Expansion of Photostable Luminescent Radicals by Meta-Substitution

Polychlorinated pyridyldiphenylmethyl radicals having substituents meta to the position bearing the carbon-centered radical (α-carbon) are synthesized. All of them are stable in ambient conditions in solutions and fluorescent in cyclohexane. The fluorescence of the radicals with bromo, phenyl, 4-chlorophenyl, or 2-pyridyl substituents are enhanced in chloroform, while the emission of the radicals with 2-thienyl or 2-furyl substituents are quenched in chloroform. DFT and TD-DFT calculations indicate that the first doublet excited states of the former are locally excited, while the first doublet excited states of the latter are charge transfer states from the π-electron-donating substituent to the accepting radical. The latter also show much higher photostability under 370-nm light irradiation compared with the first reported photostable fluorescent radical, (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM), with pronounced bathochromic shifts of the fluorescence.     

Asymmetric Michael addition of a recyclable chiral amine: inversion of stereoselectivity caused by the difference of ethereal solvents

The Michael addition of a chiral amine [(-)- 6] to alpha,beta-unsaturated esters ( 4) was attained and the stereoselectivity was inverted by changing the solvent from diethyl ether to tetrahydrofuran when alpha,beta-unsaturated esters having an aromatic ring at the beta-position were employed. In addition, the chiral auxiliary in the Michael adducts ( 9A) was facilely removed with N-iodosuccinimide to afford beta-amino esters ( 10A) and 2-methoxy- d-bornylaldehyde ( 11), which can be reclaimed to the chiral amine ( 6) by reductive amination.     

Nature of water transport and electro-osmosis in nafion: insights from first-principles molecular dynamics simulations under an electric field

The effects of water content on water transport and electro-osmosis in a representative polymer electrolyte membrane, Nafion, are investigated in detail by means of first-principles molecular dynamics (MD) simulations in the presence of a homogeneous electric field. We have directly evaluated electro-osmotic drag coefficients (the number of water molecules cotransported with proton conduction) from the trajectories of the first-principles MD simulations and also explicitly evaluated factors that contribute to the electro-osmotic drag coefficients. In agreement with previously reported experiments, our calculations show virtually constant values ( approximately 1) of the electro-osmotic drag coefficients for both low and high water content states. Detailed comparisons of each factor contributing to the drag coefficient reveal that an increase in water content increases the occurrence of the Grotthuss-like effective proton transport process, whose contribution results in a decrease in the electro-osmotic drag coefficient. At the same time, an environment that is favorable for the Grotthuss-like effective proton transport process is also favorable for the transport of water arising from water transport occurring beyond the hydration shell around the protons, whose contribution results in an increase in the electro-osmotic drag coefficient. Conversely, an environment that is not favorable for proton conduction is also not favorable for water transport. As a result, the electro-osmotic drag coefficient shows virtually identical values with respect to change in the water content.     

Accelerating and decelerating effects of metal ions on electron-transfer reduction of quinones as a function of temperature and binding modes of metal ions to semiquinone radical anions

The accelerating effect of Sc(3+) on the electron-transfer (ET) reduction of the p-benzoquinone derivative 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ) by 10,10'-dimethyl-9,9'-biacridine ((AcrH)(2)) at 233 K changes to a decelerating effect with increasing reaction temperature; the observed second-order rate constant k(et) decreases with increasing Sc(3+) concentration at high concentrations of Sc(3+) at 298 K. At 263 K the k(et) value remains constant with increasing Sc(3+) concentration. Such a remarkable difference with regard to dependence of k(et) on [Sc(3+)] between low and high temperatures results from the difference in relative activity of two ET pathways that depend on temperature, one of which affords 1:1 complex TolSQ*(-)-Sc(3+), and the other 1:2 complex TolSQ*(-)-(Sc(3+))(2) with additional binding of Sc(3+) to TolSQ*(-)-Sc(3+). The formation of TolSQ*(-)-Sc(3+) and TolSQ*(-)-(Sc(3+))(2) complexes was confirmed by EPR spectroscopy in the ET reduction of TolSQ in the presence of low and high concentrations of Sc(3+), respectively. The effects of metal ions on other ET reactions of quinones to afford 1:1 and 1:2 complexes between semiquinone radical anions and metal ions are also reported. The ET pathway affording the 1:2 complexes has smaller activation enthalpies DeltaH( not equal) and more negative activation entropies DeltaS( not equal) because of stronger binding of metal ions and more restricted geometries of the ET transition states as compared with the ET pathway to afford the 1:1 complexes.     

