Effect of crosslink density on the pressure relaxation response of polycyanurate networks

The effect of crosslink density on the pressure-volume-temperature (PVT) behavior and on the pressure relaxation response for two polycyanurate networks is investigated using a custom-built pressurizable dilatometer. Isobaric cooling measurements were made to obtain the pressure-dependent glass transition temperature (T_g). The pressure relaxation studies were carried out as a function of time after volume jumps at temperatures in the vicinity of the pressure-dependent T_g, and the pressure relaxation curves obtained were shifted to construct master curves by time-temperature superposition. The reduced pressure relaxation curves are found to be identical in shape and placement, independent of crosslink density, when T_g is used as the reference temperature. The horizontal shift factors used to create the master curves are plotted as a function of the temperature departure from T_g (T − T_g), and they agree well with their counterparts obtained from the shear response. Moreover, the retardation spectra are derived from bulk compliance and compared to those from the shear. The results, similar to our previous work on polystyrene, indicate that at short times, the bulk and shear responses have similar underlying molecular mechanisms; however, the long-time mechanisms available to the shear response, which increase with decreasing crosslink density, are unavailable to the bulk response. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2477–2486, 2009     

Origin of the relative stereoselectivity of the beta-lactam formation in the Staudinger reaction

The relative (cis, trans) stereoselectivity of the beta-lactam formation is one of the critical issues in the Staudinger reaction. Although many attempts have been made to explain and to predict the stereochemical outcomes, the origin of the stereoselectivity remains obscure. We are proposing a model that explains the relative stereoselectivity based on a kinetic analysis of the cis/trans ratios of reaction products. The results were derived from detailed Hammett analyses. Cyclic imines were employed to investigate the electronic effect of the ketene substituents, and it was found that the stereoselectivity could not be simply attributed to the torquoelectronic model. Based on our results, the origin of the relative stereoselectivity can be described as follows: (1) the stereoselectivity is generated as a result of the competition between the direct ring closure and the isomerization of the imine moiety in the zwitterionic intermediate; (2) the ring closure step is most likely an intramolecular nucleophilic addition of the enolate to the imine moiety, which is obviously affected by the electronic effect of the ketene and imine substituents; (3) electron-donating ketene substituents and electron-withdrawing imine substituents accelerate the direct ring closure, leading to a preference for cis-beta-lactam formation, while electron-withdrawing ketene substituents and electron-donating imine substituents slow the direct ring closure, leading to a preference for trans-beta-lactam formation; and (4) the electronic effect of the substituents on the isomerization is a minor factor in influencing the stereoselectivity.     

Unique sigma-bond metathesis of silylalkynes promoted by an ansa-dimethylsilyl and oxo-bridged uranium metallocene

The tetrachloride salt of uranium reacts with 1 equiv of the lithium ligand Li2[(C5Me4)2SiMe2] in DME to form the complex [eta5-(C5Me4)2SiMe2]UCl2.2LiCl.2DME (1), which undergoes a rapid hydrolysis in toluene to yield the dimeric bridged monochloride, monooxide complex [{[eta5-(C5Me4)2SiMe2]UCl}2(mu-O)(mu-Cl)*Li*1/2DME]2 (2). Metathesis of 2 with BuLi in DME gives the mono-bridged dibutyl complex {[eta5-(C5Me4)2SiMe2]UBu}2(mu-O) (3). Complex 2 was characterized by solid-state X-ray analysis. Complex 3 was found to be an active catalyst for the disproportionation metathesis of TMSCCH (TMS = SiMe3) and the cross-metathesis of TMSCCH or TMSCCTMS with various terminal alkynes. The metathesis of TMSCCH gives TMSCCTMS and HCCH, whereas the cross-metathesis of TMSCCH or TMSCCTMS with terminal alkynes (RCCH) yields TMSCCTMS, TMSCCR, and HCCH. In addition, TMSCCCH3 also was found to react with tBuCCH, yielding TMSCCBut and CH3CCH. A plausible mechanism for the catalytic process is presented.     

Do reaction conditions affect the stereoselectivity in the Staudinger reaction?

