Application to organosilanes with hydroxyl functions

The oxidation can also be performed in the presence of a free hydroxyl group. However :
* The free hydroxyl function interfered with the course of the sila-Pummerer rearrangement.
* This can be circumvented using :
- the oxidation of the thioether function to the corresponding sulfone 8 [11].
- the sulfone can then be subjected directly to the Tamao oxidation, affording the desired diol 10 in good yield, along with the cyclopropylsulfone 9 (Scheme 5).
This approach provides an extension to the concept illustrated in scheme 1 and shows that oxidations of DMPTCS do not require the protection of alcohol functions present on the substrate.

A further illustration that phenylsulfonylcyclopropyl is an excellent nucleofugal group on silicon [12] is provided by the sequence shown in Scheme 6.
The silane 12 was reacted with the alpha-diazoester 11 in the presence of Rh2(OAc)4 to give 13 [13]. Reduction of the ester function with LiAlH4 gave the unstable ß-hydroxysilane which was directly oxidized using the same conditions as above to afford the diol 14 in reasonable overall yield.

Polyhydroxylated silanes can also be obtained through Sharpless dihydroxylation [14]. For example, allylsilane 15 affords the desired diol which was purified as its acetonide 16. This latter is then oxidized as described above to afford the triol acetonide 17 [15] in 70% yield (Scheme 7).

In parallel, oxidation of DMPTCS into the sulfone [11], as before, followed by oxidation of the C-Si bond, affords 18 in good yield along with the recovered sulfone 9 (Scheme 8).
Protection of the diol is not required when the sulfone route is employed.

The C-Si bond oxidation can therefore be carried out equally well through the sulfoxide or the sulfone route, and in both cases in high yields.