Mn-NHC Electrocatalysts: Increasing π Acidity Lowers the Reduction Potential and Increases the Turnover Frequency for CO2 Reduction.

A new manganese(I) N-heterocyclic carbene electrocatalyst containing a benzimidazole-pyrimidine-based ligand is reported for the two-electron conversion of CO2. The increased π acidity of pyrimidine shifts the two-electron reduction to -1.77 V vs Fc/Fc+, 70 mV more positive than that for MnBr(2,2'-bipyridine)(CO)3; increased catalytic current enhancement is also observed (5.2× vs 2.1×). Theoretical analyses suggest that this heightened activity may follow from the preference for a reduction-first dehydroxylation mechanism.

Re(I) NHC Complexes for Electrocatalytic Conversion of CO2.

The modular construction of ligands around an N-heterocyclic carbene building block represents a flexible synthetic strategy for tuning the electronic properties of metal complexes. Herein, methylbenzimidazolium-pyridine and methylbenzimidazolium-pyrimidine proligands are constructed in high yield using recently established transition-metal-free techniques. Subsequent chelation to ReCl(CO)5 furnishes ReCl(N-methyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3 and ReCl(N-methyl-N'-2-pyrimidylbenzimidazol-2-ylidine)(CO)3. These Re(I) NHC complexes are shown to be capable of mediating the two-electron conversion of CO2 following one-electron reduction; the Faradaic efficiency for CO formation is observed to be >60% with minor H2 and HCO2H production. Data from cyclic voltammetry is presented and compared to well-studied ReCl(2,2'-bipyridine)(CO)3 and MnBr(2,2'-bipyridine)(CO)3 systems. Results from density functional theory computations, infrared spectroelectrochemistry, and chemical reductions are also discussed.

Electrocatalytic Reduction of Carbon Dioxide by Mn(CN)(2,2'-bipyridine)(CO)3: CN Coordination Alters Mechanism.

MnBr(2,2'-bipyridine)(CO)3 is an efficient and selective electrocatalyst for the conversion of CO2 to CO. Herein, substitution of the axial bromide for a pseudohalogen (CN) is investigated, yielding Mn(CN)(2,2'-bipyridine)(CO)3. This replacement shifts the first and second reductions to more negative potentials (-1.94 and -2.51 V vs Fc/Fc(+), respectively), but imparts quasi-reversibility at the first feature. The two-electron, two-proton reduction of CO2 to CO and H2O is observed at the potential of the first reduction. Data from IR spectroelectrochemistry, cyclic voltammetry, and controlled potential electrolysis indicate that this behavior arises from the disproportionation of two one-electron-reduced species to generate the catalytically active species. Computations using density functional theory are also presented in support of this new mechanism.

NHC-containing manganese(I) electrocatalysts for the two-electron reduction of CO2.

The synthesis and characterization of the first catalytic manganese N-heterocyclic carbene complexes are reported: MnBr(N-methyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3 and MnBr(N-methyl-N'-2-pyridylimidazol-2-ylidine)(CO)3. Both new species mediate the reduction of CO2 to CO following two-electron reduction of the Mn(I) center, as observed with preparative scale electrolysis and verified with (13)CO2. The two-electron reduction of these species occurs at a single potential, rather than in two sequential steps separated by hundreds of millivolts, as is the case for previously reported MnBr(2,2'-bipyridine)(CO)3. Catalytic current enhancement is observed at voltages similar to MnBr(2,2'-bipyridine)(CO)3.

Diels-Alder reactions of 12-hydroxy-9(10®20)-5aH-abeo-abieta-1(10),8(9),12(13)-triene-11,14-dione.

12-Hydroxy-9(10-->20)-5aH-abeo-abieta-1(10),8(9),12(13)-triene-11,14-dione (quinone 2) served as the dienophile in numerous intermolecular Diels-Alder reactions. These cycloadditions were conducted either thermally (including microwave heating) or with Lewis acid activation. While most dienes reacted with quinone 2 in good chemical yield, others were incompatible under the experimental conditions used.

