Hydrogen Bridged Cyclodecyl Cations

The bridging hydrogen in ion 178 is observed at a unusually high field of 8 —6.85 as compared to the adjacent methine protons (8 6.80). 

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Olah and coworkers have carried out reactions of alkanes in superacid solution (SbFs-HF or SbFs-FSOsH), leading to observable (by NMR) tertiary alkyl carbocations (17, 18, 19). This reaction is pictured as a protonation of the alkane to give a pentacoordinated intermediate which then loses H2. Thus, a plausible mechanism for the cyclodecyl -> 9-decalyl cation reaction could involve loss of the bridging hydrogen (as H ) to form decalin, and then reprotonation of the decalin to eventually form H2 + 9-decalyl cation. However, when the cyclodecyl cation 1 was prepared in SbFs-FSOsD solution, there was no evidence of H-D formation in the hydrogen gas that was produced. It was concluded therefore that the H2 gas must come directly from the carbocation hydrogens without any involvement by the superacid system. 

Sorensen and coworkers showed that at -130°C the cyclodecyl cation exists as a static 1,6- or 1,5-hydrogen-bridged structure (84c or 84c). Similar behavior was observed for the 1,6-dimethyl analog 85a.The bridging hydrogen in ion 84c was observed at an unusually high field, that is, 5 H = -6.85. 

This kind of ions with a bridged hydrogen can be generated from cycloalkanes with 8-11 carbon atoms, i.e, those for which direct transannular hydrogen shifts in carbocation reactions was postulated Ions with Cg, and -rings as distinct from a cyclodecyl cation, rearrange to contract the cycle and form tertiary ions, e.g. ... 

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