Silicon pentacovalent intermediate

Evidence that the 1Sn2 reaction at silicon does indeed go through a pentacovalent intermediate comes from the silicon analogue of the migration step in hydroboration-oxidation. Treatment of reactive organosilanes (that is, those with at least one heteroatom—F, OR, NR2—attached to silicon to encourage nucleophilic attack of hydroperoxide at silicon)... 

The immediate product is presumably the epoxide 213 which opens in acetal fashion to give 214 and transfers silicon intramolecularly to give 212. The reaction 214 is formally a 5-endo-tet reaction and would not occur if carbon were being transferred. As silicon is the atom under attack, a pentacovalent intermediate can be formed and the requirement for a linear SN2 transition state no longer applies. 

The formation of a pentacovalent silicon intermediate has been advanced frequently as the explanation of the ease of solvolysis of the silicon-halogen bond. Rochow, for example, (58) places considerable... 

This process is sometimes abbreviated to S f2 at silicon to save space. The intermediate is a trigonal bipyramid with negatively charged pentacovalent silicon. It is often omitted in drawings because it is formed slowly and decomposes quickly. This mechanism is similar to nucleophilic substitution at boron except that the intermediate is pentacovalent (Si) rather than tetrahedral (B). The hydrolysis of a boron ester at the end of a hydroboration-oxidation sequence would be an example of an analogous boron reaction. 

As displayed above there is an anomerization of 44 to 49 due to trans-(2 1)-0- (44 46) and ensuring m-(2—>l)-0- (47- 49) silyl group migration, presumably via pentacovalent silicon intermediates or transition states 45 and 48. 

The reaction (1) shows that fluorine severely catalyses hydrolysis reaction [3]. In the hydrolysis reaction, because of the smaller ionic radius of the fluorine, which approaches a molecule of TEOS in the solution forming a highly unstable pentacovalent activated intermediate. This complex rapidly decomposes, forming a partially fluorinated and hydroxylated silicon alkoxide. 

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