Silyloxonium ions

Chojnowski and co-workers have studied the polymerization of octamethyltetrasila-l,4-dioxane, a monomer more basic than cyclosiloxanes, which is capable of forming more stable oxonium ions, and thus being a useful model to study the role of silyloxonium ions.150-152 In recent work, these authors used Olah s initiating system and observed the formation of oxonium ion and its transformation to the corresponding tertiary silyloxonium ion at the chain ends.153 The 29Si NMR spectroscopic data and theoretical calculations were consistent with the postulated mechanism. Stannett and co-workers studied an unconventional process of radiation-initiated polymerization of cyclic siloxanes and proposed a mechanism involving the intermediate formation of silicenium ions solvated by the siloxane... 

Keywords cyclosiloxanes, ring-opening polymerization, silyloxonium ions... 

Recent ab initio calculations show that silylenium ions such as MesSi and EtsSr are strongly complexed even in solvents of such low nucleophilicity as CH2CI2 [15]. In the presence of bases even as weak as siloxanes, they are practically completely converted into silyloxonium ions (Table 2). 

In the system D2 + EtsSiH + PhsC B(C6F5)4 at low temperatures, not only is the generation of Et3Si - D2 observed but also its transformation to the corresponding tertiary silyloxonium ion at the chain end, i.e., Et3SiO(SiMe2)2(SiMe2)2 - D2, may be followed. This species is relatively stable at 0 °C. 

Two in situ methods for the generation of A///-dimethyl(methylene)iminium salts, which are limited to reactions with enol silanes, are also available and avoid the need to handle these moisture-sensitive reagents. In the first method, iodide (30) is prepared in situ by treating methyl(methylene)diamine (34) with chloroiodomethane (equation 5) 36,37 DMSO must be used as solvent to achieve maximum yields. Cleavage of n-butyl dimethylaminomethyl ether (35) with TMS-I or TMS-OTf in MeCN comprises the second method (Scheme 5).38 A silyloxonium ion (36) was originally proposed as the reactive aminomethylating species, but a recent 13C NMR study of the reactions of ami-nol ethers with halosilanes in CD3CN-SO2 did detect the presence of iminium salts.39 Use of these methods in reactions with enol silanes is discussed in Section 4.1.2.2.2.iii. 

Reduction ultimately involves transfer of a hydride to a silyloxonium ion. It remains unclear whether this involves direct reaction of a second borohydride complex 2 with 5, with a tetrahedral oxonium ion 6 or some other process. Irrespective, the data does not support the intermediacy of a free silylium ion intermediate [24]. 

However, kinetic studies led Toskas et al to the conclusion that silyloxonium ions may not be the dominating propagation species in the polymerization initiated by protic acids. Using Olah s initiator and the more nucleophilic monomer, octamethyltetrasila-l,4-dioxane ( 02) 7, the transformation of the primary silyloxonium ions formed in the reaction of monomer with initiator into silyloxonium ions at the end of the polymer chain was observed directly by Si NMR (eqns [30] and [31]). ... 

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