Reaction Constant Rho Research Articles

Laser flash photolysis study of the phenolic hydrogen abstraction by 1,2-aceanthrylenedione triplet

The nanosecond laser flash photolysis technique has been used to study the reactivity of 1,2-aceanthrylenedione (1) triplet towards phenols. Using acetonitrile as solvent, the quenching rate constant k q ranged from 3.3 x 10(5) L mol-1s-1 (phenol) to 1.3 x 10(7) L mol-1s-1 (4hydroxyphenol). Phenols with electron donating substituents were more reactive. The kinetic isotope effect (kH/kD) observed for the quenching of 1 with 4-methoxyphenol was 1.5. The Hammett plot for the reaction of the triplet of 1 with phenols gave a reaction constant rho = 1.04 in acetonitrile solution. The low value for the quenching rate constant measured for this triplet with 1,3-cyclohexadiene (k q=2.0 x 10(4) L mol-1s-1) and the diffusion controlled quenching rate constant obtained for beta-carotene suggest that the energy of the lowest triplet state for 1,2-aceanthrylenedione is located between 88 and 219 kJ mol-1.

Kinetics and mechanism of rhenium-catalyzed oxygen atom transfer from pyridine N-oxides to phosphines.

The oxygen atom transfer (OAT) reaction cited does not occur on its own in >10 h. Oxorhenium(V) compounds having the formula MeReO(dithiolate)PZ(3) catalyze the reaction; the catalyst most studied was MeReO(mtp)PPh(3), 1, where mtpH(2) = 2-(mercaptomethyl)thiophenol. The mechanism was studied by multiple techniques. Kinetics (initial-rate and full-time-course methods) established this rate law: v = k(c)[1][PyO](2)[PPh(3)](-1). Here and elsewhere PyO symbolizes the general case XC(5)H(4)NO and PicO that with X = 4-Me. For 4-picoline, k(c) = (1.50 +/- 0.05) x 10(4) L mol(-1) s(-1) in benzene at 25.0 degrees C; the inverse phosphine dependence signals the need for the removal of phosphine from the coordination sphere of rhenium prior to the rate-controlling step (RCS). The actual entry of PPh(3) into the cycle occurs in a fast step later in the catalytic cycle, after the RCS; its relative rate constants (k(4)) were evaluated with pairwise combinations of phosphines. Substituent effects were studied in three ways: for (YC(6)H(4))(3)P, a Hammett correlation of k(c) against 3sigma gives the reaction constant rho(c)(P) = +1.03, consistent with phosphine predissociation; for PyO rho(c)(N) = -3.84. It is so highly negative because PyO enters in three steps, each of which is improved by a better Lewis base or nucleophile, and again for (YC(6)H(4))(3)P as regards the k(4) step, rho(4) = -0.70, reflecting its role as a nucleophile in attacking a postulated dioxorhenium(VII) intermediate. The RCS is represented by the breaking of the covalent N-O bond within another intermediate inferred from the kinetics, [MeReO(mtp)(OPy)(2)], to yield the dioxorhenium(VII) species [MeRe(O)(2)(mtp)(OPy)]. A close analogue, [MeRe(O)(2)(mtp)Pic], was identified by (1)H NMR spectroscopy at 240 K in toluene-d(8). The role of the "second" PyO in the rate law and reaction scheme is attributed to its providing nucleophilic assistance to the RCS. Addition of an exogenous nucleophile (tetrabutylammonium bromide, Py, or Pic) caused an accelerating effect. When Pic was used, the rate law took on the new form v = k(NA)[1][PicO][Pic][PPh(3)](-1); k(NA) = 2.6 x 10(2) L mol(-1) s(-1) at 25.0 degrees C in benzene. The ratio k(c)/k(NA) is 58, consistent with the Lewis basicities and nucleophilicities of PicO and Pic.

Kinetics and mechanisms of catalytic oxygen atom transfer with oxorhenium(V) oxazoline complexes.

