Kazuhisa Sakakibara

A new method for evaluating the steric hindrance by substituent

Abstract New steric parameter, Ω s , evaluating the steric hindrance around the reaction center was defined on the basis of molecular mechanics.

A new steric subtituent constant Ωs. Characteristic feature and comparison with some other steric constants

Abstract A new steric substituent constant, Ωs, was proposed in order to evaluate the kinetic steric effect. Ωs is isotropic and dependent most significantly on the number of β-carbon atoms. The rates of several reactions were shown to be correlated with Ωs better than with Es.

A new steric substituent constant Ωs based on molecular mechanics calculations

Abstract A new steric substituent constant Ωs was defined and the procedure for its calculation is given. It was calculated from the geometries and the relative free energies of the conformers of substituents from MM2.

Electronic substituent effect on intramolecular CH/π interaction as evidenced by NOE experiments

In order to demonstrate the hydrogen-bond-like character of the CH/π interaction, electronic substituent effects on the equilibria between the stretched and the folded conformers of series of compounds capable of forming CH/π interactions were examined by measurements of NOE enhancements of 1H NMR signals. Nuclear Overhauser enhancement is shown to be useful to determine the abundance of the CH/π proximate folded conformer. The Hammett plots of all series of the compounds capable of having CH/π interaction gave negative ρ values. Together with other substituent effects (effects of electronegative substituents, on the CH donor, of ring size, and of α-alkyl substituent), the involvement of delocalization interaction and the hydrogen-bond-like character of the CH/π interaction were established.

Effect of CH/π interaction on the chiroptical properties of olefins and dienes

Abstract Rotatory strengths of exo-methylenesteroids, α-phellandrene and related compounds were calculated on the basis of Rosenfeld theory by using AM1 molecular orbitals. Calculation showed that a considerable change in intensity of CD band is expected to occur for the conformers where CH/π interaction is possible. Chiroptical properties of asymmetric olefins and dienes were interpreted from the viewpoint of CH/π interaction.

Rotational Barriers and 15N Chemical Shifts of N‐Acyl‐N‐alkyl‐substituted Amino Acids

Rotational barriers about the C—N bonds of several N‐acyl‐N‐methyl‐α‐amino acids and their esters R3CO—N(R4)—CR1R2—COOR5, and also N‐Boc‐protected dipeptides, were determined and the steric effects caused by the substituents R1–R5 are discussed by comparing them with the results of MM3 calculations on these amides. Bulky substituents on both the acyl group and the nitrogen atom were shown to have lower ΔG‡. In the series of N‐acylglycines with variable R3, the 15N chemical shifts were correlated with ΔG‡.

Theoretical study of CH-π interaction. Significant enhancement of CD spectra due to the participation of CH-π interaction.

Abstract Rotatory strengths of α-phellandrene and related compounds were calculated on the basis of Rosenfeld theory by using AM1 molecular orbitals. Calculated geometries and rotatory strengths indicated that significant enhancement of CD spectrum would be expected to occur for the conformers where CH-π interaction was possible.

Evaluation of reactivity for nitroxide radical trapping by correlation analysis using steric substituent parameter (?S)

The reactivity of nitroxide radical trapping (NRT), where stable nitroxide radicals react with transient carbon‐centered radicals to form diamagnetic molecules, was evaluated. This was done with the use of the steric substituent parameter of both radicals by correlation analysis on the basis of the reactivity data determined by Ingolds group. In the case where the transient carbon‐centered radicals were not resonance stabilized, the rate constant could be well correlated by using only ΩS parameters of R2NO⋅ and U⋅. If the transient radicals were stabilized by resonance, the parameter to estimate the amount of the resonance stabilization of U⋅ was necessary in the regression equation to evaluate the reactivity correctly. When the spin density, calculated by PM3 UHF molecular orbital calculations, was used as the resonance stabilization parameter, the rate constant could be well evaluated by a dual‐parameter regression equation. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 215–221, 1998.

Molecular mechanics (MM3) calculations on lithium amide compounds

The MM3 force field has been extended to deal with the lithium amide molecules that are widely used as efficient catalysts for stereoselective asymmetric synthesis. The MM3 force field parameters have been determined on the basis of the ab initio MP2/6‐31G* and/or DFT (B3LYP/6‐31G*, B3‐PW91/6‐31G*) geometry optimization calculations. To evaluate the electronic interactions specific to the lithium amides derived from the diamine molecules properly, the Lewis bonding potential term for the interaction between the lithium atom and the nonbonded adjacent electronegative atom such as nitrogen was introduced into the MM3 force field. The bond dipoles were evaluated correctly from the electronic charges on the atoms calculated by fitting to the electrostatic potential at points selected. The MM3 results on the molecular structures, conformational energies, and vibrational spectra show good agreement with those from the quantum mechanical calculations.

Synthesis and spin-trapping properties of poly[N-(p-vinylbenzylidene)-tert.-butylamine N-oxide] and its copolymers as a solid spin trap

Abstract A polymer-supported spin trap, poly[ N -( p -vinylbenzylidene)- tert .-butylamine N -oxide] (polyPBN), was synthesized by an improved procedure. Its copolymers with styrene or methyl methacrylate (copolyPBNs) were also prepared, and their spin-trapping abilities were investigated. Thus, several free radicals generated by the thermal decomposition of radical precursors reacted with the solid polymers dispersed in p -xylene and heptane. Both polyPBN and copolyPBN in the solid state can capture carbon- and oxygen-centered radicals in the heterogeneous phase (solid-liquid system).