Siloxane bond, cleavage

The extent of condensation that occurred during the experiment was simply equal to the number of silicons (no silicons are observed at early times), because all silicons were initially species. No Q species were observed during the course of these experiments. Cyclic trimers consisting of silicon atoms are expected to resonate in the region of the spectrum (12). However, because low H20/[Si-OR] ratios were used and only the early stages of the reaction were examined, no evidence of cyclic trimers was observed. Moreover, cyclic trimers could not form without siloxane bond cleavage and, consequently, would not be expected be formed in these experiments (12). Cyclic tetramers have resonances in the region (12) and were observed in these experiments. 

Cationic polymerization of D2 at room temperature initiated by EtsSiH + PhsC B(C6Fs)4 proceeds fast and chemoselectively with exclusive siloxane bond cleavage and reformation. Taking into account the similarity of cationic polymerization of D2 to that of D4, this reaction is a good model for study of the role of cyclic trisilyloxonium ions in the cationic polymerization of cyclosiloxanes. 

M2D 2. In agreement with a first-order concentration decrease and the determination of hexamethyldisiloxane only in toluene is siloxane bond cleavage the prefered reaction under both acidic and basic conditions. 

Also for the cyclotetrasiloxanediol D3D ° 2 the siloxane bond cleavage is dominating, at least at the low starting concentration due to the limited water solubiliy. 

Characteristically, the activity of the same reagents in the reactions of siloxane bond cleavage changes in the reverse order. 

Chain transfer to another chain involving siloxane bond cleavage leads to chain scrambling. It is schematically depicted by equation 2 (6,23). 

The mechanism of anionic polymerization of cyclosiloxanes has been the subject of several studies (96,97). The first kinetic analysis in this area was carried out in the early 1950s (98). In the general scheme of this process, the propagation/depropagation step involves the nucleophilic attack of the silanolate anion on the sUicon, which results in the cleavage of the siloxane bond and formation of the new silanolate active center (eq. 17). 

Cleavage of the siloxane bond and rearrangement of polysiloxanes may be accompanied by either elimination of XSiYY Y" monomers and, less frequently, pcSiYY%0 dimers. 

Upon electron impact of different energy, oligoforganylsilsesquioxanes) may polymerize with either abstraction of hydrocarbon radicals (C—C bond rupture) and formation of ladder-type molecules cross-linked by alkylene bridges or cleavage of siloxane bonds and formation of polymers of irregular structure... 

M. G. Voronkov and S. V. Basenko, Heterolytic Cleavage Reactions of the Siloxane Bond, Soviet Scientific Reviews (Ed. M. E. Vol pin), Vol. 15, Part 1, Harwood Academic Publishers, London, 1990, pp. 1-83. 

In this variant, the ipso substitution of 68 led to the cyclohexadienyl radical 69 which underwent C-P bond cleavage and expulsion of the tether to give biphenyl 70. A comparable mechanism is operative for the aryl transfer from silicon to carbon in which a comparable biphenyl 71 (c.f. 70) was synthesized from siloxane 72 (Scheme 26) [65, 133]. 

The Si—Cl bonds in the cyclic dichlorosiloxane 8 react with KOH without causing cleavage of the siloxane bonds in the ring, and so direct substitution gives a 63% yield of 9 (equation 4)43. 

Attempts from Wagner et al. to implement the same strategy [6], including hydrosilylation of an acetyl-protected-propargyl-carbohydrate followed by deprotection failed. They did not success in the deprotection of the OH groups without cleavage of the siloxane bonds. 

---