Formal total synthesis of triptolide

A new approach to the medicinally-important natural product triptolide is significantly shorter than previous syntheses, highly convergent and avoids the use of protecting groups; key features include two Diels-Alder reactions and a new deoxygenative aromatisation process.     

Rhenium(I) compounds bound by tripodal ligands of pyridine and N-methylimidazole

The reaction of Re(CO)(5)Br with tris(2-pyridyl)methanol (tpmOH) leads to unexpectedly complex chemistry with three new compounds forming instead of a single product. In compound 1, the tpmOH ligand binds to the metal in the N,N',N'-mode; 2 has tpmO(-) bound in the N,N',O-mode; while 3 is a dimer with the tpmO(-) ligand utilizing each of the four donor atoms to bridge the two metal centers. The analogous methyl ether ligands, tris(2-pyridyl)methoxymethane (tpmOMe) or tris[2-(l-methylimidazolyl)]methoxymethane (timmOMe), each yielded a single product, 4 and 5, respectively, bound in the N,N',N'-mode, and are new leads for potential radiotherapeutic agents. All compounds have been structurally characterized.     

Ultrafast photoinduced charge separation in naphthalene diimide based multichromophoric systems in liquid solutions and in a lipid membrane

The photophysical properties of multichromophoric systems consisting of eight red or blue naphthalene diimides (NDIs) covalently attached to a p-octiphenyl scaffold, as well as a blue bichromophoric system with a biphenyl scaffold, have been investigated in detail using femtosecond time-resolved spectroscopy. The blue octachromophoric systems have been recently shown to self-assemble as supramolecular tetramers in lipid bilayer membranes and to enable generation of a transmembrane proton gradient upon photoexcitation ( Bhosale, S. ; Sisson, A. L. ; Talukdar, P. ; Fürstenberg, A. ; Banerji, N. ; Vauthey, E. ; Bollot, G. ; Mareda, J. ; R?ger, C. ; Würthner, F. ; Sakai, N. ; Matile, S. Science 2006, 313, 84 ). A strong reduction of the fluorescence quantum yield was observed when going from the single NDI units to the multichromophoric systems in methanol, the effect being even stronger in a vesicular lipid membrane. Fluorescence up-conversion measurements reveal ultrafast self-quenching in the multichromophoric systems, whereas the formation of the NDI radical anion, evidenced by transient absorption measurements, points to the occurrence of photoinduced charge separation. The location of the positive charge could not be established unambiguously from the transient absorption measurements, but energetic considerations indicate that charge separation should occur between two NDI units in the blue systems, whereas both an NDI unit and the p-octiphenyl scaffold could act as electron donor in the red system. The lifetime of the charge-separated state was found to increase from 22 to 45 ps by going from the bi- to the octachromophoric blue systems in methanol, while a 400 ps decay component was observed in the lipid membrane. This lifetime lengthening is explained in terms of charge migration that is most efficient when the octachromophoric systems are assembled as supramolecular tetramers in the lipid membrane. Furthermore, the average charge-separated state lifetime of the red system in methanol is even larger and amounts to 750 ps. This effect cannot be simply explained in terms of Marcus inverted regime as the driving force for charge recombination in the red system is only slightly larger than in the blue one. A better spatial separation of the charges in the red system stemming from the localization of the hole on the p-octiphenyl scaffold could additionally contribute to the slowing down of charge recombination.     

HRMS directly from TLC slides. A powerful tool for rapid analysis of organic mixtures

High-resolution mass spectra (HRMS) of individual spots on thin-layer chromatography (TLC) slides can now be obtained quickly and easily at atmospheric pressure, with zero sample preparation, using commercially available instrumentation. The method is complementary to GC-mass spectrometry but is not limited to compounds of high volatility and high thermal stability. TLC-HRMS can be used to monitor chemical reactions in real time and has the capacity thereby to accelerate significantly the pace of synthetic organic chemistry.     

