Stereochemical environment

This example illustrates a subtle control of a chemical reaction by a delicate manipulation of tire stereochemical environment around a metal centre dictated by tire selection of tire ligands. This example hints at tire subtlety of nature s catalysts, tire enzymes, which are also typically stereochemically selective. Chiral catalysis is important in biology and in tire manufacture of chemicals to regulate biological functions, i.e., phannaceuticals. 

Similar studies have been conducted on poly(vinyl chloride) (PVC) to assign different IR signatures obtained from different stereo-configurational isomers. The sensitivity of the vC-Cl bond on the stereochemical environment has been utilized using IR spectroscopy. The characteristic vibrations of the vC-Cl bonds are inherently tied in to the configuration as well as the conformation of the... 

Magnesium(II) is a milder Lewis acid than traditionally used ones such as boron(III), alumi-num(III), or titanium(IV). A characteristic feature of Mg11 is the presence of coordination sites which are occupied by Lewis bases other than counter anions. By using bidentate Lewis-basic ligands, it is therefore possible to form rigid stereochemical environments. 

As demonstrated in the preceding Section III.A, configurations of not only the C-H bond of the alkoxide to be inserted but also the carbenoid center present is evidence for the insertion mechanism. In this regard, norcaranylidene carbenoid30 has an unsymmetrical stereochemical environment similar to those of alkylidenemethylene carbenes (Scheme 20) and, therefore, it is expected to be informative of the insertion mechanism. 

While the enzymes involved in detoxication processes are nonspecific in the classical sense of intermediary metabolism, they often have distinct specificities both for organic functional groups and for the electronic, steric, and stereochemical environments where these functional groups are located. Enzyme specificity based on organic functional groups and their environments leads to a wide diversity in the alkaloid substrates possible and therefore the products obtained from biotransformation. This section of the chapter will concentrate principally on the enzymes themselves, including general concepts of substrate specificity and mechanism. 

Hathaway has attempted to assess the value of the electronic properties of polycrystalline mononuclear copper complexes with the [CUN4], [CuNj], and [CuN ] chromophores of unknown crystal structure, in predicting the stereochemical environment of the copper(ii) atom. The value of having B. J. Hathaway, J.C.5. Dalton, 1972, 1196. 

This gap was filled by the development of NMR spectroscopy. It is Bovey s merit to have demonstrated the potentiality of this technique in the macromo-lecular field and, in particular, its sensitivity to the stereochemical environment. The applications of this technique, the methods of investigation, and the type of problem that can be solved are closely connected with the progress made in NMR instmmentation in the last quarter century. From low magnetic field spec-... 

The first antibody-catalyzed asymmetric 1,3-dipolar cycloaddition was reported recently by Janda and co-workers (382). The reaction of the relatively stable nitrile oxide 280 and dimethyl acrylamide 281 was catalyzed by antibody 29G12 having turnover numbers >50, and the product 282 was obtained in up to >98% ee (Scheme 12.89). The antibody 29G12 was formed for hapten 283 and coupled to a carrier protein by standard protocols. The hapten 283 contains no chiral center and therefore the immune system elicited a stereochemical environment capable of stabilizing the enantiomeric transition state leading to 282. 

So far, OM1 is parameterized for C, H, N, O and F, and leads to a substantial improvement in the calculation of excitation energies OM2 is parameterized for C, H, N, and O, and provides a distinct improvement with regard to the stereochemical environment of electron pair bonds, which is important for modeling conformational properties (Thiel, 2000). Recent comparative analyses of the performance of various NDDO-based schemes include also results for OM1 and OM2 (Thiel, 1998 2000), and there are also other routes of improving the NDDO scheme (Repasky et al 2002a, 2002b). 

Direct analytical applications of ehiroptical methods over the last 15 to 20 years, and most particularly with ORD and CD detection, have clearly demonstrated that a priori suspicions of serious problems from interferences are ill-founded, that sensitivities similar to those for absorption spectrophotometry are easily accessable, and, most important of all, that a much higher level of selectivity is obtained because of that very same property that makes CD a useful structural tool (namely the sensitivity of the chromophore to its stereochemical environment). 

