Determination de pKa d'acides conjugues de carbanions dans le DMSO anhydre. Reexamen des correlations entre cés pKa et les potentiels d'oxydation Eox determines dans le mĕme solvant

The pK_a values of several pseudo-acids of general formula

in anhydrous DMSO have been determined.The pK_a values have been correlated with the oxidation potential E_ox of their conjugated carbanions (enolate anions). The variation of E_ox of these enolate anions (and even those of cyclopentadienyl, indenyl, fluorenyl and triphenylmethyl carbanions) as a function of the pK_a of their acidic form is consistent with a correlation line whose slope is ?0.067 V/unity pK_a.In DMSO, all the β-carbonylated nitriles are in their enolic form, except the 2-cyano-esters which are entirely in the carbonyl form. In spite of this, their acidity is intermediate between that of the methylketones (Z = CH₃) and the aldehydes (Z = H). Thus, in DMSO medium, for β-carbonylated nitriles it appears that the acidity is not dependent on the enolization tendency.     

Stability of chemical species

In the framework of our theoretical approach of the structure and reactivity of chemical intermediates we have been led to reexamine the concept of stability which is widely used by the experimentalists often without specifying its true meaning. In this work we propose a more general definition of the concept of stabilization energy, namely, where ΔH_a is the heat of atomization of the species under consideration and the E_AB's are standard bond energy terms derived from the heats of atomization of reference compounds. Using experimental heats of formation or semiempirical ones deduced from theoretical heats of reaction of appropriate isodesmic processes, we have calculated the stabilization energies of various types of chemical species: saturated, unsaturated and conjugated molecules, free radicals, carbocations, and carbanions. The results obtained can be rationalized in terms of steric hindrance, angular strain, polar interactions, electron delocalization, and substituent effects. Moreover, we have shown that heats of hydrogenation and bond dissociation energies do not provide accurate information on the thermodynamic stabilization of unsaturated compounds and free radicals, respectively. Among other applications the concept of stabilization energy allowed us to propose a detailed classification of free radicals and to rationalize their reactivity. Considering the particular case of radical recombination reactions we have been able to deduce interesting equations showing the relations between the concept of stabilization energy and other quantities commonly used in chemical physics, namely, the bond cleavage enthalpy [BDE (C? C) if one considers alkane thermolysis], the thermodynamic stability measured by the free enthalpy change of a given reaction, and the kinetic stabilization related to the activation energy of a chosen chemical process This analysis allowed us to give a new interpretation of the adjectives transient, persistent, and stable introduced by Griller and Ingold and to show that the persistence of a radical may be due to other factors than steric ones. In conclusion, the concept of stabilization energy appears to be a good tool for rationalizing the static and dynamic properties of chemical species.     

Deaminatively generated carbocations as initiators of styrene polymerization

[reaction: see text] Addition polymerization is readily accomplished via free radicals and carbanions but conventional initiations via carbocations are limited principally by low initiator reactivity. Thermolyses of N-nitrosamides produce nitrogen-separated ion-pairs (NSIPs) containing exceedingly reactive carbocations. We report here the novel use of this facile mode of carbocation generation in the polymerization of styrene. Polystyrene of viscosity average molecular weight approximately 10(6) was obtained.     

Chemoselective Carbophilic Addition of α‐Diazoesters through Ligand‐Controlled Gold Catalysis

The chemoselective addition of arenes and 1,3‐diketones to α‐aryldiazoesters was achieved through ligand‐controlled gold catalysis. Unlike a dirhodium catalyst (which promotes C? H insertion and cyclopropanation) and a copper catalyst (which catalyzes O? H and N? H insertions), the gold catalyst with an electron‐deficient phosphite as the ancillary ligand exclusively gave the carbophilic addition product, thus representing a new and efficient approach to form “carbophilic carbocations”, which selectively react with carbon nucleophiles.     

A new method for empirical force field calculations on localized and delocalized carbocations

A new empirical force field method for localized and delocalized carbocations is described. Additional geometry parameters for carbocations were added to Allinger's MMP2 molecular mechanics program, which can treat delocalized π-systems. The effect of hyperconjugation in carbocations is introduced via a quantum chemical term into force field calculations for the first time. The calculated heats of formation are in excellent agreement with a wide range of experimental data; the largest deviations are about 3.5 kcal/mol. The calculated structures agree very well with those computed at correlated ab initio levels (MP2(full)/6-31G*). The relative energies and geometries of different conformations of representative carbocations also were in good agreement with MP4/6-31G*//MP2(full)/6-31G* results. © 1996 by John Wiley & Sons, Inc.     

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.     

