Nucleophilicity parameter carbocation-nucleophile

The roles of carbocations in commercially important hydrocarbon transformations are still not perfectly understood. The same can be said for carbocations in biological systems. Significant questions concerning reactivity still need to be explained. Why do so many reactions of carbocations show constant selectivity, in violation of the reactivity-selectivity principle Is it possible to develop a unified scale of elec-trophilicity-nucleophilicity, in particular one that incorporates these parameters into the general framework of Lewis acidity and basicity. Finally, quite sophisticated synthetic transformations are being developed that employ carbocations, based upon insights revealed by the mechanistic studies. 

In principle, these measurements represent an application of the Swain-Scott relationship to two nucleophiles only. This is apparent from Equation (28), in which nAz corresponds to n for the azide ion and the electrophilic parameter s is seen to measure the selectivity of the carbocation between azide... 

Far from confirming a dependence of nucleophilic selectivity on the reactivity of the carbocations, Ritchie observed that selectivities were unchanged over a 106-fold change in reactivity.15 He enshrined this result in an equation (29) analogous to that of Swain and Scott, but with the nucleophilic parameter n modified to N+ to indicate its reference (initially) to reactions of cations, and with the selectivity parameter s taken as 1.0, that is, with no dependence of the selectivity of the cation on its reactivity (as measured by the rate constant for the reference nucleophile, kn2o for water). 

The relative reactivity, solvent isotope effect (k /k -)) and activation parameters for the acid-catalysed hydration of allylic alcohols CH2=CR—CH2OH (R = H, Me) have been found to be similar to those for other alkenes. Whereas the results can be interpreted in terms of the conventional Ad-E2 mechanism, computed values for the life-time of possible carbocation intermediates suggest another feasible mechanism for CH2=CHCH20H, according to which the nucleophilic attack by the solvent is concerted with protonation55 56. 

The lifetime of the ionic bicyclobutane is a parameter of prime importance. An attempt was made to estimate this parameter for the thio-derivative using the clocking method . The latter involves competition between two nucleophiles e.g., PhS" and the solvent MeOH. Assuming that the PhS " reacts with carbocations at a diffusion-controlled rate and given the product ratio as a function of concentration, the lifetime of 118 in methanol was calculated to be of the order of 10" seconds . ... 

Competition experiments between two ir-bonded nucleophiles within the same molecule were studied in an attempt to identify reaction parameters and factors responsible for regioselectivity. These experiments, summarized in heme 12, demonstrated that the dithioacetal (initiating carbocation) is in competition with two nucleophilic functional groups within the same molecule, a silyl enol ether and a vinylsilane. In this instance, when the thioacetal (29) was treated with DMTSF, complete chemoselectiv-... 

Recent works of Mayr and co-workers4-14 have illustrated this trend. In fact, these authors have established, in contrast to the accepted opinion about the relative character of the experimental electrophilicity/nucleophilicity scales for many reactions in organic and organometallic chemistry, that it would be possible to define nucleophilicity and electrophilicity parameters that are independent of the reaction partners. Mayr et al. proposed that the rates of reactions of carbocations with uncharged nucleophiles obey the linear free energy relationship given by 4-14... 

A benzhydrilium carbocation series with calibrated electrophilicities, E, has been used to determine the nucleophilicity parameters, N and % for a variety of nucleophile-electrophile reactions. For example, benzhydrilium ions (16 examples, 1.48 > >-10.04) were used to evaluate the Lewis basicities of 2-imidazolines and related A-heterocyclic compounds through Mayr s equation log k = Sj N+E)3 The nucleophilicity parameters N and % of the A-heterocycles were determined by measuring the second-order rate constants for the reactions between the benzhydrilium ions and A-heterocycles. The calculated N and % parameters were shown to be useful in accurately predicting the reaction rates between the same A-heterocycles and another set of electrophiles, Michael acceptors. 

These benzhydrilium carbocations were also used to develop a comprehensive scale of hydride donor strength. Both Si-H and C-H donors were evaluated by their second-order rate constants of hydride transfers to the carbocation electrophiles. The respective nucleophilicity parameter, N and %, were then determined. The benzhydryl cation series was also used to evaluate the nucleophilic reactivities of azolium enolates (1). ... 

The kinetics of the reactions of benzhydryl cations with some heterocyclic aromatic compounds have been determined. Application of a linear free enthalpy relationship allows the determination of nucleophilicity parameters for the aromatic compounds which correlate linearly with cr+ values. " Absolute rate constants for reactions of aromatic compounds having substituents of known <7+ parameters with carbocations and diazonium ions can be calculated. Scales of nucleophilicity and electrophilicity were drawn up and can be used predictively. 

Ionization rate constants in aqueous acetonitrile were obtained for trityl chlorides, bromides, and acetates covering 21 units in the pA1r+ of the trityl cation. This study observed solvolyses with and without common ion return, solvolyses where the trityl cation could be observed to form and then decay, solvolyses where water addition occurs before complete formation of the cation, and at the other extreme, solvolyses that yield persistent carbocations. Mayr and coworkers showed how electrophilicity and nucleophilicity parameters rationalize reactivity patterns and resolve mechanistic controversies in organocatalytic cyclizations. The thermodynamic affinities of a large number of Lewis bases were computed for addition to the methyl, diphenylmethyl, and triphenyl-methyl cations. ... 

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