Gas chromatography retention data of environmentally relevant polybrominated compounds

Polybrominated organic compounds are ubiquitous throughout the environment. This generic term comprises several classes of brominated flame retardants (e.g., polybrominated diphenyl ethers, polybrominated biphenyls, hexabromocyclododecane, dibromopropyltribromophenyl ether, 1,2-bis(2,4,6-tribromophenoxy)ethane) as well as a range of marine halogenated natural products (HNPs). Here we present gas chromatography retention times and elution orders (on DB-5) of 122 polybrominated compounds that may be found in food and environmental samples. Organobromine compounds in fish samples determined with gas chromatography interfaced to electron-capture negative ion mass spectrometry (GC/ECNI-MS) are discussed. The environmental relevance and important mass spectrometric features of the compounds are described as well. Our database aims to support the closer inspection and identification of peaks in gas chromatograms and to initiate dedicated screening for less frequently studied organobromines in samples.     

High power and high efficiency green light emitting diode on free‐standing semipolar (11$ \bar 2 $2) bulk GaN substrate

We demonstrate a high power green InGaN/GaN multiple‐quantum‐well (MQW) light emitting diode (LED) with a peak emission wavelength of 516 nm grown on low extended defect density semipolar (11 2) bulk GaN substrate by metal organic chemical vapor deposition. The output power and external quantum efficiency (EQE) at drive currents of 20 and 100 mA under direct current (DC) operation were 5.0 mW, 10.5% and 15.6 mW, 6.3%, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fabrication of Silicon Carbide Nanocrystals by Electrical Discharge and Laser-Induced Processes in Solution

Plasma Chemistry and Plasma Processing - The capabilities of the liquid assisted electrical discharge technique with additional laser irradiation of colloids for the synthesis of SiC nanocrystals...     

Di‐ and trivalent rare earth complexes stabilized by sterically demanding aminopyridinato ligands as initiators in ring‐opening polymerization reactions of ϵ‐caprolactone

Salt elimination protocols using Ap*K {Ap*H = (2,6‐diisopropyl‐phenyl)‐[6‐(2,4,6‐triisopropyl‐phenyl)‐pyridin‐2‐yl]‐amine} lead to the rare earth aminopyridinato complex [Ap*LuCl₂(thf)₂], 4 . Results of X‐ray crystal structure analyses of 4 and the corresponding single THF coordinated dimer are discussed. Ring‐opening polymerization of ε‐caprolactone initiated by complexes [Ap*LaBr₂(thf)₃], 2 , [Ap*YbI(thf)₂]₂, 3 or 4 in the presence of NaBH₄ allows the preparation, in a short reaction time, of α,ω‐dihydroxytelechelic polymers with high molar mass ( up to 50,000) and moderate molar mass distributions (1.3 < / < 1.6). © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3611–3619, 2007     

Time and frequency resolved dynamics of ArBr2

We report the first spectroscopic observation of and vibrational predissociation dynamics for ArBr(2). Measurements are reported for the linear and T-shaped isomers with time and frequency resolution near the Fourier limit of a 15 ps pulse. For the T-shaped isomer, the Ar-Br(2) bond energy D(0) for the B state, nu(')=19, is found to be 200 cm(-1), yielding a D(0)(X) value of 213 cm(-1). Product appearance rates are determined for nu(')=16-25 of the B state of ArBr(2). While the rate generally increases with increasing vibrational quantum number, the trend is not monotonic. Also, obtaining reproducible rates for any given vibrational level requires very careful control of the experimental conditions. The data suggest that ArBr(2) undergoes vibrational predissociation (VP) in the sparse intramolecular vibrational relaxation regime. These observations are consistent with theoretical calculations that show that T-shaped ArBr(2) undergoes VP in the sparse regime, such that lifetimes are strongly dependent on both the vibrational and rotational quantum numbers. As for ArI(2), a linear isomer of ArBr(2) is found to contribute a quasicontinuous background to the excitation spectra. Direct excitation of this continuum results in a very broad product vibrational distribution.