Electrophilicity parameters review

Picosecond absorption spectroscopy was employed to study the dynamics of contact ion pairs produced upon the photolysis of substituted diphenylmethyl acetates in the solvents acetonitrile, dimethyl sulfoxide, and 2,2,2-trifluoroethanol.66 A review appeared of the equation developed by Mayr and co-workers log k = s(N + E), where k is the rate constant at 20 °C, s and N are nucleophile-dependent parameters, and is an electrophilicity parameter 67 This equation, originally developed for benzhydrylium ions and n-nucleophiles, has now been employed for a large number of different types of electrophiles and nucleophiles. The E, N, and s parameters now available can be used to predict the rates of a large number of polar organic reactions. Rate constants for the reactions of benzhydrylium ions with halide ions were obtained... 

In this section three main aspects will be considered. Firstly, the basic strengths of the principal heterocyclic systems under review and the effects of structural modification on this parameter will be discussed. For reference some pK values are collected in Table 3. Secondly, the position of protonation in these carbon-protonating systems will be considered. Thirdly, the reactivity aspects of protonation are mentioned. Protonation yields in most cases highly reactive electrophilic species. Under conditions in which both protonated and non-protonated base co-exist, polymerization frequently occurs. Further ipso protonation of substituted derivatives may induce rearrangement, and also the protonated heterocycles are found to be subject to ring-opening attack by nucleophilic reagents. 

Because of the different effects of electrophilic solvation of the various negative charges (i.e. Cl- for Y, "OTs for Tots I" for Z, 0 for Ej), direct comparisons between the various scales should be done cautiously. A wide variety of correlations giving clear indications of trends, has been reported by Reichardt and Dimroth (1968), but the significance of a recent general survey of scales of solvent polarity is doubtful, because of the many parameters used in the correlations (Fowler et al., 1971). The multi-parameter approach has also been adopted and reviewed by Koppel and Palm (1972). 

The perturbational MO method of Longuet-Higgins (11) and Dewar (12), which was thoroughly reviewed by Dewar and Dougherty (6), has been the pencil-and-paper method of choice in numerous applications. More recently, a modified free-electron (MFE) MO approach (13-15) and a valence-bond structure-resonance theory (VBSRT) (7, 16, 17) have been applied to several PAH structure and reactivity problems. A new perturbational variant of the free-electron MO method (PMO F) has also been derived and reported (8, 18). Both PMO F and VBSRT qualify as simple pencil-and-paper procedures. When applied to a compilation of electrophilic substitution parameters (ct+) (19-23), the correlation coefficients of calculated reactivity indexes with cr+ for alternant hydrocarbons are 0.973 and 0.959, respectively (8). In this case, the performance of the PMO F method rivals that of the best available SCF calculations for systems of this size, and that of VBSRT is sufficient for most purposes. 

Preliminary studies were carried out in order to justify, using the reactivity index machinery, the higher reactivity of Co(II) derivatives with respect to other M(II) transition metal complexes, in particular when M = Mn(II) or Fe(II). Several ab intio smdies of the ground state properties of M-N4 complexes can be found in literature, especially concerning the relative stability of the different spin states (for instance in the case of Fe(II) derivatives). Here we consider only the most stable spin state for each metal complex and analyse the effect of the metal on the reactivity indexes (i.e. hardness, softness and electrophilicity). As already mentioned, and contrary to all other calculations reported in this review, these computations were performed using the parametrized hybrid Becke three-parameter exchange correlation functional (B3LYP " ) and a smaller basis set. The same level of theory was used to compute the donor molecule, i.e. the anionic form of 2-mercaptoethanol. 

As in the previous volume, this chapter reviews kinetic and mechanistic studies of the stoichiometric reactions of coordinated hydrocarbons with nucleophiles and electrophiles, together with some related processes such as cycloadditions. Nucleophilic addition and substitution, in particular, continue to attract considerable interest, especially with dienyl- and arene-metal substrates. Even here, however, quantitative studies still lag far behind the extensive synthetic literature. However, fundamental reactivity patterns and parameters are beginning to emerge from the mechanistic studies which should assist in the design of rational syntheses. 

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