Acid-catalyzed enzymatic hydrolysis of 1-methylcyclohexene oxide

Limonene-1,2-epoxide hydrolase from Rhodococcus erythropolis DCL14, an enzyme involved in the limonene metabolism of this microorganism, catalyzes the enantioselective hydrolysis of 1-methylcyclohexene oxide. (1R,2S)-1-Methylcyclohexene oxide was the preferred substrate and it was mainly hydrolyzed to (1S,2S)-1-methylcyclohexane-1,2-diol, while (1S,2R)-1-methylcyclohexene oxide was converted more slowly and mainly yielded (1R,2R)-1-methylcyclohexane-1,2-diol. The reaction proceeded with a high regioselectivity (C1:C2, 85:15). H₂¹⁸O-labelling experiments confirmed that the nucleophile was mainly incorporated at the most substituted carbon atom, suggesting that limonene-1,2-epoxide hydrolase uses an acid-catalyzed enzyme mechanism.     

Hydrosilylation of cyclohexene, 1-methylcyclohexene,and isopropylidenecyclohexane

Hydrosilylation of cyclohexene and isopropylidenecyclohexane with chloro(methyl)silanes Me_3–n SiHCl_n (n = 1–3) gives rise to cyclohexyl- and chloro(2-cyclohexylpropyl)methylsilanes. Hydrosilylation of 1-methylcyclohexene with chlorodimethylsilane (n = 1) occurs anomalously and involves double-bond migration to form a mixture of seven compounds: the cis and trans isomers of 2-, 3-, 4-chlorodimethyl(methylcyclohexyl)silanes and chlorodimethyl(cyclohexylmethyl)silane. Chlorodimethylsilane (n = 2) adds to 1-methylcyclohexene to form a mixture of the cis and trans isomers of dichloro(methyl)(2-methylcyclohexyl)silane and dichloro(cyclohexylmethyl)methylsilane. With trichlorosilane (n = 3), no other products than trichloro(cyclohexylmethyl)silane are formed. The hydrosilylation products were reacted with ethynylmagnesium bromide to synthesize the corresponding ethynyl derivatives.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 12, 2004, pp. 2007–2011.Original Russian Text Copyright © 2004 by O. Yarosh, Zhilitskaya, N. Yarosh, Albanov, Voronkov.This revised version was published online in April 2005 with a corrected cover date.     

Kinetics and mechanism of acid-catalyzed decomposition of triazenes in aqueous ethanolic solutions

The kinetics and nature of the end products of the acid decomposition of triazenes (R-N=N-NHR, where R is aryl, and R is aryl or alkyl) in aqueous ethanolic solutions of varying composition have been studied. A hydrolysis mechanism which includes protonation of the triazene molecule (R-N+H=NH-R), formation of an _N-transition stafte (R-N=N-N+H₂-R), and dissociation of this state has been proposed to explain the literature data and the opposite ionic effects observed in the present study. This mechanism explains the roles of the acid anion and the water molecule in the hydrolysis of triazenes. An analytical expression has been obtained from the hydrolysis constant which agrees with the experimental results.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 1, pp. 23–30, January–February, 1993.     

The acid-catalyzed rearrangement of 1-N-allylindolines

Zinc chloride-catalyzed rearrangement of 1-N-allylindoline and 1-N-(2-methylallyl)indoline proceeds readily in refluxing xylene to give 7-allylindoline and 7-(2-methylallyl)indoline in 73% and 86% yields, respectively. The reaction of 1-N-2-butenylindoline and zinc chloride give rise to the mixture of 7-(1-methylallyl)indoline, 7-(cis- and trans-1-methyl-1-propenyl)indoline, and 7-(trans-2-butenyl)indoline. On the other hand, the similar reaction of 1-N-(3-methyl-2-butenyl)indoline with zinc chloride led to the formation of a mixture of 1,2,5,6-tetrahydro-4,4-dimethyl-4H-pyrrolo[3,2,1-ij]quinoline and 7-(3-methyl-2-butenyl)indoline.     

