Intermolecular oxidative dehydrogenative 3,3 '-coupling of benzo[b]furans and benzo[b]thiophenes promoted by DDQ/H+ : total synthesis of shandougenine B

With an excess of a strong acid, 2,3-dichloro-5,6-dicyano-1,4-quinone (DDQ) is shown to promote metal-free intermolecular oxidative dehydrogenative (ODH) 3,3'-coupling of 2-aryl-benzo[b]furans and 2-aryl-benzo[b]thiophenes up to 92% yield as demonstrated with 9 substrates. Based on the analysis of oxidation potentials and molecular orbitals combined with EPR, NMR and UV-Vis observations, the studied reaction is initiated by a DDQ-substrate charge transfer complex and presumably proceeds via oxidation of the substrate into an electrophilic radical cation that further reacts with another molecule of a neutral substrate. The coupling reactivity can easily be predicted from the oxidation potential of the substrate and the morphology of its frontier molecular orbitals. The intermolecular ODH coupling reaction allowed a concise total synthesis of the natural product shandougenine B.Peer reviewe

Computational investigations of 18-electron triatomic sulfur–nitrogen anions

Abstract MRCI-SD/def2-QZVP and PBE0/def2-QZVP calculations have been employed for the analysis of geometries, stabilities, and bonding of isomers of the 18-electron anions N₂S²⁻, NS₂⁻, and NSO⁻. Isomers of the isoelectronic neutral molecules SO₂, S₂O, S₃, and O₃ are included for comparison. The sulfur-centered acyclic NSN2⁻, NSS⁻, and NSO⁻ anions are the most stable isomers of their respective molecular compositions. However, the nitrogen-centered isomers SNS⁻ and SNO⁻ lie close enough in energy to their more stable counterparts to allow their occurrence. The experimental structural information, where available, is in good agreement with the optimized bond parameters. The bonding in all investigated species is qualitatively similar, though electron density analyses reveal important quantitative differences that arise from bond polarization. Most of the investigated systems can be described with a single configuration wave function, the two notable exceptions being isomers SSS and OOO that show some diradical character. The computed MRCI-SD/def2-QZVP absorption maxima for SNS⁻ and NSS⁻ are 342 and 327 nm, respectively. The corresponding PBE0/def2-QZVP values in acetonitrile are 353 and 333 nm. These data support the proposed initial formation of SNS⁻ from electrochemical or chemical reduction of SSNS⁻ based on experimental UV–vis spectra. The interconversion of SNS⁻ and NSS⁻ is calculated to be facile and reversible, leading to an equilibrium mixture that also includes the remarkably stable dianion SNSNSS²⁻. Thus, salts of either SNS⁻ or NSS⁻ with bulky organic cations represent feasible synthetic targets

Characterizing Subsiding Shells in Shallow Cumulus Using Doppler Lidar and Large-Eddy Simulation

The existence of subsiding shells on the periphery of shallow cumulus clouds has major implications concerning the parameterization of shallow convection, with the mass exchange between the shell and cloudy air representing a significant deviation from the commonly used bulk-plume parameterization. We examine the structure and frequency of subsiding shells in shallow cumulus convection using Doppler lidars at the Atmospheric Radiation Measurement Southern Great Plains facility in the central United States and at the Julich ObservatorY for Cloud Evolution in western Germany. Doppler lidar indicates that the vertical subsiding shell extent is asymmetric, while shell width is typically similar to 100 m. Large-eddy simulation can reasonably simulate the observed shell structure using a grid spacing of 10 m and suggests that much of the observed asymmetry is not a result of transient cloud evolution

Histoire et fiction : Que se serait-il passé si… ?

The synthesis and spectroscopic and structural characterization of an extensive series of acyclic, monomeric tetrylene dichalcogenolates of formula M­(ChAr)₂ (M = Si, Ge, Sn, Pb; Ch = O, S, or Se; Ar = bulky <i>m</i>-terphenyl ligand, including two new acyclic silylenes) are described. They were found to possess several unusual featuresthe most notable of which is their strong tendency to display acute interligand, Ch–M–Ch, bond angles that are often well below 90°. Furthermore, and contrary to normal steric expectations, the interligand angles were found to become narrower as the size of the ligand was increased. Experimental and structural data in conjunction with high-level DFT calculations, including corrections for dispersion effects, led to the conclusion that dispersion forces play an important role in stabilizing their acute interligand angles