Transition from concerted to stepwise processes as a function of leaving group ability: density functional theory and experimental study of lyoxide‐promoted cleavages of phosphorothioate and phosphate triesters in water and methanol

The base‐promoted solvolysis of a series of O,O‐dimethyl O‐aryl and O,O‐dimethyl O‐alkyl phosphorothioates (1) as well as O,O‐dimethyl O‐aryl and O,O‐dimethyl O‐alkyl phosphates (2) was studied computationally by density functional theory methods in methanol and water continuum media to determine the transition between concerted and stepwise processes. In addition, an experimental study was undertaken on the solvolysis of these series in basic methanol and water. The computations indicate that the solvolytic mechanism for series 1 involves lyoxide attack anti to the leaving group in a concerted manner with good leaving groups having pK_a^Lg values < 12.3 in methanol and in a stepwise fashion with the formation of a 5‐coordinate thiophosphorane intermediate when the pK_a^Lg > 12.3. A similar transition from concerted to stepwise mechanism occurs with series 2 in methanol as well as with series 1 and 2 in water, although for the aqueous solvolyses with hydroxide nucleophile, the transitions between concerted and stepwise mechanisms occur with better leaving groups than in the case in methanol. The computational data allow the construction of Brønsted plots of log k₂^?OS versus pK_a^Lg in methanol and water, which are compared with the experimental Brønsted plots determined with these series previously and with new data determined in this work. Both the computational and experimental Brønsted data reveal discontinuities in the plots between substrates bearing O‐aryl and O‐alkyl leaving groups, with the gradients of the plots being far steeper than, and non‐collinear with, the O‐aryl leaving groups for solvolysis of the O‐alkyl‐containing substrates. These discontinuities signify that care should be exercised in interpreting breaks in Brønsted plots in terms of changes in rate‐limiting steps that signify the formation of an intermediate during a solvolytic process. Copyright © 2011 John Wiley & Sons, Ltd.     

Reactions of O‐aryl S‐aryl dithiocarbonates with secondary alicyclic amines in aqueous ethanol. Kinetics and mechanism

The reactions of O‐(4‐methylphenyl) S‐(4‐nitrophenyl), O‐(4‐chlorophenyl) (4‐nitrophenyl), O‐(4‐chlorophenyl) S‐phenyl, and O‐(4‐methylphenyl) S‐phenyl dithiocarbonates ( 1 , 2 , 3 , and 4 , respectively) with a series of secondary alicyclic (SA) amines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0 °C and an ionic strength of 0.2 M. The reactions are followed spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients (k_obs) are found. For some of the reactions, plots of k_obs vs. free amine concentration at constant pH are linear but others are nonlinear upwards. This kinetic behavior is in accordance with a stepwise mechanism with two tetrahedral intermediates, one zwitterionic (T^±) and the other anionic (T^?). In some cases, there is a kinetically significant proton transfer from T^± to an amine to yield T^?. Values of the rate micro constants k₁ (amine attack to form T^±), k_?1 (its back step), k₂ (nucleofuge expulsion from T^±), and k₃ (proton transfer from T^± to the amine) are determined for some reactions. The Brønsted plots for k₁ are linear with slopes β₁ = 0.2–0.4 in accordance with the slope values found when T^± formation is the rate‐determining step. The sensitivity of log k₁ and log k_?1 to the pK_a of the amine, leaving and non‐leaving groups are determined by a multiparametric equation. For the reactions of 1 – 4 with 1‐formylpiperazine and those of 3 and 4 with morpholine the k₂ and k₃ steps are rate determining. Copyright © 2010 John Wiley & Sons, Ltd.     

