Electrochemistry of Electron Transfer Probes. Observation of the Transition from Activation to Counterdiffusion Control in the Fragmentation of α-Aryloxyacetophenone Radical Anions^1a

Abstract

The cleavage of radical anions of substituted α-phenoxyacetophenones, X-C6H4COCH2OPh, IIa−k, has been studied in DMF by voltammetric and coulometric techniques. The standard potentials (E°) for formation of and rate constants, k, for the cleavage of the radical anions were determined using linear sweep voltammetry, LSV, together with digital simulation and previously reported laser flash photolysis data. The rate constants cover a range of almost eight orders of magnitude (0.4 s-1 for X = p-MeCO- to 1.3·107 s-1 for X = p-MeO−). The relative driving forces, ΔΔG°het(RX•-), for the heterolytic cleavage of the radical anions (to give R• + X-) were estimated from thermochemical cycles. A combined plot of log(k) versus ΔΔG°het(RX•-) for the radical anions of IIa − k and of α-aryloxyacetophenones gave a curve with α = 0.5 at high driving forces and α = 1 at low driving forces, where α = ∂Δ /∂ΔG°. The plot was analyzed using a model in which reversible cleavage of the radical anions takes place inside the solvent cage followed by (counter)diffusion of the fragments out of the solvent cage. The change in the value of α is interpreted as a change in the rate limiting process from chemical activation (i.e.,, fragmentation) to counterdiffusion. The model allowed the determination of the absolute values of ΔG°het(RX•-) and the intrinsic barrier, Δ , for the fragmentation of the radical anions (8 ± 1 kcal mol-1, 0.35 eV). This leads to an estimate of the homolytic bond dissociation free energy of the C−OPh bond in the unsubstituted α-phenoxyacetophenone.

Application of a new kinetic method in the investigation of cleavage reactions of haloaromatic radical anions†

“…It is also somewhat surprising that the meta-compounds are so "well-behaved" since it is known for other types of systems such as the radical anions of nitrobenzyl halides that the cleavage rate constants are highly dependent on the position of the nitro group relative to the CH 2 X group. 7 Before analysing the plot in greater detail it should be considered whether the endergonic cleavage reactions might be kinetically controlled by out-of-cage diffusion of the dissociated products as seen in studies on radical anions of α-aryloxyacetophenones 46 and radical cations of tert-butylated NADH analogues. not completely diffusion-controlled.…”

“…anions of α-aryloxyacetophenones using Z = 10 13 s Ϫ1 . 46 In a study of the C-C bond cleavage of a number of bibenzylic radical anions and cations the intrinsic barrier was reported to be even lower 42 but the interpretation of the experimental results was subtle in this case due to kinetic control by the outof-cage diffusion for some of the cleavage reactions. 69 For a series of α-substituted acetophenone radical anions, ∆G ‡ o was estimated to be in the order of 70 kJ mol Ϫ1 .…”

“…Previous papers have mainly dealt with the influence of specific factors 4,7-9,24- 41 and not until recently have systematic investigations of families of compounds appeared. [42][43][44][45][46][47]…”

Structural Effects in Radical Clocks and Mechanisms of Grignard Reagent Formation: Special Effect of a Phenyl Substituent in a Radical Clock when the Crossroads of Selectivity is at a Metal/Solution Interface

“…A review covering the use of these tools would demand the citation of more than two thousand references. Without going to this extreme, one may select references that show the use of these mechanistic tools in organic, [6][7][8] organometallic, [9][10][11] and inorganic chemistry, [12][13][14][15][16][17] in electrochemistry, [18][19][20][21][22][23] in photochemistry [24][25][26] and in enzymatic catalysis. [27][28][29][30][31] In this work we deal with radical probes based on intramolecular additions of aryl radicals to unsaturated systems suitably positioned in substituents situated in ortho positions to the aryl radicals.…”

Cleavage Reactions of Radical Anions that Range from Homolytic to Heterolytic within the Same Family of Compounds

“…The cleavage reaction of radical ions can be classified as homolytic (eq 1a) if the cleavage leaves the charge mainly in the same region as it was in the radical ion or heterolytic (eq 1b) in which there is “regioconservation” of spin density. 2a-e Radical anions are shown in eq 1 as they are the subject of the present paper. Savéant and co-workers have described the heterolytic bond cleavage as an intramolecular dissociative electron transfer in which the electron in an orbital mainly centered on A is transferred to B with concerted bond-breaking. 2f-m In addition to the systems investigated in the papers cited, other reactions that have been studied include cleavage reactions in the radical anions of α-aryloxyacetophenones, cleavage of remote C−Br bonds in the radical anions of benzoate esters, and additional examples in the chloroaromatic family …”