Hydroxyl groups geminal

As noted earlier, three types of functional silanol groups exist on the silica surface isolated hydroxyl groups, geminal groups (two -OH groups on one Si atom), and vicinal groups (see Figure 9.6). The silanization reaction is illustrated in Eq. (9.5). 

Fig. 10.1 S ingly coordinated, doubly coordinated, triply coordinated and geminal surface hydroxyl groups on iron oxides.

From the study of a microbially mediated oxidation of arteether 28b, sufficient quantities of 7a-hydroxy 180 and 15-hydroxy derivatives 182 were obtained to employ them as intermediates for the preparation of fluorinated compounds. The hydroxyl groups were oxidized to the corresponding aldehyde 187, or ketone 188, with catalytic quantities of tetra- -propylammonium perruthenate (TPAP) in the presence of excess iV-methylmorpholine A -oxide. On reaction with DAST, 187 and 188 were converted into the corresponding geminal difluoro derivatives, 189 (63%) and 190 (42%). In addition to 190, a monofluoro olefin 191 was obtained in 25% yield from 188 on reaction with DAST <1995JME4120>. 

By contrast, the keto forms of the ketoses seem to be hydrated to only a slight extent, if at all. When the proportion of the keto form is 5% or more, as it is for the 1-deoxyhexuloses, the hydrated form should be readily detectable ip the -n.m.r. spectrum, but it has not been found. A model compound, l-deoxy-3,4,5,6-tetra-0-methyl-D-fructose, shows only the signals of the keto form in aqueous solution.16 Ketones are, on the whole, hydrated to a lesser extent than aldehydes77 in the case of ketoses having more than four carbon atoms in the chain, there would also be a 1,3-parallel interaction of one of the geminal hydroxyl groups with another hydroxyl group. 

This behavior is rather typical of gem-diols (gem = geminal, that is, with both hydroxyl groups on the same carbon atom). The few gem-diols of this kind that can be isolated are those that carry strongly electron-attracting substituents such as the following ... 

Silicagel is also called silica or bare silica. Its adsorptive properties depend on the hydroxyl groups attached to surface silicon atoms. Silicagel has a maximum silanol density of 8.0 yumolcs/ m2. Many of these silanols are buried deep in the porous structure and are available only to the smallest analytes. Silanols are either isolated, geminal, or vicinal they can be distinguished by means of Si solid-state nuclear magnetic resonance (NMR). The surface also contains siloxane bonds (Si-O-Si), which are considered hydrophobic. 

According to the rule of Erlenmeyer, compounds of type 43a with two hydroxyl groups bound to the same carbon atom are not stable and prefer to transform into ketones. The very few organic geminal diols that are isolable are those which carry strongly electron-attracting substituents at the carbon atom. 

The hydroxyl group at the allylic position has a significant effect on the syn/anti methyl stereoselectivity [67,68] and the diastereoselectivity [63,64] of the photo-oxygenation ene reaction (see Sec. II.B). To assess the effect of the hydroxyl at the more remote homoallylic position, the reaction of O with the geminal dimethyl trisubstituted homoallylic alcohols (85, 86, 89) and the cis dis-ubstituted 90 was examined in nonpolar solvents [116], The regioselectivity trend was compared with that of the structurally similar trisubstituted alkenes (87, 88, 91) [105], The results are summarized in Table 12. 

The same confusion can be found in studies using NMR spectroscopy. Whereas 29Si-NMR spectra show a distinct band at -92 ppm, attributed to Si-atoms, carrying a geminal hydroxyl group,8 H-NMR seems not to be able to distinguish between isolated and geminal silanols.9... 

Even in studies, modelling the silica surface on a theoretical basis, no consensus can be found concerning the existence of geminal hydroxyl groups. 

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