A new interpretation of the Baylis-Hillman mechanism

[reaction: see text] On the basis of reaction rate data, we have proposed a new mechanism for the Baylis-Hillman reaction involving the formation of a hemiacetal intermediate. We have determined that the rate-determining step is second order in aldehyde and first order in DABCO and acrylate. We have shown that this mechanism is general to aryl aldehydes under polar, nonpolar, and protic conditions using both rate data and two isotope effect experiments.     

Electrode reduction mechanism of aromatic nitriles in aprotic solvents: Benzonitrile

The electrode reaction mechanism of benzonitrile in anhydrous DMF has been studied by polarography, potential sweep voltammetry, macroscale electrolysis, e.p.r. spectrometry and kinetic analysis. The relatively stable radical anion formed by one-electron addition decays according to a first order kinetic law, with formation of benzene and CN^? as final products together with substantial amounts of the alkene and alkylamine corresponding to the tetraalkylammonium salt used as background electrolyte. The results are interpreted in terms of a mechanism involving protonation of the anion radical by the solvent and the back-ground electrolyte, followed by dismutation of the resulting neutral radical with the anion radical and elimination of CN^? ions from the anionic species thus formed. By phenol addition CN^? elimination is prevented and the reduction proceeds to 1-cyclohexene-1-nitrile or to cyclohexane-nitrile, depending on the reduction potential.     

Cellulose solutions in dipolar aprotic solvents

Preparation conditions and properties of cellulose solutions in DMAA containing 7–8% lithium chloride are considered. Investigation of the optical anisotropy and structure of cellulose solutions in this solvent confirms that the LS state is attained with an increase in the concentration of cellulose. On the basis of these cellulose solutions, fibers not ranking below viscose fibers in mechanical properties are prepared. The use of a mixed solution of 95% cellulose and 5% poly(amidobenzimidazole) in DMAA containing 7% lithium chloride makes it possible to prepare films and fibers whose strength is more than two times greater than the strength of cellulose hydrate fibers. The deformation-driven orientation and supramolecular structure of fibers and films are studied by spectroscopy and small-angle scattering of polarized light.     

Electroreduction of carbon dioxide: Part II. The mechanism of reduction in aprotic solvents

The mechanism of electroreduction of carbon dioxide in aprotic solvents on mercury, lead, tin, indium and platinum is studied using the photoemission method and the method of stationary polarization curves. When comparing the data of photoemission and “dark” (polarization) measurements it was found that in the first Tafel region of the polarization curves the rate-determining step is the transfer of the second electron to (CO₂)^.?₂ anion-radicals formed as a result of the interaction of initially generated CO^.?₂ anion-radicals with adsorbed CO₂ molecules. In the second Tafel region the rate-determining step is the transfer of the first electron to an adsorbed CO₂ molecule. The peculiarities of electroreduction of carbon dioxide in aprotic solvents can be explained provided that the effect of potential on adsorption of CO₂ and anion-radicals and the effect of repulsion of negatively charged radicals are taken into account.     

Polarography of organic compounds in aprotic solvents

A review of the polarography of organic compounds in aprotic solvents is presented.     

Proton solvation in protic and aprotic solvents

Protonation pattern strongly affects the properties of molecular systems. To determine protonation equilibria, proton solvation free energy, which is a central quantity in solution chemistry, needs to be known. In this study, proton affinities (PAs), electrostatic energies of solvation, and pK_A values were computed in protic and aprotic solvents. The proton solvation energy in acetonitrile (MeCN), methanol (MeOH), water, and dimethyl sulfoxide (DMSO) was determined from computed and measured pK_A values for a specially selected set of organic compounds. pK_A values were computed with high accuracy using a combination of quantum chemical and electrostatic approaches. Quantum chemical density functional theory computations were performed evaluating PA in the gas‐phase. The electrostatic contributions of solvation were computed solving the Poisson equation. The computations yield proton solvation free energies with high accuracy, which are in MeCN, MeOH, water, and DMSO ?255.1, ?265.9, ?266.3, and ?266.4 kcal/mol, respectively, where the value for water is close to the consensus value of ?265.9 kcal/mol. The pK_A values of MeCN, MeOH, and DMSO in water correlates well with the corresponding proton solvation energies in these liquids, indicating that the solvated proton was attached to a single solvent molecule. © 2016 Wiley Periodicals, Inc.     

Rotational diffusion in aprotic and protic solvents

We present the orientational relaxation times in protic and aprotic solvents for rose bengal in its lowest excited singlet state. The method uses a mode locked dye laser for polarized excitation, and time correlated single photon counting for determination of the time resolved polarized fluorescence. The observed orientational decay for the dipolar aprotic solvents and the alcohols are in agreement with the values predicted by the Stokes-Einstein diffusion equation. In the latter solvents, volume and shape corrections must be made for attachment of the alcohol to the two anion sites of the dye molecule. The solvent N-methylformamide, however, shows rose bengal reorienting much faster than the alcohols. Our interpretation of this data suggests that agreement with the Stokes-Einstein equation (stick boundary conditions) is coincidental. We propose a solvent torque model in which the solvent interaction at each anion site of rose bengal controls the deviations from an expected slip boundary condition. This qualitative model is used to correlate our data as well as relevant data in the literature. The values in picoseconds for the observed orientational relaxation times are given in parenthesis; acetone (70), DMF (160), DMSO (420), MeOH (190), EtOH (450), isopropanol (840), NMF (500).     