The taiwaniaquinoids: a review.

A comprehensive overview of the taiwaniaquinoid family of natural products is presented. A summary of the isolation, biosynthesis, and biological activity of these compounds is followed by a discussion of various synthetic strategies to the skeletal framework and a detailed discussion of 12 published syntheses of members of this family. This review covers the literature from the discovery of the first taiwaniaquinoid in 1995 until June 2009.

Total synthesis of (+)-komarovispirone.

(+)-Komaroviquinone photolytically rearranges to (+)-komarovispirone. A rationalization of this isomerization is presented.

Total synthesis of (-)-barbatusol, (+)-demethylsalvicanol, (-)-brussonol, and (+)-grandione.

The icetexane-based diterpenes (-)-barbatusol, (+)-demethylsalvicanol, and (-)-brussonol were synthesized. Synthetic demethylsalvicanol was dimerized to produce (+)-grandione using aqueous Diels-Alder conditions.

Total synthesis of (-)-salviasperanol.

The achiral enynone shown cyclized to produce a tricyclic dienone that was converted in six steps to (-)-salviasperanol.

CO2 reduction with Re(i)-NHC compounds: driving selective catalysis with a silicon nanowire photoelectrode.

The CO2-reduction activity of two Re(i)-NHC complexes is investigated employing a silicon nanowire photoelectrode to drive catalysis. Photovoltages greater than 440 mV are observed along with excellent selectivity towards CO over H2 formation. The observed selectivity towards CO production correlates with strong adsorption of the catalysts on the photoelectrode surface.

[18]Annulene put into a new perspective.

New insights into [18]annulene were gained by looking more closely at its X-ray structure, revealing a close face-to-face stacking of 3.16 Å in a herringbone-like crystal packing. Hexadehydro[18]annulene was co-crystalized in a benzene matrix, demonstrating the stabilizing role of intercalated solvent molecules in solid annulenes.

Poly(ethylene glycol) methacrylate/dimethacrylate hydrogels for controlled release of hydrophobic drugs.

Hydrogels have been successfully used to entrap hydrophilic drugs and release them in a controlled fashion; however, the entrapment and release of hydrophobic drugs has not been well studied. We report on the release characteristics of a model hydrophobic drug, the steroid hormone estradiol, entrapped in low (MW 360/MW 550) and high (MW 526/MW 1000) molecular weight poly(ethylene glycol) methacrylate (PEG-MA)/dimethacrylate (PEG-DMA) hydrogels. The cross-linking ratio, temperature, and pH ranged from 10:1 to 10:3, from 33 to 41 degrees C, and from 2 to 12, respectively. The gelation of the PEG-MA/PEG-DMA hydrogel was initiated with UV irradiation. The absence of poly(glutamic acid) in the hydrogel formulation resulted in a loss of pH sensitivity in the acidic range, which was displayed by the hydrogels' similarities in swelling ratios in the pH buffers of pH 2, 4, and 7. Use of high molecular weight polymers resulted in a higher hydrogel swelling (300%) in comparison to the low molecular weight polymers. Drug size was found to be a significant factor. In comparison to 100% estradiol (MW 272) release, the fractional release of insulin (MW 5733) was 12 and 24% in low and high molecular weight gels at pH 2, respectively, and 17% in low molecular weight gels at pH 7. On the release kinetics of the estradiol drug, the hydrogels displayed a non-Fickian diffusion mechanism, which indicated that the media penetration rate is in the same range as the drug diffusion. The synthesis, entrapment, and release of estradiol by the PEG-MA/PEG-DMA hydrogels proved to be successful, but the use of ethanol in the buffers to promote the hydrophobic release of the estradiol in the in vitro environment caused complications, attributed to the process of transesterification.

Exploring the effect of axial ligand substitution (X = Br, NCS, CN) on the photodecomposition and electrochemical activity of [MnX(N-C)(CO)3] complexes.

