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The structure of 9-barbaralyl cation was examined by Hammett-Brown plot using 13C NMR chemical shifts. 9-Aryltricylclo[3.3.1.02,8]nonan-9-yl cation (5) and 9-aryltricyclo-[3.3.1.02,8]nona-3,6-dien-9-yl cation (6) were prepared from the corresponding carbinols in FSO3H-SO2ClF solution at -120℃, and their chemical shifts of cation centers were measured. The slopes, ρC+ values, of the Hammett-Brown plot of (5) and (6) were -5.01 and -7.52, respectively. From these values themselves, it seemed that the double bonds participated in the delocalization of the positive charge. However, comparing ρC+ value and ρCα+ value in 9-barbaralyl cation (6) and those in 8,9-dehydroadamantyl cation (7), we concluded that 9-barbaralyl should be represented by the structure 4 shown in Scheme Ⅰ.
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