Nonhydrolytic condensation reactions

In this manner, both /3- and a-D-xylosyl fluoride generate a-D-xylose, in the former instance by stereochemical inversion and in the latter by stereochemical retention. The series of events is reflected in each of the inverting glycosidases listed above. The reactions are unique not only because they involve hydrolysis of both (albeit truncated) substrate anomers, but the enzymes also catalyze an uncommon nonhydrolytic condensation reaction using the activated glycosyl fluoride. 

Figure 25.8 Nonhydrolytic condensation reactions toward mixed metal oxides.

IMP does not accumulate in the cell but is converted to AMP, GMP, and the corresponding diphosphates and triphosphates. The two steps of the pathway from IMP to AMP (fig. 23.11) are typical reactions by which the amino group from aspartate is introduced into a product. The 6-hydroxyl group of IMP (tautomeric with the 6-keto group) is first displaced by the amino of aspartate to give adenylosuccinate, and the latter is then cleaved nonhydrolytically by adenylosuccinate lyase to yield fumarate and AMP. In the condensation of aspartate with IMP, cleavage of GTP to GDP and phosphate provides energy to drive the reaction. 

This method is closely related to the nonhydrolytic sol-gel method. For example, titania is prepared by etherolysis/condensation of TiCl4 by diisopropyl ether (Equation 2.4) or by direct condensation between TiCl4 and Ti(0- Pr)4 (Equation 2.5). Detailed chemistry of the reaction was examined by means of nuclear magnetic resonance (NMR), and it has been reported that the tme precursors are titanium chloroisopropoxides in equilibrium through fast ligand exchange reactions. A variety of metal oxides, " nomnetal oxides," multicomponent oxides" " were studied, and the nonhydrolytic sol-gel method was surveyed by Vioux and Leclercq. ... 

Y5 Al(0-z -Pr)4 3 on condensation with 3 mol of Al(0-i-Pr)3, whereas the mother liquor of the reaction mixture could yield the expected Y Al(0-/-Pr)4 3 in fairly high (— 70%) yield. Although similar comments were made initially by Mehrotra (479), many of these oxo-alkoxides were formed by a variety of conventional routes, which were expected to yield the normal alkoxides. This number has been increasing so fast in spite of the most stringent anhydrous conditions employed by the investigators, that concerted efforts are being made to suggest novel pathways by which such oxo-alkoxide species could result under nonhydrolytic conditions as well. 

Both stages are controlled by condensation chemistry that can include, as a first step, hydrolysis of hydrated metal ions or metal alkoxide molecules [6, 7] (hydrolytic sol-gel processing). The condensation chemistry in this case is based on olation/oxolation reactions between hydroxylated species. The hydroxylated species for further condensation can be formed also by a non-hydrolytic route, that is, by reactions between metal chlorides and alcohols with electron-donor substituents [8]. The nonhydrolytic sol-gel processing may... 

Another class of reactions leading to polycondensation of oxygenmetal precursors is the so-caUed aprotic condensation that proceeds during nonhydrolytic sol-gel processing where water is not required for precursor activation and not produced by condensation [9, 10]. It includes reactions between alkoxides of dissimilar metals differing in the polarity of M—O bonds, or of alkoxides with metal esters and metal chlorides. The condensation rate is strongly dependent on the OR/M ratio in mixed precursors, as well as on temperature, the presence of catalyst and structure of the R derivative. 

Noteworthy features of this method are that nonhydrated oxides without residual hydroxo groups are obtained, due to the aprotic conditions, and that in bimetallic systems the metals M and M have an alternate order (no phase separation), due to the reaction mechanism (Eq. (1.7)). A limitation of nonhydrolytic processes is that the M/M ratio is not freely selectable if fully condensed products are targeted. Eor this reason, sol-gel processes of mixed metal systems are sometimes initiated by nonhydrolytic reactions (to obtain a high homogeneity) and then completed by hydrolytic reactions (to obtain complete hydrolysis and condensation). 

Nonhydrolytic sol-gel routes are not necessarily completely water-free. Although the initial reaction mixture might be anhydrous, specific organic reactions are able to produce water in situ (e.g., aldol condensation and esterification reactions), rendering the system in principle hydrolytic. Therefore, nonhydrolytic sol-gel processes are often also called nonaqueous. But even in the absence of water, it is possible to have hydroxylation reactions. 

In contrast to conventional sol-gel processes, which are based on the hydrolysis and condensation of metal precursors in an aqueous alcohol-mixed solvent, the nonhydrolytic sol-gel reaction proceeds by reactions of the metal precursors in organic media [16]. This nonhydrolytic sol-gel process has unique advantages... 

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