Anilinolysis of O‐butyl phenyl phosphonochloridothioate in acetonitrile: Synthesis,characterization, kinetic study,and reaction mechanism

This paper describes a simple optimized method for the synthesis of O‐butyl phenyl phosphonochloridothioate ( 4 ) under mild conditions. The target compounds were characterized by ¹H‐nuclear magnetic resonance (NMR), ¹³C‐NMR, and ³¹P‐NMR spectroscopy, as well as mass spectroscopy. The apparent structure of 4 was confirmed by optimization using the B3LYP/6‐311 + G(d,p) level in the Gaussian 09 program in acetonitrile. The nucleophilic substitution reactions of 4 with X‐anilines (XC₆H₄NH₂) and deuterated X‐anilines (XC₆H₄ND₂) were investigated kinetically in acetonitrile at 55.0°C. The free energy relationship with X in the anilines looked biphasic concave upwards with a break region between X = H and X = 3‐MeO, giving large negative ρ_X and small positive β_X values. The deuterium kinetic isotope effects were secondary inverse (k_H/k_D < 1: 0.789‐0.995) and the magnitudes, (k_H/k_D), increased when the nucleophiles were changed from weakly basic to strongly basic anilines. A concerted S_N2 mechanism is proposed on the basis of the selectivity parameters and the variation trend of the deuterium kinetic isotope effects with X.     

Kinetics and mechanism of the anilinolysis of aryl 4‐nitrophenyl thionocarbonates in aqueous ethanol

The reactions of bis(4‐nitrophenyl), 3‐chlorophenyl 4‐nitrophenyl, and 3‐methoxyphenyl 4‐nitrophenyl thionocarbonates ( 1 , 2 , and 3 , respectively) with a series of anilines are subjected to a kinetic investigation in 44 wt.% ethanol–water, at 25.0 °C and an ionic strength of 0.2 M. Under aniline excess, pseudo‐first‐order rate coefficients (k_obs) are found. Plots of k_obs versus aniline concentration are linear, with the slopes (k_N) pH independent, k_N being the rate coefficient for the anilinolysis of the thionocarbonates. The Brønsted plot (log k_N vs. pK_a of anilinium ions) for thionocarbonate 1 is linear, with slope (β) 0.62, which is consistent with a concerted mechanism. The Brønsted plots for thionocarbonates 2 and 3 are curved, with slopes 0.1 at high pK_a for both reaction series and slopes 0.84 and 0.79 at low pK_a for the reactions of 2 and 3 , respectively. The latter plots are in accordance to stepwise mechanisms, through a zwitterionic tetrahedral intermediate (T^±) and its anionic analogue (T^?), the latter being formed by deprotonation of T^± by the basic form of the buffer (HPO). The Brønsted curves are explained by a change in the rate‐limiting step, from deprotonation of T^± at low pK_a, to its formation at high pK_a. The influence of the amine nature and the non‐leaving and electrophilic groups of the substrate on the kinetics and mechanism is also discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Reaction of poly‐L‐lysine with aryl acetates and aryl methyl carbonates. A mechanistic study

The pH profile (log k vs. pH) of the reactions of poly‐L ‐lysine (PL) with a series of aryl acetates and aryl methyl carbonates in aqueous solution show the same conformational changes as those determined by potentiometric titrations. When PL is a random coil, the most probable mechanism for the reactions studied is through the formation of a tetrahedral intermediate and its breakdown to products as the rate‐determining step. The tetrahedral intermediate is stabilized by a hydrogen bond interaction between the nitro groups in the substrate and the NH group of the principal chain or some NH₂ groups of the lateral chains. Copyright © 2007 John Wiley & Sons, Ltd.     

Concentration‐dependent Raman study of noncoincidence effect in the NH2 bending and CO stretching modes of HCONH2 in the binary mixture (HCONH2 + CH3OH)

The isotropic and anisotropic parts of the Raman spectra of NH₂ bending and ν(CO) stretching modes of HCONH₂ in a hydrogen‐bonding solvent, methanol, at different concentrations have been analyzed carefully in order to study the noncoincidence effect (NCE). In neat HCONH₂, the experimentally measured values of noncoincidence Δν_nc are ∼11 and ∼18 cm⁻¹ for the NH₂ bending and ν(CO) stretching modes, which reduce to 0.45 and 1.14 cm⁻¹, respectively at the concentration of HCONH₂ in mole fraction, χ_m = 0.1. The experimental results have been explained on the basis of two models, namely, the microscopic prediction of Logan and the macroscopic model of Mirone and Fini. The relative success of the two models in explaining the experimental data for both the modes have been discussed. It has been observed that in case of the ν(CO) stretching vibrational mode the Logan model can reproduce the experimental data rather precisely, whereas in the case of the NH₂ bending mode, Mirone and Fini model yields more accurate results. Copyright © 2006 John Wiley & Sons, Ltd.     

