Electrochemistry of Electron-Transfer Probes. The Role of the Leaving Group in the Cleavage of Radical Anions of α-Aryloxyacetophenones¹

Abstract

The formal reduction potential (E°) of α-phenoxyacetophenone has been determined from the voltammetric peak potential obtained by linear sweep voltammetry in combination with the rate constant for fragmentation of the radical anion which had been determined by laser flash photolysis. The E° values of a number of α-aryloxyacetophenones were then estimated from a correlation of the 13C chemical shifts of the carbonyl carbon and a similar correlation (E° versus 13C chemical shift) within a series of substituted α-anilinoacetophenones. Using these potentials (which vary by only 34 mV over a wide range of substituents) the rate constants for fragmentation of the α-aryloxyacetophenone radical anions were determined from digital simulation of the corresponding voltammetric waves. The fragmentation rate constants were shown to correlate with the pK a of the corresponding phenols. However, the kinetic range was too small and the experimental errors too large to allow a distinction between a linear and quadratic free energy dependence. A thermochemical analogy between the leaving group ability in reactions of radical anions and that in simple heterolysis of closed shell compounds is developed. The utility of these compounds as potential electron transfer probes is discussed.

Hammett Substituent Effect on the Cleavage Rate Constants of Anion Radicals in Dimethylformamide

“…The effects of the reactant on the cleavage rate constants k c have been discussed in terms of the linear relationships between the activation barrier and the thermodynamic driving force: the values of log k c for a series of aryl halides have been correlated [1][2][3] with the formal potentials of the radical anion formation E°A rX/ArX •-and for a series of -phenoxyacetophenones with the pK a values of substituted phenols. 9 Moreover, a non-linear dependence has also been reported. 10 Substituent effects on the k c values have been discussed only qualitatively, e.g.…”

“…The kinetics of the bond cleavage of aryl anion radicals into neutral radicals and anions has recently attracted considerable attention [1][2][3][4][5][6][7][8][9][10] as an essential step in electron-transfer stimulated reactions, e.g. the S RN 1 substitution.…”

Decomposition of β-O-4 Linked Lignin Model Compound in Anhydrous Ethanol without any Added Catalyst

“…Referring to published reports (Guthrie 1975;Netto-Ferreira et al 1990;Andersen et al 1996;Dorrestijn et al 1999;Lanzalunga and Bietti 2000) and based on the main products' distribution listed in Fig. 2, Fig.…”

“…The POPE radicals subsequently reacted along three routes, Routes Ia, Ib, or Ic, which led to further cleavage, rearrangement, or over-hydrogenation, respectively. According to the products' distribution of the decomposition of 2-PAP in AE offered in Tables 2 and S4, the POPE radicals split into phenoxy radicals and phenylethyl alcohol along Route Ia (Huang et al 1995;Andersen et al 1996;Lanzalunga and Bietti 2000). Then, the resultant 1-phenylethenol (an enol) further tautomerized to a more stable ketone form of acetophenone (AP in Scheme 2) (Huang et al 1995), while the phenoxy radicals were terminated with the formation of phenol (P in Scheme 2) by abstracting hydrogen from acetophenone or the solvent (Netto-Ferreira et al 1990).…”

“…Otherwise, acetophenone was not stable enough that it could be further transformed to 1-phenylethanol (PE in Scheme 2, Table 2 and Fig. S5) by hydrogenation along Route Ia-1 or be abstracted hydrogen atoms to form acetophenone-radicals (Netto-Ferreira et al 1990;Andersen et al 1996;Park et al 2011;Fan et al 2015). The latter underwent dimerization or coupling reactions to form products with higher boiling points that could not be detected by GC-FID procedures (Route Ia-2) (DiNinno et al 1977;Heathcock 1981;Netto-Ferreira et al 1990).…”