EFFECTS OF BRIDGEHEAD METALLOIDAL SUBSTITUENTS (MME(3), M=SI AND SN) ON THE STABILITY OF THE 1-NORBORNYL CATION

4-Metalloidal (MMe(3), M = Si and Sn)-substituted bicyclo[2.2.1]hept-1 -yl triflates (1, Y = OSO2CF3) have been synthesized and their solvoly ses in aqueous ethanol and trifluoroethanol have been examined togethe r with those of the parent and methyl derivatives (1,Y = OSO2CF3; X = H and Me, respectively). Large rate enhancement factors are observed f or the metalloidal substituents. A comparison of these accelerative ef fects with the corresponding values in the bicyclo[2.2.2]octane and ad amantane ring systems (2, Y = OSO(2)Me, and 3, Y = Br, respectively) i ndicates the substrate order of these effects is 1 > 2 much greater th an 3. Qualitative considerations lead to the pronounced effects in 1 b eing ascribed to ''back-lobe'' through-space effects (homohyperconjuga tion). A more quantitative picture of the stabilizing effects of SiMe( 3) and SnMe(3) on the 1-norbornyl cation (5) is painted by high-level ab initio (MP2/6-31G(d)[DZP]) molecular orbital calculations. The calc ulations provide strong support for the homohyperconjugative stabilizi ng effects of these groups but indicate that the weak accelerative eff ect of 4-methyl in 1 (Y = OSO2CF3, X = Me) is more likely a manifestat ion of dominant destabilizing ground-state effects. Furthermore, the h igh-level calculations expose that the usual practice of theoretical m odeling of SiMe(3) by use of SiH3 to reduce the complexity of the comp utational problem can be seriously inadequate.