Cellulose decomposition behavior in hot-compressed aprotic solvents

Microcrystalline cellulose (avicel) is treated in hot-compressed aprotic solvents,sulfolane and 1,4-dioxane,using a batch-type reaction system with a molten tin bath in a range from 290 to 390℃. The corresponding densities of the solvent are 0.25-1.26 g/cm3 and 0.21-1.03 g/cm3 for sulfolane and 1,4-dioxane,respectively. As a result,in both solvents,more than 90% of cellulose is found to be de-composed to the solvent-soluble portion in which levoglucosan is the main component with the high-est yield of about 35% on original cellulose basis. The decomposition rate to levoglucosan is,however,faster in sulfolane than in 1,4-dioxane,while levoglucosan is more stable in 1,4-dioxane. In addition,its yield is found to be solvent-density dependent to be highest around 0.4-0.5 g/cm3 for both solvents. To elucidate these decomposition behaviors,the results obtained in this study with aprotic solvents are compared with protic solvents such as water and methanol in previous works.     

Anodic oxidation mechanism of 3-arylaminoindoles at a rotating and at a (new type of) pulsing platinum electrode in dipolar aprotic solvents

The oxidation of 2-phenyl-3-arylaminoindoles (1a–e) has been studied in acetonitrile, dimethylformamide and propylene carbonate at a new type of platinum electrode with periodical renewal of the diffusion layer and at a rotating platinum electrode by cyclic voltammetry, d.c. and a.c. voltammetry, controlled potential coulometry and spectrometry (e.s.r. and u.v.). In the examined aprotic solvents and in the presence of a base (like water or diphenylguanidine) amines 1a–e undergo a two-electron oxidation to the corresponding imines. In a protophobic medium (acetonitrile or propylene carbonate) 1a–e are oxidized in two one-electron steps, the first of which leads to the formation of a cation radical (identified by e.s.r. spectrum), oxidizable at more positive potentials (second step). The second electron transfer, however, must be simultaneous with, or be immediately followed by, a very fast deprotonation reaction. The same e.e.c. sequence explains the observed bielectronic step of 1a–e in a protophilic aprotic solvent, like dimethylformamide.     

Extraction of eleutherococcus by aprotic solvents

V. V. Kuibyshev Tomsk State University. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 143–144, January–February, 1989.     

Electrochemical reduction of benzenesulphenyl chloride in aprotic solvents

The cathodic behaviour of benzenesulphenyl chloride has been studied at a platinum electrode in acetonitrile by using cyclic voltammetry and controlled potential electrolysis. It has been evidentiated that PhSCl gives rise, in an one-electron reduction process, to PhSSPh and Cl^? according to $2PhSCl+2{\mathit{e}}^{?}\to PhSSPh+2{Cl}^{?}$ The chloride ions so generated catalyze the subsequent hydrolysis of PhSCl caused by traces of water present in the organic solvent, i.e. $2PhSCl+H_{2}O\stackrel{{Cl}^{?}}{\to }PhSOSPh+2HCl$ which is concomitant with the electrodic reaction and hence accounts for the overall n_e value lower than one. The same results have been obtained in nitromethane while in dimethyl-formamide the above reported hydrolysis reaction seems to be catalyzed by the solvent itself.     

New protic salts of aprotic polar solvents

Aprotic polar solvents such as DMF, acetonitrile or DMSO can be protonated to form stable triflate salts when treated with triflic acid. The reaction of the same solvents with N,N-bis(trifluoromethanesulfonyl)imide led to the isolation of the corresponding N,N-bis(trifluoromethanesulfonyl)imide salts.     

Oligopyridones: preparation and their folding behavior in polar aprotic solvents

The oligomerization of 5-bromo-2-hydroxypyridine through a CuI catalyzed one-pot condensation tactics was developed. A series of oligopyridones, from dipyridione to penta-pyridone, were therefore synthesized via the CuI-promoted C-N coupling reactions. The oligopyridones favor to have folded conformations in polar aprotic solvents. These were confirmed by using ¹H NMR, NOESY, and 1D-NOE experiments.     

Structure of dimethylammonium bromide in solutions in aprotic solvents

The structure of the dimethylammonium bromide molecule in solution has been studied on the basis of IR absorption spectra and low-temperature¹H NMR spectra. An analysis of the spectroscopic characteristics of the compounds shows that the most preferable of the possible structures is an ion pair with equivalent NH protons and two hydrogen bonds, which form a stable four-membered ring.Scientific-Research Institute of Physics, Leningrad University. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 2, pp. 98–102, March–April, 1991.