STABLE CARBOCATIONS .220. C-13 NMR SPECTROSCOPIC STUDY OF POTENTIAL TRIS-HOMOCYCLOPROPENYL AND BIS-HOMOCYCLOPROPENYL CATIONS

The effect of methyl and phenyl substituents, in comparison with the parent systems, was determined byl3C NMR spectroscopic study of potentially homoaromatic carbocations, obtained upon ionization of the corresponding bicyclo[3.1.0]-hexyl and 4-cyclopentenyl alcohols and chlorides, respectively. Winstein's parent trishomocyclopropenyl cation 3, first obtained as a stable ion by Masamune via ionization of cis-bicyclo[3.1.0]hex-3-yl chloride 4-C1 was also independently prepared from cis-bicyclo[3.1.0]hexan-3-ol 4-OH in protic acid free SbF5/SO2ClF solution at -78°C. Ion 3, once formed, was found stable upon addition of Brjjnsted acids, such as HF or FSO3H. Methyl substitution leads to rearranged allylic ions, whereas phenyl substitution gives classical ions with little or no homoaromaticity. Contrary to the claim by Broser and Rahn, phenyl substitution at the C-l, C-3, and C-5 positions of the bicyclo[3.1.0]hexyl skeleton also gave only a classical ion with no trishomoaromatic character. Sauers claimed trishomocyclopropenylium carboxylic acid 34 was shown to be instead the corresponding protonated starting tricyclic lactone 35. All attempts to prepare the parent potentially bishomoaromatic 4-cyclopentenyl cation 2a were unsuccessful. Previously studied 7-norbornenyl and 7-norbornadienyl cations, are, however, indeed of bishomocyclopropenyl ion nature. © 1979, American Chemical Society. All rights reserved.