Nucleofugality parameters

Here Sf and Nf are nucleofuge-specific parameters, and Ef is an electrofuge-specific parameter. It is important to note that these nucleofugality parameters are a function of both the leaving group and the solvent. Selected values of Nf and Sf are listed in Table 8.17. 

Nucleophilicity parameters, N and % for electrophile-nucleophile combination based on the Mayr equation ogk = Sj (N+E), were reported for highly nucleophilic pyridines bearing amino groups at carbons 3, 4, and 5, isothioureas," and thiocarboxylates, dithiocarbamate, and dithiocarbonate." Electrophilicity parameters E were reported for triarylmethyl cations with para and meta fluorine substituents," 1,3-diarylallyl cations," and aldehydes, imines, and enones." Nucleofugality parameters Nf and Sf for ionization based upon the Mayr equation logki = Sf(Ef + Nf) were reported for acetate in aqueous methanol" and chloride in aprotic solvents." The latter study also shows that common solvent parameters do not reliably predict ionization rates in aprotic solvents. Electrofugality parameters Ef were reported for triarylmethyl cations with para and meta fluorine substituents." ... 

Electrophilicity parameters E were reported for highly reactive benzhydrylium ions. This study shows that correlations of log 2 versus E remain linear even when = 0, showing that a change from activation control to entropy control does not cause a bend in the linear free energy relationship. The correlation lines do flatten when the diffusion limit is approached. Nucleofugality parameters and for 5 1 ionization based on the Mayr equation log kjon = + N ) were reported for fluoride in protic solvents, var-... 

Thus, in a reaction of a series of nucleophiles (NU2) with A-Nuj a plot of logkoveraii vcrsus a polarity parameter of the nucleophile (for example pAa or a) will yield a break when the value of the parameter for NU2 is that for the constant nucleofuge Nuj. A similar result would occur for... 

The more stable the negative charge of the nucleophilic atom, the less nucleophilic it is. For example, due to differing resonance stabilization, an alkoxide is more nucleophilic than a phenoxide, which in turn is more nucleophilic than a carboxylate. This trend reflects the relative basicities, as well. Below we will describe LFERs for nucleophilicity and nucleofu-gality that correlate the pfCgS of the conjugate acids of the nucleophiles and nucleofuges to sensitivity parameters Pnuc and Piq). 

---