Norbomyl carbocations stability

MP2-FC/6-31G calculations reveal the symmetrically bridged 6-sila-2-norbomyl cation 1 not only to be a local minimum (Fig. 18), but also to be 17.2 kcal mof more stable than the 2-norbomyl cation (Eq. 3) at MP2-FC/6-31G + AZPE(SCF/6-31G 0.89). However, the inherently greater stability of silyl cations contributes to this difference. The Si NMR chemical shift of the bridging silicon atom, ca 1 ppm vs TMS (IGL0/H //MP2-FC/6-31G ), is very strongly shielded in comparison with ca 300 ppm expected for a free RSiH2 species [38]. Thus, the sila congener of the 2-norbomyl carbocation also possesses a nonclassical stracture which is reflected by its stracture as well as its NMR properties. 

If the classical structure were correct, the 2-norbornyl cation would be a usual secondary carbocation with no additional stabilization provided by c-delocalization (such as the cyclopentyl cation). The facts, however, seem to be to the contrary. Direct experimental evidence for the unusual stability of the secondary 2-norbomyl cation comes from the low-temperature solution calorimetric studies of Arnett and Petro.75 In a series of investigations, Arnett and Hofelich76 determined the heats of ionization (AHi) of secondary and tertiary chlorides in SbF5-SC>2ClF [Eq. (3.131)] and subsequently alcohols in HS03F-SbF5-SC>2ClF solutions [Eq. (3.132)]. 

G. M. Kramer, C. G. Scouten, The 2-Norbomyl Carbonium Ion Stabilizing Conditions An Assessment of Structural Probes", in Advances in Carbocation Chemistry (X. Creary, Ed.) 1989, 1, JAI Press,... 

K = 7W), 2 (/ and 1 10 1 mole (20 C), respectively. These results show the norbomyl and 1-adamantyl ions to be of about the same stability and only 15-30 times less stable than an acyclic tertiary ion. These facts, as well as the reaction rate constant values for the reduction of carbocation salts (R SbFg) by molecular hydrogen in strong acid media support the nonclassical stabilization of the 2-norbomyl ion. 

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