Trisilyloxonium ions

Cypryk, M. Chojnowski, J. Kurjata, J. Tertiary Trisilyloxonium Ion and Silylenium Cation in Cationic Ring-opening Polymerization of Cyclic Siloxanes. In Organosilicon Chemistry VI From Molecules to Materials Auner, N., Weis, J., Eds. Wiley-VCH Weinheim, 2005 pp 85-92. 

Tertiary Trisilyloxonium Ion and Silylenium Cation in Cationic Ring-Opening Polymerization of Cyclic Siloxanes... 

The structure of the active propagation center in the cationic ring-opening polymerization (ROP) of cyclosiloxanes is still controversial. Trisilyloxonium ions generated from hexamethyl-cyclotrisiloxane, D3, and octamethylcyclotetrasiloxane, D4, were observed at low temperature by Olah et al. [I] and their participation as intermediates in cationic ROP of cyclosiloxanes was postulated. However, the main objection against this mechanistic concept is a relatively low rate of polymerization when an initiator able to form persistent tertiary oxonium ions is used [2]. 

Studies of the polymerization of a monomer able to form more stable tertiary trisilyloxonium ions could throw more light on this problem. Octamethyl-l,4-dioxatetrasilacyclohexane ( 02) may be such a model monomer. This monomer easily undergoes the cationic ROP initiated by a strong... 

The kinetics and thermodynamics of this process show close similarities to the polymerization of D4 initiated by CF3SO3H [3,4]. Both processes involve simultaneous formation of cyclic oligomers and polymer and lead to equilibrium, with similar proportions of cyclic-to-linear fractions [3-5], They also show similar thermodynamic parameters and a similar effect of water addition on the initial rate of polymerization. The specific feature of the polymerization of >2 is that cyclic oligomers 03 and D4 are formed simultaneously with the polymer fraction, but they equilibrate with monomer much faster than the polymer fraction. This behavior is best understood assuming formation of the tertiary oxonium ion intermediate, which isomerizes by ring expansion-ring contraction [3]. These kinetic features of the polymerization of make this monomer an interesting model for deeper studies on the cyclic trisilyloxonium ion question. 

The hydride transfer from a hydrosilane to the carbenium ion leads to the transient formation of a complex of tricoordinate silicon cation with solvent. Lambert et al. showed that if a stable counterion of very low nucleophilicity is used, the complex of silylenium ion with solvent is a persistent species [12]. Tetrakis(pentafluorophenyl)borate was successfiilly explored for this purpose, and this system was later used by Olah ef al. [1] for the generation of trisilyloxonium ions derived from D4... 

The resulting trisilyloxonium ion and the ion being the product of addition of monomer (Eq. 4) were identified by Si NMR at 5 °C and by quantum mechanical calculations of the chemical shifts [6]. Signals of silicon atoms at the oxonium center appear in the range 50 - 60 ppm, in accord with previous observations for trisilyloxonium ions derived from D4 and D3. 

B(C6Fs)4 is a very weak nucleophile and a poor deactivator of the trisilyloxonium ion... 

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. 

The concept of trisilyloxonium ion received strong support from Olah et who observed such ions directly by... 

Olah GA, Li XY, Wang Q, Rasul G, Prakash GKS (1995) Trisilyloxonium ions preparation, NMR spectroscopy, ah initio/IGLO studies, and their role in cationic polymerization of cyclosiloxanes. J Am Chem Soc 117 8962... 

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