Silanol: definition isolated

On the basis of available experimental data, it is impossible to choose a definite pathway of elimination of silanol. However, study of silylation of methyl P -nitropropionate (411) with BSA in the presence of trapping agents rigorously proved that silyl nitronate D is initially formed. This compound can be detected in the [3 + 2]-cycloaddition reaction with methyl acrylate product (413). If silylation of AN (411) is performed in the presence of ethyl vinyl ether, a-nitrosoalkene E can be successfully trapped in as heterodiene a Diels-Alder reaction. Dihydroox-azine (414) is formed, and its silylation affords isolable product (415). 

The model by Trombetta and Busca (vide supra) is probably to be discarded. In the first instance. Fig. 6 provides evidence that, at least in some cases, silanols on H-[A1]-MTS systems are not more acidic than on other silica systems. Moreover, the model shows inconsistencies from the energetic point of view. The interaction between the isolated silanol and the A1 center is assumed to be repulsive. This is reminiscent of the case of boron-zeoUtes [95], in which the tetrahedral heteroatom is boron, having a definite propensity towards trigonal coordination. Indeed, interaction with the base molecule makes the boron atom to assume tetrahedral coordination. This is hardly acceptable for Al, which favors either tetrahedral or octahedral coordination and assumes trigonal coordination only exceptionally. 

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