The stereochemistry is one of the critical issues in the Staudinger reaction. We have proposed the origin of the stereoselectivity recently. The effects of solvents, additives, and pathways of ketene generation on the stereoselectivity were investigated by using a clean Staudinger reaction, which is a sensitive reaction system to the stereoselectivity. The results indicate that the additives, usually existed and generated in the Staudinger reaction, and the pathways of the ketene generation do not generally affect the stereoselectivity. The solvent affects the stereoselectivity. The polar solvent is favorable to the formation of trans-beta-lactams. The addition orders of the reagents affect the stereoselectivity in the Staudinger reaction between acyl chlorides and imines. The addition of a tertiary amine into a solution of the acyl chloride and the imine generally decreases the stereoselectivity, which is affected by the interval between additions of the acyl chloride and the tertiary amine, and the imine substituents. Our current results provide further understanding on the stereochemistry of the Staudinger reaction between acyl chlorides and imines and on the factors affecting the stereochemistry and also provide a method to prepare beta-lactams with the desired relative configuration via rationally tuning the stereoselectivity-controlling factors in the Staudinger reaction.     

Retro‐Corey‐Chaykovsky Epoxidation: Converting Geminal Disubstituted Epoxides to Ketones

Corey‐Chaykovsky epoxidation has been widely applied in the conversion of aldehydes and ketones to epoxides with sulfonium and sulfoxonium ylides. The reverse transformation is realized for conversion of geminal disubstituted epoxides to ketones in the presence of DABCO in refluxing mesitylene. The method is a weak basic transformation from epoxides to ketones with loss of a methylene group and can be applied as an alternative strategy of the acid‐catalyzed Meinwald rearrangement or oxidation for conversion of epoxides to carbonyl compounds.     

五配位铁硫化合物(Et4N)4〔Fe2(S,X-C6H4-o)4〕的合成与晶体结构

利用无水FeCl与双齿配体Na_2BDT或Na_2MP(Na_2S,X-C,H_4-o_2:X=S,BDT;X=O,MP)在乙醇溶液中反应,然后加入沉淀剂Et_4NBr,可以得到双核铁硫化合物(Et_4N)_2[Fe_(S,X-CH_4-o)_4](1,X=S;2,X=0)。它们分别具有以双硫或双氧桥联起来的五配位扭曲三角双锥构型的铁原子。分子为心对称,核心Fe_2X_2~(2+)为平面。每个铁原子上的二个苯环平面A和B相互近于平行(1,二面角10.1°)或近于垂直(2,二面角100.2°);但铁原子与XSCC平面非共面。Fe原子离平面0.05～0.5A。2是第一个以双齿配位的双氧桥铁硫化合物,它既含有桥基氧原子又具有端基氧原子,对研究含氧配位的铁原子周围环境,为模拟固氧酶中的“P簇”,有一定意义。     

Controlling the Localization of Liquid Droplets in Polymer Matrices by Evaporative Lithography

Localized inclusions of liquids provide solid materials with many functions, such as self‐healing, secretion, and tunable mechanical properties, in a spatially controlled mode. However, a strategy to control the distribution of liquid droplets in solid matrices directly obtained from a homogeneous solution has not been reported thus far. Herein, we describe an approach to selectively localize liquid droplets in a supramolecular gel directly obtained from its solution by using evaporative lithography. In this process, the formation of droplet‐embedded domains occurs in regions of free evaporation where the non‐volatile liquid is concentrated and undergoes a phase separation to create liquid droplets prior to gelation, while a homogeneous gel matrix is formed in the regions of hindered evaporation. The different regions of a coating with droplet embedment patterns display different secretion abilities, enabling the control of the directional movement of water droplets.     

Effect of temperature on the enantioselectivity in the oxazaborolidine-catalyzed asymmetric reduction of ketones. Noncatalytic borane reduction, a nonneglectable factor in the reduction system

The effect of temperature on the enantioselectivity of the oxazaborolidine-catalyzed asymmetric borane reduction of ketones has been investigated carefully using alkyl aryl ketones with a variety of functional groups and a B-methoxyoxazaborolidine derived from trimethyl borate and (S)-alpha,alpha-diphenylprolinol as a catalyst. The reductions were carried out over a range of temperatures in THF and toluene with or without the catalyst. The reductive rates increase along with increasing reaction temperature with or without the catalyst by determining the conversion of the ketones to alcohols by GC analysis. However, the rates of the catalytic reductions increase faster than those without the catalyst. The results indicate that the noncatalytic borane reduction is an important factor to the enantioselectivity in the reduction. The highest enantioselectivities were usually obtained between 20 and 30 degrees C in the asymmetric reduction.