The kinetics of the reactions of ethenesulfonyl fluoride (ESF) with sulfonium and pyridinium ylides were measured photometrically to determine the electrophilicity parameter of ESF according to the ...correlation lg k20 °C=sN(N+E). With E=−12.09, ESF is among the strongest Michael acceptors in our comprehensive electrophilicity scale, which explains its excellent performance in reactions with many nucleophiles. Its predicted usability as a reagent in electrophilic aromatic substitutions with electron‐rich arenes was confirmed by uncatalyzed reactions with alkyl‐substituted pyrroles.
Kinetic studies of the reactions of ethenesulfonyl fluoride and styrenesulfonyl fluoride with sulfonium and pyridinium ylides reveal that the SO2F group activates C=C bonds 106 to 108 times more strongly towards nucleophilic attack than an SO2Ph or SO2Tol group. Ethenesulfonyl fluoride is thus one of the most electrophilic monosubstituted ethylenes ever reported (compare the electrophilicity parameters E).
Kinetics and mechanisms of the reactions of p-quinone, 2,5-dichloro-p-quinone, 2,3,4,5-tetrachloro-p-quinone (chloranil), 2,3,4,5-tetrafluoro-p-quinone (fluoranil), and 3,4,5,6-tetrachloro-o-quinone ...with π-nucleophiles (siloxyalkenes, enamines) and amines have been investigated. Products arising from nucleophilic attack at all conceivable sites, that is, at C and O of the carbonyl groups (pathways a, b) as well as at halogenated and nonhalogenated conjugate positions (pathways c, d), were observed. The partial rate constants for the C-attack pathways (a, c, d), which are derived from the photometrically determined second-order rate constants and the product ratios followed the linear free energy relationship log k (20 °C) = s N(E + N) ( Mayr, H. ; J. Am. Chem. Soc. 2001, 123, 9500−9512 ). It was, therefore, possible to calculate the electrophilicity parameters E of the different positions of the quinones from log k (20 °C) and the N and s N parameters of the nucleophilic reaction partners, which have previously been derived from their reactions with benzhydrylium ions. Almost all rate constants for the C-attack pathways (a, c, d) were considerably larger than those calculated for the corresponding SET processes, indicating the operation of polar mechanisms. SET mechanisms may only account for the formation of the products formed via O-attack. With the E parameters determined in this work, it is now possible to predict rate constants for the reactions of these quinones with a large variety of nucleophiles and, thus, envisage unprecedented reactions of quinones.
The rates of the epoxidation reactions of aldehydes, of the aziridination reactions of aldimines, and of the cyclopropanation reactions of α,β-unsaturated ketones with aryl-stabilized ...dimethylsulfonium ylides have been determined photometrically in dimethyl sulfoxide (DMSO). All of these sulfur ylide-mediated cyclization reactions as well as the addition reactions of stabilized carbanions to N-tosyl-activated aldimines have been shown to follow a second-order rate law, where the rate constants reflect the (initial) CC bond formation between nucleophile and electrophile. The derived second-order rate constants (log k 2) have been combined with the known nucleophilicity parameters (N, s N) of the aryl-stabilized sulfur ylides 4a,b and of the acceptor-substituted carbanions 4c–h to calculate the electrophilicity parameters E of aromatic and aliphatic aldehydes (1a–i), N-acceptor-substituted aromatic aldimines (2a–e), and α,β-unsaturated ketones (3a–f) according to the linear free-energy relationship log k 2 = s N(N + E) as defined in J. Am. Chem. Soc. 2001, 123, 9500–9512. The data reported in this work provide the first quantitative comparison of the electrophilic reactivities of aldehydes, imines, and simple Michael acceptors in DMSO with carbocations and cationic metal−π complexes within our comprehensive electrophilicity scale.
As close as you can get: Since Breslow intermediates usually exist in their keto form, their O‐protected tautomers may be considered as their closest isolable relatives. A series of these compounds ...have been synthesized, their structures determined, and the kinetics of their reactions with electrophiles investigated.
