Negative Rho Value Research Articles

Toward a Chemiluminescent Molecular Device: Metal Ion-Enhanced Chemiluminescence of Benzylidenacridan with 15-Monoazacrown-5

The benzylidenacridans (1a-d) with one or more methoxy groups emit light from the excited N-methylacridone (NMA) when treated with a solution of sodium hypochlorite and alkaline hydrogen peroxide, in which a Hammett relationship with a negative rho value was applied between the chemiluminescence quantum yields and the sigma values. However, the dimethylamino group drastically inhibited the chemiluminescence in spite of its low sigma value. While the compound (2) with 15-monoazacrown-5 also showed a very weak chemiluminescence upon treatment with tetrabutylammonium hydroxide and hydrogen peroxide in acetonitrile, the chemiluminescence was enhanced up to 20 times in efficiency when divalent metal cations, such as magnesium and calcium ions, were added. The chemiluminescence inhibition is proved to be due to the shortening of the fluorescence lifetime of NMA due to energy dissipation by the amino substituents, but the incorporation of calcium ion into the cavity of the azacrown significantly prolonged the lifetime. Thus, the possibility of metal ion sensing by a chemiluminescent molecular device is suggested.

Borate‐Catalyzed Reactions of Hydrogen Peroxide: Kinetics and Mechanism of the Oxidation of Organic Sulfides by Peroxoborates

The kinetics of the oxidation of substituted phenyl methyl sulfides by hydrogen peroxide in borate/boric acid buffers were investigated as a function of pH, total peroxide concentration, and total boron concentration. Second-order rate constants at 25 degrees C for the reaction of methyl 4-nitrophenyl sulfide and H(2)O(2), monoperoxoborate, HOOB(OH)(3) (-), or diperoxoborate, (HOO)(2)B(OH)(2) (-), are 8.29 x 10(-5), 1.51 x 10(-2) and 1.06 x 10(-2) M(-1) s(-1), respectively. Peroxoboric acid, HOOB(OH)(2), is unreactive. The Hammett rho values for the reactions of a range of substituted phenyl methyl sulfides and hydrogen peroxide, monoperoxoborate or diperoxoborate are -1.50 +/- 0.1, -0.65 +/- 0.07 and -0.48 (two points only), respectively. The rho values for the peroxoborates are of significantly lower magnitude than expected from their reactivity compared to other peroxides. Nevertheless the negative rho values indicate positive charge development on the sulfur atom in the transition state consistent with nucleophilic attack by the organic sulfides on the peroxoborates as with the other peroxides. The kinetic parameters, including the lack of reactivity of peroxoboric acid, are discussed in terms of the differences in the transition state of reactions involving peroxoboron species with respect to those of other peroxides.

Aerobic oxidation of benzyl alcohols catalyzed by aryl substituted N-hydroxyphthalimides. Possible involvement of a charge-transfer complex.

A series of aryl-substituted N-hydroxyphthalimides (X-NHPIs) containing either electron-withdrawing groups (4-CH(3)OCO, 3-F) or electron-donating groups (4-CH(3), 4-CH(3)O, 3-CH(3)O, 3,6-(CH(3)O)(2)) have been used as catalysts in the aerobic oxidation of primary and secondary benzylic alcohols. The selective formation of aromatic aldehydes was observed in the oxidation of primary alcohols; aromatic ketones were the exclusive products in the oxidation of secondary alcohols. O-H bond dissociation enthalpies (BDEs) of X-NHPIs have been determined by using the EPR radical equilibration technique. BDEs increase with increasing the electron-withdrawing properties of the aryl substituent. Kinetic isotope effect studies and the increase of the substrate oxidation rate by increasing the electron-withdrawing power of the NHPI aryl substituent indicate a rate-determining benzylic hydrogen atom transfer (HAT) from the alcohol to the aryl-substituted phthalimide-N-oxyl radical (X-PINO). Besides enthalpic effects, polar effects also play a role in the HAT process, as shown by the negative rho values of the Hammett correlation with sigma(+) and by the decrease of the rho values (from -0.54 to -0.70) by increasing the electron-withdrawing properties of the NHPI aryl substituent. The relative reactivity of 3-CH(3)O-C(6)H(4)CH(2)OH and 3,4-(CH(3)O)(2)-C(6)H(3)CH(2)OH, which is higher than expected on the basis of the sigma(+) values, the small values of relative reactivity of primary vs secondary benzylic alcohols, and the decrease of the rho values by increasing the electron-withdrawing properties of the NHPI aryl substituent, suggest that the HAT process takes place inside a charge-transfer (CT) complex formed by the X-PINO and the benzylic alcohol.

