Complex sites

However, addition of (+ )-(7 )-l-methyl-4-(mcthylsulfinyl)benzene, to aldehydes and ketones proceeds with low stereoselectivity. An improvement of the 3-syn diaslereoselectivity was found with the zinc reagent obtained by transmetalation of the lithiated sulfoxide with anhydrous zinc chloride38. An improvement of the stereoselectivity was also attained by exchange of the 4-methylphenyl substituent for a 2-methoxyphenyl or 2-pyridinyl substituent. Thus, the introduction of an additional complexing site into the aromatic part of the sulfoxide reagent enhances the stereoselectivity35. 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

Hsi C, Langmuir D (1985) Adsorphon of uranyl onto ferric oxyhydroxides applications of the surface complexation site-binding model. Geochim Cosmochim Acta 49 1931-1941 Ingri J, Widerlund A, Land M, Gustafsson O, Anderson P, Ohlander B (2000) Temporal variation in the fractionation of the rare earth elements in a boreal river the role of colloidal particles. Chem Geol 166 23-45... 

Precise placement of metal complexing sites within the infrastructure of a cascade molecule is of importance from a variety of perspectives. In the construction of the above noted Micellane family (cf. Sect. 3.1), we reported the construction of dendrimers with four alkyne moieties at sites equidistant from each other in the interior (17, Fig. 8) [60]. These were treated with decaborane (B10H14) to afford 1,2-dicarba-closo-dodecaboranes (o-carboranes) [71]. Rendering boron clusters soluble in water is of interest because of their use in cancer treatment by Boron Neutron Cancer Therapy. First and second generation water-soluble dendrimers containing four and twelve precisely located boron cluster sites, respectively, were synthesized (e.g., 18). These water soluble dendrimers and their precursors were characterized by 13C-, and nB-NMR spectroscopy (Fig. 8). 

The production of highly isotactic PPs with Zr- and Hf-FI catalysts//-Bu3Al/ Ph3CB(C6F5)4 (phenoxy-amine complexes site-controlled polymerization with 1,2-insertion) is in sharp contrast to that of highly syndiotactic PPs with Ti-FI cata-lysts/MAO (phenoxy-imine complexes chain-end controlled polymerization with 2,1-insertion), which will be described later [64]. 

To cast the equations in general terms, we use the label Ap to represent each type of surface site. In the case of hydrous ferric oxide, there are two such entries, >(w)FeOH and >(s)FeOH. There are Mp total moles of each site type in the system, divided between uncomplexed and complexed sites. This value is the product of the mass (in moles) of the sorbing mineral and the site density (moles of sites per mole of mineral) for each site type. 

Label Aq represents each possible surface complex, including protonated and deprotonated sites (e.g., >(w)FeO- and >(w)FeOH ) and complexes with cations and anions (>(w)FeOZn+ and >(w)FeP04-, for example). The molalities of the uncomplexed and complexed sites, respectively, are mp and mq. 

SN NMR spectroscopy is still underexploited in structural analysis of nitrogen-containing heterocycles. It can however be a powerful tool, for instance in tautomerism studies <2002JP2126>. Some 1SN NMR investigations of lanthanide induced shift have been performed on bicycles 50-52, showing that the complexing site of these 4,5-dihydro-l/f-l,2,3-triazoles is located on N-3. 

In the simplest case, it can be assumed that the water exchange reaction with a complexing site of a heterogeneous ligand will be governed by the Eigen-Wilkens mechanism as for complexes with simple ligands. In this case, equations (26) and (29) can be combined to yield ... 

Equation (5.17) is more useful for this system than for most others, because for isobutene there is only one complexing site so that W = 1, and (5.17) takes the form... 

Any carbenium ions which are not paired have both their complexation sites occupied by the most polar or polarisable species available, which can be the solvent or the monomer, according to their relative polarities, polarisabilities, and concentrations for paired cations, the picture applies to their other, still vacant, site. Such a situation will generally prevail in nonpolar solvents because in these the concentration of paired cations is dominant. In a polar solvent, both sites at an unpaired cation can be occupied by solvent, or one by solvent and one by monomer, or both by monomer. In the radiation polymerisations, one sees clearly that as the monomer concentration is reduced from bulk monomer, the kinetics change and they eventually become first order in monomer, whatever the solvent the critical monomer concentration at which this happens depends on the polarity of the solvent [12]. 

Qy-boronate derivatives have the ability to form covalent bonds with d,v-diol sugars at alkaline pH. Unlike lectins, which typically bind only the terminal sugar in a complex carbohydrate, boronates can bind any physically accessible CM-diol on a glycoprotein.23-25 When hydrophobic boronate derivatives are used, the hydrophobicity of each complexation site is enhanced. To the extent that the protein components of a sample are differentially glycosylated, this can provide a tool for exploiting those differences. 

The ligand may act not only via its complexation site, but also by separating the bound substrate from the external medium and thus acting as a layer characterized by certain features ... 

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