酰基过氧化物的化学 VII: 过氧化环丙基甲酰与硝基烷烃阴离子的单电子转移反应

过氧化环丙基甲酰(CPP)能将硝基仲烷烃阴离子氧化成相应的酮. 产物分析和ESR研究揭示该反应经过由硝基烷烃阴离子向CPP的单电子转移引发步骤及随后的一系列自由基反应步骤, 生DN 主要产物酮和典型的自由基副产物, 如双环丙基、硝基烷基二聚体和能被ESR直接检测的双环丙基氮氧自由基等.     

Electron transfer reactions relevant to atom transfer radical polymerization

The continuous development of more active and stable catalysts in atom transfer radical polymerization (ATRP) has increasingly required a thorough knowledge of concurrent electron transfer reactions that can affect catalyst performance. Special attention is provided in this short review to such processes, including disproportionation, most pronounced in Cu-mediated ATRP, the reduction of radicals to carbanions or oxidation to carbocations, and radical coordination to the metal catalyst resulting in the interplay of controlled radical polymerization mechanisms.     

A kinetic study of the electron-transfer reaction of the phthalimide-N-oxyl radical (PINO) with ferrocenes

A kinetic study of the one-electron oxidation of a series of ferrocenes (FcX: X = H, CO2Et, CONH2, CH2CN, CH2OH, Et, and Me2) by PINO generated in CH3CN by reaction of N-hydroxyphthalimide (NHPI) with the cumyloxyl radical produced by 355 nm laser flash photolysis of dicumyl peroxide has been carried out. Ferrocenium cations were formed, and the reaction rate was determined by following the decay of PINO radical at 380 nm as a function of the FcX concentration. Rate constants were very sensitive to the oxidation potential of the substrates and exhibited a good fit with the Marcus equation, from which a lambda value of 38.3 kcal mol(-1) was calculated for the reorganization energy required in the PINO/ferrocenes electron-transfer process. Knowing the ferrocene/ferrocenium self-exchange reorganization energy it was possible to calculate a value of 49.1 kcal mol(-1) for the PINO/PINO- self-exchange reaction in CH3CN. Moreover, from the Marcus cross relation and the self-exchange rates of ferrocene and dimethylferrocene, the intrinsic reactivity of PINO in electron-transfer reactions has been calculated as 7.6 x 10(2) M(-1) s(-1). The implications of these values and the comparison with the electron-transfer self-exchange reorganization energies of peroxyl radicals are briefly discussed.     

Carbocations and Electrophilic Reactions

The general concept of carbocations encompasses all cations of carbon containing compounds which can be differentiated into two distinct classes: trivalent (“classical”) carbenium ions and pentacoordinated (“nonclassical”) carbonium ions. In this paper the preparation and structural study (by NMR, IR, Raman and ESCA spectroscopy) of stable carbenium ions and carbonium ions are discussed. As is well known, trivalent carbenium ions play an important role in electrophilic reactions of π- and n-donor systems. Similarly, pentacoordinated carbonium ions are the key to electrophilic reactions of σ-donor systems (single bonds). The ability of single bonds to act as σ-donor lies in their ability to form carbonium ions via triangular two electron, three-center bond formation. Consequently there seems to be no principle difference between the electrophilic reactions of π- and σ-bonds except that the former react more easily, even with weak electrophiles, whereas the latter necessitate more severe conditions. The role of carbocations in electrophilic reactions of π- and σ-donor systems is discussed.     

Calculations of van der Waals interaction energies for Al, Cr, Fe, and Ir acetylacetonate crystals

Experimental data have been analyzed and interpreted for four volatile acetylacetonates of trivalent metals Al, Cr, Fe, and Ir. The crystal lattice energies were calculated by the atom-atom potential method. The lattice energies obtained by using the Buckingham potential are in better agreement with the sublimation heats of these metal complexes than those calculated from the Lennard-Jones potential. The experimental dependences of vapor pressure for the complexes are in satisfactory agreement with the values obtained from the calculated lattice energies and entropies of crystal-gas transitions.     

Sandwich Compounds of Transition Metals with Cyclopolyenes and Isolobal Boron Analogues

A series of sandwich compounds of transition metals (M=Ni, Fe, Cr) with cyclic hydrocarbon (M(CH)_n) and borane (M(BH₂)_n), ligands (including mixed hydrocarbon/borane sandwiches) has been studied using density functional theory (B3LYP/6‐311+G(df,p)). Multicenter bonding between the central metal atom and basal cycloborane rings provides stabilization to planar cycloborane species. Large negative NICS values allude to aromatic character in the cycloboranes similar to the analogous cyclic hydrocarbons. The ability of cycloborane sandwiches to stabilize attached carbocations, radicals and carbanions is also assessed.     

