Activation parameters nucleophilic substitutions

A more unusual fact observed in thiazole chemistiy is that also the other positions (4 and 5) are activated toward the nucleophilic substitution, as found independently by Metzger and coworkers (46) and by Todesco and coworkers (30, 47). Some kinetic data are reported in Table V-2. As the data in Table V-2 indicate, no simple relationship between nucleophilic reactivity and charge density, or other parameters available from more or less sophisticated calculation methods, can be applied. As a... 

Rappoport and Topol investigated the displacement of the halogen of bromo- and chloromethylenemalonates (287 X= Br, Cl) by several substituted anilines and that of the brosyloxy group of (4-nitrophenyl)(4-bromo-phenylsulfonyloxy)methylenemalonate (289) by morpholine and piperidine, in acetonitrile. A rate-determining nucleophilic addition of the amines was suggested as the mechanism for these reactions. Activation parameters (AH, AS ) were determined [72JCS(P2)1823]. 

Nucleophilic substitution on methyl / -nitrobenzenesulfonate in CH2CI2 has been studied with a series of chloride salts with different structures and solvations BU4NCI, PPNCl [bis(triphenylphosphoranylidene)ammonium chloride], KCl complexed by 18-crown-6 or Kryptofix 2,2,2, and for comparison PPNBr. ° Rate constants and activation parameters are in accordance with an S 2 mechanism. The results were treated by the Acree equation. There are two reaction paths the first, involving the chloride ion, has the same rate for all the salts, whereas the second slower path, involving the ion pair, has a rate related to the dissociation constant of the salt. 

Enthalpies of reaction for nucleophilic substitution of ethyl iodide by a series of 27 nucleophiles in acetonitrile have been determined.147 Various empirical correlations were developed. Partial desolvation accompanying activation has been identified as the major contributor to activation thermodynamic parameters, while the propensity of the reacting central atom in the nucleophilic anion plays a crucial role in determining reaction thermodynamic parameters. 

There has been a review of relationships between activation parameters and mechanisms for biomolecular reactions in solution, including both nucleophilic substitutions and additions.5 Several studies have been reported involving substitutions by amine nucleophiles where both electronic and steric effects may be important. Kinetic studies of the reactions of 2,4-dinitrophenyl 2,4,6-trinitrophenyl ether (1) with ring-substituted... 

Picosecond absorption spectroscopy studies of the contact ion pairs formed in the photo-initiated, S N 1 reaction of three substituted benzhydryl acetates (18) provided the rate constants for the k and k2 steps of the reaction (Scheme 10), in acetonitrile and DMSO.83 The activation parameters for the k and k2 steps were obtained from the temperature dependence of these steps and the transition state energies were calculated from the rate constants. This allowed the energy surfaces for three substituted substrates to be calculated in each solvent. The effect of solvent reorganization on the reactions of the unsubstituted and methyl-substituted benzhydryl contact ion pairs (CIP) was significant, causing a breakdown of transition state theory for these reactions. The results indicated that it will be very difficult to develop a simple theory of nucleophilicity in, S N1 reactions and that Marcus theory cannot be applied to SnI processes. 

Substitution reactions of Co2(CO)g show more variation than the group 7 dimers. Reaction with CO, AsPhs, and H2 show a rate law that is independent of the concentration of the incoming nucleophile at temperatmes from -15 to 30°C. The activation parameters (A// = 22kcalmoR and AS% = 10 calK mol ) and the lack of dependence on... 

Kinetic studies ou reactions of Pt(ll) complexes with biologically relevant nucleophiles have been reported. The substitution of both coordinated water molecules by a series of nucleophiles (namely, thiourea (tu), L-methionine (L-Met), and gnanosine-5 -monophosphate (5 -GMP)) was investigated as a fimction of concentration, temperature, and pressure using stopped-flow techniques and was found to occur in two subsequent reaction steps. The activation parameters for all reactions suggest an associative substitution mechanism. 

The vertical ionization potential for a solvated chemical species can be the measure of its reactivity in the solution phase, especially for a single electron transfer reaction. It has been reported that the ionization potentials of anions in solution are conelated with the kinetic parameter for nucleophilic substitution reaction. This implies that an important aspect of the activation process of the reaction is a single electron transfer from anion to substrate. The ionization potential for solvated species has been available as the threshold energy E by photoeiectron emission spectroscopy for solution (PEES). This spectroscopic technique is able to provide the , values of almost any solvated species, such as organic, inorganic, cations, anions and neutral molecules in aqueous and nonaqueous solutions. 

In a manner similar to that of olefins, bicyclobutanes substituted by electron-withdrawing groups at the bridgehead position can undergo facile nucleophilic attacks. Studies of alkoxide-catalyzed additions of alcohols to bicyclobutanecarbonitrile (35, R = H) (equation 73) using crotononitrile as a reference olefin (equation 74), showed that the two substrates resemble one another very much in both their reactivity and activation parameters. ... 

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