Selectivity steric

Steric Selectivity. In addition to the normal regularities that can be rationalized by electronic considerations, steric factors are important in coordination chemistry. To illustrate, 8-hydroxyquinoline, or 8-quinolinol (Hq) [148-24-3J, at 100°C precipitates both Mg " and AE" from aqueous solution as hydrated Mg(q)2 (formulated as Mg(q)2(H20)2 [56531 -18-1]) and as Al(q)3 [2085-33-8] respectively. 2-Meth54-8-hydroxyquinohne [826-81-3] (6),... 

Another example of steric selectivity involves the homopoly and heteropoly ions of molybdenum, tungsten, etc. Each molybdenum(VI) and tungsten(VI) ion is octahedraHy coordinated to six oxygen (0x0) ligands. Chromium (VT) is too small and forms only the weU-known chromate-type species having four 0x0 ligands. The abiUty of other cations to participate in stable heteropoly ion formation is also size related. 

The two techniques differ in that HDC employs a nonporous stationary phase. Separation is affected as a result of particles of different size sampling different velocities in the interstitial spaces. Size exclusion chromatography is accomplished by superimposing a steric selection mechanism which results from the use of a porous bed. The pore sizes may vary over a wide range and the separation occurs as a result of essentially the same processes present in the gel permeation chromatography of macromolecules. 

Normal-phase liquid chromatography is thus a steric-selective separation method. The molecular properties of steric isomers are not easily obtained and the molecular properties of optical isomers estimated by computational chemical calculation are the same. Therefore, the development of prediction methods for retention times in normal-phase liquid chromatography is difficult compared with reversed-phase liquid chromatography, where the hydrophobicity of the molecule is the predominant determinant of retention differences. When the molecular structure is known, the separation conditions in normal-phase LC can be estimated from Table 1.1, and from the solvent selectivity. A small-scale thin-layer liquid chromatographic separation is often a good tool to find a suitable eluent. When a silica gel column is used, the formation of a monolayer of water on the surface of the silica gel is an important technique. A water-saturated very non-polar solvent should be used as the base solvent, such as water-saturated w-hexane or isooctane. 

Steric selectivity in the C — H insertion process has been studied in some detail51. The cyclization of methyl (/ )-2-diazo-3-oxophenvlheptanoate, for instance, proceeds53 to give exclusively 1. 

The original objective in binding metal complexes to insoluble supports was undoubtedly to get over the solubility problems of homogeneous catalysts which made their separation after reaction such a problem. However, there have been other spin-off advantages. In particular the selectivity of the supported catalysts is often greater than their homogeneous counterparts because, in addition to the electronic and steric selectivity present in the free complex, further selectivity arises because the solvent channel leading up to the active site has both a size restriction and polar properties. This results in the diffusion rate of two... 

In the wide field of possible applications for MTS, the use of their properties of adsorption and steric selectivity is still to be explored. However, such applications require well-defined particles, especially spherical particles in the micrometric range. The synthesis of MSU-X silica that exhibits these shapes allowed us to test their properties in adsorption HPLC. Non polar solvent such as hexane are suitable to allow a significant separation. Further analyses and testing for size exclusion separation processes are under progress. 

The steric selectivity of this reagent is indicated by the fact that the A-nng double bond of progesterone remains intact, whereas the -iing double bond readily undergoes attach, as shown in Eq. (79). 

In situ reductive dechlorination has been cited as an important pathway for biochemical weathering of PCBs in sediments (2, 5). The reductive dechlorination hypothesis is based on the premise that microbial action on PCB congeners is sterically selective for the chlorines in the meta and para positions on the biphenyl rings (i.e., 3, 3, 4, 4, 5, and/or 5 ). Such selectivity would result in the cleaving of the chlorines in these positions (5). If this process occurs, the terminal dechlorination products of the microbially resistant ortho-substituted congeners in the anaerobic environment should be 2,2 -DCB, 2,6-DCB, 2,2, 6-TrCB, and 2,2, 6,6 -TCB. Thus, the weight... 

Fig. 9 Hexagon visualizing the desired properties of a C18-bonded silica column efficiency (1), symmetry (2), capacity (3), hydrophobicity (4), steric selectivity (5), metal shielding (5), and silanophilic properties (6). (Reproduced with permission from Merck, Darmstadt, Germany.)...

Polymeric phosphin-Ni complex also has a selectivity in its catalytic activity (154). Such a sterically selectivity is shown in an intrapolymer electrophilic reaction of Fe-carbonyl-olefin complex (155,156). 

