Configurational exclusion

A rare species of salts consisting of a heteroatom-stabilized carbocation and a heteroatom-stabibzed carbanion has been formed by deprotonating methyl (Z)- or (E)-3-hydroxy-2,3-dimesitylpropenoate with tetrakis(dimethylamino)methane the resonance stabilization of the cation [(CH3)2N]3C+ and enolate anion, which is of E-configuration exclusively, since the guanadinium ion is incapable of forming a chelate, prevents a spontaneous O- or C-alkylation.12... 

In addition, during our studies on the MBH reaction of a-keto esters and cyclic enones, we found that the catalysts have a significant effect on the MBH adduct. The MBH reaction of a-keto esters with cyclopent-2-enone catalyzed by diphenylmethylphosphine provided the corresponding MBH adducts 155 in higher yields in the presence of p-nitrophenol, whereas the similar reaction of a-keto esters with cyclopent-2-enone catalyzed by tertiary-amine of DBU furnished the corresponding aldol adducts 156 with n-configuration exclusively (Scheme 1.67). " ... 

Tin(II) triflate has been used to catalyse the condensation between tetra-O-acetyl-a-D-glucopyranosyl bromide and primary and secondary alcohol groups on other monosaccharide derivatives. The products had the 3-configuration exclusively but the yields were modest (30-65%). In a related study the influences of catalyst, solvent, and temperature on the reaction of 0-(tetra-O-benzyl-a-D-glucopyranosyDtrichloroacetimidate with primary and secondary sugar alcohols were examined. With boron trifluoride in dichloro-methane at -I8—good yields (e.g. 80 for a secondary alcohol) and anomeric ratios (a,3, ca. 1 4) were obtained. 

The Morita-Baylis-Hillman reaction of imines with ynones in the presence of iodotrimethylsilane was efficiently catalyzed by ZrCU (Equation 15) [20]. Compared with the other Lewis acids, ZrCU showed much higher activity for the reaction. The newly formed olefin moiety has (E) configuration exclusively. 

The butadiene units in the copolymer prepared with a vanadium catalyst system have 1,4-trans configurations exclusively, but no information is available concerning the tacticity of the propylene inits. 

The both systems give the identical results testifying to the formation of equilibrium mixture of isomerization products (Table 1.4). Thermodynamic control is confirmed by repeated heating of the isomerization product (the same conditions), the structure of which does not change. The system t-BuOK/DMSO is more active owing to much better solubility of t-BuOK in DMSO in comparison with KOH, while the basicity of both systems is approximately the same (at equal concentration of the base in DMSO) [210]. Composition and structure of isomerizate components have been proved by H NMR technique. The values of spin-spin coupling constants ( /cp= 15.4-15.6 Hz) indicate that the alkenyl substituents are of fi-configuration exclusively. 

HNOa gives the corresponding L-methionine sulphoxide with retention of configuration (exclusive attack at the sulphoximide nitrogen is notable). ... 

The simplest many-electron wave function that satisfies the Exclusion Principle is a product of N different one-electron functions that have been antisymmetrized, or written as a determinant. Here, N is the number of electrons (or valence electrons) in the molecule. HyperChem uses this form of the wave function for most semi-empirical and ab initio calculations. Exceptions involve using the Configuration Interaction option (see page 119). HyperChem computes one-electron functions, termed molecular spin orbitals, by relatively simple integration and summation calculations. The many-electron wave function, which has N terms (the number of terms in the determinant), never needs to be evaluated. 

Ug2/)) (n 2p) (Tig 2p) ground electron configuration. Photolysis by radiation from the sun, at wavelengths below about 310 nm, produces O2 almost exclusively in the a state whereas a wavelength as low as 193nm produces about 23% of O2 in the b state. 

Many different configurations of diffusion flames exist in practice (Fig. 4). Laminar jets of fuel and oxidant are the simplest and most well understood diffusion flames. They have been studied exclusively in the laboratory, although a complete description of both the transport and chemical processes does not yet exist (2). 

Lithium-ammonia reductions of most steroidal enones of interest create one or two new asymmetric centers. Such reductions are found to be highly stereoselective and this stereoselectivity constitutes the great utility of the reaction. For conjugated enones of the normal steroid series, the thermodynamically most stable products are formed predominantly and perhaps exclusively. Thus the following configurations are favored 5a, 8/ , 9a, and in certain cases 14a (see page 35). Starr has listed numerous examples illustrating these facts and Smith " and Barton have tabulated similar data. 

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