One-step versus stepwise mechanism in protonated amino acid-promoted electron-transfer reduction of a quinone by electron donors and two-electron reduction by a dihydronicotinamide adenine dinucleotide analogue. Interplay between electron transfer and hydrogen bonding

Semiquinone radical anion of 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ(*-)) forms a strong hydrogen bond with protonated histidine (TolSQ(*-)/His x 2 H(+)), which was successfully detected by electron spin resonance. Strong hydrogen bonding between TolSQ(*-) and His x 2 H(+) results in acceleration of electron transfer (ET) from ferrocenes [R2Fc, R = C5H5, C5H4(n-Bu), C5H4Me] to TolSQ, when the one-electron reduction potential of TolSQ is largely shifted to the positive direction in the presence of His x 2 H(+). The rates of His x 2 H(+)-promoted ET from R2Fc to TolSQ exhibit deuterium kinetic isotope effects due to partial dissociation of the N-H bond in His x 2 H(+) at the transition state, when His x 2 H(+) is replaced by the deuterated compound (His x 2 D(+)-d6). The observed deuterium kinetic isotope effect (kH/kD) decreases continuously with increasing the driving force of ET to approach kH/kD = 1.0. On the other hand, His x 2 H(+) also promotes a hydride reduction of TolSQ by an NADH analogue, 9,10-dihydro-10-methylacridine (AcrH2). The hydride reduction proceeds via the one-step hydride-transfer pathway. In such a case, a large deuterium kinetic isotope effect is observed in the rate of the hydride transfer, when AcrH2 is replaced by the dideuterated compound (AcrD2). In sharp contrast to this, no deuterium kinetic isotope effect is observed, when His x 2 H(+) is replaced by His x 2 D(+)-d6. On the other hand, direct protonation of TolSQ and 9,10-phenanthrenequinone (PQ) also results in efficient reductions of TolSQH(+) and PQH(+) by AcrH2, respectively. In this case, however, the hydride-transfer reactions occur via the ET pathway, that is, ET from AcrH2 to TolSQH(+) and PQH(+) occurs in preference to direct hydride transfer from AcrH2 to TolSQH(+) and PQH(+), respectively. The AcrH2(*+) produced by the ET oxidation of AcrH2 by TolSQH(+) and PQH(+) was directly detected by using a stopped-flow technique.     

Crystallization-driven constitutional changes of dynamic polymers in response to neat/solution conditions

Dynamic polymers (dynamers) based on reversible imine interactions were generated and found to respond to changes in neat/solution environment, thus displaying adaptive behavior through modification of their constitution in order to maximize the stability of their mesoscopic state as a function of conditions.     

Determination of vanillin and related flavor compounds in cocoa drink by capillary electrophoresis

In the first part of this study, the stability of five terpenes (alpha-pinene, limonene, camphor, citronellol, and carvacrol) under subcritical water conditions was investigated. The stability studies were carried out at four different temperatures (100, 150, 200, and 250 degrees C) with two different heating times (30 and 300 min). When water temperature was increased, the degradation of terpenes became more serious. Prolonged exposure time to each heating temperature also caused decreased terpene stability. The terpene recoveries were determined by conducting subcritical water extraction of sand spiked with terpenes. The recoveries are typically around 70 to 80% for extractions at 100 degrees C. Terpene recoveries were decreased with increasing water temperature due to poorer stability of terpenes. After the degradation and recovery studies, basil and oregano leaves were extracted using water at both 100 and 150 degrees C. The concentrations of each individual terpene in the water extract generally ranged from trace quantity to 65 microg terpene/g herb. However, the concentration of carvacrol in the oregano-water extract at 150 degrees C was found to be as high as 4270 microg carvacrol/g oregano.     

Iterative addition of carbon nucleophiles to N,N-dialkyl carboxamides for synthesis of α-tertiary amines

A protocol for the synthesis of α-tertiary amines was developed by iterative addition of carbon nucleophiles to N,N-dialkyl carboxamides. Nucleophilic 1,2-addition of organolithium reagents to carboxamides forms anionic tetrahedral carbinolamine (hemiaminal) intermediates, which are subsequently treated with bromotrimethylsilane (Me₃SiBr) followed by organomagnesium (Grignard) reagents, organolithium reagents or tetrabutylammonium cyanide, affording α-tertiary amines. Employment of (trimethylsilyl)methylmagnesium bromide as the 2^nd nucleophile allowed for aza-Peterson olefination of the resulting α-tertiary (trimethylsilyl)methylamines with acidic work-up, resulting in the formation of 1,1-diarylethylenes.

We herein report a concise protocol for iterative addition of carbon nucleophiles to N,N-dialkyl carboxamides for the synthesis of α-tertiary amines.     

The RATIO method for time‐resolved Laue crystallography

A RATIO method for analysis of intensity changes in time‐resolved pump–probe Laue diffraction experiments is described. The method eliminates the need for scaling the data with a wavelength curve representing the spectral distribution of the source and removes the effect of possible anisotropic absorption. It does not require relative scaling of series of frames and removes errors due to all but very short term fluctuations in the synchrotron beam.