Cyclooctyl cations

The main reaction path is stereospecific, with the trifluoroacetate being added syn to the proton. This implies that the reaction proceeds through a discrete Itydride-bridged intermediate rather than a conformationally mobile cyclooctyl cation. 

The observed proportions of do mjz 96), (mjz 97), and 2 mjz 98) ions result from a normal kinetic isotope elfect on the order of 3.5 for the Bronsted acid base reaction followed by a k jk of 2.8 for the subsequent atom abstraction. Cyclooctyl cations acquire symmetry due to the bridging hydrogen, as depicted, which accounts for the fact that both of the tetradeuterated precursors give the same product ion intensity ratios. After the Bronsted acid-base step, however, the cyclooctene-containing complexes are discrete and non-interconverting, which accounts for why the randomization model (equation 3) fails for this reaction. 

According to the rules of orbital symmetry, vicinal hydride shift in cations is a thermally allowed suprafacial 1,2-sigmatropic shift. Migration of a more distant hydrogen can occur by successive 1,2-shifts, via a bridged structure (such as the cyclooctyl cation in Scheme 7 ), or by corner-to-corner transfer in a protonated cyclopropane (which give net 1,3-shifts). An EBFlow experiment has been able to measure the competition between 1,2- and 1,3-shift in the 1-fluoropropyl system. 

Hydride-bridged ions of this type are evidently quite stable in favorable cases and can be observed under stable-ion conditions. The Itydride-bridged cyclooctyl and cyclononyl cations can be observed at — 150°C but rearrange, even at that temperature, to methylcy-... 

In contrast, secondary p-H-bridged cations are much more fragile, and only the cyclooctyl 8, cyclononyl 9, cyclodecyl 10 and cycloundecyl 11 cations have been characterized. 

The instability of the cyclooctyl and cyclononyl cations is due to a rapid rearrangement to the lower ring tertiary cation, the 1-methylcycloheptyl 12 and 1-methylcyclooctyl 13 cation, respectively. 

There is a clear preference for 1,5-hydride bridging in these cations. Qualitatively, the most stable cation of the series is the 1,5-dimethyl-cyclooctyl, but the preference is also shown by the slow rearrangement of... 

This dichotomy is quite evident in the reaction of cydooctane with nitronium trifluoroacetate. The products obtained were cyclooctyl triflu-oroacetate, cyclooctyl nitrate and nitrocydooctane. Conversion of adaman-tane to 1-fluoroadamantane in 95% 3ueld on reaction with nitronium tetrafluoroborate in pyridine polyhydrogen fluoride indicates that formation of adamantyl cation by formal hydride abstraction is a significant alternative to the nitration-protodenitration pathway. 

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