Application of monodentate secondary phosphine oxides,a new class of chiral ligands,in Ir(i)-catalyzed asymmetric imine hydrogenation

Secondary phosphine oxides were prepared from R(1)PCl(2) and R(2)MgBr, followed by hydrolysis. They were obtained in an enantiopure form by preparative chiral HPLC. These new monodentate ligands were tested in the iridium-catalyzed hydrogenation of imines at 25 bar. Enantioselectivities up to 76% were obtained at L/Ir = 2. Addition of pyridine (Pyr/Ir = 1:2) raised the ee to 83%. Using pyridine as an additive allowed reduction of the L/Ir ratio to 1 without reduction of ee. [reaction: see text]     

Mono- versus bidentate ligands in rhodium-catalyzed asymmetric hydrogenation. A comparative rate study

[reaction: see text] Bidentate chiral phosphines are no longer essential for achieving a fast and highly enantioselective hydrogenation of alpha- or beta-dehydroamino acid derivatives. In particular, a readily accessible and stable monodentate phosphoramidite can be highly effective in these asymmetric hydrogenations.     

On the correlation for Kac-like models in the convex case

The aim of this paper is to stu the behavior asm tends to of a family of measures exp[- ^(m)(x)]dx ^(m) on ^m , where ^(m) is a potential on ^m which is a perturbation in a suitable sense of the harmonic potential _j x _j ² .     

Variation of the pH of the background electrolyte due to electrode reactions in capillary electrophoresis: Theoretical approach and in situ measurement

Electrode reactions during the electrophoretic process may change the pH of the buffer and subsequently the migration behavior of solutes with resultant loss of reproducibility. A theoretical treatment of pH variations due to electrolytic processes is presented. The choice of buffer appears to have a dramatic influence on the pH variations observed, even if substantial buffer action is expected at the pH chosen. The experimental evaluation of the separation of 4-hydroxy-3-methoxycinnamic acid and 3-hydroxybenzoic acid reveals that the quality of the separation decreases continuously from a baseline separation observed in the first experiment to a comigration of the two solutes (resolution = 0) in the ninth experiment. A pH decrease of about 0.05 pH units accounts for the observed changes in mobility. A novel in situ pH measurement approach is presented, in which the mobility, peak area, and peak height of an indicator dye are related to the pH in the capillary. This enables the identification and quantification of pH variations during electrophoretic runs: the pH decreases at the anodic side already after the first experiment and pH variations as small as 0.02 pH units can be measured. The variations in peak height appear to be less suited. The calculated pH variations are in close agreement with the ones obtained experimentally.     

Biosynthesis of Vitamin B(6) in yeast. Incorporation pattern of trioses

The biosynthetic origin of the C(3) unit, C-6,5,5', of pyridoxamine was investigated in two yeasts, Candida utilis ATCC 9256 and Saccharomyces cerevisiae ATCC 7752. The incorporation patterns within pyridoxamine bishydrochloride derived from variously multiply (13)C- and (2)H-labeled samples of glycerol and glyceraldehyde, established by NMR spectroscopy, indicate that the three-carbon unit C-6,5,5' of pyridoxamine is derived intact from a triose.     

Vibronic coupling and double excitations in linear response time-dependent density functional calculations: dipole-allowed states of N2

The present study serves two purposes. First, we evaluate the ability of present time-dependent density functional response theory (TDDFRT) methods to deal with avoided crossings, i.e., vibronic coupling effects. In the second place, taking the vibronic coupling effects into account enables us, by comparison to the configuration analysis in a recent ab initio study [J. Chem. Phys. 115, 6438 (2001)], to identify the neglect of double excitations as the prime cause of limited accuracy of these linear response based TDDFRT calculations for specific states. The "statistical averaging of (model) orbital potentials (SAOP)" Kohn-Sham potential is used together with the standard adiabatic local-density approximation (ALDA) for the exchange-correlation kernel. We use the N2 molecule as prototype, since the TDDFRT/SAOP calculations have already been shown to be accurate for the vertical excitations, while this molecule has a well-studied, intricate vibronic structure as well as significant double excitation nature in the lowest 1Pi(u) state at elongated bond lengths. A simple diabatizing scheme is employed to obtain a diabatic potential energy matrix, from which we obtain the absorption spectrum of N2 including vibronic coupling effects. Considering the six lowest dipole allowed transitions of 1Sigma(u)+ and 1Pi(u) symmetry, we observe a good general agreement and conclude that avoided crossings and vibronic coupling can indeed be treated satisfactorily on the basis of TDDFRT excitation energies. However, there is one state for which the accuracy of TDDFRT/ALDA clearly breaks down. This is the state for which the ab initio calculations find significant double excitation character. To deal with double excitation character is an important challenge for time-dependent density functional theory.     

