N‐Heterocyclic Silylenes in Boron Chemistry: Facile Formation of Silylboranes and Silaborinines

Reaction of a N‐heterocyclic silylene (NHSi) with PhBX₂ (X=Cl, Br) readily afforded six‐membered silaborinines through an insertion/ring expansion sequence. Increasing the sterics of the borane from phenyl to duryl enabled the selective generation and isolation of the highly colored silylborane intermediates. Theoretical studies on the mechanism and energetics of the silaborinine formation were fully consistent with the experimental observations.     

Synthesis of Stereochemically Flexible Cyclic Biphenylbisphosphinite Ligands: Control of the Dynamics and Selectivity

Novel l -tartaric acid based stereodynamic biphenylbisphosphinite ligands with anilino linkers were prepared as cyclic diamide compounds. The influence of the constitutional substitution pattern of the amino group at the aniline moiety on the diastereoselective formation of the ligand was investigated. While the meta-anilino bridged ligands showed a highly dynamic behavior and no discrimination between (R_ax) and (S_ax) configuration, l -tartaric acid substitution on ortho-anilino bridged ligands leads to diastereoselective self-alignment of the central biphenol axis, which controls the stereoselectivity of the ligand and catalyst. The experimental findings were corroborated by theoretical calculations. In the rhodium-catalyzed enantioselective hydrogenation of methyl 2-acetamidoacrylate (MAA), an enantiomeric ratio of 75 : 25 (R/S) was obtained for the ortho-substituted ligand.     

Thermal and acid labile polyurethanes as a new class of responsive materials in polymeric nanoparticles and nanocapsules

A new class of polyurethanes has been designed, containing tertiary carbamate groups in the main chain of the polymer, which enable the resulting polymer to degrade completely under acid and thermal treatment. The decomposition temperatures of the polymers were determined by measuring the evolution of carbon dioxide and other decomposition products using TGA‐MS. Until decomposition of the polymer, no glass transition was found. The polymers exhibit excellent solubility in common organic solvents like chloroform and tetrahydrofuran, making them to suitable materials for film formation. From the obtained polymers, nanoparticles were synthesized by the solvent evaporation method combined with the miniemulsion technique. The resulting nanoparticles can be used as intelligent fillers in films and sensors, since they degrade at temperatures of above 180 °C, which can be detected by a color change reaction with ninhydrin. Polymeric nanocapsules were prepared by an interfacial polyaddition reaction from 2,4‐toluene diisocyanate and tertiary diols performed at the droplet's interface in inverse (water‐in‐oil) miniemulsions. These nanocapsules with an encapsulated photoacid generator can act as a release system, whereby an acidic release through irradiation with ultraviolet light can be triggered. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Molecular Recognition in Glycolaldehyde,the Simplest Sugar: Two Isolated Hydrogen Bonds Win Over One Cooperative Pair

Carbohydrates are used in nature as molecular recognition tools. Understanding their conformational behavior upon aggregation helps in rationalizing the way in which cells and bacteria use sugars to communicate. Here, the simplest α-hydroxy carbonyl compound, glycolaldehyde, was used as a model system. It was shown to form compact polar C₂-symmetric dimers with intermolecular O–H⋅⋅⋅O=C bonds, while sacrificing the corresponding intramolecular hydrogen bonds. Supersonic jet infrared (IR) and Raman spectra combined with high-level quantum chemical calculations provide a consistent picture for the preference over more typical hydrogen bond insertion and addition patterns. Experimental evidence for at least one metastable dimer is presented. A rotational spectroscopy investigation of these dimers is encouraged, also in view of astrophysical searches. The binding motif competition of aldehydic sugars might play a role in chirality recognition phenomena of more complex derivatives in the gas phase.     

Toluene as a novel carrier of xanthates—preparation,use and surrogate of S-tri- and di-chloromethyl xanthates

Toluene has been identified as a novel carrier of xanthates. Their corresponding fragmentative precursors proved to behave efficiently in radical group transfer reactions. As examples, unprecedented S-tri/di-chloromethyl xanthates could be prepared, isolated and further used in radical additions to olefins. Their precursors (de-aromatized toluene upon which is grafted, at one end, a tri/di-chloromethyl-group and, at the other end, a dithiocarbonyl group) can also be used directly in the transfer of both groups to olefins. The re-aromatizing loss of toluene by radical initiated fragmentation of the precursors brings thus new opportunities to the chemistry of xanthates, exemplified here in the intermolecular additions to olefins of new S-tri/di-chloromethyl xanthates.     

Enantioselective Catalytic [4+1]-Cyclization of ortho-Hydroxy-para-Quinone Methides with Allenoates

The first highly asymmetric catalytic synthesis of densely functionalized dihydrobenzofurans is reported, which starts from ortho-hydroxy-containing para-quinone methides. The reaction relies on an unprecedented formal [4+1]-annulation of these quinone methides with allenoates in the presence of a commercially available chiral phosphine catalyst. The chiral dihydrobenzofurans were obtained as single diastereomers in yields up to 90 % and with enantiomeric ratios up to 95:5.