Yasushi Ohga

Selective oxidation of tertiary-secondary vic-diols to α-hydroxy ketones by dioxiranes

Abstract Isolated dimethyldioxirane ( 1a ) and methyl(trifluoromethyl)dioxirane ( 1b ) efficiently afford the conversion of bicyclic diols bicyclo [2.2.2]octane-1,2-diol ( 2 ) and (+)-pinane-2,3-diol ( 6 ), of tricyclic diols adamantane-1,2-diol ( 3 ), homoadamantane-3,4-diol ( 4 ), and of homopentaprismane-3,4-diol ( 5 ) into the corresponding α-hydroxy ketones in high yields and under mild conditions. In the oxidation of (+)-pinane-2,3-diol ( 6 ), (−)-2-hydroxy-3-pinanone ( 11 ) is obtained in optical yield > 97% with retention of configuration.

Structural effects of the Grunwald–Winstein correlations in the solvolysis of some simple tertiary alkyl chlorides

The rates of solvolysis in various solvents at 25 °C were determined for five tertiary alkyl chlorides: 2-chloro-2,4,4-trimethylpentane (4), 2-chloro-2,4-dimethylpentane, 2-chloro-2-methylpentane, 1-chloro-1,3,3-trimethylcyclopentane (7) and 1-chloro-1-methylcyclopentane. The rate data were analysed on the basis of the original and extended Grunwald–Winstein-type equation [log(k/k0)=mYCl+c and log(k/k0)=lNT+mYCl+c] and the results were compared with those reported for 2-chloro-2-methylpropane (1) and 2-chloro-2,3,3-trimethylbutane (3). The rate data for 4 in 18 solvents give an excellent correlation with l=0·00±0·02 and m=0·74±0·01. The neopentyl group in 4 more effectively shields the rear-side of the reaction center than the tert-butyl group in 3 that is correlated by l=0·10±0·04 and m=0·81±0·04. The rate ratio between 4 and 1 at 25 °C is 275 in TFE and predicted to increase to 950 in TFA. The previous 4/1 rate ratio of 21 in 80% ethanol evidently underestimates the B-strain effect on the solvolysis rate of 4 by a factor of at least 40. The remote methyl groups in 7 works to increase rear-side shielding without increasing B-strain. The marked difference in the effect of the remote methyl groups between 4 and 7 suggests that the leaving chloride ion in 4 takes a locus that is nearly antiperiplanar to the tert-butyl group.

Dynamic solvent effects on the fading of a merocyanine formed from a spironaphthoxazine. A case of a strong coupling between chemical and medium coordinate

The pressure dependence of the rate constant for the fading of a merocyanine formed from a spironaphthoxazine was measured in three pairs of nonviscous/viscous solvents of similar polarity, such as ethanol/2-methylpentane-2,4-diol, methyl acetate/glycerol triacetate, and methylcyclohexane/2,4-dicyclohexyl-2-methylpentane. The results unequivocally demonstrated a necessity to treat the medium and the chemical coordinate as two independent coordinates. These coordinates were found strongly coupled in the present reaction, especially in protic solvents. The results also revealed that hydrogen-bond solvation to the oxygen atom in the initial state and its desolvation during the activation step play important roles in both reaction mechanisms and dynamics.

Dynamic solvent effects on the thermal cyclization of a hexadienone formed from a diphenylnaphthopyran: An example of a system with distinctly separate medium and chemical contributions to the overall reaction coordinate

Pressure effects on the rate of thermal fading of a colored hexadienone formed from 3,3-diphenyl-3H-naphtho[2,1-b]pyran were measured at various temperatures in three highly viscous solvents, i.e., 2,4-dicylohexyl-2-methylpentane, glycerol triacetate, and 2-methylpentane-2,4-diol, as well as in their chemically similar nonviscous counterparts, such as methylcyclohexane, methyl acetate, and ethanol. Both experimental data and quantum-mechanical calculations are in agreement with formation of a cyclic activated complex. In the viscous solvents, high viscosities resulted in retardations of solvent thermal fluctuations which, in turn, slowed down the cyclization. The analysis of experimental results unequivocally demonstrated that the dynamic solvent effects in the present systems have to be described by a two-dimensional model that includes separately defined chemical and medium coordinates. Predictions of an alternative one-dimensional model with a diffusion-controlled reaction coordinate are not supported by the experimental data.