The concept of hard and soft acids and bases (HSAB) proved to be useful for rationalizing stability constants of metal complexes. Its application to organic reactions, particularly ambident ...reactivity, has led to exotic blossoms. By attempting to rationalize all the observed regioselectivities by favorable soft–soft and hard–hard as well as unfavorable hard–soft interactions, older treatments of ambident reactivity, which correctly differentiated between thermodynamic and kinetic control as well as between different coordination states of ionic substrates, have been replaced. By ignoring conflicting experimental results and even referring to untraceable experimental data, the HSAB treatment of ambident reactivity has gained undeserved popularity. In this Review we demonstrate that the HSAB as well as the related Klopman–Salem model do not even correctly predict the behavior of the prototypes of ambident nucleophiles and, therefore, are rather misleading instead of useful guides. An alternative treatment of ambident reactivity based on Marcus theory will be presented.
Hard or soft? The prediction of ambident reactivity by the concept of hard and soft acids and bases fails in the majority of cases and has to be abandoned. In this Review an alternative approach is presented to rationalize the regioselectivities of ambident systems which differentiates between kinetic and thermodynamic product control as well as between reactions proceeding with and without activation energy. The predictive power of qualitative Marcus theory is demonstrated.
Do general nucleophilicity scales exist? Mayr, Herbert; Ofial, Armin R.
Journal of physical organic chemistry,
July ‐ August 2008, Letnik:
21, Številka:
7-8
Journal Article
The nucleophilicity and Lewis basicity of DBU and DBN toward C(sp(2)) centers have been measured: nucleophilicities increase in the series DMAP < DBU < DBN < DABCO while Lewis basicities are DABCO < ...DMAP < DBU < DBN.
Aromatic or nonaromatic? Kinetic investigations show that structurally analogous saturated and unsaturated N‐heterocyclic carbenes have almost identical nucleophilic reactivities, while the ...corresponding deoxy Breslow intermediates differ dramatically.
Equilibrium constants for the associations of 17 diaryliodonium salts Ar2I+X– with 11 different Lewis bases (halide ions, carboxylates, p-nitrophenolate, amines, and tris(p-anisyl)phosphine) have ...been investigated by titrations followed by photometric or conductometric methods as well as by isothermal titration calorimetry (ITC) in acetonitrile at 20 °C. The resulting set of equilibrium constants K I covers 6 orders of magnitude and can be expressed by the linear free-energy relationship lg K I = s I LAI + LBI, which characterizes iodonium ions by the Lewis acidity parameter LAI, as well as the iodonium-specific affinities of Lewis bases by the Lewis basicity parameter LBI and the susceptibility s I. Least squares minimization with the definition LAI = 0 for Ph2I+ and s I = 1.00 for the benzoate ion provides Lewis acidities LAI for 17 iodonium ions and Lewis basicities LBI and s I for 10 Lewis bases. The lack of a general correlation between the Lewis basicities LBI (with respect to Ar2I+) and LB (with respect to Ar2CH+) indicates that different factors control the thermodynamics of Lewis adduct formation for iodonium ions and carbenium ions. Analysis of temperature-dependent equilibrium measurements as well as ITC experiments reveal a large entropic contribution to the observed Gibbs reaction energies for the Lewis adduct formations from iodonium ions and Lewis bases originating from solvation effects. The kinetics of the benzoate transfer from the bis(4-dimethylamino)-substituted benzhydryl benzoate Ar2CH–OBz to the phenyl(perfluorophenyl)iodonium ion was found to follow a first-order rate law. The first-order rate constant k obs was not affected by the concentration of Ph(C6F5)I+ indicating that the benzoate release from Ar2CH–OBz proceeds via an unassisted S N1-type mechanism followed by interception of the released benzoate ions by Ph(C6F5)I+ ions.
