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Assessment of Nucleophilicity and Electrophilicity of Radicals, and of Polar and Enthalpy Effects on Radical Addition Reactions1

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journal contribution
posted on 10.11.1998, 00:00 by Károly Héberger, Antal Lopata
Principal component analysis (PCA) was performed on experimental rate constant and theoretical barrier height data of radical addition reactions involving various carbon- and sulfur-centered radicals and vinyl-type alkenes. Altogether six data sets were analyzed. In three cases the reactivity data were completed by certain descriptors, i.e., the electron affinity (EA) and negative ionization potential (−IP) of alkenes, as well as the exothermicity (−ΔHr) of reactions. It was found that in each case the first two principal components account for more than 93% of the total variance in the data. The scores of the first principal component correlate with EA and (−ΔHr), whereas those of the second principal component with (−IP). It is concluded that PCA is able to decompose both experimental and theoretical reactivity data into nucleophilic and electrophilic components, as well as into polar and enthalpy terms. In the plots of component loadings the radicals form significant groups depending on their character. Thus, PCA can classify radicals according to nucleophilicity and electrophilicity. The PCA results were validated by significant correlations of experimental and theoretical reactivity data with Hammett σp as well as with the descriptors EA, (−ΔHr), and (−IP). The hydroxymethyl radical is classified as strongly nucleophilic, the methyl radical as moderately nucleophilic, the tert-butoxycarbonylmethyl and cyanomethyl radicals as weakly nucleophilic, the phenylsulfonyl and tosyl radicals as moderately electrophilic, and the 2,2-dimethyl-4,6-dioxo-1,3-dioxan-5-yl radical as strongly electrophilic. It is concluded that the reactivities of tert-butoxycarbonylmethyl and cyanomethyl radicals are mainly governed by enthalpy effects. This conclusion is in agreement with the findings of Giese et al. [Chem. Ber. 1988, 121, 2063−2066] and Fischer et al. [Helv. Chim. Acta 1995, 78, 194−214]. A symmetry pattern of correlations is proposed:  the reactivity correlates with EA for strongly nucleophilic radicals, with EA and (−ΔHr) for moderately nucleophilic radicals, with (−ΔHr) for weakly nucleophilic or weakly electrophilic radicals, with (−ΔHr) and (−IP) for moderately electrophilic radicals, and with (−IP) for strongly electrophilic radicals. On the basis of the symmetry pattern of correlations, it is concluded that the dominant factors influencing radical addition reactions are polar effects alone for strongly nucleophilic or strongly electrophilic radicals, polar and enthalpy effects for moderately nucleophilic or moderately electrophilic radicals, and enthalpy effects alone for weakly nucleophilic or weakly electrophilic radicals.