Silanorbomyl cations

The internally stabilized silyl cation (36), formed by hydride transfer as shown, has a 2-silanorbomyl cation structure, and is not coordinated to die solvent or the counterion.78 NMR chemical shifts calculated on the basis of die bridged structure shown are in agreement with the experimental values. The autiiors describe it as free but internally re-stabilized .78 The silicon-stabilized oxonium ion (37) shows considerable stereoselectivity in its reactions (38) is the preferred product isomer by a 92 8 ratio, and (39) by 98 2.79... 

Further support comes from calculation of 1 Jch spin-spin coupling constants at IGLO-DFPT/BIII level. The data were interpreted as giving strong evidence for intramolecularly TT-stabilized silanorbomyl cation structure lacking coordination to solvent or counterion. The species can also be regarded as an almost symmetrically bridged /J-silyl carbocation with siliconium ion character. 

The identification of an almost unlimited stable silanorbomyl cation shows that in principle a C=C double bond might be nucleophilic enough to 7i-stabilize a Si center [6],... 

Summary The synthesis of intramolecularely Jt-stabilized silylium ions such as 7-sila-norbomadienyl- and 2-silanorbomyl cations was attempted. While benzo-7-sila-norbomadienyl cation 7 could not be detected at room temperature in hydrocarbon solvents, its donor-acceptor complex 8 was identified by NMR spectroscopy. In contrast, persistent 2-silanorbomyl cations could be synthesized and characterized by NMR spectroscopy and modem quantum chemical methods. Silanorbomyl cations 5 and 25 are stable at room temperature and have a bridged stmcture in which the silicon adopts a [3+2] coordination. 

While in the course of the HCl addition silanorbomyl cations were formed along the a-route (Scheme 6), we choose the Jt-route for the synthesis and spectroscopic characterization of a persistent silanorbomyl cation (Scheme 6), i.e., the intramolecular addition of a preformed silylium ion to the C=C double bond. 

Scheme 6. Ssmthetic approaches to stable silanorbomyl cations.

In summary, the silanorbomyl cations 5 and 25 are stable compounds in arene solvents at ambient temperature, lacking direct coordination to solvent or counterion. They are stabilized by intramolecular Jt-interaction and can be therefore regarded as an almost symmetrically bridged p-silyl-substituted carbocation with siliconium ion character. 

The intramolecular jc-stabilization of silyl cations has been successfiilly verified by the synthesis of 2-silanorbomyl cations and will be extended to other silyl, germyl and stannyl cations in the near future. The synthetic potential of intramolecularly Jt-stabilized silyl cations in polymerization and hydrosilylation reactions is currently under investigation in our laboratories. 

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