Solvatochromism of 2‐(N,N‐dimethylamino)‐7‐nitrofluorene and the natural dye β‐carotene: application for the determination of solvent dipolarity and polarizability

The effects of solvents on chemical phenomena (rate and equilibrium constants, spectroscopic transitions, etc.) are conveniently described by solvation free‐energy relationships that take into account solvent acidity, basicity and dipolarity/polarizability. The latter can be separated into its components by manipulating the UV–vis spectra of two solvatochromic probes, 2‐(N,N‐dimethylamino)‐7‐nitrofluorene (DMANF) and a di‐(tert‐butyl)‐tetramethyl docosanonaen probe (ttbP9) whose synthesis is laborious and expensive. Recently, we have shown that the natural dye β‐carotene can be conveniently employed instead of ttbP9 for the determination of solvent polarizability (SP) of 76 molecular solvents and four ionic liquids. In the present work, we report the polarizabilities of further 24 solvents. Based on the solvatochromism of β‐carotene and DMANF, we have calculated solvent dipolarity (SD) for 103 protic and aprotic molecular solvents, and ionic liquids. The dependence of SD and SP on the number of carbon atoms in the acyl‐ or alkyl group of several homologous series (alcohols; 2‐alkoxyethanols; carboxylic acid‐ anhydrides, and esters, ionic liquids) is calculated and briefly discussed. Copyright © 2013 John Wiley & Sons, Ltd.     

Solvent,salt and high pressure effects on the rate and equilibrium constants for the formation of tri‐n‐butylphosphoniumdithiocarboxylate

Solvent, salt and high pressure effects on the rate and equilibrium constants for the formation of tri‐n‐butylphosphoniumdithiocarboxylate at 298.2 K are reported. This equilibrium is shifted to the phosphobetaine in polar solvents, salt solutions and under high external pressure. The reaction volume changes dramatically on going from less polar diethyl ether (?69 cm³ mol^?1) and tetrahydrofurane (THF) (?66 cm³ mol^?1), to more polar acetonitrile (?39 cm³ mol^?1) and acetone (?38 cm³ mol^?1). Copyright © 2010 John Wiley & Sons, Ltd.     

Determination of N and O‐atoms and N2 (A) Metastable Molecule Densities in the Afterglows of N2‐H2, Ar‐N2‐H2 and Ar‐N2‐O2 Microwave Discharges

Early afterglows of N₂‐H₂, Ar‐N₂‐H₂ and Ar‐N₂‐O₂ flowing microwave discharges are characterized by optical emission spectroscopy. The N and O atoms and the N₂ (A) metastable molecule densities are determined by optical emission spectroscopy after calibration by NO titration for N and O‐atoms and measurements of NO and N₂ band intensities. If an uncertainty of 30% is estimated on N‐atomic density, an inaccuracy of one order of magnitude is obtained on the O and N₂ (A) densities. In N₂‐(0.05‐2.5%)H₂ and Ar‐(1‐50%)N₂‐(0.05‐2.5%) H₂ gas mixtures, the O‐atoms are coming from O₂ impurities in the discharge. Concentrations of N and O‐atoms and of N₂ (A) densities are compared to the ones obtained in Ar‐(5‐50%)N₂‐(0.2‐2.5%) O₂ gas mixtures in which a controlled amount of O₂ is added. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reactivities of mixed organozinc and mixed organocopper reagents. Part 13 Kinetic study for phosphine‐catalyzed acylation of alkylarylzincs and effect of residual group on the transfer rate of alkyl group