Packed capillary columns with chirally selective stationary phases (e.g., flr acid glycoprotein), as well as wall-immobilized, CD-based stationary phases, have been successfully used in CE chromatographic separations. Also, macro-cylic crown ethers, forming sterically selective complexes with the guest molecule, have been used for the resolution of optically active amines. 

Activated carbon is commonly acknowledged to be very efficient as a VOC adsorbent. However, in the last decade, interests arose to develop new materials, such as alumni-silicate molecular sieve, towards operating conditions for which activated carbon was inappropriate due to its inflammability and adsorption capacity dependence on effluent relative humidity. Apart from hydrophobicity, the advantages of High Silica Zeolites (HSZ) are notably a thermal and chemical stability, a high steric selectivity and a complete regeneration at low temperatures [11]. Yet, for adsorption and separation processes development, organic compounds properties impact on adsorption and crystalline framework influence on selectivity are to be clarified and efficiently modeled. 

Enantiomeric separations of amino acids and short peptides are performed using either a direct or the indirect approach [10]. The indirect approach employs chiral reagents for diasteromer formation and their subsequent separation by various modes of CE. The direct approach uses a variety of chiral selectors that are incorporated into the electrolyte solution. Chiral selectors are optically pure compounds bearing at least one functional group with a chiral center (usually represented by an asymmetric carbon atom) which allows sterically selective interactions with the two enantiomers. Among others, cyclodextrins (CDs) are the... 

Microporous and, more recently, mesoporous solids comprise a class of materials with great relevance to catalysis (cf. Chapters 2 and 4). Because of the well-defined porous systems active sites can now be built in with molecular precision. The most important catalysts derived from these materials are the acid zeolites. The acid site is defined by the crystalline structure and exhibits great chemical and steric selectivities for catalytic conversions, such as fluid catalytic cracking and alkane isomerization (cf. Chapter 2). In Section 9.5 we discuss the synthesis of zeolites and, briefly, of mesoporous solids. 

The major physical properties affecting biological potency of a molecule are supposed 1 3) to be electronic, steric, and hydrophobic. Electronic properties influence the reactivities of the compounds and steric properties are clearly important in view of the steric selectivity of interactions between the compound and components of the biological system. Hydrophobic properties of a compound appear to affect its distribution throughout the body organs and tissues as well as the interaction of the compound with its target (receptor) in the biological system 12). 

This type of restricted, steric selectivity must therefore be introduced into the single-events methodology. Fortunately, a certain number of exclusion rules can be introduced at the reaction network generation step, to take into account this type of restriction. 

The complex formed by a Co(II) salt and NaBH4 (most likely a Co hydride species) reduces alkenes with high steric selectivity mono di tri- and tetrasubstituted olefins. Steric effects can best explain the observed selectivity. The reduction of limonene, 9, demonstrates the synthetic utility of this reducing agent ... 

In the same study, remarkable steric selectivity was demonstrated in the hy-drosilylation of dienes with varying substitution patterns [35]. Virtually complete selectivity was observed for the reaction of monosubstituted olefins in the presence of disubstituted alkenes (Eqs. 19,20) and for the silylation of a 1,1-di-substituted olefin in preference to a trisubstituted double bond (Eq. 21). 

Effective control of chemo and steric selectivity of a chemical reaction lies in our ability to manipulate nano-environment of active sites. Due to difiGculty in manipulating the active site on the nanoscale and lack of fxmdamental nderstanding of the reaction mechanism, development of chemo- and stereoselective catalysts has relied heavily upon empirical studies. One successful xample of fine-tuning steric environment of the active site is the use of chiral diphosphite ligands to control the selectivity of styrene hydroformylation on Rh omplex catalysts (1-3). 

An unusual excunple of steric selection has been demonstrated following the addition to. sulphur to racemic (9-(-)-menthyl phenylphosphinothioate (526) to give 0-(-)menthyl phenylpho-sphinodithioic acid, iodine oxidation then afforded... 

Electronics overcomes sterics-selective even for bulky substituents ... 

For the user of HPLC columns, the most important features are the chromatographic selectivities of the different packings. In the following, we will categorize reversed-phase packings using measures of their hydrophobicity, silanophilic interaction at pH 7, hydrogen-bond formation, and their steric selectivity. 

Another important feature of a packing is its steric selectivity. It reflects the ability of a packing to discriminate between compounds that differ only in shape. The best-documented examples are polycyclic aromatic hydrocarbons, but the same feature is also... 

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