A value of pKR = −5.1 for the anthracenonium ion in 50:50 (v/v) aqueous trifluorethanol is reported based on a ratio of measured rate constants kH for acid‐catalysed dehydration of the 9,10‐hydrate of anthracene and k H 2 O for the reverse hydrolysis of the carbocation (KR = k H 2 O/kH). Comparison with pKR = −15.9 for the fluorenyl cation indicates that the latter ion is less stable by more than 10 log units (15 kcal mol−1). This difference is (a) considerably larger than that between the benzhydryl (pKR = −11.7) and fluorenyl cations (ΔpK = 4.2), which has been considered too small to indicate antiaromatic character for the fluorenyl cation, and (b) comparable to that between pKas for the ionization (in DMSO) of fluorene (22.6) and diphenylmethane (32.2), which has been interpreted as implying aromatic character for the fluorenyl anion. It is shown that a difference in stability of anthracene hydrate and 9‐hydroxyfluorene makes only a minor contribution to the difference in pKR values and that the fluorenyl cation is destabilized by ca 10 kcal mol−1. A smaller difference in pKas for protonation of fluorenimine (5.85) and benzhydrylimine (7.0) is consistent with the expected moderating effect of an electron‐donating substituent on relative carbocation stabilities. Evidence from calculations relating to the antiaromaticity of the fluorenyl cation is reviewed in the light of these measurements. An additional comparison between equilibrium constants for the ionization of aralkylazides (Kaz) and alcohols (KR) reveals the influence of differences in geminal σ‐bond interactions for the hydroxy and azido groups in their respective reactants. Copyright © 2002 John Wiley & Sons, Ltd.

A study of the enolization of phenylacetylpyrazine (PzCOCH(2)Ph) catalyzed by acid, base and metal ions in aqueous solution shows, unusually, that metal ions are more effective catalysts than protons, e.g., for zinc k(Zn)/k(H) = 600. Such behavior contrasts with that of the structurally related phenacylpyridine (PyCH(2)COPh) for which k(Zn)/k(H) = 0.0065. To interpret this difference, equilibrium constants for the tautomerization of phenylacetylpyrazine and for binding of protons and metal ions to its keto tautomer and enolate anion have been measured or estimated and are compared with existing measurements for phenacylpyridine. A tautomeric constant, K(E) = 1.2 x 10(-3) (pK(E) = 2.9), is derived by combining forward and reverse rate constants for enolization measured, respectively, by iodination or bromination of the keto tautomer and relaxation of the less stable enol. For the keto tautomer, NMR measurements yield a pK(a) = -0.90 for N-protonation, and spectrophotometric measurements give pK(a) = 11.90 for ionization to an enolate anion. For the enol, pK(a) values of 0.44 and -4.80 for mono- and diprotonation are obtained from the pH profile for ketonization and absorbance measurements for the transient enol reactant. Binding constants for metal ions (Cu(2+), Ni(2+), Zn(2+), Co(2+), and Cd(2+)) are derived from the saturation of their catalysis of the ketonization reaction. It is found that ketonization is efficiently catalyzed by metal ions but inhibited by acid. These findings, and the striking difference from phenacylpyridine, are ascribed to differences in thermodynamic driving force arising from stronger binding of the proton to the more basic pyridine than pyrazine nitrogen atom in both the reactant keto tautomer and in the enaminone or zwitterion product of the rate-determining (proton transfer) step of the enolization.

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1997 photochemistry, radiation chemistry, chemoluminescence photochemistry, radiation chemistry, chemoluminescence O 0160 -072Photochemistry of Some Trialkylsilyloxybenzylidenemalonic Acid Derivatives. C-Si Bond Fragmentation in a Polarized Excited State.-While photoinduced cleavage of title compounds (I) and (II) is not observed, photofragmentation of title compound (III) in polar medium leads to C-Si bond cleavage, generating tert-butyl radicals, which are trapped by the starting compound, while in apolar solvent slow photodimerization takes place to yield cyclobutane derivative (VII). -(COURTNEY, M. C.; MELLA, M.; ALBINI, A.; J.