The rhenium(V) monooxo complexes (hoz)2Re(O)Cl (1) and [(hoz)2Re(O)(OH2)][OTf] (2) have been synthesized and fully characterized (hoz = 2-(2'-hydroxyphenyl)-2-oxazoline). A single-crystal X-ray structure of 2 has been solved: space group = P1, a = 13.61(2) A, b = 14.76(2) A, c = 11.871(14) A, alpha = 93.69(4) degrees, beta = 99.43(4) degrees, gamma = 108.44(4) degrees, Z = 4; the structure was refined to final residuals R = 0.0455 and Rw = 0.1055. 1 and 2 catalyze oxygen atom transfer from aryl sulfoxides to alkyl sulfides and oxygen-scrambling between sulfoxides to yield sulfone and sulfide. Superior catalytic activity has been observed for 2 due to the availability of a coordination site on the rhenium. The active form of the catalyst is a dioxo rhenium(VII) intermediate, [Re(O)2(hoz)2]+ (3). In the presence of sulfide, 3 is rapidly reduced to [Re(O)(hoz)2]+ with sulfoxide as the sole organic product. The transition state is very sensitive to electronic influences. A Hammett correlation plot with para-substituted thioanisole derivatives gave a reaction constant rho of -4.6 +/- 0.4, in agreement with an electrophilic oxygen transfer from rhenium. The catalytic reaction features inhibition by sulfides at high concentrations. The equilibrium constants for sulfide binding to complex 2 (cause of inhibition), K2 (L x mol(-1)), were determined for a few sulfides: Me2S (22 +/- 3), Et2S (14 +/- 2), and tBu2S (8 +/- 2). Thermodynamic data, obtained from equilibrium measurements in solution, show that the S=O bond in alkyl sulfoxides is stronger than in aryl sulfoxides. The Re=O bond strength in 3 was estimated to be about 20 kcal x mol(-1). The high activity and oxygen electrophilicity of complex 3 are discussed and related to analogous molybdenum systems.

Structure−Activity Relationships for the Glutathione Conjugation of 2-Substituted 1-Chloro-4-nitrobenzenes by Rat Glutathione S-Transferase 4-4

In the present study structure--activity relationships (SAR's) are described for the experimentally determined kinetic parameters (Km, kappa cat, and kappa cat/Km) of the GST 4-4-catalyzed reaction between GSH and 10 2-substituted 1-chloro-4-nitrobenzenes. Steric, lipophilic, and electronic parameters were correlated with the kinetic parameters. Moreover, charge distributions and several energy values were calculated for the substrates and the corresponding Meisenheimer intermediates with MeS- as a model nucleophile for the thiolate anion of GSH and used in the regression analyses. The correlations obtained were compared with the corresponding SAR's for the base-catalyzed GSH conjugation reaction at pH 9.2. A high correlation coefficient was found between the kinetic parameter kappa s for the base-catalyzed reaction and the Hammett substituent constant (sigma p). Much lower correlation coefficients were obtained with kappa cat and sigma p and with kappa cat/Km and sigma p. Moreover, the reaction constant rho was significantly higher for the base-catalyzed than for the enzyme-catalyzed reaction. Also, high correlations were found between the kinetic parameters and the charges on the p-nitro substituent in the substrates. When kappa s was plotted against these charges, a linear relationship was found in which the slope was larger than the slope of a corresponding plot with kappa cat/Km. The Hammett sigma p can be divided into an inductive (F) and a resonance (R) component. With multiple regression between the kinetic parameters and F and R, higher correlation coefficients were obtained than with sigma p alone. Our observations suggest that the transition states for the base-catalyzed and the GST 4-4-catalyzed GSH conjugation reaction are different. Moreover, single classical physiochemical and computer-calculated molecular parameters and combinations of them can be an alternative approach for examining SAR's for spontaneous and GST-catalyzed GSH conjugation reactions.

