Die Reaktion des [(Me3Si)2CH]2Al?CH2?Al [CH(SiMe3)2]2 mit LiCH(PMe2)2; Bildung eines fünfgliedrigen Al2C2P-Heterozyklus

The Reaction of [(Me₃Si)₂CH]₂Al? CH₂? Al [CH(SiMe₃)₂]₂ with LiCH(PMe₂)₂; Formation of a Five-membered Al₂C₂P Heterocycle The recently synthesized methylene bridged dialuminium compound [(Me₃Si)₂CH]₂Al? CH₂? Al [CH(SiMe₃)₂]₂ 3 reacts with one equivalent of LiCH(PMe₂)₂ in the presence of TMEDA to give an adduct with one aluminium atom coordinated by the carbanionic carbon atom and the second one coordinated by one phosphorus atom. A five-membered heterocycle 5 is formed, which was characterized by a crystal structure determination showing a strongly bent ring with the phosphorus atom located above the plane of the four remaining atoms (Al₂C₂). 5 is unstable in ethereal solution decomposing under ether cleavage to the educt 3 and the diphosphinomethane derivative CH₂(PMe₂)₂.     

Diboramacrocycles: reversible borole dimerisation–dissociation systems

We report that the outcome of the tin–boron exchange reaction of a mixed thiophene-benzo-fused stannole with aryldibromoboranes is associated with the steric bulk of the aryl substituent of the borane reagent, leading to either boroles or large diboracycles as products. NMR spectroscopic studies indicate that the two products can reversibly interconvert in solution, and mechanistic density functional theory (DFT) calculations reveal boroles to be intermediates in the formation of the diboracyclic products. The addition of Lewis bases to the diboracycles leads to the corresponding borole adducts, demonstrating that they react as “masked” boroles. Additionally, the reaction of the title compounds with a series of organic azides affords complex heteropropellanes, formally 2 : 1 borole-azide adducts, that deviate from the usual BN aromatic compounds formed via nitrogen atom insertion into the boroles.

Diboramacrocycles are a new form of borole dimers, participating in various addition reactions as “masked” boroles. The reaction of a less crowded diboramacrocycle with organic azides affords unprecedented complex heteropropellanes.     

1,4,6,9‐Tetraoxa‐5λ4‐selena‐spiro[4.4]nonane – A Combined Theoretical and Experimental Study

The symmetric spiro‐selenurane derived from ethylene glycol, 1,4,6,9‐tetraoxa‐5λ⁴‐selena‐spiro[4.4]nonane, was prepared from selenium tetrachloride and ethylene glycol and its molecular structure was determined by single crystal X‐ray diffraction. NBO analyses for the title compound and a related compound were conducted to assess the role of the stereochemical active lone pair on the selenium atom on the structure.     

Studies on the manganese-mediated isomerization of alkynyl carbonyls to allenyl carbonyls

[reaction: see text] A study of the role of base in the isomerization of manganese-coordinated conjugated alkynyl carbonyls to the corresponding allenyl carbonyls is described. The use of phosphine additives indicates that manganese requires a ligand prior to isomerization with amine bases. A series of amine bases were also examined for their efficacy in this isomerization reaction revealing a strong dependence on pK(a). By contrast, potassium tert-butoxide led to rapid isomerization in the absence of added manganese ligand.     

MRI-induced stimulation of peripheral nerves: dependency of stimulation threshold on patient positioning

Fast gradient systems enable the imaging of moving organs but also restrict imaging possibilities by stimulating the peripheral nerves. Several studies have determined the stimulation thresholds of the peripheral nerves. However, no study has explicitly examined whether stimulation thresholds are dependent on the position of the patient in the MR imager. In this study, combinations of one, two and three gradient axes were switched with sinusoidal wave forms (rise time 300 micros). Stimulation thresholds were determined for five patient positionings: gradient isocenter at the bridge of the nose, jugular fossa, 10 cm cranial of the umbilicus, umbilicus, and 10 cm caudal of the umbilicus. Experiments were performed in two volunteer groups, one for triple gradient switching (17 male, 14 female) and the other for single and double gradient switching (14 male, 11 female). The dependence of the stimulation threshold on the position of the volunteers in the MR imager was statistically significant for most of the gradient combinations, with the highest stimulation thresholds for positions umbilicus+10 and umbilicus at isocenter. Similar patterns of position dependence were obtained for gradient pairs whose axes were axially symmetric to the human body. Mean stimulation thresholds of male volunteers were lower than those of female volunteers. These findings indicate that stimulation thresholds should be defined according to the position of the patient in the MR scanner.     

Thermal and photochemical decomposition of cis- and trans-3,5-diphenyl-1-pyrazoline

The decomposition mechanism of the title compounds 8c and 8t is discussed. Dibenzylic 1,3-biradicals are suggested as intermediates.