The Grunwald-Winstein type correlation for 2-chloro-2,4,4-trimethylpentane: A simple tertiary alkyl chloride that shows essentially limiting behaviour in solvolysis

Evaluation of the rates of solvolysis of 2-chloro-2,4,4-trimethylpentane (4) in 17 solvents on the basis of the Grunwald-Winstein type equation [log(k/k0) = lNT + mYCl + c] gives an excellent correlation with l = −0·01 ± 0·02 and m = 0·74 ± 0·01. The neopentyl group in 4 more effectively shields the rear side of the reaction center than the tert-butyl group in 2-chloro-2,3,3-trimethylbutane. The rate ratio between 4 and 2-chloro-2-methylpropane (3) at 25°C is 275 in trifluoroethanol and predicted to increase to 950 in trifluoroacetic acid. The previous 4:3 rate ratio of 22 in 80% ethanol evidently underestimated the B-strain effect on the solvolysis rate of 4 by a factor of at least 40.

Dynamic solvent effects on the thermal isomerization of zinc dithizonate.

The pressure dependence of the rate constant for the thermal isomerization of zinc dithizonate was measured at various temperatures in highly viscous solvents, i.e., glycerol triacetate and 2,4-dicyclohexyl-2-methylpentane, as well as in their chemically similar nonviscous solvents, such as methyl acetate and methylcyclohexane. In the nonviscous solvents, the reaction was slightly retarded upon external pressure suggesting the validity of the transition state theory. On the other hand, in the viscous solvents, the reaction, even at ambient pressure, was strongly retarded by an increase in external pressure suggesting the slow thermal fluctuation of the solvent molecules upon external pressure in highly viscous systems. This made it possible to observe the dynamic solvent effects on the isomerization of zinc dithizonate in viscous solvents. The viscosity dependence of the rate constants was rationalized by the two-dimensional reaction coordinate model.

Quantitative treatment of rate enhancement due to F-strain in the solvolysis of (Z)-2-ethylidene-1-adamantyl mesylate and halides

Abstract The Z:E ratios for the trifluoroethanolysis at 25°C of the title mesylate, chloride, bromide, and iodide are 126, 1020, 2230, and 9680, respectively, whose logarithms are linearly correlated with MM2 steric energies and Hanschs Es values.

Effects of γ-cyano substituent in allylic bridgehead solvolyses: evidence for cyano π conjugation in carbocations

Abstract Slightly flexible 4-methylene-3-homoadamantyl mesylate solvolyzes 2 × 10 5 times faster than rigid 2-methylene-1-adamantyl mesylate. Placement of a cyano substituent on the ( E ) position of the vinylic carbon of these systems hardly alter the rate ratio (6 × 10 5 ). The result suggests fortuitous cancellation of the inductive destabilizing effect of the cyano substituent by its mesomeric stabilizing effect toward the partially conjugated allylic cation. PM3 calculations on simple allylic cations as models supported the experimental results.

A dynamic-route study of solution reactions: ring closure of a hexadienone

Pressure effects on the thermal rate process from trans-1 to 2 shown in scheme 1 were studied in ethanol and highly viscous 2-methylpentane-2, 4-diol. In the former, the reaction was moderately accelerated as expected from the transition state theory (TST). In the latter, the TST became invalid at high pressures and low temperatures and a so-called dynamic solvent effect was observed. The results clearly demonstrated that the medium and the chemical coordinates have to be treated independently.

Long-Time Relaxation of Stress-Induced Birefringence of Microcrystalline Alkali Halide Crystals

Alkali halide single crystals are most commonly used as the diluent matrix in the tablet method or disk technique for spectroscopic measurements. However, stress-induced birefringence (SIB) of alkali halides as well as intrinsic birefringence manifest during the disk formation process. Thus, the true chiroptical measurement is disturbed by optical anisotropies (OA) containing SIB and intrinsic birefringence, except in the case of optical homogeneity. SIB is generally larger than intrinsic birefringence and has a value of several thousand millidegrees in the ultraviolet-visible wavelength range, although this varies with disk type. Here, to investigate the SIB origin, alkali halide crystals were examined using polarized light, X-ray diffraction, Fourier-transform infrared, and electron backscattering diffraction spectroscopic measurements. It was found that, after stress release, the SIB exhibited nonlinear long-time relaxation, which roughly converged within several hours, with the only time-invariant intrinsic birefringence remaining being due to OA. This behavior was strongly related to an increase in the quasi-amorphous domain and the generation of an air gap between the crystallite boundaries and their pellets. Further, a straightforward correlation was found between amorphization and an increase in the disk water content caused by deliquescence. Thus, the OA of alkali halide single crystals was found to have two different origins yielding intrinsic birefringence and SIB.