Nitrogen state-selected ions

In 2005, both Rueping et al. and List et al. reported the first transfer hydrogenation with Hantzsch ester 1 of several N-protected ketimines catalyzed by chiral Bronsted acids derived from l,l -binaphthol [17, 18]. The reaction typically requires 1 to 20 mol% of catalyst, is performed in benzene at 60 °C, and enantio-selectivities of up to 90% are obtained. The chiral Bronsted acid protonates the lcetimine at nitrogen, giving an ion-pair which is reduced by Hantzsch ester 1. (For experimental details see Chapter 14.21.2). A preferred transition state has... 

The mode of the diastcrcofacial selectivity is completely reversed in the case of reactions with A -methyl A-acyliminium precursors 4176. Now the nitrogen atom of the A-acyliminium ion is not able to chelate with the tin atom and the lower diastereoselectivity is explained by the less rigid nonchelation-controlled transition state 5. An electronic effect, such as n-iz attraction between the electron-deficient carbonyl group of the acyliminium ion and the electron-rich phenyl group of the phcnylthio substituent R, may account for the somewhat higher diastereoselectivity in the case of arylthio substituents R. 

Macrocycles attached to redox responsive groups such as ferrocene (78) can give selective transition metal ion receptors. The X-ray structure reveals a five-coordinate zinc with distorted square-pyramidal geometry bound to the four macrocycle nitrogens and an iodide. In the solid state the two ferrocenyl groups are positioned on the same side of the ligand with distances to the metal center of 5.347(7) and 6.120(8) A and these distances can be related to the redox behavior.689... 

The metal ion, e.g. Fe or Co, when in its lower oxidation state can share electron charge with the oxygen molecule adduct. Several iron and cobalt prophyrin derivatives and cobalt-Schiff bases show the necessary reversibility and rates for successful application [e.g. 23]. a, a, a", a" -weso-tetrakis[(o-piralamidophenyl)-po phinato] Co (II) has been complexed with 1-methylimidazole to make a complex (CoPIm) which, when mixed with polybutyl methacrylate gave oxygen permeabilities on the order of 10-9 sec-cm/(cm2-s-cmHg) with a selectivity of about 5 over nitrogen [23]. 

Due to the choice of nitrogen- and oxygen-containing ligands, the metal ions are found mostly in an electronic high-spin state in all of the proteins. In the following, a few classes of non-heme iron proteins will be discussed on the basis of selected examples of representative enzymes. Where appropriate, mechanistic details obtained from model structures will also be reviewed. 

The recorded differences in adsorption capability indicate a different mechanism of interaction between the carbon surface and the ionic metal species pre.sent in the aqueous solution (aqua and hydroxy complexes, hydroxide ions, and electronegative complexes). To discover the state of the adsorbed. species, some independent measurements of the surface layer of adsorbent were carried out. The selected carbon samples were studied by the XPS method in powdered form following copper uptake (Figs. 42 and 43). Several peaks attributable to carbon, oxygen, nitrogen, and copper were present. The XPS survey spectra of the initial modified carbons (before adsorption) were discussed in the previous section. The surface elemental composition estimated from XPS data for modified D43/1 car-... 

The tetrazole residue of the catalyst is coordinated by one of the reagent s nitrogen atoms via a hydrogen bond. In the cyclic transition state, this coordination leads to a relatively fixed conformation, which is important for the selectivity of the reaction. The initially formed imminium ion deprotonates to give the enamine. Upon electron donation of the pyrrolidine nitrogen, the enamine adds to the N=N double bond which is protonated by the tetrazole to form an a-modified imminium intermediate. After hydrolysis, product 27 is formed. 

---