Silanols highly sterically hindered

Treatment of highly sterically hindered iodosilanes with a silver salt in a solvent containing water has also been used successfully to prepare bulky silanols (Scheme 6). The introduction of the OH group seems to occur not only by hydrolysis of the silyl perchlorates formed, but also by reaction of the bridged silicocationic species that are formed as intermediates in this type of reaction (62). The reaction... 

Scheme 5. Preparative routes to highly sterically hindered silanols.

C. Compounds Containing One Si-OH Group 1. Highly Sterically Hindered Silanols... 

Some highly sterically hindered silanols such as tetracyclohexyldisilane-1,2-diol (139) and tris(trimethylsilyl)silanol (59) are readily obtainable and resistant to dehydration. 

Another approach to improve the stability of bonded phases at acidic pH is the nse of sterically hindered silanes, which use, for example, isopropyl groups as side chains, instead of methyl groups. Due to the somewhat lower surface coverage, this technique leaves a larger amount of silanols on the snrface than a high-qnality bonding with a dimethylsi-lane, but this is balanced against the definite improvement in hydrolytic stability at acidic pH. 

Concluding, the effect of the substrate structure on chemisorption is controlled by three parameters the number of surface silanols, the presence of strained siloxanes and the average pore size. Chemical bonding in dry conditions is localized on the surface silanols. Only hydroxyl-specific adsorbed silane molecules are chemically bonded during curing. On dehydroxylated silica a anomalous high level of physisorption is found, probably due to interaction with strained siloxanes. In pores smaller than 4.0 nm, the reaction with APTS is sterically hindered. 

These observations are easily explained by another simple reaction mechanism, nucleophilic substitution of an alkoxide on silicon (12). In this case, the basic alkoxide oxygens tend to repel the nucleophile, OH, and the bulkier alkyl groups tend to crowd it. Therefore, more highly hydrolyzed silicons are more prone to attack. Because this mechanism would have a pentacoordinated silicon atom in the activated complex, hydrolysis of a polymer would be more sterically hindered than hydrolysis of a monomer. Reesterification would be much more difficult in alkaline solution than in acidic solution, because silanols are more acidic than the hydroxyl protons of alcohols and would be deprotonated and negatively charged at a pH lower than the point at which the nucleophile concentration becomes significant (ii). Thus, although hydrolysis in alkaline solution is slow, it still tends to be complete and irreversible, if extensive polymerization does not occur first. 

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