Persistent alpha-CF3-substituted (1-pyrenyl)dimethyl-, (1-pyrenyl)phenylmethyl-, (4-pyrenyl)dimethyl-, and (9-phenanthrenyl)dimethylcarbenium ions: Enhancing arenium ionic character by increasing electron demand at the carbocation
Laali, KK; Tanaka, M; Hollenstein, S; Cheng, M; ,
To examine the influence of carbocation destabilization (increasing electron demand) on the magnitude of pi-participation by an alpha-pyrene (or alpha-phenanthrene) moiety, a series of regioisomeric PyC+(CF3)R (R = Me, Ph) [Py = pyrene], and (9-Phen)C+(CF3)Me [phen = phenanthrene] carbocations having a CF3 group alpha to the cation center were generated from their carbinols, 1-OH to 5-OH, by low-temperature protonation and ionization with FSO3H/SO2ClF. Multinuclear (C-13, F-19, H-1), NOED, and 2D NMR (H/H COSY, C/H HETCOR) were used to deduce the mode and magnitude of charge delocalization and the extent of Ar-C+ double-bond character in the carbocations; AM1 calculations were used to examine their energies and charges (and conformations) for comparison with NMR. Since charge delocalization into the 1-Py-, 4-Py-, and 9-Phen-substituents is quite effective, the presence of alpha-CF3 greatly increased pi-participation and the arenium ion character in 1(+) and 2(+) as well as in 3(+) and 5(+). On the other hand, diminished charge delocalization into a a-Py-substituent, coupled with destabilization by alpha-CF3, prevents the formation of 4(+) and ring protonation occurs to give 4H(+). It was hoped that these (PAH)C+(CF3)R carbocations (PAH = polycyclic aromatic hydrocarbon) may serve as models of PAH-epoxide ring opening where decreased carbocation stability augments arenium ion character to an extent that nucleophilic quenching (by DNA nucleotides) at a remote site may become feasible (-->PAH-DNA adduct). Model quenching reactions with pyridine, Et3N, MeOH, and H2O did not, however, support the occurrence of nucleophilic attack on the PAH.