Keizo Ikai

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.

Stereomutation of endo-2-phenyl-endo-6-tropyliobicyclo[2.2.2]octane to the exo-6-tropylio isomer. Steric repulsion between the phenyl and tropylium rings showing intramolecular charge-transfer

Abstract endo-2-Phenyl-endo-6-tropyliobicyclo[2.2.2]octane 2 exhibits intramolecular charge-transfer (CT) interaction, but rearranges to exo-6-tropylio isomer 3 in acetonitrile with the 2/3 ratio of ca.0.07 at equilibrium. This ratio indicates that the steric repulsion between the face-to-face phenyl and tropylium rings at ca.2.5A is estimated to be at least ca.1.6kcal/mol.

Evaluation of the π-conjugative effect of the 2-methylene and the 2-oxo substituent on the stability of carbocations in the solvolysis of bicyclic bridgehead derivatives

Abstract The incipient 2-methylenebicyclo[3.2.2]non-1-yl cation is stabilized by allylic conjugation by 4 kcal/mol compared with the more rigid 2-methylenebicyclo[2.2.2]oct-1-yl cation. In contrast to this, no appreciable stabilization due to carbonyl π-conjugation was detected in the 2-oxobicyclo[3.2.2]non-1-yl cation, indicating unimportance of π-conjugative stabilization in tertiary α-keto cations.

Syntheses of bicyclic 1,2-diols via the ring-expansion of bridgehead aldehydes of bicyclo[3.2.1]octane and bicyclononanes with benzoyl triflate

Abstract Bridgehead aldehydes of bicyclo[3.2.] loctane (11a), bicycio-[3.3.1]nonane (12a), and bicyclo[3.2.2]nonane (13a) have been subjected toacylative ring-expansion by using benzoyl trifluoromethanesulfonate (trif-late) to give mixtures of two or three bicyclic 1,2-diol monobenzoates containing the hydroxyl group on the bridgehead carbon. Control experiments have shown that the reaction is kinetically controlled. The direction of the ring-expansion has been found to be predictable by comparing the strain energies calculated by molecular mechanics of the parent hydrocarbons corresponding to the produced diol monobenzoates. The major monobenzoates, easily isolated in practical yields by crystallization or column chromatography, have been converted to the corresponding diols, and their structures determined. For synthetic purposes, 11a gives bicyclo[3.3.1]nonane-1,endo-2-diol (27), and 12a the endo and exo isomers of bicyclo[4.3.1]decane-1,2-diol (33 and 35, respectively). From 13a is furnished bicyclo[4.2.2]decane-1,2-diol (42). Diols 27, 33. and 42 are conveniently oxidized to the corresponding ketols by using silver carbonate in 50 – 80% yields. The oxidation of diol 35 to 1-hydroxybicyclo[4.3.1]decan-2-one (34) failed under similar conditions, but it was achieved by the Corey oxidation by using dimethylsulfide -chlorine complex.

Retentive solvolysis. Part 14. The methanol-perturbed phenolysis of optically active 2,2-dimethyl-1-(p-methoxyphenyl)propyl p-nitrobenzoate. The mechanism and the structure of the second ion-pair intermediate

The polarimetric and titrimetric rate constants have been measured for the solvolysis of optically active 2,2-dimethyl-1-(p-methoxyphenyl)propyl p-nitrobenzoate (ROPNB) in phenol–methanol (97:3 w/w). The salt effect of added (Bun)4NClO4 indicates that all products are derived from the second ion-pair intermediate (Int-2), not from the first (Int-1), as for the phenolysis in pure phenol. Competitive solvolysis, i.e., methanol-perturbed phenolysis, produced partially inverted ROPh, o- and p-RC6H4OH, and partially retained ROMe, whereas phenolysis in pure phenol afforded partially retained ROPh and o- and p-RC6H4OH. This stereochemical outcome demonstrates that Int-2, the key intermediate of these phenolyses, has an ion-pair structure shielded by a phenol molecule from the rear-side. The absolute configurations and the maximum rotations of all products have been determined.