Marie-Françoise Ruasse

Electrophilic Bromination of Carbon—Carbon Double Bonds: Structure, Solvent and Mechanism

Publisher Summary This chapter discusses the experimental conditions, under which it is possible to obtain data relevant to the study of the bromination mechanism, and presents the evidence for the occurrence of charge transfer complexes and cationic intermediates on the reaction pathway. The chapter presents the kinetic substituent and solvent effects on this cation-forming halogenation; these effects are discussed in terms of kinetic selectivity, transition-state shifts and solvation. From an energetic point of view, free bromine addition can be considered as an ionization process leading from a neutral reagent to a cation-anion pair in a single elementary step, most work on this electrophilic addition can be used to understand how charge separation and stabilization occur in the organic reactions. Bromination thus appears as a suitable model, and is complementary to the conventional heterolytic substitutions. The association of kinetics and stereochemistry is particularly useful for obtaining data on the structure, open or bridged depending on the substituents of bromination intermediates.

Nucleophilic substitution at sulfonyl sulfur atom: aminolysis of 1-tosyl-3-methyl imidazolium chloride in aqueous medium

Abstract Kinetics of the reaction of 1-tosyl-3-methyl-imidazolium chloride with various amines were measured to examine the nature of sulfonyl transfer in enzymatic reactions. The activation parameters and the value of the bronsted exponent, β = 0.48, are consistent with a small degree of bonding between the entering amine and the sulfur atom in the transition state. Similarities in the nucleophilic behavior of sulfonyl and carbonyl groups are detected.

Reactivity and selectivity control by reactants and products. A general relationship between the selectivity and the position of the transition state.

Abstract Although the reactivity-selectivity principle and the frontier molecular orbital theory lead to opposed reactivity-selectivity relationships, they lead to an identical transition state position-selectivity relationship. The transition state position is generally reactant- and product-dependent. The RSP which neglects reactant effects and the FMO which neglects product effects can be considered as the two limits of the general rule.

The polar reaction constant for alkene chlorination. similarity and differences in the chloronium and bromonium ion like transition states.

Abstract The polar reaction constant, ϱ*cl2, for the addition of free chlorine to alkenes, obtained in methanol at 25°C by direct kinetic methods, is -2.9 whereas ϱ*Br2 is -3.1. The change from bromination to chlorination isassociated with a large reactivity increase but a small drop in selectivity. This result is discussed in terms of the Harmond postulate and dependence of the charge distribution on the bridging atom in the halomium transition states.

Kinetics and mechanism of phosphate-catalyzed hydrolysis of benzoate esters: comparison with nucleophilic catalysis by imidazole and o-iodosobenzoate

Phosphate-catalyzed hydrolysis of 2,4-dinitrophenyl 4-X-benzoate, and 3- or 4-Y-phenyl 3,5-dinitrobenzoates, where X and Y are substituents, has been studied spectrophotometrically. The following conclusions are based on catalytic rate constants, solvent kinetic isotope effect, detection of a mixed anhydride by FTIR, and application of the Hammett equation: (i) the catalytically active species is HPO42−, (ii) the mechanism of catalysis is nucleophilic, the reaction proceeds via the irreversible formation of acyl phosphate, (iii) the formation of a tetrahedral species by attack of the HPO42− on the ester CO group is rate limiting. Comparison of these results with our previous data on hydrolysis of benzoate esters with imidazole, or o-iodosobenzoate anion showed that: (i) the catalytic efficiency observed, o-iodosobenzoate > imidazole > phosphate, is due to a combination of steric and solvation effects, and the enhancement of the nucleophilicity of the former catalyst by the α-effect, (ii) the nonlinear Bronsted-type plot between log (catalytic constant) and pKa of the leaving 4-Y-phenol indicates that the rate-limiting step for o-iodosobenzoate changes as a function of changing Y, (iii) the reaction rate-limiting steps are different for the above-mentioned nucleophiles.

Stereo-, regio- and chemoselectivity of bromination of ethylenic compounds

Publisher Summary Systematic studies of the selectivity of electrophilic bromine addition to ethylenic bonds are almost inexistent whereas the selectivity of electrophilic bromination of aromatic compounds has been extensively investigated. This surprising difference arises probably from particular features of their reaction mechanisms. Aromatic substitution exhibits only regioselectivity that is determined by the bromine attack itself, i.e. the selectivity- and rate-determining steps are identical. Whereas the three possible selectivities, stereo-, regio- and chemo-selectivity, of bromine addition are determined in steps posterior to the formation of the ionic intermediate. Bromine addition is, therefore, more complex than bromine substitution, as regards the variety of the selectivities and as regards the mechanistic aspects which determine the product formation.

Self-association mechanisms in aqueous media: dimerization versus micellization of alkyl-ellipticinium derivatives

Self-association mechanisms in aqueous media of ellipticinium (E), 2-methylellipticinium (2-NME), oxazolopyridocarbazolium (H-OPC), 10-pentyloxazolopyridocarbazolium (10-pentyl-OPC), 2 aminopentyloxazolopyridocarbazolium (2-aminopentyl-OPC) and 2-pentyloxazolopyridocarbazolium (2-pentyl-OPC) acetates have been investigated by spectroscopic and kinetic studies. Whereas E, 2-NME, H-OPC, 10-N-pentyl-OPC and 2-aminopentyl-OPC dimerize by self-stacking, the 2-pentyl-OPC displays micellar behaviour (with a critical micellar concentration of 20 µmol dm–3), as a direct consequence of its amphiphilic character. As regards dimerization, both aliphatic substitution (Kd= 7.6 × 103 and 2.4 × 104 dm3 mol–1 for E and 2-NME, respectively) and extension of the π-electron aromatic system (Kd= 1.4 × 105 and 4.5 × 105 dm3 mol–1 for H-OPC and 10-pentyl-OPC, respectively) significantly increase the dimer stability. From kinetic analysis, dimer stability seems to be controlled mainly by the reverse rate constants which vary from 4.9 × 104 to 1.9 × 103s–1.

Solvation and steric effects on electrophilic reactivity of ethylenic compounds. Part 4. Bromination of oct-1-ene in anionic microemulsions

The kinetics and product distribution of the bromination of oct-1-ene in anionic sodium dodecyl-sulfate (SDS)–butanol–hexane–water and sodium bis (2-ethylhexyl)sulfosuccinate (AOT)–isooctane–water microemulsions are reported. Dibromide and solvent-incorporated products are formed. In both kinds of microemulsion, the dibromide yield decreases smoothly from 100% to 10% as the water content of the reaction medium increases from 0% to 65%, whereas in pure water or butanol it is greater than 80%. The regioselectivity of the water- or butanol-incoporated products is 70:30 Markownikoff:anti-Markownikoff, a ratio identical with that found in pure methanol, butanol or water. Kinetic bromide-ion effects on the reaction in a water-rich (75%) SDS microemulsion, show that bromination occurs in the interfacial oil–water region, and not in one of the two microphases, the only brominating agent being molecular bromine and not the tribromide ion. The overall bromination rate constant in this SDS microemulsion (k= 1.6 × 104 dm3 mol–1 s–1) is smaller than that in pure water (2.3 × 107 dm3 mol–1 s–1) and in SDS micelles (2 × 105 dm3 mol–1 s–1), in the same range as that in a 80–20 methanol–water mixture, and greater than that in butanol (2 × 102 dm3 mol–1 s–1). These results are discussed in terms of the particular characteristics (ionization and dissociating abilities, aquation and water properties) of the microemulsion interfaces.