16804-70-9Relevant articles and documents
Experiments and calculations for determination of the stabilities of benzyl, benzhydryl, and fluorenyl carbocations: Antiaromaticity revisited
Amyes, Tina L.,Richard, John P.,Novak, Michael
, p. 8032 - 8041 (1992)
The following pKR values for the formation of benzyl, benzhydryl, and fluorenyl carbocations in 50:50 (v:v) trifluoroethanol/water at I = 0.50 (NaClO4) were determined as pKR = -log (kHOH[H2O]/kH), where kH is the second-order rate constant for acid-catalyzed reaction of the alcohol to form the carbocation and kHOH is the second-order rate constant for capture of the carbocation by water: (R+, pKR); PhCH2+, ≤-20; PhCH(Me)+, -15.4; PhC(Me)2+, -12.3; Ph2CH+, -11.7; Ph2C(Me)+, -9.3; 9-fluorenyl carbocation (9-Fl+), -15.9; 9-methyl-9-fluorenyl carbocation (9-Me-9-Fl+), -11.1. The pKR for Ph2CH+ is in fair agreement with the value estimated using acidity functions,1a but the pKR for 9-Me-9-Fl+ is ca. 4 units more positive than that from the acidity function method,1a so that the difference in the acidity of benzhydryl and fluorenyl carbocations is smaller than estimated in earlier work. The 12 π-electron cyclic fluorenyl system in 9-Fl+ and 9-Me-9-Fl+ causes only 5.7 kcal/mol and 2.4 kcal/mol, respectively, destabilization of the corresponding acyclic carbocations Ph2CH+ and Ph2C(Me)+. The pKR values show that "antiaromatic" destabilization of the 9-fluorenyl carbocations must be small. Ab initio calculations of the structures and energies of 9-Fl+ and Ph2CH+ and of the corresponding alcohols at the 3-21G//3-21G and 6-31G*//3-21G levels indicate that Ph2CH+ is ca. 8-10 kcal/mol more stable than 9-F1+, which is in good agreement with the stability difference calculated from the pKR data. This indicates that electronic factors play the major role in determining the relative energies of these carbocations. Force field calculations were performed to estimate the contribution of van der Waals and ring strains to the difference in the pKR values for Ph2CH+ and 9-Fl+. Assuming hypothetical structures for Ph2CH+ and 9-Fl+ which are free of van der Waals and ring strains, it is then estimated that there is an 8-11 kcal/mol decrease in π-electron stabilization on moving from Ph2CH+ (C2v) and 9-F1OH to 9-Fl+ and Ph2CHOH. It is concluded that 9-fluorenyl carbocations are not antiaromatic. The difference in the energy of the 9-fluorenyl and benzhydryl carbanions relative to the alcohols was calculated to be -13.2 kcal/mol at the 6-31G*//3-21G level. This difference is attributed to the difference in the energies of the HOMOs for the two carbanions.
Evidence for significant through-space and through-bond electronic coupling in the 1,4-diphenylcyclohexane-1,4-diyl radical cation gained by absorption spectroscopy and DFT calculations
Ikeda, Hiroshi,Hoshi, Yosuke,Namai, Hayato,Tanaka, Futoshi,Goodman, Joshua L.,Mizuno, Kazuhiko
, p. 9207 - 9215 (2008/12/21)
Photoinduced single-electron-transfer promoted oxidation of 2,5-diphenyl-l,5-hexadiene by using N-methylquinolinium tetrafluoroborate/ biphenyl co-sensitization takes place with the formation of an intense electronic absorption band at 476 nm, which is attributed to the 1,4-diphenylcyclohexane-1,4-diyl radical cation. The absorption maximum (λob) of this transient occurs at a longer wavelength than is expected for either the cumyl radical or the cumyl cation components. Substitution at the para positions of the phenyl groups in this radical cation by CH3O, CH3, F, Cl, and Br leads to an increasingly larger redshift of λob. A comparison of the ρ value, which was obtained from a Hammett plot of the electronic transition energies of the radical cations versus σ+, with that for the cumyl cation shows that the substituent effects on the transition energies for the 1,4-diarylcyclohexane-1,4-diyl radical cations are approximately one half of the substituent effects on the transition energies of the cumyl cation. The observed substitu_ent-induced redshifts of λob and the reduced sensitivity of λob to substituent changes are in accordance with the proposal that significant through-space and -bond electronic interactions exist between the cumyl radical and the cumyl cation moieties of the 1,4-diphenylcyclohexane-1,4-diyl radical cation. This proposal gains strong support from the results of density functional theory (DFT) calculations. Moreover, the results of time-dependent DFT calculations indicate that the absorption band at 476 nm for the 1,4-diphenylcyclohexane-1,4-diyl radical cation corresponds to a SOMO-3-SOMO transition.
Lifetimes and UV-visible absorption spectra of benzyl, phenethyl, and cumyl carbocations and corresponding vinyl cations. A laser flash photolysis study
Cozens, Frances L.,Kanagasabapathy,McClelland, Robert A.,Steenken, Steen
, p. 2069 - 2082 (2007/10/03)
Benzyl (4-MeO, 4-Me, and 4-methoxy-1-naphthylmethyl), phenethyl (4- Me2N, 4-MeO, 3,4-(MeO)2, 4-Me, 3-Me, 4-F, 3-MeO, 2,6-Me2, parent, and 4- methoxy-1-naphthylethyl) and cumyl (4-Me2N, 4-MeO, 4-Me, parent) cations have been studied by laser flash photolysis (LFP) in 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). In most cases styrene or α-methylstyrene precursors were employed for the phenethyl and cumyl ions, the intermediate being obtained by solvent protonation of the excited state. Benzyl cations were generated by photoheterolysis of trimethylammonium and chloride precursors. While a 4-MeO substituent provides sufficient stabilization to permit observation of cations in TFE, cations with less stabilizing substituents usually require the less nucleophilic HFIP. Even in this solvent, the parent benzyl cation is too short-lived (lifetime 6H4C+(R)-CH3 (R = Me, Et, i-Pr, t-Bu, cyclopropyl, C6H5, 4-MeOC6H4) were generated in TFE via the photoprotonation route. The alkyl series shows that steric effects are important in the decay reaction. The cation with R = cyclopropyl is a factor of 1.5 less reactive than the cation where R = phenyl. Several vinyl cations have also been generated by photoprotonation of phenylacetylenes. ArC+=CH2 has a reactivity very similar to that of its analog ArC+H-CH3, the vinyl cation being slightly (factors of 2-5) shorter-lived. For the various series of cations, including vinyl, substituents in the aryl ring have a consistent effect on the κ(max), a shift to higher wavelength relative to hydrogen of 15 nm for 4-Me, 30 nm for 4-MeO, and 50 nm for 4-Me2N.
