shifts of sp3- and sp−sp2-hybridized carbon atoms, respectively. This multi-standard approach (MSTD) performs better than TMS in terms of accuracy and precision and also displays much lower dependence on the level of theory employed. The use of mPW1PW91/6-31G(d)//mPW1PW91/6-31G(d) level is recommended for accurate 13C NMR chemical shift prediction at low computational cost.
TL;DR: A PhI(OAc)(2) mediated selective functionalization of sp(3) C-H bonds adjacent to a nitrogen atom has been reported and alpha-amino nitriles can be obtained by oxidative coupling of amines with malononitrile.
Abstract: A PhI(OAc)(2) mediated selective functionalization of sp(3) C-H bonds adjacent to a nitrogen atom has been reported. When piperidine derivates were used, direct diacetoxylation of alpha and beta sp(3) C-H adjacent to a nitrogen atom were observed to afford various cis-2,3-diacetoxylated piperidines. On the other hand, tetrahydroisoquinoline derivatives gave various alpha-C-H functionalized products in the presence of PhI(OAc)(2). Nitroalkanes, dialkyl malonates, and beta-keto ester are active participants in this coupling reaction. Meanwhile, alpha-amino nitriles can also be obtained by oxidative coupling of amines with malononitrile.
TL;DR: An array of carbazoles (23 examples) can be synthesized from substituted biaryl azides at 60 degrees C using substoichiometric quantities of Rh(2)(O(2)CC(3)F(7))(4).
Abstract: An array of carbazoles (23 examples) can be synthesized from substituted biaryl azides at 60 °C using substoichiometric quantities of Rh2(O2CC3F7)4 or Rh2(O2CC7H15)4.
TL;DR: Comparative investigations of the analogous bicyclic guanidine 1,4,6-triazabicyclo[3.3.0]oct-4-ene reveal it to be a much less active acylation catalyst than TBD, implying that the higher reactivity of TBD is a consequence of both its higher basicity and nucleophilicity than TBO.
Abstract: The bicyclic guanidine 1,5,7- triazabicyclo[4.4.0]dec-5-ene (TBD) is an effective organocatalyst for the formation of amides from esters and primary amines. Mechanistic and kinetic investigations support a nucleophilic mechanism where TBD reacts reversibly with esters to generate an acyl-TBD intermediate that acylates amines to generate the amides. Comparative investigations of the analogous bicyclic guanidine 1,4,6-triazabicyclo[3.3.0]oct-4-ene (TBO) reveal it to be a much less active acylation catalyst than TBD. Theoretical and mechanistic studies imply that the higher reactivity of TBD is a consequence of both its higher basicity and nucleophilicity than TBO as well as the high reactivity of the acyl-TBD intermediate, which is sterically prevented from adopting a planar amide structure.
TL;DR: The present coupling procedure is applicable to the synthesis of other various heteroarenes possessing di-, tri-, and tetracyclic cores and some of the products exhibit intense fluorescence in the solid state.
Abstract: The selective synthesis of 1,2,3,4-tetrasubstituted carbazoles can be performed effectively through the palladium-catalyzed oxidative coupling reactions of N-substituted indoles or their carboxylic acid derivatives with alkynes. Unsymmetrically octasubstituted carbazoles can also be obtained by the stepwise couplings of 1-methylpyrrole-2-carboxylic acid with two different alkynes. In addition, the present coupling procedure is applicable to the synthesis of other various heteroarenes possessing di-, tri-, and tetracyclic cores. Some of the products exhibit intense fluorescence in the solid state.
TL;DR: Imines and secondary amines were synthesized selectively by controlling reaction conditions for the Pd-catalyzed one-pot reactions of benzyl alcohols with primary amines.
Abstract: Imines and secondary amines were synthesized selectively by controlling reaction conditions for the Pd-catalyzed one-pot reactions of benzyl alcohols with primary amines. The reactions did not require any additives and were effective for a wide range of alcohols and amines.
TL;DR: The results indicate that the nucleophilicity of the competing functional groups, polarization of the alkyne triple bond, and the cationic nature of the intermediate are the most important factors in determining the outcome of these reactions.
Abstract: The relative reactivity of various functional groups toward alkyne electrophilic cyclization reactions has been studied. The required diarylalkynes have been prepared by consecutive Sonogashira reactions of appropriately substituted aryl halides and competitive cyclizations have been performed using I2, ICl, NBS and PhSeCl as electrophiles. The results indicate that the nucleophilicity of the competing functional groups, polarization of the alkyne triple bond, and the cationic nature of the intermediate are the most important factors in determining the outcome of these reactions.
TL;DR: Indium-catalyzed C-S cross-coupling of aromatic and alkane thiols with aryl halides proceeds smoothly in the presence of In(OTf)(3) (10 mol %), TMEDA, and KOH as a base in DMSO at 135 degrees C.
Abstract: Indium-catalyzed C−S cross-coupling of aromatic and alkane thiols with aryl halides proceeds smoothly in the presence of In(OTf)3 (10 mol %), TMEDA (20 mol %), and KOH as a base in DMSO at 135 °C. When this protocol was utilized, a variety of thiols could be cross-coupled with aryl halides to afford the corresponding aryl sulfides in good to excellent yields.
