Slow degenerate rearrangement

This conclusion is in accord with the results of MINDO/3 quantum-chemical calculations. Cation 7 is also prone to undergo a rather slow degenerate rearrangement by 1,2-bridge shift (AG is about 60 kJ/mol) (Scheme 8). This rearrangement was probed via 2H NMR spectroscopy, when the 9-CD3-analog of carbinol 5 was used as precursor for carbocation generation. At -50 °C cations 6-8 transform firstly into l,l,2,3,3a,4,8b-heptamethyl-l,3a,8b-trihydrobenzopentalenium ion (9) and then (partially) into 1,1,2,3,4,4,8b-heptamethyl-l,4,8b-trihydrobenzopentalenium ion (10) (Scheme 9). 

The success of a carbocation preparation in superacid is frequently very technique dependent. Despite this fact, rather few detailed accounts have been published of the special techniques that have been developed. To be able directly to observe reactive (unstable) ions by nmr, for example, the ions not only have to be studied at low temperatures (where non-degenerate rearrangements are slow) they also have to be prepared at low temperatures for the very same reason. Furthermore, side reactions have to be suppressed. Common reactions of this type are dimerizations and polymerizations water and oxygen also have to be excluded. 

Fivefold degenerate reversible [3,3]-sigmatropic shifts were first reported in 1988116,117 in the CPD-amidine system 257, where AG g = 117 to 120 kJmol-1 (equation 88) (for aza-Cope rearrangements see Section IV.E.2). In addition, a slow accumulation of a colored by-product was observed at elevated temperatures. This was identified as a product of a novel intramolecular carbon to nitrogen 1,4-shift of the methoxycarbonyl... 

The spectrum at — 50°C is in accord with a slow rate for the degenerate Cope rearrangement and in addition to four vinylic protons at 65.1, two allylic protons at 6 2.8, there is also a single cyclopropyl proton, 6 0.3 two tertiary cyclopropyl protons absorb between 61-2 and a second cyclopropyl proton is thought to be deshielded by the double bonds. 

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