SCF and CI Studies of Hund's rules for the effects of electronic correlation and delocalization. I

In connection with the reinterpretation of Hund's multiplicity rules for molecules, a detailed study has been made of the energy differences in the total energy and its components for the triplet and singlet Π_u states of the hydrogen molecule and the analogous states of the four- and six-membered hydrogen atom rings. For the hydrogen molecule, both SCF and CI studies indicated that the outer electron is considerably more contracted in the triplet than in the singlet state. In both approximations, the energy difference is dominated for all bond distances of chemical and physical significance by the electron-nuclear attraction component and not by the electron repulsion component as predicted by simple first-order perturbation theory. Although the correlation energy for each of the states is of the same magnitude as the energy differences considered here, the difference of the correlation energies is much smaller. It had little effect on the qualitative differences between these states of the hydrogen molecule. For the four- and six-membered rings, SCF studies were made on the lowest singlet and triplet states where one electron was promoted from the σ_g to a Π_u orbital. Even though the coupled electrons were more delocalized in these cases, the electron repulsion became relatively more important. However in all cases, the lower state had the highest electron repulsion energy and lower electron-nuclear attraction. The triplet state continued to have the more contracted outer open-shell orbital.     

Synthesis and conjugation of oligosaccharide analogues of fragments of the immunoreactive glycan part of the circulating anodic antigen of the parasite Schistosoma mansoni

The gut-associated circulating anodic antigen (CAA) is one of the major excretory antigens produced by the parasite Schistosoma mansoni. The immunoreactive part of CAA is a threonine-linked polysaccharide composed of long stretches of the unique repeating disaccharide-->6)-[beta-D-GlcpA-(1-->3)]-beta-D-GalpNAc-(1-->. Previously, using surface plasmon resonance and ELISA techniques, it has been shown that some anti-CAA IgM monoclonal antibodies (MAbs) also recognize members of a series of bovine serum albumin (BSA)-coupled synthetic di- to penta-saccharide fragments of the CAA glycan. To generate information on the molecular level about the glycan specificity of the relevant IgM MAbs, two series of oligosaccharides related to the CAA disaccharide epitope were synthesized, and coupled to BSA. The first three analogues, beta-D-GlcpA-(1-->3)-[small beta]-D-GlcpNAc-(1-->O), beta-D-GlcpNAc-(1-->6)-[beta-D-GlcpA-(1-->3)]-beta-D-GlcpNAc-(1-->O), and beta-D-GlcpA-(1-->3)-beta-D-GlcpNAc-(1-->6)-[beta-D-GlcpA-(1-->3)]-beta-D-GlcpNAc-(1-->O), wherein the native beta-D-GalpNAc moiety was replaced by beta-D-GlcpNAc, were synthesized to investigate the specificity of the selected MAbs to the carbohydrate backbone of CAA. The second series of analogues, beta-D-Glcp6S-(1-->3)-beta-D-GalpNAc-(1-->O), beta-D-GalpNAc-(1-->6)-[beta-D-Glcp6S-(1-->3)]-beta-D-GalpNAc-(1-->O), and beta-D-Glcp6S-(1-->3)-beta-D-GalpNAc-(1-->6)-[beta-D-Glcp6S-(1-->3)]-beta-D-GalpNAc-(1-->O), wherein the native beta-D-GlcpA moiety was replaced by beta-D-Glcp6S, was synthesized to evaluate the importance of the type/nature of the charge of CAA for the MAb recognition.