Stereospecific Migration of P from N to C: Ring-Expansion Reaction of Chiral Diazaphospholidine Oxides

An addition-pseudorotation-elimination pathway is proposed for the first stereospecific ring-expansion reaction of chiral diazaphospholidine oxides by a 1,3-migration of P from N to C [Eq. (1)]. R=H, Ph; LDA=lithium diisopropylamide.     

Electron transfer between two stable radicals

Electron spin resonance (ESR) is a sensitive tool to detect the electron-transfer processes in some specified systems from the change of the ESR signals. In this paper, the interaction between poly(N,N-dimethylaniline) (PDMA) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) was investigated by ESR spectra based on the stability of their radicals in ethanol. It is shown that an electron-transfer process occurs from nitrogen atoms of PDMA molecules to those of DPPH molecules which makes PDMA form more stable radical cations and eliminates the radicals on DPPH.     

Reactions following electron transfer: Origins of selectivity in the reaction of radicals with carbanions

The regiochemistry observed upon arylation of ambident carbanions by phenyl radicals can be readily explained by consideration of the site of greatest basicity. Thus 1,3-diphenylindenyl anion is arylated to give predominantly 1,1,3-triphenylindene despite the presumed greater stability of the intermediate leading to 1,2,3-triphenyl indene, that is, the 1,2,3-triphenylisoindene radical anion. This explanation may also account for the preference of C-alkylation rather than O-alklation of nitrocarbanions and for the absence of O-arylation in the S_RN1 arylation of enolates. The lack of reactivity of other carbanions is rationalized on thermodynamic grounds.     

Key counter ion parameters governing polluted nafion membrane properties

Some properties of perfluorosulphonated ionomer membranes contaminated by a series of 10 counter ions were investigated by infrared spectroscopy (FTIR), thermogravimetric analysis coupled to mass spectroscopy (TG‐MS), and dynamic mechanical spectrometry (DMA). Distinctive parameters were extracted and regarded as a function of the cations' properties. An optimum interaction between sulfonate group and cation was found for cations with Lewis Acid Strength (LAS) in the 0.2–0.3 range. This critical value is found to be the Lewis Basic Strength (LBS‐) of the sulfonate anion in Nafion membrane. Thermal stability analyses also point out the influence of this cation parameter on the polymer degradation process. Cations with LAS values lower than LBS‐ improve the thermal stability of the side chains while cations with LAS values higher than LBS‐ enhance the thermal degradation. Moreover, the temperature of the modulus drop increases with the LAS of the counter ion. For cations with values lower 0.5, the transition is attributed to the glass relaxation of the polymer while for cations showing LAS values higher than 0.5, the loss of stiffness originates from the polymer thermal degradation process. The overview of the experimental data allows the definition of calibration curves as a function of the cations' LAS. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1381–1392, 2009     

Towards Predicting the Magnitude and Direction of Radical Substituent Effect:The Importance of Polar Interaction in Radical System

Substituent effect of reactive intermediates has long been a central issue in organic chemistry^[1]. While quantitative knowledge on structure-property relationship of close-shell species (e.g., carbanions, carbocations, and neutral molecules, etc.) has already been well documented in the literature and textbooks, the present understanding of substituent effect on radicals is still largely limited to the information drawn basically from the (σ and BDE (bond dissociation energy) studies of a single family, the benzyl radical^[2]. Naturally, the general applicability of these scales as indicators of radical stability to other families bearing different structural features needs to be examined. The present systematic experimental and computational investigation on radicals of the benzylic type centered on carbon atoms of vaiying degree of electronegativity shows that i) there are actually three (rather than one as previously well claimed) basic patterns of radical substituent effect for carbon radical; and ii) prediction of the direction and magnitude of radical substituent effect should be workable on the basis of quantitative understanding of polar interactions within a radical system^[3]. Experimental evidence and detailed analysis on this respect are presented.     

General method for the photogeneration of benzolated cationic and anionic systems in aqueous solution. Test of relative stability of these systems in the excited state.

A general method for the photogeneration of dibenzolated carbocations and carbanions is described with the aim of testing the relative stabilty of these species as a function of number of pi-electrons in the excited state.     

Theoretical study of reaction of trifluoromethyl radical with hydroxyl and hydrogen radicals

Ab initio molecular orbital theory and density functional theory calculations have been carried out on the reactions of the trifluoromethyl radical with the hydroxyl and the hydrogen radicals. These reactions are key reactions that underlie a new fire extinguishing mechanism of non-bromine-containing halon replacements. The activation energies calculated by the MP2 and QCISD methods are in good agreement with the experimental values. The B3LYP, as well as MP2 and QCISD, give good results for the calculations of the heats of reactions. The GAUSSIAN-1 and GAUSSIAN-2 theory calculations present the most acxcurate results on both the activation energies and the heats of reactions. The effects of the scaling factors on the activation energies and the heats of reactions are also evaluated. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 277–289, 1998