A dynamic supramolecular system exhibiting substrate selectivity in the catalytic epoxidation of olefins

A dynamic supramolecular system involving hydrogen bonding between a Mn(III) salen catalyst and a Zn(II) porphyrin receptor exhibits selectivity for pyridine appended cis-beta-substituted styrene derivatives over phenyl appended derivatives in a catalytic epoxidation reaction.     

Experimental evidence for multiple oxidation pathways in the (salen) Mn-catalyzed epoxidation of alkenes

The substrate electronic effects on the selectivity in the catalytic epoxidation of para-substituted cis stilbenes 2a-i were investigated by using (R,R)-[N,N'-bis(3,5-di-tBu-salicylidene)-1,2-cyclohexanediamine]manganese(III) chloride 1 in benzene as the catalyst with iodosobenzene as the terminal oxidant. A Hammett study of the selectivity results reveals a stronger electrophilic character than previously assumed in the (salen)Mn-catalyzed reaction. In general, the best correlations with the experimental values were obtained by using the Hammett sigma + values, which gave rho = -1.37 for the rate of cis-epoxide formation and rho = -0.43 for the rate of the stepwise process leading to the corresponding trans product. The reaction involves two separate pathways as indicated also by the competitive breakdown of the intermediate on the path to trans epoxide for methoxy-substituted substrates. The asynchronicity in the concerted pathway leading to cis epoxide is apparent for 4-methoxy-4'-nitrostilbene, which yields cis epoxide with 75% ee entirely as a result of electronic effects.     

On the mechanism of the copper-catalyzed cyclopropanation reaction

The selectivity-determining step in enantioselective copper-catalyzed cyclopropanation with diazo compounds has been studied by experimental and computational methods. The addition of the very reactive metallacarbene intermediate in an early transition state to the substrate alkene is concerted but strongly asynchronous, with substantial cationic character on one alkene carbon in the neighborhood of the transition state. Evidence from isotope effects and Hammett studies supports the nature of the transition state. Formation of a metallacyclobutane intermediate by a [2+2] addition is kinetically disfavored. Ligand-substrate interactions influencing the enantio- and diastereoselectivity have been identified, and the preferred orientation of the alkene substrate during the addition is suggested.     

Investigation of the metal binding site in methionine aminopeptidase by density functional theory

All methionine aminopeptidases exhibit the same conserved metal binding site. The structure of this site with either Co²⁺ions or Zn²⁺ions was investigated using density functional theory. The calculations showed that the structure of the site was not influenced by the identity of the metal ions. This was the case for both of the systems studied; one based on the X-ray structure of the human methionine aminopeptidase type 2 (hMetAP-2) and the other based on the X-ray structure of the E. colimethionine aminopeptidase type 1 (eMetAP-1). Another important structural issue is the identity of the bridging oxygen, which is part of either a water molecule or a hydroxide ion. Within the site of hMetAP-2 the results strongly indicate that a hydroxide ion bridges the metal ions. By contrast, the nature of the oxygen bridging the metal ions within the metal binding site of eMetAP-1 cannot be determined based on the results here, due to the similar structural results obtained with a bridging water molecule and a bridging hydroxide ion.     

First order transitions to a lyotropic biaxial nematic

Abstract

The phase diagram of the sodium dodecylsulphate/decanol/water system is studied by²H NMR spectroscopy in the range between the calamitic nematic (N⁺ _C) and discotic nematic (N^? _D) phases. In this narrow range a nematic biaxial phase (N_BX) is observed. The phase transitions between the nematic phases are all of first order. The shape of the surfactant aggregates in the nematic phases varies with composition and temperature.     

Computational investigation of the role of fluoride in Tamao oxidations

The Tamao oxidation of alkoxysilanes was investigated computationally to determine the role of fluoride, a key additive, in this reaction. A sequence of fluoride equilibria as well as possible transition states, mediated by basic and neutral peroxide, respectively, were examined, and a potential energy surface was calculated which was consistent with the typical synthetic methods required for the conversion of alkoxysilanes to alcohols.     

γ- and δ-lactams through palladium-catalyzed intramolecular allylic alkylation: enantioselective synthesis, NMR Investigation, and DFT rationalization

The Pd-catalyzed intramolecular allylic alkylation of unsaturated amides to give γ- and δ-lactams has been studied in the presence of chiral ligands. Ligand (R)-3,5-tBu-MeOBIPHEP (MeOBIPHEP = 6,6'-dimethoxybiphenyl-2,2-diyl)bis(diphenylphosphine)) afforded the best results and allowed the cyclization reactions to take place in up to 94:6 enantiomeric ratio. A model Pd-allyl complex has been prepared and studied through NMR spectroscopic analysis, which provided insight into the processes responsible for the observed enantiomeric ratios. DFT studies were used to characterize the diastereomeric reaction pathways. The calculated energy differences were in good agreement with the experimentally observed enantiomeric ratios.     

Pyranoside phosphite-oxazoline ligands for the highly versatile and enantioselective ir-catalyzed hydrogenation of minimally functionalized olefins. A combined theoretical and experimental study

A modular set of phosphite-oxazoline (P,N) ligands has been applied to the title reaction. Excellent ligands have been identified for a range of substrates, including previously challenging terminally disubstituted olefins, where we now have reached enantioselectivities of 99% for a range of substrates. The selectivity is best for minimally functionalized substrates with at least a moderate size difference between geminal groups. A DFT study has allowed identification of the preferred pathway. Computational prediction of enantioselectivities gave very good accuracy.     

New insights into the stereoselectivity of the aryl zinc addition to aldehydes

The addition of Ph(2)Zn to aldehydes has been investigated by DFT calculations. The experimentally observed increase in enantioselectivity upon addition of Et(2)Zn to the reaction mixture is rationalized from calculations of all isomeric transition states. Spectator ethyl groups in the transition state do not lower the intrinsic activation barrier, but instead increase it. In the presence of a bulky ligand, the inherently preferred all-phenyl transition state is selectively disfavored. The paths with less sterically demanding spectator ethyl groups will experience a more drastic ligand acceleration, and thus the influence of the ligand would be expected to be stronger in the presence of Et(2)Zn, in agreement with experimental observations.