Kinetics of polyesterification. II. Foreign acid-catalyzed dibasic acid and glycol systems

Polyesterification of diacid and diol in the presence of the foreign acid p-toluene sulfonic acid as catalyst was carried out under constant reaction temperatures of 140–166°C (rather than at the usual constant oil-bath temperature) and at molar ratios r of diol to diacid of 1.2–3.5. The experimental data obtained do not fit conventional rate equations as they appear in the literature. On the basis of ion pair formation, consideration of proton transfer from acid to alcohol, variation in dielectric constant of the reaction mixture as conversion increases, and inclusion of reverse reaction due to presence of unremoved water, we proposed a reaction mechanism and rate equations. The rate equation fitted our experimental data quite well. In addition the self-catalysis (absence of foreign acid) also showed promise, as confirmed by the experimental data measured under constant reaction temperatures, which appear in Part I.     

Kinetics of acid-catalyzed O-atom transfer from a hydroperoxorhodium complex to organic and inorganic substrates

Oxygen atom transfer from (NH(3))(4)(H(2)O)RhOOH(2+) to organic and inorganic nucleophiles takes place according to the rate law -d[(NH(3))(4)(H(2)O)RhOOH(2+)]/dt = k[H(+)] [(NH(3))(4)(H(2)O)RhOOH(2+)][nucleophile] for all the cases examined. The third-order rate constants were determined in aqueous solutions at 25 degrees C for (CH(2))(5)S (k = 430 M(-)(2) s(-)(1), micro = 0.10 M), (CH(2))(4)S(2) (182, micro = 0.10 M), CH(3)CH(2)SH (8.0, micro = 0.20 M), (en)(2)Co(SCH(2)CH(2)NH(2))(2+) (711, micro = 0.20 M), and, in acetonitrile-water, CH(3)SPh (130, 10% AN, micro = 0.20 M), PPh(3) (3.74 x 10(3), 50% AN), and (2-C(3)H(7))(2)S (45, 50% AN, micro = 0.20 M). Oxidation of PPh(3) by (NH(3))(4)(H(2)O)Rh(18)O(18)OH(2+) produced (18)OPPh(3). The reaction with a series of p-substituted triphenylphosphines yielded a linear Hammett relationship with rho = -0.53. Nitrous acid (k = 891 M(-)(2) s(-)(1)) is less reactive than the more nucleophilic nitrite ion (k = 1.54 x 10(4) M(-)(2) s(-)(1)).     

Kinetics and mechanism of acid-catalyzed oxidation of bromide ions by the aqueous chromyl(IV) ion

The reaction between the aqueous chromyl ion, CraqO2+, and Br- is acid-catalyzed and generates Br2. Kinetic studies that utilized a superoxochromium ion, CraqOO2+, as a kinetic probe yielded a mixed third-order rate law, -d[CraqO2+]/dt=k[CraqO2+][Br-][H+], where k=608+/-11 M-2 s-1. Experimental data strongly favor a one-electron mechanism, but the reaction is much faster than predicted on the basis of the reduction potential for the Br*/Br- couple. The reduction of CraqO2+ by transition-metal complexes, on the other hand, exhibits "normal" behavior, that is, k=(1.37x10(3)+1.94x10(3) [H+]) M-1 s-1 for Os(1,10-tris-phenanthroline)(3)2+ and <10 M-1 s-1 for Ru(2,2'-bipyridine)3(2+) at 0.1 M H+. The reduction of CraqOO2+ by Br2*- takes place with a rate constant k=(1.23+/-0.20)x10(9) M-1 s-1, as determined by laser-flash photolysis.     

Kinetics and mechanism of the acid-catalyzed hydrolysis of [carbonatoethylenediamine(L)2cobalt(III)]+ ions

