A computational study on the role of chiral N-oxides in enantioselective Pauson-Khand reactions

Density functional calculations were carried out to ascertain the origin of enantioselectivity in the brucine N‐oxide (BNO)‐assisted enantioselective Pauson–Khand reaction (PKR) of norbornene with 2‐methyl‐3‐butyn‐2‐ol. The computed ee value in acetone is 68 % (R), which compares well to the previously reported experimental value of 58 % (R). In DME the computed ee value of 76 % (R) is in excellent agreement with the experimentally determined value of 78 % (R). The mechanism of enantioselectivity consists of several steps. First, the dicobalt complex is activated by BNO with chirality transfer from enantiopure BNO to the dicobalt complex. Second, competition occurs between a racemization process and complexation with the olefin reagent, which leads to the products. The lower ee value in acetone is due to the lower energy barrier of the racemization process. Calculations show that replacement of BNO by a hypothetical more enantioselective chiral N‐oxide will hardly increase the ee value beyond 90 %.     

Unraveling the origin of regioselectivity in rhodium diphosphine catalyzed hydroformylation. A DFT QM/MM study

The origin of regioselectivity in rhodium diphosphine catalyzed hydroformilation was investigated by means of hybrid QM/MM calculations using the IMOMM method. The roles of the diphosphine bite angle and of the nonbonding interactions were analyzed in detail by considering rhodium systems containing xantphos-type ligands, for which a correlation between the natural bite angle and regioselectivity has been recently reported. From the pentacoordinated equatorial--equatorial HRh(CO)(alkene)(diphosphine) key intermediate, eight possible reaction paths were defined and characterized through their respective transition states (TS). We succeeded in reproducing the experimentally observed trends for the studied diphosphines. By performing additional calculations on model systems, in which the steric effects induced by the phenyl substituents of xantphos ligands were canceled, we were able to separate, identify, and evaluate the different contributions to regioselectivity. These additional calculations showed that regioselectivity is governed by the nonbonding interactions between the diphenylphosphino substituents and the substrate, whereas the effects directly associated to the bite angle, what we call orbital effects, seem to have a smaller influence.     

Metal–Arene Interactions in Dialkylbiarylphosphane Complexes of Copper,Silver, and Gold

Dialkylbiphenylphosphane–Au^I complexes exhibit only weak metal–arene interactions with the covering arene ring. However, the contacts in isoleptic Ag^I and Cu^I complexes are shorter than the limiting values of 3.03 Å (Ag^I) and 2.83 Å (Cu^I). Strong metal–arene interactions were also found in the two Ag^I aquo complexes and in two acetonitrile? Cu^I complexes with dialkylbiphenylphosphane ligands. Arene–Ag^I complexes with these bulky phosphane ligands show the strongest Ag^I? arene bonds known.     

Investigating interfacial parameters with platinum single crystal electrodes

The concepts of total and free charge of platinum single crystal electrodes are revised in this paper, together with the associated concepts of potential of zero total and free charge. Total charges can be measured from CO displacement method. Results on solution of different pH are described. A novel buffer composition is used to attain pH values close to neutrality while avoiding interferences from anion adsorption processes. Stress is made on the fact that free charges are not accessible through electrochemical measurement for systems at equilibrium since adsorption processes (hydrogen and hydroxyl) interfere with free charge determination. Still, a model is described that allows, under some assumptions, extract free charge values and the corresponding potential of zero free charge for Pt(111) electrodes. On the other hand, fast measurement outside equilibrium can separate free charges from adsorption processes based on their different time constant. In this way, the laser induced temperature jump experiment allows determination of the potential of maximum entropy, a magnitude that is intimately related with the potential of zero free charge. Values of the potential of maximum entropy as a function of pH are given for the different basal planes of platinum.     

Qualitative method for determination of aflatoxin B1 in nuts

The proper characterization of a commercial qualitative method for determining aflatoxin B1 in some nuts is described. A qualitative method that provides binary responses of the yes/no type means that the performance parameters have been properly adapted and defined. Performance characteristics such as the cut-off limit, the detection limit, sensitivity, specificity, false-positive and negative rates, and the unreliability or uncertainty region are defined and then estimated by means of the performance characteristic curves. The commercial test kit showed the cut-off limit at 1.6 ng/g, with a sensitivity rate of 95% and a false-negative rate of zero. A modification can be performed to shift the cut-off to 2.0 ng/g, keeping the same values for the sensitivity and false-negative rate.     

Experimental and theoretical investigations of new dinuclear palladium complexes as precatalysts for the amination of aryl chlorides

A series of new palladium dinuclear species with general formula [Pd2X(mu-X)[mu-P(t)Bu2(Bph-R)]] (X = Cl, Br; Bph = biphenyl; R = H, Me, NMe2) have been prepared. The two palladium centers in these species are bridged by one of the aromatic rings of the biphenyl group present in the corresponding phosphine. The X-ray crystal structure of one of these complexes has been obtained, providing a clear picture of the bonding pattern. The stability of these dimers in solution is shown to be highly dependent on the nature of the phosphine R group and also on the bridging halide. When R = NMe2, the dimers dissociate, yielding the palladium(II) compounds PdX2[P(t)Bu2(BPh-NMe2)] (X = Cl, Br), and the X-ray crystal structure of one of them (X = Br) has shown that the biphenyl group from the phosphine interacts directly with the metal center. This interaction seems to play an important role in stabilizing the otherwise coordinatively unsaturated palladium(II) complex. In contrast, when R = H or Me, the analogous monomeric palladium(II) complexes are unstable and undergo cyclometalation to generate a palladium(II) dinuclear species in which each of the two phosphines cyclometalates with the palladium centers forming a strained four-membered ring. In addition to their unusual structures, these aryl-bridged dimers have also proven to be excellent precatalysts for the amination of aryl chlorides. To rationalize some of the experimental results, a detailed DFT computational study has been carried out and is presented herein.     

Linear‐dendritic polymeric amphiphiles as carriers of doxorubicin—In vitro evaluation of biocompatibility and drug delivery

In our recent work, we have explored the formation of chemotherapeutic delivery vehicles constructed from four different amphiphilic linear‐dendritic hybrid block copolymers. These micelles were found to form about 100‐nm‐sized structures that were capable of sequestering doxorubicin at loading efficiencies up to 22%. Here, the cellular toxicity of these biocompatible and biodegradable linear‐dendritic hybrid materials was evaluated on two breast cancer cell lines and primary human macrophages. The micelles were found not to affect the cellular viability at concentrations below 35 μg mL^?1. After drug loading, these constructs could deliver an efficient dose of drugs, resulting in significant decreases in cell viability. Kinetic studies indicated that the drug formulation in the polymer micelles slowed down the cell uptake compared with the nonformulated drug, but similar efficacy in viability reduction and cell apoptosis were found. Taken together, these linear‐dendritic hybrid materials represent an interesting novel architecture for the construction of drug delivery systems. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012