Evidence for the role of tetramethylethylenediamine in aqueous Negishi cross-coupling: synthesis of nonproteinogenic phenylalanine derivatives on water

The structure of the alkylzinc-tetramethylethyl-enediamine (TMEDA) cluster cation 3 has been determined in the gas phase by a combination of tandem mass spectrometry, infrared multiphoton dissociation (IRMPD) spectroscopy, and DFT calculations. Both sets of experimental results establish the existence of a strongly stabilizing interaction of TMEDA with the zinc cation. High-level DFT calculations on the alkylzinc-TMEDA cluster cation 3 allowed the identification of two low energy conformers, each featuring a four-coordinate zinc atom with a bidentate TMEDA ligand, and internal coordination from the carbonyl group of the Boc group to zinc. The experimental IRMPD spectrum is reproduced with an appropriately weighted combination of the IR spectra of the two conformers identified by theory. DFT calculations on the structure of the alkylzinc halide 2 with coordinated TMEDA using the PCM model of water solvent suggest that TMEDA can promote ionization of the zinc-iodine bond in organozinc iodides under aqueous conditions, providing a credible explanation for the role of TMEDA in stabilizing the carbon-zinc bond. Reaction of the serine-derived iodide 1 with aryl iodides "on water", promoted by nano zinc in the presence of PdCl(2)(Amphos)(2) (5 mol %) and TMEDA, leads to the formation of protected phenylalanine derivatives 4 in reasonable yields. In the case of ortho-substituted aryl iodides and aryl iodides that are solids at room temperature, conducting the reaction at 65 °C gives improved results. In all cases, the product 5 of reductive dimerization of the iodide 1 is also isolated.     

Temoporfin (Foscan®, 5,10,15,20-tetra(m-hydroxyphenyl)chlorin)--a second-generation photosensitizer

This review traces the development and study of the second-generation photosensitizer 5,10,15,20-tetra(m-hydroxyphenyl)chlorin through to its acceptance and clinical use in modern photodynamic (cancer) therapy. The literature has been covered up to early 2011.     

Acoustic resonators for far-field control of sound on a subwavelength scale

We prove experimentally that broadband sounds can be controlled and focused at will on a subwavelength scale by using acoustic resonators. We demonstrate our approach in the audible range with soda cans, that is, Helmholtz resonators, and commercial computer speakers. We show that diffraction-limited sound fields convert efficiently into subdiffraction modes in the collection of cans that can be controlled coherently in order to obtain focal spots as thin as 1/25 of a wavelength in air. We establish that subwavelength acoustic pressure spots are responsible for a strong enhancement of the acoustic displacement at focus, which permits us to conclude with a visual experiment exemplifying the interest of our concept for subwavelength sensors and actuators.     

Dehydrogenation of dodecahydro-N-ethylcarbazole on Pd/Al2O3 model catalysts

To elucidate the dehydrogenation mechanism of dodecahydro-N-ethylcarbazole (H(12)-NEC) on supported Pd catalysts, we have performed a model study under ultra high vacuum (UHV) conditions. H(12)-NEC and its final dehydrogenation product, N-ethylcarbazole (NEC), were deposited by physical vapor deposition (PVD) at temperatures between 120 K and 520 K onto a supported model catalyst, which consisted of Pd nanoparticles grown on a well-ordered alumina film on NiAl(110). Adsorption and thermally induced surface reactions were followed by infrared reflection absorption spectroscopy (IRAS) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) in combination with density functional theory (DFT) calculations. It was shown that, at 120 K, H(12)-NEC adsorbs molecularly both on the Al(2)O(3)/NiAl(110) support and on the Pd particles. Initial activation of the molecule occurs through C-H bond scission at the 8a- and 9a-positions of the carbazole skeleton at temperatures above 170 K. Dehydrogenation successively proceeds with increasing temperature. Around 350 K, breakage of one C-N bond occurs accompanied by further dehydrogenation of the carbon skeleton. The decomposition intermediates reside on the surface up to 500 K. At higher temperatures, further decay to small fragments and atomic species is observed. These species block most of the absorption sites on the Pd particles, but can be oxidatively removed by heating in oxygen at 600 K, fully restoring the original adsorption properties of the model catalyst.     

Bridging the gap between porphyrins and porphycenes: substituent-position-sensitive tautomerism and photophysics in meso-diphenyloctaethylporphyrins

2,3,7,8,12,13,17,18‐Octaethyl‐5,15‐diphenylporphyrin ( 1 ) is characterized by an inner cavity with a rectangular shape and small NH???N distances. It resembles porphycene, which is a constitutional isomer of porphyrin known for its strong intramolecular hydrogen bonds and rapid tautomerization. Such distortion of the porphyrin cavity leads to tautomeric properties of 1 that are intermediate between those of porphyrin and porphycene. In particular, a tautomerization in the lowest excited singlet state of 1 has been discovered, occurring with a rate three orders of magnitude lower than that in porphycene, but three to four orders of magnitude higher than that in porphyrin. An isomer of 1 , 2,3,7,8,12,13,17,18‐octaethyl‐5,10‐diphenylporphyrin ( 2 ), exhibits a different kind of geometry distortion. This molecule is nonplanar, but the inner cavity shape and dimensions are similar to those of the parent porphyrin. The same hydrogen‐bonding strength as that in porphyrin is observed for 2 . In contrast, the nonplanarity of 2 significantly influences the photophysics, leading to a decrease in fluorescence quantum yield and lifetime. Absorption, magnetic circular dichroism, and fluorescence spectra are similar for 1 and 2 and resemble those of parent porphyrin. This is a consequence of comparable energy splittings of the frontier orbitals, ΔHOMO≈ΔLUMO. The results demonstrate that judicious selection of substituents and their position enables a controlled modification of geometry, hydrogen‐bonding strength, tautomerization rate, and photophysical and spectral parameters of porphyrinoids.