The synthesis, electrochemical activity, and relative photodecomposition rate is reported for four new Mn(i) N-heterocyclic carbene complexes: [MnX(N-ethyl-N'-2-pyridylimidazol-2-ylidine)(CO)3] (X = Br, NCS, CN) and [MnCN(N-ethyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3]. All compounds display an electrocatalytic current enhancement under CO2 at the potential of the first reduction, which ranges from -1.53 V to -1.96 V versus the saturated calomel electrode. Catalytic CO production is observed for all species during four-hour preparative-scale electrolysis, but substantial H2 is detected in compounds where X is not Br. All species eventually decompose under both 350 nm and 420 nm light, but cyanide substituted complexes (X = CN) last significantly longer (up to 5×) under 420 nm light as a result of a blue-shifted MLCT band.

A synthesis of (+)-salvadione-A.

[reaction: see text] The p-benzoquinone shown is converted to the novel hexacyclic triterpene salvadione-A in four steps.

Electrophilic Aromatic Bromination Using Bromodimethylsulfonium Bromide Generated in Situ.

It has been shown that bromodimethylsulfonium bromide, generated in situ by treating dimethyl sulfoxide with aqueous hydrobromic acid, is a milder and more selective reagent for electrophilic aromatic bromination than elemental bromine.

Carbodiimide-Promoted Olefin Epoxidation with Aqueous Hydrogen Peroxide(,).

Commercially available carbodiimides in hydroxylic solvents containing hydrogen peroxide with mildly basic or acidic catalysts have been found to promote the epoxidation of olefins. A commercially available 30% aqueous solution of hydrogen peroxide serves as the oxidant for this process. The presumed reactive species is a peroxyisourea generated in situ by the addition of hydrogen peroxide to the carbodiimide.

Use of Conjugated Dienones in Cyclialkylations: The Total Syntheses of (+/-)-Barbatusol, (+/-)-Pisiferin, (+/-)-Deoxofaveline, (+/-)-Xochitlolone, and (+/-)-Faveline.

Concise syntheses of five tricyclic diterpenoids are reported. The key reaction in each synthesis is a cyclialkylation of a functionalized arene with a Lewis acid-activated conjugated dienone to generate a 6,7,6-fused tricycle.

A concise synthesis of (+)-salvadione B.

The novel hexacyclic triterpene salvadione B was synthesized in seven steps from the chiral quinone shown. The insight gained from this synthesis permitted a two-step, one-pot sequence to introduce the three additional rings and seven chiral centers.

Total synthesis of (+)-19-deoxyicetexone, (-)-icetexone, and (+)-5-epi-icetexone.

The first asymmetric total syntheses of 19-deoxyicetexone, icetexone, and 5-epi-icetexone was achieved from epimeric tricyclic dienes.

Determination of capsinoids by HPLC-DAD in capsicum species.

Capsicum fruits contain a newly discovered phytochemical called capsinoids. Because little is known about the quantities of these compounds in both sweet and pungent pepper fruits, a high-performance liquid chromatography (HPLC) method was developed to identify and quantify the capsinoids (naturally present E-capsiate and dihydrocapsiate) utilizing fruit obtained from a variety of Capsicum spp. in the U.S. Department of Agriculture's Capsicum germplasm collection. Capsinoids were extracted with acetonitrile, filtered, and analyzed using an HPLC system equipped with a C(18) monolithic column, gradient pump, and diode array detector. The elution solvents were acetonitrile and water (60:40) with an isocratic flow rate of 1.0 mL/min. Forty-nine samples representing distinct morphotypes of four cultivated species ( C. annuum var. annuum, C. annuum var. glabriusculum, C. baccatum , C. chinense , and C. frutescens ) contained detectable levels (11-369 microg/g) of E-capsiate quantified at a wavelength of 280 nm. Nine of the E-capsiate-containing samples also had dihydrocapsiate (18-86 micro/g). Gas chromatography with a mass spectrometry detector (GC-MS) confirmed the presence of these compounds in the Capsicum spp.