Structure-reactivity relationships for the inhibition mechanism at the second alkyl-chain-binding site of cholesterol esterase and lipase.

Alkyl-N-phenyl carbamates (2-8) (see Figure 1), alkyl-N-phenyl thiocarbamates (9-15), 2,2'-biphenyl-2-ol-2'-N-substituted carbamates (16-23), and 2, 2'-biphenyl-2-N-octadecylcarbamate-2'-N-substituted carbamates (24-31) are prepared and evaluated for their inhibition effects on porcine pancreatic cholesterol esterase and Pseudomona species lipase. All inhibitors are characterized as transient or pseudo substrate inhibitors for both enzymes. Both enzymes are not protected from inhibition and further inactivated by carbamates 2-8 and thiocarbamates 9-15 in the presence of trifluoroacetophenone. Therefore, carbamates 2-8 and thiocarbamates 9-15 are exceptions for active site binding inhibitors and are probably the second alkyl-chain binding-site-directed inhibitors for both enzymes. The inhibition data for carbamates 2-8 and thiocarbamates 9-15 are correlated with the steric constant, E(s), and the hydrophobicity constant, pi; however, the inhibition data are not correlated with the Taft substituent constant, sigma. A comparison of the inhibition data for carbamates 2-8 and thiocarbamates 9-15 toward both enzymes indicates that thiocarbamates 9-15 are more potent inhibitors than carbamates 2-8. A comparison of the inhibition data for cholesterol esterase and Pseudomona species lipase by carbamates 2-8 or thiocarbamates 9-15 indicates that cholesterol esterase is more sensitive to the E(s) and pi values than Pseudomona species lipase. The negative slope values for the logarithms of inhibition data for Pseudomona species lipase by carbamates 2-8 and thiocarbamates 9-15 versus E(s) and pi indicate that the second alkyl-chain-binding site of Pseudomona species lipase is huge, hydrophilic, compared to that of cholesterol esterase, and prefers to interact with a bulky, hydrophilic inhibitor rather than a small, hydrophobic one. On the contrary, the second alkyl-chain-binding site of cholesterol esterase prefers to bind to a small, hydrophobic inhibitor. Both enzymes are protected from inhibition by carbamates 16-23 in the presence of trifluoroacetophenone. Therefore, carbamates 16-23 are characterized as the alkyl chain binding site, esteratic site oxyanion active site directed pseudo substrate inhibitors for both enzymes. Both enzyme inhibition data for carbamates 16-22 are well-correlated with sigma alone. The negative rho values for these correlations indicate that the serine residue of both enzymes and carbamates 16-22 forms the tetrahedral species with more positive charges than inhibitors and the enzymes and follow the formation of the carbamyl enzymes with more positive charges than the tetrahedral species. Carbamates 24-31 are also exceptions for active site binding inhibitors and probably the second alkyl chain binding site-directed inhibitors for both enzymes. However, the enzyme inhibition constants for carbamates 24-31 are correlated with values of sigma, E(s), and pi. The negative rho values for these correlations indicate that both enzymes and carbamates 24-31 form the tetrahedral species with more positive charges than inhibitors and the enzymes and follow the formation of the carbamyl enzymes with more positive charges than those tetrahedral species. Therefore, carbamates 24-31 may bind to both the active sites and the second alkyl chain binding site and follow the evacuation of the active sites. A comparison of the rho values for cholesterol esterase and Pseudomona species lipase by carbamates 24-31 indicates that cholesterol esterase is much more sensitive to the sigma values than Pseudomona species lipase. The negative sensitivity values, delta, for the cholesterol esterase inhibitions by carbamates 24-31 indicate that the enzyme prefers to bind to a bulky carbamyl group rather than bind to a small one. The hydrophobicity of carbamates 24-31 does not play a major role in both enzyme inhibitions.