Kinetics of reactions of di‐n‐butylzinc, n‐Bu₂Zn, and mixed n‐butyl(substituted phenyl)zinc reagents and n‐Bu(functional group (FG)?C₆H₄)Zn with benzoyl chloride in the presence of tri‐n‐butylphosphine have been investigated. Reaction rates of transferable n‐butyl group have been determined in tetrahydrofuran at 0 °C to compare the transfer rate of n‐butyl group in homo and mixed diorganozincs. Rate law is consistent with a third‐order reaction, which is first order in diorganozinc, benzoyl chloride, and n‐Bu₃P, and a mechanism was proposed. The lower reaction rate of n‐BuPhZn than that of n‐Bu₂Zn and negative reaction constant in Hammett plot are in accordance with the carbanionic charge of transferable n‐butyl group in the acylation reaction. These findings support the hypothesis that the reaction rate of transferable group, R_T, changes depending upon the residual group, R_R, in R_RR_TZn reagents. Copyright © 2016 John Wiley & Sons, Ltd.     

Recovery of the depolarization ratio of single lines from overlapping isotropic and anisotropic Raman profiles and assignment of molecular vibrations,with special reference to toluene and toluene‐d8

If a Raman line consists of several overlapping components, any attempt to perform a quantitative analysis of the wavenumber‐dependent depolarization ratio ρ(ν) fails, since ρ(ν) of a composite line is not an additive function of ρ(ν) of its constituents, and for a composite line representing an envelope of i single lines, ρ_comp(ν) ≠ ∑ρ_i(ν). This situation causes significant difficulties in the analysis of Raman spectra, especially in the case of weak lines (overtones, combination bands, etc.) situated in the close vicinity of strong fundamentals. In this paper, a specific algorithm is employed to recover isotropic and anisotropic Raman profiles. Using toluene and toluene‐d₈ as examples, it is shown that the ρ(ν) spectra of all components of a composite spectrum can be easily restored, facilitating the assignment of overtones, hot bands, and combination bands. Copyright © 2006 John Wiley & Sons, Ltd.     

NMR spectroscopy of organolithium compounds,part XXXIII: trans‐2,3‐dimethylcyclopropyllithium aggregation in diethylether: An equilibrium between three different complexes of comparable energy and the influence of LiBr on aggregate structure

The aggregate formation of trans‐2,3‐dimethylcyclopropyllithium ( 2 ) was studied in diethylether (DEE). With the help of the isotopic fingerprint method three clusters, a monomer, a dimer, and a fluxional tetramer in the ratio 1.00:0.53:0.22 were identified at 187 K. In the presence of 1 equivalent of LiBr cyclopropyllithium ( 1 ) forms in DEE at 163 K a mixed dimer and a mixed tetramer, while in THF a mixed dimer dominates. Under the same conditions 2 and 2,2,3,3‐tetramethylcyclopropyllithium ( 3 ) form in the solvent mixture DEE/THF (1:1) mixed dimers, as does 2 also in THF.     

Raman spectroscopic study of the mineral gerstleyite Na2(Sb,As)8S13·2H2O and comparison with some heavy‐metal sulfides

The mineral gerstleyite is described as a sulfosalt as opposed to a sulfide. This study focuses on the Raman spectrum of gerstleyite Na₂(Sb,As)₈S₁₃·2H₂O and makes a comparison with the Raman spectra of other common sulfides including stibnite, cinnabar and realgar. The intense Raman bands of gerstleyite at 286 and 308 cm⁻¹ are assigned to the SbS₃E antisymmetric and A₁ symmetric stretching modes of the SbS₃ units. The band at 251 cm⁻¹ is assigned to the bending mode of the SbS₃ units. The mineral stibnite also has basic structural units of Sb₂S₃ and SbS₃ pyramids with C_3v symmetry. Raman bands of stibnite Sb₂S₃ at 250, 296, 372 and 448 cm⁻¹ are assigned to Sb S stretching vibrations and the bands at 145 and 188 cm⁻¹ to S Sb S bending modes. The Raman band for cinnabar HgS at 253 cm⁻¹ fits well with the assignment of the band for gerstleyite at 251 cm⁻¹ to the S Sb S bending mode. Raman bands in similar positions are observed for realgar AsS and orpiment As₂S₃. Copyright © 2010 John Wiley & Sons, Ltd.