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

摘要

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 solvolyses in aqueous ethanol and trifluoroethanol have been examined together with those of the parent and methyl derivatives (1,Y = OSO2CF3; X = H and Me, respectively). Large rate enhancement factors are observed for the metalloidal substituents. A comparison of these accelerative effects with the corresponding values in the bicyclo[2.2.2]octane and adamantane ring systems (2, Y = OSO(2)Me, and 3, Y = Br, respectively) indicates the substrate order of these effects is 1 > 2 much greater than 3. Qualitative considerations lead to the pronounced effects in 1 being ascribed to ''back-lobe'' through-space effects (homohyperconjugation). 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 calculations provide strong support for the homohyperconjugative stabilizing effects of these groups but indicate that the weak accelerative effect of 4-methyl in 1 (Y = OSO2CF3, X = Me) is more likely a manifestation of dominant destabilizing ground-state effects. Furthermore, the high-level calculations expose that the usual practice of theoretical modeling of SiMe(3) by use of SiH3 to reduce the complexity of the computational problem can be seriously inadequate.