The reaction between dimethyl sulfoxide and Me 2 SiCl 2 leads to the formation of cyclic siloxanes, principally the more highly strained six-membered ring hexamethylcyclotrisiloxane (D 3 ), via linear R,ω-dichlorosiloxanes. At short reaction times (∼15 min), the reaction was shown not to be undergoing equilibration reactions (ligand metathesis) to a significant degree. A mechanism for the formation of D 3 and D 4 (octamethylcyclotetrasiloxane) is proposed that invokes conversion of a chlorosilane group into a sulfonium ion intermediate 12. The preferential formation of D 3 over D 4 is attributed to the greater steric encumbrance of the activated chain termini (SiOS + Me 2 ) than in the corresponding hydrolysis reaction (SiOH). Both the chain extension reactions 12 f 3 and cyclization reactions 12 f D 3 are retarded, resulting in a higher selectivity (with DMSO as the oxygen source) for the intramolecular reaction producing D 3 than in the case of hydrolysis. The experimental results are inconsistent with silanone formation.

Under cationic conditions using triflic acid as the initiator, it is possible to oligomerize Ptrichlorosilylstyrene 2 to low molecular weight oligomers 14 with a maximum degree of polymerization of about 9. Termination of the process was shown to occur by an intramolecular FriedelLCrafts reaction, leading to highly functionalized, indane-terminated oligomers 9, 12, etc. At lower temperatures, the reaction is diastereoselective. The oligomerization process was shown to require electron-withdrawing groups on silicon; the replacement of C1 spectator ligands with alkoxy or alkyl groups led to protiodesilylation. The mechanism for formation of the indane-terminated oligomers is discussed.Key words: cationic polymerization, P-silylstyrene, stereoselective oligomerization, indene termination.RCsumC : Sous des conditions cationiques utilisant de l'acide triflique comme initiateur, il est possible de provoquer une oligomCrisation du P-trichlorosilylstyrkne, 2, en oligomkres 14 de bas poids molCculaire et comportant un degrC maximal de polymCrisation d'environ 9. On a montrC que le processus de terminaison se produit par une rCaction de FriedelLCrafts intramolCculaire conduisant B des oligomkres 9,12, etc. se terminant par des indanes hautement fonctionnalisCs. A des tempkratures plus basses, la rCaction est diastCrCosClective. On a montrC que le processus dLoligomCrisation nCcessite la presence de groupes Clectroattracteurs sur le silicium; le remplacement des coordinateurs C1 agissant comme spectateurs par des groupes alkoxyles ou alkyles conduit t i une protiodCsilylation. On discute du mecanisme de formation des oligomkres se terminant par un indane.Mots clks : polymCrisation cationique, P-silylstyrkne, oligomCrisation stCrCosClective, terminaison par un indkne.[Traduit par le rCdaction]

Electrophilic Additions to Styrylsilanes: The Effect of Changing the Ligands on Silicon.-The products of the addition of carbon electrophiles and protons depend upon the nonparticipating substituents on silicon. (Ic) readily reacts with electrophiles. The reaction products do not contain silicon or new C-C bonds, protiodesilylation is favored. In contrast, ( Ia) does not participate in the reaction with carbon electrophiles or strong protic acids. However, with triflic acid (Ia) dimerizes. Under the same conditions (Ib) produces a different diastereoisomer together with a trimer. The reasons for the changes in the mechanism are discussed. -(BROOK, M. A.; HENRY, C.; JEFFERSON, E.; JUESCHKE, R.; SEBASTIAN, T.; TOMASZEWSKI, M.; WENZEL, S.; Bull. Soc. Chim. Fr. 132 (1995) 5-6, 559-568; Dep. Chem., McMaster Univ., Hamilton, Ont. L8S 4M1, Can.; EN)