Murakami et al. have utilized Mayer vesides to study aldolase-type reactions [48]. Formation of [i-phenylserinc from glydne and benzaldehyde proceeded effectively by cooperative catalysis of a hydrophobic pyridoxal derivative (47) and Zn(n) ions in the bilayer vesicle formed with 32. The threo isomer was dominantly produced over the erythro form. A marked enantioselectivity was observed in the co-veside of 32 and 35 in combination with 47 and Cu(ii) the ee for formation of (2S,3R)-P-phcnylscrinc over its enantiomeric (2R,3S)-isomer was 58%. Enantioselectivity also arose with another bilayer assembly, formed with 32, 35, and 37 in the presence of Cu(ii), where the (2R,3S) isomer was dominant over the (2S,3R) species in 13% ee. The opposite enantioselectivity performed by the second system, as compared with that for 47, might reflect a different stereochemical environment around the quinoid intermediate that allows the attack of benzaldehyde. 

Infrared spectroscopic studies regarding the adsorption of pyridine on both anatase and rutile have been reported (136, 176, 194, 216,217). Hydrogen-bonded pyridine is readily desorbed on pumping at room temperature, whereas pyridine held by coordinatively unsaturated Ti4+ ions is thermally stable up to approximately 400°C. As ammonia, pyridine forms two distinct coordinately held species (176, 217) indicating the existence of two types of Lewis acid sites, which should correspond to Ti4+ ions in different stereochemical environments. According to Primet et al. (176), the more stable species is chemisorbed on type... 

The p.m.r. spectra of simple derivatives of D-glucopyranose are often complex, because of the similar stereochemical environments of the ring protons. Thus, the 220-MHz spectra of the enamine derivatives 9 and 10, formed by condensation of 2-amino-2-deoxy-D-... 

Their properties can be further improved based on the coordination chemistry strategy for supramolecular systems (31, 38, 39, 46, 56, 116, 191, 262-265). For example, one can exploit the electron donor-acceptor and/or ET properties of the ancillary complexes or make use of them as cofactors, creating new pathways for the oxidation of organic substrate (183, 266, 267). In particular, transition metal complexes connected to the periphery of we.so-pyridylporphyrins (268, 269) can modify the stereochemical environment or even modulate the electronic characteristics of the metalloporphyrin center. This aspect was explored in the case of the Mn(4-TRPyP) and Mn(3-TRPyP) systems, because of their contrasting stereochemistry (Fig. 27). 

Similar procedures are also successful in the construction of cyclobutane rings in other stereochemical environments, including the formation of bicyclo[3.1.1]heptane (67)and bridged tricyclic systems such as (68), (69) ° and (70). ° A variety of annelated cyclobutanes have been prepared by the photochemical procedure as above, yields are sometimes poor or only moderate. Examples include bicy-clo[2.1.0]pentanes (71), °° bicyclo[2.2.0]hexanes (72)"° and cyclobuteno aromatics (e.g, 73), " o-nor-steroids e.g. 74)," A-bisnorsteroids" and triterpenes" as well as the highly strained tricyclo[4.2.0.0 ]octane system (75)," and the [4.4.4.5]fenestrane (76)." ... 

Fox and Soiia-Ruiz [9] found that the decomposition rate may be influenced by the mobility of dislocations. They concluded that the increased reactivity in the vicinity of a dislocation arises ifom changes in the stereochemical environment of the ions concerned and not from the strain field associated with the distorted lattice at the dislocation core. Impurities may also be preferentially incorporated in the disorganized regions of the lattice [10] and exert some influence on reactions occurring there. 

FRAU features were applied to classify and predict reagent functions and to distinguish 3D stereochemical environments of atoms. 

Satoh, H. (2007) Numerical representation of three-dimensional stereochemical environments using FRAU-descriptors. Croat. Chem. Acta, 80,217—225. 

The stereospecific polymerization of ot-olefins is one of the best examples which illustrate the possible applications of high resolution NMR to the determination of reaction mechanisms. As a matter of fact, the stereoregular structure of the reaction products and the sensitivity of chemical shifts to stereochemical environments make it possible to obtain considerable and important information about very subtle details of the reaction mechanism. This is especially true when NMR is used in conjunction with isotopic substitution. 

The mechanism by which benzene solvent molecules are able to produce differential solvent shifts in the proton resonances of the same molecule has been widely interpreted in terms of a benzene-solute collision complex. The term collision complex is meant to imply a short-lived orientation of the benzene molecule(s) that has been brought about by dipole-induced dipole, dipole—quadrupole, or other weak chemical association. Any mechanism that is proposed for benzene-induced solvent shifts should take into account the nature, stoicheiometry, strength and time-averaged stereochemical environment of the interaction and we shall deal with each of these points in turn. 

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