Silicenium ions caged

The photochemical reduction of 1-methylquinolinium ions by (TMS)3SiH proceeds regioselectively to afford the corresponding 1,4-dihydroquinones in a water-acetonitrile solvent system (Reaction 4.47) [83]. Mechanistic studies demonstrated that the reactions are initiated by photoinduced electron transfer from the silane to the singlet excited states of 1-methylquinolinium ions to give the silane radical cation-quinolinyl radical pairs, followed by hydrogen transfer in the cage to yield 1,4-dihydroquinones and silicenium ion. Silyl cations are quenched by water. 

The tris-(9-borabicyclo[3.3.1]nonyl)silicenium ion, (9-BBN)3Si+ (49), adopts a D3h equilibrium geometry, perhaps due to the greater rigidity of the bicyclic cages. Based on non-bonded H—H distances it was estimated that an opening with a diameter of 2.34 A, through which nucleophiles might interact with the silicenium ion centre, exists in 49113. 

The experimental search for free stable silicenium ions in solution seems to have finally succeeded, at least for triaryl-substituted species4,5. However, computational studies will continue to augment, to verify and to guide the interpretations of experimental findings, as well as to suggest new experiments for preparing other silicenium ions. Schleyer and coworkers, for instance, have recently explored a silicenium ion within a cage , namely... 

FIGURE 24. B3LYP/6-31G optimized geometry of a fully caged silicenium ion 51. Bond lengths in A. Non-bonded H—H and H—Si contacts (A) shown as dashed lines... 

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