TL;DR: The enantioselective hydrogenation of 2-benzylquinolines and 2-functionalized and 2,3-disubstituted quinolines was developed by using the [Ir(COD)Cl](2)/bisphosphine/I(2) system with up to 96% ee.
Abstract: The enantioselective hydrogenation of 2-benzylquinolines and 2-functionalized and 2,3-disubstituted quinolines was developed by using the [Ir(COD)Cl]2/bisphosphine/I2 system with up to 96% ee. Moreover, mechanistic studies revealed the hydrogenation mechanism of quinoline involves a 1,4-hydride addition, isomerization, and 1,2-hydride addition, and the catalytic active species may be a Ir(III) complex with chloride and iodide.
TL;DR: The direct oxidative coupling of 2-amino- and 2-hydroxybenzoic acids with internal alkynes proceeds efficiently in the presence of a rhodium/copper catalyst system under air to afford the corresponding 8-substituted isocoumarin derivatives, some of which exhibit solid-state fluorescence.
Abstract: The direct oxidative coupling of 2-amino- and 2-hydroxybenzoic acids with internal alkynes proceeds efficiently in the presence of a rhodium/copper catalyst system under air to afford the corresponding 8-substituted isocoumarin derivatives, some of which exhibit solid-state fluorescence. Depending on conditions, 4-ethenylcarbazoles can be synthesized selectively from 2-(arylamino)benzoic acids. The oxidative coupling reactions of heteroarene carboxylic acids as well as aromatic diacids with an alkyne are also described.
TL;DR: Cyclic aminals were prepared through a Brønsted acid-promoted reaction that involves iminium ion formation, 1,5 H-transfer, followed by ring closure.
Abstract: Cyclic aminals were prepared through a Bronsted acid-promoted reaction. This redox neutral process involves iminium ion formation, 1,5 H-transfer, followed by ring closure.
TL;DR: 2-Octynoic acid and phenylpropiolic acid were employed for the palladium-catalyzed decarboxylative coupling reaction and with a variety of aryl halides and showed high reactivities and tolerance of functional groups such as vinyl, ester, ether, ketone, and amine.
Abstract: 2-Octynoic acid and phenylpropiolic acid were employed for the palladium-catalyzed decarboxylative coupling reaction and with a variety of aryl halides. The former needed 1,4-bis(diphenylphosphino)butane (dppb) as a ligand and the latter tri-tert-butylphosphine (P(t)Bu(3)), and both required 2 equiv of tetra-n-butylammonium fluoride (TBAF) for full conversion. These reactions showed high reactivities and tolerance of functional groups such as vinyl, ester, ether, ketone, and amine.
TL;DR: An efficient method for the synthesis of functionalized benzoxazoles is described that involves a copper(II)-catalyzed regioselective C-H functionalization/C-O bond formation protocol and the presence of a directing group at the meta position markedly improves the reaction efficacy.
Abstract: An efficient method for the synthesis of functionalized benzoxazoles is described that involves a copper(II)-catalyzed regioselective C−H functionalization/C−O bond formation protocol. The use of dichlorobenzene as a solvent at 160 °C allows the use of air as the terminal oxidant in the catalytic synthesis of benzoxazoles in a process that has high functional group tolerance. The presence of a directing group at the meta position markedly improves the reaction efficacy and a variety of 7-substituted benzoxazoles are selectively produced under mild reaction conditions. The mechanism of the reaction is also discussed in this report.
TL;DR: Operationally simple Au(I)-catalyzed intramolecular hydroarylation reactions of terminal alkynes that proceed in high yield and under very mild conditions are described.
Abstract: Operationally simple Au(I)-catalyzed intramolecular hydroarylation (IMHA) reactions of terminal alkynes that proceed in high yield and under very mild conditions are described. These processes involve low catalyst loadings, mild reaction temperatures, and short reaction times, require no cocatalysts or additives, and allow for the generation of a number of important heterocyclic motifs from readily accessible starting materials.
TL;DR: It is found that the oxygen-carbon bond dissociation enthalpy is substantially lowered by oxygen substituents situated at the phenyl ring adjacent to the ether oxygen, which could be a competitive initial reaction under high-temperature pyrolysis conditions.
Abstract: Lignin is an abundant natural resource that is a potential source of valuable chemicals. Improved understanding of the pyrolysis of lignin occurs through the study of model compounds for which phenethyl phenyl ether (PhCH2CH2OPh, PPE) is the simplest example representing the dominant β-O-4 ether linkage. The initial step in the thermal decomposition of PPE is the homolytic cleavage of the oxygen−carbon bond. The rate of this key step will depend on the bond dissociation enthalpy, which in turn will depend on the nature and location of relevant substituents. We used modern density functional methods to calculate the oxygen−carbon bond dissociation enthalpies for PPE and several oxygen-substituted derivatives. Since carbon−carbon bond cleavage in PPE could be a competitive initial reaction under high-temperature pyrolysis conditions, we also calculated substituent effects on these bond dissociation enthalpies. We found that the oxygen−carbon bond dissociation enthalpy is substantially lowered by oxygen subs...