Kunio Okamoto

Linear free-energy relationships between stability parameters of substituted tropylium and cyclopropenylium ions as measured by reduction rate with chromium(II), reduction potential, charge-transfer energy, and pKR+ values

Abstract Nine tropylium ions have been synthesized and the empirical stability parameters (i)–(iv) have been measured along with the parameters of known tropylium and cyclopropenylium ions: (I) the Cr(II)-ion reduction rate constant ( k 2 ) in 10% aqueous hydrochloric acid at 25.0°; (ii) the charge-transfer frequency ( v CT max ) with pyrene in 1,2-dichloroethane; (iii) the reduction peak potential (E red ) in dichloromethane or in acetonitrile; and (iv) the pK R + values in 23% aqueous ethanol, in 50% aqueous ethanol, and in 50% aqueous acetonitrile. When the linear free-energy relationships (LFER) of these and previously reported results were examined (a) between the charge-transfer energy {(E CT)}, 119.5 × v CT max (cm −1 ), kJ mol −1 and the E red {(96.36 × E red (V), kj mol −1 )}; (b) between the log k 2 {(5.70 × log k 2 (1 mol −1 s −1 ), kJ mol −1 )} and the E red ; and (c) between the log k 2 and the E CT , satisfactory interrelations were found. On the basis of these LFERs the electron affinity scales (log k 2 , E CT , and E red ) are interchangeable for each other as the stability parameters for the cyclopropenylium and tropylium ions. When the pK R + values (5.70 × pK R + , kJ mol −1 ) of 15 cyclopropenylium and 21 tropylium ions were compared with the E red values (kJ mol −1 ), not one but eight correlation lines were shown within the range of slopes −0.25 to −1.05 for each of (1) 12 cyclopropenylium ions in 50% aqueous acetonitrile, (2) nine cyclopropenylium ions in 23% aqueous ethanol, (3) five cyclopropyl-substituted tropylium ions in 50% aqueous acetonitrile, (4) four t-butyl-substituted tropylium ions in 50% aqueous acetonitrile, (5) 10 tropylium ions in 23% aqueous ethanol, (6) nine polymethyl tropylium ions and others in 50% aqueous acetonitrile (7) six 1-aryl-8-tropylionaphthalenes in 50% aqueous ethanol, and (8) five bicycloalkanes fused with the tropylium ion in 50% aqueous ethanol. Thus, the pK R + scale, i.e. the water-affinity scale of the cations, is not generally interchangeable with the E red , i.e. an electron-affinity scale of the cations, even though the pK R + values in a single solvent were compared with the E red values (see categories (1),(3),(4) and (6)). The implications of such divergence of the pK R +- E red correlation are discussed.

Syntheses, properties and some reactions of 8-phenyl- and 8,8-diphenyl-heptafulvenes

Abstract 8 Phenyl- ( 2 ) and 8,8 diphenyl-heptafulvenes ( 3 ) have been synthesized and fully characterized. The 220 MHz NMR data on 2 and 3 , as well as the 13C NMR data on 3 , indicates only a small contribution of the dipolar structure for this system. The electrophilic and nucleophilic reagents react at different sites of this system while protonation of 2 and 3 occurs at the exocyclic methylene carbon butylation occurs at the 7 -membered ring, i.e. the 1- and 3-positions of 2 and only at the 3-position of 3 , when treated with n-BuLi. Cycloaddition with tetracyanoethylene gives the [8 + 2]-cycloadduct for 2 and the [4 + 2] adduct for 3 .

Effects of t-butyl substituents on valence tautomerism of 7-t-butyl-7-cyano-1,3,5-cycloheptatrienes to the norcaradiene form

Abstract Introduction of t-butyl groups to the 2-, 3-, 2,4-, or 2,5-positions of 7-t-butyl-7-cyano-1,3,5-cycloheptatriene dramatically shifts the cycloheptatriene - norcaradiene equilibrium to the norcaradiene form. The nonbonded interaction is an important factor.

Primary dependence of 7-aryl-2,5-di-t-butyl-1,3,5-cycloheptatriene valence tautomerism on the π-acceptor strength of the 7-aryl substituent

Abstract 13 C NMR studies showed that the population of the norcaradiene form of the title systems containing p-CH 3 O, H, and p-CF 3 on the 7-aryl group increases in this order. The result is consistent with the prediction from the π-acceptor strength of the aryl group estimated by INDO calculations.

The tricyclopropylcyclopropenium ion

Abstract A novel non-benzenoid aromatic cation, tricyclopropylcyclopropenium ion, was synthesized by smooth addition of photochemically generated cyclopropylchlorocarbene to dicyclopropylacetylene, and was shown to have remarkable stability.

