Borate bond stability

FIGURE 5.3 Borate minerals stabilize ribose under conditions in which it is formed from glycolaldehyde and glyceraldehyde. Wedges are used to describe bonds that project above and below the plane of the paper. 

Although a number of factors affect structural evolution in sol-gel processing, many of the observed trends can be explained on the basis of the stability of the M-O-M bond in its synthesis environment. Structures which form in solution are the products of a successive series of hydrolysis, condensation and restructuring reactions. M-0-M bonds which are unstable with respect to hydrolysis and alcoholysis are absent. In borates this criterion precludes gelation for systems with insufficient fractions of 4-coordinated borons. In silicates... 

The intrinsic inertness of the peptide bond is demonstrated by a study of the chemical hydrolysis of N-benzoyl-Gly-Phe (hippurylphenylalanine, 6.37) [67], a reference substrate for carboxypeptidase A (EC 3.4.17.1). In pH 9 borate buffer at 25°, the first-order rate constant for hydrolysis of the peptide bond ( chem) was 1-3 x 10-10 s-1, corresponding to a tm value of 168 y. This is a very slow reaction indeed, confirming the intrinsic stability of the peptide bond. Because the analytical method used was based on monitoring the released phenylalanine, no information is available on the competitive hydrolysis of the amide bond to liberate benzoic acid. 

Diazomethylene)phosphoranes 33 (Scheme 8.10), which represent another type of diazocumulenes (12) are easily obtained by the oxidative ylidation of the corresponding phosphanyl(trimethylsilyl)diazomethane with CCI4. The increased stability of these compounds as compared with diazocumulenes (R2C=C=N2) is probably due to the ylidic character of the P=C bond. These diazo compounds exhibit the expected dipolar reactivity toward electron-deficient alkenes, alkynes, phosphaalkenes, and heterocumulenes (12). Thus, 33 reacts with TCNE to form A -pyrazoline 35 (60). Furthermore, 33 could be converted into the phosphonio-borate-substituted diazo compound 34, which underwent subsequent cycloaddition with electron-deficient alkenes (e.g., 34 36) (61). 

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