Ionic state of tetrahedral intermediates

Imidate salts hydrolysis, 118-144 syn and anti, 120 isomerization, 142 B-lactam, 142 Iminium salts, 211-221 Imino-ethers, 147 lodolactonization, 169 Ionic state of tetrahedral intermediates, 65, 105-106, 119 Ionophore A-23187, 13 Isochromane-3-one, 70 Isocyanate, 300 Isonitrile, 296... 

The ionic state of the hemi-orthoamide tetrahedral intermediate must also be considered (9-12). In acidic medium, the intermediate will exist in... 

The ionic state of the hemi-orthoamide tetrahedral intermediate must also be considered (9-12). In acidic medium, the intermediate will exist in the protonated form T, in slightly basic medium (near the pKa of T°, pH = 10), it will exist as a mixture of T° and T-, and in basic medium (pH > 11), it will exist as T. ... 

Since 1971, he has developed a new stereoelectronic theory in which the precise conformation of the tetrahedral intermediate plays a major role. In other words, the stereochemistry and the ionic state of the tetrahedral intermediate, the orientation of nonbonded electron pairs, and the relative energy barriers for cleavage and for molecular rotation are the key parameters in the stereoelectronically controlled cleavage of the tetrahedral intermediate formed in the hydrolysis of an ester or an amide. He postulated that the precise conformation of the tetrahedral intermediate is transmitted into the product of the reaction and that the specific decomposition of such an inter-... 

The cis-trans isomerization of PtCl2(Bu P)2 and similar Pd complexes, where the isomerization is immeasurably slow in the absence of an excess of phosphine, is very fast when free phosphine is present. The isomerization doubtless proceeds by pseudorotation of the 5-coordinate state. In this case an ionic mechanism is unlikely, since polar solvents actually slow the reaction. Similar palladium complexes establish cis/trans equilibrium mixtures rapidly. Halide ligand substitution reactions usually follow an associative mechanism with tbp intermediates. Photochemical isomerizations, on the other hand, appear to proceed through tetrahedral intermediates. 

Most transition states involve charged intermediates, which are stabilized within the active site of an enzyme via ionic bonds in pockets or holes bearing a matching opposite charge. Such charges are derived from acidic or basic amino acid side chains (such as Lys, Arg, Asp, or Glu) ° or are provided by (Lewis acid-type) metal ions, typically Zn +. Computer simulations studies suggested that in enzymes electrostatic effects provide the largest contribution to catalysis [107]. As a prominent example, the tetrahedral intermediate of carboxyl ester hydrolysis is stabilized in serine hydrolases by the so-called oxyanion hole (Scheme 2.1). 

Tetrahedral Anions. The hydrolysis of the [BHJ anion in aqueous DMSO has the kinetic form for a general-acid-catalysed reaction, but with the catalytic coefficients and k-s.+ very different to the values observed for aqueous media. This difference is ascribed to the inability of DMSO to act as a proton relay system. A series of molecular orbital calculations suggest that BH5 could be a metastable intermediate in the hydrolysis of [BH They indicate the presence of identifiable BH3 and Hg subunits in the structure. The authors conclude, however, that limitations in the theoretical methods used do not yet allow experimental and computational results to be reconciled (at least to within a few kcal). The acid-cat ysed ammonolysis in liquid ammonia is first-order in both [NHJ+ and [BHJ, but is appreciably slower than the corresponding reaction in water the rate is markedly decreased by increases in ionic strength. This may be due to the participation of a water molecule in the latter case. The rates of reduction of a variety of substrates by [BHJ have been reported, and for the reduction of ketones it is suggested on the basis of H/T isotope effects that a four-centre transition state is involved. 

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