Previous studies of phenyl substituted tropylium3 and cyclopropenylium4 cations have indicated relatively little interaction of the phenyl substituent (s) with the central kharged ring In the ground state. In a few cases %,c,e phenyl substitution was reported to exert an apparent destabilizing effect particularly when the phenyl group could not achieve coplanarity with the charged ring. In order to further examine this curious "phenyl effect" and to provide more quantitative data on twisted phenyl interactions with stable carbonium ions, we have prepared the interesting hexaphenyl-and tetraphenyltropylium ions I and II and now describe their spectral properties and relative stabilities compared to the previously reported heptaphenyl ion III?
After limited success with other synthetic routes the following method was developed for the synthesis of the hexaphenyltropylium cation (I). Hexaphenylcycloheptatrienecarboxylic acid (IV),5 m.p. 252.5 -254Q 2; 260 mu (loge 4.311, prepared from the Ciels-Alder, reaction of 2,3-diphenylcyclopropenecarboxylic acid and tetraphenylcyclopentadienone; was oxidatively de-.' '
.* carboxylated with lead tetraacetate according to Corey's procedure 6 ':c' to give !:Lgt: yields (>90$) of hexaphenylcycloheptatrienyl acetate (V),' m.p. 185cX 2," 262 rnp (c.19). Thir ace'late wa; . quantitatively converted to the bright yellow hexaphenyltropylium perchlorate (I, Cl~i),' m.p. 228' hMe: 244 (4.97), 273 (4.281, 322 mu (4.131, by addition of concentrated perchloric acii to a cold solution of V in 50% acetonitrile-acetic anhydride.