Rearrangements long-lived carbocations

Rearrangements of long-lived carbocations formed via ewfo-9-hydroxy-1,8,9,10,10,11,12-heptamethyltricyclo[6.2.2.02,7]dodeca-2(7),3,5,11 -tetraene (5) in FSO3H-SO2CIF-CD2CI2 at low temperature (-95 °C) were studied by H and... 

According to the preliminary data the correlations obtained make it possible not only to perform similar estimations for the rearrangements of long-lived carbocations, but also to predict the rates of anchimerically assisted solvolytic reactions. 

As mentioned above, persistent carbocation 9 underwent rearrangement into cation 10 which rearranged further into cation 11. To reveal general relations/factors governing cationic rearrangements in benzopentalene derivatives, the behavior of 5,5,10,10-tetramethyl-5,10-dihydroindeno[2,l-fl]indene (12) in superacids was studied (52). It had been expected that hydrocarbon 12 would transform into the long-lived 5,5,10,10-tetramethyl-4b,5,9b,10-tetrahydroindeno[2,l-a]inden-4b-yl cation (13). However, H and 13C NMR data showed that hydrocarbon 12 transformed firstly into isomeric ion 14 which transformed further into cation 15 (Scheme 11). 

Timberlake and coworkers have studied the degenerate rearrangement of pentacyclo-propylethyl cation 56 (involving 1,2-cyclopropyl shifts) under long-lived stable ion conditions81 82 (equation 39a). The rearrangement could not be frozen even at -80 °C. However, additivity of 3C NMR chemical shift analysis7 indicates the classical trivalent nature of the carbocation. 

The alternative explanation assuming the transition state of the 1,2-shift to be similar to the initial one (cf Ref >) is hardly acceptable for the case of rearrangements of long-lived carbonium ions. The 1,2-methyl-shift-retardation effect of the phenyl group and of the substituted phenyl groups in arenonium ions may be, to a certain degree, due to the predominance of conformation with the aryl group in the quasi-axial position this is favourable for the intramolecular donor-acceptor interaction with the carbocation centre (cf. Ref. 

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