Concurrent electrophilic and oxidative pathways for reactions of .alpha.-hydroxyalkyl and .alpha.-alkoxyalkyl complexes of chromium(III) with mercury(II) ions

The reactions of Hg/sup 2 +/ with a number of ..cap alpha..-hydroxyalkyl and ..cap alpha..-alkoxyalkyl complexes of chromium(III) were studied. The lowest members of both series, CrCH/sub 2/OH/sup 2 +/ and CrCH/sub 2/OCH/sub 3//sup 2 +/, react with Hg/sup 2 +/ in an acid-independent electrophilic substitution reaction with the reaction constant rho = 10. The organomercurials formed are unstable and decompose rapidly to yield Hg/sup 0/. All the other organochromium complexes studied react with Hg/sup 2 +/ in acid-dependent electron-transfer reactions. (..cap alpha..-Hydroxyisopropyl(chromium ion, CrC(CH/sub 3/)/sub 2/OH/sup 2 +/, undergoes a one-electron transfer reaction, producing Cr/sup 2 +/ and Hg/sup +/ with k (M/sup -1/ s/sup 1/) = 166 + 467/(H/sup +/), while (..cap alpha..-ethoxyethyl)chromium ion, CrCH(CH/sub 3/)OC/sub 2/H/sub 5//sup 2 +/, reacts in a two-electron redox process, yielding Cr/sup 3 +/ and Hg/sup 0/, k (M/sup -1/s/sup -1/) = 0.535/(H/sup +/). (..cap alpha..-Hydroxyethyl)chromium ion, CrCH(CH)/sub 3/OH/sup 2 +/ reacts by both routes, k(M/sup -1/ s/sup -1/) = 4.11/(H/sup +/), with approx. 15% participation from the one-electron path and approx. 85% from the two-electron path. The effect of Cr/sup 2 +/ and Co(NH/sub 3/)/sub 5/F/sup 2 +/ (a specific scavenger for Cr/sup 2 +/) on the kinetics and stoichiometry ofmore » the reactions is discussed.« less

The solvolytic rearrangement of some 1,2-diphenyl-3,3-dichloroaziridines

The solvolytic rearrangement of 1,2-diphenyl-3,3-dichloroaziridine has been studied by stoichiometric, kinetic and isotopic tracer techniques. The heterogeneous reaction occurring in water suspension results in a quantitative yield of α-chloro-α-phenylacetanilide. The homogeneous reaction in dioxan-water mixtures yield α-chloro-α-phenylacetanilide and α-hydroxy-α-phenylacetanilide in nearly equal amounts, also in high total yield. Tracer results using Cl 36 and C 14 give information on the chlorine atom shift and on the point of ring cleavage respectively. The rate constant increases markedly with increase in the %H 2O in the mixed solvent (Grunwald-Winstein m = 0·88). Kinetic investigations are reported for a series of 1-( para-substituted phenyl)-2-phenyl- and 1-phenyl-2-( para)-substituted phenyl)-3,3-dichloroaziridines. The Hammett reaction constant rho (ϱ) for the former series is −3·5, while a value of −1·9 was obtained for the latter series. Mechanistic interpretations based upon our results are discussed in terms of an ion-pair intermediate.

THE MECHANISM OF THE CARBONYL ELIMINATION REACTION OF BENZYL NITRATE KINETIC ISOTOPE EFFECTS AND DEUTERIUM EXCHANGE

The carbonyl elimination reaction (ECO2) of benzyl nitrate has been investigated with the object of distinguishing between the concerted and carbanion mechanisms. A deuterium exchange experiment resulted in a very small amount of deuterium pickup. The nitrogen isotope effect, k14/k15, associated with formation of the nitrite ion was found to be 1.0196 ± 0.0007 at 30 °C. The two results taken together exclude the formation of a carbanion intermediate but are consistent with a concerted mechanism.Benzyl-α-d2 nitrate has been prepared and the rate of its carbonyl elimination reaction compared with that of the undeuterated compound. The deuterium isotope effect was 5.04 ± 0.25 at 60 °C. The significance of the magnitude of the nitrogen and deuterium isotope effects and of their interrelationship with the Hammett reaction constant rho is discussed in terms of the nature of the transition state and a comparison is made with other E2 elimination reactions.