Dramatic Change of Carbonyl Oxide Reactivity by the Potent Electron-Withdrawing Trifluoromethyl Group.

alpha,alpha,alpha-Trifluoroacetophenone oxide, generated by (1)O(2) oxidation of Ph(CF(3))CN(2), reacted electrophilically with sulfoxides (rho = -0.74 vs sigma; r = 0.95) affording sulfides as the major product. The intermediacy of persulfoxides formed via cyclic peroxidic sulfuranes in the novel deoxygenation was evidenced by an (18)O-tracer study and by the formation of benzaldehyde in the reaction with benzyl phenyl sulfoxide. Olefins were oxidized to epoxides during the photooxidation of Ph(CF(3))CN(2). From the fact that the epoxidation proceeded almost stereospecifically and the substituent effect on styrenes resulted in the negative rho-value of -1.66 vs sigma (r = 0.97), a dioxirane was proposed as the second intermediate which might be formed by the isomerization of carbonyl oxide. Trapping experiments with styrenes and sulfoxides suggested that the cyclization of carbonyl oxide is competitive with the reaction with sulfoxides. In contrast to the known nucleophilic nature of carbonyl oxides, the present oxide is shown to have an electrophilic character, indicating that the reactivity of carbonyl oxides could be controlled by substituents.

Mechanistic studies on the reaction of monopersulfate with a manganese(III) porphyrin in the presence of phenates and benzoates as axial ligands and subsequent epoxidation of olefins in homogeneous solution

The oxygen transfer reaction from Ph 4PHSO 5 to manganese(III) porphyrins in the presence of a series of tetrabutylammonium phenates and benzoates leading to a manganese-oxo species and the subsequent epoxidation of olefin has been studied in dichloroethane. When benzoates act as axial ligands of the catalyst, the Hammett plots obtained by changing the substituents on the phenyl ring of the ligand give negative rho values for both oxene formation and epoxidation. On the contrary, when phenates are employed the Hammett plot for olefin epoxidation provides a positive rho value. This is rationalized by assuming that the variation of the nature of the ligand produces a change of the rate determining step of the catalytic process. In addition, the oxo species formed in the presence of phenates behaves as an electrophilic oxidant whereas that formed in the presence of benzoates has a definite radical character.

Mechanisms of hydroxylation by cytochrome P-450: metabolism of monohalobenzenes by phenobarbital-induced microsomes.

The monohydroxylation of halobenzenes by phenobarbital-induced rat liver microsomes was studied. The p-halophenol was found to be the major metabolite from all four halobenzenes; o-halophenol formation decreased as the halogen atom size increased. Vmax for total hydroxylation (ortho and para products) correlated well with the sigma + Hammett constant with a negative rho value. This implies a positively charged intermediate in the rate-determining step. Vmax for either ortho or para hydroxylation alone did not correlate with a Hammett constant, implying that the product-determining step occurs after the rate-determining step. Rate-determining formation of a radical cation intermediate is postulated to explain this data.

The 3-fluoromethylphenyl group. a useful moiety for the study of solvolysis reactions with large negative rho-values

The 3-fluoromethylphenyl group. a useful moiety for the study of solvolysis reactions with large negative rho-values