Cobalt carbonyl-mediated carbocyclizations of enynes: generation of bicyclooctanones or monocyclic alkenes

Depending on the thermolytic conditions, dicobalthexacarbonyl-complexed enynes underwent cyclizations to provide different carbocyclic frameworks. Bicyclopentanones were formed from enyne-Co2(CO)6 complexes, or from enynes that were treated with Co2(CO)8, or more effectively, with Co4(CO)12 in an alcoholic solvent under a H2 or N2 atmosphere. This transformation proceeded via a sequential cyclocarbonylation and 1,4-reduction and is the first account using the cobalt carbonyl cluster. Under these conditions a cobalt hydride was presumably generated, which mediated reduction of the enone to the saturated ketone. In contrast, thermolysis of dicobalthexacarbonyl-complexed enynes under a hydrogen atmosphere in toluene resulted in their reductive cyclization to form monocyclic alkenes in moderate yields, in addition to the bicyclopentenone product. In some cases, addition of a hydrosilane to the reaction induced a complete suppression of the bicyclopentenone formation. While the former results demonstrate a reaction that occurs after the cycloaddition, the latter depicts another example of an interruption of the normal route in the Pauson-Khand reaction pathway.     

A sterically-encumbered,C2-symmetric chiral acetal for enhanced asymmetric induction in the Pauson-Khand reaction

High levels of diastereoselection were achieved in the PKR of 1,6- and 1,7-cyclopropylidenynes bearing a bulky propargylic C(2)-symmetric acetal.     

Synthesis of ‘inside-outside’ medium-sized rings via ring-closing metathesis

Functionalized and unfunctionalized dienes were synthesized from lactones 7 and 6, respectively, which were subjected to ring-closing metathesis (RCM) using Grubbs’ catalysts. These dienes provide a systematic study of the stereochemical requirements for efficient cyclization. Moreover, it has been established that, within a uniquely defined oxabicyclo[3.3.0]octane template, medium sized rings bearing an ‘in-out’ intrabridgehead stereochemical relationship can been synthesized in good yield.     

Tubular microstructures made from nonchiral single-chain fluorinated amphiphiles: impact of the structure of the hydrophobic chain on the rolling-up of bilayer membrane

The nonchiral, single-chain fluorinated amphiphiles derived from dimorpholinophosphate, C(n)F(2n+1)(CH(2))(m)OP(O)[N(CH(2)CH(2))(2)O](2) (FnCmDMPs), form hollow tubular bilayer-based self-assemblies when dispersed in water, ethanol/water mixtures, and dimethylformamide. The fluorinated tubules are highly stable and sturdy. Upon heating, they transform reversibly into giant multilamellar vesicles. Uncommon U-shaped and V-shaped coiled membranes were obtained from mixtures of FnCmDMPs. Depending on conditions, fluorinated tubules can evolve with time into collapsed flattened crystallized needles. The successive steps involved in the formation and evolution of these tubules were identified, and the specific features of fluorinated chains that are relevant to membrane coiling and tubule formation are discussed.     

Zur Ausbreitung der Entladung in selbstlöschenden Zählrohren. IV

Measurements with a Geiger counter with inserted grid between wire and cathode solved the problem whether the discharge process is built up by the photoeffect in the gas or at the cathode of the counter. For negative voltagesU _g of the grid (cathode grounded) only the volume inside of the grid is sensitive to discharges. It was found, that within the limits of error, the threshold did not depend on the masking degree of the cathode. From this it follows that mainly the photoeffect in the gas contributes to the discharge process. But the distributions of time of the discharge spread along the wire atU _g<0,U _g>0 and in a counter with a massive cathode has shown that also the photoeffect at the cathode contributes to the discharge process.     

Spectral distributions of thomson-scattered photons from high-intensity pulsed lasers

General formulas for the far-field spectral distribution of photons Thomson scattered by a single electron have been obtained. Effects due to the pulsed nature of the laser beam are explicitly allowed, simultaneously with intensity high enough that harmonic generation is possible. For realistic pulsed photon beams, the spectrum of backscattered radiation is considerably broadened because of changes in the longitudinal velocity of the electrons during the pulse. Such ponderomotive broadening is especially pronounced at higher harmonics, eventually leading to a continuous emission spectrum.