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.     

La3+-catalyzed methanolysis of O,O-diethyl S-(p-nitrophenyl) phosphorothioate and O,O-diethyl S-phenyl phosphorothioate. Millions-fold acceleration of the destruction of V-agent simulants

The La3+-catalyzed methanolysis of two phosphorothioate derivatives, O,O-diethyl S-(p-nitrophenyl) phosphorothioate (4a) and O,O-diethyl S-phenyl phosphorothioate (4b) were studied as a function of [La3+] and pH in methanol solvent. In both cases the kinetics of catalyzed methanolysis maximize at pH 9.1 and a detailed analysis indicates that the dominant species responsible for catalysis are dimers formulated as La3+(2)(-OCH3)2 and La3+(2)(-OCH3)4. The catalysis is compared with that seen for the corresponding phosphate esters, namely paraoxon (3a) and O,O-diethyl phenyl phosphate (3b) for which La3+ catalysis is slightly better and markedly worse than for 4a and 4b respectively. Overall, at s(s)pH 9.1, a 2 mmol dm-3 solution of La(OTf)3 with equimolar NaOCH3 provides accelerations of 2.2x10(8)-fold, 9.7x10(6)-fold and 9.3x10(6)-fold for methanolysis of 3a, 4a and 4b, relative to the background reaction of methoxide reacting with the three substrates. In each case, the P-containing product of the reactions is exclusively diethyl methyl phosphate. Turnover experiments with 6-fold and 100-fold excesses of 4a and 4b respectively, methanolyzed in the presence of approximately 10 mmol dm-3 La3+ and equimolar NaOCH3, indicate that the reactions are essentially complete within 103 s and 70 min respectively. The latter turnover experiment with 4b corresponded to 100 turnovers in 70 min and an overall reaction t1/2 of 8 min. A common mechanism of reaction is postulated for each of the substrates which involves Lewis acid coordination of one of the La3+ to the P=O unit, followed by nucleophilic attack by the second La3+-(-)OCH3.     

Cu(II)-Mediated decomposition of phosphorothionate P=S pesticides. Billion-fold acceleration of the methanolysis of fenitrothion promoted by a simple Cu(II)-ligand system

The kinetics of methanolysis of the title compound (3) were studied in the presence of Cu(2+), introduced as Cu(OTf), in the presence of 0.5-1.0 eq. of methoxide and in the presence of 1.0 eq. of a ligand such as bipyridyl (5), phenanthroline (6) or 1,5,9-triazacyclododecane (4). In all cases the active species involve Cu(2+)((-)OCH(3)). In the case of added strong-binding ligands 5 or 6, a plot of the observed rate constant for methanolysis of 3 vs. [Cu(2+)](total) gives a curved line modelled by a process having a [Cu(2+)](1/2) dependence consistent with an active monomeric species in equilibrium with an inactive dimer i.e.[LCu(2+)((-)OCH(3))](2) <==> 2LCu(2+)((-)OCH(3)). In the case of the added strong binding ligand 4, the plot of the observed rate constant for methanolysis of 3 vs.[Cu(2+)](total) gives a straight line consistent with the catalytically active species being Cu(2+)(OCH(3)) which shows no propensity to form inactive dimers. Turnover experiments where the [3] > [Cu(2+)](total) indicate that the systems are truly catalytic. In the optimum case a catalytic system comprising 1 mM of the complex 4Cu(2+)((-)OCH(3)) catalyzes the methanolysis of 3 with a t(1/2) of approximately 58 s accounting for a 1.7 x 10(9)-fold acceleration relative to the background reaction at near neutral (s)(s)pH (8.75).     

Syntheses and reactions of some fluorinated dienes,cyclobutenes and furans

There are many reports¹ of the pyrolysis of fluorinated organic compounds, including the defluorination of cyclic fluorocarbons over iron to give aromatic compounds. Extending this technique we have investigated the flow pyrolysis of some readily accessible unsaturated fluorocarbons, such as I, II, and III, and found these to be synthetically

useful routes to fluorinated dienes, cyclobutenes, and furans. Pyrolyses were carried out using a nitrogen flow over platinum, iron or caesium fluoride heated at 430–700°. The various products can all be rationalized in terms of intermediate allylic radicals, and the solid substrate influences which allylic radicals are formed.We are also investigating the chemistry of those now accessible compounds, such as IV, V, and VI, and some of the preliminary results are described.

For example the fluoride ion induced dimerisation of IV gave two major products VII and VIII via a particular interesting mechanism.

     

Use of computers to calculate the mechanical characteristics of relaxation processes in polymers

A method of calculating the constants found in Kohlrausch's empirical function describing relaxation processes is proposed, which is based on the application of a well-known algorithm for solving nonlinear problems on a computer using the conjugate gradient method. All the experimental information is used by this method and the degree of calculation accuracy is increased. The reproducibility of calculations to find these constants is found to be satisfactory in the case under investigation when the duration of the experiment is varied.     

Magnetic breakdown and quantum magnetotransport with a constant pseudospin under tilted magnetic fields in an n -In x Ga1? x As/GaAs double quantum well

A procedure is proposed for precise scanning of the (B _⊥, B _‖) plane between the magnetic field projections that are perpendicular and parallel to (quasi-)two-dimensional layers when measuring their longitudinal and Hall magnetoresistances. Investigations of a n-In _x Ga_1−x As/GaAs double quantum well (x ≈ 0.2) performed using this procedure make it possible to reveal a number of the features of the magnetoresistance, which appear due to a complex energy spectrum of the double quantum well in a parallel field, and to separate them from the structures associated with the magnetic breakdown. The trajectories representing the features of the magnetoresistance in the (B _⊥, B _‖) plane are described by the semiclassical calculations of the quantization of the energy spectrum of the double quantum well under the action of the perpendicular field component. The structures appearing due to the magnetic breakdown are amplified with increasing the total magnetic field magnitude and, in the samples with low mobility, completely suppress the features caused by the motion of an electron with a constant pseudospin component. The peaks corresponding to the magnetic breakdown are split in a strong parallel field due to the spin splitting of the Landau levels. These splittings correspond to the effective Landé factor |g*| ≈ 3. Original Russian Text ? M.V. Yakunin, S.M. Podgornykh, V.N. Neverov, 2007, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 132, No. 1, pp. 241–249.     

Influence of molecular clustering on the interpretation of diffractograms of hydrocarbon films from tokamak T-10

The influence of molecular clustering on the formerly suggested interpretation of diffraction patterns of hydrocarbon films formed in the vacuum vessel of the tokamak T-10 is analyzed numerically. The simulation of clustering of simple hydrocarbon molecules C(D, H)₄, C₂(D, H)₄, and C₆(D, H)₆ and molecules composed of curved graphene (fullerenes and toroidal nanotubes) is carried out with the rigid body molecular dynamics method. It is shown that formerly neglected atomic correlations C–C and C–D(H) in the amorphous hydrocarbon component decrease the calculated values of the scattered intensity in the range of scattering vector modulus 5 < q < 20 nm^–1 because of homogenization of scatters on the spatial scale of ~1 nm. The allowance for these correlations does not change the diffraction patterns in the range q > 20 nm^–1. The results suggest the necessity to introduce to the procedure of determining the structural content of the films, similar to those from the tokamak T-10, the clusters formed by the van der Waals adhesion of hydrocarbon molecules to “graphene” nanoparticles. This simplifies the mathematical optimization to the former level of complexity—but for an extended ensemble of objects—and makes it possible to calculate the diffraction patterns of these objects using the distributed computing resources. A modified algorithm of structural content identification on the basis of joint X-ray and neutron diffractometry is suggested.