Asymmetric conjugate addition of thiols to (E)-3-crotonoyloxazolidin-2-one by iron or cobalt/pybox catalyst

The enantioselective conjugate addition of thiols to (E)-3-crotonoyloxazolidin-2-one was effectively catalyzed by the Fe(BF₄)₂·6H₂O/(S,S)-ip-pybox catalyst, and the addition product was obtained with up to a 95% ee. Furthermore, the Co(ClO₄)₂·6H₂O/(S,S)-ip-pybox catalyst also catalyzed the same reaction with a high enantioselectivity.     

Effective waste heat recovery from engine exhaust using fin prolonged heat exchanger with graphene oxide nanoparticles

Waste heat recovery is an important alternative to reduce the energy consumption in industrial processes. Heat Exchangers are used effectively for heat recovery. Thus, the role of heat exchangers for waste heat recovery system is crucial. The exclusive of heat transmission of a heat exchanger can be improved by many methods such as by modifying the geometries and using nano-additives of different concentration. In this continuation, a modified geometry of finned heat exchanger is developed with CFD analysis. Modified heat exchanger includes the fins in the internal pipe to improve heat transfer. Nanoparticles of graphene oxide with various concentrations are introduced in working fluid. A steady numerical study is performed by using ANSYS Fluent with k-omega turbulence model for exhaust flow. Variation at inlet velocities of exhaust gas and water, particles concentration and internal fin geometry are considered. The reduction in hot fluid temperature from 6 m/s to 2 m/s enhanced the effectiveness by approximately 33.3%. The decrease in hot fluid velocity to 2 m/s and 6 m/s can reduce its outlet temperature by 100 K and 14 K at 0.03 m/s cold fluid temperature. The inclusion of nanoparticles at 0.1% can enhance the effectiveness by maximum of 7%.     

Necessary extremum conditions for differential inclusion with aftereffect

Necessary extremum conditions for optimization problem on the minimum of a functional specified at the right-hand end of the trajectory of differential inclusion with aftereffect are obtained. The constraints, specified at the left-hand end of the trajectory, imply that the initial function belongs to some rnultivalued mapping. The problem is considered at the fixed interval of time     

Density-functional computation of 53Cr NMR chemical shifts

53Cr chemical shifts of CrO4(2-), Cr2O7(2-), CrO3X-, CrO2X2(X = F, Cl), and Cr(CO)5L (L = CO, PF3, CHNH2, CMeNMe2) are computed, using geometries optimized with the gradient-corrected BP86 density functional, at the gauge-including atomic orbitals (GIAO)-, BPW91-, and B3LYP levels. For this set of compounds, substituent effects on delta(53Cr) are better described with the pure BPW91 functional than with B3LYP, in contrast to most other transition-metal chemical shifts studied so far. For selected cases, 53Cr NMR line widths can be rationalized in terms of electric field gradients (EFGs) computed with the BPW91 functional, but in general other factors such as molecular correlation times appear to be dominating. 53Cr chemical shifts and EFGs are predicted for CrO3, Cr(C6H6)2, Cr(C6H6)CO3, and, with reduced reliability, for Cr2(mu2-O2CH)4.     

Globally convergent Polak-Ribière-Polyak conjugate gradient methods under a modified Wolfe line search

It is well known that global convergence has not been established for the Polak-Ribière-Polyak (PRP) conjugate gradient method using the standard Wolfe conditions. In the convergence analysis of PRP method with Wolfe line search, the (sufficient) descent condition and the restriction β_k?0 are indispensable (see [4,7]). This paper shows that these restrictions could be relaxed. Under some suitable conditions, by using a modified Wolfe line search, global convergence results were established for the PRP method. Some special choices for β_k which can ensure the search direction’s descent property were also discussed in this paper. Preliminary numerical results on a set of large-scale problems were reported to show that the PRP method’s computational efficiency is encouraging.     

Co-ligand effects in the catalytic activity of Pd(II)-NHC complexes

Three cis-chelating di-N-heterocyclic carbene palladium(II) complexes [PdX₂(diNHC)] (X = I, 1; X = SCN, 2; X = CF₃CO₂, 3) bearing different anionic co-ligands were synthesized and fully characterized. A comparison of their catalytic activities in the Mizoroki-Heck reaction and conjugate addition of arylboronic acids to cyclic enones revealed increasing efficiency in the order SCN⁻ < I⁻ < CF₃CO₂⁻. The di(trifluoroacetato) complex 3 showed the best activity in both transformations highlighting the importance of co-ligands effects in catalysis. In addition, the molecular structure of an unusual poly-heteronuclear complex salt 4 is reported, which has been isolated as a byproduct in the synthesis of complex 3.     

Synthesis and reactivity of 5-methylenehydantoins

5-Methylenehydantoin, as well as the N-mono- and N,N-di-protected derivatives, can be obtained by different synthetic routes. These compounds can undergo a large variety of reactions, such as Diels-Alder, epoxidation, methanol addition and conjugate addition reactions of different types of nucleophiles, including carbon (cyanide), nitrogen (piperidine) and sulfur (thiols, thioacetate) nucleophiles. The reactivity with electrophilic reagents, such as m-CPBA or methanol in acidic medium, and the need for Lewis acids to promote the conjugate addition reactions indicate that hydantoin is a poor electron-withdrawing group.     

桥链冠醚卟啉胆固醇酯的表观质子化常数

用光度法测得三种桥链冠醚卟啉和二种卟啉胆固醇酯的质子化常数，并用ＨＭＯ法进行了量子化学计算，讨论了取代基的电子效应及空间效应对常数的影响。     

Study on Separation Method of Overlapped Peaks in Differential Thermal Analysis with Computer Simulation

With the computer simulations, it has been found that in Differential Thermal Analysis (DTA) the overlapped peaks do not satisfy the linear overlapping principle, and the relationship between the overlapped peaks is rather complex that is a function of sample thermal conductivity, specific heat, quantity and latent heats, etc. If the first point where the two peaks begin to overlap is at the posterior half of the first peak curve and the two overlapped peaks can be identified by two different peaks, from the curve of overlapped peaks we can know that the temperature corresponding to the first point deviating from the sample's first phase change curve is the sample's second phase change temperature. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal conditions of growth and the necking evolution of Si,GaSb AND GaAs

The configuration of thermal gradient is illustrated for various types of crucible rotation, which is important for the creation of dislocations, which decreases along the grown axis of crystal. A new mechanism for dislocation elimination during the growth is proposed to explain this phenomenon, which provides a good agreement with the experimental results. The concentration of etch pits rapidly decreased from the beginning to the end of the crystals and the dislocation densities in the middle portion of all investigated crystals were found less than 102 cm^-2. The shallow vertical temperature gradients and virtually flat solidification interface prevented thermal stress from their building up in the crystals. As a result, the dislocation formation had random distribution. Using good necking procedures and choosing an appropriately oriented starting crystal with the shoulder angle <38.94° (assuming growth in <111> direction) it is possible to produce almost dislocation-free crystals without resorting to additional doping normally employed to reduce dislocation formation. This revised version was published online in July 2006 with corrections to the Cover Date.