Structural effects on valence tautomerism : VII. An NMR and molecular mechanics study of the steric effects of t-butyl substituents on the cycloheptatriene-norcaradiene equilibrium of 7-cyano-7-m

Abstract 7-Cyano-7-methylcycloheptatrienes containing one t-butyl group on the 1-position ( 2 ) or two t-butyl groups on the 1- and 3-, 1- and 4-, or 1- and 5-positions ( 3 , 4 , or 5 , respectively) were synthesized and their cyclo- heptatriene (CHT)-norcaradiene (NCD) equilibria measured by variable-temperature 1 H NMR for CS 2 -CD 2 Cl 2 solutions. The 1 H NMR chemical shifts of the 7-methyl group indicate that these compounds are composed of essentially one CHT and one NCD tautomer with an endo geometry of the methyl group. The introduction of a t-butyl group at the 1-position of 7-cyano-7-methylcycloheptatriene ( 1 ) markedly shifts the equilibrium to the NCD side and the addition of the second t-butyl group further favors the NCD form, with the NCD populations for 2 , 3 , 4 , and 5 at 25 °C 70.9, 96.5, 92.3, and 99.3%, respectively. An application of molecular mechanics (MMPI) calculations to various t-butylated CHT-NCD systems suggests that the t-butyl groups sterically destabilize the CHT form more than the NCD form, bringing about increased NCD populations.

The tropylium ion annelated with bicyclo[2.1.1]hex-2-ene: stabilization due to σ-π conjugation versus destabilization due to Mills-Nixon type π-bond localization

Abstract In the tropylium ion annelated with bicyclo[2.1.1]hex-2-ene, stabilization due to the σ-π conjugation of tropylium p-orbitals with the highly strained bicyclic σ-framework was shown to be more effective than destabilization due to the Mills-Nixon type π-bond localization.

Ionic Dissociation of Carbon—Carbon σ Bonds in Hydrocarbons and the Formation of Authentic Hydrocarbon Salts

Publisher Summary This chapter deals with the ionic dissociation of carbon–carbon σ bonds in hydrocarbons and the formation of authentic hydrocarbon salts. Carbon compounds are most commonly constructed using covalent bonds, and the nature of the bond is of prime importance in all organic compounds. Among the compounds of carbon, hydrocarbons, which are made up of only hydrogen and carbon, are the simplest known group of organic compounds having exclusively covalent bonds. The search for such novel hydrocarbons depends primarily on the synthesis and examination of highly stabilized hydrocarbon cations and anions. The details of structural features such as the length of the carbon–carbon bond σ that easily undergoes heterolysis and the spatial arrangement of ions in hydrocarbon salts are intriguing subjects. Electrical characterization of the hydrocarbon salts, though they are not stable enough to air and light, might be useful as new functional materials.

Syntheses and racemization via intermolecular prototropy of optically active alkyltropylium ions. A novel scale for the kinetic brønsted basicity of organic solvents

Abstract Optically active (1-methylpropyl)tropylium ion ( 1 ) and (2-bicyclo[2.2.2]octyl)tropylium ion ( 2 ) have been synthesized. The first-order rate constants of racemization via intermolecular prototropy of 1 provide a novel scale for the kinetic Bronsted basicity of organic solvents.

Reaction of 1-adamantyl cation with carbon monoxide in the presence of adamantane and trifluoromethanesulfonic acid: a convenient route to 3,4-homoadamantanediol

Abstract The carbonylation of the l-adamantyl cation with carbon monoxide in carbon tetrachloride at atmospheric pressure in the presence of trifluo-romethanesulfonic acid (triflic acid) and adamantane affords 3-hydroxy-4-homoadamantyl l-adamantanecarboxylate ( 2 ) in 70% yield under appropriate conditions. Among various l-adamantyl cation precursors tested, l-adamantyl trifluoromethanesulfonate (triflate) and methanesulfonate (mesylate) have given the best, comparable results. As to the acid catalyst, fluorosulfonic acid is less effective than triflic acid, and 100% sulfuric acid and methanesulfonic acid are completely ineffective to produce 2 . It is recommended to use five mol each of triflic acid and adamantane per mol of l-adamantyl mesylate or triflate. This reaction proceeds via the addition of the l-adamantanecarbonyl cation to l-adamantanecarbaldehyde, a transient intermediate, followed by the Wagner-Meerwein rearrangement. The hydroxy ester 2 is easily converted into 3,4-homoadamantanediol which is a promising starting material for 3,4-bifunctional homoadamantane derivatives.