The kinetics of acid-catalyzed hydrolysis of the [Co(en)(L)₂(O₂CO)]⁺ ion (L = imidazole, 1-methylimidazole, 2-methylimidazole) follows the rate law –d[complex]/dt = {k ₁ K[H⁺]/(1 + K[H⁺])}[complex] (15–30 or 25–40 °C, [H⁺] = 0.1–1.0 M and I = 1.0 M (NaClO₄)). The reaction course consists of a rapid pre-equilibrium protonation, followed by a rate determining chelate ring opening process and subsequent fast release of the one-end bound carbonato ligand. Kinetic parameters, k ₁ and K, at 25 °C are 5.5 × 10^–2 s^–1, 0.44 M^–1 (ImH), 5.1 × 10^–2 s^–1, 0.54 M^–1 (1-Meim) and 3.8 × 10^–3 s^–1, 0.74 M^–1 (2-MeimH) respectively, and activation parameters for k ₁ are H₁ = 43.7 ± 8.9 kJ mol^–1, S₁ = –123 ± 30 J mol^–1 deg^–1 (ImH), H₁ = 43.1 ± 0.3 kJ mol^–1, S₁ = –125 ± 1 J mol^–1 deg^–1 (1-Meim) and H₁ = 64.2 ± 4.3 kJ mol^–1, S₁ = –77 ± 14 J mol^–1 deg^–1 (2-MeimH). The results are compared with those for similar cobalt(III) complexes.     

Lewis acid-catalyzed rearrangement of 2,2-dichloronorbornane to 1-chloronorbornane

The mechanism for the unusual AlCl(3)-catalyzed rearrangement of 2,2-dichloronorbornane to 1-chloronorbornane in pentane has been elucidated; the reaction, which also yields four isomeric dichloronorbornanes, occurs in three steps: (1). ionization to form the 2-chloro-2-norbornyl cation, which was fully characterized by two-dimensional (1)H and (13)C NMR in SbF(5)/SO(2)ClF; (2). Wagner-Meerwein shift to yield the 1-chloro-2-norbornyl cation, which was partially characterized by (1)H NMR; and (3). hydride abstraction.     

1, 2-丙二醇酸性条件下反应机理的量子化学研究

用MNDO方法研究了1, 2-丙二醇在酸性条件下反应机理, 寻找到了反应过渡态, 并从理论上确证了它最可行的反应过程和最终产物。     

Kinetics of bacterial potentiometric titrations: the effect of equilibration time on buffering capacity of Pantoea agglomerans suspensions

Several recent studies have made use of continuous acid-base titration data to describe the surface chemistry of bacterial cells as a basis for accurately modelling metal adsorption to bacteria and other biomaterials of potential industrial importance. These studies do not share a common protocol; rather they titrate in different pH ranges and they use different stability criteria to define equilibration time during titration. In the present study we investigate the kinetics of bacterial titrations and test the effect they have on the derivation of functional group concentrations and acidity constants. We titrated suspensions of Pantoea agglomerans by varying the equilibration time between successive titrant additions until stability of 0.1 or 0.001 mV s(-1) was attained. We show that under longer equilibration times, titration results are less reproducible and suspensions exhibit marginally higher buffering. Fluorescence images suggest that cell lysis is not responsible for these effects. Rather, high DOC values and titration reversibility hysterisis after long equilibration times suggest that variability in buffering is due to the presence of bacterial exudates, as demonstrated by titrating supernatants separated from suspensions of different equilibration times. It is recommended that an optimal equilibration time is always determined with variable stability control and preliminary reversibility titration experiments.     

Straightforward acid-catalyzed synthesis of pyrrolyldipyrromethenes

Three for one: Pyrrolyldipyrromethenes having different functional groups were efficiently synthesized from POCl(3)-promoted condensations between 5-chloro-2-formylpyrrole or isoindole derivatives and suitable pyrrole or indole fragments through a novel nucleophilic aromatic substitution of the initially formed protonated azafulvene rings.     

Attempted acid-catalyzed transannular reactions in the cembranoids

Several cembrane diterpenes were treated by various acids under different experimental conditions. All tested compounds were found to be acid sensitive, leading either to local chemical transformations (e.g. opening of epoxides), to transannular reactions and/or to unidentified mixtures. Sarcophine (7), the principal cembrane tested, was found to afford either all types of possible epoxide opening products (11–22), or, by a transannular reaction, when treated with SnCl₄, two tetrahydrooxepine derivatives (23 and 24). The structure determination of the various derivatives of sarcophine was based mainly on the ¹H and ¹³C-NMR spectra and also on several chemical transformations.Flaccidoxide (8), another cembranoid examined, was found to yield, with Zn/Cu couple, the expected deoxygenation product (sarcophytol-B, 32) together with an unexpected transannular reaction product (33). The structure, including the stereochemistry, of the latter THF-derivative of 8 (33) was elucidated by NMR and chemical reactions.